C3 Diagnosis question

[Gary S.]

Hi to all. I drove the 200 miles from home to Carlisle and the 72 LT1 ran beautifully at highway speed of around 65-70 mph. Driving the stop and go traffic from the turnpike to the fairgrounds on route 11 was another question. It idled in a less than an impressive manner and bucked occasionally. The car is a 36000 mile 72 LT1 with all original components and a one year old Jerry MacNeish rebuilt original distributor and carb. New Delco spark plug wires and AC plugs (44s??). Recent dwell check and timing check. The car is set up as close to original specs as can be expected and is a 96+ point Top Flight car.

Vacuum leak? Float set too high? Screwed up flux capacitor?

Hey engine gurus, any ideas here?

Thanks in advance.

Gary

[Mark #28455]

Re: C3 Diagnosis question

Is this new since the carb & dist rebuild? Then you know where to start.

On the other hand, my Holley equipped cars always have acted up after a long, hot drive. I used to commute from Baltimore to Washington 20 years ago and every time I would get off the highway, the engine wouldn't want to idle and low RPM drivability was lousy. I eventually added a factory heat shield as used on the LS6 and ducted cold air to the air cleaner and it finally ran ok.

Good luck,
Mark

[Duke W.]

Is your heat riser wired open? *NM*

[Klaus Friedrich #33362]

Re: C3 Diagnosis question

Hi Gary,

that's the mess with the open airfilter equipment.
After driving on highways the engine and all parts in the engine bay are very hot. In this way the air that goes into the carburator is also hot. The mixture between gas and air is getting bad. After driving awhile all parts are cooling down and the engine is running better.
Try it on a cold winter day and your car is running much better than in summer.
The Chevrolet engineers knew this problem: Take a look to more modern cars - the air intake is in front of the radiator to get cool air.

That's all

Greetings from Germany
Klaus #33362

[Gary S.]

Re: Is your heat riser wired open?

After the carb and distributor rebuild, I wired open the heat riser. That way, should I ever decide to judge this car again, a pair of side cutters would put me back in business.

Gary

[Gary S.]

Re: C3 Diagnosis question

This is the first long distance trip taken since the rebuild. Typically I run 10 miles or so to the nearest drive in for an old fashioned burger and shake. Car is ok, more or less, after that 10 mile jaunt down the road.
Gary

[Tom D.]

heat riser down in the summer?

What's the downside of latching the heat riser down in the summer? Just a little rougher cold engine operation?

Tom D.
4889

[Gary S.]

Re: heat riser down in the summer?

Wiring the heat riser open doesn't really affect the engine idle quality after the first 10-15 seconds of idling in the summer time. The up side is you never have to worry whether or not the riser opens. If you don't have the car judged, you can replace the riser altogether and put a spacer in there. I don't remember what the spacer is called but I know Paragon sells one.

Gary

[Gerard F.]

Could it be the gas?

Gary,

My 67 with a Holley on it, does the same thing. Runs great when cold and at high speeds, but when it's hot it begins this rough idle routine. Last time it did this I pulled the air filter, and saw gas dripping into the air horn from the secondary bowl. Either the float gets stuck when hot or the gas is boiling in the bowl.

I have seen some recent posts about the lowered boiling point of today's gas. Maybe that's the problem. Have a post into Clem on this very subject.

I know in making Grappa, the higher alcohols start coming off the still at about 160 degrees and I'm about done when the pot reaches 200 degrees. Maybe it's similar to gasoline. (Tastes like it )

Jerry Fuccillo
#42179