Topic: 73 CB350f idles fine, cylinders 3+4 dont fire under acceleration Please help!!

[751]

     Hi everyone! Im new to these forums, first of all I am a decent mechanic, I race dirt bikes and know fcr carbs and modern dirt bike engines. This cb 350f is kicking my ass.
   
Main issue is the bike idles fine, all 4 cylinders the pipes are hot at idle, idles perfect, if you quick hit the gas upper high end it kinda cuts out, idles fine, pull in clutch and put in gear and get 100 feet and cylinders 3+4 completely cut out, bike runs funny, wont even move itself, when your sitting and trying to keep it running, the pipes 3+4 start to sputter a bit, and try to fire and then it must start to be burn the unburt  gas in it. Then when it is sitting there in neutral is idles fine again.
   
   I got this from my girlfriends dad who has owned it since the early 80's, i got it, cleaned the carbs, sat in a parts washer for a week, then left them sit in seafoam, then scrubbed them good and thouroughly cleaned as much as i could with aerosol carb cleaner, installed a new carb kit, except the needles, have tried different plugs, switched the coils with a older cb750 and it run the same, took it to a mechanic he replaced the points and condensors, said he checked the valves and the timing. Dont think he synched them.

  Whats next? is there a voltage regulator /rectifier? bad ground? corroded connection? plug caps bad? Head gasket? I sprayed intake cleaner all around the cylinder and intake connections, nothing, runs the same without the air filter.
    But it idles perfect???      Please help me if you can, I would kinda like to ride it before winter gets here in mn. Thanks, sam.

[strangedaze39]

Triple check your points/timing. Make sure the contacts are 100% clean still. Most definitely sounds like some kind of ignition issue. Are you 100% sure it's 3&4 and not 2&3 or 1&4 lagging?

Had a similar problem with my CB350F last summer. A full cleaning and reassembly/retiming of the points fixed it right away.

[Mdub]

1-4 are on one coil and 2-3 are on the other.
!-2 and 3-4 each have their own fuel supply.
That tells me it's probably a fuel delivery issue.
I believe there is a fuel tee between 3-4 that is prone to cloggage.
Check it out.
Mike

[751]

Triple check your points/timing. Make sure the contacts are 100% clean still. Most definitely sounds like some kind of ignition issue. Are you 100% sure it's 3&4 and not 2&3 or 1&4 lagging?

Had a similar problem with my CB350F last summer. A full cleaning and reassembly/retiming of the points fixed it right away.

 Thanks for the fast reply, I am 100% sure its 3+4, I have short pipes on it right now, the 1+2 are running and making noise, 3+4 are just pushing air out with no noise,it comes back to life later on, at idle, puffs out the unburnt gas and then idles fine, is there a set of points just for 3+4? Do I just use electrical contact cleaner, fine sandpaper to clean points? I had pauls cycle in marshall mn, replace the points and condensors, check the valves and timing, he couldnt figure it out and I picked up a unworking motorcycle. I wasnt to happy, maybe a different shop soon, I am gonna try to clean the points later, but i did a search on here, One guy had said a resistor in the plug caps can go bad and cause a no spark issue also, IDK? This am the bike started up and idled, but the number 1 cyl had a cold pipe, i pushed the plug cap on, then pulled it off and reinstalled and the pipe got hot asap. I think new plugs and new caps will be done today also, I have been battling this issue all year. Thanks, sam.

[751]

1-4 are on one coil and 2-3 are on the other.
!-2 and 3-4 each have their own fuel supply.
That tells me it's probably a fuel delivery issue.
I believe there is a fuel tee between 3-4 that is prone to cloggage.
Check it out.
Mike

   Thanks mike! I am on this bike all day, this will be checked asap, i will be on and off the pc all day, thanks! sam.

[751]

1-4 are on one coil and 2-3 are on the other.
!-2 and 3-4 each have their own fuel supply.
That tells me it's probably a fuel delivery issue.
I believe there is a fuel tee between 3-4 that is prone to cloggage.
Check it out.
Mike

   The bowls have gas in them, just checked by losening the screw, Would the 3+4 carb have just enough gas for idle, then acceleration it runs out of fuel, creating the no fire, then i struggle to keep it running in neutral, until the carb bowls fill up very slow due to the passage is alsost clogged? Is that your thoughts? I didnt not seperate the carbs when i cleaned them, just did them on the rail. Do I have to separate them to clean the t? Thanks, sam.

[751]

   I just looked at the points, the points under the cover looks really clean, but the set of points on the top looks like it doesnt open as much as the other set, also it has spark but doesnt look as big as the other, I havnt cleaned or checked the gap yet either. Thanks!

[flybox1]

+1 on the fuel delivery issue.
just wait.... the OCD (obsessive carb disorder) will set in soon.  
pull your carbs and check all of the main jets and float valves for good flow of fuel.
3&4 main jets might be clogged causing fuel starvation when they are supposed to kick in.
while the bank of carbs are off, double check float levels are at 22mm's when at rest against (but not depressing)the float pins.
air screws should be in the 1 turn out (+/- 1/4 turn)

btw, what exhaust/airbox systems do you have?
mine has the 4into 2 Jardines with stock airbox, and when i switched from the stock 75 main jets to 80's, the higher rpm's came to life.

good luck

FB1

[751]

+1 on the fuel delivery issue.
just wait.... the OCD (obsessive carb disorder) will set in soon.  
pull your carbs and check all of the main jets and float valves for good flow of fuel.
3&4 main jets might be clogged causing fuel starvation when they are supposed to kick in.
while the bank of carbs are off, double check float levels are at 22mm's when at rest against (but not depressing)the float pins.
air screws should be in the 1 turn out (+/- 1/4 turn)

btw, what exhaust/airbox systems do you have?
mine has the 4into 2 Jardines with stock airbox, and when i switched from the stock 75 main jets to 80's, the higher rpm's came to life.

