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Discussion Starter · #1 ·
Hi Everyone,

I apologise in advance if this post is hard to understand. I've had some trouble while working on my cb350 for a while now and come across quite a few issues so I'm going to try do my best at putting them all down here.

So in its current state my cb350 runs - but not very well. The bike starts relatively easy but sounds rough with the right side occasionally backfiring out the exhaust. after it has warmed up it sounds a little better but will randomly race off and sit around 5000-6000rpm until i turn it off. If it is idling 'fine' and i try to rev it will either race up to high revs and sit or bog out (is this pilot issues - i was very thorough with cleaning these). If i start with the choke on it seems to idle well but if i open the choke it will race up again.

I have done a bench sync for the carbs. when i run the each side individually they seem to run a lot better than when they are run together.

possibly unrelated but another observation is when the bike shoots up to high revs the left side starts spitting white smoke - burning oil? smoke often comes out of the left exhaust header after it has warmed up - I'm not sure if this is helping the issue.

I am currently pouring fuel directly in to the fuel line which connects to the bowl and then leaving it open when i run the bike. one thing i have noticed about this is i can see the fuel in the line moving around quite aggressively while the bike is running - meaning the engine is moving the fuel around through the carb - does this sound fine or do i need to think about replacing the float valve? I think the left side is using more fuel than the left side because after a while the pressure out of the left exhaust is much lower (this is after it has sat at high revs and i start it again)

The bike is also running with no air filter.

some of the things I've done:
  • set cam chain tensioner, valve tappets, and timed to what i think is good. although i have been hearing a lot about the advancer, but i am not sure how to check this.
  • cleaned the carbs multiple times and replaced the o-rings. i also read something about the felt butterfly valve? is this something i sure be aware of?
  • replaced the intake manifolds. I am quite certain that there are no air leaks. i have sprayed ignition fluid around the manifold to check this.
  • the bike has a new battery but i have an old charger.. the voltage sits around 14v. i have replaced the plugs boots and condenser so i feel like my spark is fine but i have not physically tested this.
  • i have checked the throttle cable and this is not sticking

again, sorry if this post is a mess. I don't get to work on this much because of work but i am happy to provide any more information if needed.

Thanks in advance :).
 

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Put your air filters on
sunch throttle cables adjust idle & make sure there is enough slack

time ignition with a strobe to make sure timing advances and retards smoothly on the marks
 

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Discussion Starter · #3 ·
thanks! seems to run a bit better with the air filters on but still getting the issue where it sticks at high revs. any ideas around this one? would anyone be able to provide some detailed information around the spark advancer? I've been searching for a while with no luck :(
 

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thanks! seems to run a bit better with the air filters on but still getting the issue where it sticks at high revs. any ideas around this one? would anyone be able to provide some detailed information around the spark advancer? I've been searching for a while with no luck :(
Take the points base plate off to look at the advancer. Check the springs to see if they have continuous tension from first movement to full advance, and be sure the springs return the weights fully with no slack allowing the weights to swing out unnecessarily at low revs. If there is any slack in the springs, the advancer will begin to advance the timing before it is supposed to and if your overall timing is not spot on (if slightly retarded at idle), once the engine gets revved up it will want to stay there.
 

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Discussion Starter · #5 ·
Thanks for the reply and sorry for the late response. Only just got time to work on the bike.

Cleaned the advance and checked the spring.... Seems like they still have a bit of tension but I don't know how to truly check this. Also set the valve tappets and reset the ignition gaps and timing.

I'm still getting the issue of high revs. I mentioned earlier that it ran fine with choke but high without (with choke it runs fine for longer but will eventually run high revs) this sounds like a fuel issue??

Little bit lost. Next step would be re jetting the carbs. I think it is stock filters and exhaust, but it is really loud - removed baffles so may require a re jet?

Please let me know of you require more info.

Morgs
 

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Thanks for the reply and sorry for the late response. Only just got time to work on the bike.

Cleaned the advance and checked the spring.... Seems like they still have a bit of tension but I don't know how to truly check this. Also set the valve tappets and reset the ignition gaps and timing.

I'm still getting the issue of high revs. I mentioned earlier that it ran fine with choke but high without (with choke it runs fine for longer but will eventually run high revs) this sounds like a fuel issue??

