YJ runs rough on startup?

Hello everyone. 258, Manual, 1988. Last weekend I swapped out my Weber K551 for my newly rebuilt Carter. It starts great and, after it warms up, runs great, but when I start it cold, it runs at fast idle for 2-3 seconds, then drops to about 600, running very, *very* rough for about a minute. Situation stays the same if I rev it higher. Then it goes back to fast idle and runs fine from there. Sounds like detonation, or like it's only firing on two cylinders - a neighbor heard it and thought I was revving a Harley V-twin. Can't figure it out. Almost positive I have all my vacuum hoses connected right, and I replaced my O2 sensor at the same time as the carb. Installed new spark plugs today, (the cap, rotor, and wires only have a few thousand miles on them) new air filter, I can't figure out what the problem is. Any suggestions as to the cause of the trouble? Thanks all.

-Micah

Reply to
Micah
Loading thread data ...

From foggy memory.

Check the choke pullback. Should be a small wire connecor on the choke which you bend to increase the pull-back. The setting should be about .140 inches. Bend the choke connector wire until you get enough clearance. You can check by attaching a hose to the choke and sucking on it or using a vac pump.

Reply to
DougW

Good idea. I'll check it and let you know.

Thanks.

Reply to
Micah

Well... when mine does that I usually am in need of a new distributor cap and rotor and it is usually on a damp day.

I think it is condensation inside the distributor cap affecting dirty parts.

You might want to give the insides of the cap a spray clean with WD40, that does it wonders sometimes by cleaning out the carbon bits.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
formatting link
(More Off Road album links at bottom of the view page)
Reply to
Mike Romain

Reply to
L.W. (Bill) Hughes III

Bill, I just don't know how to answer that.

Mike, it's been 70-80 degrees here in Texas the past couple of weeks and low humidity. And it ran fine after starting cold with the Weber on it a week ago.

I did check on the choke pull-off earlier today, it doesn't seem to be functioning correctly (though I can't figure out what could possibly be causing it to malfunction). I started my engine today, let it rumble for a few seconds and stall, then I wedged the choke plate open about 1/4 inch, started it, and it ran perfectly at fast idle. Then, around 120 degrees I pulled the wedge out of the choke and it settled down to its curb idle. Seems to run fine if I manually do the job of my choke pull-off. The vacuum hose on it is brand new. Anything I can do or test?

Reply to
Micah

Bill I might have just understood your question. I replaced it because my Weber was falling apart. The idle mixture screw was missing, I was getting 8mpg, and for the life of me I couldn't maintain 70mph on a flat highway with no wind. Also, I have to pass my emissions inspection this month and my exhaust made me slightly woozy. As Mike argues, the Carter will run fine with some minor adjustments. The Weber, it was old, and I wanted to go back to my stock setup.

Reply to
Micah

When the engine first starts, there is a choke pull off valve that pulls open the choke just slightly so it doesn't chug out.

This valve is on the choke side of the carb and is hooked to direct manifold vacuum via the nipple that is dead center and the bottom back of the carb.

To test this, you can unplug the line to the carb base and suck on the line. The pull off is supposed to hold vacuum.

When this pull off is working right, it should open the choke between

0.130 and 0.140 inches open as soon as the engine turns on. The setting calls for the choke to be turned rich 'while it is being set', that is just to make sure there isn't any slop in the main choke linkage which would allow the choke to open too much.

This choke pull off is powerful. It will pull my manual choke cable handle in as it fires up. I can feel the cable move.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
formatting link
(More Off Road album links at bottom of the view page)
Reply to
Mike Romain

Which reminds me. If your carb isn't sitting flat or the gasket it torn, you will never get enough vac to pull in that choke. A bit of carb cleaner sprayed around the base will let you know if there is a leak going on.

Reply to
DougW

I tested it pretty thoroughly yesterday. Sucked on the hose to the pull-back and it... surprise... pulled back. That section of vacuum hose is brand new. It seems I'm just not getting any manifold vac on startup. I checked all the carb-intake bolts, they're all at the 16 foot pounds recommended by my Haynes manual. Both base gaskets are new (above and below the spacer). I also sprayed carb cleaner around the base like Doug recommended and didn't find any leaks. Perhaps I have another leak elsewhere? Just can't figure this one out.

On the bright side, when I start it, then immediately prop the choke plate open it runs fine. I then have to wait for the choke to open enough on its own before I can pull my wedge (a bottle cap-ha), put the air filter lid back on, and drive away.

I'm considering installing a remote control wire (I have a few spares laying in a box somewhere) so I can pull the choke back on my own after the engine starts. Not entirely desirable as a long-term solution, but it could hold me until I track down the source of the problem/leak.

Thanks for your help, everyone. Any more suggestions, and I'd be glad to hear them...

Reply to
Micah

Do you have a vac assisted brake? If so, plug that line off and try again. Same thing with every other vac line that comes from the intake rail.

hmm... does you Jeep have a vac reservoir under the battery? IIRC those used to rot out after a few years.

I use a vac gauge and a small hand-pump to test lines.

formatting link
never go over 20in of vac or you risk causing problemsin the system. (20in is below what a normal engine pulls ondeceleration)

Reply to
DougW

I would next verify you have vacuum coming out that nipple on the carb. It would be obvious with your finger.

If there is no vacuum, maybe the carb base plate didn't get in right side up. It has notches at the edge of the main passages that need to be up. These provide the channel for the manifold vacuum.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
formatting link
(More Off Road album links at bottom of the view page)
Reply to
Mike Romain

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.