Carb synching... what the heck is the problem??

[hobbsy84]

Y'know, I used to read threads of people having trouble with getting the carbs synched and running well on these machines, and wondered how it got to the stage of people wanting to ditch their bikes in favour of something less frustrating. I'm starting to see how they felt...

Thing is, I'm so close to being able to ride this thing, I just need to get a reliable tick over at idle, which is proving hard. I managed to get my hands on a set of mercury sticks in order to sync the carbs with, and as far as I can tell I have it pretty spot on. Providing I'm actually doing this right.
Basically, bike's on the centre stand, fuel tank is facing backwards for access to the screws etc, and all the vacuum points on the manifolds are plugged (except when synching, then they're all plugged into the sticks).

Thing is, while I'm trying to sync, I can't get a reliable idle. I can get all four sticks to line up pretty well, which is where I have them now, but now it'll either idle low and die by itself, or when I adjust the idle screw up, at a certain point it'll start to run away by itself, like the vacuum advance is kicking in and taking the idle too high.

Add to this, I'm also getting backfires (back into the carbs) occasionaly which is killing the idle as well, and it makes it hard to sync the carbs too, since the engine keeps dying. As a result the engine is getting a bit hot while running, I'm thinking this can't be making the sync any easier.

As for the idle mixture screws, I've tried multiple different settings, ranging from 1/4 turn out on all, to 2 1/2 turns out, and it does't seem to help the situation much. The only thing I can think of in these that might be a problem is that the tips of them have been a bit deformed from someone over tightening them beforehand. No tips broken off though. I posted about this not long ago.

I'm thinking I 'should' be able to rule out ignition in this, since I just installed a set of MikesXS coils along with new spark leads and caps, along with a new set of spark plugs. I'm also not getting any cold headers, so I'm pretty sure I'm getting a good spark on all four.

I can't ever remember getting so frustrated over something like this, its starting to get me down a bit. Does anyone have any tips as to what might be going on here? I haven't had a whole lot of experience in four cylinder bike tuning, I guess it really can be four times as hard...

Cheers all.

-Hobbsy

[bikerphil]

1. Have the carbs been thoroughly cleaned and float levels set correctly?
2. Have you checked for vacuum leaks around the carb boots.
3. Silly question, is there enough fuel in the tank and the petcocks turned on?

If the vacuum advance is making the bike rev too much, you could temporarily disconnect it and plug the port on the #2 carb.

[Ivan]

I'll second the vacuum leak theory.

[TopCatGr58]

Hey there Hobby,

I just went back and reread all of your previous 4 threads regarding this bike. You said the carbs were pretty gummed up. You said you cleaned them, triple cleaned them even. However, you didn't mention much about how you cleaned them? I hope you didn't soak the entire carb bodies in carb cleaner because if you did, then you could have damaged the butterfly shaft seals! That's one possible vacuum leak source.

Hopefully you replaced the pilot/idle screws, apparently yours were a bit mangled! However, IF turning them from 1/4 out to 2+ out doesn't really make much difference in how the bike idles/runs, sounds like the pilot circuit may still not be completely clear and clean?? Did you replace the mains and pilot jets with new actual Mikunis? A while back, it was suggested to disconnect vacuum advance hose, plug the #2 carb vacuum port to put the advance out of the equation.

Also as mentioned, be sure you don't have leaks either in the walls of the intake manifolds as well as where they bolt to the engine/head!
T.C.

[79XS11F]

My own experience with carb syncing is this. The bike MUST be at full operating temp. I do my sync after a 20 min run. You will have to adjust the idle as you sync them. While making the adjustment to the set screws DO NOT BE PUSHING DOWN WITH THE SCREW DRIVER. a very light touch is required. Turn the screws a VERY small amount at a time, give the throttle a little twist and let it return to it's static position every time you make an adjustment. Check it out yourself. The weight of even just the screwdriver setting in the set screw will have an effect. Also before you even start make sure the throttle cable is set right of you will never get them spot on. Make sure that cable is lubed and working smoothly as well.
It goes without saying that the other systems also need to be working properly before you start. That's timing, cam chain adjustment, valve clearances, good plugs,wires and coils.
Rob

