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VM tuning (Read 179 times)
verslagen1
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VM tuning
06/24/19 at 10:40:00
 
I'm having an issue with a VM carb.
It runs great (around 12-13) up to almost full throttle then it suddenly run super lean (above 17).
Just like there's a hole in the body and once the slide uncovers it, it goes lean.
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hotrod
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Re: VM tuning
Reply #1 - 06/24/19 at 11:53:58
 
Is it a 36 or 38 mm ? What do you mean 12,13,17 ?
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Re: VM tuning
Reply #2 - 06/24/19 at 12:12:48
 
hotrod wrote on 06/24/19 at 11:53:58:
Is it a 36 or 38 mm ? What do you mean 12,13,17 ?

Yes, it's one of those.  (I'll have to check)

A/F ratios.
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Re: VM tuning
Reply #3 - 06/24/19 at 12:18:24
 
The 12, 13, 17 means A/F ratio.  I suspect Versy is using an A/F meter.

Any slide type carburetor (CV or VV) can have a problem where WOT fuel is limited by the Needle Jet (NJ) instead of the Main Jet (MJ).  It's unusual, but I have personally experienced it and also read about it.  You can eliminate the NJ as a possible source of the lean-out by simply replacing the main jet with a grossly oversize jet.  A main jet so large that it should be virtually impossible for the mixture NOT to go rich.  If the engine still goes lean at WOT, you may need a larger needle jet, or possibly a smaller needle, or a compound needle.

I read one article where the author actually removed the main jet entirely to facilitate the test.  That seems pretty stupid to me since the needle jet could come out and go through the engine.  I bet that would be ugly.

You also might have an issue with fuel delivery.  The needle and seat might be too small to allow sufficient flow to support WOT, but large enough to support part throttle operation.  It could also be an issue with restricted delivery through the petcock or fuel line.

Sounds like you've got one thirsty motor.  Those are generally a lot of fun Cool
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verslagen1
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Re: VM tuning
Reply #4 - 06/24/19 at 12:39:58
 
Thanks, I'll check the main jet size and see if I have a larger one.
While I'm at it, I'll check fuel flow.

The beast is capable of busting the ton uphill   Cool
Lately, I can only get it up to 70.
Can 2 carbs have the exact same problem?
If this doesn't resolve the problem, I'll have to tear into the engine.
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Re: VM tuning
Reply #5 - 06/24/19 at 14:55:12
 
What if it's a fuel delivery problem ?( I assume your still using the tuned intake and with a VM you might be using a lot of fuel at WOT) Is it possible to gauge the bowl level ,say with a clear plastic tube ,at WOT?  If it showed the level dropping  ,it might be the restriction is the float valve seat (orifice), You might be able to drill it slightly larger ,and still leave enough that the valve seats .  This is all a guess ,as I've never seen a VM.
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Re: VM tuning
Reply #6 - 06/24/19 at 22:41:14
 
Wow!  Breaking the ton on an incline.  That's impressive.  My compliments.

I can't imagine any mechanical issue with the engine that would result in lean WOT operation.  The issue has to be related to the carburetor or fuel delivery.

Can you provide some background?  Are we lookin at somethin that was runnin fine and then suddenly went south, or is this a new carb install that is problematic?
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Re: VM tuning
Reply #7 - 06/25/19 at 02:47:52
 
Are you using one of those UFO things on the bottom of the slide ? Try without it.
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Re: VM tuning
Reply #8 - 06/25/19 at 07:13:03
 
The vm 36 and 38 are out of the same body, the manual has all the dim's the same... don't know how you tell one from the other.

I found a vacuum port that wasn't plugged but it did the same thing.

No UFO.

I swapped carbs to try and identify the problem, same problem with both.

It feels like rpm limiter, you get to 70/75 and the a/f meter shoots up.

I'll try swapping the TCI and coil next.
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« Last Edit: 06/25/19 at 08:56:01 by verslagen1 »  

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LANCER
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Re: VM tuning
Reply #9 - 06/25/19 at 09:11:28
 
Did you get both carbs brand new or used ?
If used, did you verify that they have the correct needle jets ?
What size main jet are you using ?
What other sizes have you tried ?
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Re: VM tuning
Reply #10 - 06/25/19 at 09:22:44
 
LANCER wrote on 06/25/19 at 09:11:28:
Did you get both carbs brand new or used ?
If used, did you verify that they have the correct needle jets ?
What size main jet are you using ?
What other sizes have you tried ?

Both bought used.
Picked up from forum members that had used them on savages.
An edelbrock with a 5-k needle
The vm was originally from Lancer (if I remember correctly)
It has a 190 main and 55 pilot.
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Re: VM tuning
Reply #11 - 06/25/19 at 15:03:32
 
Bump it up, try 200.
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Re: VM tuning
Reply #12 - 06/25/19 at 21:22:56
 
Ok, so the inlet to the float valve was 1.5 mm is now 2mm   Cool

the float height was 22mm is now 19

clear tube on the drain nipple

swapped TCI
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Re: VM tuning
Reply #13 - 06/25/19 at 22:12:37
 
Wide band a/f meter temporarily mounted in a cup holder.


Bosch sensor in an adapter that fits in between the header and muff.
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Re: VM tuning
Reply #14 - 06/26/19 at 07:26:21
 
The morning ride report.

Problem is still there although much better.  I can hold it wide open and while it goes up from 12.5 to 14.5, it only goes a little higher.  

Wasn't able to set up the camera this morning to monitor the bowl level line.  And it's a little too tucked away to see by eye on the go.
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