Starting 650 Maxim over-revvvving have to shut it down

I am having problems getting my XJ 650 Maxim starting. When it starts it acts like somebody is holding the throttle wide open. It revs up to 8,000 rpm if I let it. HOLD UP before you tell me to check my throttle cable! I have checked and rechecked the throttle cable and butterfly valves, I even unhooked the throttle cable from carbs. I also have the butterfly valves set at with a slit of light coming through. (Bench-synched) Carbs are freshly cleaned (disassembled and cleaned/rebuilt).

This bike is a project I bought to fix up and ride. Now the funny thing is I had this thing running good 2-3 years ago, but I never got around to vacuum sync'ing and colortuning is all. After letting it sit, I went through the carbs again and cleaned them up.

I adjusted the pilot fuel mixture screws to test. I started out at 2 1/2 on all four and adjusted up 1/2 turn all the way to 5 full turns out. No good. So I started at 2 1/2 again and worked my way down and finally at 1/8 of a turn out from bottom the bike idled better but it was choppy.

Now I'm trying to figure out why it is acting up. Could it be due to running far to lean? Or is it simply a matter of too much fuel at idle?

 

You have a vacuumed leak some where, or one of your slides are stuck. I had one of my slides get stuck one time and scared the hell out of me. I cured it by opening and closing the throttle a couple of times to free it up. Then I promptly cleaned the carbs when I got home.

 

Hi Seth,

I have a 84 750RL as a project & just finished my carbs for the 3rd time!!
I cleaned them & had it running ...but not very well.
I cleaned them, had them ultrasonic cleaned & had it running ....still not very well.
I "Church of cleaned" them & replaced ALL o-rings & throttle shaft seals & she's much, much happier!!!!

No short cuts & all will be great!!

Good luck buddy.

 

You need alot of air to get to 8,000 RPM - "had this thing running good 2-3 years ago" - I suspect your bench sync or throttle shaft seals.

Can you proceed to the running sync at this point?? (and you're supposed to have your valve adjustment done at this point)

 

Thanks for the good advice guys. I think I'll take the carbs apart for the 5th time and re-check things. I had "church of clean" cleaned my carbs, but I assumed my throttle shaft seals were ok, so I'll be sure to look at them and order new ones if need be. (I already shimmed the valves to spec)

I have a hard time believing though that a vacuum leak from the throttle shaft seals would cause such an increase in rpm??? Could they have worn out that much over a period of 2-3 years?

Also, I kinda took a short-cut when setting the float heights. :roll: I found a tutorial on here that set the float height by measuring from a surface. Would a bad float height be a contributor/cause of my problem?

 

Until you do a running synch all sorts of things might seem to be a problem which actually aren't. Out-of-synch engines exhibit all sorts of issues that can seem to be actually caused by other issues. That being said, make sure that none of the vac pistons are hung up (you can lift them with a finger and see if they drop back down easily) and make sure the choke plunger valves aren't somehow sticking open (don't just rely on the choke lever on the handlebars being closed, make sure the plunger valvaes actually are closed).

The purpose of the "bench synch" is to merely get the carbs roughly aligned with each other in order to be able to get the engine capable of being started.......period. That has been accomplished. A good "bench synch" means NOTHING beyond that, and only by doing a vac (running) synch can you start to eliminate tuning issues....

 

Running sync... what does that entail?

Is it the process of using the yics tool with a vac gauge to equalize the 4 carbs? I have both of those tools ready, but I can't let the bike run for more than a second. As soon as it starts the rpms go sky high.

 

Mine did that until I replaced the idle mix adjust screw o-rings, bench synced, and turned the idle adjustment screw way down. Then it was still crappy, and wouldn't idle without me physically running the throttle, but it ran well enough for me to attempt a color tune. I found that not all cylinders were firing on idle, this is what forced me to set my idle too high in the first place. Once I set it high enough to actually idle on its own, my carbs were no longer running off the idle circuit at all, they had hit the main circuit which caused the sliders to raise and the engine to rev.

If you are sure your carbs are clean, with replaced o-rings and such - get a colortune. This was the turning point for my bike resurrection. As you get all cylinders tuned to fire consistently, you will be lowering your idle adjustment quite a bit, vacuum sync the carbs. Then you'll be lowering the idle some more.

I then colortuned again and then did another vacuum sync. Purrs like a kitten and no more weird misfires/backfires.

Once everything is running as it should, and all cylinders are doing their job at idle - you should finally be actually idling on the idle circuit of the carbs instead of the main jets. Then you can set your idle anywhere you want.

EDIT: I did not disable the YICS when I did my vacuum sync(s) - I will do that later to fine-tune the sync. For now, I just needed the bike running and idling

 

Forgot to mention.........is your idle SPEED screw backed off?

 

what is a colortuned?

 

color tune plug cong

 

-Yep, my idle speed screw is backed off completely
-The slides also slide easily (performed "clunk test")

After racking my brain, I feel like something silly is causing it. I will go back through the carbs in a couple days to search for anything suspect.

 

Just a curious side note: I had the petcock turned to the "PRI" position or "prime" position. Would this cause any issues?

Why is there a "prime" position on the petcock anyway?

 

bet its junk on the carbs.......just me

 

My 750 was high revving like that due to leaking intake boot on #1. The prime position is to fill the fuel bowls with fuel when the bike has been ran for a while, then turn it back to the run position. If you have to use pri position for the bike to run, then the petcock needs rebuilt.

 

"If you have to use pri position for the bike to run, then the petcock needs rebuilt."


That's very possible, but it could also be that the petcock is fine but the vacuum hose leaks, which could also contribute to lean running condition & revving.

 

I shouldn't have to worry about the petcock, it's brand new! I had just ordered one from chacal. I also checked the vac hose the other day for clogs/leaks.

I have the carbs broken down and I'll be going through and checking all passages. I have some good news though! I had forgotten when I rebuilt the carbs the first time 2-3 years ago, I replaced the v-seals on the throttle shafts.



Chacal, as far as doing a vac-sync, you mean this process? http://www.xjbikes.com/Forums/viewtopic/t=2132/highlight=one+gauge+sync.html Since I can't let the bike run at a safe level should I screw the pilot mixture screws down to a 1/4 turn out where it runs safely and then start the vac-syncing? (That's if I have the same problems after I get the carbs back together, cross fingers!)

 

That link is one way to do it.

Did you bench sync the carbs prior to trying to start it? Your idle speed screw could be backed all the way out and one or more blades still open enough to run the engine on main fuel.

I'd start there and then the sync but only if your valves are in spec.

 

Well, I have the bike running perty good!

I cleaned the carbs real good again although I didn't find any blocked passageways. I did make one adjustment. I changed the setting on the main valve needle jet by moving the e-clip. I moved it from 3 from the top to 2 from the top. I thought by doing that it would prevent the carbs from going from the pilot circuit to the main circuit too soon.

I then vac-sync'd and would take the plugs out every so often and adjust the pilot mixture. All I have left is to colortune, but she's already purring nice!

The idle will sometimes sit at 2,000-2,500 rpm but eventually come down to normal idle. I won't worry too much about this though until I get it colortuned.

Thanks guys for all the advice!