1981 Yamaha TR1 popping exhaust

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Q: Hi, great article in a recent mag on the TR1. I own a 1981 model which I rebuilt from a basket case. It’s a lovely bike and as John Nutting says the torque is wonderful. However since I built it I get a ‘tut-tut’ in the carbs when ticking over and what appears to be a popping in the exhaust on final stages of over-run when stopping. I took it to be a problem with the fuel valve (I read on the web to dis-connect it due to this popping, however that had no effect.) Unfortunately I can’t buy a new one as Yamaha don’t make them anymore, and Fowlers (main dealers) don’t have any info on a bike more than 10 years old! I’ve bought a replacement carb and exhaust system (they are in a better condition) and hope to fit them soon. The thought there is the usual old beliefs that it could be a leak on the carb or exhaust connection, but I doubt it as I put a lot of care in the rebuild. The bike pulls like a train, so I’m not thinking timing is out: the ignition’s fixed, and the valve I’m sure is correct. Any suggestions? I can live with it but would like to sort it.

Jim Wood

A: As always, you would be advised to get hold of the Yamaha manual if you can. This popping can be caused by a number of things and as you say, the mixture control valve is one of them. It does seem to be a source of unreliability but you have removed that possibility. For owners who wish to keep it fitted, the manual gives a test procedure which ensures that it is creating a vacuum during operation. The other point is to ensure that the vacuum hoses connected to it do not have any air leaks.

The carburettors are the second suspects and, as on any other similarly equipped bike, they need to be to the correct specification in terms of jet sizes, fuel levels etc, correctly adjusted and fully serviceable so that includes items such as the diaphragms and of course the carb mountings. If you intend to replace the carbs with your new-to-you ones then naturally you will have to carry out all these checks before fitting them. You will be fitting a different exhaust so make sure that it fits properly and there are no leaks anywhere.

Incorrectly adjusted valve clearances usually cause rattles or loss of compression but another fault associated with clearances which are too tight is this popping.

Finally, it is possible that a slight ignition fault such as a bad earth or another slightly dodgy connection might be the cause.

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