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Unread 06-13-2010, 08:37 AM   #1
walkerhoundvm
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1981 CJ8 
 
Join Date: Apr 2010
Location: The Little Apple, KS
Posts: 3,090
Occasional stumbling +/- stalling - carb related?

Hoping someone here can help me with this, it's driving me slightly nuts.

The basics: newly rebuilt MC2150, has run great for the past 1000 miles since professional rebuild and calibration. Less than 1000 miles since TR upgrade, with everything from coil to plugs to wires replaced along with the cap and rotor. Engine has less than 7K miles since rebuild. All vacuum and fuel lines are fresh, the only vacuum line is to the distributor advance. The distributor diaphragm is less than 1K miles old. Fuel filter is less than 1K miles old. I've retorqued all manifold bolts and carb attachments to the 6=8 clifford manifold. She has dual exhaust and cats and a 9" K&N filter, which is clean. Timing was performed based on vacuum pressure because I'm at altitude, and it's way overshooting the timing marks (i.e. it's 14 or so at idle). She's an '81 CJ8, so my understanding is that a Nutter is not possible.

Just in the past two weeks, and only occasionally, whether she's hot or cold, there'll be stumbling on acceleration. Usually I can tell there's effort, but then she trips over herself. It's most obvious in 1st gear coming off a stop, but I can tell it's doing it when cruising in 2nd or 3rd when I give her a quick shot of gas. I can manually pull the throttle and see gas spurt into the carb. I have always noticed a smell of gasoline in the garage after I get home, I'm not sure if this is related, and it's coming right from the top of the carb when I pull the air filter. When I let her sit, she'll start right up like nothing happened. I can't tell what will set it off, but it's dangerous when you're trying to make a left turn across traffic and she just about dies! It helps somewhat when I close the choke down and let her idle high. If I ease on the clutch and give her a bit more gas, she'll catch up and make it across the intersection. I sprayed carb cleaner and WD-40 all over the place: manifold, base of carb, no changes in idle that I could pick up.

What's really odd to me is that she ran perfect for nearly 1K miles. Nothing changed over the course of those miles as far as engine or fuel, so I don't think it's anything I've done. Any ideas?

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Unread 06-13-2010, 08:56 AM   #2
Mike Romain
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1986 CJ7 
 
Join Date: Nov 2007
Location: Off the Grid in Chester Grant, Nova Scotia, Canada
Posts: 13,781
A plugged up air filter on, or a just plain unhooked or blocked gas tank vent can cause your symptoms. If the charcoal canister is still there and not hooked up to purge, it will saturate with fuel which puts a vacuum on the gas tank causing bogs. The $2.00 air filter on the bottom can also cause the vacuum if it gets too dirty. If the canister is removed and the line blocked, same deal. The canister is a 'good' thing to have....

The fast check for this is to just loosen off the gas cap and see what happens.

The gas filter is also supposed to have two outlets wit hthe top outlet going to a return line so on a hot shutdown any vapors produced get blown back to the tank rather than vaporlocking the gas line or carb.
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86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread 06-13-2010, 09:20 AM   #3
walkerhoundvm
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1981 CJ8 
 
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Location: The Little Apple, KS
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Thanks, Mike, I'll try the loose gas cap check. By plugged up air filter do you mean the one on top of the carb (it's freshly cleaned as best I can tell) or somewhere else?

Any thoughts on why it only just started happening, after nearly a thousand miles of great running with the same setup? Really the only thing I can think of that's changed is maybe that the weather has gotten a little warmer, though she certainly has put a lot of miles on in hot weather in the stretch before the past two weeks.

Also, my fuel filter is single inlet/outlet. If I had a dual outlet filter, where should the second hose go (and which one!)? I still have my charcoal canister, but the only line still attached to anything as far as I can tell is one in the middle heading back towards the tank. Not sure if it makes it all the way there. There are two coming off the front, from a little circular jut that are about 2' long and just sucking air. One of these two (the larger diameter one coming more from the bottom of the circular jut) was attached to a strange S-bent aluminum pipe, about 1/4" diameter, which I still have but pulled since it seemed to be doing nothing more than rubbing against the steering column. Perhaps this is where the other outlet hose should be going? What about the other one, a smaller diameter tube coming more from the top of the jut? There is also another port towards the firewall that has no tubing coming out at all.

