Carb tuning, last part of the project

[1980cm400t]

I am having some carburetor issues with my recently finished project bike.

The good:
The bike runs well when throttle is more than 10% open, Hard or light acceleration is no issue, the bike is goes like its namesake.
The bike is not terribly lean as the pipes are not bluing

The bad,
The bike will not start without ether. It will typically fire (maybe one or 2 cylinders) a single time but after that nothing. It does not matter what the throttle position is, or whether the choke is on or off. Give it the tiniest shot of starting fluid and it fires right up, and will easily restart even if shut down immediately after starting.
The bike won't shut down with the idle screw out. When the idle screw is adjusted the idle goes up and down as expected up to a certain point then the idle goes no lower (around 1000 rpm) every other bike I have worked on shuts off when the butterfly valves are completely closed
With the throttle closed or less than 10% the engine does not run evenly, idle is rough. After sitting at a stoplight for a time there is an odd drop in power the first time you take off; it goes for a second, slows down for a second, and then takes off normally.
When the fuel petcocks are shut off and the engine is revved, the idle will stay 3000 rpm or so, opening the fuel valves back up brings the idle back to normal.

The details,
The bike an xs1100 "special" is a complete rebuild project. It has velocity stacks with pod filters, 4 into 2 exhaust with straight pipes, the fuel octo has been removed and replaced with a gravity feed system. The carbs have been completely and thoroughly cleaned. The float valve screens were removed (I added a fuel filter and the new petcocks have a screen as well). The float valves, the carb diaphragms, and the inlet boots were all replaced. The air jets have been replaced with plugs (pod filters and velocity stacks cause the air jets to inject too much fuel and cause the engine to run so rich it shuts down once hot). Adding the plugs corrected this issue although I didn't need ether to start it then. The diaphragm springs were stretched to ~1/2" above spec which helped smooth out the idle somewhat (they were originally too short due to age). Jetting is currently stock (new jets in the stock size). I tried turning the pilots jets in half a turn from standard position, it made the lowest idle speed lower but made the engine prone to shutting off at stoplights and made the idle rougher. The fuel levels are all equal and fairly close to where they are supposed to be according to the manual.

At this point I am suspecting that the fuel level is too high and is causing flooding at idle and low speeds (even though the fuel level matches the manual). Does anyone have any alternative theories/are these bike typically that sensitive to fuel level? For those of you that have heard of the band Meatloaf here are a few pictures of the bike. Bat out of hell album art was the inspiration for the project.

[Bonz]

About your pipes... You may be way-lean...

Blue pipes are NOT from lean mixture! They are from a RICH mixture as extra gas is burned on the way out or running premium when the XS needs regular (slower burning premium isn't completely burned as it goes out the exhaust valve and completes the burn on the way out). Similar for hi-temp (but not ceramic) powder coated headers. Rich mixture will ruin the finish, lean mixture not so much.

[Bonz]

Sounds like an, ummm, a "unique" carb set up...

You have straight pipes going out the exhaust, and pods on the carbs with new stock size jets and seats along with diaphragms. Are the diaphragms sealing /seated fully under the carb tops?

- Mikuni brand jets or another brand? Big difference in what supposed equivalent jets do vs Mikuni. That carburetor/exhaust combo screams to me the bike is running lean.

-You plugged the air jets because other wise they inject too much fuel with pods... I don't understand "air" jets injecting "fuel". Never heard of that. Jet the carbs with the correct size jets for pods, not by plugging stuff.

-Stretched the slide springs... That isn't a known idle problem fix, that would simply mess with slide lift as you get in the throttle from a start and alter slide lift everywhere else too. Probably make your stock float settings, if they are set correct to stock specs, incorrect for the way the slides lift now and how the fuel is timed coming out of the bowls.

-Turned in the pilot jets... You mean the pilot/idle mixture screws are turned in, that leans the bike out (they control fuel, not air), will shut down the idle if they are turned more than "standard", even more so with your carb/exhaust set up lean to begin with.

