Need to review as i forget where we are with this. By idle screw do you mean the idle speed screw or the pilot [mixture] screw? By burried do you mean turned all the way in or all the way out?

Does the idle speed change if you turn the handle bars from one side to the other? Cable may be binding somewhere?

How did you test for intake leaks? Find any? How fix?

If the idle speed screw is turned all the way in something big is definitely wrong. Watch the throttle plate at the back of the carb as you adjust the idle speed screw from all the way out to all the way in. And at the same time watch the contact point between the screw and the lever that it contacts. At a few turns in the plate should be open a very small amount and the screw should contact the lever,which would be the correct setting if all else is OK.

 

Mick,
No change when I turn the bars. Cable seems free.
I used propane to check for leaks, none found. Would you recommend WD40 instead.
Yes, the idle speed screw has to be turned just about all the way in the keep a decent idle.
No plate on the VM38 as it is a slide carb. However when I turn the screw in it appears to be funtioning as it should. As I turn it in the slide raises.
Any thoughts?
Thanks

 

Bucaman,if it helps,I use the same carb and when I upright Her I get about 1-150 rpm increase.just enough to notice! close to factory settings also,I have a 6" extended front end,not sure if that effects anything. ( the carb is level) I think theres simply trash in Danny's carb..running dry with crud in the tank does that.

 

Propane is best for intake leaks check. You can aim it everywhere, and hold it in each place for as long as you need. WD-40 etc are far too messy, and do not have the fine particles of propane.

 

Bucaman.........
I just saw your post (it is buried in another OP's post) regarding your Mikuni problem.

The throttle stop screw is simply a screw with a beveled end.
The beveled end rides against a ramp built into the throttle slide. As you turn in the screw, the end rides on the ramp and lifts the slide upwards..... just the same as turning the throttle grip.
There is a spring on the screw to keep tension so it does not move easily.
From the sound of it, you have screwed in the throttle stop screw so far that it has coil bound the spring. Correct?
Here is a pic of my Mikuni which is set for a normal idle speed as you see it in the pic. Notice the gaps in the coil spring. It is far from being coil bound.
As a reference point, my screw sits turned in 10 full turns as it is shown here........



Now as far as the stop screw is concerned, only 2 things could be wrong. Either the screw has worn down or the screw is the wrong item. There are 3 different throttle stop screws available, and they are intended for different size Mikuni carbs.

I must say right here that both concerns mentioned are w-aaaay out in left field, but I thought I would mention it anyway.
The screw is unlikely to wear enough to coil bind the spring. The stop screw is almost certainly the correct one for a VM-38, unless you have changed out the screw for some reason.

Here is a pic of the ramp that the screw rests against on the throttle slide. The reddish dot is where my screw hits the slide ramp.....



Here is another pic of the inside of the carb barrel with the throttle slide removed. See how far the screw sticks inside? Also see that the coils are far from binding on the spring.......



And finally, here is a pic of the overall length of my throttle stop screw for comparasion with yours. Mine measures just a tad under 1.447 inches (1.4467 actually)........



So check your needle length. Look at the ramp on your slide. There is nothing else that could be the problem.
pg

 

The truth and reality is I had several problems going on but blamed everything on the carburetor.

I would check these easy things first and make sure your 100% about what you find.

Static timing-(make sure you know the timing is close to dead on)

Intake manifold connections-(when bike is running lightly mist a combustible spray like WD-40 around the manifold; if engine speed increases you have a leak)

Exhaust port connection-(make sure the bands that secure the pipe to the cylinder head are tight, it’s easy to feel a leak) I personally didn’t check this one right and did NOT have enough back pressure on my motor to properly tune the carb.

Exhaust pipes-(if you have 2” drags I would most definitely add a lollipop or baffle)

Pushrod adjustment-(the setting changes as the bike gets hot, bottom line is they should be able to spin when bottomed out in the tappet guide with no up and down play. If you adjust them till they are tight and can’t spin they will most definitely be out of adjustment once the bike is warm. The result is the exhaust and intake valve will remain open and throw off the right percentage of air, spark, and fuel. You will need more fuel due to the open valves, meaning burying the idle screw to keep the bike running.

Spark plug check-(having too hot of a plug will can cause preignition once the bike is hot, it can be masked as a timing or carb problem)

Vapor Lock-(do you have good gas flow from the tank? Does it run better if you remove the gas cap?)

