cb350 carb help

Uniac

Uniac

Been Around the Block
Ok, so the deal is I have really no idea about carbs. I'm trying to learn as I go, but not having much luck.

My bike ran great for 300km, It all of a sudden decided it didn't like to idle, so tried to sort that. I got it to the point to where it idled great until it got hot. Then it would want to slowly die. Fast forward a few days, riding home in a light rain, the bike decides it's going to hold revs when you clutch in. They hold for say 5 seconds, then slowly drop. I spot an intake boot rip and replace the boots.

Today I installed the new intake boots, tried to get the bike to idle, Succeeded at 1 1/2 turns each on the air/fuel screws (v-stacks and new exhaust). It's a rough idle but it idles, so I take it for a ride and the revs still stick. The throttle cable isn't sticking, the throttle bodies are fully closed. I checked for leaks using some either, can't seem to find any. I'm just at a loss.

What would cause the revs to stick like this and slowly drop? Do I have the throttle stops adjusted to high?

Thanks for any help
 
Have you sync'd the carbs? Could be out of balance.

Also your idle mixtures might be off. Running better when the bike is cold as opposed to hot indicates a rich mixture.
 
1 1\2 turns ...on your air screws,,means nothing.bang it over on one cylinder at a time and set your throttle stop and air screw,,,,,,,,then do the same for the other side,,,,then engage both cylinders and adjust both throttle stops,,,,,if done right ,it will tick over at 800 rpm +-
 
I did a little search around the forum, found the following:

This is how I make those adjustments.
Place a fan pointing at the engine to cool it during the following.

1. Start and warm the engine up completely. 5 minutes of running will do it. It must be running with the choke off.

2. Choose a cylinder , left or right as your starting point.

3. Turn the idle speed screw on the arm at this cylinders carb clockwise about 1/4 turn to get the idle speed of it up just a little bit.

4. Disconnect the sparkplug wire at the other cylinder so that it is running on only the cylinder that you are adjusting. It should be idling just fast enough that it does not stall out on you.

5. Turn the idle mixture screw for this cylinder either in (clockwise/richer) or out (counterclockwise/leaner) until the engine idles at the highest RPM as observed on the tach. Make small adjutments and wait a few seconds between them.

6. Now turn the idle mixture screw in slowly (clockwise/richer) until the engine RPM just begins to drop back down by about 100 Rpm. This is a setting that is just a touch rich to aid in cold starting and decreased operating temps of the engine.

7. Now adjust the idle speed screw back down to where the engine just barely stays running.

8. Plug opposite plug wire back onto sparkplug and repeat steps 3 thru 7 on the other cylinder.

9 If the engine is idling too high or too low when both plug wires are plugged back onto the plugs at the end of the operation , (it should be about 900 RPM) turn the idle speed screw (the screw on the arm) for each carb clockwise (increases RPM) or counterclockwise (decreases RPM) by an equal amount of turns until the RPM is at the desired RPM.

I assume this is the method you're describing Wee Todd?

I'll give this a shot this afternoon if it doesn't rain.

Thanks
 
I know im probably gonna get smacked by the old timers here ;D but i dont like shutting down one cylinder to sync the carbs. Its hard on the engine, you dump fuel into a cylinder thats not firing, and i personally find it harder to get them synced properly. On a twin, its not too bad to leave both cylinders firing and listen for the changes in each individual cylinder if you get your ear close.

I made a post in the Toronto local board about syncing carbs, maybe check that out (I think its under the cyclewerx topic)?
 
Well some news. I was working on the bike outside my building and this little old british man stops and starts asking questions. Turns out in another life he was a mechanic, showed me how to sync the carbs and set the idle. Then when that didn't solve the problem he suggested checking the slides. Sure enough one of the slides was sticking really bad. I cleaned it up, cleaned the other one just in case, started it up, reset everything, idle was great. Went for a small ride and then the RPM went through the roof. Throttle plates closed and it still rev's, worse than before.

I suppose this is a sign that the carbs need to come off and have a good cleaning.
 
Doesn't high RPM's mean that there is not enough fuel and more air, where a closed choke (less air and more fuel) bogs engine out?
 
Wee Todd, I don't know if you were saying ya to me or not? But I was just thinking that maybe not enough fuel was getting to carb. Check float level and petcock screen and bowl? Also check advance unit springs. I've heard those can go bad.
 
Odd, all sorts of grit in the fuel bowls. It can't be coming from the tank because it's fiberglass and has a new epoxy liner. I'm honestly surprised how dirty these are, they've only been run 300km, were boiled in lemon juice and sonic cleaned. Is there any way that some dirt could have got sucked into the fuel system through the stacks?

Float levels are dead on, petcock screen i haven't checked but there is a shit-ton of fuel when you open it. I'll check that and the advancer after I finish cleaning the left carb.

Again, many thanks for the help.
 
Uniac said:
Odd, all sorts of grit in the fuel bowls. It can't be coming from the tank because it's fiberglass and has a new epoxy liner. I'm honestly surprised how dirty these are, they've only been run 300km, were boiled in lemon juice and sonic cleaned. Is there any way that some dirt could have got sucked into the fuel system through the stacks?
Click to expand...

You want to buy gas in a can and run it through a coffee filter in a funnel.
You will be shocked by the crap that comes out.
If you want it to be really bad, wait until a tanker has re-filled gas station underground tank.
I've seen rust up to 1/2" and paint 1"x3/4" or larger (didn't actually measure it)
Get a couple of inline filters, good (replaceable filter) billet can be had from around $10.00~15.00 in most automotive parts stores

PJ
 
That makes a lot of sense PJ.

It has inline filters, just the little plastic ones, apparently they don't filter very well though, so i'll be replacing them.

An update, after cleaning everything completely, still the same story, the motor revs for no reason to redline as soon as you blip the throttle. I double checked float heights they're fine, slides aren't sticking, jets were all clean, no holes in the floats, diaphrams are good(new 300km ago) and carbs are mechanically synced. Short of pulling the carbs off again and cleaning a second time, what's my next option?
 
bobjohnson said:
Is the throttle cable itself sticking?
Click to expand...
Nope, that's the first thing I checked.

interceptor said:
Sorry dude. BTW, did you pull out the needle jet and check that? Maybe this will help:
http://cb350.blogspot.com/2007/05/carb-rebuild.html
Click to expand...

Thanks for the link, yes I pulled them completely apart, but I know i must have missed something because It ran worse on the left side after cleaning. Looks like i'll be pulling them off again.
 
Well that was fun. Everything cleaned, scrubbed and generally looking spic and span. It fired right up and runs nice.

It's a little sluggish just off throttle stop, but then it comes alive. I find it to have a bit more vibration at the moment then I remember; which hopefully someone will correct me if I'm wrong, means that the carbs are slightly out of sync mechanically still? Easy enough to correct I suppose, As long as it's running like this I can fiddle and tinker to get it perfect.

Again, many thanks to all for the hints and help.
 
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