Rebuilt 427 Still Not Quite Right

"The culprit:
'68 Ford 42

"The culprit:
'68 Ford 427 (Eaton) 300 HP
MSD Ignition
Autolite 4100 Carb (From Pony Carbs)

I set the timing based on suggestions from the engine shop - 34 degrees at full advance.

When the engine starts up, and when making quick little runs, it runs beautiful. It idles @ 700 out of gear, down to 600 or so in gear, and will roar up to 4000 RPMS and back again with no issues.

At some point, something "happens". There's nothing noticable to indicate it happened, like a backfire or the such. All of a sudden, when trying to bring the engine back down to idle, the thing starts to run rough and wants to cough out below 1000RPMs. It'll still run fine at higher RPM's, I just can't get it back down to idle without fighting with it.

Eventually, the problem simply vanishes. This appears to be an on-or-off situation, as there's no gradual decline or recovery of performance. I'm eyeing the carb, and have thought of ordering a Holley Marine Carb. This would obviously be an expensive "guess". Any thoughts on where to start (or continue) on this?

One other thing, which may or may not be related. The thermostat is a 160 degree version, which appears to be the only one available for this motor. Outside of the thermostat housing is a constant flow of fresh (cold) lake water. As a result, the thermostat won't pop open until around 190-200 degrees. I seem to recall this was always the case (pre rebuild), but the smell of fresh engine paint makes this a little more obvious after the rebuild. Is there a solution to getting the thing to open earlier?

Thanks!"

"Alright - I'll help the d

"Alright - I'll help the discussion along

Is it possible that an overheated engine would cause the rough (impossible) lower RPM operation? I read on a car site about the gas vaporizing on a hot manifold and causing too rich a mixture, causing difficult restarts on a hot engine.

I purchased a "Balanced Sleeve" 160 degree thermostat from Summit Racing today - hoping the added surface area will cause the thermostat to pop at the right temperature. I also ordered one that has three bypass holes in it, thinking this may also help heat the thermostat properly.

http://store.summitracing.com/partdetail.asp?part=MIL%2D16400&autoview=sku

http://store.summitracing.com/partdetail.asp?part=EMP%2D301&autoview=sku

If I can disprove the overheating theory by keeping the engine running in the 160-180 range, then I guess I'll have to turn my attention to the carb (a pricier "guess")."

"I'm continuing this threa

"I'm continuing this thread, in hopes it may help someone else in the future...

I tossed the new thermostat with the three bypass holes in last weekend. The bypass holes were obviously too much, as I couldn't get the engine temp up above 140 or so.

Today I installed the other thermostat with no bypass holes. Running it with a fake-a-lake in my driveway, the temp rose to about 180, then dropped to 160. This seems to be acceptable, so I'll see what happens out in "real water".

Last night I borrowed a holley carb from the shop that rebuilt the engine. I'm hoping this eliminates the rough (impossible) idle after running for a while. If it does, I'll proceed to purchase a new holley marine carb.

Based on the formula, it looks like my CFM needs is about 500 (427 cui / 4000 max rpm)...
427 / 2 * 4000 / 1728 = 494

I figure I'll go with one of the 600 or 650 CFM flavors.


I'll report back after my outing. If this becomes the last message in this thread, assume I made a critical error while installing the temporary carb "

Sounds like an advance problem

Sounds like an advance problem in the dist. check to see if the advance under the rotor isnt rusted

"In response to Michael: I&#39

"In response to Michael: I'm running an MSD electronic ignition with an MSD distributor. I'm pretty comfortable stating it's operating correctly.

My trial run with the new thermostat and the loaner-carb was a huge success. The engine temp rose to 160, which is where it stayed (+/- 10 degrees). More importantly, the new carb showed no signs of the low-idle problems I had been experiencing. I pushed the motor pretty hard, going from idle to full-throttle 4000 RPM, then slowly back down to idle. I also took it out for an extended haul at 2500 RPMs, and then slowly took it back down to idle. Both times, the thing just purred back at idle, begging for a little more fun."

did you check and make sure th

did you check and make sure the carb isn't flooding. mine did that and would sometimes the float would not stick and the motor would work perfectly