After you do all that and still have a problem

What I did to make a difference;

I wasted money on the above things even after checking them and proving they were ok, I would go back on a forum and be told to check the same things again, it was frustrating but without seeing everything it's hard for these guys to pick up on something if they aren't there standing in front of it.

I believe the real problem were my valves. They were carbonized and holding open just enough to screw up the process of combustion. I pulled the jugs, cylinder heads, and rocker covers, and I am glad I did. These things can come off the bike without removing the motor, just the gas tank and carb and pushrods.

My rocker gaskets had blown out causing an oil leak, my valves were in desperate need of being lapped in and more importantly my pistons and inner cylinder wall were slightly scored right near the oil return hole.

I don't have a micrometer to measure the cylinders, or the necessary tool to hone them. I brought them to a shop and they needed to go from 10 over to 20. I lapped the valves, replaced the piston and rings (in my case I had too), and replaced the gaskets (using copper spray on them to insure a sealed fit).

After doing this I have a one kick bike.

P.S. I never owned an ironhead, after the top end work was done it feels like there is a 50% increase in the required pressure to kick the bike through.

My advice is to get a lapping stick, lapping compound, top end gasket kit, and copper spray for the gaskets and a valve spring compression tool to remove the springs.
After you lap the valves you flip the heads over, drop the valves into the valve guides with no spring tension and fill with gas. The gas should slowly weep however the valves themselves should be able to hold.

While you have the jugs, cylinder heads, and rocker covers off you can hit it with flat black stove paint as well as check other clearances in your service manual.

I don’t fix bikes for a living, and the guys who have already commented know much more than me, but this is what I needed to do in order to reset the carb to it’s settings without burying the idle screw.

 

Hello PG,
I had the same thought that perhaps I had the wrong screw. I bought this carb as NEW OPEN BOX. But I ask the same question why would someone change it.
Thanks for the pics. I can at least take some measurements for comparison.
As I said before everything appears to be correct but stranger things have happened.
Bucaman

 

Danny,
I have also checked and rechecked all of the above. I maintain a solid 125lbs. compression in both cylinders. I had the rear head off to repair a rocker box leak and it looked great. Looks like top end had been done recently. Seller made that claim and now I can believe him.
All thats left is to keep playing wiith it until I stumble upon something.
Not the best plan I know but its all I have for now.
Run the bike every weekend but it just frustrates me when I sit at idle and it falls out from under me.

 

You were right I don't think it was my carb. I know this because like an idiot I bought a new Mikuni off ebay for 95 bucks, put it in and it ran good for a day or two. Then I gave up on Mikuni and spent a bunch of money on a S&S E. Guess what? I still had excessive idle problems.

So I spend about 600 bucks on carb crap, and 100 or so the the push/pull cable set up when all I needed to do was pop open the top end, lap the valves, hone the cylinders, replace the pistons, rings and gaskets. I went with K&B pistons but if I hadn't the repair would have been much less than all these spare carbs. lol

I still regret running out and buying the S&S.

Ahhh frustration and desperation!

 

Yes, no reason for anyone to change out the screw.
The carb is new, so wear is not involved.
I did not want to even mention these two things because the chances were almost nil that either could be the cause.

The Mikuni should be trouble free and idle without a problem though.

With the air cleaner removed, stick your finger inside to touch the slide. Turning in the idle speed screw should raise the slide. If it does not, then the screw is too short.
Remove the spring and try it again. Does the slide now raise because you can turn the screw in further? If it does, then I have to say the idle screw is too short.
The correct screw is Sudco part number 002-183.
There is no way to ID the screw however, so that info is not very useful.

If the screw moves the slide upward without having the spring on it, you may want to try this.........
Obtain a metric nut of the correct size/thread to fit the throttle screw and put it on the screw. I looked and did not find I have such a nut, but found something even better, and is easy to find one.
This is a ring nut off a bicycle inner tube (Presta type) and it fits perfectly, plus it is thin.



Substitute the ring nut for the spring and lock it down with pliers.
If you cannot find the nut (look at a bike store) then I will send you this one. It is close to an M6-.70 thread as best as I can tell.


pg

Then screw in the screw and obtain the correct idle, then lock down the nut against the carb body.
That will work provided you have enough thread to accommodate the nut, which replaces the spring. If you use the ring nut there should be plenty of thread.
pg