2000 ZR EFI 600-erratic idle

[lcfcats]

Hello all,
recently purchased 2000 ZR 600 EFI. After top end rebuild, sled has a very erratic idle. It will start,(sometimes with a backfire) then the idle will go from 1500 rpm to 2500ish and stay there for a bit, then back to 1500 and it may stay for a bit, then repeats the cycle. I can't tell, but there might be a miss in the pto side, as the head on the mag side is warmer. Its like it has a mind of it's own. Here I must inform that I have just the engine running, with no belt, as the suspension is being rebuilt, so I know that could affect some of it, but I figure the idle shouldn't care as much. However, I should add that sometimes, any application of the throttle in even the smallest bit will cause the engine to stall, and if held in that position, the engine will die. If the throttle is released it will go back to erratic idle pattern.

At this point, I must note that this sled was beaten without mercy by the previous owner(s). It has the roughest 3300 miles I have ever seen on a sled in 15 years of riding, and I have done a lot of wrenching on this thing. After working on it the last few weekends, I can say with confidence was no preventative maintenance performed on it whatsoever. (BTW is a project sled)

So far, I am not really sure where to go. My dealer is thinking maybe a throttle position sensor? I have an ohmmeter, but am having difficulty getting any kind of a reading. I have sprayed carb cleaner around all the base gaskets, crank seals and throttle body flanges(yes I replaced the boots) but could not find any evidence of a leaking seal. Both cylinders measure 118psi using a conventional compression checker. No trouble codes from the ECU. Nothing out of the ordinary on the rebuild. I have had many of these fuel injected fast track chassis and have never had a problem like this. They always ran flawlessly.

I am thinking an electrical issue somewhere. Both plugs have hot spark, headlights are bright. Ant ideas would be helpful. After posting this, I am going to go out and check all of the electrical connections to make sure there is no corrosion anywhere that could cause issues.
Thanks in advance to anyone who has any ideas!

 

[JimAndros]

However, I should add that sometimes, any application of the throttle in even the smallest bit will cause the engine to stall, and if held in that position, the engine will die. If the throttle is released it will go back to erratic idle pattern.

in regards to your stall/die problem, your throttle cable is too tight causing the safety switch to operate. Turning the handlebars from right to left will take up slack, so the problem can be intermittent.

Regarding the surging idle speed, vac leak would be my 1st guess. Have you verified fuel pressure remains constant? Can you pull one plug wire at a time & see if the speed slows but becomes stabilizes?

 

[lcfcats]

Its been in my garage and the bars have been pointed straight ahead the whole time. Today when I ran it, the stall problem seemed to dissapear. It responds to throttle like it should. I will check the cable tho.
I have not checked the fuel pressure yet, since I don't have a gauge. I did pull a plug wire, and the engine just ran rough, like it should on one cylinder. I am going away from the idea that it is missing. It is in my garage and has a can on it, so it is kind of loud. I may have been hearing things. When I give it throttle, it accelerates like it should. However, when I let the throttle back to idle, the engine stays at a higher rpm before falling back down. I am partially attributing this to the engine not having any load, but it still sounds weird.
In regards to the possible vacuum leak, where can I buy an actual 2 stroke pressure tester, if anyone knows? I haven't found one here in Aberdeen yet.
Thanks!

 

[Wyelde]

It could be something as easy as a bad Spark plug wire/cap. The easy way to test for this is to simply switch the wires between the 2 cylinders. Since they both fire at the same time, they can be plugged in to either cylinder. If after you do that, the opposite cylinder is cooler, that would indicate a bad wire/cap On older sleds, the cap can get carbon buildup in it that acts like an insulator, And the end of the wire that screws into the cap can get corroded. Often, the fix is to trim a half inch off the wire and reinstall the cap (it twists in)

Also, You should tale a look at your Reed Valves to make sure they look good.

 

[lcfcats]

Ok I will do the wire switch and see what happens.
I looked at the reeds when I put the new throttle body boots on. No broken petals or anything.
What about the idle?
Thanks for the ideas!!

 

[Wyelde]

Well, the other thing you should do is to disconnect the Throttle Safety system for a test. I don't think it would be causing your throttle variation, but could be what is causing the sled to stall when giving it a bit of throttle. To test this, all you have to do is unplug the 3 prong connector at the steering column. It is the wire coming from the throttle. Keep in mind that this will disable your kill switch too.

 

[lcfcats]

Thanks all for the advice!
I did nip off the plug wires an inch or so since they were all green and corroded when I pulled the boots off. That evened out the idle and the hesitation seems to be gone. I unhooked the throttle safety switch, and that didn't seem to affect anything. After putting the track and other components back on, I was able to test it on the stand under load. It engages just like it should and accelerates, but the idle is still high and varies, and after you let the throttle off, sometimes, the engine will still stay in that 3500-5000 rpm range for up to 10 seconds or so.
I am trying to rule out a vacuum leak, since it would seem that would be constant.

Ok here is dumb question.... Since I tore the mill down, is it possible all the air didnt bleed out and there is an air pocket behind the coolant temp sender , and that makes it idle weird? I have had situations like this with vehicles. Just a thought?

I think the next step is maybe ride it a few miles and see what else happens, but I need snow for that.......
Any more ideas are helpful! Thanks in advance!