[hobbsy84]

Thanks for the quick replies fellas, much appreciated. Now for a few answers...

bikerphil, I'm going to have to tick all three there, I've cleaned the carbs more times than I'd like to remember. I set the floats using a ruler and I'm pretty certain they're all even. Unless there's a better way of setting the heights? I checked the boots (used engine silicon to seal the boots to the head, also put a coat of it around the outside, even though none of the cracks in the rubber were through all the way. I've tried spraying WD-40 around the boots, no pickup of revs, so I'm assuming there's no leaks there. And I ran into the petrol problem yesterday, but picked up on it pretty quick. And I figure the silliest questions are the ones not asked, since we all do dumb things...

Topcat, luckily I didn't soak the bodies in carb cleaner. Only carby spray and a series of softish brushes (nothing metal) was used. I'd considered it once, but never did it, turns out laziness on that one paid off... As for the mixture screws I posted about last time, I'm ashamed to say they're still in there. I did neaten them up a bit with a flat surface and a few grades of sandpaper though, so they're not AS bad as they were. Still a bit bad though, but I priced out a set from the local dealer here, and $41 each was more than a little steep. I'm thinking I need to look at an aftermarket option there.
The mains and pilots were replaced with the ones I took out, so as to not disturb the sizing. However, since the 1179 bore kit was carried out, would a main jet or pilot jet re-sizing be worthwhile?

79XS, since I've never really done a sync like this before, I'll give it another shot keeping those tips in mind. I really haven't been giving the engine much of a rev after an adjustment, it's been hard enough to keep the thing going during adjustments. I'm pretty sure valve clearances and cam chain adjustments are all good, the engine was just re-built a couple of months or so ago, which included a head re-condition. My only concern is the noise the engine is making during running at lowish rpm. It almost sounds like cam chain chatter, but I can't really be sure. Would it be worthwhile to remove the cam chain tensioner and check just in case?

Cheers again.

-Hobbsy

[dbeardslee]

The carb synch should be the last thing you do in a tune-up. I'm a big advocate of colortunes - some people swear by 'em, others swear at 'em. I like them because it allows you to adjust the mixture by what's going on in the combustion chamber, not by number of turns on the mixture screws. Every one of my carbs is set to a different number of turns, but my pipes are all even colored and my plugs all look the same. The other thing you might check is your timing. I tried advancing mine a little, and Betsy didn't like it. Set it right dead on 10 degrees btdc (with the vacuum advance disconnected and plugged) and it made a big difference, even though it was only off a little. You've got the big guns lookin' at this one, but that's my $.02.

[Ken Talbot]

I'm putting my bet on dirty low-speed circuits. Even after three or four cleanings. One of our earlier members, stckyfingers I believe did the full triple clean at least three times. That makes 9 or 10 times through the process until he finally got it right. Short of pulling the bodies off the racks, every last part that could possibly be removed has to be rmoved. That is every part. Not just most of them, all of them. And you must verify that you have sprayed cleaner through every orifice in every area. Verify as in actually rig up whatever needs to be rigged up so you can watch and clearly see cleaner coming out of every hole at some point.

Than again, I could be wrong...

[hobbsy84]

Gah, just woke up after falling asleep on the couch in frustration. Rolling over to see anything related to 'clean the carbs' isn't what people should wake up to...
As much as I don't really want to say it, I might just have to give the cleaning another shot. It'd be good to hold out on that for just a bit though. I have a few things I want to try. One thing though, would anyone have recommendations on where mixture screws might be bought? And I mean somewhere other than at a Yamaha dealer, since $41 each is a little bit steep.
I meant to ask before, but what was people's opinion on using resistor plugs with resistor caps? For some reason finding a set of BP6ES plugs was somewhat hard the other day, everywhere just had the BPR6ES plugs. Would the resistance really be that much of a problem, seeing as it's just to filter RF for radios?
I think I should have slept for longer...

Cheers all.
-Hobbsy

[randy]

Sorry to hear you're having such problems, must be extra frustrating when you know you're so close to finishing.

