Pilot jet size & carb ID

I've been trying to track down a popping/coughing sound at idle on my '79 KZ750 twin. This is a recent purchase and I don't know much about the bike's history.

It only happens on the right side of the bike.

I've checked for air leaks as best I can and can't find any. I've also taken apart and cleaned the carbs with no luck. Timing, point gap, plugs, etc. all set.

I pulled the float bowl again today and when I removed the pilot jet to triple check that all was clean in there, I noticed a little stamp on the side of the jet.

I can barely read it, but it sure looks like a 40. Stock pilot jet size for a '79 KZ750 is supposed to be a 45 and I've had people tell me they run even better with a 50.

So my first question seems a little dumb, but I'm assuming that 40 stamped on the side means that's the jet size right?

My second question is, how to I identify whether or not the carbs are stock? Someone either replaced this jet with the wrong one on accident or maybe they replaced the whole carb? I'm just guessing at this point and I'd like to know what I've got.

The bike has after market exhaust so it's possible that some previous owner rejetted on purpose after installing the new exhaust.

It's dark and rainy now so I can't go pull the other bowl to see if the pilot jet in the other carb has the same stamp.

Any thoughts on this would be appreciated.

The bike has the correct pilots installed when it can achieve the correct pilot mixtures within 3 turns on the mix/air screws. The correct pilot mixture is very close to where max rpm is achieved with screw adjustments.

The number would be the jet size, but going from 40 to 45 is a very large jump, 40 to 50 would be rediculous.

At most, you should only require one size increase on a pilot.

Tune your mixture screws correctly and your popping will likely dissapear fine.

Dont know what carbs you have unless you post a pic.

Good luck.

Thanks Tek9. I've tried the pilot screw procedure you suggested in my other thread and can't get the popping to go away.

I'll take some pics of the carbs tomorrow if I get home before dark.

Regarding the pilot size, so baby steps huh? More like a 42.5, if I decide to try a change?

I actually found a photo where the carbs are pretty easy to see.

I'm now fairly confident these are the stock carbs. They're stamped Mikuni Koyota Co Japan LTD on the top and they look pretty much exactly like the carbs on the bike on the cover of my manual.

I tried to remove the other pilot jet just to verify they're both 40s. Unfortunately the head of the other jet was a little stripped before I started. No matter how careful I tried to be, I still didn't get it all the way out before the head stripped the rest of the way.

It's still too far down in there to get out with plyers. I guess I'll be drilling it out. (That might require its own thread.)

So back to the jetting question. I've got after market pipes (pic below), stock carbs and it's looking like my pilot jets are 40s instead of the stock 45s. Does it make sense that the previous owner might have needed to go down to 40s when he installed the after market exhaust?

I got the other jet out and fortunately they did match. Both 40s.

The local shop didn't have my jet in a 42.5, but they did have some 45s. That's what was stock for my bike and carbs, but the previous owner put aftermarket exhaust on and I guess rejetted at that time.

I still wanted to find out what would happen with a larger pilot jet and for $3 each I figured I'd see what happened. So here's what happened...

The bike started up right away just as usual. Full choke, no throttle, no problem. Idle was a little high, but I adjusted the set screw to compensate.

As it warmed up it started to falter. When I touched the choke it really seemed like it would die so put it back on full. Shortly there after it stalled and died. Then it wouldn't restart right away.

I let it sit for about 15 minutes restarted it and got the same behavior again.

Is that pretty typical for too large a pilot jet?

I've still got two 40s and will swap back to those tomorrow.

As you've probably guessed, I'm fairly new to this stuff. I gather that when you change exhaust systems you usually rejet, but I don't really understand why. Do you usually have to change the main jet too or just the pilot?

Whoa... sorry, I was gone there for a bit



The Pilot is a 45.

OK, so you said that as it started up on choke the idle was higher - thats good. DO NOT adjust the idle control knob while the choke is activated, or your defeating the choke circuit.

Typically when you start a bike on full choke, it will rev up slightly to say 2000rpm or higher for about 7-10 seconds, then it will start to faulter and sputter slightly. That is when you begin to adjust the choke (NOT IDLE) closed some to increase the engine sped back up some to help it warm up. Continue to manipulate the choke as you like, until you can close it fully and the engine idles.

DO NOT adjust the idle until you have ridden the bike for at least 30 minutes.

The bike can start and die from to much fuel through the choke circuit, even if the carbs are closed too much to idle with the choke off. ( BY ADJUSTING THE IDLE SCREW TOO LOW)

When you just change the exhaust system, typically you would change the main jet only, and maybe bump the needle one notch if your midrange was sluggish, but the pilot would generally still be stock. If there is a popping sound when you release the throttle from high rpm, then an adjustment to the mixture screws would solve that.

If it pops all the time, even at idle - one/both of the cylinders are not reveiving enough fuel in the air stream to fire properly. This could be an intake manifold leak, or a seriously mal-adjusted air screw.

When you put on a high performance exhaust it enables more air to move through the engine wuicker, and thus a larger jet is typically used to add more fuel to compensate.

