Author Topic: CB350F carb sync problem SOLVED (Read 8553 times)

55fbomb · « **on:** June 25, 2011, 01:50:13 PM »

I tried to sync the carbs on my Dads 350f today and it was a nigthmare. There is no air leaks, checked several times. Cylinders 3 and 4 wouldnt even come close to keeping the fluid in the sync tool, everytime we started the bike they would shoot up until i shut the bike back off. i tried turning the adjusters a little each way but i didnt help. i disconnected the hoses to 3 and 4 and got 1 and 2 right in no problem. here is the problem, if i adjust 3 and 4 to where it will not suck the fluid out of the tool, it idles at about 5K and will not come back down. the idle screw was backed out until it wasnt even touching and it was just setting there screaming. these carbs were dissassmbled by the PO and cleaned and look like new inside and out. even now the idle is wandering and i have the adjusters turned the whole way in on all four and it still idles high and hangs when you open it up, like it has a vacuum leak but it doesnt. everything looks OK inside of the carbs, im at a loss here, i have never seen this before, anyone got any ideas?

WarwickE36 · « **Reply #1 on:** June 25, 2011, 01:52:41 PM »

Not sure about the 350 carbs, is there 1 (non adjustable) that the other 3 adjust to?

If not pull carbs, bench sync with a 1/8 drill bit, re-install and sync again

camelman · « **Reply #2 on:** June 25, 2011, 01:55:12 PM »

Pull the carbs apart again and verify you have the correct jets. You must also pull the emulsifier tubes out and check them. Most people miss them when cleaning carbs, and they are a major problem if dirty. Verify your float levels, and then try syncing the carbs.

Make sure your mixture screws are set correctly too. Some of the 350f bikes had black plastic caps on the mixture screws that must be removed to set the mixture correctly.

55fbomb · « **Reply #3 on:** June 25, 2011, 02:02:44 PM »

the idle mixture screws are set to 1 turn out, and the carbs have the stock jets in them and are clean. the bike runs fine if you have it tached up, its just the idle is crappy and it wanders. i runs, i just cannot get the 3 and 4 carbs in the ballpark of the other 2 without the idle going up to 5K and not coming down. all 4 have an adjuster so i dont think its a matter of settin 3 to the master carb, i think thats a yamaha trick.

WarwickE36 · « **Reply #4 on:** June 25, 2011, 04:07:12 PM »

Unless you have air leaks, 3 and 4 are pulling too much air, therefor why wouldn't you want to set them where 1 and 2 are? Isnt that exactly what a sync does? A bench sync will tell you they are all the same, then you can adjust for a cylinder that pulls more air. Sync should be a fine adjustment not a base one.

55fbomb · « **Reply #5 on:** June 25, 2011, 04:40:18 PM »

Yeah, thats what im saying, if i set them all the same the engine idles at 5K, thats the problem. there is no air leaks, i checked several times. i know how to sync carbs, there is a problem here i have never seen before, this is not normal and i wanted to see if someone else has seen this before. you cannot get 3 and 4 anywhere near 1 and 2 on the guages, without the idle going up and out of control.

Kevin400F · « **Reply #6 on:** June 25, 2011, 05:23:56 PM »

Is the throttle cable holding the carbs open slightly? When you look at the carb slides, is the "silde cutaway" on the airbox side.

You can get the sync adjusters so far out of whack that even with the idle screw backed completely off, the slides are still being held open slighty. Make sure the idle speed screw is screwed in enough to start rotating the linkage, then bench-sync the slides so there is just a tiny gap on the engine side. You should be able to back out the idle screw and make the gap disappear.

