Eratic idle and cylinder #1 not consistently firing

I have used the search function and read some other threads about similar problems, but I can't really find an answer.

I have an '86 xj 700 maxim, it doesn't seem to be idling properly and I'm guessing its because the #1 cylinder only fires about 50% of the time. It seems like when I open it up cylinder #1 will fire, but when I back off it doesn't like to. Depending upon how many cylinders are firing it seems to either idle high, ~2,500 rpms, or below 1,000 (which results in it dying).

We are pretty sure that the first cylinder isn't firing because the spark plug is about half as black as the other three. Any ideas why this might be happening?

edit: carbs are perfectly synced, the problem still occurs. Here is a video of it, the volume is a bit too quiet to hear on a laptop probably.

http://www.youtube.com/watch?v=R8URye9gYTQ

 

Have you replaced the plug ?

Have you checked the spark plug caps?
There are resistors inside of them. You need to get a meter and ohm out the plug cap, the coil\wire(s) primary and secondary.

 

We checked the plug and it was working fine, they all were. Haven't checked the caps though, I guess thats next.

 

Dawsoner, welcome to the site. A few suggestions/questions,
1. have you pulled & cleaned the carbs? don't forget the clunk test.
2. have you bench synch'd the carbs?
3. do you have an inline fuel filter?
4. have you checked vavle clearences?
5. have you vacuum synch'd the carbs.
6 how does it idle, smooth at 1050RPMs or ruff?

Just about all of these can lead to the apparent misfire on your #1 I believe. the black soot is because the mixture is to rich in that cyl.

If anyone else disagrees on this sound off, I am not an expert but all these items helped with my engine issues.

Again, welcome to the site Dawsoner & good luck, we are all here to help another XJ stay young.

Gregg

 

My bet is #1 carb not letting enough fuel through to run at low speed. If it kicks in at higher RPMS that means the pilot circuit is restricted. Clean the carbs and see if that works. Then get a good sync on those carbs.

 

Thanks for the welcome

 

Edit, see video

One of my friends thinks it might be that it idles on either 3 or 4 cylinders, generally 4 cylinders at or above 2,000rpms and then 3 cylinders below 2,000rpms (which would explain the rougher idle at low rpms I think).

 

1. Clunk test?
2. Yes
3. No, I have 4 individual air filters, one on each carb
4. ?
5. Yes
6. Seems smooth above 2000rpms (on 4 cylinders maybe?) and rough below 2000rpms (3 cylidners maybe?

 

Just a quick question: what idle jets do you have in your carbs?

 

Not sure, I bought it second hand, whatever it came with I guess. I do live at elevation (5,400 ft above sea level), but I bought it here.

 

Might wanna check into your jets. Pods cause the air flow to be different in your carbs. Dont forget about the Idle mixture screws too! If carb one is Black, close the mixture screw a bit, (clockwise) and cut off some gas. You may be flooding the cylinder and it cant fire 100 percent of the time.
And yes, clunk test!! Its important

http://xjmadness.myphotoalbum.com/slide ... me=album01

There is a video in there about your pistons in your carbs. If it doesnt sound like the video, you gotta fix it. Easy fix
-SLKid

 

Here is one I learned the hard way (actually 2). Pull the fuel mixture screw and shoot some carb cleaner down there (or as the one guy I helped with this did, an air hose). The other thing might be a loose plug...I had an eratic idle like you said, and after I tightened the plug down, she idled like a content kitty.

 

Here is one I learned the hard way (actually 2). Pull the fuel mixture screw and shoot some carb cleaner down there (or as the one guy I helped with this did, an air hose). The other thing might be a loose plug...I had an eratic idle like you said, and after I tightened the plug down, she idled like a content kitty.

 

Ok, then do yourself a favour and remove the carbs and then the fuel bowls and check those jets. Right now we're just guessing. After the correct jet sizes we can have a better guess

 

I was so excited about your photoalbum, but then I realized I have Mikuni carbs. Anybody know where to find their mix screw?