Timing belt / rough running Q

Post Fri Feb 28, 2014 7:08 pm

OK so since me and a couple of other Zoners reassembled my touring a couple of years ago although it runs OK it doesnt run great.
It had a new HG and pressure tested/skimmed & rebuilt head

However since it all went back together although it starts fine and runs ok it doesnt run as sweetly as I think it should.

In particular the idle is rough when warm but sounds OK as soon as the throttle is opened slighty - I've been all over it looking for air leaks and cant find any - all the hoses and gaskets are new or good.

Also although the top end is quiet at idle at anything over 3k it is noticeably noisy - tappy
All of my previous M20's have been quiet through the rev range

Finally its just not as quick as I think it probably should be.

Ive done a few hundred miles in it since the head went back on but a thought has occurred to me is it possible the timing belt is one tooth out ?
Would this explain the poor idle, underpower and tappy top end particularly at higher revs ?
I hope its not but even if it is can it have done damage as it still seems to run OK - passed MOT emissions twice - albeit with a lean CO figure

If it is out is it possible to check this without taking it all to bits - can the belt be de-tensioned, the crank be lined up and then the cam moved one tooth and the belt then refitted at the top ?

I have been meaning to check the valve clearances as they were set by the machine shop who did the head and I havent re-done them since - but if they were out would that explain the top end being quiet at idle and noisy when revved ?

The oil spray bar was blasted through and the holes opened out slightly when refitted and oil pressure is good.

Any input or ideas welcome

Post Fri Feb 28, 2014 7:20 pm

It's bad practice to slacken off and re-tension a cam belt, but to 'de-tension' the belt and do what you say, everything you need to dismantle to change the belt will be dismantled, so it would be a very false economy to reuse a belt that is half way through its life. A belt is hardly expensive!
One tooth out is unlikely to cause damage, but will take the edge of of the performance, but there's loads of other things that could be responsible as well.

Post Fri Feb 28, 2014 7:30 pm

Are the timing marks visible without taking it all to bits so I can at least see whether its lined up or not ?

Post Fri Feb 28, 2014 8:42 pm

If your one tooth out. I think its advanced will give you lots of low down power with no top end power or revs and quite slow cranking also the exhaust note will be quite booming.
Retarded is the other way weesey until the revs are high then turns into a top end rocket pulling beyond the red until it pops.