I am at the end of my rope with a hot idle problem. It sounds just like a vacuum leak at the exhaust ( that tell-tale chuff chuff sound) and the engine is shaky. It happens only when hot and run for awhile. Sometimes (rarely) it idle OK when hot. The Engine , a 350, runs great at all other throttle positions. The timing is a 7 BTDC, electronic ignition conversion, new tank ,new fuel pump, rebuilt quadra jet (professionally) and the throttle bushings and well plugs were done, new filters, new intake gasket (valley pan), new plugs and wires.

I tested all the vacuum lines, intake, pwr brake booster, trans modulator by pinching them off or propane. The idle screws are 4 turn out to get it to idle reasonably.They should be 1 and 1/2 I believe. Out richens it so I suspect a vacuum leak I just can't find. Or possibly that quadra- jet is in trouble. It runs perfect until its hot at an idle and then the idle is "shaky".
The only thing I am unsure of is the vacuum line routing. I don't have a diagram to verify....Any thoughts would be welcome...John

 

Well, it does SOUND like a vac leak. It might be a crack that opens up as the engine gets hot. It might be the underside of the intake manifold. It might be the hot idle compensator.

 

What idle RPM are you looking for....? Maybe just an adjustment or fine tuning is required between curb idle, and mixture screws......just a thought.

 

it's very easy to check for a bad hot idle compensator. on the rear of the carb there is a small, spring-loaded pin, push it in straight with your finger. if the rebuilder overlooked the h.i.c. gasket-that would cause a vacuum leak.


bill

 

Valve clearance problem when hot maybe ?

 

I will check that out when I get the car back on thursday from getting the dual exhaust installed.

 

Could be....I haven't checked. They don't rap. I haven't checked the compression yet either. It runs soooo nice off idle though. I don't understand why compression or valve lash would be it.

 

Done that , but thanks.

 

It would be good to check those things, just to make sure.
My '71 Ford F-250 with a 390 would act up like that when the points and condenser started to get old.

 

Had a 73 Delta that would act like that after it got good and warm. It was the Accel coil I had put on a few months earlier. Only coil I ever had to just absolutely fail.

My Toronado would act up warm too and that one ended up being a wore-out distributor.

Something to check- does the electronic ignition require a full 12v to operate, or can it run off the resistance wire?

 

Is the choke set correctly and working? rebuilt isn't always good, I went through three "rebuilt" on mine. The butterfly isn't hanging and the pull off okay? If it were me and the carb is okay externally I would borrow a carb or get another one that I know is right and make sure it isn't that, sell it or return it if yours is okay, start eliminating things one by one. Engines chug when the choke is stuck, no heat riser valve on those is there? those really chuff when they get stuck.


Allan

 

The choke functions perfectly....its only has a problem at a hot idle. Something is "running home to mama" when the engine is hot, that is operating temperature. Its important to note that it had the same issue before the carb was rebuilt....it improved a bit.

 

Check the other post "rough idle after 20 seconds or so", sounds like you may have a similar problem. His was corrected with spark advance adjustment.

 

OK. For those interested, I did a compression check this morning...........1) 115 3) 115 5) 100 7) 105
2) 100 4) 100 6) 100 8) 115
When I checked the rings by compression test w/oil, it only raised them 3-5 lbs each.

I think a valve(s) or seat(s) is/are in trouble. I adjusted the idle mix screws WAY out ( 6 turns ) on the "iffy side", 4 or so on the other side to get the best possible hot idle and I will live with it.

At this point rather than put the money into this engine, I will start looking for a rebuildable big block..

 

BTW...the dual exhaust with flowmaster 40's sound heavenly!!