'78 XS1100E, '80 carbs low rpm funk

[gtem]

Hi all,

Apologies for not posting this in the correct technical forum but seems that I don't have the ability to do so (yet?) as a pretty new member to the forum.
I've got a '78 XS1100E I've been working on since the winter, got it as a (poor) runner. I received it with the original '1978 carbs disassembled and jumbled up in a box, and a set of (as I've deduced) 1980 Special(?) carbs on the bike. I rebuilt the stock carbs only to realize a previous owner had wallowed out/damaged two of the four idle mixture screw orifices on the carb bodies. I'm still experimenting with repairing these, but in the near term I've been trying to get these 1980 carbs to work right.

As a "baseline:" the pickup coil wires have been repaired, confirmed that the carb boots do not leak, carbs have been vacuum sync'd. #110 Mains, #42.5 pilots, idle mixture screws set to 1.0 turns out. Stock pilot air jet for 1980 carbs (I just forget off hand what they were). Stock airbox and filter, RC Engineering 4-1 exhaust with a packed baffle. Current issue is that off idle, low throttle inputs say under 3000/3500 rpm the bike just feels weak-kneed with mediocre response, smells somewhat rich, plugs read fairly rich but not sooty. 4000rpm and up the response is *perfect* the bike absolutely rips. Fuel economy is I'd say lower than I'd expect, at around 35mpg even with easy riding. I guess I'm surprised that (IMO) I have the idle mixture screws turned out "only" 1.0 turns on a stock(ish) bike and it's still rich in the pilot/off-idle circuit. Currently going to try going even leaner on the idle mixture screw, but I'm wondering what else I might keep an eye out? At one point I was suspicious that my choke/enrichening circuit wasn't closing down fully, wondering if there's a good way to isolate/remove that as a variable.

Any and all advice is appreciated!

[DiverRay]

If you do have the 1980 carbs, there can be a few problems you may not know about. One is there was a change mid-year, and the main jet size should be the 110, not the 145.5 of earlier models. Also, if someone moved the needle position from the center, that will cause problems. I would try and do an "idle drop" adjustment of the idle screws first. Starting at #1, with the engine warm and a fan blowing over it to keep it from over heating, adjust the mixture screw for the highest RPM you can, then back it off about 1/8 turn. Go to #2, 3, and 4 and to the same thing. You may have to turn down the idle when doing this. Once done, you will need to sync the carbs with vacuum gauges or a carb stix to balance them.

[gtem]

Yes I mentioned that I have the correct #110 mains installed. The bike runs spectacularly at mid-upper RPMs, this is definitely an off-idle/transition issue. I'm going to fiddle with the idle mixture screws a bit more but I wonder whether that will cover it or not, from reading the FAQ and my previous experience with this type of Mikuni CV carb on Suzuki GS bikes and my own XS500C, the idle mixture screw is only affecting one of three or four orifices fed by the pilot jet. I may try putting the 210(?) pilot air jets on these 80 carbs assuming they have 180s now as a way to lean things out a bit more. But I had assumed if I kept all of the stock 1980 settings in place that things would work well enough on a stock '78, even with the known differences in valve sizes and cams between the 78s and later year bikes.

Another variable is the needle taper: if I have '80 "Special" carbs, doesn't that imply the more aggressive initial taper at smaller throttle openings? Could I simply try swapping the '78 needles and needle jets (emulsion tubes) into the '80 carbs as a work-around?

[jkorn]

Have you verified timing and that the centrifugal advance is working. This could also be part of the issue. You probably have but just mentioning. Also the clip on the needle position could be an issue. On my 78 stock carbs the needle is up on position.

sounds like it’s rich at lower rpm. I would suspect idle screw settings.

[Schming]

gtem, on the later carbs some get rubber passage plugs if the main jet cavity has a hole bored into the pilot jet cavity. Are yours this type and do you have the passage plugs in place?

[Ranger_xs1100]

Hi gtem, Jetting the carbs to the 4 into 1 exhaust system is a challenge. For me it has always been a trial and error event with limited success. I have been able to minimize the low rpm bog and the 2.5k-3k rpm stumble but have not been able to eliminate it completely. I usually end up going back to the factory exhaust if I plan on keeping the bike because I can’t handle the bikes not running up to the high standard that they are capable of. However, others more talented than me have been successful, so it can be done. But I would advise only trying one trial at a time versus changing pilot jet sizes and main jet sizes plus moving needles simultaneously. Here is a link to what others have suggested as a good starting point:
Jetting Recommendations - XS11.club Forums
Good luck and keep us posted.
Bob

[Radioguylogs]

Are the pilot jets genuine Mikuni?

