TOPIC:

This is the most agravating thing I've ever run into. I got this 1977 KZ650 10 yrs ago with the pilot screw type carbs which I've never had before. Bike was pretty junky for $150 but after a bit of work, cleaning the carbs ect, it ran real good so I thought I'd use it. A couple thousand miles later it started running and idling rough so I removed the carbs, disassembled and cleaned them although they looked spotless. Problem solved, then about 2000 miles later same thing. I removed the carburetors and cleaned them every 2000 miles or so for about 15,000 miles. Finally I figured I'd just pour the Yamaha carb cleaner in the carbs when it started running bad, so I did this every 1000 miles or so for a while as the time was gradually lessening between good and bad running. Finally I figured if I removed the pilot screws from each carb after the problem surfaced, I could simply spray cleaner in there and reinstall the screws and it would rectify the problem, but it would return in about 500 miles or so. Now I'm down to about 75 miles between running good and removing the pilot screws which is getting a little rediculous. I ordered new screws for $100, hopefully they are the cause of this. I know darn well gas is not dirty as the bowls are clean, just the pilots clog. Anybody seen this?

Sounds like you need an inline fuel filter.

yes, an inline filter. you might be getting that powdery rust from the tank accumulating in the carbs...

I should have mentioned I am running the filter and the fact I've used 4 different sets of bodywork on this bike. A couple tanks had been sealed, a couple had not, but all did the exact same thing. Also the fact that I fill multiple 5 gallon cans of gas and use the same gas to fill all my motorcycles and this one is the only one that ever has this problem. I think the pilot screws are all screwed up at the tips and bent slightly. I'm hoping this is impeding gas flow and causing this aggravating thing to happen. I've ridden this bike now 33,000 miles and have had zero problems except this one about 20 times so far. If the screws don't fix it, I'm throwing these carbs in the trash.

Is it possible the pilot screws are changing their adjustment, and when you r&r them you're mainly re-setting the adjustment? Are all the springs present underneath them?

I would also try to isolate which cylinder or cylinders this is happening with. It seems unlikely that all four are experiencing the exact same carb-related problem. Next time remove the left two and see if that helps, then the right two. You could do the center two and the outside two. You should be able to narrow it down to a carb or maybe a pair of carbs. (Unless the pilot screw springs are in fact missing, which would explain all four going out of adjustment together).

This might be a good time to start actually taking notes as you troubleshoot the problem. If none of the ababove works then it could be a bit of a stumper.

RonKZ650 wrote:

I should have mentioned I am running the filter and the fact I've used 4 different sets of bodywork on this bike. A couple tanks had been sealed, a couple had not, but all did the exact same thing. Also the fact that I fill multiple 5 gallon cans of gas and use the same gas to fill all my motorcycles and this one is the only one that ever has this problem. I think the pilot screws are all screwed up at the tips and bent slightly. I'm hoping this is impeding gas flow and causing this aggravating thing to happen. I've ridden this bike now 33,000 miles and have had zero problems except this one about 20 times so far. If the screws don't fix it, I'm throwing these carbs in the trash.

don't toss them, i'm sure someone could use them for parts..etc. beides, you'll figure it out..

How does the bike perform at highway speed? If it runs okay above quarter throttle, perhaps this would help confirm your suspicion about the pilot air screws which lose their influence by quarter throttle when the slide cutaways and tapered needles come into play. Still a mystery why the spray in carb cleaner would temporarily help, and for progressively shorter times. Could the problems possibly be resulting from deterioration of the rubber intake manifolds or possible leakage past the rubber vacuum tube caps? Saw somewhere in the archives about a neat quick test by spraying with propane gas from a burner (unlit of course) in and around the intake manifolds while the motor is running, which if sucked in causes a noticable difference in rpm (this being alternative to the WD40 spray treatment while watching for smoke in the exhaust). As I recall you have already done or are satisfied with all the pre-carb tuning procedures(timing, plugs, valve lash, spark, air filter) so as to give you the best clear shot at the carbs. My experience is limited to the stock 26mm Mikunis on a '76 KZ900ltd and 29mm smoothbore Mikunis on a '73 Z1. Have never experienced the same symptoms you describe but wanted to at least submit some food for thought. Please don't despair.

