Hitchcocks Forum

Hopefully somebody can help.

I have a 1959 350cc which often cuts out at cruising revs. I will be travelling along at around 55mph and the engine is singing along nicely and all of a sudden it just dies and produces an awful rattling noise. If you open the throttle it lets out a big backfire. If you just leave it for about 5 seconds of coasting it comes back to life and runs well.


Standard Amal carb at factory settings.
Standard points setup
Timing set with a DTI & disc at full advance (32 degrees BTDC)
Fully rebuilt bike and engine including rebore in the last 500 miles
All wiring replaced with good connections

Any ideas ??

I would take a look at the rockers - especially the exhaust rocker - it might be it / they are getting tight when hot and holding a valve open. When the engine cuts out and you open the throttle, perhaps it cools enoght to become free again and allow the engine to run again for a while.

Thanks for the reply Paul.

Im sure they are adjusted quite loose but it certainly seems like a logical explanation so willing to loosen them even more to test the theory.

I've heard of Indian Bullets' exhaust valve seats coming loose when very hot, has this ever been known with a Redditch engine? A.

If the barrel was rebored recently, its possible the piston is nipping up due to the extra heat generated by new ring/bore friction. Aircooled engines dont have the same dissipation potential as one which is water cooled, and the extra engergy produced by the bedding in rings goes into expanding the piston, instead of being absorbed by the water jacket. When the piston skirt catches, the rattle isnt uncommon,and the backfire could be self ignition due to a localized hot spot. Regards Paul.

Hi, had exactly the same experience, caused by a sticking exhaust valve. Vince.

As B W says, sounds like the exhaust valve sticking open, rocker tight and sticking when hot or valve stem sticking in guide, modern exhaust valves are stainless an will stick if you use a cast iron guide.

The rattling noise certainly points to something sticking in the valve train. The danger is you could have a push rod jump off. I'd be looking at this sooner rather than later.

Thanks all, i appreciate your input. Just to answer a few point which have been raised. The problem was present with the old piston prior to the rebore so I am doubtful the problem is nipping up. The valve guides are brand new bronze items from our host with their performance valve set. They have been installed and honed by a professional engine builders so again I don't think the problem is with the actually valve sticking. New valve seats also inserted and machined at the same time. I am leaning towards the first suggestion of the clearances being too tight so i will loosen them off and report back. Keeps the suggestions coming tho as I appreciate your input.

Flick, you misunderstood me, I meant the rocker arm [s] might be tight in the rocker block [s]. Any valve 'clearance' on a Bullet will increase when the engine is hot, so that won't be an issue. I would open up the rocker bearing blocks and check they are getting oil and look for any signs of seizure in them, which if present, could be the culprit.