1982 Maxim 750 revs up to 5000 rps and sticks

I've recently purchased a 1982 Maxim 750. The bike idles fine at startup, but once the throttle is advanced it races up to 5000 RPM and it sticks. I has been over/through the carb several times and outside of a couple of clogged ports, I am unable to find anything wrong. The carb moves freely on/off the bike until it's revved up after start. It seems like it might possibly be vacuum related, but any ideas would be appreciated, thanks in advance.

 

try this

 

To achieve rpm's over 25-Hundred, ... the Throttles have to be open.

It takes Main Jet Supplied Fuel to run at 5-Grand.

Disconnect the Throttle Cable.
Make sure the Linkage PULL isn't stuck under the Head.
Look for anything preventing the Linkage from Closing the Throttles.
Test the Diaphragm Pistons according to the instructions in the Post: CLUNK TEST"

 

and if the cable isn't the culprit follow Rick's directions, while they're off from that, bench sync or at least do the inspection portion of a bench sync. One throttle closing way early will hang the others open as surely as the idle knob does.

 

I had the same problem when I first got my '82 Maxim 750 running. Turns out I had knocked the throttle cable out of the groove in the cam on the throttle on the handle bar. Simple check / fix. Hope this helps!

 

Thanks for the advice and the links. I'll be trying out these suggestions tomorrow after work and post the results.

 

Is it doing this with the choke on? or is this after it's completely warm and running without choke?

 

I'll try to be a little more descriptive:

This happens when the bike is warmed up and the choke is off. When the clutch is released while shifting through the gears, it revs up to 5K. When the clutch is engaged and there is drag on the engine, the RPMs return to a somewhat normal state, but it will still not engine brake as I downshift. It acts as if the throttle is stuck, but the cable has full travel and good spring-back return.

 

A vacuum gauge or a Carbtune-type manometer would tell you more about what's going on when you're experiencing this behavior. If you've got a bad seal somewhere or a butterfly that's closing too soon and keeping the others open, it'll be pretty obvious which one it is from the differences in vacuum between the cylinders.

The unlit propane torch trick might help as a diagnostic method as well, but it's only going to find leaks through seals and boots, not extra air being drawn through butterflies.

 

Burnish the Diaphragm Piston Bores with ScotchBrite Gray and White.

Polish the Bores if you have to remove the Carbs, again.

Slack the Cable.
Operate the Linkage by hand.
See if the Linkages fully return or are binding.

 

I ganked the carbs off after work and gave them a quick "clunk" test after removing the caps. Two of the pistons dropped freely and the other two had a slight hang. I will be polishing the bores tomorrow (given the time). I will also operate the linkage as you've mentioned and see if there is anything binding.

I am also going to invest in a vacuum gauge as well to give me a little more information with regard to each carb's situation.

 

Okay, I burnished the diaphragm piston bores and cleaned the pistons: clunk test like a bag of cement. Operated the linkage by hand with slack in the cable and the linkage operated within the full range and there was no binding whatsoever. I started it up, took it for a spin and it's still having the same issue with the throttle running away. Anytime the clutch handle is in, the RPMs run up above 3K and stick, I actually have to shut the bike down when I'm at a standstill and it isn't under power. Once I restart it will idle until I engage the throttle then runs away.

 

Your carbs are out of sync and/or you have improperly adjusted float levels.

IN ORDER:

Valves in spec, or you won't be able to sync.

Carbs clean, slides go clunk.
Float levels set using fuel and clear tubing.
Bench sync.

-back on the bike-

Running vacuum sync, YICS blocked.

Skip any, and you'll be back to do it before it will run right.

 

Based upon your descriptions, ... what I think you ought to do first is visit how the Clutch and Throttle Cables are routed and interacting with each other.

Try anchoring the Cables to the Frame.

 

Since you have said that this only happens when the bike is warm, I suspect it's not cables.

Have you checked compression? Are you getting any popping sounds when the bike is warmed up? As Fitz always asks: are the valves in spec?

How are the throttle shaft seals?

It really sounds like you have a mechanical problem and the idle is raised to compensate for it. When it's warm, if you turn the idle down, does it idle correctly at that point, and not hang?

 

If the Bike runs-up to 3000 rpm's ... "Something is moving the linkage" to let in some Fuel.

Since the high revving happens with the Clutch Lever being pulled, ... see if the whole Rack might be moving some when the Clutch Cable is Pulled.

Have someone work the Clutch Lever while you watch to see if the Cable's too tight and the tension causes the Carbs to react by getting moved some.

 

To answer a couple of your questions and to clarify the points that I wasn't clear on:

No popping sounds as the bike warms up.

The throttle shaft seals do not seem to be bad, as I have sprayed them with carb cleaner as the engine is running with no change in RPM.

I have yet to check the compression and will get a gauge tomorrow.

SQLGuy when you say: "if you turn the idle down, does it idle correctly at that point, and not hang?", do you mean the idle screw? I will have to get a friend to assist me and determine if the linkage is actually hanging up, but the throttle handle has full travel as it's rotated and I cannot see that it is hanging up on anything.

The throttle shaft seals look good, they are not brittle or cracking. I have sprayed carb cleaner around the seals and there is no change in engine performance.

I adjusted the valves out (the best that I could) via the shop manual (Intake 1-15 mm Exhaust 16-20 mm), as several had been tightened all the way down (probably never having been adjusted), it will be in need of shims,:

#1 Intake 13mm Exhaust 10 mm

#2 Intake 10mm Exhaust 15 mm

#3 Intake 15mm Exhaust 13 mm

#4 Intake 10mm Exhaust 13 mm

So it looks like I will be taking it to the shop for a valve adjustment.

All, I want to thank you for insight and I intend on working through this issue based on the sequence bigfitz52 provided. I have purchased new cables (clutch, accelerator, and choke), as the clutch cable had a kink in it from the previous owner and when I replace them I will make sure that they are properly routed. I've orded a YCIS tool and will pick up a sync gauge, so once the valves are properly adjusted, the carb float levels are adjusted and it's bench synch'd, I can sync the carbs. I realize that there is a proper order to things, I have just recently purchased the bike and wanted to get a few weeks of riding in before the bad weather hit here in Utah, so I wanted to see if I could try a few simple things first. As it turns out this carb setup is anything but simple and it's one of the reasons that I replaced the dual carbs on my 67 Mini Cooper S with a single Weber 48 DCOE.

Anyway, onward and upward, I'll keep you posted and thank you again for your help.

 

Yes, I do mean the idle screw. I've seen quite a few bikes that had problems that could be masked when cold by raising the idle, but would then have problems when warm.

I currently own two such bikes. The first was my GPz 750. The real problem there was bent valves and nearly no compression on #3. But that also would pop once it warmed up. The second is my daughters KDX80, where the problem was also low compression, due to the engine having apparently chewed on a ring at some point in its past.

Both of them now start and idle perfectly... the GPz after getting four new intake valves, the KDX after getting a new piston and rings.

 

"Adjusted the valves out?" You checked clearances, which is good. Adjusting is via shims, so I don't quite understand this quote.

They've tightened up via pounding into their seats; and from the looks of what you posted, all except intakes one and three are horribly tight.

Get the valves in spec before you do a compression test or you'll get all worried for nothing.

As for the throttle shaft seals, you can only actually see two of the eight seals (the two outside ones.) The rest are "buried" in the rack. If those two are good, all you really know for sure is that those two are good; others within the rack could still be bad. If the outer two were toast, then it would be a safe assumption that the others are too; but not the other way around.