Rough running when warm,

Joined
Mar 13, 2003
Messages
192
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Raleign / Durham, NC
I've been experiencing some engine idle issues with my ring for some time now, and I still haven't been able to figure out what the root cause is. Here's the deal:

When I first start it up, it idles just fine (I have it set at about 800 rpm, even though it's supposed to be lower). It runs great all day long on surface streets. When I take it on the express way and drive 60mph for about 20 minutes (I don't let the tack get above 3000 rpm), I have a strange idle situation. When I get off the express way and sit at the end of the ramp, it idles at about 1200 and runs really rough (the temp gage reads about 1/2 deflecction). It takes a while of driving on surface streets to get it back running normal again.

'79 2F in a FJ40.
All emissions controls removed several years ago.
Cat converter has been removed.
Original carb was replaced with a Rochester 2 barrel.
The wiring for the IGN system is all new (and separately fused).
The carb has been adjusted recently (it was a little rich, but didn't seem to fix the problem).
The valves have been adjusted recently (also didn't seem to fix it).
I run 89 octane gas.
The fuel pump is an electric rotory type (with a regulator set to 4.5 psi).
The choke control on the carb has been removed.
The temperature gage wiring is new.
Compression check showed all near the lower limit.

Would a vacuum leak cause this condition?
Would an exhaust leak cause this condition?

Any ideas are appreciated.
 
Joined
Mar 13, 2003
Messages
192
Location
Raleign / Durham, NC
76fj40can,
I didn't think of that. I do notice A LOT of vacuum pressure when I take the gas cap off to refill. I do not have any venting for the fuel system. I'll vent it and see what happens.

Thanks for the suggestion!
 
Joined
Apr 11, 2003
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i am trying to track down a similar problem. I did have the venting problem -- i used to have major pressure release when i openned it up to refuel. Now that's totally remedied -- you should get little to no pressure -- but the poor running problem is still there.
 
Joined
Mar 13, 2003
Messages
192
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Raleign / Durham, NC
To test the potential solution, I only engaged the gas cap half way to allow the tank to vent. Unfortunately, that did not solve the problem. I did, however, notice that I had an exhaust leak at the head and thought that it may be a contributor to the problem. Two of the 4 bolts that hold the manifolds on were loose. I tightened them to minimize the leak and found that it helped a lot. As a note, I do not have a stock carb or air filter. The filter just sits on top of the carb and pulls air from the engine compartment (and thus may have been pulling exhaust in as well).

I had been told by a friend (knows a lot about engines) that a vacuum leak would also be a contributor. He said that as the engine warms up, components expand and open up leak paths.

I may pull off the manifolds, check them for flatnesss, then reinstall them with a new gasket... We'll see what happens.
 
Joined
Mar 1, 2003
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239
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Wasilla/Bangkok
I recently had a similar problem. Check to make sure the ingnition system is working right. :slap:With a timing light check each plug wire to make sure your intermitant fault is'nt there. My problem turned out to be a faulty didtributor cap, the center contact was making intermitant contact after it warmed up.
I found that one out after rebuilding the fuel pump, carb and checking the intake and fuel venting. Drove me crazy for a couple of days. :doh: :beer:
 
Joined
May 28, 2003
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cafe.rapidus.net
...  Two of the 4 bolts that hold the manifolds on were loose.  I tightened them to minimize the leak and found that it helped a lot.
... that a vacuum leak would also be a contributor.  He said that as the engine warms up, components expand and open up leak paths....
Bonjour,
Upon your description and my honest opinion, I would bid on the warming-open-up vacuum leak.
P.O.
 
Joined
May 5, 2003
Messages
70
Location
Ruston, Louisiana
if you are running the stock exhaust manifold you could have a cracked intake manifold. the cast iron exhaust manifold is open to the bottom of the aluminum intake for warming purposes. over time the intake is prone to crack. you can check it by simply removing the carb and and the pcv suction port and looking at the bottom flat surface of the inside of the intake. This could be the cause of an intermitent but sizeable vacume leak. the crack can be welded. I put on headers and j.b. welded my crack....in my manifold that is.
 
Joined
Mar 13, 2003
Messages
192
Location
Raleign / Durham, NC
Thanks for the hints. I can take some time this weekend and check things out. I have never messed with the distributor, so I have some learning to do there.

I'm curious to see what the manifold will tell me. They have never been taken off, so I better be real careful and use the advice from previous posts to slowly back the bolts out.

Cheers
 

STR

Joined
Jun 20, 2003
Messages
2
It could also be something as simple as the linkage from the throttle to the carb sticking. I use to have to put my foot under the throttle and pull back on the pedal with my foot to bring the idle back down on my old 76 Cruiser. The linkage always got hung up in one particular spot and a quick snap of the pedal where it brought the rpms up and then came back down didn't always fix it. It was like that from the day I first drove it to the day I sold it. HTH.
 

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