Mikuni vm38 round slide carb setup

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Mikuni vm38 round slide carb setup

Postby RunninWild » Sun May 06, 2018 6:05 pm

Hey all, been struggling to get my 54 KH to start and have gone through many motions but still no luck. I'm hoping to get some base line jetting on the vm38 for my engine, anyone have info on what they run or where to start?

Backstory, engine was running previously but very hard to start. It was setup with points and the same carb. Since then I've had the intake manifold fully off and replaced the seals with peek seals and replaced one of the nipples so now it has no intake leak. I put a Fairbanks mag from a 69 XLCH on it. Using the same cap, wires, etc that was being used on the XLCH. Brand new copper core spark plugs. The bike it came off of was running and I can still get a visual spark on each plug. I bought a mag timer and used it today to get the timing set. Side note; I have an adjustable base but not the proper nuts and washers to tighten it down and still have adjust-ability so I just have it locked down so it won't move. Timing dash in window while front cylinder is coming up and the points in the mag are just pulling open per the annoying buzzer of the box, all should be good.

I was able to get it to cough out of the carb and then backfire out the exhaust a couple times kicking it. I then tried to bump start it multiple times on a good long steep hill. I still cannot get it to start. After going through the kicking and bump starting I rechecked timing and it's still right where it needs to be.

The only variable left to me is carb settings and at this point I'm working off people that have vm38s on ironheads and panheads. For pilot jets I've tried 25, 27.5, 30, and I have a 40 I haven't tried yet.

Any help would be great fellas, thanks.
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Re: Mikuni vm38 round slide carb setup

Postby Ferrous_Head » Sun May 06, 2018 7:14 pm

If I am usnure about a carb working OK I resort to Aerostart. A good strong squirt of aerostart will allow the engine to run no matter what state the carb is in. As long as t lets air in the engine will run.
If it doesn't, the peoblem is going to be electrical.
The carb cough and bacjfire in the pipes would leas me to suspect the timing 180 out. I know you know what your doing but the fact your posting here tells me your frustrated at this point.
You need to make absolutely sure the timing is set when the front cylinder is coming up on the compression stroke. Even if the timing is out by 10 degrees either way the engine will attempt to run on Aerostart, not just cough and fart.
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Re: Mikuni vm38 round slide carb setup

Postby Lisa » Sun May 06, 2018 8:50 pm

Sounds like he said 180 off bet that is your problem.
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Re: Mikuni vm38 round slide carb setup

Postby RunninWild » Mon May 07, 2018 10:47 am

Ferrous_Head, I'll give the starting fluid a shot. For timing I'm using the Morris Magneto instructions

a) Remove your rear spark plug only.
b) Kick until you feel front cylinder compression; rear piston will be on the way up.
c) Continue turning the motor until the rear piston gets to the top. At this point, the correct front cylinder advanced timing mark will be appearing in the inspection hole.

I have the bike on a stand and in 4th gear using the rear tire to control the engine but otherwise i'm following these instructions. Should be pretty fool proof right?
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Re: Mikuni vm38 round slide carb setup

Postby Ferrous_Head » Mon May 07, 2018 11:28 am

Generally, that will work. I pull both plugs and put my thumb over the front plug hole. You'll feel when the piston is going down (creates vacuum) and then feel the piston coming u (pressure will try to blow past your thumb.
But I've been caught out myself before. And I always go back to basics. The engine only needs tree things to run.
Compression, air and fuel, a spark at the right time.
None of these needs to be perfect. Compression can be down a long way, like 90 PSI as opposed to 150-165. The A/F ratio can be way off, like anywhere from 8 or 9 to 1 to 18 or 19 to 1 and the engine will still run (just not very well).
The spark can be weak and off by 5 to 10 degrees and the engine will still fire.
So, when an engine simply won't at least "hit" a few times even if it won't continue running - especially with a bump start - something is way off.
Compression you can tell just by kicking it. A gauge is better (and what I use - it tells you a lot).
I get around the carb problem with the Aerostart. If you have some compression and a spark close to the right time they will run for a couple seconds on the Aerostart. As long as the throttle is working even half way and the engine gets some air, it will fire.
But if the ignition timing is out by more than 20 degrees you'll get nothing. Occasional carb farts and backfires. When using Aerostart those carb farts and backfires are generally much more impressive.
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Re: Mikuni vm38 round slide carb setup

Postby RunninWild » Mon May 07, 2018 2:03 pm

Well I double checked timing with the finger in the spark plug hole trick and confirmed that timing is dead on. Out of curiosity I checked compression and found 30 on the rear and 10 on the front. This engine was rebuilt and has less than 200 miles on it and last I tried a year ago I was able to get it to start. Any ideas on how I could lose compression just sitting?
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Re: Mikuni vm38 round slide carb setup

Postby Lisa » Mon May 07, 2018 3:44 pm

What lobe are you putting it on - wide or narrow. I found out in some of the early repair manuals it was wrong still it sounds like 180 off - change it and try it what have you got to loose ? Narrow lobe times front cylinder
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Re: Mikuni vm38 round slide carb setup

Postby Maxcapacity » Mon May 07, 2018 4:35 pm

Check your pushrod adjustment.
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Re: Mikuni vm38 round slide carb setup

Postby Ferrous_Head » Mon May 07, 2018 5:24 pm

Well, you have found the root of the problem at least. The engine won't run without sufficient compression.
If it was running fine a year ago and you haven't touched anything in the cam chest nor played with the valve lash there is probably only 1 thing left. If you had a leak down tester it would confirm it. There exists the possibility that you have rust on the valve seats. I used to see this problem a lot with Honda stationary engines. They often stopped with the exhaust valve open and if left sitting for a long period rust would form on the seat and the valve wouldn't seat properly. No compression, no go.

If it were mine I would beg borrow or buy a leak down tester. Because if nothing else can be found at this point you have to pull the heads off.

The compressed mixture can only escape two ways. Through the valves or past the rings. If the rings were that bad I would have expected an indication long before this. Would have smoked like Bob Marley. But you can confirm the rings are OK by simply pouring a small amount of oil into the plug holes and repeating the compression tests. If the readings go up you have a ring problem.
At the very least you will need to check the valve adjustment. But like I said unless you played with them just before you laid the bike up you would have to be extremely unlucky for both cylinders to lose an adjustment while just sitting.
But you do need to do the check.

One thing also has occurred to me. Possibly sticky valve guides ?

So yes, if your valve adjustment is now way too loose it would indicate the valves aren't seating for one of those two reasons.
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Re: Mikuni vm38 round slide carb setup

Postby RunninWild » Mon May 07, 2018 6:21 pm

Thanks ferrous, time to do a leak down and check valve adjustment
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