69 cj5 spits and sputters in third gear

[69 cjfun]

I've got a 69 CJ5 . It's got the Dauntless Buick 225 V6 v6, and it absolutely falls on it face as soon as I try to accelerate in third gear. It runs perfectly at all rpm ranges in first and second gear as well as in neutral. Here's whats been done to it ...hei distributor conversion, heads had new seats and guides put in, I've run it with two different carbs and have the same issue with both.
So... what I'm thinking is that when I shift into third and change the load on the motor it loses vacuum causing the vacuum advance to fail causing a significant enough change in timing to cause it to run like it is out of time, popping and sputtering... push in the clutch it smoothes right out. down shift to second and it immeadiately recovers and roars to life.
Am I way off on my theory? Has anyone else run into this? How do I fix it?
Im going to recheck my vacuum lines, check for leaks in the carb and intake, but I've been anally thorough through my last inspections and found nothing. I'm going to add a vacuum canister to it to see if that helps.

 

[Torxhead]

I would say you are on the right track by going into the distributor. There could also be advance springs that need to be changed.

 

[hagar]

Check the carb base gasket for proper fit/positioning.
I think that will be your problem.

 

[Thorwon]

I would start by changing the fuel filter. It sounds like you're running out of gas.
I had a 67 Chevy that gave me fits until I had an old racer told me to check my fuel system. I ended up adding a electric fuel pump. That took care of it.
I don't think you will need to go that far.

 

[69 cjfun]

just changed the fuel filter, spilled a little gas and started it on fire. now I've got a whole host of other problems. lol! good thing I carry a fire extinguisher. It could have been so much worse.

 

[Thorwon]

just changed the fuel filter, spilled a little gas and started it on fire. now I've got a whole host of other problems. lol! good thing I carry a fire extinguisher. It could have been so much worse.

I'm glad you had the fire extinguisher. I keep one in every vehicle.
As far as the host of other problems, take them one at a time and you can fix them all.

 

[69 cjfun]

so I've cleaned up the fire damage. New cap and plug wires. new wire harness to the ignition module, waiting on a new radiator. I also replaced the carb adapter, carb base gasket, adapter flange gasket, installed a vacuum canister, changed the fuel filter, and replaced all of the fuel lines, and replaced the vacuum line to the pcv valve, as well as to the vacuum advance. Still when I hit 50mph in third gear it cuts out and sputters and will not accelerate anymore. Thinking back it's been doing this since around the time I had my heads re-done. I've cautiously checked for vacuum leaks with brake parts cleaner and found none. I've been through the carb, this is the second carb I've run and it does the same thing with both the stock and aftermarket carb. The only thing I can think of now would be low fuel volume??? I think I'm going to try an electric fuel pump next. I 'm perplexed. It runs flawlessly, starts easily, accelerates fast, and then falls on its face at 50 mph

 

[Thorwon]

so I've cleaned up the fire damage. New cap and plug wires. new wire harness to the ignition module, waiting on a new radiator. I also replaced the carb adapter, carb base gasket, adapter flange gasket, installed a vacuum canister, changed the fuel filter, and replaced all of the fuel lines, and replaced the vacuum line to the pcv valve, as well as to the vacuum advance. Still when I hit 50mph in third gear it cuts out and sputters and will not accelerate anymore. Thinking back it's been doing this since around the time I had my heads re-done. I've cautiously checked for vacuum leaks with brake parts cleaner and found none. I've been through the carb, this is the second carb I've run and it does the same thing with both the stock and aftermarket carb. The only thing I can think of now would be low fuel volume??? I think I'm going to try an electric fuel pump next. I 'm perplexed. It runs flawlessly, starts easily, accelerates fast, and then falls on its face at 50 mph

You have pretty well covered common problems. You might try a fuel pressure gage between the pump and carb. Mount it where you can see it from your seat. Take it for a drive and see what it's doing at speed.

 

[Torxhead]

I would also suggest what Thorwon did. An electric fuel pump can lead to a host of other problems, a mechanical pump on a carbed engine will work just fine. If you have at least 4 psi you are good. There could also be a low float level in the carb that could make your fuel supply a bit weak also. Determining whether this is a fuel or ignition issue would help too.

 

[shamrockarms]

new to forum and don't know if I'm answering or complaining about my own gm151.im having the same issues in a newly reman.jasper that I have only put 100 miles on it over the past 5 months(been working on other issues)my problem seems to happen only after it gets hot and then in all gears.my mechanic drives around the block and says its fine.well it is fine until you drive for awhile and then the peddle gets to a point it feels like I hit a wall.mine has a hei ign.put in a tach.and notice it has high idle like normal drops to 900 at curb idle but under a load after getting hot if I could use your quote ,falls on its face.my only idea is vapor lock,my gas is new and could it and your 69s gas be vaporizing when it gets to the carb.again sorry if im stomping on your post,rich

 

[69 cjfun]

I think I'll give that a try. I'm going to hook my vacuum gauge up to it at the same time and check both when it starts to stumble.

 

[69 cjfun]

you're not stomping at all! I've got a phenolic spacer/adaptor under my edelbrock carb so it should be insulated enough from the intake to not be a candidate for vapor lock. I had the same problem with the stock carb so I went to a thick base gasket under it to try to eliminate any chance of vapor lock. It didn't change anything on my jeep but it sounds like that might be worth trying. Fel-pro makes a gasket that is about 1/4 inch thick for the Rochester 2g carb. that might give you the insulation you need if yours is vapor locking. I'm working on my first cj so it's been a lot of trial and error, a fair amount of guess work, and a bit of luck.

 

[gutthans]

My first guesses would have been fuel pressure (filter/lines) and then vapor lock.

The engine doesn't take much fuel until demand loads up (usually high gear and higher speeds). That's why low gears run smooth. But vapor lock typically doesn't get in the way until you turn the motor off and it has a 'hot soak' while waiting for you to get back in...and try to start it again. But carbs can 'percolate' fuel in the bowl if they get hot enough and cause rough running too when the demand sucks the fuel level down enough to where the bubbling interferes with the flow.

So insulate the carb base...route lines away from the block or headers or wrap then in foil./(yes, kitchen aluminum foil!).