Bonneville America Forums Forums Technical Forums Performance Mods An interesting main jet question.

I made the mistake of starting my latest mod without double checking my parts. It seems the shop sent me the large head mains instead of the small ones. What would be the problems if I used them. the only difference in them is the large head is about 1/16th inch longer. It would go that much deeper into the bowl. There isn't any obstructions and the threads are the same. I am only asking as I already did everything else first of course. At 60 you would think I would know better.

What is the manufacturer & part # of the ones you have?

They are EBC KJ127.5 slot head type for Keihin carbs. The orifice is the size I need but the head is taller and protrudes about a 1/16 or more down into the float bowl. They work, but.......... just wondering if there could be any complcations later?

I tried some of the tall main jets in the CVKs once. If i recall correctly, it caused a fuel level problem and I had to take them back out.

can't the floats be adjusted to compensate?

Yea, you can bend the thin bracket to adjust the float elevation.

I am not sure how that could happen! The jet is not in the way of the metering valve or the mechanism to stop it from going up and closing. If it did, it would flood all the time. The small space it takes up merely means there is a little less fuel in the bowl. My main concern was that the jet would be closer to the bottom of the bowl and not pick up as well.

I think the bigger head on the jet is at the wrong level in the bowl creating a fuel level problem and if you adjust the float then you will end up with a pilot and mid range level issue.

According to my manual the fuel level is just above the mating surface of the float bowl and the main body, that puts it well above the main jet. I know there is probably a good reason for not using them and I would like to know it. the short ride I took down the road a mile and then back went fine. Does anyone know what will happen if I leave them in?????????

I was going to look at my notebook for the speedmaster last night to see what the issue had been, but totally forgot about about.
rummaging thru my memory (sketchy at best) it seems that the fuel level problem I mentioned was more related to the main jet bore height relative to the needle tip. It would be like shimming the heck out of the needle.

Will it hurt anything to leave them in? no reason it should, worse case it runs crappy and you have to change them out.

You may stumble onto something you like. Never know till ya try it.

That's what my brain wasn't comming up with. The distance from the needle end to the main! So I just went out to the shop and put the caliper to them. Using my old eyes, contact lenses, reading glasses and the magnifing glass.(damn it's crap getting old) It looks like the difference is between 1/2 and 1 mm. A shim is .65mm. Not to much difference. I figure at worse it will run a little rich.

It is wednasday and I rode to work today, 108 mile round trip. I looded at the plugs and they look good, might be a tad lean, probably could have put a 130 in and been fine, but a 126 would have deffinetly been lean. I'd post a pic here but I don't have a way of doing that right now.

If you can judge a half mill,theres not to much wrong with your sight