Sputtering Under Hard Acceleration

I was just looking at the wiring diagram for yours. I don't see a resistor in the points/ coil circuit like some other models with points. Kinda surprising actually. Seems like most of the points stuff would have one, to lower the voltage across the points to prevent pitting/ arcing. When you got your new coil, did you get a 5 ohm or something else? What does the Primary loop measure on your new coil?

Ok, re-checked timing and it looks good. Advances smoothly and stays there.

Battery Voltage: 12.05v
At the Coil: 11.45v

Primary coil winding Ohms: 4 (spec is 3.2-4.
Secondary coil winding Ohms: 13.6 (spec is 10.4-15.6)

I am running with resistor plug caps (5k each) but not resistor plugs.

So other than the possibility of seeing if running without the resistor plug caps fixes this, I'm back to thinking this is a carburation issue.

Topper wrote: Ok, re-checked timing and it looks good. Advances smoothly and stays there.

Battery Voltage: 12.05v
At the Coil: 11.45v

Primary coil winding Ohms: 4 (spec is 3.2-4.
Secondary coil winding Ohms: 13.6 (spec is 10.4-15.6)

I am running with resistor plug caps (5k each) but not resistor plugs.

So other than the possibility of seeing if running without the resistor plug caps fixes this, I'm back to thinking this is a carburation issue.

Ignition aspects look good.
If not already done, would also visually confirm a fat blue spark on plug tips.

As to carburetion, if not already done, would perform clear tube test to assure fuel levels are within specs, because the fuel level will impact the mixture.

Given correct fuel levels, would check condition and fitment of the throttle slide diaphragms. And that the slides move freely.

Yet to be determined is whether the mixture is too rich or too lean when under hard acceleration, and if so, what is causing the condition.

Good Fortune!

I have set the fuel level with the clear tube method. (I've also synced the carbs)

Slide diaphragms look good. The slides seem to move up and down normally. I do seem to have a nice spark at idle.

I moved the clip on the needle. It was at position 4 and I moved it to 3. If I understand the needle clip positions they work basically like this:

1 lean
2
3 <- I'm now here
4 <- I was here
5 rich

This seems to have made some difference though honestly it's a little hard to tell. The problem didn't go away, but its nature changed a little. The stutter and misfire under hard acceleration seemed a little more smooth, more like it was running on one cylinder.

I'm not sure if that means I made it better or worse. But I guess the next step is to move the clip up one more notch and see what happens.

If not already done -- would test run with the fuel tank cap ajar to see what if any effect it might have on the stutter under hard acceleration. This test is to rule out the possibility of an obstructed fuel tank vent as causing the problem.

Is it for sure that gasoline flows freely through the petcock?

Good Fortune!

Topper wrote: I have set the fuel level with the clear tube method. (I've also synced the carbs)

Slide diaphragms look good. The slides seem to move up and down normally. I do seem to have a nice spark at idle.

I moved the clip on the needle. It was at position 4 and I moved it to 3. If I understand the needle clip positions they work basically like this:

1 lean
2
3 <- I'm now here
4 <- I was here
5 rich



This seems to have made some difference though honestly it's a little hard to tell. The problem didn't go away, but its nature changed a little. The stutter and misfire under hard acceleration seemed a little more smooth, more like it was running on one cylinder.

I'm not sure if that means I made it better or worse. But I guess the next step is to move the clip up one more notch and see what happens.

Is the 4th clip the stock position ? Don't think you'd want to lean this out from stock. What do you guys think ?

I'm not 100% sure, but I think position #4 is stock. Pipes are aftermarket 2 into 2 Mac Exhaust. They were as close as I could get to stock.

I haven't tried position #2 yet but hope to later today.

Google or bing 1979 KZ750 carb jetting sizes KZ forum and look for what Bountyhunters wrote. I think it looks like the clip is in the 3rd mid position stock so it might have been one step richer in the 4th.

I'm really sure the 2nd clip is going to be too lean for you so I don't think you should even try that. We need Bounty hunter and Steele to chime in on this as they really know the 750 twin.

Since you have the Mac pipe.... what cleaner do you have? Maybe you do need a little jetting help but these two guys are gonna help you most on that.

Looks like you are right, middle position #3 is stock.

www.kzrider.com/forum/3-carburetor/44245...8-on-a-76-kz750-twin

Seems like trying one slot lean (#2) is worth a go.

I'm at a loss here. In position #2 it pretty much behaved like it did in position #4.

In the middle position (#3) it seemed to run on 1 cylinder under hard acceleration. In 2 & 4 it stutters and sputters under hard acceleration.

Could this be a main jet issue?

I've never had them out and have no idea if they're stock. I did some experimenting with pilot jets a while back. My memory is fuzzy on where they ended up. But I wouldn't think they'd be playing a roll in this.

What do you think, could this be a main jet issue or is it a fuel supply / petcock issue?

Does it make any difference with the petcock on "Prime" or "Reserve"?
It might be hard to have a look, but after this stumble, pull over and look at the Filter you have and see if it is still full of fuel.
With the change in pipes you made it could well be a main jet change is in order. If you use brand new plugs, you can do a plug read at that throttle where it occurs. But you may need a long stretch of road where there is no traffic, or cops. Read the plugs for a sign of rich, or lean. Or if you had some main jets you could swap and experiment. Best to go richer first, then see where that goes.

You should pull the carbs off and make sure your mains haven't rattled loose. I went picking on Harley D's one afternoon and the same symptom occurred for me. Take the rack off and I will put money on a jet being loose. If it continues to happen you can use a small amount of clear coat nail paint for women to secure the jet. Let us know. Remember, carbs are the last thing to adjust when troubleshooting.