Can't get her running right

[Catmaigne]

I don't mean to be that guy whose first post on a forum is a cry for help, but I could really use some. Here goes nothing...

The bike is a 1981 XS1100 Special I picked up as a titled project for fairly cheap. A few months of cleaning + rebuilding old parts and the bike is nearly road ready, but I can't get the tuning perfect.

Important information (sorry about the jumbled mess): The airbox was replaced with pods and the main jets were upped to 115. Octy was deleted and the carb boots are all capped, but the #2 carb still has vac advance. The carbs have been thoroughly cleaned and the plugs have been replaced. Both coils are good and the vac advance is working. The carbs have been sync'd with a tool I made from a 4-way aquarium valve + vac gauge and I attempted to tune the idle screws but ultimately set each to 2.5 turns. I also replaced 7 of 8 throttle shaft seals (carb #3 gave me trouble). Float heights are also in spec. Float needles were replaced and seat properly (no gas leaks here). I examined the carb boots and they don't appear to be cracked. I also cleaned the mating surfaces and gaskets.

There are still 2 major problems I see:

The bike backfires out of random carbs at random intervals and also when a short burst of throttle is given. I thought this was an indication that the bike was running lean so I tried a number of methods to check for vacuum leaks. I sprayed the area around the carb boots with WD40 and saw no change in RPM, then used an unlit propane torch with no success, and finally tried starting fluid but still found nothing. I'll try again next time, but I think an intake leak is unlikely.

The bike idles fine for a while but won't return to idle when revved after fully warmed up. Sometimes it gradually revs up on it's own to around 3-4K RPM. I noticed that the vac advance is jumpy at idle and fully advances around 2.5-3K when it revs on its own. The throttle cable isn't binding because the assembly still rests against the idle speed knob while all of this is happening. When I pull the vac advance tube from the #2 carb body and cap it the bike still backfires intermittently but always revs back down.

Are my carbs out of sync and tuned improperly because I suck? Is it definitely a vac leak somewhere causing a lean condition? Could there be something else causing this that I'm unaware of? I remember reading about the cam chain adjusting process and valve adjustment but am unsure how it relates to my problems. There appears to be blue sealant around the valve cover so I'm guessing it's been opened some time in the last 31 years.

[Catmaigne]

Compression is good too.

I can also hear a bit of a slurping sound from the carbs when I pull on the throttle but I'm not sure if that indicates a leak.

[trdriver71]

I had that problem with mine until i adjusted my valves. All my valves were tight I think that was the first time they were ever adjusted.

[natemoen]

Welcome!

You sort of stated it both ways but just to make sure, your vacuum advance needs to be connected to the nipple on the #2 carb and not on any of the 4 intake boots.

[BigRed]

+1 for what Nate said. I think the high/climbing idle when hot could indicate that your mixture screws are out too far. See if you've got someone local who has a Colortune. I think tuning by ear is potentially better if you can do it, but a Colortune will get you really close first.

[DGXSER]

First and formost, welcome to the forum!!! No worries, almost all of us were once THAT guy who comes in first post with an issue to be resolved. After all, that is how we found the forum in the first place typically, trying to find a resource to help resolve our issue.

If you have not personally checked the valve clearances, I would do that now. If they are not properly set (and never trust that the PO set them correctly), you will need to do so soon, and then all your tuning needs to be repeated anyway.

The bike backfires out of random carbs at random intervals and also when a short burst of throttle is given. I thought this was an indication that the bike was running lean so I tried a number of methods to check for vacuum leaks. I sprayed the area around the carb boots with WD40 and saw no change in RPM, then used an unlit propane torch with no success, and finally tried starting fluid but still found nothing. I'll try again next time, but I think an intake leak is unlikely.

Vaccum leaks show up as lean running because they are letting to much air into the mixture. The other option for a lean running condition is that your lean on your mixture at the carbs. Have you checked your plugs to see what they look like? 2.5 turns is a good starting point to get the bike running, but often more is needed to get it tuned out correctly. First check the plug condition, then try adding a full turn to the mix screws. Repeat until it gets better or your more than 5-6 turns out from seated.

The bike idles fine for a while but won't return to idle when revved after fully warmed up. Sometimes it gradually revs up on it's own to around 3-4K RPM. I noticed that the vac advance is jumpy at idle and fully advances around 2.5-3K when it revs on its own. The throttle cable isn't binding because the assembly still rests against the idle speed knob while all of this is happening. When I pull the vac advance tube from the #2 carb body and cap it the bike still backfires intermittently but always revs back down.

This is a pretty text book indication of carbs not properly synched. I never found one of those aquarium valves that would work for me. So I always used the plenum setup with my Mity-vac for a gage, and just moved the tube from carb to carb. This method will give you good results and you should be able to get the bike running pretty well. I will say that I now have a Morgan Carbtune and it is alot simpler and does produce better results, for me anyway.