If it helps I will be taking pictures and posting them. Thank you so much for your advice so far.
-Nick
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Unread 06-13-2010, 09:46 AM   #4
walkerhoundvm
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Promised pics

OK, here's more an idea what I've got that may be contributing to a vapor lock or bogging situation.

Single inlet/outlet fuel filter:


Removed "S" shaped aluminum hosing, previously doing nothing more than rubbing against steering column:


The grand tamale, the charcoal canister, with key to hosing:


Is this information in a Haynes manual? I buy into the vapor-lock theory. It would explain why when I shut her down she may re-equilibrate and then be good to go. I thought at first it might be grit in the needles, so tried Mike's "throttle then choke it down, like blowing your nose approach." Not sure if it helped, the problem still seems to be occurring.
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Unread 06-13-2010, 10:10 AM   #5
mcmud
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1983 CJ8 
 
Join Date: May 2008
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The 's' line is the fuel return line and it was connected to the dual outlet fuel filter mentioned earlier at the one end and the the tube that you have identified as the "bottom hose". That is the hose connected to the fuel return to the fuel tank line.

The "aft" tube is the fuel tank vent.

The open nipple on the canister is for the carb bowl vent tube.
The "circular" cap is the canister purge signal valve.

Not at all sure what the "smaller diameter" tube is. Where does the other end lead.
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Unread 06-13-2010, 11:00 AM   #6
walkerhoundvm
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That's helpful, mcmud, thanks. Mike, is that your opinion too, or can you ID what the smaller diameter tube going nowhere is? Perhaps it was supposed to connect into the old air filtration system, pulling unspent gas off?

As far as the open port that is for the carb bowl vent - I have no other connections on the MC2150, so I assume it will just stay there open, or should I cap it?

Last thing, I traced the "aft" tube all the way back, and I think it ends up here, in this unholy, rust-laden, crud filled area of my driver's side rear corner. It looks like there may be something missing - an air filter perhaps? It looks like this whole unit, whatever it is, may need to be replaced along with the hosing going to it, if it would be any help.

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Unread 06-13-2010, 11:22 AM   #7
walkerhoundvm
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OK, I did some more searching. Looks like I'll have a heck of a time finding and replacing the rollover and liquid check valves, though both look spent (pictured above). I found great threads going over the canister and check valves here:

http://www.jeepforum.com/forum/f8/fu...roblem-768433/

and here:

http://www.jeepforum.com/forum/f8/cj...blems-1015391/

In any case, as far as I can tell the charcoal canister is completely disconnected from anything important in the engine, so likely isn't the issue. If the two hoses running back to the liquid check valve are blocked, well obviously they need to dealt with. At some point if I want to revert back to stock, which may be an issue come emissions time (only a sniffer test next year), I'll look into getting all of this rigged up over the winter, maybe.

In the meantime, it looks like I could check my gas cap to see if it's vented, and if so clean it out, if not get a new vented gas cap, right?
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Unread 06-13-2010, 02:37 PM   #8
walkerhoundvm
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1981 CJ8 
 
Join Date: Apr 2010
Location: The Little Apple, KS
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OK, here's what I've done based on some sound advice: I bought a two-outlet fuel filter and 3' length of hosing to run from the auxiliary outlet on the filter to the gas tank return. As Mcmud pointed out, the S bent tubing was the original connector between the filter and return, but for the life of me I can't figure out how it's supposed to be mounted on the body...at least how it's supposed to be mounted and not in the way of the steering column and exhaust headers.

I now have the new filter in, and the aux. hose running across but out of contact with the manifold and carb down to the return. I checked first to make sure the return wasn't plugged, and it was wide open. Took her for a test spin and so far so good. I also bought a new vented gas cap, but won't install it unless the problem returns. I'll let everyone know how it goes after she's had a few more runs, but one thing I have noticed is that the overwhelming smell of gasoline is nearly gone from the carb after our test run.

Thanks for your help,
-Nick

Last edited by walkerhoundvm; 06-13-2010 at 03:02 PM..
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Unread 06-14-2010, 06:43 AM   #9
Mike Romain
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Posts: 13,781
If you are going to hook the canister up, all you need it the gas tank vent line with the MC carb. The canister gets a turn on signal from the S port or the line to the distributor and it purges into the PCV line via a T fitting. Some canisters use the CTO valve so they don't turn on until the coolant has warmed up.
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86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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carburetor , motorcraft , stalling

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