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erratic idle, carb farts :(


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hey all,

I've had my 81 750c for about five years now, and it's always been a bit of a pain. my friends and coworkers say i need a better mechanic (true) but i'm determined to understand the mechanics and get it sorted on my own.

the bike is hard to start, sometimes requiring a bit of throttle to get her going, sometimes i make a decent adjustment (1 turn) on the idle screw. mixture screws are at 1.5 turns out. after about a minute sitting just at 1k rpm, i hear a loud PFFT and the RPM jumps to 2500, requiring me to undo the idle adjustment to bring it back down. seems to run fine, sometimes requiring further backing out of the idle screw over time. occasionally it will pfft again and then the idle will increase a bit, but nothing as dramatic as the initial period.

i replaced the spark advancer as the old one had all its index pins either bent or snapped off, and made sure to set the timing properly. rest of the electrical system is working fine, though it's worth mentioning i've been through two rotors in as many years. I had it running real hot before due to some poor valve adjustments (since fixed).

probably worth mentioning that it sure does smell like gas when it's running


valves are in spec
timing is good
carbs are clean
spraying WD40 around boots and fuel rails has no effect. boots are new.
throttle cables are smooth and snap back fully
airbox is stock, filter is same part but replaced
prev. owner swapped to 4-into-1 exhaust
jets are stock sizes
vacuum petcock or whatever system affectionately referred to as the spawn of satan has been removed by previous owner

haven't resync'd the carbs since changing the advancer, so i'll do that today, but my confidence in that fixing anything is low since i've been through that a lot when trying to address this exact problem to no avail.

i've read a few hundred forum threads about this at this point on various engines and haven't been able to fix it, so i figured it's time to get my hat in my hands and ask for help. everything i've read is pretty confident it's a vac leak, and my current theory isn't that there isn't a vac leak, but that if there is one i haven't found it. should i keep it up with the WD40 or look elsewhere?
Very much sounds like carb sync to me. #2 cylinder is your base and cannot be adjusted. 1,3,4 need to be adjusted to match 2.
Is your choke working properly?
the choke actuates properly but has minimal impact on these problems. so...i think that's probably a no?

lemme give her a carb sync right quick and report back. appreciate the help. us brets & bretts gotta look out for each other lol 🤝
they look pretty sync'd to me, but i fine tuned them a bit. it's running nicely now as it's warm so i'll have to wait for another cold start (this issue seems to manifest after hours of the bike being off)


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luckily for me i can't keep anything straight without a spreadsheet. not that it matters much but 4.1 is cyl 4, towards the stator. 4.1 is cyl 4, towards carb 1. ex is exhaust and in is intake

Position namecurrent shim thicknesscurrent clearance
Those are inches or mm? .12-.13mm is ideal (= .005"). Bike should run ok on your numbers. But those .9s and .1s will need adjustment again sooner than the others.
mm! i'm hoping to get it running "ok" and then revisit the shims. even brand new ones kept coming in a little thinner than expected so i went through several rounds and got pretty frustrated, haha. was hoping to revisit in the winter since it'll be off the road anyway. tbh i was really hopeful the advancer was going to be it but no such luck :(

i was shooting for 0.085 per the manual tho so maybe i can move things around
.003" or .085mm is NOT what you want. It will burn valves. There was an addendum to the service manual and the new number is .005" (or 0.125mm). But again, the bike should run fine with those numbers.

Let us know how the cold start goes
got a similar result i'm afraid. I got cocky and upped the idle screw ("look, it's great now! maybe i just need to set the idle where i like it!") but you can see i had to bring it right back down after a few minutes. it seems to happen in at least two stages, you can hear it here:

again, appreciate your help a lot! is there an upper range on that valve spec, or should i just try to keep it precisely at 0.125 mm?
Mine will behave just like that when cold if I don't use the choke. I use the choke, and it goes steadily up until I push the knob in. But the stages are not as drastic as yours.

Definitely adjust the idle when it is warm, then leave it. Don't adjust it when the bike is warming up.

Curious what others say, but it sounded like a cylinder finally 'caught' and then started to go, which raised the idle. When is the last time you had the carbs apart and looked at the passageways? I ask because if your choke doesn't affect how the bike idles, then it sounds like a fuel delivery or air passage issue.
it's been a few months now, but i broke them all down and followed ... some PDF i found on a forum that everyone said was the keihin guide to use, and this problem was there right away...again. i've cleaned them at least once a year for the past six years and had this problem every time -- that said, the first time i really messed it up.

would burnt valves do it? i think i had a couple wayyy too tight at one point because i didn't read the manual properly and measured a couple with the camshaft in the wrong spot, so i'm roughly 100% sure i've done some damage (though the bike has travelled only a few hundred kilometres that whole time)
Wow, I don't know. If there was a way to get a scope up there to see, that would be ideal
i seem to remember the choke being weird, actually. I bought a new cable (all the cables are new...cables are cheap), but if i clamp it on the metal bit the choke isn't fully open, so i clamped it a bit higher.

that was a midnight revelation, sorry i didn't mention it earlier. i'm giving my neighbours a bit of time to sleep in before i fire it back up today but i'll adjust it to the metal bit and see if that helps
update: choke is not working. no working choke and behaving pretty similar to your bike when not using choke certainly seems like something. i'll try to chase that today
aight, choke was completely dinked. had the levers in the wrong spots. pull out clutch halfway, fires up after about half a second, holding idle steady at 1K. let it idle a few minutes and pushed the choke in and everything seems great, honestly. resynced and things are a LOT smoother. i'll report back tomorrow if anything changes or if things seem to be fixed (verrrry slight hanging idle i'm not sure yet if i'm worried about), but i think we got her. but i'll be sure to do a final update for posterity of the thread

thanks again for all the help, i really really appreciate it.
final update:

my sync screws are acting a bit weird so i threw it all out of sync. pulled the carbs to bench sync first, and discovered that #2's adjustment screw wasn't as far out as the others. set all to 2 ÂĽ after some trial and error (any further in and hanging idle returns), resynced, and it's running great. used to have some bogging around 5500rpm, but no more, rides like the wind.

thanks again!