Minor backfire after jet change?

Just got my old 1982 KZ750H3 with Keihin CV34's running again. After installing pods and drilling the mufflers, I fooled around with jets, finally changing the stock 62/125's to 68/140's. Left the pilots stock with 2-1/2 turns out. Runs quite strong and idles quite nice! Seems almost ideal.

The only problem is when I shift down a gear to let the engine slow the bike while approaching a slow turn, or a stop. Then it mildly (almost too quite to hear-but I can) backfires through the mufflers. I figure that I must be slightly off on one of the jet sizes.

Any ideas?

So was it the cable causing you grief?
Back fire is usually a lean condition in the exhaust, do you have the air system with the reed petals on the head? It could be the valve is bad? Vacuum not hooked up properly or a bad reed petal/ carbon causing to not seat.

Had you changed the jet needle clip position?

What rpm when down shifting... mid range?

Little tip Larry and Plum taught me....
Seems Pilot and mains are close or spot on so get your self running ~ 5K-6Krpm then release the throttle letting her snap back to idle position...

First thing first... Carbs synced?

* If she pops and backfires, still too lean...

** On mid range acceleration, any hesitation, fall flat on it's face... still too lean...

*** Hold at a constant 4.5Krpm, is she stuttering as if running out of gas, clogged fuel line, missing etc... still too lean...

1 clip position richer and you'll feel the difference...

BTW, condition of plugs? Carbonized, sooty, oily, bright white etc...

Good luck,

OMR

Mine pops on deceleration when the pilot jets are too lean.

Turns out the cable was fine. Where the cable wraps around the grip, there is a yellowish nylon piece the cable wraps around on. This fits into a channel in the black plastic switch housings. That channel was full of crap, and after I cleaned it out and lubed it, the high rev problem went away!! I had been sure it was closing before, as I could hear a click when I released the grip. Turns out it was clicking when the cable came taught.

Anyway, these are Keihins from 1982, and don't have adjustable positions for the rod. Yea, carbs are cleaned, synced, etc....I've gone over it with a fine tooth comb. It idles very smoothly, and accellorates nicely with no flat spots. The backfire happens if I'm using the engine to slow down for a turn or a stop. Example is I'm doing maybe 40 in 4th gear and begin dragging it down with the engine. Then it backfires. But let me explain that better, it doesn't BACKFIRE per say, but just pops a little, so quietly that wind noise alone sometimes keeps me from hearing it unless I'm listening for it (I wear a full helmet, which doesn't help hearing much). And oddly, it seems to come a little bit more from the left side then the right.

Now, admittedly, I tried jets that were too big the first few times, and being very poor (and I mean poor), I solder filled the old ones and drilled them out. It is my guess from the drill sizes that they are closest to 68/140, and like I say, it runs great, but I guess I'm not quite there yet.

"I solder filled the old ones and drilled them out. It is my guess from the drill sizes that they are closest to 68/140, "...

Whoa .... That's a new one I never thought of... :blink:

Well clip position change is nothing but time, no $$$ required... Go one richer on the clip and take her for a spin....

Good luck, let us know how it turns out for ya...

I think an occasional pop is normal. If you hear it out one side more than the other, maybe an exhaust leak at the header port?

I would expect this with "drilled out mufflers"..

Also I hear more of what the bikes noises are with a helmet on then with it off with the wind noise.

Could be a header port leak I suppose, but it sure seems to be coming from the back. As I mentioned, this year Keihin's don't have the adjustable needles. Only one setting available. Actually, I'm coming around to the drilled jets ending up a little too small (lean), and until I can afford new ones I'll have to live with it. Anyone have any real 68 and 140 jets they will sell me cheap? :laugh:

polkat wrote:

Could be a header port leak I suppose, but it sure seems to be coming from the back. As I mentioned, this year Keihin's don't have the adjustable needles. Only one setting available. Actually, I'm coming around to the drilled jets ending up a little too small (lean), and until I can afford new ones I'll have to live with it. Anyone have any real 68 and 140 jets they will sell me cheap? :laugh:

The header leak will cause the poping at the back/end of the muffler.

You can shim the needles with small washers to make things richer.

PS. Its called afterfire. Backfire is when it comes through the pods/intake.

Yes, the term "afterfire" makes more sense to me. I have asked before about 'shimming' the needles. Got a lot of responses, but no real instructions (like what washers to use and how).

Okay, I can sort of understand how a leak at the exhaust ports would cause a back....er, afterfire at the muffler ends, but if there is indeed a leak at the headers, why doesn't it afterfire more often, instead of just when slowing the engine with a load?

I have run my hand around the exhaust ports (at the head of course) feeling for blowing hot air (did this at idle) but can't feel anything. How do you check for this?

Do you have your reed valves in place on the heads? If these are not working properly it can cause back firing.