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General Category >> Rubber Side Down! >> 36mm Mikuni VM carb idle issues
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Message started by Celeste on 05/04/23 at 05:19:05

Title: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/04/23 at 05:19:05

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?

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/04/23 at 06:06:57

Whoops forgot context:

Stock air box
No black smoke at all.

Pilot size, Air Screw Turns and reactions
15 Lean (2.5) Back fire
15 Mid (1.5) Backfire
15 Rich (.5) Backfire

No idle on choke or closed throttle

17 Lean Back Fire
17 mid Sparse Carb Fire, Strong Backfire
17 Rich Carb Fire, Back Fire

No idle on choke or closed throttle

20 Lean rapid backfire and carb fire
20 mid rapid back fire and carb fire
20 rich carb fire strong, back fire sparse.

No idle on choke or closed throttle

Exhaust, is muffled with DB killer and fiberglass baffle.

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/04/23 at 12:58:09

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

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/04/23 at 12:58:57

Or is there a idle screw adjustment that I need to set it at to
Get idle? Currently it’s screwed all the way in. Throttle is at maximum position

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/05/23 at 09:55:00

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?

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/05/23 at 09:58:20

You have turned your idle stop screw all the way in (clockwise) as far as it will go and the engine still will not idle. Do I have that correct?

This is the idle stop screw.

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/05/23 at 10:03:12

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.

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/05/23 at 10:07:41

You are adjusting your idle mixture screw in one-turn increments. So, you try 0.5, then 1.5, then 2.5 turns counterclockwise from the fully seated position. Do I have that correct?

Is this the screw you are using to adjust idle mixture?

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/05/23 at 10:16:10

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.

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/05/23 at 10:29:41

You installed a new timing chain and set the cam timing. Are you absolutely confident that the cam timing is correct?

You have 180 psi cranking pressure. That seems awfully high. Do you have a stock piston or a high-compression piston?

If you have a stock piston, and your cranking pressure is 180 psi, you might want to double check your cam timing. The extra 25 psi compression might be due to carbon accumulation on the top of the piston and combustion chamber, but it might also be due to cam timing. This is a long shot, but you should take another look at the cam timing.

The excellent cranking pressure indicates that you don't have any valves hanging open, so bent valves are pretty much ruled out. And it doesn't look like you have any valve adjusters too tight.

Why did you change the cam chain? Was the engine running OK before you changed the chain?

Did you change the carburetor? If so, why did you change the carburetor? Was the engine running OK before you changed the carburetor?

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/05/23 at 14:40:24

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.

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/05/23 at 14:41:02

The “Mikuni Carb”

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/05/23 at 14:42:11

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

Title: Re: 36mm Mikuni VM carb idle issues
Post by Celeste on 05/05/23 at 16:08:08

:) Anddddd the intake valves were tight as hell… this is the second time I’ve adjusted them. [ch128580]

Title: Re: 36mm Mikuni VM carb idle issues
Post by ThumperPaul on 05/06/23 at 07:41:14

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.

Title: Re: 36mm Mikuni VM carb idle issues
Post by DragBikeMike on 05/06/23 at 11:24:21

As Thumperpaul observed, all that liquid in your carb throat almost appears to be oil, but hard to tell from picture. Either way, oil or fuel, it ain't right. It's appears black too. Might be a result of backfire through the carb. If you have an intake valve bent or hanging open, it will backfire through the carb. What exactly do you mean when you use the term "Carb Fire"? Is it backfiring through the carb?

You stated that the valve adjustment went tight after just a bit of run time. You said it's the second time you have adjusted them. Your timing chain jumped. You said you took it apart and cleaned the piston and combustion chamber. You had the valves out of the head. Did you verify that the valves aren't bent? Bent intake valves would result in backfire through the carb, failure to idle, and valve adjustment problems. Did you verify your valves are not bent when you had it apart? Were there any marks on your piston that would indicate that the piston and valves collided when the timing jumped? Did you check your valves for proper seating when you had it apart?

I'm not seein your idle stop screw in the picture. Where is your idle stop screw?

Title: Re: 36mm Mikuni VM carb idle issues
Post by ThumperPaul on 05/07/23 at 11:21:48

This may be really elementary....but do you have enough fuel in the tank? Have you tested the petcock valve to make sure it works in all positions? Do you have the stock petcock valve? If so, make sure it is set in the "Prime" position and working in the "Prime" position. This is the gravity fed mode that the VM36 needs. The "On" position won't work - that's for the stock CV (vacuum) carb.