Alright I'm losing it here, someone give me some ideas, heh..
I picked up this 79 CB650 with 5k miles, it wasnt charging when I got it, already fixed that. It wouldnt run at all without the choke being full-on.. you could rev it in neutral with the choke off as long as you kept twisting the throttle.. Im guessing the extra fuel from the power valve squirting in there was the only reason...
So I said im not gonna screw around with seafoam on this one, and totally dissassembled the carbs, checked to make sure the floats were working good, dipped them in the cleaner, blew them out with air, put em back together. Everything was looking great. No change in problem, lol.

I dont understand how it could run that massively lean if theres nothing clogged up.... and theres not. When you open the choke up, right before it dies, it sounds alot like a car when you've turned the distributor back too far... you know like the carbs are going to suck you in, heh... so I dont know I thought maybe the timing was off, so I checked the static timing and thats ok as far as I can tell. The boots on the engine are mint, theres no cracks in them.. The holes in the carbs for the sync tool are plugged also.

With the choke on it runs ok, all 4 cylinders are firing, spark plugs were brand new when we first discovered it ran this way.. I need ideas before I light something on fire.

Do you have a vacuum leak maybe one of the carb holders are leaking?

I'm sticking to a low carb diet. Sorry I just couldn't resist.

Good luck tuning your carb!

No vacuum leaks I can find.... you would think it would have to be, this thing will die without, i dunno at least 90% choke. But there it runs smooth and fine, In fact I can ride around the block as long as I leave the choke on, even get her up to 5k before she doesnt have enough air to go any faster :P

Carbs are not evil, it is the bike owners who are neglect the them. A bike with carburetors will never leave you stranded on the side of the road due to some electronic gremlin in the FI or ECM.[/rant]

The #1 problem is improper long term storage. The carburetor bowls have drain screws for a reason. It takes only a few minutes to drain the bowls. If you leave the gas in them say over a winter it turns to a nasty brown varnish. I have also seen green slim which is really gross and smelly.

Fast forward... It's spring or in your case a new owner. You are anxious to fire the bike up. Guess what? All that crap sitting in the bowl(s) now gets sucked into the carburetor(s). Nothing left to do except give them a thorough cleaning. The main jet, slow speed jet, idle jet, are only part of the equation. More important are the tiny orifices and passage ways inside the carburetor. I use fine wire stainless steel to clean stubborn clogged holes.

I'm guessing a partially blocked slow speed circuit (idle to appox. 1/8 throttle opening) is causing your problem. The slow speed jet is usually a long brass tube with a number on it. Inspect it with a magnifying glass if ok proceed to blow out the hole where it threads into. Follow it up with the SS wire. A word of caution "be careful" If the wire breaks your screwed.

Above all, have patience...

I spent the better part of a day thoroughly cleaning them, theres nothing in them except fresh gas. They looked like new when I was done. Wont just not idle either, wont run holding the throttle open at any position. I was also smart enough to install a filter before I let anything in the tank back into my freshly cleaned carbs.

I've had this battle many times before with my 81, im telling you these things just arent worth a damn.. lol. You know if cars all had multiple carbs, well we just would have invented another means of transportation because noone would have liked that... hard enough to get one working good, forget 4.

MisterMike is on the money with his advice, but did you actually remove the four pilot jets (slow jets) from the carbs to clean them? And when you reassembled everything, how many turns out did you set your fuelscrews?
Carbs are incredibly simple and quite easy to work on when you understand them. Good luck.


PS - What size are your pilot jets? I might have some new ones for you.

Removed everything that was possible to remove. Slow jet is a press fit. I get what your saying, I understand it. I mean when i opened them up and saw just how much grime was in them thats when I said i was going to go hardcore on these ones. I made them spotless, put them back on. Absolutely 0 change. maaaaaybe slightly better idle, I cant really tell. No passageway as far as I can tell is clogged, I used light to try and confirm that on all of them. Blew every hole out invidually with 120 psi, if thats not going to free up some gook then nothing is. Its starting to be my understanding that once these carbs get gooked, its over.

I was hoping someone was going to have another suggestion, but having done this before I knew this was going to be it. See at least you all know my pain, lol. Just doesnt seem right though..

The pilot jets, not sure what size.. Ive played with the screws at different positions but its not making any difference as far as being able to run without the choke. There 2 turns out right now.

You have done a remarkable cleaning.
Quick questions:
1. Is the stock air box and filter in place when you did the test ride?
2. Did you sync. all 4 carbs. with either a Hg(mercury manometer) or one of the newer non-Hg models?