Try letting the bike choke rev to about 2500 by adjusting the idle back up. get the choke off, and manipulate the idle at the same time so it stays around 1500, then use just the idle knob to drop the idle to 1000.

Now you can adjust your air/mix screws...

Good luck!

Let me know...

PS. Are both exhaust pipes getting real hot? Check with a mist of water - should sizzle instantly after a minute of running.

Have you checked the fuel levels in the carbs? Verified flow into the bore with spray cleaner?

Yes I've checked the fuel levels at least twice. But given that I'm new to this I might do it again just to be sure.

I don't know how to verify flow into the bore (and can only guess at what you mean). Do you just mean spray some carb cleaner into the air intakes? When I do that the engine bogs down.

I have done the water mist test and both pipes sizzled.

Sounds like I've been going about my idle adjustment all wrong. I'm definitely not waiting till I've ridden half an hour anyway. I'll try what you suggested tomorrow before I try swapping the pilot jets back to 40s.

I'm confused as hell now about why the bike had 40s in for pilot jets and how it was running at all. I had figured they'd been changed to compensate for the aftermarket exhaust but it sounds like that probably wasn't the case.

The bike seems to be mostly stock except for the exhaust and little cosmetic things like the hand grips.

Topper wrote:

Yes I've checked the fuel levels at least twice. But given that I'm new to this I might do it again just to be sure.

If you've done it twice, it probly isnt necessary again.

I don't know how to verify flow into the bore (and can only guess at what you mean). Do you just mean spray some carb cleaner into the air intakes? When I do that the engine bogs down.

If both pipes are getting nice and hot, you should be ok, but what I meant is when you have the carbs off the bike, you spray cleaner through the pilot passages and look down the bore to make sure that the spray is passing through the carb just like the fuel stream.

I have done the water mist test and both pipes sizzled.

Right, very good.

Sounds like I've been going about my idle adjustment all wrong. I'm definitely not waiting till I've ridden half an hour anyway. I'll try what you suggested tomorrow before I try swapping the pilot jets back to 40s.

Very good idea. Should work ok, then you can turn your mix/air screws to find the highest idle point. Should be with 3 turns from lightly seated, and if not, the a pilot change is required.

I'm confused as hell now about why the bike had 40s in for pilot jets and how it was running at all. I had figured they'd been changed to compensate for the aftermarket exhaust but it sounds like that probably wasn't the case.

Who knows... PO is a tool maybe he ran 40's cause he used colder plugs, or low fuel levels, or retarded ignition timing. Could be many reasons.

The bike seems to be mostly stock except for the exhaust and little cosmetic things like the hand grips.

Shouldnt be too hard to get going nice then

Checked timing?

Valve gaps?

Compression?

:laugh:

Cheers.

Topper wrote:

...bike started up right away just as usual. Full choke, no throttle, no problem...As it warmed up it started to falter. When I touched the choke it really seemed like it would die so put it back on full. Shortly there after it stalled and died. Then it wouldn't restart right away.
I let it sit for about 15 minutes restarted it and got the same behavior again.
Is that pretty typical for too large a pilot jet?
....

If not already done, would assure valve clearances being set to specs.

A too tight valve may cause the symptoms as described, where cooler engine momentarily seems to run okay, but then falters as the engine warms and loses compression when valve clearance disappears.

Good Luck!

I've set the valve clearance. I have not performed a compression test. I'm borrowing a comp tester from a friend tomorrow night and will test on Sunday.

The plugs that were (and are) in the bike when I bought it are B9ES which is the coldest of the three plugs recommended in the manual. I was told they were new when I bought the bike.

Topper wrote:

I've set the valve clearance. I have not performed a compression test. I'm borrowing a comp tester from a friend tomorrow night and will test on Sunday.

The plugs that were (and are) in the bike when I bought it are B9ES which is the coldest of the three plugs recommended in the manual. I was told they were new when I bought the bike.

Excellent on the valve clearances...

A good compression tester is a required tool for maintenence on these bikes. The numbers increase considerably on these bikes when hot and as such, that is when you should perform the test.
A quicky way to see if a bike has enough compression to run is to remove the spark plugs and press your thumb down hard over the plug hole and turn the starter over. No matter how hard you press, it should pop your finger off hard. Dont worry, it wont suck your finger in

There you have it. Some guys liked to run lower fuel levels and colder plugs with lean jetting. Makes some bikes more "peppy", but not all, and it can be dangerous, as certain points in your fuel map can be unknowingly, seriously lean.

Swap the plugs for the B8ES. The performance with the stock jeting will be much more in tune with a street machine.

Right on... getting there :laugh:

Off to work now... ttyl.

Oh, one more thing. If you have the means, buy yourself a colortune plug, you'll love it - aand it makes tuning the mixture @ idle and testing carbs a blast.

Also... :laugh: If you have the carbs off the bike again, reseal the carb to engine holders to the engine with some good Permatex Motoseal, or another gas resistant gasket or epoxy. Leak proof re-assurance.