TOPIC:

What performance-related symptoms would indicate that the fuel isn't at the correct levels in one or more of my bike's float bowls?

1982 KZ750-N1 Spectre, shaft drive, 4-cylinder, U/S. version

I'm reviving the 30 year old bike after buying it from a guy who let it sit since 2009. I cleaned the gas tank, disassembled & cleaned the fuel petcock, removed and cleaned the carbs except didn't pull the air mixture screws, reset the float levels to 17mm, cleaned and checked all the rubber boots, new battery, new air filter, new plugs, cleaned all electrical connectors (had to re-solder some).

Finally put it all back together today. It took a LONG time to start; had to hook up a car batter to jump it because it cranked the juice out of my new battery.

Once it started I was back to the same old problem: doesn't want to hold its idle consistently.

I also used an unlit propane torch to see if the intake boots are leaking. no change in RPM with the propane.

When I do get the bike running it can rev up nice when I twist the throttle. But then come two problems.

1. Sometimes, after twisting the throttle, it doesn't want to go backdown to an idle -- it races at 1300 or so. If I turn the ignition keo off then back on real quick the idle will settle down again.

2. If I can get it to hold an idle, after awhile it wants to climb back up again. Not always, but maybe 3/4 of the time. It'll idle for maybe 30 seconds or so, then climb up and start revving high again.

Would my failure to remove & clean the aair mixture screws cause these symptoms?

Would my failure to measure the actual fuel level itself (using a clear plastic tube) but instead just setting the float bowls to 17mm also cause these symptoms?

Starting to scratch my head over this one.

I forgot to mention: it's not a vacuum leak in any of the emissions control things -- because I removed that whole system (based on a post I found on this site).

there's only one vacuum line left on the bike: from one carb to the fuel petcock. All other vacuum lines and systems are removed.

check float levels just to be sure, a float level out of adjustment can cause a cylinder to drop or load up and flood. after checking that if your model uses cv carbs and a slide isnt opening its a diaphram. this is all assuming that the carbs are synced properly after the tear down, chris

rustbuckett wrote: 2. If I can get it to hold an idle, after awhile it wants to climb back up again. Not always, but maybe 3/4 of the time. It'll idle for maybe 30 seconds or so, then climb up and start revving high again.

sounds like a vacuum leak

yea vacuum is the issue i had with a gsxr due to a unsealed diaphram

You probably ought to go ahead & do the fuel level check. Sometimes the dry method doesn't get the fuel level correct.

Thanks guys for the replies. I also forgot to mention this, as I look back on my post, this part is probably important:

It can "sometimes" hold an idle. It idled for a good 5 minutes straight yesterday, no stutters or rises or falters.

But, half or more of the time, when it's idling ok but then I twist the hand throttle to rev it back up, it has trouble going back down to the idle it was on before the throttle-twist.

It's not 100% consistent. It's one of those intermittent things. Sometimes ok, but sometimes not; and sometimes WAY out of whack to the point of me wondering if I'll ever dare ride this monster on a highway if it's gonna act so random like that (I picture an 18 wheeler riding my tail behind me, another one on my left in the passing lane, a guardrail on the right and a braking dump truck in front of me and this bike picking that exact moment to decide it's time to rev up to 1500 rpm in 5th gear...). Sorry 'bout the drama but you know how scary some riding scenarios can become in just a flash of time.. so I have to fix this or I'll never ride it -- too dangerous, I have kids to support.

If I remember, Patton replied to one of my earlier posts and recommended pulling the air screw plugs and removing/cleaning them; and I never did that step. I think he also might have recommended measuring the fuel level (not just the float height) and I didn't do that, either. I think I'll remove the carbs (again,,,) and do a second, more thorough cleaning and will inspect the diaphrams even closer.

Related: Tell me about synching the carbs.... I figured that since I never disassembled the carb bodies from the 4-rack system; and since I didn't touch any of the butterfly valve screw settings for any of the carbs -- well that meant to me that i didn't UN-synch the carbs. Then i figured if i didn't UN synch them, there's no need to re-synch them. That's probably a bad assumption isn't it.

I don't have the fancy vacuum tubes and all that stuff to synchronize each carb. I tried that once back in 1989 when I put dual Mikuni 40mm sidedraft carbs on my little Suzuki Samurai mini-Jeep, and a friend who had the synching tools helped me measure & balance the airflow through each carb to get them all equal. I don't have those tools.

How can I synch them without proper tools?

Is the throttle cable sticking? Disconnect the throttle cable and see what happens.

I'll do that. I wondered if maybe I didn't have enough slack in the cable, and as the machine heated everything up it might have started slowly tugging the cable more snug -- so I loosened the cable to give it some more slack. didn't change anything.

So before I do anything else, I'll unhook the throttle cable and see if that changes anything. Thanks for the suggestion.

Then I think I'm going to remove the carb rack, buy a new gallon can of Berrymans carb cleaner, and I'll give those carbs a 2-day detail cleaning. Last time I didn't want to damage any O-rings or whatever might be inside the air screw areas (carbs I've worked on in the past seem to all have a rubber O-ring or at least a nylon washer), so I didn't soak the carbs in actual carb cleaner -- I just boiled them on the stove in a pot of vinegar water with a little Super Purple degreaser/cleaner.

This time I'll also use fine copper wires to poke thru all the little passageway holes in the carb bodies and will also blow them out with compressed air.

Then I'll reassemble them slowly and carefully like I'm doing brain surgery. When I reassemble them on the bike I'll pay closer attention to the throttle cable too. Then I'll also do the fuel level check.

I think what you said above is exactly the right thing(s) to do - take the carbs off and do it all again and do it thoroughly including the air mix screws. Carb cleaner wont dissolve clogged crud in a little passageway or jet so poke each one through. You need some slack in the throttle cable. You should have maybe a 1/4 inch free play at the throttle grip. I would suggest leaving all the carbs on the rack unless you need to replace some of the connecting tubes and o rings. When you take the cars apart place the bits from each carb in small bowls or glasses numbered 1, 2, 3, 4 for the # of each carb so you can reassemble in the same order.

Before you take the carbs off the bike actually measure the fuel height with a clear tube and a ruler. Not difficult to do. If you do this first and anything need adjustment you can do the math and adjust each float to give the right fuel level when you re assemble.
Good luck

here is a Patton special. hope it helps.

Start with checking your valve clearances, then make sure your carbs are clean, fresh o-rings throughout, fuel level set properly, then synchronize carbs after making sure there are no air leaks. Intake manifolds get hard and leak. After that it’s air fuel mixture fine tuning. Check the resistance on your coils if you suspect weak spark. The “hang throttle” you’re experiencing sounds like an air leak to me.