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Unread 01-18-2014, 12:39 PM   #1
mefish
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1987 YJ Wrangler 
 
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87 YJ 4.2L Conundrum

Hey Guys,
Wondering if anyone has any additional thoughts on this miss I have.

Here is the status:
Compression check shows 138 to 145 in all cylinders
Nuttered
Team Rush ignition upgrade
new timing set
New exhaust and intake gaskets
Weber 32/36 needs a little tuning yet trying to get the guys at Redline to respond to my conditions
New motor mounts
New PCV Valve connected to manifold vacuum port.
New EGR valve connected to ported vacuum

When it starts cold it has a pretty significant miss and runs pretty rough (shakes the whole Jeep) after warm-up it improves but is still not a real good idle. High speed it is not noticeable but hard to tell if it is still not "quite" firing on all cylinders. Plenty of power though running down the road.

No signs of a head gasket or cracked block as I am not losing coolant and there is only a trace of condensation on the oil dipstick end and around the oil cap but I attribute this to normal condensation.

When I connect my vacuum gauge the gauge fluctuates back and forth rapidly with only 16 inches of vacuum indicated. It jumps maybe 2 inches each way as it fluctuates and does not settle down at higher RPMs.

This could be indicative of a vacuum leak still but I have checked all the lines (disconnected them and plugged them all also when testing). I have no vacuum at the ported vacuum port on the weber at any throttle position. On the weber settings it indicates that I am rich as I am only out 1 turn on the mixture screw and one turn in on the idle screw at 800 rpms. If it was a vacuum leak wouldn't I be leaner? The exhaust stinks of a too rich condition as well. Can you lose a head gasket with no sign of water loss or contamination of the oil or white smoke? Also the coolant level is holding steady. On start up it does release quite a bit of condensation thru the exhaust but when I check it the water/moisture appears very clean.

This fluctuation could also be indicative of a valve problem but with the compression test so good the only thing I can think of is maybe a bad push rod or broken rocker that is not opening a valve since any other condition ie burnt valve or sticking valve would cause a low compression reading right?

I have not pulled the head yet (it is about the only thing I have not done)but as I run through all the scenarios the only things I can come up with that would make it miss with having such good compression is something in the top end not working quite right. Anyone have any other thoughts on this?

As always, your help is much appreciated,
Mitch

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Unread 01-18-2014, 01:13 PM   #2
Old4X
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Intake manifold bolts tend to work loose and cause vacuum leaks. Check torque.

The mounting plate/gasket area for those carbs are notorious for having vacuum leaks. Inspect and reinstall with good gaskets.

Your vacuum gauge is telling you there are vac leaks.
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Unread 01-18-2014, 01:15 PM   #3
Old4X
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Also, you may have a bad intake gasket
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Unread 01-18-2014, 01:28 PM   #4
GrEmLiN
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Absolutely second old4x's diagnosis. I had the same problem.

Snug up the manifold bolts after a couple warm up/cool off cycles then pull the carb and adapter plate, deck the manifold lighty (I carefully used a large flat file, keeping shavings out of the inlet) and new gaskets with a skim of RTV.

Ultimately I had the damnedest time trying to tune the Weber and since every one of my race cars has had a Holley over the years I had all the Holley parts handy to change out my Weber. I've got an extra Weber I rebuilt that I'd part with.

Good luck, you're on the right track. The problem is most likely in the adapter plate stack.
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Unread 01-18-2014, 01:30 PM   #5
GrEmLiN
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Double check the intake/exhaust gaskets to make sure they didn't shift when installing them. Those two piece dealios can be a pain in the neck.
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Unread 01-18-2014, 02:22 PM   #6
Old4X
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Quote:
Originally Posted by GrEmLiN View Post
Double check the intake/exhaust gaskets to make sure they didn't shift when installing them. Those two piece dealios can be a pain in the neck.
And is the reason for the one piece gaskets
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Unread 01-18-2014, 07:40 PM   #7
mefish
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I hear all of you but this same condition existed before the gasket changes also which is what started me down this "fix stuff" path when I bought the rig.
I will quadruple check the torque on the manifold bolts but I have been thru that but may have a problem there still. I've tried spaying carb cleaner around the manifolds and carb connections with no change. Also the weber is a brand new install so I don't think there are any leaks there but I will double check that too.
Any other thoughts if we were to assume that these things have been done correctly? Not sure why but my gut feels like I am chasing my tail...
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Unread 01-18-2014, 08:24 PM   #8
jay-h
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I had a similar problem when I got my YJ (previous owner had a head gasket replacement, then had idle problems).

