Left cylinder problems

[Boxy]

Greetings all,
Just got a 650B for my son. Has been restored. Engine has had a top end rebuild-1st oversize, all new seals, cam chain, valves lapped, carb kit etc. Not by me but by previous owner-a fitter and turner, so I am assuming all good. BS38 carbs-XS pod filters and K&N rain socks, NOS stock exhaust. Pilot jet 42.5, main 130.
Problem:
Can't sync carbs to get motor to run well on left cylinder.
Air screw when fully seated still allows left cylinder to run----behavior indicates that it is running rich.
I have cleaned the carbs, reset the float levels etc etc.
I have replaced the condensers as condenser test showed sparking on left points.
Ohms tested coils-OK there.
Cannot seem to cure the running rich at idle symptom on the L cylinder.
Pops and splutters with puffs of white smoke from that pipe-does not smell of oil.

Any ideas where to from here? Gotta get this old girl running 100%.

 

[peanut]

It could be something really simple like a choke valve sticking on, or not seating properly ,or a float needle valve leaking , or an incorrectly set float level, or a tiny air leak on the inlet or carb body or the slide on the LH carb sticking and not returning properly . Syncing should be done on the bench initially and can be set quite accurately.
Run the engine at idle and try pressing down on the left throttle shaft with your thumb see if that reduces the revs. press down on both choke valves to make sure that both are seating properly.

Given that you've removed and cleaned the carbs already I would suggest that you remove them again and check them over really thoroughly for tiny air leaks aspecially around the diaphrams, choke assembly etc .Check the rubber manifold gaskets for leaks. I apply liquid gasket to both sides of mine to totally eliminate that possibilty.

Reset the float levels and sync the carbs on the bench. Make sure that your floats are air tight etc .
I'm assuming that you have no other issues with the left cylinder like a gasket leak, inlet air leak, HT lead and cap etc.

Do a search for the excellent carb guide on here written by some of our members.

 

[5twins]

Sometimes these older carbs just plain wear out or have been damaged internally by a ham fisted P.O. If the mix screw was tightened too hard instead of lightly seated, that could damage it's metering seat. But, there are a few things you could check. It appears to be a pilot circuit problem so we'll concentrate mostly on that.

There is an air feed jet on the lower right side of the intake bell .....

If it's plugged, the pilot jet will have no air to mix with the fuel. It will deliver only fuel. You will also want to verify that the pilot jet is the correct BS30/96 type. VM22/210 style pilots were used in the later BS38s and while they will fit in your set, they don't flow the same .....

The float bowls were also changed when the VM22/210 pilots were adopted. The way the air was delivered to the pilot jet was changed. Check to insure you have the proper BS30/96 float bowl .....

 

[Boxy]

Thanks.
I'll clean the left carb again. Jets etc are all correct. Valve clearances all good, checking compression tomorrow. If it still won't run smoothly then I reckon you're right-someone has fiddled too much and that air screw seat is buggered.
What replacement carbs would you recommend or would it be better if I sourced another pair of BS38's?

 

[5twins]

I like the stock carbs. If they're in good shape, I think they work just fine. If I was in your situation and needed another set, I would look for '76 or newer ones, the linked sets. They're bolted together as a "bank" and use a single throttle cable. Syncing is easier and they hold the sync better (longer). If you go that route, you may need to change your cam chain tensioner assembly to a later version as well. The later carb "banks" have an angle bracket across the bottom between the two carbs and that may not clear your older style, larger tensioner assembly.

Something else you can try is to remove the air filters and test run it. I'm pretty sure the XS pods you mentioned are the pleated K&N style filter. That type doesn't work well on CV carbs. The straight foam UNI pod is better. I don't know why your filters would affect just the one carb, you would expect them to hinder both, but it's worth trying.

 

[Boxy]

I like the stock carbs. If they're in good shape, I think they work just fine. If I was in your situation and needed another set, I would look for '76 or newer ones, the linked sets. They're bolted together as a "bank" and use a single throttle cable. Syncing is easier and they hold the sync better (longer). If you go that route, you may need to change your cam chain tensioner assembly to a later version as well. The later carb "banks" have an angle bracket across the bottom between the two carbs and that may not clear your older style, larger tensioner assembly.

Something else you can try is to remove the air filters and test run it. I'm pretty sure the XS pods you mentioned are the pleated K&N style filter. That type doesn't work well on CV carbs. The straight foam UNI pod is better. I don't know why your filters would affect just the one carb, you would expect them to hinder both, but it's worth trying.

Thanks for the advice.
Re cleaned left carb. Used thin fuse wire in pilot jet as poor flow with carb cleaner.
Idle now much better, air screw on 1 and a bit turns out.
Might put original jet in as running 42.5 now and raise needle one notch on a 130 main.
Cheers.

 

[5twins]

Yes, your jetting is a little off from stock. I can understand the possible need for one size larger main but don't know why the pilots were reduced a size. The stock needle setting for your carb set was a bit odd. Most carbs come from the factory with their needles set in the middle #3 slot. Yours spec the 4th slot from the top. However, the one step leaner #3 slot may be needed with your larger mains. Only testing will tell you for sure. And I still recommend switching to the UNI foam pods. They just plain work better. The bike will run better and tuning the carbs will be easier. Here's a list of stock jetting specs to aid you in your tuning efforts .....