Skipping and chugging at idle when hot (1 Viewer)

[myobsessions]

Hey all
I have a bit of a strange one.

Background: 1973 fj40 with the engine, transmission, transfer case and driveshafts out of a '78.

I kept the following parts from the '73:
Valve cover, distributor (vacuum line is connected), fuel pump, intake & exhaust manifolds, & carb (Holley 7440 2bbl).

It starts and idles perfectly. It drives perfectly, for about 5 minutes until it really gets warm. Then it starts to chug and feels like it's idling on 5 cylinders. It never stalls, and when I give it some gas it smoothes out and runs strong even when hot.

It has brand new spark plugs, wires, cap and rotor. Older coil (hrm...).

It runs perfectly every time when it's cold, and runs like s*** every time when it's hot.

I did an admittedly lazy forum search and didn't see anything that immediately jumped out at me as a match for my symptoms. What have I done wrong (besides the possible blasphemy of transplanting a 2f I to a 1f rig)?

Thanks in advance!

 

[charliemeyer007]

How old is your gas and filter? Run some fuel treatment stuff. Scrape the contacts inside the distributor cap if they are aluminium. How is the fuel pressure. For like $20 you can add in an electric fuel pump to runs as a backup/aux unit.

Maybe you are vapor locking.

 

[pb4ugo]

Are you using the choke on cold start?

 

[bkfj40]

Is your intake manifold cracked under the carb?

 

[Green Bean]

How hot is "hot?" In other words, once it warms up, is it running at normal temperature? Or is it overheating?

 

[myobsessions]

Are you using the choke on cold start?

No choke needed on cold start, and it's wide open.

 

[myobsessions]

How hot is "hot?" In other words, once it warms up, is it running at normal temperature? Or is it overheating?

Normal operating temp not overheating at all. Center of gauge.

 

[myobsessions]

Is your intake manifold cracked under the carb?

I don't think so but that's a good thing to check. I have a spare intake and carb that came with the 2f, but I kept the 1f kit because it all bolted right up to the existing exhaust. My intent has been to clean up the long tube header that came with the 2f and have an exhaust fabbed up that sounds better. I've done all of the work on the truck to this point but I don't have the tools or desire to bend and build an exhaust.

 

[myobsessions]

How old is your gas and filter? Run some fuel treatment stuff. Scrape the contacts inside the distributor cap if they are aluminium. How is the fuel pressure. For like $20 you can add in an electric fuel pump to runs as a backup/aux unit.

Maybe you are vapor locking.

All fresh gas, new fuel pump and line. I ran a new hard line from the mechanical fuel pump to the carb, it's definitely not getting too hot. It also runs smoothly under load, even when hot. It only runs like crap when it's idling or under minimal load when hot.

 

[myobsessions]

Also thanks to everyone for you ideas and input so far! I'll look at the manifold to make sure it isn't cracked or pulling extra air from somewhere.

 

[charliemeyer007]

Ok. Vacuum leaks have a giant impact at idle compared to open throttle. Grab a spray can of WD-40 with a pee tube and check for leaks at all the gaskets when it hot - rpm's will kick up when the spray hits the leak.

 

[1911]

What did the spark plugs you took out look like? Photos would help.

 

[bkfj40]

Ok. Vacuum leaks have a giant impact at idle compared to open throttle. Grab a spray can of WD-40 with a pee tube and check for leaks at all the gaskets when it hot - rpm's will kick up when the spray hits the leak.

Along the same line - get a rental vacuum gauge from the parts store and see what vacuum it is pulling.

 

[charliemeyer007]

I still have a big 3 or 4 inch vacuum/pressure gauge hanging on a stainless steel GI issue dog tag chain from the middle dash knobs. It really helps with gas mileage - there is a point when more gas pedal just makes more noise not more speed

I got the gauge for free as part of getting paid to clean out a chemical engineering lab room at the university. I still 3 brand new standard taper glass water cooled condensers and a pint of distilled mercury.

 

[myobsessions]

Ok. Vacuum leaks have a giant impact at idle compared to open throttle. Grab a spray can of WD-40 with a pee tube and check for leaks at all the gaskets when it hot - rpm's will kick up when the spray hits the leak.

That's a great idea. I do have a vacuum gauge, though it's for balancing Stromberg carbs so I'm not sure it's the right type for this.

 

[charliemeyer007]

Start the engine, run the engine at a what ever rpms it will do steady. Spray the intake gaskets linearly right at the mating seams - any leak will cause rpm's to jump - fix that gasket. Intake manifold, carb, and maybe even fuel vapor recovery system

 

[myobsessions]

Start the engine, run the engine at a what ever rpms it will do steady. Spray the intake gaskets linearly right at the mating seams - any leak will cause rpm's to jump - fix that gasket. Intake manifold, carb, and maybe even fuel vapor recovery system

Indeed my little Stromberg vacuum gauge is not able to read anything but that type of carb. So instead, I took it for a spin this afternoon and isolated the leak to be on cylinder 5 by pulling that plug wire when hot, and the idle didn't change. I'm going to pull the manifolds and look closely once it cools down.

I have the intake manifold and stock carb from the '78. Anyone know if I can mate up the exhaust manifold from a '73 with the intake manifold from a '78?

Worst case I can clean up the (stainless?!) long tube header that came in the '78 and have a new exhaust made up that bolts to the header (attachment point is much lower on the header obviously).

 

[bkfj40]

When was the last time that you checked valve clearances on the engine? Spraying it down to check for leaks would be good as well. It may be as simple as a new gasket or a valve adjustment

 

[myobsessions]

When was the last time that you checked valve clearances on the engine? Spraying it down to check for leaks would be good as well. It may be as simple as a new gasket or a valve adjustment

I had the head off when I was refurbishing the engine. I did check all the valves at tdc to make sure none of them were too tight. And while I didn't put a gauge in them, they all have a similar amount of play on the pushrods.

I'll spray for leaks, couldn't find my wd40 this afternoon so I have to go out for a new can

 

[charliemeyer007]

I been setting valve lash for like 60 years. I would never do it without the correct tool - a good set feeler's gauges is cheap. In fact buy 2 sets, keep one set whole and strip one set to the ones you need say 0.006, threw 0.015, then you might have whatever the points gap is set for, spark plug gap...

I set the valve lash on the loose end of the range - better to hear them than to burn them.