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Discussion Starter #1
Hello!

I have a ‘73 CL350 that I bought as a non runner with Mikuni VM30’s, velocity stacks and cheap shorty cone mufflers installed. It took a lot of time, jets, plug chops, etc to get close to being dialled in, and was for very fun to ride, but was quite rowdy!

The main issue was a flat spot just after idle. If you held the throttle open at idle and then worked up the revs progressively it was fine. But if you had your hand off the throttle, letting the idle settle, and then opened up the throttle to accelerate, it would stall or die.

For a comparison, I have recently installed a set of Keihin 722 carbs, new cable and air filters. I replaced the float valve and needle, and rubber o rings and gaskets, etc, new manifold intakes.... and also a set of pair of replica CB350 dunstall style silencers. Again, the bike runs great apart from this flat spot after idle. If I hold the revs up when stationary 1300-1500rpms, and then pull away with a smooth opening of the throttle it pulls away beautifully, but leave the idle at 900-950rpms and then open the throttle a little too quickly and it stalls. Leaving me stranded at traffic lights. Embarrassing :)

If anything, I would say that it appears to improve after a long ride at high rpms.

So, different carbs but same issue. I’ve assumed it was carb/fuel related, hence posting it here, but wondered what else it may be? I was planning a run through of cam chain tension, valve clearances and ignition timing. I put a strobe on it and the advance is working and timing is okay, but haven’t checked for slack in the springs or sticking weights. If anything it is a fraction too advanced, but just a slight amount. The battery holds 12.5-12.6V. I know the 350’s need a strong battery to keep them running at idle.

I guess my question is what could be the main culprits?

I’m keen to get an electronic ignition it, but what with the current situation can’t justify the extra expense.

Any suggestions to add to the list would be greatly appreciated! Thanks!
 

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Normal idle speed setting is 1250 to 1350 RPM............Try that.......
 
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Hi 66sprint. Thanks for the quick reply! I know Clymer list it as 1000-1200. Will tweak it up.

I have also been considering the order of how to sync the carbs. I’ve been running them with the air boxes off and using a air flow meter to Balance both at the correct idle speed. Then, when happy, I adjust any play in the cable to make sure they’re sync’d. Does that sound right.
 

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Check your conversations.....(PM sent)...
 

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Discussion Starter #5
So spent a bit of time tweaking the carbs. Balanced them with the air filters off and an air flow meter to idle at 1200 rpm. Made a slight adjustment to cable. Took it out for a ride and it was a vast improvement. No stalling or flat spot straight off the throttle. Thanks 66sprint, it’s good when it’s a simple fix!
 

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Did you make adjustments to the idle mixture screws?

On these carbs, the idle mixture screw is usually an air-bleed (EDIT: correction - a fuel bleed) adjustment. If the adjustment screw is behind the slide (closer to the air box) it adjusts the amount of air - so turning it IN provides a richer mixture. If it's in a more forward position or at the bottom it's a fuel-bleed - so turning it IN provides a leaner mixture.

With the engine fully warm, adjust each screw to obtain the highest RPMs. That is the optimum setting, but it will change with temp. I also like to go about 1/8 turn richer (EDIT: correction - turn the screw OUT on this type of carb), which smooths the transition off-idle. Then adjust the idle speed back to the correct setting (it will most likely be too high after adjustment).

You'll be surprised at what a difference it makes to have the idle mixture set properly.
 

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Did you make adjustments to the idle mixture screws? On these carbs, the idle mixture screw is usually an air-bleed adjustment.
The mixture adjustment screws on the Keihin carbs used on the CL350's, 3D and 722A, are fuel adjustment, NOT air. So turning the screw in leans the mixture, out enriches it.
 

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The mixture adjustment screws on the Keihin carbs used on the CL350's, 3D and 722A, are fuel adjustment, NOT air. So turning the screw in leans the mixture, out enriches it.
Yes - thanks.

I see I was mistaken about that, and made edits and corrections to my post.

The procedure for adjustment is now correct.
 
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