Vacuum Advance Trouble, 79 CJ5, 258 -
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post #1 of 5 Old 05-22-2020, 01:57 PM Thread Starter
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1979 CJ5 
Join Date: Aug 2012
Posts: 18
Vacuum Advance Trouble, 79 CJ5, 258

I could use some help figuring out what I think is a vacuum advance issue.

79 CJ-5, 258 ci, HEI (no Nutter needed), EMPI 32/36 carb, T-18 4 speed, 119th owner after many previous hack owners.

Under load (driving up a hill), engine will choke out and sputter. If you saw me driving up a hill, it would look like a teenager learning a clutch for the first time. I think Vac advanced is way off, causing a delay (or premature) spark.

Just sitting, I can rev all the way to red line and back to idle until I run out of gas. No problems at all. Down hill or on the flat, I can slow climb up to speed or jam through gears like I'm running from a T-Rex with Goldblum in the back...

I hit a hill, somewhere around 1,300 RPM it starts to sputter and choke out. Sometimes I can push through if I have it wrapped up high over about 2,400 RPM. When it's good and hot, the RPM range is about 1,600 - 2,200 RPM where I have the issue.

I sorted out the vacuum mess (I think) to it's simplest form. One vacuum tube from the distributor to port vacuum on the carb, and one tube from the front of the valve cover to manifold vac. Done - everything else is capped and removed. No change.

Smog pump, charcoal canister, EGR, spark COT and EGR COT vac switches all stripped out and capped off.

I can occasionally get the symptoms to occur if I load up with 4 people on the flat. My carb is running a little too rich, so I'll back that down next and then see if the plugs have a story to tell. I live at 6,400 feet. will also see if I can get a vac guage to see what's it's pulling...could have a clog I guess (same symptoms when I unplug all vacuum).

Guessing it's a stock 79 258. I haven't pulled the cam and measured lobes or anything. Odometer says 33K, so probably 133K.

Any tips you have to save me some brain damage? Half tempted to yank and build a 304...

I cut it twice and it's still too short
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post #2 of 5 Old 05-22-2020, 04:12 PM
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1956 CJ5 
Join Date: Apr 2015
Location: Boise
Posts: 1,481
Hi Not vacuum advance related.. its more carburetor issue. Float would be more accurate. I have heard that the Weber type carbs have a problem with hills and their float locations and levels. So I think your on the right track to lower it, as you say it a little rich. On a hill it may be dumping fuel into the carb OR it could be to low and causing the fuel to move away from the jets when on an angle like hills or a lot of folks in the back.. So I'd say adjust the float up or down and see if one or the other helps..
Some round d round drivers have the same problem when cornering fuel moves away from the jets and motor dies (been there done that and had that problem once on my round d rounder)
good luck
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post #3 of 5 Old 05-22-2020, 07:04 PM Thread Starter
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1979 CJ5 
Join Date: Aug 2012
Posts: 18
Thanks - I adjusted the float up (bent the little tab) and had improvement. I'll keep tweaking and see if I can find the sweet spot. I looked up specs - 35 MM from the bottom of the float to the closest flush point on the lid for the plastic float. Jeep is lifted a little, so the motor sits a little at an angle I went to 33 MM. Still had sputter, but like I said, I'll keep playing.

Other food for thought. If I put it in neutral on the same hill, I can wrap the motor from idle to red line without issue. I wonder if adding the vehicle momentum and jostling the jeep around while driving is adding to the woes.

Now semi-off-topic - I did have a bad fuel pump - so I swapped to a new mechanical. Then I added an electric fuel pump in line (just to charge the lines if it's been sitting a while). Electric fuel pump on or off has no impact.

I'll keep at it and post when I'm done. Guessing I probably have a mixed bag of minor tweaks to get it all square.


I cut it twice and it's still too short
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post #4 of 5 Old 05-22-2020, 07:11 PM Thread Starter
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1979 CJ5 
Join Date: Aug 2012
Posts: 18
Check out this guy's post: has a similar carb and says that the fuel climbs up through the vent in the top of the bowl cover. He plugged it and relocated the vent. Pretty low risk mod - I'll add that to my list of tricks.

I cut it twice and it's still too short
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post #5 of 5 Old 05-22-2020, 08:51 PM Thread Starter
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Ok - last status of the night. 33 MM float measurement, better, still sputtered a bit. Could do short hills just fine, no dice on long steep hills. Read that other guys post linked above, and I adjusted to 37 MM. Nope, worse it's ever been. Back at it, went as high as I could (31.5 MM) to where the float just started to make contact with the cover plate, yet still close the needle valve. Still sputtered a bit, but now pretty much the best it has ever been, and I'm out of adjustment options.

Contrasting to the other fellas post, my bowl cover has both the rectangle vent and tube vent. I covered the rectangle vent from the top with a little chrome on a roll, just to test. I did about 10 miles, half hills. Sputtered just a hair on one of the 6 hills I climbed. The rest I accelerated through the hills just fine. I think that's as good as I can make this issue with this carb.

I'll drive it a while and see how it tests my patience. Maybe upgrade the chrome on a roll to JB Weld. Worse case, I'll go find a motorcraft 2100 and let someone else enjoy this carb.

I cut it twice and it's still too short
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