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Author Topic: Help tuning Micarb VM28 for my 350  (Read 221 times)

TejK

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Help tuning Micarb VM28 for my 350
« on: December 13, 2013, 12:53:20 AM »
Hi,
I recently changed the VM24 on my 350cc to a VM 28. However with a 130 main the bike doesn't start. It runs really good upto 1/2 throttle with a 120 main however at large 1/2 throttle opening the bike pops and sound too lean. The pilot is 27.5 and the stock needle is there in the richest setting ( lowest notch).

Could it be the needle causing this ? Has any one run the 500cc carb on a 350cc and does the needle need to be changed ?

Couldn't find any carb setting for a VM28 for a 350cc - I guess most of the members here have a 500.

The bike has a mild ported head, a high comp piston (8.5:1), K&N air filter and a free flow exhaust.

Blltrdr

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Re: Help tuning Micarb VM28 for my 350
« Reply #1 on: December 13, 2013, 01:02:08 AM »
Here is a manual.
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High On Octane

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Re: Help tuning Micarb VM28 for my 350
« Reply #2 on: December 13, 2013, 07:28:07 AM »
Going from a 24 to a 28, it is a very good possibility that your cut away on the slide is too large and causing an over-all lean condition.  That is pretty damn big jump in carb sizes, especially if there are no performance mods done to the motor.  If you can't get it to run right with new jets and adjusting the needle position you'll probably need to replace the slide to a smaller cut-away.  Read thru the manual Blltrdr posted and see if you can get it straightened out.  Keep in mind, every replaceable part in the carb effects the engine differently at different throttle positions.  So make sure you are tuning the carb in the correct order, otherwise you're just wasting your time.

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ace.cafe

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Re: Help tuning Micarb VM28 for my 350
« Reply #3 on: December 13, 2013, 09:06:11 AM »
Half-throttle is primarily governed by a combination of the needle jet and the needle. Work with those to get past the hiccup.

125 main would be a good try for a main, because that is usually a decent jet for the Mikarb. If it needs a little more, then try the 127.5.
Usually, the problem jet on the Mikarb 28 is the needle jet being too lean, so a richer one is typically needed.
Unfortunately, that is the jet that is not compatible with any Mikuni jets, and it costs a lot more money to get one.
But, it is usually needed.
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