Your symptoms almost sound like fuel starvation. The petcocks are vacuum actuated, and during a synch, you've taken away their vacuum source. You do have the petcocks turned to PRIME, right? Is there a problem with your fuel cap vent? Try it with the cap unlatched.

Lastly, make sure your plugs are clean. Or if you're not sure, put in fresh ones, fouled plugs can really screw up a synch session! DAMHIK!

[boyat68]

hobbsy84:

I know it is a pain in the old keister, but tripple clean means more than three as at first we all think that everything is clean as we can get it. Myself, I failed to clean the orifice that goes from the bottom of the float chamber, up the side and connects to the upper chamber by a brass tube. This feeds the starter jet. I thought that I saw cleaner coming out of all of the orifices but two of the float bowls were still blocked and I had to use a very small needle to break up the crude that was lodged in those two. Many on the site that say that it is not clean until you get an eyeful are right as those passages are very small and when you stick the cleaner straw in that hole you should get velocity out of the other side. Once those carbs are surgically clean and the fuel flow is right, your synching dials everything in almost on the first try, It seems like a miracle how it works.

[hobbsy84]

Hmm, opinions are stacking up on the recleaning carbs option. Truth be told I can't completely rule it out myself... Oh well, whats another day on the bench with a can or two of carb cleaner, eh? I'll quickly try the timing suggestion, dbeardslee, seeing as the advance unit settings were fiddled with during disassembly and reassebmly of the engine, something could have been moved.

Randy, as for the petcock and fuel tank bit, I've got the taps on prime, and tried with the cap off back when I first got the bike back together, ended up finding the vent line in the tank all but completely blocked with mud, funnily enough. Cleaned all that out though. Wish it was something that simple, I'd rather look like an idiot and be riding than the alternative.

Well, looks about time to clear off the bench and get the carbs off the bike. On the plus side, I'm getting pretty good at getting them off and on. Oh, while I'm thinking of it, is 26-27mm a decent height for the floats? Probably wouldnt hurt to check them again while I'm in there. If memory serves, I set them to 26mm last time, as far as I know this is about the ballpark figure for the correct height. Would that be right?

I'll let you all know how it goes.

Cheers all.
-Hobbsy

[bikerphil]

is 26-27mm a decent height

No. Your carbs should have the plastic floats. The correct setting for the 80-81 carbs (BS34-III) is 23mm, upside down, bowl gasket removed. If you had 26mm, that is too lean. 26mm is the correct setting for the brass float (early) carbs.

[John]

One place to check closely is the three small orifices on the top of the carb throats, where the throttle plates close off the air flow. They are in the pilot circuit also. The orifice closest to the engine side of the carbs should be slightly uncovered when the throttle plate is at rest (idle), it also supplies a constant supply of fuel at idle. The pilot screw just provides a means to adjust the fuel at idle. The other two orifices supply fuel under throttle, until the main jet circuit takes over. To check the pilot circuit, bottom out the pilot screws, remove the pilot jets, and while the throttle is held WOT, and plug the air bleed for the pilot circuit. It is located in the inlet bell of the carb. Spray some cleaner into the tower for the pilot jet, and see if the spray is exiting all three orifices. If not, let the carb sit upside down for a while with the cleaner sitting in the passages. This should help to soften the crud, and a good blast of compressed air into the pilot jet tower should help blow it out. It is imperitive that theses three orifices be clean, or it will never run right on the pilot circuit, and will only run well over 3k rpm.

[webbcraft2150]

Be patient, or you will be a patient!!! Carbs are the most finiky part on these bikes to get dialed in, but once they are they are pretty trouble free. I use the carbs sticks also. I will dial in the idle mixture with the carb stick first and then go back and forth syncing the carbs. It can take a while. If your carbs are truly, truly, truly (triple truly), frigging with the idle mixture screws should show some change in the running with even a 1/4 turn. Try to have a fan blowing on the engine while you are syncing carbs.

Remember you are working on a 30+year old bike and they can be cranky. You could go buy a turn-key, fuel injected computer controlled bike but it would not be nears as cool as a good running XS11.