Kevin

WarwickE36 · « **Reply #7 on:** June 25, 2011, 05:32:09 PM »

What did you do to the bike since the last time it was running correctly

55fbomb · « **Reply #8 on:** June 25, 2011, 06:54:48 PM »

Nothing, it didnt run when i got it. the carbs were cleaned and rebuilt in a box. i put new plug boots and a pamco ignition in it and put the carbs back on. Stock air box is in place and a 4 into 1 with a baffle. Im gonna have to take them back off and tear them apart and see what is going on, and bench sync them with the idle screw in, like Kevin400f said. As far as 1 and 2 are from 3 and 4, i think something may not have been put back together correctly. So as far as what i have done to make it not run correctly, nothing.

55fbomb · « **Reply #9 on:** June 26, 2011, 08:59:49 AM »

400f carbs are the same except the pilot jet size correct?

camelman · « **Reply #10 on:** June 26, 2011, 09:17:07 AM »

I believe the idle screws are supposed to be 7/8 turns out.

Kevin has a good point about the throttle cable being tight. Try grabbing the throttle cable by hand and shaking it while the engine is running. If the engine revs, then you need more slack in the cable.

If you are going to pull the carbs apart again, then make sure you check the emulsifier tubes. They are very often missed even by "professional" shops. Your problem is not typical, so look for something out of place. check your timing again too.

Camelman

Spanner 1 · « **Reply #11 on:** June 26, 2011, 09:21:00 AM »

Drive you nuts wouldn't it

......... Here's my 2c. Assuming your float heights are set correctly, one of your carbs may be overfilling. With fuel welling in the carb throat that cyl. will tend to increase in rpm and will increase rpm/vacuum ( obviuosly

) in the other 3 cyls and now were 'off to the races' and engine rpm will shoot up. More prone to happen during a sync. as throttle cables are backed-off.
Watched this happen on an F 750, the slides actually rise, have to push down on the linkage to restore idle rpms........... might be that .

55fbomb · « **Reply #12 on:** June 26, 2011, 03:22:33 PM »

Spanner, that is interesting, i will check into that also. thanks. camelman, i always clean the emulsion tubes, i have had problems with them before. the timing is dead on and the throttle cable was backed off until i had slack at the linkage. I am going to have to take them back off, im hoping something jumps out at me once i have them apart. thanks for the ideas guys.

WarwickE36 · « **Reply #13 on:** June 26, 2011, 03:50:01 PM »

Float level set to spec?

camelman · « **Reply #14 on:** June 26, 2011, 04:36:41 PM »

Another thing that could cause erratic revs and hanging throttle is a sticky spark advance unit.

55fbomb · « **Reply #15 on:** June 26, 2011, 06:58:21 PM »

I tore the advance unit apart and lubed it up when i put the pamco in, it seemed fine and i verified it worked with the timing light. the floats were OK but im gonna check them out again while the carbs are off.

Bodi · « **Reply #16 on:** June 27, 2011, 06:12:49 AM »

There are two things that affect the throttle other than the carb balance adjusters: the throttle wheel and its idle stop plus the choke cam. Make sure all four throttles move freely to fully closed with the carbs off the bike: this eliminates the throttle cables at least. When the wheel is turned all four must move simultaneously from fully closed or with identical small rods inserted between the slide and bore. Make sure the slides are in correctly - the cutout must face the choke. You can reverse them in pairs since the keying slots are opposite sides with 1 & 2 different from 3 & 4.
Ensure the choke cam is clear at idle with choke off. Roughly adjust the sync screws so that the idle screw is most of the way out with the slides fully closed. Then set a "bench sync" idle with the slides open slightly and all four visually identical. Set the air screws to spec and all four the same. You should have verified that the internal air passages are clear already. Check the pilot jet emulsion tube holes are clear. Check that the needle jet emulsion tube holes are clear.
It seems that your 1 and 2 slides are being held up, not dropping enough. This means you are idling on 1 and 2 at their lowest possible setting, your idle goes way up when you pull up 3 and 4 to match.

flybox1 · « **Reply #17 on:** June 27, 2011, 07:13:45 AM »

carbs were rebuilt and in a box.....
who rebuilt them and did you inspect the work inside?
if you didnt do it, its time to crack them open. first thing...pull the main jet and the emulsion tube.
they need to be shiney brass with clean and clear holes. unscrew your pilot jet, clean all of its holes. set your float heights to 24mms.
complete a bench sync of your slides before you attempt another vacuum sync.

