Hi Peter, What you are describing sounds very like a problem I had with an
MGB.
When you mention that the exhaust valves open much further than the
inlets I think you should check the actual lift of the inlet lobes. On
the MGB 3 of the inlet and one of the exhaust lobes was worn so badly
that the valves were barely lifting off their seats.
If the cam has worn that badly the engine will have to be disassembled
and thoroughly cleaned out because those flakes of hardened steel from
the cam and lifters will wreak havoc throughout the entire engine..
Don't ask me how I know!!!
Michael Salter
On Fri, Jun 25, 2010 at 4:33 AM, Peter Linn <greylinn@ozemail.com.au> wrote:
> G'day list
>
> Today I spent 6 hours trying to get a friend's BN1 to run properly. He's
had
> persistent problems with the car misfiring under load. He hasn't had the
car
> long and doesn't know a great deal of its history, except that it has
> apparently been used as a race car and was at one time supercharged. I went
> through everything today - valve clearances, compression test, points,
dwell
> angle, carbs etc. We found that, apparently, it has a non-standard cam as
the
> lift for the exhaust valves seems considerably higher than that for the
inlets
> (looking at the compression of the valve springs, the exhausts appear to
> compress around 1/2" more on opening than the inlets) We set the exhausts
at
> 18 thou, the inlets at 12. This appeared to improve things with the car
> stationary, but under load it still misfired and backfired. We changed out
the
> condenser, the distributor cap and leads. The coil is new. Points seemed OK
&
> dwell angle 60 deg. I was unable to check the valve timing as there's no
mark
> on the pulley and we didn't have time to establish TDC. Carb tune is OK and
> the plugs looked right. The car has 1 3/4" HS6 SUs. The vacuum advance is
> working. The only thing that didn't look right was the compressions, which
> were between 105 and 125 psi (another car I did this week had 165 - 175).
We
> wondered if the cr had been reduced when the car was blown. The car doesn't
> blow oil smoke nor does there apppear to be excessive crankcase pressure.
>
> Bottom line - after all this attention the car would rev cleanly when
> stationary, but on the road under load would misfire and backfire to the
> extent that it was virtually undriveable.
>
> Any suggestions listers?
>
> Cheers
>
> Peter Linn
> Brisbane Oz
> BN1 Ward Spl coupe
> BN1 Holden V6
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