Chugging in low RPM

Hey all,

I have a 1980 KZ750 LTD and when it's below about 3000 RPM, it chugs and is down on power (it's more noticeable the higher gear it's in). So far when this has happened, I've just shifted down, but now that I've cleaned the carbs, set the float levels, set the valve clearances, and replaced the coils and wires, I figured I should track down what's actually causing it.

If anybody has a suggestion on what I can try, let me know. I know carb issues are usually related to throttle position and not RPM, but so far they're my best guess - it's quite possible I'm posting this in the wrong category.

Thanks in advance for any advice!

Have you put new spark plugs in it, does sound ignitionish.

These motors are made to spin. You can get more grunt with a larger bore but it won't make peak power until it get into the range it was designed for. I got Andrews 1X cams for the low end gain they offered. It helped a little but not as much as I had hoped. Going from 903 to 1015 did more and the cams made it run better all the way through the red line.
If you're looking to have low end power there are other bikes that will give that to you. :whistle:
Steve

Julian, it's had a couple new sets of spark plugs since I've owned it, and haven't noticed a difference. Currently, it's the hotter of the factory-recommended types for colder climates, which seem to burn better (but then again, I'm not very experienced with reading plugs). I can pull them and have a look - anything in particular I should be looking for or trying?

Steve, I do spend my time above 8000 RPM when speed is a factor. But the fact that it sort of shudders below 3000 RPM, and there's a noticeable step in power from "barely anything" to "decent" after that mark, makes me think something is amiss.

Does the bike have the stock air box and air cleaner installed? Ed

Hi Ed, it has the stock airbox with a K&N filter at the moment.

Have you checked to make sure your ignition advance mechanism is working properly? It should be free to twist and snap back when youwork it by hand. Are you still running the stock needles? I've had good luck placing a thin washer under the head of the needle to raise it up a touch. Also, make sure the holes in the slide line up with the holes in the needle holder. That helps.

I'll take a look at the ignition advance. When the bike is running, I can hear a bit of a ticking noise that sounds like it's coming from that side; I wonder if it could be related to the ignition advance.

Edit: realized I totally forgot to answer your question. I believe it still has the stock needles.

When I cleaned the carbs, I remember the screws holding the needles in place being pretty stubborn, so I left them in place instead of risking stripping them or scoring the vacuum valves trying to hold them in place. I don't suppose there's a handy trick for getting the screws out?

Another edit: after some searching, looks like leaving some penetrant in there and making sure the screwdriver fits tight should let me get the screws out. Maybe I can use my bench vise (with some cloth over it) with the flat bit on the vacuum valve to keep it from turning while I unscrew them. Sounds like a winter project!

Hi Shimmy!
If the bike idles nice but has no power or struggles to push the bike below 3000RPM: you need to make sure the 2 trasition ports behind the throttle butterfly valve are super clean. This trasition ports feed fuel from the pilot jet at really small opening of the butterfly valve (lets say around 1/16" open throttle, before the primary main jet comes into place). More to say, adjust your fuel screws to 3 turns out from slightly seated. I used a Gunson Colortune spark plug to adjust idle mixture. To my surprise, every carb had a different adjustment.

For example:
Carb 1: 2.5 turns out
Carb 2: 3.0 turns out
Carb 3: 3.5 turns out
Carb 4: 2.0turns out

This is related to cylinder compression, valve gap adjustment, ignition, etc.

If you have a good ear,,, you might do it with out the ColorTune. Search for the mixture that gives you the highest RPM.

Carbs synced?
Steve

Thanks undiablo, I'll try playing around with the air/fuel screws, and check those ports next time I have the carbs off. I don't have a colortune, but maybe that'll get added my the ol' Christmas list.

Steve, good thinking; I forgot to mention it, but I've sync'd the carbs a couple times while I've owned the bike with no noticeable change to this issue.