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International carburation

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International carburation

Posted by geoffrey_stovold at June 17. 2017

I have got an Inter special. It is a 1949 iron engine in an early 50's wideline frame. The guy who built the bike fitted it with a 930R concentric carb with a 270 main jet. The bike starts and runs well on this set up until you get to high end revs on wide open throttle then it starts to stutter and misfire a bit. I thought at first, fuel starvation and tried larger jets. They just made the problem worse and the bigger the jet the worse it got. So I am now thinking too rich? and have ordered some smaller jets to experiment with. I am wondering what others think on this. I don't know what the compression ratio is but I guess quite high because it is really quite hard to kick it over compression. (no it hasn't got a valve lifter!)

Also I am considering a 10TT carb. Expensive but would look right on a bike like this. Does anyone have any views or experience with these? Will it improve performance? Do they tick over? Currently the bikes ticks over very nicely. I guess you can flood it with the separate float chamber to aid starting?

Nearly forgot. It is a Model 30

Re: International carburation

Posted by Michael Rettie at June 17. 2017

This may get you in the ballpark:


Re: International carburation

Posted by Mikael Ridderstad at June 18. 2017

I've experienced full throttle problems with too big carbs on a couple of bikes. My -56 ES2 goes quite well with a 928 Concentric.The CB250 twin racer is happy with 30mm Mikuni's. The CB350 racer goes like you described it with 32mm Mikuni's (sh*t). The Vincent Comet is not entirely happy with 930Amal.

There is a reason why so many Gold Star owners has replaced the TT with other carbs. Even experts seems to find it difficult to get idle and small throttle openings go well on a TT carb.


Re: International carburation

Posted by David Cooper at June 18. 2017

Have you tried a plug chop as per the book?  Not all that easy on today's roads perhaps...

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