Mixture screw deforming?

[hobbsy84]

I've been having all sorts of dramas getting my recently (almost) complete project bike to sustain a decent idle. Seems to be fighting me all the way. Thing is, I'm getting spark on all 4 cylinders, no leaking fuel back into the airbox, and it runs, but it will tend to rise in rpm and not come back down for ages (if at all) if the idle is adjusted to around 1100rpm or so. But if the idle is lowered to bring it back again, it stalls out. The idle is extremely uneven, yet I'm not getting the problem with a cold header upon starting.
As far as I can tell, it sounds like a vacuum leak problem, but I spose it could also be the pilot circuit with crud in it. Mind you, I've done the trusty clean x 3 trick, and it just doesnt seem to be making a difference. I also need to give it a better sync than a bench sync with a kitchen tie, I reckon, but I'm without a vacuum gauge or mercury manometers.

One thing I did notice though, is the idle mixture screws are a bit of a strange shape on the end. Sorta deformed from overtightening if you ask me. But would this cause the poor idle symptoms I'm getting? I got a shot of the mixture screws for all to take a gander at... (I'm hoping that link works, I couldnt get the photo to insert properly, I'm bit of a cluster with that sorta thing)

http://1.bp.blogspot.com/_GlFNi4L8c7...1600-h/059.jpg


They don't seem to be an even shape, and this photo doesn't really show the slight bend in the tips very well either. When lightly seating one of them, I reckon I can feel the tip entering the hole unevenly.
Any thoughts?

[TADracer]

The end of the screws do look a bit funky but should still give you a decent idle mixture. I had some that looked like that and still worked.

What you are describing usually is due to carb syncronization. Even if one carb is out of sync it can drag the others along for the ride. You really need to use a sync tool of some sort and get them in balance.

[81xsproject]

I highly recommend replacing those screws. When they are smashed like that, the adjust-ability becomes extremely narrow because the needle taper has been smashed. Also, not every needle is smashed the same amount which removes the ability to get consistent mixtures across all the cylinders even though they are the supposed to be the same number of turns out. You could buy a colortune, or for the same price or less, you could just replace the screws and fix the problem instead of just putting a band-aid over it.

As far as syncing the carbs, if you want to give it a really decent sync w/o sticks a manometer, here is the absolute best method I have found. The results I have gotten have been VERY close to the quality of idle after an on bike sync.

With the carbs off, spend a little time figuring out how the sync screws move the butterflies relative to one another. Basically, they are all sync'd to the #3 butterfly position. The best order of operations is to get to the point where as you adjust the throttle position/idle speed knob-screw down (butterflies closing), the number 3 butterfly closes first. Get it to where it is JUST totally closed. Now, use the butterfly adjustment screw between 3 and 4 to get it to where #4 is now also just closed. Now adjust the 1-2 screw to get 1 and 2 butterflies to be at roughly the same open position. This is the trickiest part. Because now you need to go the the 2-3 screw and adjust it. It will move BOTH 1 and 2 butterflies at the same time. Turn it until either 1 or 2 (or both if you set them perfectly the same in the last step) comes to the just totally closed position. If either 1 or 2 is still open, fine tune the adjustment between them. No you basically go back and forth between the 1-2 screw and the 2-3 screw until all the butterflies are totally closed. Use the idle adjustment knob to open the butterflies. They should all have and indistinguishable gaps relative to one another. If not, some thing went wrong.

I think I will try to take some pics on this subject and try to make a full on tech tip when I get a chance, as good beach sync is really very important.

[Cobia]

As we all know synching the carbs is absolutely crucial as is having good parts in them. Yet it has been my experience that when the engine revs up after it's warm it is a sign that it's running lean. You may want to re-visit your float height when you take them out to replace the idle screws and perform the bench synch procedure 81xsproject detailed below. Good luck!

[boyat68]

hobbsy84:

I would say that definitely you need to synch the carbs as was posted prior. When I rebuilt my carbs, the same thing would happen untill I synched each carb precisely to the #3 position. It took several tries as the #3 has to be lowered/raised in relation to the others to be sure that you have a good starting point. As was described in 81xsproject answer. The bread tie method is a good swag starting point but it is a miracle if you happen to hit it on the first try. You can use a single gauge but it makes it more difficult. Two gauges makes it simpler as you have the reference of one gauge on the #3 position and the other one moved between the other carbs. Of course 4 gauges makes it even simpler. The first time you do it seems a lot of work, but you learn the relationship and it becomes easier.

