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Engine goes LEAN after jumps/long runs

Engine goes LEAN after jumps/long runs

Old 07-10-2014, 09:41 AM
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Default Engine goes LEAN after jumps/long runs

I have an Nvision(1.5 gallons) on a MBX6T and after longer runs the motor goes extremely lean. Runs fine up until about 6 mins or so and comes off at about 260ish when it happens.

It's really noticeable on the 'big jumps'. Hit the jump and in the air the motor goes up in RPM quite a bit. Had to drop out of my last race it got so bad.

Pre-heating the motor and let it run for about 3 or 4 mins and then tune the top and bottom. Idles for about ten seconds and pull full wood, get a nice puff of smoke and WOT gets good smoke. Made the low end a little rich but it's still having the issue.

Thanks,
Dennis
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Old 07-10-2014, 10:11 AM
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My guess is idle gap too big. HSN too tight, LSN too loose.

You don't see it on the bench because you aren't running off the HSN long enough for the issue to appear, plenty of fuel from LSN for WOT pull there, and to drop back down to a good idle.

This is assuming that everything else in the system is good.
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Old 07-10-2014, 10:25 AM
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I agree with Billy, could well be an in balanced lsn (too rich and too big an idle gap compensating)

The amount of people i see thinking their engines are lean when in fact they are rich.. very common

Could be also in the hsn been too lean.. im not familiar with NVision tuning enough to really know

I would lean lsn two hours and drop idle to suit, richen hsn one hour and see how she goes, make small adjustments from there




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Old 07-10-2014, 10:40 AM
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Thanks! I'll check it out tomorrow at the track.

DEnnis
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Old 07-10-2014, 03:42 PM
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+1 on what the guys have said.

What you describe (especially the revs hanging up over jumps) is a classic sign of your idle gap being too big.
Motor will start out a little rich, but come in at the end of the tank lean and smoking hot - again what you describe.

Check out the recent post put up in the GO Engines thread.
This is a good way to tune so you ensure your idle gap is correct from the get-go.
This will alleviate the current issues you are having.
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Old 07-10-2014, 04:56 PM
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Could be a bad tank as well.
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Old 07-10-2014, 08:44 PM
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I've had fuel lids go bad, engine runs great till you take a jump or a roll over and the lid comes open just enough to stop the building of pressure causes motor to lean out. Also make sure that your not sucking air in anywhere? Just look every thing over, many a tines I thought I had a running issue only to find it was caused by something else.
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Old 07-11-2014, 01:39 AM
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Sometimes when you're list in your tune, the easiest way to rectify it is to start over. Richen the top and bottom and turn up the idle so it stays on and tune from there. Hsn first, then lsn and keep lowering the idle as you work to lsn so idle rpms sound right.
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Old 07-11-2014, 01:53 PM
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Originally Posted by rustyfingers View Post
I have an Nvision(1.5 gallons) on a MBX6T and after longer runs the motor goes extremely lean. Runs fine up until about 6 mins or so and comes off at about 260ish when it happens.

It's really noticeable on the 'big jumps'. Hit the jump and in the air the motor goes up in RPM quite a bit. Had to drop out of my last race it got so bad.

Pre-heating the motor and let it run for about 3 or 4 mins and then tune the top and bottom. Idles for about ten seconds and pull full wood, get a nice puff of smoke and WOT gets good smoke. Made the low end a little rich but it's still having the issue.

Thanks,
Dennis
Its the tank Dennis
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Old 07-11-2014, 05:12 PM
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Thanks guys.

Houston,
What's up with the tank that causes this issue?

Thanks,
Dennis
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Old 07-11-2014, 06:34 PM
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Originally Posted by rustyfingers View Post
Thanks guys.

Houston,
What's up with the tank that causes this issue?

Thanks,
Dennis
Tanks go bad , old mugen tanks arent the best but truck tanks arent too bad

loose tank lids , broken pickup tubes in tank
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Old 07-11-2014, 06:55 PM
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Yeah it sounds like a textbook tank issue to me.

I don't understand Mugen's occasional tank problems because their onroad tanks are some of the best on the planet but the offroad ones are better than some but not the best.

Also a bad exhaust seal can act like this sometimes but it's usually more severe than what you are describing.
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Old 07-11-2014, 07:16 PM
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Thanks for the help guys.

Dennis
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