1 peice of my 2 peice airbox is there But no it came with K&N pod filters, but as far as testing it in my driveway while im trying to get it working I just leave the filters off, to eliminate any variation. On or off doesnt produce a noticeable difference though.

Ive not been able todo a sync yet because I cant get a steady idle. I mean I can but as the bike warms up with the choke on the idle will race, I bench synced them but thats it so far.

Gonna take them off tommorow again and open the float bowls and have another look, going to check those connecting boots really good too but they looked great. They are exceptionally stiff though, I didnt think i was going to be able to get the carbs back on those suckers.

FWIW: Years ago I had a CB550 in the shop with a similar problem. The owner and I worked on this bike for 2 whole days. Did all the trouble shooting without the air box. Put the stock air box on with a new filter and presto, the bike ran like a champ.

Put the K & N's on and give it a try. It still might run like crap unless someone with lots of experiance tuned the carbs for the increased air flow.

Just to eliminate the boots while the bike is running spray a little starter fluid around the boots and listen for a change in the way it is running.

Yeah, i gave that a try, still not happening. Im thinking you could be right so im going to find something more restricting to cover them up with and see if i cant get something to work and go from there.

V65rider, I gave your idea a try, and it did make a difference but, I think it was just blowing around and being sucked in the front, im not real sure :P


So I looked around and it looks like 90 is the stock main jet size, and thats what I have. And then im looking around and see what some people are using, and they are claiming 102, 110, stuff like that on their cb650s.. is there something about slide carbs as opposed to CV carbs that require more extreme changes in jetting? This bike just has K&N's and a 4-to-1 that was put on because the original exhaust had rotted out. From what most people are claiming thats probably good for 3 or 4 jet sizes, so I dont know im confused here.

uh...i haven't had a set of those carbs apart since last summer, but the pilots come out...
they do look remarkably clean! what color are your plugs? if i remember, those also have that annoying vaccum line that goes to an automatic fuel shut off of sorts? i've had them give me fits before... another problem i've seen is voltage; it seems kinda goofy, but if it doesn't put out a good, clean voltage from the alternator, it will give a weak spark; you should be able to jump at least a half inch of spark while you crank it. as the motor speeds up, it takes more juice to fire them fast enough, so make sure you have a good voltage at the coils. for some reason, when you choke it, it seems to help, but only hides the actual symptoms and sends you on a goose chase! my sisters bike had this same problem, and i ended up beating my head up against the wall for a long time trying to figure it out.
have you checked the fuel screen that goes into the tank? i've had to clean/ replace those before, also...
yes, you'll have to jet up 3-4 main sizes (from stock) for the exhaust and air cleaners, if they are the emgo or cheaper type of pod filters, they don't effect your pilot jet as much as a spendy filter or the foam pods (uni-filter type)
should you decide you want to try a stock airbox and can't find one, i think i have one in a box somewhere... i hate moving! anyway, PM me and i can get it to you.
good luck! don't burn it up, just be patient!

I've been in this exact situation. I rebuilt a barn fresh 81 maxim 2 years ago. 4 carbs are such a PITA. Make absolutely sure every pin hole is not clogged. Plugs are super important, and without a good gas/air mix, they'll foul quick. Voltage is also really important for cold starts.

Here are a few links that helped me through my madness. They're all yamaha/mikuni specific but there's a ton a basic info to be had.

http://members.tripod.com/dave_jack/id38.htm
(I don't think the YICS will apply unless Honda used a similar design)

http://xjbikes.com/Forums/viewtopic/...ol+method.html

http://xjbikes.com/Forums/viewtopic/...nch+synch.html

Make sure you're not using the choke too much. My maxim will fire right up at 50+ degrees no choke. Start turning the motor over and gently apply more choke until it starts to fire.

One last thought, I had cleaned my carbs for "the last time" and was about to set the bike on fire when I had a friend look at it. There was a simple spring that holds tension on the butterflies that wasn't seated just right. Simple fix, but my frustration had got the best of me. Take it slow

Hope it helps. I know how frustrating carb work can be. Good luck.

Thats funny BlackRig because I traded an 81 Maxim for this bike, lol... at least that one ran though.

scrat; the slow jets on these are pressed in, I mean I could remove them but it would involve pliers and probably damage
I will have to try and find out if im getting enough spark.. my plugs end up looking lightly carboned and some gas on them, its rich or nothing I guess, I can only keep it running on 95% choke.

If anything I'm probably getting more volts then usual, because my spliced in car regulator is charging high.. dont ask, the new regulator either came in bad, or blew itself immidiately because of a bad rotor I had to replace... shrug.

Im going to try a fresh source of gas too, maybe there is water in it? I wouldnt think the choke would help much if that were the case, shrug.