[fredintoon]

Hi Hobbsy,
those needles can get their thin tips busted off to jam in the tiny hole in the carb body.
If that's happened to even one of your carbs you can synch them until hell freezes over and it won't go right.
There's links on this site about how to get the busted tip out again.
If those tricks don't work, carefully drill the hole clean with a 1mm drill bit.
DON'T use a hand drill. Centre a drill press on the needle hole with a cone bit.
My drill press chuck won't grip that small of a drill bit so I used a 1/16" instead.
The bigger hole means that needle's setting will be different but the carbs will still synch OK.

[hobbsy84]

Cheers all, I think I'll take that good advice and replace the idle screws. Realistically I spose I probably should have done it long ago when the bike was in bits, but ah well.
I'll give the sync method you explained a go 81XSproject, I pretty much just need to get things close enough until I get onto the bloke who has a set of mercury manometers. He's usually away driving trucks around, hard person to get a hold of.
From the looks of the ends of those screws, I think I've been lucky the tips didn't snap off in the hole, and I can only imagine from hearing other's stories of removal, how badly that would ruin my day.
As for float heights, I did check them last time I had the carbs out, and I measured 25.5mm from the gasket sealing surface, as far as I know this is about where it needs to be. But seeing as I've been doing these things by eye and a ruler, I might just double check that too...

[Crazcnuk]

Just for curiosity, when you have it set up and the idle climbs up, try pinching or unplugging the vacuum advance tube.

I had the same problem, either the idle was too low and it would slowly die, or it would be perfect then climb up to 2000-2500rpm or so.

The problem was that the #2 carb was opening too soon and the vacuum port was opening and pulling the advance mechanism on.

My other question is about those screws. Those look like 78-79 mixture screws, but your sig says you have an '81?

[81xsproject]

Good catch, Crazy. I missed the signature info (even though that's why it's there) So, are these transplant carbs or is there some other lack of info? This is easy to tell by how many holes there are in the rear bell of the carbs. 3 for 70's carb and 4 for later carbs.

[petejw]

hey '81,
I also sync my carbs that way,
only thing i would add, i hold the carbs
up to a lite when i adjust them, this
seems 2 b more accurate than using those
cheap vacuum guages which i think are useless.

Hobbsy84,
Id also replace those mixture screws, i doubt
you would get a decent idle with them.

[XSPastor]

Sync'ing while vacationing?

I think I will try to take some pics on this subject and try to make a full on tech tip when I get a chance, as good beach sync is really very important.

I agree. I like to sync my carbs at the beach too. I just love all that sand and the gentle sound of the surf...

[hobbsy84]

The carbs fitted to the bike confused me a bit at first, it would indeed seem they're from an earlier model bike. But if I remember rightly, the fiche at the Yamaha store which also shows my bike as a 1981 XS1100H 'Police' model, also had the older style carbs as part of it, which is a bit strange. I might go check that with them just out of curiosity, seeing as I'll have to try and get a set of mixture screws out of them too.
Craz, I'll give that vac. advance line trick a go and see what happens too, but wouldn't the 2nd carby butterfly have to be a fair whack out compared to the others to cause that much difference in vacuum?

[Crazcnuk]

Not really, as I found out.

It just has to 'crack' slightly before the others and it will start pulling the advance. Then it becomes self-perpetuating, as the idle increases so does the vacuum and the advance.

It was actually another user here that told me about it, when I had the same question as you!

[hobbsy84]

Just came back from giving the vac. advance line a pinch, and wouldn't you know it Craz, you were right! The idle would come down eventually if the line was pinched (which makes me think maybe still a vac. leak somewhere, the revs hold up for a little bit) but when the line was open and operating the advance diaphragm, the revs picked up and wouldnt come back. I guess that second butterfly does need a bit of adjustment, eh?
At least now I have a starting point for tomorrow, once I'm not going to be waking neighbors up with the noise. Cheers for the advice everyone, it's unbelieveable how helpful the people on this forum are. I don't think I would have my bike at the stage it's at without the info found here.

[Crazcnuk]

What this does is confirm that all those that suggested a synch were correct.

Once the carbs are in sync all the carbs will open at the same time, getting rid of the high idle issue.

It's hard to pinch the hose properly, which likely accounts for the problem not going away completely. To be sure, just unplug the hose at the carb end and plug the little tube, on the carb, with a rubber cap or your finger.