I had the pilot jets in a K&L kit that were labeled 42.5, but they were actually much larger than the Mikuni 42.5. The bike was so rich it would hardly idle.

[bikerphil]

Check your pilot jets, if they have 8 holes in the side, they are the wrong type, they should have 6 holes.

[jkorn]

Lots of good thoughts above me on the thread….

But I put a Kerker 4-1 on my brand new 78 after just one year. Played around with the jetting for several years but it’ runs like crazy. Just for me I never thought about going back to my stock pipes. Still got ‘em, never crossed my mind to reinstall them. Besides they sound great…

good luck with issue. They can be very illusive. Like above. Only change one thing at a time.

[gtem]

Thanks all for all of the feedback and ideas. I can confirm that all of the jets are OEM-Yamaha sourced mikuni. I haven't done an actual timing check using an ignition light but have visually confirmed that the mechanical and vacuum advance seem to be operating "normally." I'll also confirm that my '80 carbs do NOT have the crossover passage from the main to pilot jets, therefor I am running without the rubber plugs.

That's interesting input on the 4-1 exhausts. Bob I'm right there with you, if the bike isn't running *perfectly* with ideal carburation it drives me up the wall. I'll either get it right or I'll sell the bike! Jeff, if you have your stock exhaust and have any interest in selling it.... let me know! I enjoy the look and tone of my R-C pipe, but I'm a total sucker for stock.

For starters, I'm going to keep tinkering with the idle mixture screw will back off another quarter turn. I have a Gunson Colortune I may investigate with that as well to better understand where my rich condition comes in (right at idle or once more of those pilot holes become uncovered by the throttle plate). I'll also borrow my friend's timing light to confirm that things are in spec (I have a factory service manual).

I was actually able to solder up and then redrill the idle mixture screw orifices on my original '78 carbs last night using a .85mm drill bit (a tad larger than stock unfortunately but it's what I've got). So I've got hope yet to try using my stock '78 carburetors. Another path forward will be to see if the 210 pilot air jets from the '78 carbs might help improve the mixture (leaner) on the low end on the '80 carbs.

[gtem]

A small update, some test riding with the mixture screws turned down to just .75 turns out from seated (down a quarter turn from the 1.0 they were set to), it feels a bit better but the low rpm light stumble/bog is still present, bike is obviously still quite rich (based on smell and minimal warm-up time). I think I'm gonna try swapping to 210 pilot air jets to see what that gets me.

[gtem]

A further update: received my new idle mixture screws for the '78 carbs so I buttoned them up and did a bench leak test, seeing a bit of fuel come out of the air jets so I need to take another look at the float valves, but will be trying these out on the bike once that is hopefully sorted out. Fingers crossed!

An important note: when I pulled the '80 carbs I noticed there was some signs that perhaps I'm getting some leakage past the float valves on this set as well! This may very well be my rich running condition issue. I've checked and rechecked the float levels on the '80 carbs a number of times (rubber tipped style valves with the wire "cage" holder and plastic floats).

Another *possible* suspect on the '80 carbs is that the threads on the needle jets are a bit buggered up where the main jets screw in on a few of the carbs. Does this thread normally keep fuel from seeping past, is it possible for fuel to get sucked past and leak on the *outside* of the needle jet? I also have a bit of traces of fuel on the outside of the carb bodies where the throttle shafts are, what would this possibly point to? Where would that fuel be coming from? Thanks again!

[DiverRay]

The '80 has an "O" ring to seal the float needle seats. That seems to be the weak point as far as fuel into the float bowels.

[gtem]

The '80 has an "O" ring to seal the float needle seats. That seems to be the weak point as far as fuel into the float bowels.

Yes these have been replaced as needed, I did just check for leaks by pressurizing the fuel line with air (slightly) and sprayed around the base of the float needle seats and they looked good. Also confirmed that on the '80 carbs the float heights are all set to stock spec, and seem to seal well during that same air pressure experiment.

I refilled the '80 carbs with gas on the bench, no notable leaks or wetness anywhere, I did notice a bit of gas around the #1 pilot air jet, cleaned it off and didn't see any more, not sure if that was a fluke but I will keep an eye on it. Cyl #1 is the most rich reading plug, and when I experience my low rpm stumble it seems like a cylinder cutting out sort of roughness

[Radioguylogs]

Another *possible* suspect on the '80 carbs is that the threads on the needle jets are a bit buggered up where the main jets screw in on a few of the carbs.

Is it possible that the buggered threads have prevented the main/needle jet from resting in the proper spatial relationship with the needle?