Author Topic: Slight flat spot just above idle (Read 2122 times)

regcheeseman · « **on:** February 24, 2008, 03:32:42 pm »

As the title says I've a bit of a flat spot just above idle.

The car will kangaroo and coming of the throttle to swap cogs is not a smooth as it should be.

She's a 30,000 mile GTD, now with near 20psi of boost, no fuel screw adjustment, boost pin maxed.

Would advanced pump timing cause this? she starts and idles perfect. No smoke, low egts.

I set the pump to 1.00mm but that was with engine out using mark scribed on crankcase (by me using measurements taken from gearbox) - so it may not be spot on.

I've pulled the cold start, it seem to have no effect.

The problem appears to be slightly worse when the engine is cold and slightly better if engine is warm or the ambient air temp is low. i.e. a cold engine in the middle of the day is the worst it'll be.

regcheeseman · « **Reply #1 on:** February 27, 2008, 01:26:48 pm »

Anyone? Anyone got any suggestions as it's begin to do my head in.

zukgod1 · « **Reply #2 on:** February 27, 2008, 01:51:21 pm »

By flat spot do you mean you can give it some peddle and it does nothing for the first few inchs of peddle movement then starts to take off?

regcheeseman · « **Reply #3 on:** February 28, 2008, 12:29:23 pm »

Let the clutch out at idle - it'll pull away, add some throttle and it'll hesistate slightly then go.

But hold it on a steady throttle at low revs - i.e. cruising in traffic at less than 2000 rpm and the engine will 'kangaroo' and shudder unless you accelerate or come of the throttle.

Rolling off the throtle in top and allowing it to coast down (clutch engaged) through the rev in top it will hit the flat spot and dramatically slow like there is no fuel, then as it slows more down to near idle it will pick up again and just continue to chug along at idle with no throttle.

This flat spot in the rev range is a pain because it's where the engine would spend a lot of it's time in normal traffic and it's hit every gear change unless I'm nailling it.

I've three suspects at the mo - air in pump(no bubble in line, poor timing, or the gov mod (the spacer used was 0.4mm longer than the spring it replaced)

But I can't see how these would have any effect apart from timing but it runs/starts perfect apart from the flat spot.

regcheeseman · « **Reply #4 on:** March 05, 2008, 02:44:29 am »

Seem to have just about cured it.

Answer - a gearbox swap.

Dunno why but the problem has all but gone, the only reason I can think of was something to do with the engine mounting.

I didn't change any mounts just changed the short five speed for a longer five speed.

Easy 100mph at 3500rpm and I haven't touched the fuel screw yet.

Waahay!