-Fuel levels are all fairly equal and where they should be... Is that to say you measured fuel level in the bowls with clear hose or is that based on the float heights being fairly equal? Fairly equal is not equal, and a 1mm diff in measured float height translates to multiple mm's difference in fuel level in the bowls.

-What is your float height? Proper float height set to stock numbers on stock carbs will never run like what you described, and float height with pods doesn't necessitate a change when jetted correctly for pods.

What is your needle clip set at? Older carbs have adjustable needle clips and are important to have correct.

Glad to see you on the forum and sharing what you have! I simply believe the things you have done are not playing well together. But it is fixable for sure.

Now, start by getting the main jet size correct for your carb/exhaust combo by getting the best high rpm (6000-6500 rpm on up) power and pull you can. It can run not as well as you want everywhere else, per se, but once on the main jets the other things don't matter a lot.

Once you have the main jet size, how is the mid-range? Your needle clip position controls this. Probably is close as it sits right now, but can never tell. You want crisp roll-on response from around 2500 rpm up to around 6000 rpm, and with the main jets now being correct on the top end it should keep on pulling to 8500 rpm.

Now that the mid-range and top end is good, you are adjusting the pilot screws to get best throttle response from just off idle to around 2500 rpm.

The kicker could be float height with the stretched slide springs affecting fuel draw from the bowls, but start with the float height at stock and shouldn't need to mess with it if you do the above in order I shared.

Keep us posted!

[3Phase]

Like A Bat Out Of Hell!

[...]
The bike won't shut down with the idle screw out. When the idle screw is adjusted the idle goes up and down as expected up to a certain point then the idle goes no lower (around 1000 rpm) every other bike I have worked on shuts off when the butterfly valves are completely closed

This is a 1980 SG, right?

If it starts with ether and then starts every time after the initial fire it looks like the Enrichener jets in the carburetor float bowls may be plugged, there may be one or more air leaks somewhere the intake tract and/or the carburetors are most egregiously out of synchronization so backing out the main Idle screw only closes some of the throttle plates.

The air jets have been replaced with plugs (pod filters and velocity stacks cause the air jets to inject too much fuel and cause the engine to run so rich it shuts down once hot). Adding the plugs corrected this issue although I didn't need ether to start it then.

Hmm, air jets don't inject fuel, they allow air to flow through them but you say you did what! to the air jets? Which air jets? Is it the Pilot air, Main air or both?

The Pilot air jet is removable and it unscrews, the Main air jet is pressed in and has to be drilled out to remove or change it.

The diaphragm springs were stretched to ~1/2" above spec which helped smooth out the idle somewhat (they were originally too short due to age). Jetting is currently stock (new jets in the stock size). I tried turning the pilots jets in half a turn from standard position, it made the lowest idle speed lower but made the engine prone to shutting off at stoplights and made the idle rougher. The fuel levels are all equal and fairly close to where they are supposed to be according to the manual.

You must be describing adjusting the Idle Mixture screws, Pilot fuel and air jets are not adjustable.

What type of new jets did you buy in the stock size? You're going to have problems if they're Internet kits and they aren't Genuine Mikuni jets. I'm surprised the engine starts and runs at all with the air jets plugged and magical mystery size stock fuel jets.

At this point I am suspecting that the fuel level is too high and is causing flooding at idle and low speeds (even though the fuel level matches the manual). Does anyone have any alternative theories/are these bike typically that sensitive to fuel level?

I think you have out-thunk yourself with the slides, diaphragms and springs, the fuel levels and the carburetors.


With pod air filters and shorty mufflers with no exhaust crossover pipe the engine is not going run very well even with larger Pilot and Main fuel jets but your carburetors may be having electrical problems.

Does your bike still have the stock Fuse Block? It's over thirty years old and the fuse clips get weak, then heat up and get brittle and they can't handle their electrons like they used to be able to do back in the day.

