Carb Woes...

[lumbee]

Hey guys...I have a ground-up project I'm finally getting running and am struggling to get it running right. I'm 90% sure it's a carb issue. I fully rebuild the engine (split the cases) so I feel confident in the valve train. It has good compression for what that is worth. B38 carbs, and the symptoms are these:

• will only run on choke; as soon as I turn choke off it dies, even after having been warmed up
• lots of popping out of the left carb; if I look down into the carb from the back I can see it firing/popping (pre-ignition?)
• even on choke, if I crack the throttle even a little it will die; won't take any throttle whatsoever
• uneven idle when it runs on choke...rpm wanders up and down

Things I have tried:

• sprayed starter fluid around the boots and there is no change in the rpm; so I don't think there is any vacuum leak
• as I said, fresh engine rebuild, so I'm confident the valves are set correctly
• cleaned the carbs...twice; if its dirty carbs or carb passages, then I've done all I can
• adjusted air/idle screws and no change
• standard/stock jet sizes, I realize I probably need bigger jets due to running pods, but would it be this bad? (unable to run on anything but choke)

Anything else I can check? I'm going to buy another set of carbs if I can't get these running but wanted to exhaust all troubleshooting before I go that route.

Thanks in advance!

 

[jpdevol]

What carbs are you working with?

 

[lumbee]

Sorry, B38's

 

[jpdevol]

So the BS38's present a range of possibilities as to model and jetting and there can be mismatches. Can you tell what all the jet sizes are?

 

[Brassneck]

Here are my first thoughts:

Are you getting enough fuel? Did you set the float height correctly?
Did you happen to put the slides in backwards? I can't recall if that's a possibility with BS38's (Like if you can put the left slide on the right side and vice-versa?).
I would check the spark timing...and maybe re-check the timing chain--could it be off by a tooth?

 

[lumbee]

The markings on the main jets are unreadable. @jpdevol you are thinking its lean?

@Brassneck fuel bowls are full and yes, I checked the float level. Also, slides are are right I think; the angle is facing the rear of the carb.

I will say, I noticed the advance doesn't quite snap back all the way if I open/close the advance with the bike not running. Would this cause it to idle high?

 

[jpdevol]

The "only runs on choke" issue typically involves a carb problem, but you've been through them a couple times. I was hoping you'd confirm decent jetting to eliminate that cause. Unless the jetting is way off, it will usually run w/o choke - just not well.

The second guess involves timing - both cam and ignition. So if your advance is sticking, remove it and the rod and clean/lube all that. Ensure that the advance unit snaps back after serviced. While the advance is off, verify cam timing: compression stroke Left Cyl. TDC the advance pin in Right side of cam should be at exactly 12 O'clock.

 

[Brassneck]

Having the advancer move freely is important...especially if you're setting the spark timing at idle, but if the advancer is...well, advanced, it may make it hard to start...however, I'm not sure that would cause it to not accept more throttle.

If you're cam timing is correct, and the spark timing is also correct... and it will only run on choke, generally, that means you're not getting enough fuel for the amount of air. And if you're running pod filters, that is even more airflow. My guess is your pilot jet is not allowing enough fuel in... either too small or somewhat clogged, etc.

Another thought may be that the butterflies aren't sealing completely when closed...and allowing more air in...causing a lean condition. And of course, there is always the possibility that the BS38's have been worked on prior and the wrong parts were used (I believe the early version of the BS38's vs. the later version are not interchangeable regarding slides, needle jets, etc... and that can make things hard to run correctly if those parts were used). But this is unlikely if the bike ran prior...and/or if you know the history of them.

 

[lumbee]

The "only runs on choke" issue typically involves a carb problem, but you've been through them a couple times. I was hoping you'd confirm decent jetting to eliminate that cause. Unless the jetting is way off, it will usually run w/o choke - just not well.

Agree 100%; if the jetting is off it wouldn't run great, but would not completely die with the slightest amount of throttle.

@Brassneck This is a true Frankenstein build, including the carbs. No guarantee on any of it being compatible. Thats part of the fun right?

I statically timed the ignition, but I'll double check and I guess I need to go ahead and get the advance running smooth...its going to need to be addressed at some point anyway, but I may just go ahead and get another set of carbs just to see if I can rule out or confirm whether or not that is the issue.

Thanks all. I will report back when its all resolved.

 

[jpdevol]

Thanks all. I will report back when its all resolved.

Yes, please let us know - it adds to the knowledge base - carbs, advance unit, cam timing?

 

[Brassneck]

Sorry I couldn't be more help.

I like the BS38's for the tune-ability of them and the classic "Stock" look...that being said, they can be finicky. I ended up switching to VM34's a while back. They are super easy to tune and work well with pods. CV carbs can of course be tuned for pods, but because of the vacuum slides, they prefer smoother air flow that pods don't readily provide unless you have a velocity stack or something similar to aid in that process....not impossible, but not ideal.

Good luck!

 

[650Skull]

The markings on the main jets are unreadable. @jpdevol you are thinking its lean?

Mine were to. Fine 1000 grit wet and dry with some W40 and they come up enough to be read.

 

[lumbee]

Just wanted to follow up on this. I purchased another set of carbs on eBay and zero issues with them. Idles great and no popping. Did a couple of laps around the block yesterday also. Fresh engine rebuild (tore it all the down to the cases) so I'm stocked that it runs good and shifts through all gears.

Plugs are lean so need to do some rejetting, but other than that I'm good.