Carb sync

[red bandit]

Stock bike & engine except for larger velocity stacks. Stock 79 carbs with all Mikuni parts. 42.5 low speed jets, 137.5 mains. I got her in 04 with 35000 on the clock. Now 66000. In the past when I would sync the carbs I would have the idle mixture screws out about 1.75 turns on all 4. Now I have them out about 2.5 turns. I went through the sync procedure several times to be sure. I use 4 gauges to sync. It runs great. Any thoughts on this? Poor quality gas these days? I meant to put this in the regular discussion section. Would someone please move this. Thanks

[DiverRay]

The ethanol content of the gas makes a BIG difference! I'm able to use regular, ethanol free gas here in Idaho. My sync is all from here, as the bike was not running when I moved here.

[red bandit]

Ethanol content around here is10% and has been for years.

[Bax]

red bandit a few small things could cause the setting shift you describe. Since it's across all 4 carbs it's probably something common. First thoughts would be air filter restriction, second would be the tank vent, try again with the gas cap open. Next would be to read the plugs after idle period to verify the correct mixture with the +3/4 turns setting.

---Bax

[bikerphil]

Ok, my theory is a bit different. Over time, the rubber throttle shaft seals shrink and get brittle and start to leak. This leans out the idle mixture which requires a bit more fuel to be introduced, which would result in opening the mixture screws more. I just changed the 40 year old seals on both of my bikes with brand new Yamaha OEM seals and I noticed a huge improvement in low speed/idle performance. The old seals just fell right out so I'm convinced they had to leak a bit. Unfortunately I did not take note of the number of turns out of the mixture screws before and after. Does this make any sense?

[cajun31]

Ok, my theory is a bit different. Over time, the rubber throttle shaft seals shrink and get brittle and start to leak. This leans out the idle mixture which requires a bit more fuel to be introduced, which would result in opening the mixture screws more. I just changed the 40 year old seals on both of my bikes with brand new Yamaha OEM seals and I noticed a huge improvement in low speed/idle performance. The old seals just fell right out so I'm convinced they had to leak a bit. Unfortunately I did not take note of the number of turns out of the mixture screws before and after. Does this make any sense?

I agree with Phil. The butterfly shaft seals can be a source of problems with the pilot jet sync. When I start working on any carbs the first thing I do is completely disassemble them down to just the bodies. I inspect everything and all the rubber parts get replaced. From there I bench sync them utilizing the bread tie method. Once that is done I dial them back down with the main idle adjustment screw to where all the butterflies are about half way splitting the largest hole at the top of the carb throat. There is a tip in the maintenance section on what I am talking about on this step. I turn all the pilot jets in completely until seated and then come out 2.5 turns on each of them. This gets the carbs very close and I have never had a problem with the bike starting from here. Once mounted on the bike and and at operating temp I sync them without touching anything else. From there I utilize my color tune ( and many don't like this tool ... but I do) I get a reading on each cylinder starting with 1 and work from there through 4. I adjust the color until I see the first hint of orange and then turn the screw back in and out until I settle on where the screw just hits orange. If your shaft seals are not good this step is very hard to attain with the color tune ... or if there is anything in the pilot circuit that is impeding anything. I forgot to mention that when I have the carbs apart I soak each carb body and all jets and metal components in a gallon can of Berryman for about an hour and then wash them in clear water and blow dry them with 80 PSI air through all orifices. This not only makes the carbs spotless it also guarantees that they are clean inside and out including all passage ways. Back to the sync... I know I am getting long winded. Any way... from the color tune step I check the sync again and normally it is still spot on or needs very minor adjustment. Like others have stated I try my best to avoid ethanol gas ... not always possible when on the road. I just never let my bike sit stored with that crap in my carbs. The shaft seals are not cheap but are a needed update to an old bank of carbs. Another thing that wasn't mentioned but can really mess with trying to syjnc the carbs are intake leaks. The shaft seals are a source as well as the rubber intakes on the bike. I fought one for a while until I finally discovered I had a leak on my number 1 intake manifold.

[cajun31]

Here is the post I spoke of about adjusting the butterlies using the main idle adjustment. Easy Bench Sync Method - XS11.club Forums. Of course I don't do this step until after I have adjusted them using the bread tie methiod.Pre-Synchronizing Carburetors - XS11.club Forums

[red bandit]

It was running great before hand I just thought it was time to clean the carbs and re-sync. A couple years ago I had these same results. I'll take a look at the plugs since the engine is cold and I haven't started it since the sync. Then I'll check for intake/shaft seal leaks. I don't think it's restricted air intake flow because of modified air box, or gas cap vent since it runs great down the road but definitely worth keeping in mind. Thanks for the knowledge.

[bikerphil]

Just in case anyone needs them, here are the part #'s for the OEM shaft seals...

78-79, 2A2-14997-00-00

80-82, 256-14997-00-00

partshark.com

[red bandit]

Here is a pic of the plugs in order left to right as on the bike. When I spray the shaft seals I don't get any response but as soon as I spray the intake boots the idle drops immediately on all 4 boots. I replaced them around 06 or so with Parts-n-more boots. I would have thought they would have lasted longer than that but what do I know. Any way, any suggestions on where to buy new ones or where not to? I see them on fleabay for $50-60. Or should I spend more for good ones?

[DiverRay]

Did the carb boots have a gasket? If you installed them without putting on some Yamaha non-hardening sealer, that may be the problem.

[red bandit]

I don't think gaskets are on. That may be part of the problem but the boots are definitely bad.

[red bandit]

I got new carb boots on and the screws are 1 3/4 turns now just like they used to be. THANKS GUYS. When I got done syncing the carbs I spayed the boots and throttle shaft seals and all look good. When I got the old boots off I capped off both ends and applied about 15 lbs of air pressure thru the nipple and sprayed the boots with soapy water. Lots of bubbles. I would have taken a pic but I needed three hands. Lets see if the mpg go back up to where it used to be.

[cajun31]

I got new carb boots on and the screws are 1 3/4 turns now just like they used to be. THANKS GUYS. When I got done syncing the carbs I spayed the boots and throttle shaft seals and all look good. When I got the old boots off I capped off both ends and applied about 15 lbs of air pressure thru the nipple and sprayed the boots with soapy water. Lots of bubbles. I would have taken a pic but I needed three hands. Lets see if the mpg go back up to where it used to be.

Glad to hear you got it worked out. Part of the joy of owning these old beasts.