Have you checked the system voltage with the engine running above 2,000 RPM? It should be able to hit and hold over 13 Volts with the battery fully charged. If the battery is not fully charged then the alternator may not be able to feed the battery and the bike at the same time and the system voltage may be lower.

Have you checked Primary voltage at the Ignition Coils?

Have you checked the Pick-up Coil wires to see if they're good or if they're broken? The engine will spit, snort and generally misbehave as if it were only running on two cylinders at certain times:-

The Pickup Coil Wire Repair-Pictorial
by Ken Talbot

For those of you that have heard of the band Meatloaf here are a few pictures of the bike. Bat out of hell album art was the inspiration for the project.

I LiKE iT! (0:11)


.

[mack]

I can't understand

how it runs at all. Plugging the air jets would make it super rich, if you plugged them because they were leaking fuel, then the fuel levels are too high. Stretching the diaphram springs was a mistake. They are designed to run off vacum and have a design tension. Your pods may have been plugging the hole at the top of the bell. Straight pipes don't work well with these bikes. And I concur with 3phase, in that the enrichement circuit meter in the bowls are probably plugged. As for the electrical aspect, since this was a complete rebuild, I would assume that the harness was off and all the connectors were cleaned and the all the components checked before it was reinstalled!

[1980cm400t]

Wow thanks for all the responses! Lets see If I can answer everyone's questions.

Bonz

Bluing pipes is caused by high exhaust temperatures. Usually this is a result of running exactly on ratio or slightly lean as there is no unburned fuel to provide the necessary heat capacity to lower the exhaust temperature. I suppose if you were running rich and had an exhaust leak you could complete combustion in the exhaust pipes and cause them to blue but I have new exhaust gaskets so that is unlikely. I am also running regular 87 octane fuel.

The carb diaphragms are seated properly, I did them the same way as shown on this thread: http://www.xs11.com/forum/showthread.php?t=33250. I did not use silicone to seal them, is silicone necessary when the diaphragms appear to fit snugly?

The jets currently installed are genuine Mikuni (I heard all the horror stories about the off brand jets).

The Air Jet I plugged is the removable brass screw not the pressed in jet. I found a very detailed cutaway diagram of the carbs that showed where that air stream actually goes (I can't seem to find the diagram now but it was on the internet somewhere). Anyway as piped the air jet acts as an eductor and draws fuel into the engine. This was a problem with the low pressure drop intake as the pressure and flow of air through this jet went up substantially which caused it to pump more fuel. The result was the engine would start but would be extremely rich and would shut off once hot and could not be restarted. Replacing the air jet with a plug fixed this problem.

I adjusted the carb slide springs after reading this post: http://www.xs11.com/forum/showthread.php?t=20389. Someone got a hold of a NOS carb spring and reported that it was 5.14" long. Mine were ~1/2" shorter (and not all shorter by the same amount) so I stretched them so they were all the same and about 1/2" longer than stock. It did seem to improve the idle smoothness although I suspect it was more due to spring uniformity rather than absolute length.

Fuel levels I measured and adjusted by the clear hose method. They are all exactly the same from carb to carb. They are also within 2 mm of the manual's fuel height. I say within 2 mm because there is not exactly a mark on the carbs for where the fuel is supposed to be, there is only a picture in the manual saying fuel level should be just below the carb bowel screws which is where it is.

I have the original non adjustable needle installed right now, I do have a set of the adjustable needles I can swap it with but I don't see reason to do so as it runs so well under acceleration.

In case it wasn't clear the bike runs perfectly in the high, mid, and most of the low range. The issues I'm having are apparent only at the very low range, when idling, and when starting cold (which is the most perplexing problem).