Ill make a video later so you can hear this thing.

Are the slides on this bike vaccum operated or driven by a rod ?

By rod, thats what made me curious if it were more susceptible to mods somehow, maybe the vacuum main jet can compensate.. shrug

mostly I was trying to figure out what people could have done to there bikes to require 20-25+ size on there main jets then I have :P

This project is not dead, I've just not worked on it in awhile.. I do have a ton of other things to work on to so it happens alot

Anyways, my neighbor found the stock airbox, I'm charging up the battery and will find out later today or tommorow if its going to make any difference.

You think maybe I have a thing for Honda CB's?

I had a 1980 CB900c and my father-in-law still has his 1979 CB750 Limited. We had to go through the carbs every spring regardless if we drained the carbs or not. Syncing the carbs is a must for them to operate properly. My carbs work great once we had them jetted properly for my 4-2 hooker header exhaust and K&N filters. Stock main jets were I think 105 and I had to go to a 120 with the exhaust and filters. The stock airbox is Very restrictive warranting the jump in jets. While the stock airbox is easier to tune, it is not fun to take off anytime you need to get to the carbs. Make sure your idle is not too low. Blow carb cleaner in every orifice in the carb and then blow with your compressed air. Play with the main jet as the pilot jet only supplies fuel during start up and as a compensator when you jump on the throttle. Make sure your floats are floating and not sinking in the fuel. There is a little metal tab that allows you to adjust the float height. Be patient...

ComStar Wheels forever !

ok so I was actually able to keep it running by holding the revs really high, even with the choke off, now that the box is in place. Its still running like garbage, but I wasnt able todo that before. So progress has been made.... but yes this box is terrible and I dont want it in there, one of the boots for the carb is totally deformed from some other peoples troubles with getting them in and out. I masking taped around it to seal it up, lol.

If it were me, here's what I'd do...

1.) Get air filter box to original setup with new filter
2.) New Main and slow jets
3.) Adjust floats with precision
4.) Replace intake boot

From your description, you have a big air leak somewhere, my guess is intake boots. The carbs look clean.

Good luck!

Been through this many times. You either have clogged idle jets, an intake air leak, or a high exhaust leak (not likely, but it can happen.) Most likely, given the cleaning you've done, it's the idle jets. Especially, since the idle screws were still plugged, the PO put on the K&Ns and then it wouldn't idle. That's when he parked it. Since you've already put the stock breather back on, it's almost certain that it's the idle jets. If you have the carbs off, pull the bowls and slides, then hold the carbs up to light. You should see pinholes of light through all the jets. I bet you don't. The jets may be down in the tubes and you'll have to get a skinny screwdriver to get them out. You might be able to unplug them with a welder's tip cleaner without removing them. Seafoam and the like won't work if they're completely clogged, because if the fuel can't get through the circuit, it can't dissolve the crap. If you can get them out, soak them in Berryman's B-12, that you can get at WalMart. (Army Chopper mechs use it--- stronger than Seafoam--- careful, it'll peel paint!) If not, use the welding tip cleaner or a strand of wire from a wire brush to (gently) unplug the idle jets. The jets are brass, so be careful if you use the wire method, 'cause if you wallow the holes, they're shot and need to be replaced. Once you can see through the jets, you can put the carbs back on and it'll start and likely idle. You can get the sync in the ballpark by looking at the slides as you work the linkage--- they should all have the same size opening as the slide goes up and down. By the way, check the diaphragms on top of the slides while you have them off. Stretch the rubber and look for pinhole tears, especially around where they're bonded to the metal. Sometimes you can fix them with liquid tape. Replacing them is better.

You can put the Berryman's in your first tank--- use about 3-4 oz. It'll run a little rough until you fill up again. Just don't get any on your paint. Once they're clean, you may not even have to touch the idle screws, especially if you're going to run the stock airbox. If you kept the original settings, you won't have to do much adjusting--- Honda does it right, and I always recommend cleaning the linkage with WD-40 or something on any old Honda that runs before you touch any of the adjustments on the cables or anything. Only adjust or repair after you've rode it a couple of hundred miles, because some things are just stuck and will work loose by themselves.

Sorry this is a bit long winded. And if you've already done this stuff, disregard!!!

Yamaha dealers carry a really good stabilizer/cleaner that you can mix with the gas, but the advice given is spot on -

But I kind of gave up on cleaning pilot and main jets, I just replace them -

Then soak the old ones forever, clean them up, and use them as swapping material the next time this comes up....

And it will, as long as I plan to be working on motorcycles, there will always be a set of gunks carbs waiting for me, somewhere......