Aussie Pontiac problem

[aussiecowboy]

Hi all,

Greetings from Australia. I am the proud owner of a 28 tourer, a 29 landaulette and a 48 Silver Streak. I'm currently having some dramas with the 29. I drive thia car a lot, just not long finished a 2000 mile trip through the desert with it.

For some reason the engine won't rev out cleanly, gets to maybe 2000rpm and it's like it hits a govenor. I thought it was the carb (original updraft) leaning out initially, I had some spare manifolds which I modified and changed to a stromberg downdraught. I also rebuilt the mechanical fuel pump and added an electric boost pump. It started easier and idled smoother but the same problem remains.

I then moved to the electrics and changed from 6v to 12v, again better starting and idling but problem remains. I suspected a burnt valve or broken valve spring for a bit but generally that presents as a miss at idle and disappears higher in the revs as the engine is moving faster than the valve can leak. I need to make a fitting to do a compression test, the plugs are too big for all my fittings.

So, timing was next on my list, I cleaned up and lubed the mechanical advance mechanism, no difference. I then held the engine wide open against the flat spot and moved the distributor either way until it started to die but the flat spot remains so it can't really be timing. It runs beautifully otherwise, responsive to throttle until that point then it just makes a flat barking noise back through the carb. I put an adjustable jet in the new carb and have tried all sorts of tunes from very lean to almost blowing fuel out of the exhaust.

I'm now starting to think possibly softened valve springs leading to valve bounce? An internal earth fault in the distributor? Other than that I'm lost. I would be grateful for any suggestions. My email is aussiecowboy@gmail.com

Cheers