440 6-Pack misfire... Driving me crazy!

[440-6pack]

Pick up coil was good.

I measured temperature as the engine was warming up. Lots of variance in exhaust manifold temperature. Not sure what that means...

 

[440-6pack]

Also, I put the timing light on a spark plug wires and no intermittent spark/flash, it was very consistent.

 

[Cudakiller70]

Leak down test done for all cylinders? Was it constant across each/all cylinders?
During leak test did you go to the tail pipe and listen or hear any air?
Or hear any air through carb?

 

[440-6pack]

I’ve never done a leak down test before this one. All cylinders were pretty even between 15-17psi. I didn’t listen in the tailpipe or carb.

New developments tonight:
1- rad cap must be toast. It is not holding pressure.
2- While cruising tonight and accelerating mildly there was a backfire through the carb. Not a bang, more of a puff sound.

 

[Rich G.]

If the compression is good and all the valves are going up and down and it still runs rough it has to be ignition or fuel related.

 

[440-6pack]

Hey Rich,

I haven’t pulled the driver’s side valve cover yet to check that all the valves are opening the correct range.

I’ve replace every ignition component 3 times with no change:
- Coil
- ignition box
- distributor and pick up coil
- voltage regulator
- alternator
- ballast resistor
Also
- new wiring harness
- new ignition switch
- ran power straight to coil

Fuel
- installed another set of carbs that were rebuilt with new needle jets
- adjusted floats best I can with engine shaking
- new mechanical pump
- fuel pressure is 5-6psi on start up

 

[Cudakiller70]

Disconnect outer carbs and see how it runs.
Changed spark plugs?
Disconnect or pinch off vacuum to brake booster, any change?

 

[AUSTA]

Was it the same with the old carburetors
was it loosing water or just a faulty cap as brian6pac suggested
When it is idling try using a squirter with some fuel in it give each venturi a small squirt to see if the idle evens out after each ,squirt let the engine settle it may help to isolate weather it is a fuel spark or mechanical problem it is fuel the engine should smooth out for a brief period.

 

[440-6pack]

Disconnect outer carbs and see how it runs.
Changed spark plugs?
Disconnect or pinch off vacuum to brake booster, any change?

Hey CudaKiller,

I changed the plugs 3 times in less than 200 miles. No change.

I have pinched the brake booster hose. No change.

i have not disconnected the outboards but the base gaskets between carb and throttle body are wet until it warms up...see attached. Do I have to remove them completely and blank off the openings to disconnect them? Or just disconnect the fuel lines and linkage?

 

[440-6pack]

Was it the same with the old carburetors
was it loosing water or just a faulty cap as brian6pac suggested
When it is idling try using a squirter with some fuel in it give each venturi a small squirt to see if the idle evens out after each ,squirt let the engine settle it may help to isolate weather it is a fuel spark or mechanical problem it is fuel the engine should smooth out for a brief period.

Hey Austa,

It never idled rough before the misfire started. I tried dribbling/squirting some fuel in the main carb when it was warmed up. It was hard to tell if it smooth mes out or was slightly flooding. It smithed out but dropped rpm then revved slightly higher. It definitely vibrated much less.

In regards to the rad cap. The hose was still soft and not pressurized when hot after driving for 30 mins. I put another old cap I had on. No change. I’ll pick up a new cap this week... weird, I had a new cap. Only 3500 miles on it.

 

[440-6pack]

If the compression is good and all the valves are going up and down and it still runs rough it has to be ignition or fuel related.

Hi Rich,

I still have to pop off the driver’s side valve cover and measure the valve movement, something tells me I have a valve that is not opening enough or broken spring.

 

[dolphin3111]

First of all - 15% to 17% leakdown is definitely NOT good. It should be 1/2 of that. My race engines normally leak down at 4 to 6%.

I feel sure that there are valve problems. Possibly a bent or burned valve, mushroomed valve head, bent stem, etc.. That would explain many of the symptoms.

Pull the heads and take them to a machine shop that you really trust and have them do a good 3 angle valve job.

 

[wedg2go]

I'm just curious, but when you did your burnout did you keep an eye on the tach? If so, where did you rev it to?

E-bodies are so ass-end lite, sometimes it's easy to get a little carry away...Especially with a big block with "some" 50 year old components?...Just saying. 440s are such torque monsters. I know.

So,

In a greater sense, I would agree with dolphin on this one.

 

[440-6pack]

First of all - 15% to 17% leakdown is definitely NOT good. It should be 1/2 of that. My race engines normally leak down at 4 to 6%.

I feel sure that there are valve problems. Possibly a bent or burned valve, mushroomed valve head, bent stem, etc.. That would explain many of the symptoms.

Pull the heads and take them to a machine shop that you really trust and have them do a good 3 angle valve job.

Hi Dolphin311,

Thanks for the info. The forums I was reading said that anything over 20% leakdown was bad and that race engines operate around 5%. Removing the heads was going to be a spring project as I’m putting the car away for the winter. Question: I have 452 heads now from I suspect a motor home which have hardened seats. Should I rebuild them OR find rebuilt 902 heads OR buy aluminum heads? I’d appreciate your advice. I’ll also be changing cam, lifters pushrods etc..

 

[dolphin3111]

440-6pack,

If Aluminum heads won't break your budget, I'd do that. Even mildly modified 440's respond very well to 2.14/1.81 valves.

Otherwises good valve job on the 452 heads would be OK. If any of the valves need replaced, I would replace all of them and go to the oversize valves if that is possible with hardened valve seats. IDK. A little light porting in the bowl area under the valves helps too. Make sure they skim cut the gasket surface flat. You'll be surprised what just a good valve job would do in performance, especially when it fixes that high leakage.

Best of Luck,
ED

 

[DetMatt1]

Call me crazy but iron heads haven’t yet given me a reason to change to aluminum but I don’t go racing either so...
I run 915s with the bigger exhaust valves in my Imperial and I drive it daily during the season on pump gas with no issues. I had the original 346s rebuilt for the 383 in my convertible and again have no issues, although I kinda wish I would’ve used the ‘68 906s I have instead. I will be using another set of 915s on whatever I put in my Road Runner. The rebuild on each of these sets of heads has cost me less than a set of aluminum heads and the iron heads have a proven track record of 80 + years so why use anything else(at least for my street driving applications).