Hitchcocks Forum

My new 1962 350 has a slight back fire on the exhuast, but only now and again. It starts first time every time and does not feel that quick at all. Does not rattle or smoke, 26,000 miles on clock. Any help would be nice.

Is the backfire occuring when tghe engine is under load (Pulling away or accelerating) or on the over run (slowing with the throttle closed)?

If it is on the over-run with closed throttle then it is likely to be an airleak at the exhaust/ head joint.
There was a recent topic on exhaust sealing.
Feeling "does not feel that quick at all" may be relative to what you've been riding before. A few years ago at a Watsonian Squire Open Weekend I test rode a new 350. It felt gutless after riding my 500 to the event.

A slight valve leak might also cause a back fire and a lack of performance. Does it hiss when you kick it over very slowly?

Thanks for the replies chaps.
Normally pops on over-run e.g. changing gear or slowing down. No hissing noises detected. Only does this now and again. Exhaust seems a sound fit to the head, no signs of a leak or soot around the head joint to exhaust. ??

Hi Colin

I agree that the problem is almost certainly a leak at the exhaust port/pipe joint. You may not see any actual soot if the problem is not too great. The famed "bathroom sealant" fix will work, but I am now going to put a spanner into the "universal admiration works" for this fix. While it is certainly effective initially, no matter how carefully I apply the stuff, I have found it to be only a temporary solution and the popping and banging always returns. However, if you are happy to dismantle the pipe and re-apply at intervals, it's fine.

Cheers

TimG

I use high temperature silicone gasket stuff that really is not expensive. Once the joint has been assembled I try to wait 24 hours before firing up.
Works for me. There will of course be other opinions that may be different. Merely my 2d

Well you learn every day, I thought the timimng was out. I do have to take the exhaust off to do another job, so silicon will go on when I refit the pipe.

thanks again chaps

Colin-------make sure that you clean the exhaust port with no trace of black carbon left---try some course wire wool / scotchbright, and the same on the pipe-end that fits inside. Firmly smear a little at a time to ensure it takes to the aluminium and the pipe. You might find a some water or spit helps tool it along. Aim to use more than you need---the excess will ooze out and you can remove with a damp cloth. The pipe has a long way to go before the first support bracket and it's a wobbly one at that. I suggest an extra support using a heavy duty spring at the pipe exit to pull the pipe inwards, like the scramble boys do. I've yet to do mine but it's a known mod. especially on the 2-strokes.