3Phase

This is a 1980 special model which to my knowledge is an SG

What are you referring to when you say enricher jets, the slow jets or the choke jets? I figured that new boots would not leak, but if there are air leaks that would explain why it won't shut off. I'll have to get some propane and see if I can find any leaks. I set up the initial synchronization with a set of feeler gauges, I do have a carb synch manifold but the last time I checked it the idle was too rough to give very useful readings. It is somewhat better now so I may give that another shot.

It was the pilot air jet that I replaced with a plug see my above response to bonz.

They are genuine Mikuni Jets, and yes I turned in the idle mixture screws, it did not improve the idle and seems to have made it worse.

As for electronics and wiring, I replaced the entire wiring harness with a new homemade one. All the electrical equipment has been replaced with the exception of the ignition unit and coils. The new harness does not appear to have any charging issues but I can check the pick up coil wires for shorts. This would not explain why it won't start without ether or why idle picks up when the fuel valves are turned off.

Mack

See my above response to Bonz, the Air jet acts as a fuel eductor. With velocity stacks, pod filters, and low pressure drop exhaust the air flow to the eductor goes up substantially causing it to draw more fuel and causes the bike to run so rich it will shut down once warmed up. Replacing the air jet with a plug prevents the educor from flooding the the engine with fuel. I did not plug them because they were leaking fuel. I have seen the claim that straight pipes and pod filters don't work with these bikes many places on this forum and others. I believe that this air jet is the root of most of the tuning misery experienced when adding straight pipes and pod filters. I spent many afternoons changing jet sizes and needle positions trying to get it to run initially and nothing worked. Replacing the pilot air jets with plugs completely changed everything for the better.

Also see my above response to Bonz. The carb diaphragm springs have a spec tension which is analogous to spring length just as a fork springs tension is analogous to length.

I am still using the stock velocity stacks so there is no obstruction to the air inlet on the top of the carbs intake.

You are correct that all the electronics are new and have been checked.

When you refer to the enricher jets/circuit are you referring to the choke jets or the slow jets? To me it seems as if it is flooding as the idle adjustment will not shut the bike off when it is turned out all the way, and the idle speed picks up substantially if the fuel petcocks are turned off while the bike is running.

[3Phase]

Cm,

Never assume that the electrical system is working.

For the starting/ether issue, there are small brass tubes in the carburetor bodies that go into holes in the float bowls. At the bottom of those holes in the float bowls is a tiny brass jet that meters the fuel for the Enrichener circuit. If that tiny jet is plugged up, the Enrichener can't supply fuel to start the engine when it's cold.

The Pilot circuit is SNAFUBAR. I understand what you did and why you did it but it does not work the way you think it works so you have low speed, idle and engine shut-off problems because you killed off the Pilot circuit's air supply.

Normally, plenty of fuel is supplied by the Idle Mixture screws but you've almost completely eliminated that by shutting off the air jets. Fuel can't burn without air so you have to open up the throttle plates to give the engine the air it needs to run.

Opening the throttle plates for more air uncovers the vacuum advance port in the #2 carburetor and that starts to add advance to the ignition, increasing the idle speed as you try to add the missing air to the mixture from the carburetor intake throat, which is very similar to trying to slake your thirst by holding your head under a four foot diameter pipe.

Opening the throttle plates also pulls even more fuel as the three Transition ports are uncovered and eventually from the Main jet/needle if you open them a little more to add even more air to the fuel that's dumping into the engine at what's supposed to be an unloaded Idle speed.

Now instead of a small and very finely-controlled amount of fuel being supplied to the engine at idle by the Idle Mixture screw and a hairline throttle plate opening, you're trying to control the idle speed with the throttle plates.
That's like holding garbage can lids over one end of those four foot diameter pipes with your head underneath them and the equivalent of three or four fire hoses all spraying fuel into each pipe.

Since the throttle plates have to be opened so far there is absolutely no way you're going to be able to synchronize the carburetors.

If shutting off the fuel taps doesn't kill the engine, the taps are leaking and need to be fixed. Should I even ask if the original vacuum-controlled fuel valve, the Octopus or 'Octy', is still installed or if it has been binned?

