Strange runnings...

[FFMan]

1994 Griff 500 - remapped a few years ago by Mark adams, new injectors etc and has been running great ever since.

used it 2 weeks ago - all ok

Used it friday, executing my favourite Griff past time, 3rd gear overtake and engine gets to about 4k and dies,i change up to drop the revs, it comes back on and I continue the journey running a few small test. It seems doesn't want to rev past 3.5 - 4k revs, getting all fluffy and stumbly until the revs drop back. I did give it a quick wash before I went out in it and o sort of convince myself it got damp though I don't think I really believe that.

So at lunch time at work, check all the leads etc, all seems ok and on the way home a couple of short blasts to 4k revs and its ok. i convince myself it's ok. Then on a dual carriageway section I open it up having some fun with a Porsche, 3rd and 4th gear full power, seems ok, but then as we coast down it gets all fluffy and misfiring. So i cruise at 2k revs for a minute or so and it all comes good again. I limp home.


First though, fuel starvation ? Seems like fuel flow not keeping up with high revs ?

What should I check next ? Fuel pump runs when I turn on the ignition. I've got rovergauge and I'll plug it in later.

[Jobster]

What comes to mind are: ignition module, coil, rotor, leads.

[FFMan]

Jobster said:

What comes to mind are: ignition module, coil, rotor, leads.

all these were replaced by Mark Adams when remapped bar the ign module. Coil is a decent Bosch item.

For me, the fact it mis fires the settles after a period of low speed running suggested fuel but i'm open to all things.

[Phud]

Might be water in there fuel or dirty fuel, lines and filter.

Had similar, when I got above 3.5K no power and would run up to that.

[blitzracing]

The HT voltage required to keep a reliable spark goes up as you open the throttle. Problem is the available HT drops as the revs rise as there is not enough time to fully charge the coil between sparks on V8 engine, but it's designed to have enough spare capacity in the coil to still provide a spark if as all is well. If not it can do as described.

[FFMan]

thanks for the replies but none of that explains why after a hard gas in 3rd and 4th it continues to misfire (on all cylinders) until it's had 30 secs or so at 2k revs.

[Jobster]

FFMan said:

all these were replaced by Mark Adams when remapped bar the ign module. Coil is a decent Bosch item.

For me, the fact it mis fires the settles after a period of low speed running suggested fuel but i'm open to all things.

I still think it's the ignition module. not going above 4k revs suggests a failure there.

[FFMan]

i think i may have a spare but in case i don't i seem to recall there is a difference between 2 & 3 wires versions ?

Are either readily available ?

[FlipFlopGriff]

I'd agree on ignition module. Probably a DAB118 if its a 500.
Plenty on f-bay.
FFG

[blitzracing]

The only difference between the two and three pin is the two pin is earthed through the body, and the three pin has an extra earth pin. Both bolt on the same way. You can get a lucas cable up to convert between the two, or just use spade connectors. Drop me a mail if you want the details.

[FFMan]

thanks - IIRC i have a spare in the boot bought when we went to classic lemans but i'll check this evening.

[blitzracing]

Have a peek at the rev counter when the issue occurs- if the amp is not switching correctly the reading is likely to be erratic.

[FFMan]

i have a spare amp in the boot - when i came to change it one of the wires on the amp didn't seem very secure.

so i attached it firmly and will test in day or so, keeping the spare amp and spanner handy so i can change if not resolved.

[andy43]

Long shot but could be an original spiral wound induction hose collapsing at high revs.

[ianl]

As others have mentioned, coil is a strong possibility. I had exactly these symptoms on my Griff 500 shortly after fitting a brand new Bosch coil during a service. Joolz diagnosed the issue on the rolling road & swapping to an Intermotor coil from the local autofactor fixed the issue. The Bosch coil was returned for a full refund from the supplier.

[Steve_D]

We recently had a Bosch coil new out the box which had a short from primary winding to the case.

Steve

[Jobster]

FFMan said:

i have a spare amp in the boot - when i came to change it one of the wires on the amp didn't seem very secure.

so i attached it firmly and will test in day or so, keeping the spare amp and spanner handy so i can change if not resolved.

And......?

[FFMan]

not had time to test it yet, rain and traffic but Tuesday looks good.

will report back

[Hedgehopper]

My old Opel Manta used to run out of breath beyond 4500 rpm or so and then would recover itself as I slowed down. It turned out to be that the fuel tank vent tube was blocked so much so that the tank had taken on a flattened shape due to suction.

[FFMan]

took it for a blast and all seems good. must have been that not-so-tight wire on the amplifier

thanks for pointing me in the right direction.