Hey Thumpers.
I finally got a new timing chain on the motorcycle and timed it correctly during the rebuild.

Blue Spark
180 compression
87 Gas
36mm Mikuni VM Carb with jetting stated on Lancers post.
180 Main, 20 Pilot. 1/2 turn Air. Position 2 on needle from top.

Anything else I should be looking at?

Do I need to close up a pilot jet or expand it?

Greetings Celeste.  I need you to clarify something.  Your problem is that your bike will not idle.  Never mind all the backfire stuff, it has to idle in order for you to be able to ride it, and it simply will not idle.  With or without the enricher on, the engine will not idle.  Do I have that correct?

Have you verified that the idle stop screw is actually touching the bottom of the slide.  These screws have a tendency to break off.  It's rare but it does occur, especially when an operator gets in the habit of opening the throttle and releasing it just to hear the pleasing "thud" when the slide hits the stop.  Have you verified that the adjustment screw actually raises the slide when you turn the screw in?  Have you looked into the venturi and verified that the stop screw is protruding into the throat of the carb?

It should look like this.

Is this a new carburetor?  Is the manifold that connects the carburetor to the engine a Mikuni manifold?  Is the manifold a new manifold or a used manifold?  Have you checked the manifold for leaks?

The Mikuni manifolds that come with VM carbs are flimsy and they warp easily.  They leak between the cylinder head and the manifold.  Sometimes the deformation gets so bad you can actually see the opening between the manifold and cylinder head with your naked eye.  

Examine the manifold joint.  Try to see if there is a gap between the head and manifold.  Probe around the joint with a feeler gage.  You should not be able to insert a feeler gage into the joint.  If the engine would idle, you could spray the joint with carb cleaner to test for leaks, but your engine doesn't idle.

This is an example of how the Mikuni manifold deforms.

The full story:

Got the bike from a guy who claimed he paid a lot work on it.
He didn’t, he hacked at the bike before his wife wrecked it.

I bought it, cleaned the stock BS40 carb and got it running. And it ran like a scalded cat. Rode it 150 miles home only to find the next day, it would crank, but wouldn’t fire up.

Going thru the carb again, it was clear it pretty bad off, and I knew the repair kits were nuts at best. So I found Lancers post, bought one off Amazon, which had the specifications that matched lancers post, starter tinkering with it.


At first, it fired up, lagged a bit. Gave heavy thuds like it was going rich, and died. Since then cracks and pops. No idle.

I decided since it’s a 87 to crack it open and peek around. See if anything was amiss. Lo and behold the infamous tensior. Grabbed a new chain, rebuilt the engine, popped the valves out for an inspection and cleaned. Got into the head and got all the carbon nuts out of it. Cleaned the piston too.

Followed the timing posts on here to set it, did hand crank tests to listen for trouble, didn’t hear it. Put everything back together and been struggling to find idle since.


So… Correct on everything.
However one huge difference I’ve noticed is your Idle screw (the big one) is on the opposite side of the carb body. I’m starting to suspect Amazon sold me a knock off.

No matter what cleaning or screw position or pilot I have it at, it doesn’t seem to get enough fuel.

Note this is what it looks like after it seemed to be flooding.

It's hard to tell in the photo, but what you describe as flooding almost looks like oil to me.  Maybe it's the photo.  Super tight intake valves (less than minimum clearance) obviously isn't good.  But excessive heat is less of a problem on the intake side than the exhaust side - the fuel and air coming in act as a bit of a refrigerant.  The exhaust side doesn't have that luxury.