ported heads/ airt jet?

[boediddly]

I had some light porting done on my heads basically just cleaned up. the bike idles like a champ but when i give it throttle it falls flat ignitions good compressions is good and after going up and down on the jet needle and up and down on the jets (still falls flat) ive come to the conclusion that my emulsion tubes( main jets )are not picking up any fuel (will run with choke on with throttle response)so ive come to the realization that I need to adjust my air jet its a 79 with stock 180s does anybody have any experience with this sort of thing?and is the air jet on the 79 the same kind as the 80?

[petejw]

hey there,
the pilot air jet supplies air to the pilot (idle) circuit,
it doesnt supply air to the emulsion tube,
air from the emulsion tube is supplied by a
port on the right side intake bell and is not adjustable.

if you do a search, theres a cpl of good posts with diagrams
of the carby including fuel and air flow, once you understand
how the carbys function it will allow you to diagnose problems
much easier.

what are you carb settings atm?

possibly a rich fuel float setting/pilot jet could give you the problems
you describe.

[TADracer]

You did not say what mods you have other than the light porting nor did you say if it runs well after you get up over say, 3500-4000 RPM. ANYTHING you do to heads, cams, air cleaner, ignition timing or exhaust, can affect your low/mid performance right off the bat and give you poor idling and/or an off-idle stumble.

I have ported heads with some rough-up work done on the intake runners and the exhaust runners are opened to 1/8" diameter less than the exhaust crush gasket and my valves have a 30 degree back cut and that alone made my machine want one size up on the pilots.

Considering if all your other things are set right, if you have an off-idle stumble that spits back through the carbs and it improves when the enrichener (choke) is on a notch or it improves when your motor warms up then you are probably a bit lean on the pilot circuit. Pilot jets will be involved all the way up past 1/4 throttle opening, which is most of your riding and cruising throttle position.

If you have a 79 then your stock pilot jet will be a 42.5 and your stock main jet will be 137.5. If your idle mixture screws are turned out 3 or more turns to get a smooth idle that might be another sign you need to go to a higher pilot jet and solve your problem. Next size up is 45 on the pilots.

If you do go up a size then your will need to syncronize your carbs and reajdust your idle mixture screws again and you might find the best idle is around 1-1/2 turns out from lightly seated which would be normal. Also, put your jet needles back on the center setting and check your float level is right. Stock setting is 25.7mm above the gasket surface without the gasket then go from there.

If you change any jets, use Mikuni jets. The number on them represents CC's per minute of flow at a pre-set pressure. Other jet makers might measure their jet flow differently and that can throw you off too.

Its trial and error but when you hit that sweet spot you will be well rewarded.

[dbeardslee]

Another test you can do for rich or lean is to set your idle at 900 rpms with the motor warm, blip the throttle (maybe 1/8 turn) and watch what the rpms do. If it hangs up high for a few seconds and then drops back down it indicates lean. If it goes the other way (rpms drop) it indicates rich. If it goes up and immediately drops back down to 900 rpms, it's correct. That's only going to tell you what slow circuit is doing, but main jets do affect the low rpm range.

[Elevenator]

This is a typical problem with port jobs....great or small.

I had the head ported on Zilla years ago. When we got it buttoned up it with all the mods it made about 33% more power over stock. But it had a bad stutter from right about idle (2000 rpm ) to 3200 rpm. Then it would come alive and feel like you were shot out of a cannon.

We tried everything....we changed the pilot jets...the mains...up and down and round and round.

Finally we went to different needles. The needles that came in a stage 2 dynojet kit were close....they helped alot but they didn't fix the problem completely. We wound up putting a set of needles froma dynojet kit designed for the FJ1200 and got it worked out.

Of course I was running Megacycle Stage 2 cams in the bike and FJ1200 carbs...so that likely made the trouble shooting routine a bit more intense.