Originally Posted by
kael
Could be a clutch issue, but I'm guessing it's the infamous fat-bottom tune problem.
On the box with engine warmed up, rev up to clear the motor out and then drop to idle. If the idle starts out high and then drops after a number of seconds, the low end is rich, top end is lean. Do the pinch test (at idle, pinch the fuel line closed), you should get 3-5 seconds before the engine dies with minor change in RPMs. If it's much longer, lean the bottom an hour or so at a time.
A lean top and rich bottom cause the run on problem. Running wide open throttle burns off all the fuel, when dropping to idle the mixture is initially very lean (so idle is very fast), but after a couple of seconds becomes very rich as fuel loads up. Fought this quite a while last weekend myself.
See the excellent tuning guide here:
http://go-racing.vpweb.co.nz/Tech-Tips.html
Grizz: question about the tuning guide, when doing pinch test for initial LSN setting, is that after idling for a bit or right after clearing out the engine and dropping to idle?
Hi Kael,
Give it a rev to clear it out if required, then let it settle back to a steady idle.
If you are doing the initial LSN pinch test (7 sec test) for the GX or earlier series motors with the long LSN, this must be done just after the motor has been started from cold to get the right results.
Start it, let it run on the box for 30 sec to get a stable idle, then pinch the fuel line off and set the idle gap going on the results. Once you have the idle gap set, pinch the fuel line off again for the 7 sec LSN setting before the motor warms too much. If you do this test when the motor is warm, you will not get the right results due to more efficient fuel vapourisation in the crankcase as the motor heats up. Once hot you will probably get a 2 - 3 sec pinch at best.
Remember this is a "ball park" LSN setting (normally a little on the rich side). Once you lean the HSN, which you normally will have to do to get nice WOT performance, the LSN will be leaned at the same time, bringing it very close to where it should be for a good crisp tune.
Hope that helps.