Edelbrock 750 - Very Slow Return to Idle

[EddieE]

I have an Edelbrock 750 that was on the car when I bought it 7 months ago.  340 was completely rebuilt and stroked by Ryan Johnson at Shady Dell Speed Shop. Now it's a 416.

Engine was dyno'd with this carb, and I've put about 700 miles on it since I put it in the car.  Carb has run well, although I still think it has too much of a stutter coming out of the hole in normal driving with a 4-speed.

Enough history.  Now the engine runs fine, although it runs at about 220 degrees while driving and about 190 while idling.  The symptom I have is a very slow return to idle after acceleration.  If I run the engine up to 2500 or 3500, it SLOWLY comes back to idle.  I looked in the carb and the butterfly seems to be snapping tight quickly, and I checked the accelerator pedal/linkage for any binding and found nothing.  I also checked for any obvious vacuum leaks.  I'm not running any vacuum advance to the distributor.  All ports are plugged at the carb, except the vacuum to the brake booster.  The brakes work fine.

I've heard that a lean condition can cause the high operating temp and a slow return to idle.  I've also heard that this could be a timing issue at the distributor.

OK Einsteins, what obvious cause am I missing? 

[Chryco Psycho]

the first thing I would check is the link to the secondaries & make sure it si closing as well , I would disconnect all vacuum hoses & cap them & see if that changes it , if so you may have a leak into the brake booster or similar .
 Eddy carbs are typically lean & yes you are running too hot , do you have a clutch fan ? A rigid fan will block air flow , the air flow through the rad would seem to be a problem if it runs hot at speed or you are running lean , if it idles cool at idle then you do not have a cooling problem . A larger squirter on the acclerator pump will help , of course you have to buy 3 to get the one you need or use numbered drills & open the holes up . Check flaot level they are notorious for being low , they set at 7/16 , I usually set them closer to 3/8 . Sometimes the gasket under the carb will barely cover the channel in the base & can cause a leak under the carb so check that also.
 The other issue is the advance springs in the dist , , they can cause weird idle drop ifthey are too light & the dist stays advances as RPM drops .

[EddieE]

First, sorry for putting this in the wrong area on the forum.

I will make all of the checks referenced above.  What temp (idle and highway) should a setup like this run?  It's an original A/C car so it has the larger radiator which is clean inside.  I'm in the middle of replacing the original AC with Vintage Air, although I've only gotten as far as the compressor (which is basically just an idler pully now).  There no condenser in front of the radiator yet, so I guess I've got plenty of airflow.

One weird thing regarding the highway temp...I just installed the stock front tag bracket and tag.  Could that have a significant effect on the highway temp?

[Chryco Psycho]

It should be more reversed , sitting close to the thermostat temp 180-195* on the highway & maybe rising a bit when sitting still . I doubt your front bracket will have any effect .

[dodge freak 2]

Great points, not sure what I could add.

I have heard bad talk about the 750 Edelbrocks. The booster design flows better in the 500 and 600 cfm carbs. If you are going to run a 750, I would get a Holley carb.

Make certain the timing is staying put--make sure the springs on the dist weights are there and semi tight, they do break from rust.

I would still look for a vacuum leak, hoses crack, leak.

The springs under the metering rods controls the power richest. When the engine is idling, the rods should be down. You can loose the cap and slide them over and look..do it while the engine is idling.

Running hot is often a clog radiator. My 340 runs NO thermostat in the summer, I found it runs 10--15 degrees cooler. Yes, I'm sure a nice $500 brand new rad and high flow water pump would be best but I'm not rich.

With even a high flow 160 stat, engine would run 190--200 driving around in 85 temps outside. No with no stat it runs 180-185 and never goes over 200 no matter what..well it never hits 100 up here, well it did, 3 days so far--big deal. Michigan has seen the hottest temps, its all downhill from here.   



 

[EddieE]

I think I got the return problem solved, but time will tell.  I had a two-spring setup from Mancini on the special bracket for the Edelbrock Air Gap manifold.  They must have weakened over the past two months, because when I removed both springs then reinstalled one of them a a point further back on the bracket (I know it doesn't like right) the carb snapped back right away.  When I tried it with both springs (one is inside the other) it was way too much resistance on the trottle.

So I believe that the slow return to idle may be solved.  Now I need to concentrate on the temp rising a little to high at cruise.  Ryan eliminated the bypass hose at my request to get it out of the way of the Sanden compressor for the AC.  He drilled small (1/8th inch?) holes in the actual thermostat to bleed air.  Should my next step be to replace the stat with a new low-temp stat with the same holes drilled?

[dodge freak 2]

i would try it with no t stat first