good luck

FB1

  Thanks for the input, stock airbox, drag pipes, have a aftermarket carb kit from jerrys cycles on ebay, I read somewhere to install the old oem jets because some aftermarket ones are incorrect, I have my old mains and pilots soaking in seafoam for the last 3 days, They will be goin in place of the aftermarket ones, are these carbs pos items? also mdub said the 3-4 carb t might be clogged? what is the best way to clean them without removing the rail? this bike has oem ignition also, the bike used to be able to go a mile before slowing up, now its 50-75 feet, so it sounds like the passage might be getting slowly but surely plugged. Thanks, sam.

[flybox1]

yeah, the aftermarket (keyster brand) carb kits will cause nothing but headaches.
especially the needles.  bad tapers they have.
be sure to pull out the emulsifers underneath the mains
the tiny holes along them can get clogged as well.
thin brass wire can unclog them.
if the o-rings on the oem mains are good and seal well, stick them back in. you may want to go with larger mains with your drag pipes because of less backpressure.
keihin jets are easily obtainable and a only few bucks each.
only way to clean the fuel connections between the carbs is to dissasemble.

[751]

yeah, the aftermarket (keyster brand) carb kits will cause nothing but headaches.
especially the needles.  bad tapers they have.
be sure to pull out the emulsifers underneath the mains
the tiny holes along them can get clogged as well.
thin brass wire can unclog them.
if the o-rings on the oem mains are good and seal well, stick them back in. you may want to go with larger mains with your drag pipes because of less backpressure.
keihin jets are easily obtainable and a only few bucks each.
only way to clean the fuel connections between the carbs is to dissasemble.

   I read on here another guy put 85s in a helped, I plan on doing that as soon as I figure out what the main issue is, with 75s I should be at least able to go 15mph, i live on a farm and cant even get to the driveway, the bike sat for 4 years, and i got it, cleaned the carbs, had sediment in it, used the old parts at first, and I remember it worked ok for a bit, then i bought a pos kit, at least the needle and seats work, I didnt know they were junk till later on, im gonna clean the carbs now, It will be a new experience splitting the carbs, install the oem jets main and pilots, and go from there. I will repost back here later tonight! Thanks, sam.

   

[flybox1]

hopefully you have a sediment-free tank and fresh fuel to start with so you arent clogging it as you start.
if you havent pulled the carbs yet, check fuel flow to 3 & 4 by opening the bowl screw on 4 and turn your petcock on.  replace screw on #4 and remove #3 screw.
if fuel flows well for each, you wont need to split the bank.
then its on to float valves, float height, emulsifers, mains.

[751]

  Im finally getting somewhere, The t in the middle of the carbs is not plugged, carbs are apart, fuel flies right through there with small air presssure, Biggest note!!! " the keyster jets and the oem jets the numbers are the same, but you can see size differences with the naked eye on both the pilot and mains! The oem jets are bigger!!!  " WTf do they even sell the stuff for? carbs are still clean, I have a new fuel strainer in the tank petcock, new fuel line, the tank isnt rusty, Gonna replace the gas with new, it is 3 months old, so probably bad, i noticed the float looked lower in the third cylinder, the rest looked level across, guess 1.5 mm, needle and seats are new, floats dont sink in a pail of gas, Im getting new plugs caps today also, and 3-4 cans of carb cleaner!! Gonna let it fly. haha lol, I will keep you updated! Im also gonna clean and put diaelectric grease on every connection under the tank right away. Thanks, sam.

[r6100mph]

Verify the float heights are correct, at 22mm I believe for the 350F.  The "tang" on the float should just be resting on the pin which has a spring underneath.  Tilt the carbs at an angle so the float tang is just resting on the pin.  At this point the valve jet should be seated in the valve seat and spring is not compressed.  This distance to heighest point on the floats should be 22 from the face of the float bowl to top of the actual float. 

If they are set too low, a larger number, 30mm or something above 22mm, then there isn't enough fuel in the bowl and the main jet and secondary jet will starve for fuel. 

Forgive me if this is old news.

[751]

    Last night I removed the carbs, switched the keister mains and pilot jets, gave the already clean carbs a super clean, reinstalled and I drove it 10 miles, not 10 feet. I am happy! This has been a miserable ordeal since last winter, here it was the jets! you can visually see the difference in the size of the holes in both jets! also put in diff plugs, cut a 1/4 inch off the old wires, used some diaelectric grease, fresh gas, cleaned the points with a can of electrical cleaner and a buisness card. Rode it this am and it seems fine. carbs were sinked when points and condendors were installed. I think its gonna need some air screw work, since it was sinked, I have messed with them since then. Seems kinda doggy down low? maybe the pipes? Runs perfect on top.
   Does anyone know what screw setting should be used? 3/4 or 1.5 out. I have 93 octane gas, drag pipes, oem air filter, minnesota altitude. like 1500-1600 feet. Thanks, sam.

   

[fmctm1sw]

I think its gonna need some air screw work, since it was sinked, I have messed with them since then. Seems kinda doggy down low? maybe the pipes? Runs perfect on top.
   Does anyone know what screw setting should be used? 3/4 or 1.5 out. I have 93 octane gas, drag pipes, oem air filter, minnesota altitude. like 1500-1600 feet. Thanks, sam.

Your air screws are for fuel/air mixture at 0-1/8 (about) throttle opening.  Factory setting was 7/8 +/- 3/8.