After much fussing, I found a loose nut on the intake manifold. The biggest improvement from a 3 minute repair... EVER.
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Unread 01-18-2014, 08:54 PM   #9
GrEmLiN
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Is the Weber brand new?
The adapter plates may not be flat and thus not seal well.
I had the same issue on mine, but the brake cleaner method didn't work to find it, I had to try different things until it went away.

Ultimately the adapter and carb gaskets were to blame. I decked everything with a file, it was pretty far from flat.

It could've been one problem before, that you fixed, but now created a new problem?

Personally I found my Weber less responsive to tuning and idle quality not as good as a Holley. YMMV.

I hope you don't give up looking for it. I'm truly interested to see where this goes considering I've been in the same boat.

After slapping an entirely-too-large Holley on mine and getting it reasonably dialed in, I didn't bother to further tune it because it's not my daily driver anymore. It runs better than it ever has and that's good enough for now.
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Unread 01-18-2014, 10:43 PM   #10
jeepguy85
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I would also check your carb mounting surface for vac leaks.
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Unread 01-19-2014, 05:26 AM   #11
CB10101
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Instead of using a file on the carb adapters, lay a sheet of 60 grit on the cement floor in the gargage and deck them that way. Easier to make sure you are level across the entire surface and don't drop shavings into it.
The 3 weber adaptor plates are known for not being flat and causing leaks. You can also use a little bit of permetex #2 with the paper gaskets.
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Unread 01-19-2014, 07:19 AM   #12
fishadventure
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[QUOTE=CB10101;19544633]Instead of using a file on the carb adapters, lay a sheet of 60 grit on the cement floor in the gargage and deck them that way. Easier to make sure you are level across the entire surface.../QUOTE]

I do something like this but a little more precise. I have a used tempered glass display shelf 3/8" thick by 14"x36" I picked up from a store remodel scrap heap. It is PERFECT for things like this.

I use rolls of adhesive-backed sandpaper which I stick on the flat glass plate or I tape 8-1/2"x11" sheets to it depending on what I need to do. When parts are scuffed on this setup in a rotating figure eight pattern you get a machine-quality truly flat surface. I mostly use 180 grit but 120 is a close second. Any plate glass will do but the shelf was perfect. Before I had the easily moved glass I had a piece of broken polished granite countertop which was nearly surface plate flat but weighed like 250#

Just thought I would pass that along...it's been very useful for me.
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Unread 01-19-2014, 12:44 PM   #13
GrEmLiN
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Excellent suggestion. I did the file method because I discovered how unflat the brand new plates were when I did a carb rebuild and already put them on. The file didn't work as well as your flat plate glass method, but it wasn't too hard to see where the file had been and hadn't been.

Still... I'm gonna keep my eyes out for a piece of plate glass. Good tip.
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Unread 01-19-2014, 01:55 PM   #14
jay-h
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Truth is, though, if it's a regular miss, I would be looking at something that affects one cylinder. A leak at the carb plate will lean all cylinders equally causing general idle problems and running issues.
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Unread 01-19-2014, 02:13 PM   #15
GrEmLiN
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^^^ Excellent point. Doesn't really explain the vacuum gauge readings but vacuum gauge WILL fluctuate under normal operation so who knows...

I also had that problem a few months after my carb leaks. You may have more than one issue.

Pull the spark plugs. Inspect them for broken piece of ceramic insulator or cracks anywhere. Dropping a spark plug is like dropping a lightbulb. It's pretty much done for once you've dropped it on the ground.

Then I would say pull one plug wire at a time with the engine running to determine if the miss is limited to one cylinder in particular, if the spark plugs aren't already an indicator. If one plug shows obvious signs of misfiring, then swap the plugs around to different cylinders and see if its a bad wire or that particular plug. Could also get new plugs and wires, changing one at a time to narrow it down to which plug/wire was failing. Cheapest route though is pick up 6 or 8 new plugs (Autolite 985) and try them. Don't drop any and you've got extras if you find one that's bad.

It also may help to make sure you have good grounds tying the engine, chassis and body all to the negative terminal of the battery. Some of that spark energy could be finding its way to places its not supposed to.
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