Cam Timing

[Bob Tucker]

Hi,

all this year I've had a strange phenomenon with my Red Top.

 

The power has started to come in higher & higher up the rev band.

When the engine was first built, it came in about 4K then hit hard up to 7750 then tailed off.

 

Eventually It started to feel gutless until at least 5.5k, then hit even harder than usual, up to 8k.

It made Barkstone & Lydden track days a real chore coming out of any corner.

 

Compression was down on all 4 too, so I planned to strip the engine expecting to find broken rings, blown head gasket etc.

But Good News.!!!

 

The inlet cam timing had changed maybe due to slack Allen bolts on the vernier pulley. Instead of 110 degrees, it was retarded by just 5 degrees. Reading up on a few cam websites, apparently that can cause all the symptoms I had. Luckily Ray has a remote camera, so I could check the pistons, bore & valves...no sign of any damage, & the compression is back where it was.

So no need to remove & strip the engine!!!

Just a new sump gasket & cam cover gasket (it spouted oil everywhere) & wait until March or April to try it out..

Best Christmas present this year...

Bob

[Longboarder]

That's a bit lucky. I suppose it tightens the bolt as it creeps. Is your red top an interference engine? I wonder how many degrees it has to turn before a valve taps a piston top. Doesn't bear thinking about. Shows how cam timing is critical. If you advance it 5 degrees does the power come in at 3000rpm?

 

Nigel

[Snapperpaul]

You can swing any can a few degrees, normaly only 2 either side of manufacturers recomendation for bhp or torque.

Having the inlet valves close later after bdc would indeed reduce combustion pressure on all 4

[Bob Tucker]

Yup its an interference engine. Someone lost a belt on the way to the Lemans classis last year & bashed everytghing

I think its lucky its the inlet that was loose...the inlet valve will open as the piston is descending & close at or just after BDC, so nowhere near the piston.

But the exhaust will stay open as long as possible on the upward exhaust stroke.....5 deg retarded may well have holed a piston..

 

This site helped a lot esp the final paragraph..

 

.http://www.iskycams.com/pdfcatalog/2004-05/page195.pdf

 

 

Cheers, Bob