Post edited by: Patton, at: 2006/10/21 14:55

Post edited by: Patton, at: 2006/10/21 14:56

Post edited by: Patton, at: 2006/10/21 14:57

Post edited by: Patton, at: 2006/10/21 14:58

Thanks for all the responses. The springs are all there, I know that for sure because I've removed 4 springs about 20 times now, and each time I turn in the screws to see how far they are adjusted out before I remove them and they have never changed. The motorcycle runs pretty darn good on the highway all the time, but more surging will be felt after the carbs start going bad. I started the engine, let it idle for a few seconds and determined cyl #1 was not running, so did my carb cleaning on all 4. Ran real good for about 75 miles. Repeated this and #3 was not running this time, so I removed only #3 pilot out of laziness and cleaned it. The problem is still there as of last time I rode. Need to clean all 4 everytime I guess. I'll have my new screws next week and see what happens from there. The thing that aggravates me is these carbs run real great when working right. I've got probably 15 sets of carbs I could clean and put on this bike, all the airscrew type, not pilot screws, and I've run hundreds of thousands of miles with this design and normally have had no problem with them, but I've already tried this motorcycle with 2 sets and never could get them to run right. They both had completely different problems. I'm not a carb guy, I hate them.

More food for thought. Wondering how your spark plugs keep staying at their best through all this and also wondering if you have checked the float levels. Just maybe the spray job has enough influence to keep things going for awhile until a too high float level passes enough excessive gas through to flood the cylinder so much the spark plug cannot overcome the ultra rich condition. My personal experience involved #1 cylinder on a '76 KZ900 than ran just perfect with a bran new plug for about 20 miles before throughly fouling the #1 plug and dropping the #1 cylinder. This drove me to new coils and plug wires, etc until there was just nothing else left but the carbs. It was the float level in #1 carb -- too high = too much gas. Easy to check the levels. Once carb bank is removed the level is easily changed -- checked -- changed until getting it just right. One of WG's helpful hints worked for me -- just one float bowl screw is sufficient when bench testing the actual float level. I'm sure you know about the clear plastic tube to be rigged into the bowl drain screw hole, etc. (or maybe you have or have access to the special factory tool for this). Anyhow the float level fix on #1 carb worked like a miracle on my bike and maybe -- just maybe -- it could work for you. Please don't despair.

Everything you're describing is consistent with some kind of contaminant getting into the fuel system. This would affect the pilot circuit first, but it could also affect the main circuit after more time has passed.

Why is it taking less and less time to return after you clean the pilots screws? Two possibilities, as I see it:

1) The contaminant is being produced and entering the carbs more quickly, maybe because of a progressive rate of deterioration in whatever is causing the contamination.

2) The other carb circuits have also become somewhat affected, so that when you clean the pilot screws it is only a partial solution. When is the last time you had the bowls off for a look?

Whatever the reason, it seems something is getting in there and if I were you I think I would try to zero-base the problem at this point. I would suspect a problem with the fuel filter, the fuel line itself, possibly something that is getting into the gas tank, and possibly something that is already in the carbs. It is even possible, I suppose, that the air supply is introducing crud into the system.

I would:

Remove the carbs, take them completely apart and look for anything not correct. Maybe somebody at some point used an improper sealant to keep the bowls from leaking and it is dissolving and crudding up the system - or who knows what else.

Get a new fuel filter, like the cheap visu-filter type you can buy for a few bucks at a bike shop.

Get a fresh length of proper fuel line.

Completely drain the fuel tank, clean it out then run some clean gas through it and use it in your lawn mower or another vehicle.

Check that the air filter and inside of the airbox are nice and clean on the filtered side. It is conceivable that small contaminants could enter the pilot system from the air supply.

If you do all that and still have a problem I'd be surprised.

Post edited by: glenncarpenter, at: 2006/10/22 11:59

Thanks for all the good ideas. Unfortunately I've done them all multiple times. I've had the bowls off at least 10 times, checked and cleaned and replaced filters ect. The bowls are clean. You can take the bowls off and no dirt will be seen. Floats are set ok, I've checked them both with measurement and fuel level. This is what threw me when I first got this motorcycle after I got it running and running real great a couple months later this happened for the first time and I dissassembled the carbs expecting to find dirt, opened them and said WTF, no dirt here, but I'll blow the passages out ect anyway as long as I'm here. Problem gone, happily ride around for a month or 2 then back again. My screws are here, I'll put them in next week, other than that I really think I've tried everything.

hey ron what are you using for fuel lines??? just a thought from an inquisitive guy??? steve