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MG MGB Technical - What happens when the cam is two teeth out

This was interesting to me, a race Magnette had been fitted with a new crank. The owner had put the cam back in two teeth out, it showed up very obviously as lack of power. Last year it showed 118 bhp at the wheels. The owner stripped it down and we reset the cam for him.....back to 118+ at the wheels ( 149bhp at the flywheel)

I attach a pic of before retiming cam and after, solid lines bhp and dotted lines torque.

Peter
Peter Burgess Tuning

Oooops, forgot to attach the pic!

Peter

Peter Burgess Tuning

Peter-
Excellent! Thanks.

We once had an AH 3000 which started and idled perfectly, but couldn't pull the crust off a cheese sandwich. After much investigation, we checked cam timing. It turned out that the crank nut was loose, evidently repeatedly, and so the guy had welded it in place, still loose. (only those who have worked down in that wretched hole could appreciate this!) So it sheared the key and the thing retarded itself until the key jammed at about 90+ degrees late. Now I do routine cam timing checks as soon as I have problems after basic tune.

Did a Spitfire 1500 that was assembled incorrectly at the factory - 14 late and another 2 from chain wear, picked up about 20% power and economy, after 60,000 miles, no other changes or adjustments! Mileage went from 27 to 32+ M/USG in the same service.

Anyone who works on a bunch of the same car knows that some are spectacular, some are horrid, and most are in the middle. I now think that cam timing is a major factor in this.

FRM
FR Millmore

Thanks Fletcher, I thought it good to actually measure the differences rather than seat of the pants that we usually get. The real frightener, in a way, was the fuelling. With the cam retarded it ran as if on choke ( someone could have altered this to try and sort 'the problem' and then be too weak when re-timed!)re-timing the cam and the fuelling came back to when I set it it 2012.

Peter
Peter Burgess Tuning

When I bought my 68 GT it had a recently rebuilt engine, but it didn't perform correctly and only had 105 psi cranking compression. It turned out to have a custom grind cam on a Crane billet, but I had no specs for it. I took an eyeball guess and advanced it two teeth. It wasn't right, but it was better. The cranking compression went to 145 psi.

Charley
C R Huff

Just reading 'Short way down' by Steve Wilson about a 5000 mile solo bike ride through Africa on a 50s Ariel. A 'bitsa' bike built for the trip, it's a nightmare from start to finish, one of them being lack of power and compression. Part way round some investigations are done and the timing marks are found to be two dots off, and the inlet valve isn't closing until the piston is halfway up the compression stroke.
PaulH Solihull

It sure makes adifference, doesn't it?

I once built a race Twin Cam engine and had miscalculated the necessary depth of valve reliefs in the pistons. To get the clearance I wanted I had to retard the timing about 4 deg. It still went very well, but significantly less so than when I stuck in the next set of pistoins and the correct timing.
Bill Spohn

I have to agree. Camshaft timing is critical. So much so that I check mine every winter and have the engine equipped with a Vernier-adjustment timing sprocket in order to simplify the fine-tuning of the camshaft timing. Worth every penny!
Stephen Strange

This thread was discussed between 25/02/2013 and 02/03/2013

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