The diaphragm slide springs you tweaked aren't too bad but (you knew that was coming didn't you) the two middle springs are supposed to be slightly longer than the two outer springs to delay the rise of the slides and the jet needles to keep the mixture richer in the middle cylinders. It won't hurt anything the low and intermediate speed is pretty much gone with the pod-style air filters and the shorty exhaust.

.

[Fearthepack]

The bike will not start without ether. It will typically fire (maybe one or 2 cylinders) a single time but after that nothing. It does not matter what the throttle position is, or whether the choke is on or off. Give it the tiniest shot of starting fluid and it fires right up, and will easily restart even if shut down immediately after starting....

Just a side note are your valve shims within spec? A lot of bikes area nearly impossible to start if valves shims are off but will run and start all day after the first start....

[mack]

Well

Sounds like you've worked on lots of carbs before, unfortunately they weren't CV carbs. Your understandings of how they work differs dramatically to what I've grown to understand with these bikes.
Your set up may work ok with drag bikes and such where you only have 5K and full throttle in play but these are a multi functional motor and the pilot circuit is the means of transition between throttle plate settings. It's always running in the background making the motor respond smoothly from one jet to the other. Plugging the air jet will prevent atomization in these carbs.
I never read the thread about stretching the diaphram springs, I'm not sure how they would ever become compressed, but I'm sure who ever wrote that thread meant well.
I have a lot of carbs here and haven't experienced that problem yet, but I'll be on the look out for it!

[TopCatGr58]

Well,

All of the experts have spoken, and I concur with their analysis that plugging/capping off the pilot AIR jet opening was a serious Faux Paux!

Here's the photo of the sliced view of the carbs....80-81 series



[img]http://i44.photobucket.com/albums/f17/topcatgr/Carbs/80_81_Sliced_Cross_02_zps36462679.jpg
[/img]

The Pilot Air Jet supply joins the pilot jet flow from the carb inlet via that larger hole at the top of the Y shaped channel just above the Pilot Jet as seen in the right half of the bottom picture. Then it mixes with the fuel supply as it goes upwards via that vertical channel seen in the left half piece of the bottom image...the other end of the Y channel is capped off at the carb body. The vertical channel is also capped where it meets the vac. slide chamber. The larger round hole near the top of the vertical channel is where it then moves forward in the carb body to the 3 small ports that empty before where the butterfly seals against the top of the carb body throat...and then eventually goes to the channel where the Pilot jet SCREW sits...where it then can METER BOTH the Fuel/Air mixture that is coming thru that channel. There is NO other place where AIR can enter the Pilot circuit along that path. WITH the Pilot AIR JET port closed....it can't supply ANY Air mixed fuel, and as has been stated....the butteflies have to be opened up more to provide the AIR, and then the whole venturi affect kicks in drawing some fuel thru the Pilot jet circuit....but it will be extra rich because NO AIR can mix with it from the capped Pilot Air Jet...and so it's sucking ALL FUEL!

You have the 80 carbs...at least we think they are the 80's, you said they have the non-adjustable vac. slide needles. WHAT SIZE jets did you install...mains and pilots???

ALSO, did you inspect the pilot jet/main jet tower closely to see if the carb bodies were those funky bastardized ones that had the pilot to main jet tower sharing tunnels??? In those rare early model 80 transition carbs.... they used the smaller Main jets, AND they capped off the Pilot JET TOWER with rubber plugs so that the pilot jet would feed via the sharing tunnel and the main jet. IF it does have the sharing tunnel....and you did NOT install the rubber pilot jet tower caps....THEN it would be running excessively rich in the Pilot circuit because it would be getting fuel directly from the bowl AND from the main jet!

T.C.

[1980cm400t]

Let me clarify, when I shut off the fuel petcocks it does eventually shut off. Before it shuts off the revs go high 3-4k and adjusting the idle screw out will bring the revvs back normal (and for a brief period while the fuel is at that particular level it idles smoothly).

