Chasing my Carb Tail

Hope you guys can help me diagnose my fuel problem. I'm trying to resurrect my 1980 KZ750 twin in the Projects Forum www.kzrider.com/forum/11-projects/591126...-twin-revival#613071 I've been banging my head against a funny problem and I'm hoping someone here can shine some light through my frustration.

The bike kick starts in 3 kicks or less even after sitting for weeks.

With the choke on(lever up for my year) it warms up and idles well but will quickly sputter and stumble with very little throttle(1/8 turn)

If I lower the choke half way the rpms increase significantly...?

Choke off and the rpms drop almost to a stall and the littlest throttle instantly kills the engine. Adjusting the idle screw didn't make a difference; I'd turn in all they way in an iddle wouldn't increase, just sputter

I've never troubleshooted an engine that starts up easily and idles well on choke but quickly stalls once I put her in gear and apply the slightest throttle

I first thought I had a float level problem being too low. So I checked my levels and adjusted and the engine would stall when I take off the choke. Put the float level back to it's original position (almost 0mm) and now the engine stays alive when I take off the choke. The problem of sputter and stall persists across the board.

Note: I've cleaned the carbs by disassembling, removing the jets, boiling all metal parts in water, soaking in gas for a week, copper wiring all ports and finishing with lots of carb cleaner and compressed air. I forgot to remove the air mixture screw so that needs to be checked.

ANY IDEAS?

Thanks.

Can someone tell me if the choke bypasses the idle pilot jet on these carbs?

Sounds like you're not getting enough fuel. The choke would make the engine artificially RICH. You've also got the CV carburetors. They open with engine vacuum. Not with throttle position. You could have a pin hole in a diaphragm..Where are you located? Hopefully someone is nearby and can give you a hand. This kind of thing is very hard to diagnose long distance.

KZ750 twin Service Manual (takes a few minutes to download)

bublicpucket.s3.amazonaws.com/Manuals/ka...llwith80and81sup.pdf

From the Index --


Pilot system at page 133



Supplement for 1980 KZ750-G begins on page 220.

Despite efforts to-date, would suspect that the carbs' pilot circuit passageways are not yet perfectly clean.

Good Fortune!

Does the problem persist after letting the engine warm up for 5 minutes?
Does the bike have the stock airbox and air filter?

Sounds like you have dirty / clogged carburetors. Yes the choke bypasses the idle circuit. That is why on clogged / dity carbs the bike will run on full choke but when you close the choke it dies, the idle circuit is clogged. And no pouring some carb type cleaner in the gas tank and running it through the system WILL NOT work.

Problem in the pilot circuit, jets may be clogged.

You may also have a vacuum leak making everything too lean.

Best to clean the whole fuel system top down (tank, petcock assy, install inline filters, clean carbs and adjust float levels).

You can TRY an old trick: take the gas lines off the carbs and remove the drain screws. Then get some carb cleaner spray and hose the carbs down through the gas inlet to clean the crud out.

Install the drain screws and then shoot them full of carb cleaner and let sit overnight. This might help, but it's better to clean them right but the rinse does remove some dirt.

Hemp wrote: . . . first thought I had a float level problem being too low . . . checked my levels and adjusted . . . Put the float level back to it's original position (almost 0mm) . . . forgot to remove the air mixture screw so that needs to be checked. . . .

If not already done, should perform the "clear tube test" to confirm that the actual true fuel level is within specs (typically 3mm below surface where float bowl meets carb body).
The process of physically measuring some distance between whatever with a dry carb held upside down and float bowls removed is an unreliable means toward achieving correct fuel level, and is usually unsuccessful.

The air mixture screw should be removed incident to cleaning the passageways.
(See carb diagram earlier posted.)

Good Fortune!

Thank you guys for the fast responses,

I originally suspected the pilot jets and have been removing and dusting them every time I drop the bowls. So far they're coming up clean.

The engine is definitely warmed up. I let her run for about 10 minutes every time I try to figure out symptoms/causes.

I'll take your advice and reconsider vacuum leak. I never thought a small leak would cause such a rapid stall but what do I know.

Question: I'm running with the stock air box; but I never plugged the port that elbows down to the engine case for emissions control. I wonder if this has anything to do with it?

Question: I replaced my original Petcock with a simple NPT valve on my gas tank. Because I can idle all day long on choke I assume this has nothing to do with my insta-stall problem..?

My check list so far goes...

Clean carbs again, especially fuel mix screws and pilot jet,
Test for vacuum leaks around air box, carb body & diaphragm.

Thanks for your input.

The little elbow shaped hose that normally runs between the top of the crankcase and the stock airbox is the crankcase breather hose. I don't think the fact that it is not in place would cause the symptoms you describe, but the port on top of the crankcase should not be plugged up or the crankcase will not be able to "breathe" as the pistons rise and fall. Ed

Not just the jets -- also clean the passageways.




When carb cleaner is sprayed into any of the "red arrow" locations, the carb cleaner should exit through both orificies into the carb bore.

It will of course be necessary to remove the pilot mixture screw #3 in order to spray carb cleaner into and through the orifice regulated by the pointed tip of the mixture screw.

And remember to wear eye protection, because carb spray always backfires -- Always.


Good Fortune!