Help please - V8 backfire at 3000RPM only

I hope someone out there can help me with this one, it's got me baffled!

I have recently taken the engine out of my boat (Volvo 5.7 GS) to
replace the damaged ring gear, the engine went back in with no problems
however, it has developed a backfire that occurs only between 2700 and
3200RPM. At all other revs there is no problem and it runs better than
ever at WOT. The engine does not have electronic ignition and has a
twin choke holley carb which I have stripped and found to be spotlessly
clean.

Can anyone shed any light on the possible problem please?

Chris

"Chris Twibell" <chris.RemoveThis@nospam.twibs.co.uk> wrote in message
news:Xns99575DA4056A2chrisnospamtwibscouk@80.5.182.99...
>I hope someone out there can help me with this one, it's got me baffled!
>
> I have recently taken the engine out of my boat (Volvo 5.7 GS) to
> replace the damaged ring gear, the engine went back in with no problems
> however, it has developed a backfire that occurs only between 2700 and
> 3200RPM. At all other revs there is no problem and it runs better than
> ever at WOT. The engine does not have electronic ignition and has a
> twin choke holley carb which I have stripped and found to be spotlessly
> clean.
>
> Can anyone shed any light on the possible problem please?
>
> Chris


Chris A long shot.

What distributor?

Bosch, Marcel and Lucas have all made distributors that can be assembled in
such a way that they occasionally short out the LT. If its "continental" and
the points are held in place by a "hairgrip" type clip make sure there is an
insulating washer under the clip.

If its Lucas with the flat square sided cap check the place where the two
wires "clip" under the points spring. If the wires do not have a twist in
them you may have the plate fixed to the two wires in the wrong way round
and that can touch the baseplate.

If it is starting and running well (whatever WOT is) I can not see the
timing being too far out, but does it have a vacuumed advance retard pipe
on it between inlet manifold/crab and distributor - has it fallen off.

Can you check to see if the centrifugal advance is working (I expect it is
otherwise you would complain about loss of power). Yankee distributors are
not unfamiliar with seizing up.

Not much help I am afraid.


--
Tony Brooks
www.TB-Training.co.uk

Maintenance & Electrical training for ordinary boaters