55fbomb · « **Reply #18 on:** June 27, 2011, 01:44:37 PM »

Thanks guys, i have to get back down to my Dads and get the carbs pulled of and do a lot of checking. I got the bike from a member on here and the carbs were supposed to be ready to put on and run. everything inside is spotless clean, i think the were probably ultrasonicly cleaned and all the screws for the caps and bowls were replaced with stainless allen heads, they look great inside and out, they just dont work so hot right now. Having the throttle cables off makes no difference and i backed the idle screw for the choke off also, so thats not holding it up. i just gotta pull em off and tear em apart and see what is going on in there. thanks again guys.

gregk · « **Reply #19 on:** June 27, 2011, 02:26:46 PM »

Quote from: 55fbomb on June 25, 2011, 04:40:18 PM

Yeah, thats what im saying, if i set them all the same the engine idles at 5K, thats the problem. there is no air leaks, i checked several times. i know how to sync carbs, there is a problem here i have never seen before, this is not normal and i wanted to see if someone else has seen this before. you cannot get 3 and 4 anywhere near 1 and 2 on the guages, without the idle going up and out of control.

i've got a similar problem with a 550. A climbing idle which seems to settle in at about 4000rpm. When i tried to sync the carbs, i couldn't get any decent readings at less than 3500rpm and these were very low. Checked for air leaks with a propane bottle, checked the advance unit, went thru all the tune-up procedure, have had the carbs off many times etc. No remedy to the problem. I have to go along with the float level or a carb(s) over filling. This would mean that the float valve leaks. The bike runs rich and everything smells like gas, even the oil.

What vaccuum readings do you get ?

55fbomb · « **Reply #20 on:** June 27, 2011, 03:44:20 PM »

I have a motion pro sync tool, for now, and i was getting less than a quarter of the way up on 1 and 2 and if i let it go 3 and 4 would have shot out of the tool. they are extremely far apart. this bike doesnt seem to run rich, it doesnt stink or smoke and the plugs look OK. I am eager to get the carbs off and get them torn apart, i will post the findings and if i figure anything out i will let you know gregk.

Kevin400F · « **Reply #21 on:** June 27, 2011, 04:41:16 PM »

Have you checked compression in each of the cylinders?

55fbomb · « **Reply #22 on:** June 27, 2011, 07:16:20 PM »

lol, never even gave it a thought, it runs so good once ya get it moving a little i never even gave it a thought.

55fbomb · « **Reply #23 on:** July 02, 2011, 01:16:47 PM »

I got my hands on the carbs today, my dad had them ripped off when i got there and was trying to bench sync them but said that when he tried to drop the slide for #1 it was lifting the rest of the slides. i started tearing into the linkages and unhooking stuff one thing at a time and trying to get the slide to go the whole way down until all that was left was the slide itself. So once the slide was out we found a burr on it and smoothed it out and still nothing. So i started checking the body itself and found a small piece of metal lodged in the little slot that the slide is supposed to rest in. it took some light digging with a small screw driver but it finally came loose and the slide fell right into place. the piece of metal looked to have been there for a while, it was starting to look corroded. where it was you could not see it from either end of the carb and it didnt show up in the bench sync since it was about the size of the wire we were using to sync them. Only when you backed the idle screw off and held the carbs up to the light could you see that the slide was not returning down to its proper place. Once it was out and the carbs were back on it fired right up and ran great, a little adjusting of the mixture screws and a good vacuum sync and it should be good to go. thanks for all your help guys.

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Author Topic: CB350F carb sync problem SOLVED (Read 8553 times)