Strange jets...

[hobbsy84]

Doing a little top-end work to my '81 H, (little big bore kit) and thought I'd better get around to the jetting side of things. Now, the bike I've seen run, but it wasn't pretty. As such, it's now been a bit of an on-going complete re-build.

I've cleaned the carbs out, there's no problem there, but now I'm looking at jet sizes for the jetting up for the bore kit. Thing is, the Clymer manual states 120 / 115 main jet sizes for a H model, and 137.5 main jets for earlier model carbs. The problem is that my carbs seem to have 132.5 mains in it. Can anyone think why smaller than stock jets would be in the carbs? Am I missing something obvious? (Wouldnt be the first time )

Also, in this case would it be correct to assume roughly 140 mains would be good for the big bore kit? Cheers.

[DiverRay]

You will NEED to find out what carbs you are running!! An easy way to tell the early model carbs is the vent tube above the fuel inlet tube between carbs 1&2, and 3&4. If that isn't there, you probably have the later model carbs. If you DO have 132.5 on later model carbs, I would NOT go larger!!
Once you are done with the engine, run the bike and see how the spark plugs look at ALL RPM ranges. If idle and low RPM, to about 3,500 look good, you know the pilot jets are correct. If the plugs stay light tan all the way through the range, you are good to go. If you have sooty or dark plugs at the higher RPM range, you will probably need to go SMALLER, say a 130 main jet.
Remember, this is all based on how the bike was running BEFORE the work, so if it was rich before, it will still be rich.

[hobbsy84]

Pretty sure they're the earlier model carbs, for starters the floats are the round brass style ones, including the earlier style float needles / seats. That, and the vent line you were talking about is there too.

[TopCatGr58]

Hey Hobbs,

It does sound like someone has swapped out the later model 81 carbs for some early model ones, but we don't have access to the AUS. models microfiches! You jet according to the carb, not the year. The early model carbs share fuel from the MAIN jet, thru that little hole in the side of the main jet needle/nozzle/emulsion tube, thru a small tunnel between the main jet tower and pilot jet tower, the pilot jet tower should be capped off. IF it wasn't, that would partly explain the poor performance, sucking up too much fuel directly from the bowl, instead of just thru the tunnel!

SO.. stock for these carbs were 137.5 IIRC, rejetting isn't necessarily needed as much just for the big bore kit, as for intake/exhaust mods. If you're running Indy intake filters, and aftermarket exhausts, then you'll probably need to go up a few sizes, per the jetting guide in the tech tips, ~145.0 max. As has been stated, if stock intake/exhaust, just clean them, synch them, and then test them with new plugs....however, you'll want to get thru the ring break in period first, and then rejet/adjust according to the plug color, or lack thereof!

Hope you get it running soon, or you'll be out of nice summer weather down there before you know it!
T.C.

[hobbsy84]

You're right, the pilot is capped off, I think the terrible running was mainly because of dirty carbs before the re-build. I've heard the carbs are a big enough problem on these bikes without half of the passageways being blocked...

The exhaust system was more than a little rusty, so the old mufflers are probably going to have to go, The headers are rusty, but no holes, so they'll do for now until I find something better. Just a set of slip on mufflers is going to be the only difference.

Still not sure why the main jets fitted at the moment are smaller than stock spec, but it'll be easy enough to remedy with a new set of jets.
Cheers for the advice, greatly appreciated. I'll have to get some photos up once she's up and going. Hopefully will be done in the next couple of weeks (Famous last words...).