I checked the valve gaps when I installed the new valves and springs but I can recheck it.

I do not remember if I saw a sharing tube when I had these carbs apart but there were no plugs, of course I am not the original owner so that may not mean anything, I will have to recheck and see if I have the sharing tube.

I will also take a look at the enricher circuit although I would be surprised to find anything plugged as I gave all the carbs a proper soak in carb cleaner and a run through the dishwasher. If the enricher circuit were plugged wouldn't the bike start once the choke were applied or the throttle opened as that would introduce enough fuel to compensate?

I am currently running 42.5 pilots and 110 mains all around.

There is still air available for mixing with the pilot fuel in the main air stream (which is how carbs that do not have air jets function). I have worked exclusively on CV carbs and most of the ones I worked on did not have pilot air jets. When I get back from vacation I'll have to shoot a video of running with the pilot air jets and without. It was flooding horribly with the pilot air jets installed. I was expecting to have to increase the pilot jet size with the loss of the eductor effect from plugging the air jet but it does not seem to be lean, if anything it still seems to be rich.

[natemoen]

Are you turning the fuel off to turn the bike off (that is sort of what your explanation sounds like)? If that is the case then yes it will run for a bit because it needs to use up the fuel in the carbs before it dies. You should be turning the bike off with the key not the petcocks.

When you turn the fuel off the bike starts to starve for fuel causing a lean condition. When engines run lean the tend to rev up some and have issues with hanging idles.

I totally agree with others, you need to remove those plugs and start back from stock. Something is screwed up and plugging those jets is not at all a solution to your problems.

[Bonz]

I digress on the exhaust temp issue. Overly rich mixture carries the burn out the exhaust valve and finishes in the header, causing bluing. Seems counter- intuitive, but true story.

This can also happen with an otherwise healthy mixture if timing is retarded and combustion is started too late, still-burning mixture goes out the exhaust valve and continues the burn.

In the end, I just don't think the color of the pipes is a good way to tell if mixture is lean or not, especially with those carbs... There is no sound basis in CV tuning for those adjustments.

[3Phase]

The biggest problem is trying to control the Idle and Low Speed Air/Fuel MIXTURE with the throttle plates instead of the Pilot Air and Fuel jets and the Idle Mixture screw.

Without the Pilot Air, the fuel comes out of the Idle port and the Transition ports as raw fuel instead of at the correct Air/Fuel Ratio and the fuel is not correctly atomized. When it is mixed with the air swirling around the throttle plates it's in drips, drops, and blops. At the very best it's going to be unstable and erratic.

Once the engine load and speed is past the Idle and Low Speed range in the carburetors when the slides/needles lift up and move into the Intermediate and High Speed range it gets up and flies like, well, like a bat out of Hell.


.

[1980cm400t]

I got back from vacation and tried a few things. First I removed the plugs and installed a set of stock size pilot air jets (size 185). It still would not start without ether (maybe it was sitting to long over vacation). Once I gave it a shot of ether it started and immediately revved up to 3500 rpm and stayed there with some intermittent backfiring (which is different from what it was doing before with the pilot air jets installed). I tried adjusting the idle screw and even with it all the way out and the butterfly valves closed the engine stayed at 3500 rpm. So i shut it off and pulled the carbs off again. I looked for a sharing tube and if it is supposed to be below the pilot jet it is not there. I looked at the enricher jets as well. They appear to be brass tubes that have a horizontal hole drilled in them near where they mate up to the aluminium carb body. The direction this hole faces appears to be random i.e. they all face in different directions. Also only three of the carbs have the hole in the enricher tube. The lead carb (#2) does not have the the hole. The enricher tubes do not appear to be ported to any other location, I tried spraying carb cleaner into them and the only place it comes out is that horizontal hole. Any thoughts?