[James England]

I'd also do the pickup wire check anyway. It's a known fault and it would be worthwhile to eliminate it from the possible causes.

I would invest in the cheap vacuum gauges available on eBay. We pay about $50 for them here but they are a good investment and you're going to need them.

I'm personally not into pods and prefer the OEm setup with a freeflow K&N filter. If it were my bike, I'd be looking for an OEM airbox on eBay. I realise many people like pods and I'm not knocking it but I don't like the empty space, or the performance of pods.

[Catmaigne]

I put pods on my bike because the stock airbox was cracked and missing pieces when I bought it. The main jets were upgraded to 115 to accommodate the increased flow but I'm going to swap the stockers back in later for testing purposes.

I devised a little experiment with starting fluid to finally rule out any vac leaks. I ripped the 4 tubes out of the airbox that look like velocity stacks and installed them on my carbs. I cut 4 holes in a plastic sheet, pushed it over the stacks, and sealed around each opening to act as an air barrier for the intake side of the carbs. Then I attached a straw to the nozzle on my can of starting fluid and hosed down the boot area with the bike idling. There were no substantial RPM changes so I think a vac leak can be ruled out.

After that, I dug up my Mityvac and checked the carb sync but everything was already spot on. I also tried tuning the carbs by ear which decreased the backfiring but the revving problem persists and only worsens as the bike gets warmer. How sensitive are these bikes to improper syncing and tuning? I didn't think it would be this tricky to get everything running right, maybe it's time to invest in some better tools... unless there's something I'm overlooking because it's extremely hard to find a sweet spot.

Is it time to pull the valve cover and start checking clearances along with a cam chain adjustment?

[TopCatGr58]

Hey there Cat,

The others have suggested checking your valve clearances. You stated the vac. adv. hose is properly connected to the #2 carb body brass tube, and when you remove the hose, cap the carb, the engine DOESN'T keep revving, but idles back down. This points to too much vacuum from the port during idle, which could mean that the port was drilled out so that the hole is too large? OR, you have the throttle plates too open...too high of an idle, and so it's sending an increased vac. signal to the vac.adv. which is speeding up the idle, not letting it rev back down. WHAT RPM are you idling at?

When the bike warms up, what is your idle speed BEFORE the engine starts running up the idle? Try setting the idle speed with the main thumbscrew down a bit once the bike warms up. Also as suggested by DGXser, turn the pilot screws out another turn each and then see how it behaves.

You didn't mention what spec you set the floats to....should be ~23mm for the later carbs, not the 25.7 of the early series...there are addendum notes in the Clymer's manuals for the later carbs in the back of the manual, sometimes folks don't realize they are different specs!? JAT!

T.C.

[jetmechmarty]

Michael,

Welcome to the forum! First, leave the jets alone! They aren't your problem.
115 main jets are fine. Even if they were 125, they would not be the problem. Follow the given advice. Check the valve clearance. It seems your sync method may have gone awry. Vacuum leaks are out, so you're getting close. Pull the carbs and do a visual sync. That's all I've done to mine and it trys to rip my arms off. Try this: Easy Bench Sync Method

I Hope you've taken everything apart and thoroughly cleaned everything. The smallest bit of crud and you're running poorly.

[Catmaigne]

I had the idle RPM set to ~1800 when syncing but I'll lower it.

The float heights were set to the recommended spec from this tutorial:

http://www.mtsac.edu/~cliff/storage/...d_Tutorial.pdf

I guess I need to bring each up to 23.

[motoman]

I had the idle RPM set to ~1800 when syncing but I'll lower it.

The float heights were set to the recommended spec from this tutorial:

http://www.mtsac.edu/~cliff/storage/...d_Tutorial.pdf

I guess I need to bring each up to 23.

If float needles are new, or if sealing good, a 23.5mm will have fuel levels at the recomended 3mm down from carb boby edge with bike idleing. Major importance that all eight floats are set identically same height. Set idle mix screws where just the highest in. vacuum is on each carb. Now, if it's idleing decent at 1000-1100rpm, sync carbs. Drop idle down to 800, re-set idle mixes at highest in. vacuum, then re-sync again. Shut it off and blip the throttle several times to seat sync screw threads, re-start and re-sync again. If all is good in the idle circuit, it should idle smooth down to 450rpms before it starts to lope. set idle speed to its normal 1100rpm and go ride some 20miles, do a throttle chop, hit the kill switch and pull plugs looking for at least a bit of color on the porclien. Want to aviod plugs looking like they come out of the box new, as this can lead to unwanted holes in pistons.

[DGXSER]

One more thought, if you say removing the vac advance line and plugging the carb opening resolves the issue, have you checked thoroughly for a leak in the vacuum hose?

[Catmaigne]

Yep, hooked the Mityvac up to that and there was barely any leak down.