stumble at WOT and tach jumps way up? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 10-23-2002, 09:48 AM
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stumble at WOT and tach jumps way up?

OK, so I've rebuilt my BBD, installed a brand new coil and brand new plugs. 10,000 miles on rotor, cap and wires. Charged battery, removed batt cables, cleaned inside ends, and batt posts. Cleaned ground lug at block. Cleaned all coil connections w/ sandpaper on loom.

Jeep starts and idles great, if a bit low(500 rpm, but I'll fix that later). Pulls semingly fine thru first gear up to 3000 rpm.

Shift into second and floor it and it pulls to about 2000 then starts missing badly and the tach will jump and bounce and hover around 3000rpm as long as you keep it WOT, but it is really only at 2000 rpm +/-. Does the same stumble in 3rd. Haven't taken it any other gears but that round the block a couple times.

I've spent several hours looking thru search and can't find much that seems to apply to my situation.

What do I need to check?

Mike
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post #2 of (permalink) Old 10-23-2002, 10:10 AM
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found the problem

If any of you care I just found my problem.

I hd left the vacuum line to the roll-over check valve on top of the carb off.

I just took it around the block and it seems to run great up to 3000 rpm in 2nd and 3rd. Haven't taken it onto the highway yet tho.

Pulls more smoothly and stronger than it ever has in the 8 years I've owned it so far :-)

Mike
post #3 of (permalink) Old 10-23-2002, 10:12 AM
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Re: stumble at WOT and tach jumps way up?

Just so someone else with more insight into your problem can have some more data:

Can you achieve higher then 3000 rpm in first?

What happens if you ease it up to 2000 rpm and beyond instead of WOT?

The only reason that I'm asking is that you've kinda put it into a narrow band of operation with the WOT ..and given your tach anomoly could indicate issues that may not be encountered if you "ease it up there".
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post #4 of (permalink) Old 10-23-2002, 10:25 AM
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Re: stumble at WOT and tach jumps way up?

Well, my 88 YJ has 120,000+ miles on it and has led a hard life. I don't think it has ever seen over 3000 rpm in the 8 yrs I've owned it. Granted before it felt VERY unhappy to approach 3K, but now it is so much smoother. I just don't want to break something and the power drops off so much up there I can't see any reason to rev it higher. I just drive my 300ZX with underdrive pulley, lightweight flywheel, aftermarket clutch, and lightened 1 pc driveshaft when I want to feel high-rpm power :-)

I did try running at about 2000 rpm and opening the throttle, but not WOT. It did the same stumble thing. Even easing it up past 2200 or so seems to be a pretty constant sunble.

I'm guessing it was robbing too much vaccum and causing the timing to act up(vacuum advance and all...)??

I still do not know why the tach was jumping like that tho, it has never jumped before, always been stable at any throttle opening.

Mike
post #5 of (permalink) Old 10-23-2002, 11:28 AM
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Re: stumble at WOT and tach jumps way up?

The tach thingie is strange ...I don't know if an electronic ditributor can be subject to something like "point bounce" ..and due to the way the tach integrates the data ..interpret it as higher rpm. I wonder if lame distributor bushings could do something like that. [img]images/graemlins/confused.gif[/img]
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post #6 of (permalink) Old 10-23-2002, 01:44 PM
 
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Re: found the problem

what is the roll over check valve?

i would like to know. do you see it on this schematic (barrowed from the adventures under the hood web site.)






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post #7 of (permalink) Old 10-23-2002, 01:48 PM
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Re: found the problem

Follow the left-most pink line comming from the carbon canister on the right of the schematic.

It goes to the carb and "Tees". You can't tell, but one goes up and one goes down. The one that goes up goes to the roll-over check valve(I think that's it's technical term) on the top of the carb.

Physically it is located on the top front portion of the carb. It looks to be made of nylon abd has a large vacuum line going into it. Like 3/8" ID or so. I'll try to dig up a pic...

Mike
post #8 of (permalink) Old 10-23-2002, 01:53 PM
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roll over check valve

Here's a pic of what I am talking about. It is the yellowish thing in the pic with the large vacuum line comming off it and 3 rivets holding it to the carb.

Mike
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post #9 of (permalink) Old 10-23-2002, 01:54 PM
 
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Re: found the problem

cool, thanks. I get it now.
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post #10 of (permalink) Old 10-23-2002, 04:04 PM
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Re: stumble at WOT and tach jumps way up?

The tach jumping may or may not be related to the miss. I had a problem with the tach jumping and fought it a long time. running the ground cable from the battery to the block to the frame and to the body as well as a ground from the alt to the block helped, but did not elliminate the tach problem. Shortly afterward the alternator had to be replaced (the brushes were worn out) and the problen went away. You may also want to add a ground for the ignition. I don't remember what wire to splic into, but a search should answer that. Team Rush is a big proponent of the added ground.
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