Carb Troubles

[Redbaron]

Had carbs rebuilt, had to clean more than once, they have been adjusted and synched, with CO2 analyzer and digital synch meter. problem is now the bike stutters badly at 5500 to 6000 Rpms. Starts at 5500 and i haven't found a way to overcome this as yet. Any suggestions as to starting points would be greatly appreciated. I am thinking mains too big but not sure these symptoms indicate such. Thanks in advance.

Also: New plugs, new air filters to replace old ones singles, cleaned and treated gas tank, have performed the coil pick up fix (Checks good with OHM meter). Idle mixture screws are set about 2 turns out, cranks and runs fine with this exception. Sputters slightly at blip of throttle at idle, but only at rpms indicated before. Coils test good, good spark all plugs, plug color fairly dark tan.

[beechfront]

are the mains stock? what about the air filter - is it still a stock setup as well?

Does it ONLY stutter from 5500 - 6000 or is that where it is the worst?

I was having a similar problem when trying to fit my bike with pod filters. My 79 special has stock 137.5 mains. With the pods I tried 147.5 down to 142.5. It seemed to get a little bit better but I didn't have any 140's at the time so I put it back to stock and havne't gotten back to it yet.

[Snow]

Also, in addition to which jets you re running, what type of exhaust system do you have? What year is the bike? Did it run better before you had the work done on it?

From my limited yet ever growing understanding of these carbs, it does sound like it *could* be either your main jets are too big or you pilots are too small - I am actually trying to cure a similiar problem on my bike, hopefully I will be able to resolve it completely today.

Btw, very important here, how were the carbs rebuilt? If they just slapped in a K&L rebuild it, jets and all, that could very well be your problem. The jets that come in the K&L rebuild kits are *HUGE* and definately not right for our fine bikes - though I would expect you'd have problems stuttering even early than 6 grand.

[skids]

Re: Carb Troubles

There are a couple of things to check:

Baffles, pipes, mufflers, poorly matched aftermarket exhaust system.

Do the plugs foul at the rpms where it misses, are they wet, or are they white? Headers warm or scortching hot.

Does the bike run OK at the upper range when on prime, where it doesn't depend on engine vacuum? I stretched the spring in a standard's petcock once to help eliminate leaking when the bike isn't running, but it restricted fuel flow at speeds above 65 mph.

If you have a vacuum advance unit (78, 79, 80), take the cover off, start the bike, and manually rotate the ignition plate fully to see if it has a fault. Maybe you tested the unit already with an ohm meter by rotating the plate (flexing the wires) with the engine off and if you did, there is no need to test it again, unless you think the wire(s) insulation could have worn away somehow.

[skids]

Also check the filter towers for clogging and as a last resort, there are small screens (stock mics for 78 and 79's) on the fuel inlets of the fuel valves that can become clogged with bits of rubber, etc.

[sixtysix]

Pilots have zero effect above 4K, so it is not them. A bad stumble sounds like fuel starvation not, XSive amounts of fuel. Mains are really only for WOT, so that leaves needles, needle seats (emulsion tubes), and diaphram condition as the likely culprits.

Check out this site for more info:

Factory Pro carb tuning

Click on bikes with redlines above 8K.

[Snow]

Sixtysix, I've got 145 mains and 45 pilots in 78 standard carbs with pod filters and 4into2 aftermarket exhaust and get bad stuttering almost like I've got an rpm limited on right at 4500 rpm and it goes away around 6000rpm, does that sound like anything you've heard about before?

[jchigbee]

I am no carb expert, but with all the help and links from this site and people here i am learning. I would try this link http://www.factorypro.com//tech/carb...m_engines.html this is the best online info on carb problems/tuning I have found yet!

[sixtysix]

When you cleaned the carbs, did you knock out the needle seats (emultion tubes) and clean out all the holes, lots of gunk packs in behind these too. You may need to shim up the needles a little (only on 78's in US I think) to allow more fuel in mid-range. It is hard to say what is wrong without the bike right here in front of me.

[Snow]

Quite understandable - I've actually since last posting lowered my needle height (raised the clip) and the stuttering is a bit better, but still there. I have the idea in my thick-skull that I should try going on the final leanest position on the needles and seeing if that helps more or not. I'm thinking that if that doesn't fix it all the way, but is even better, I should try either replacing my 145 mains with 142.5 mains (140 mains made it so that no matter what my idle screws were set at, I couldn't idle below around 4 grand) or adjust the float height.

I *belive* I have triple dipped this set of carbs (hard to remember since I have 3 sets b/c my brother got rid of his XS and parts and gave me two sets)

I've not heard of taking out the emulsion tubes before, is there a tech tip on the subject?

[xssiveone]

Taking out the emulsion tubes isn't hard. Remove the top cover and slide/needle assembly. Put a punch or small bolt on top of the emulsion tube where it comes into the throat(you should see a small lip sticking up) of the carb and GENTLY push the tube out. It may be stuck in there so GENTLE tapping may be needed to jar it loose. Of course, this is done after removal of necessary parts under the float bowl first. When you put it back in, it "should" only install one way as there is a slot cut in one side and an alignment pin in the body of the carb.

[jkorn]

I have had my carbs apart a million times and could probably do it blind folded now, though not recommded. After trying many variations on my 79 equiped with a Kerker 4 to 1 and a Oil type air filter in the stock box I am using 140 mains with the needles up a notch (clip lowered raises the needles and more fuel). Tried up to 150's but the motor stubled too much and hated warm weather. I also have 180 air jets. Not sure what the ilde jets are but just a bit larger than stock. The bike runs GREAT. I have used the choke in the past to help determine if the bike could use a little more gas or less.

After letting the bike sit for way to many years several years ago I found that there were HUGE gum deposits everywhere. I pulled the carbs apart yet leaving them all connected and emersed them in a gallon of carb cleaner and blew out all of the million or so passage ways. Still needed to run carb cleaner for a tank or too to get them completely cleaned. WHAT T LESSON THAT WAS...

[Snow]

jkorn, you are my hero! After keeping my 145's in and loweing the jet needle to the final notch and starting teh bike, she immediately jumped to 5,000 rpm...many words that cannot be repeated spewed forth. Then, in desperation I hopped on the computer and saw your new post and decided to give it a whirl and threw my 140's back in and set the needles 1 notch richer than default...much more power, but *still* just a little stumble left, nothing like before - I guess I have to lean the needle up one notch at a time now...but finally some hope!

[Snow]

Ok, after my ride home and the ride to work this morning, I've noticed a couple things:

Before she is fully warmed up, she stutters pretty badly but when she is fully warmed up, she now only stutters between 5k and 6k rpm. Next, I will see if putting the choke on makes it better or worse, but I am thinking it will make it worse in which case I believe I shall lower the needle one notch, back to factory default.

[Snow]

Ok, here's something interesting for those watching the thread:

Yesterday, I decided to bypass the arduous ritual known as "adjusting the Jet Needles" in favor of trying yet smaller main jets, my original 137s (they are stamped 137, prolly really 137.5s, right?). To my suprise, she runs even better now.

What has gotten my mind in a pickle though, is if I have Emgo individual filters and what I believe to be a MAC 4-2 exhaust, wouldn't I need larger mains? The big still has just a tiny sputter around 5 grand, just a split second - I need to check the color of the plugs asap...btw, does anyone know the size socket/wrench /spanner needed to remove sparkplugs? I 've always just used this american standard sized socket I have lying around.