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Main Jet...

Bazlerker

Well Known and Active Forum Website User
VOC Member
Having fitted Triumph/BSA style air cleaners on adaptors to my "C".. I find that the plugs are a bit sooty, Is there anyone else running the same air cleaners that can give me a suggestion as to main jet size?..
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
that can give me a suggestion as to main jet size?..
Although what follows doesn't directly answer your question (actually, it doesn't even indirectly answer your question...), several years ago I measured twelve Amal 500 main jets on my SuperFlow flow bench. Nine of them were within one jet size of the marked values (i.e. between '491' and '509'). However, three of them were as much as 3-1/2 jet sizes too large or too small (i.e. the smallest was equivalent to a '465' and the largest a '535'). The implication is that, no matter what the correct size someone tells you is necessary for your machine as it is presently configured, there's a ~25% chance one of the replacement jets could be significantly too large or too small, thereby compounding your tuning issues.

And, since since you didn't ask about needle jets, this is a good place to address that non-question as well. Again a few years ago I came across an eBay auction for 27 NOS Amal needle jets, each sealed in a plastic bag from the U.S. and Canadian distributers and printed with the part number. Twenty-two of the 27 were marked '106', which some people will insist means their ID is 0.106", but I claim it means they are 0.1065" (more on this in a moment).

I measured all of these with my Starrett bore micrometer, which has an accuracy and a resolution of 0.00005" (a half of a ten-thou.). Similar to what I found with the main jets, 17 of the 22 were nicely bunched at 0.1065". That is, 77% had the correct diameter. However, 5 of the 22 (23%) were significantly too small or too large. These five outliers clustered near '105' and '107' making me think they were mismarked, not incorrectly bored.

However, it is important to note that all of my measurements were on jets made by the original Amal company, not by one of the several that have traded using that name since then. I have no way of knowing at present whether the quality of newer Amal jets are better, worse, or the same as the originals. Also, keep in mind that aftermarket suppliers also have produced jets, some of which are even marked "Amal," so it's really the luck of the draw if you don't have instruments to measure them.
 
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Howard

Well Known and Active Forum Website User
VOC Member
Having fitted Triumph/BSA style air cleaners on adaptors to my "C".. I find that the plugs are a bit sooty, Is there anyone else running the same air cleaners that can give me a suggestion as to main jet size?..

Is this after a high speed plug test? If not it may be down to air screw adjustment - I think the Triumph air cleaners cover the pilot air hole.

H
 
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