Little bit lost. Next step would be re jetting the carbs. I think it is stock filters and exhaust, but it is really loud - removed baffles so may require a re jet?

Please let me know of you require more info.

Morgs
One cause of high revs is a ruptured carburetor slide diaphragm. Did you check these for holes when the carbs were apart?
 

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Discussion Starter · #7 ·
Checked the diaphragms and couldn't see any holes and the slide seemed to functioning properly / smoothly.

Could this be a ignition timing issue - i've only done static timing, how do i use a strobe to test that the advancer is working correctly?
 

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Discussion Starter · #8 ·
So been searching a lot and have started to think that the issue could just be the idle mixture setting? I've got both screws set at stock turns (can't remember this off the top of my head..) Do i need to follow the manual with these settings religiously or does it vary per bike? I'll check the plugs this weekend to see what my fuel mixture is like, can anyone help me on this one? what am i looking for with the plugs?

As for bogging out I'm still a bit stumped. Might need to tear in to the carbs again and clean them out.
 

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Idle mixture settings will vary from bike to bike.
Actually they vary from the "EXACT" Float Settings Carb to Carb.
Very small (.1 mm) differences in Float Height will result in slight differences in the mixture settings.

The FSM Adjustment here is a STARTING Point not a Fixed Point.
The mixture should be adjusted for the highest RPM - then the Idle Speed Screw gets reset to approx. 1100 - 1300 RPM.
Then Adjust the opposite Side Highest RPM & Reset the Idle Speed Screw to approx. 1100 - 1300 RPM.
This is a back & forth process until both side are at the highest RPM for the Mixture & the Idle Speed is 1100 - 1300 RPM.
Make sure there is sufficient SLACK in the Throttle Cable to allow the Throttle Plates to Close.

The FELT Washer are located on both sides of the Throttle Shaft for each Carb.
The purpose is to SEAL the Hole the shaft rotates in to prevent an air leak behind the throttle plates which results in Air NOT Mixed with fuel entering the Intake.
If you dunked the entire Carb Body in cleaner and did not remove &/or re-oil these seals they can leak air.
The Carb Cleaners solution will Dry Dissolve any Oils that swell the Felt.
The Oils also make the shafts move smoothly and prevent sticking.
There is a REALLY good Sticky Post in the 350 Fuel/Carb Section you should read.

Timing & Advance is best checked with the Strobe Light.
(which will also tell you "IF" you are getting reliable SPARK to the Plugs.

Strobe Lights connect to Battery Power and have a Magnetic (Inductive) Pickup that Clips OVER the spark Plug wire.
The Inductive Pickup senses the Energy of the Coils discharging to the Spark Plug and results in a Flash from the strobe light at the time the Energy is released. (SPARK).
The Mechanical Advance process can be seen as the timing Mark MOVES in relation to the RPM and should max out at the FULL ADVANCE Mark at approx. 3500 RPM.
The Advance and Retard process should happen smoothly in the relationship to the RPM.
If it doesn't happen either the springs are weak and allow advance too early or they are stuck and not moving to advance the spark.

IF you have an unreliable SPARK the Flashes from the Strobe will be inconsistent.
They should occur on a consistent basis. Not Random.
Inconsistent Flashes typically means one of (2) things:
A failing condenser that is NOT suppressing the Arcing that occurs when the points Open & Close. (Replace Condenser)
Or severely Pitted Point Contacts. (replace the Points)
It is possible for the Coils to also cause this issue but unlikely.
Points & Condenser are the 1st things to try.
 

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Discussion Starter · #10 ·
Thanks for the info @Yendor!

Started playing around with the mixture and idle screws, don't know if i got this 100% correct but the bike idles a lot better. The issue now is the it does not respond very well to throttle. Initially, the bike will bog if i try give it throttle, i assume this is just because the bike isn't warmed up enough? After the bike does warm a bit it can throttle fine but it seems sort of sluggish? After the bike revs up it takes a bit of time (~3 seconds?) to return back to idle. Is this ok? does this have something to do with the advancer - maybe i should get on to testing the advancer with a light.

The idle also seems a bit temperamental. kinda jumping between 1000-1300 rather than staying consistent. is this normal?
 
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