The Mi Managed 300miles, Then It Stopped!

[yorkdave 1]

now then fellas, went out and about yesterday to run in my mi (all new liners, rings, shells,etc, also rebuilt head with three angle valve seats, r1 bike carbs, piper re-ground cams, 8valve ignition, converted off vac advance).

Got it up to about 270miles with no problems, then it started to miss fire below 2500rpm when under any load in any gear, this got slowly worse untill it got to be miss firing up to about 3200rpm. after this it would clear its throat and be on. Then coming out of a village it did a big backfire out of the exhaust when i hit the throttle and had no power, when i dipped the clutch it died! Wouldn't start again.

Me and steve bogg (up at bogg brothers near malton) had a look at it and decieded the cam timing must have jumped as the timing belt was slack. we retimed it up and it would try to start but wouldn't actually go! Put a compression tester on it, number3 was at 150psi, the others where much lower, ranging from 120 up to 140. Thought a piston must have kissed a valve so towed it home and pulled the head off.

There is no obvious damage to any of the valves, although the valves for each clyinder seem to close a fraction after each other, i thought each pair of valves should close together?

The only other thing i found is that the dowel in the ignition adaptor (the one which actually goes onto the end of the inlet cam) stayed in the cam not the adaptor when i pulled the adaptor off. It had been running slightly out of true because it has cut a slight groove into the inner part of the aluminuim part of the adaptor.

I am going to have the head checked over by park engineering who over hauled the clyinder head in the first place and see if they come up with any thing.

We did manage to get it just running at one point at Bogg brothers when old man Bogg messed with the timing alot, he said the cam or ignition had become massively retarded.

Thank you to any one who takes the time to read all this and any pointers would be very gratefully recieved.

[pug309twin40s 1]

sounds like the timing belt jumped and has bent some valves.

 

strip the cyl head down and check the valves.

 

also have a look on the top of the pistons to check if you can see any marks where the valves have hit them.

 

 

 

bad luck anyway after which looks like a full rebuild.

[yorkdave 1]

i have ripped off the head and there is nothing obvious, the valves look fine and there are no marks in the pistons.

I have a new theory of what happened, when i pulled off the dizzy adaptor, the dowel peg which is ment to be solid in the adaptor had come out and had cut a groove into the aluminuim bell, i think this has caused the ignition timing to slip, due to the adaptor not being driven correctly by the end of the inlet cam! This would explain why the problem gradually got worse as the timing slipped.