Maxim 650 Idle races and won't rev past 7000 RPM

I recently got an 81 Maxim 650 that had sat for about a year runnin and on the road. I have flushed out the old gas, ran seafoam with every tank, changed oil & filter, and changed the plugs. The bike runs good when cold, but when you ride it for a while the idle races to 2000 rpm and when you jump on it it will no go past 7000RPM, it just feels like you are hitting a rev limiter. I thought the intake might be leaking (vacuum leak) but all of my boots look good, and I snugged the clamps up which seemed very loose, but the bike runs exactly the same. I don't know where else to look and don't have the cojones or know how to start tinkering with the carbs.

Please Help.....

 

hi jmk welcome to the site! .)

about the reving u should check for cracks

on the boots or the vacum line.

i rubbed black silicone on my boots and it solved the problem,

theres a better post than this about that somewhere here!

 

Welcome aboard JMK! Glad to have you here. You can go hunting for a vaccum leak with a can of carb cleaner or WD 40 (which I prefer, it helps rubber, not like brake cleaner which drys it out). Spray with the little red straw (tube) supplied with the can at the joints around the carbs and boots. When and if it finds a leak, the liquid will be sucked into the crack, temporarily sealing it. The rpm's should smooth out and run nicely for a few seconds. Here is another thought. Have you a new or fairly new air filter? A restricted filter could also cause you grief. How about your fuel filter? Is it unrestricted? Same problems. Get busy, your going to need to verify this stuff. If you haven't a book yet, get one, your job will be much easier with the information it will provide. Good luck to you and let us know where you land.

 

Im having the same exact issue. I just cleaned my carbs and stuck it back on. It starts however, it immediately races to 7000-8000 rpm.. With choke on. It goes up higher and faster as i close the choke. It would probably go higher except that I put the kill on. Does this mean its getting too much air or gas. Anyway, before i strapped the carbs back on I set the butterfly valves to have a clearance that is about the size of a twisty tie wire. I turned the mixture screws 2.5 turns out. Could I have not put the carb back in the intake boots incorrectly. Here is another thing. I left the fuel on pri (since i think there is something wrong with the petcock cause when I suck on the vacuum line no fuel comes out). I saw it was not drawing anymore gas so I assume that the floats did their thing. But I still left it on pri though. I did not connect the vaccuum line to the manifold. Could this cause the high idle. I will try the wd40 trick but the bike revs up so high so fast its a little scary.


Oh, can you get those fuel filter thing-nga-ma-bobs (clear triangular shaped apparatus between fuel and carb--just in case im calling it the wrong thing) at say autozone, pep-boys or kragen...

seca550_sf

 

throttle/s stuck open?

 

The vacuum line needs to be attached or plugged. This is a contributer to your high idle. THe setting of 2.5 turns is a starting point for tuning and not "THE" correct setting. I would assume that one or more carbs are lean and also contributing to the high idle. Soem don't ge the carbs back in just right so by all means check this too.
I would take care of the vacuum line and then colortune and sync.

 

I took the tank off and pulled the petcock out to make sure that the filter was not plugged - it looked decnt but I cleaned it with some solvent and a toothbrush. put it back together and it is still running the same. I noticed that these carbs have never been adjusted though - the holes are not even drilled out for the mix screws - I'm thinking that after 25 years this may be my problem.....

 

thanks for all of the ideas - but I think I figured this thing out. The carbs are not the problem it seems. The previous owner had put that nasty nylon/rubber coating inside the gas tank and it looked like it was peeling off when I looked in the tank yesterday. It was coming off in huge sheets so I peeled off as much as I could with my long needle nose pliers. I pulled the tank off and drained it, and it took forever because a sheet of it must have been sticking to the petcock tube (later verified when I pulled the petcock). I flushed out a ton more with my garden hose and now have the tank filled up with acetone which eats the stuff very well. I think my liberal dosing of Seafoam (which has lots of acetone in it) is what caused this crap to peel, but its for the better because the tank is not even that bad once that stuff peels off. Once I finish flushing it out I'll use some acid rust treatment and see how she runs. I'll post with an update when I get it back together.

 

Ok - this thing really has me baffled. I pulled the carbs and cleaned them out (cleaned jets, passages, made sure floats worked properly, pulled pilot screws and cleaned them, etc.), cleaned the fuel tank and the screen, and made sure the petcock works properly. The bike now idles much better, runs smoothly, doesn't seem to get hot anymore, and the idle stays good even after the bike is warm. However, it still does the evact same thing at 7000 RPM - it kind of sputters and won't rev anymore. I don't think this is a fuel problem anymore, could I be having an ignition problem or could it be something in the valvetrain. I am leaning towards ingition but am not too familiar with the ignition on these as this is my first bike. Please Advise!!

 

checked air filter?

 

just put a brand new one in - no difference

 

Hmm... I'm sensing a disturbance in the force. Are you by chance carrying a rag under your seat? Or a kick stand plate? Still sounds like an air restriction. At idle this will have an overly rich mixture (thus high idle) and at speed this will bog you down. I would look at what has been going on under your seat. Have you had it modified recently or was it modified by the PO? Possible blockage comes to mind.

 

nothing under the seat - just the factory tray with the tool bag in it - nothing else...

 

Ok, I'm flat out of ideas. Wish you were in my neck of the woods, I'd love to look at your beast. Sorry jmk, I'm stumped.

 

7000 rpm in any gear? stuttering is a missfire or backfire? if not i'd guess and only a guess as can no ride or hear it , still sounds like a fuel thing

 

Sounds like you have tried everything. Are you sure the PO didn't install a rev limiter?

If it's electrical then it would probably be a coil breaking down or ignition module.

For my money I'm still picking fuel. 7000RPM is about the range the main jets are kicking in. Have you checked what size they are? Are they standard size or not. Are they all the same size. My guess would be that they are either too small(starving for gas) or too big(essentially too rich at high RPM)

Also make sure the sliders are totally free and the diaphragms have no holes. Also, Did you remove and clean the emulsion tubes?

All the above relate to high rev running.

Will it rev over 7000 with no load. Eg, just revving it stationary.


Hope these ideas help.

 

it is a miss - no backfiring at all, and to answer a previous question jets were all the same size - the mains were 110 and the secondary were 40.