CB350 - cylinders fire inconsistently at idle - float levels?

[Madmusk]

Hi there. Long time lurker, first time poster. Figured I'd post this problem up here since high quality advice seems to flow freely and I haven't had any luck asking elsewhere.

So, basically my wife's stock 1970 CB350 has been giving me issues with inconsistent firing of the cylinders at idle. I've gone through a long list of possible fixes including rebuilding and cleaning the carbs. All the usual routine maintenance has been completed - valves, etc.

Most recently I checked the spark plug boot resistance. 5.72 ohms and 5.27 ohms. I assume those are close enough to spec but if not please speak up. Then I clipped away some of the old wire to get fresh copper.

Then I put a brand new set of points in, static set the timing/gap to be spot on, and while I was there verified once again that the advance mechanism was acting properly, and that nothing was grounding out. New set of properly gapped plugs, strong battery, stock mixture settings, and fired her up...

Here's a video

Basically, left side didn't want to fire with the choke on while the right would purr away. Turn the choke off and the left side kicks in while the right side starts sputtering and popping. After looking at the plugs (left dry with black carbon, right pretty lean looking though light gray) it would seem the left is running rich and can't handle the choke, while the right is lean and needs the choke. With the air filters off I could see clear backfiring and little puffs through the throat of the right carb with the choke off.

Seems like a clear case of fuel levels right? I don't think a clogged idle passage/jet could cause both cylinders to be so opposite at stock settings. I have the carbs back off but I'm wondering what, besides very carefully setting the floats (which I already did) I can do to make sure the levels end up correct. Both the floats and the needles and seats are new aftermarket parts from the Sirius kit. Should I swap the old float needles and seats back? Do you sometimes just need to keep messing with the floats repeatedly before the levels take?

On my own bike I'm used to setting the float levels with actual gas and a clear tube which gives positive verification that the levels are correct.

Thanks guys.

 

[Sonreir]

First up, do the usual checks of timing and compression. Issues with either of those will mask problems and confuse matters.

Next, check the battery voltage. It should be right around 12.5V. If it's lower, try hooking up some jumper cables to a second battery and trying it then. Classic sign of a dying battery on the Honda twins is one cylinder starts to cut out and misfire.

After you've done the first two, it's time to sync the carbs. Vacuum gauges will tell you how close they are to one another. I fought a similar issue to you for days before I realized the sync was the problem. Bike loved running at any throttle setting except idle.

Finally, check for air leaks by spraying starter fluid around the manifolds and see if you can get the revs to jump. If so, you've found an air leak.

If none of those things comes back with any faults, you know you have a carb issue. That could be anything from a blocked jet or passage, loose jets, float height issues, or butterfly valve problem. It shouldn't be related to the slide or diaphragms though, those only come in after idle.

 

[Madmusk]

First up, do the usual checks of timing and compression. Issues with either of those will mask problems and confuse matters.

Compression was good last fall. Both low 160's IIRC and probably higher since that was cold. New points are installed and static timed/gapped to be spot on. Would it be important to get a strobe on it while working out the idling issue? I don't have access to one currently.

Next, check the battery voltage. It should be right around 12.5V. If it's lower, try hooking up some jumper cables to a second battery and trying it then. Classic sign of a dying battery on the Honda twins is one cylinder starts to cut out and misfire.

Before every attempt I've had the battery (1 yr old) on a tender right up until starting. Usually sits at around 12.8v.

After you've done the first two, it's time to sync the carbs. Vacuum gauges will tell you how close they are to one another. I fought a similar issue to you for days before I realized the sync was the problem. Bike loved running at any throttle setting except idle.

No vacuum ports on these carbs/manifolds. I'd love to hook up my DIY sync tool as I hate the old school guess and check methods. I'll be bench syncing the carbs this time around rather than relying on baseline idle screw adjustments.

Finally, check for air leaks by spraying starter fluid around the manifolds and see if you can get the revs to jump. If so, you've found an air leak.

I did do this with WD40 and found no place that would up the revs. I soaked every boot/manifold joint a bunch, but I probably missed some of the rubber surfaces so I 'spose I'll give that another try.

Thanks for the tips. I think my best bets at this point are an air leak or completely wonked out fuel levels.

 

[Sonreir]

Once you do get the carbs synced (I'd tap a vacuum port, personally, I don't trust bench sync) be sure to tweak the adjustment screws as well. Starting point is two turns out and then dial them in for maximum idle revs.

 

[Madmusk]

Ok, I'll look into what's involved with tapping vacuum ports. I'd like to have them.

I believe the stock mixture for these carbs (3-D) is 1 turn out +/- 1/4. Can you verify which direction is richer? I've read some statements that it's in and others that it's out. The screws are on the motor side of the carbs.

 

[Madmusk]

Oh, one more question. Is it bad to run the bike for a short period with one spark plug lead unplugged just to compare how each cylinder runs independently?

 

[Madmusk]

Just wanted to follow up in case someone else runs into a similar issue.

First off, success! I don't know exactly what it was that did it, but here's what I did:

Went through the carbs again carefully inspecting/re-cleaning the idle circuit. Didn't seem like anything was out of sorts there. Next I carefully set the floats once again and wet set them by letting fuel pour through with the bowls off and using a DIY float height gauge to push up on the floats. Then I bench synced the carbs using a cut out strip of index card as a gauge. One thing I noticed... setting the throttle butterflies to bisect one of the tiny fuel/air inlet ports in exactly the same amount gave the same result as far as accuracy, and is much easier and quicker. Might be worth a try for other folks.

Put the whole thing back together and this time spent some extra effort making sure the newly rebuilt air filters had no possible place to leak.

Fired it up and she purred with not the slightest pop or cough. So easy to dial in the mixture screws now that they actually have definite effect. They ended up at 1.5 turns each. Seems too far given the stock settings, but since they're both the same I'm happy. A quick and dirty sync was pretty easy as well. Still some more tuning in the works, but so far it hasn't skipped a beat in about 20 miles of riding. I suspect my issue was a difficult to find air leak.

Thanks for the help guys.