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Matt
Matt, Engineer
Category: Motorcycle
Satisfied Customers: 20590
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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I have restored a 1971 Triumph T120 Bonniville Starts easy,

Customer Question

I have restored a 1971 Triumph T120 Bonniville
Starts easy, Idles fine. But when acceleerating it misses. No audable backfire.
I have cleaned and balanced the carbs....
Could this be a condenser/points problem? Seems to be missing at all speeds....
Thanks
David
Submitted: 1 month ago.
Category: Motorcycle
Expert:  Matt replied 1 month ago.

This could be an airleak after the carbs, any air dragged in here isn't 'seen' by the system and so not compensated for and can lean the engine out causing rough running.

As its a mechanical fault you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.