Persistent Problem at Speed.

[ferret76]

G'day All,

I've been wrestling with my car for the better part of a month now and I'm at my wits end.
At the start of August I went camping in my mog which involved approximately 90km of highway driving. It did the trip down beautifully, sitting on about 90km/h the whole way. 2 days later on the return, much the same for most of the trip until I got about 25km from home and I started to notice an occasional 'blub' in the exhaust note, but no other real change in performance. The day after getting home I drove to work, approximately 15km, and I have to say the car has never run so well since I've owned it. I parked it up in the usual spot and then drove home after work. On the trip home, the occassional 'blub' in the exhaust from the previous day became a persistent 'blub' and by the time I was almost home I was also getting the occasional backfire.
That night I inspected the plugs, which were biscuit brown, inspected and regapped the points and took it for a quick test drive which seemed to indicate success.
The following day and every day for the next week consisted of 'blubbing', persistent backfiring, no power and on a few occasions complete breakdown (once after entering a roundabout, which was hilarious. I laughed and laughed).
It got to the point where fiddling with the timing, mixture, changing the condenser etc had no effect on the car whatsoever. I did a compression test on the head with no conclusive results other than all cylinder pressures were OK.
Not satisfied with that, I took the head off. Head gasket OK, replaced it anyway. While the cylinder head was off I did a decoke and lapped the valves.
Reassembled the head, started the engine and it idled beautifully (as it has all the while through this saga) went for a test drive and once again, 'blubbing', backfiring, the works.
Here's what I have done so far (the engine is a very standard 948 by the way):

New spark plugs
New condensor
New points
New distributor cap
Decoked the head
Lapped the valves
New head gasket
New exhaust gasket
Set timing with stroboscope
Set and reset the mixture
Checked all vacuum hoses for tightness
Checked manifold bolts
Cleaned the air filter
Cleaned the carburettor

The backfiring issue has resolved. However the 'blubbing' remains. The car idles beautifully, pulls well through 1st and 2nd gear but once in 3rd - blubbing, loss of power, 4th blubbing and surging power.
The poor running usually starts after about 10 minutes of the engine running.
I'm sorry that I don't have a more technical description of 'blubbing', but that is exactly what it sounds like. Everynow and then it feels as though I'm losing one cylinder, sometimes (less often) it feels like I'm losing two.

I'm on the verge of buying a new distributor, even though I have replaced all the 'consumable' parts and the vacuum advance seems to be operating normally, it just feels to me like this is where the problem lies. Like I said, runs beautifully at idle, pulls nicely through 1st and 2nd, but runs like a new born giraffe at speed.

Any suggestions or advice would be greatly appreciated.

[philthehill]

I am sorry to hear of your problems and the efforts you have undertaken to try and resolve the situation.
The one thing that you have not referred to is the fuel supply.
Is the pump delivering its correct quota of fuel and is it delivering fuel constantly?
You could have dirt in the system - check the filter located in the bottom of the fuel pump for starters.

[oliver90owner]

As PTH, but in addition check the tank filler for proper ventilation. An outsider is valve lift, but if the backfiring has ceased and it does not do it from cold, it really is an outsider. No mention of plug leads either.

RAB

[martin418]

have you tried an old original condenser rather than a new one , i had exactly the same fault on my car and i cured it by fitting an old original condenser i found in a box of bits , the new replacements are not good

[midget]

For what it's worth,I had similar problem with mine. I did all the things that you mention plus fitting new fuel tank and line.
Gave up eventually and took it to a recommended diagnostic chap who, after rolling road assisted examination, threw away the electronic ignition set up, fitted a "flamethrower" coil and an M needle to the carb.
The result is sensational. Apparently he uses some competition grade points (only supplied with a complete competition package). Wouldn't tell me the name or location of supplier. The engine is mildly upgraded (flowed head, HS4, Maniflow) but ran pretty well before the problem started, but is now superb.
He wasn't cheap, but looks after many successful competition cars particularly Historics.

When I asked what the problem was, he opined that it was a catastrophic collection of ALL the above systems.

[liammonty]

It would be worth swapping the coil. - I've had them give similar symptoms only beginning after a period of running. Also, check the little low tension lead in the dizzy. A long shot, but worth checking.

[palacebear]

Similar problem here a while back. Traced to a badly fitted condensor. The little wire was trapped between dizzy cap and body. Insulation on wire was damaged causing intermittent arcing.

[NOEL]

as per some of the above posters, try the condenser with an old spare if you have one, I had a similar issue with a dodgy low tension connection which resulted in the back firing and lack of power at speed but it would idle ok
best of luck !

[JOWETTJAVELIN]

Going back to basics, the oil level in the carb. dashpot.

[ferret76]

Worn piston rings.

New rings and big end bearings fitted, runs like a charm. Why do we do this to ourselves?

[oliver90owner]

Amazing!

Good to hear the problem was sorted.

[amgrave]

I wonder why the pressure test did not show the worn rings up? Glad it's sorted now.