Ford 302 runs rough at 2500RPM. What's wrong with it?

I have a Ford 302. I have an issue with it running rough at 2500 RPM. With the car parked, freely revving the engine to about 2500 RPM, it shakes violently as though at least two cylinders are missing. There are no unusual noises while it is shaking, no popping, no obvious sounds of missing spark. At 3000RPM, it smooths out somewhat. Oil pressure stays high, and within normal spec.

Historically, the engine has had issues at 2000-2500 RPM. It would lose power, as though the engine only had 4 cyl. Again, no missing sounds, no popping. This would only happen at 200F engine temp and above. If I let off the gas, it would smooth out, and if I punched it, it would be ok. This has happened intermittently, almost randomly, but only above 200F.

The engine has an Edelbrock Performer carb (newly rebuilt) and intake. Unknown performance cam. Crane Pro-Curve distributor (with newly replaced module), Crane HI-6 ignition box.

List of parts that have been replaced:
MSD ignition and coil replaced with new Crane HI6 and coil. New plugs gapped to .45. Changed plug wires 2yrs ago, didn't fix it. New mechanical fuel pump, new fuel pressure regulator, new fuel lines from front chassis to carb. Replaced all wiring for alternator charging system, all grounds at front.

Any ideas on what could be going on? It's driving me crazy!

 

Hard to pinpoint something that you have been fighting for so long, but a couple of thoughts.......
Have you watched advance curve thru RPM range with a timing light? We had a SBF w/ a loose timing chain where you could watch the advance get very erratic off idle. Also maybe put the light on a couple of different cylinders to verify consistency of spark. Heat sounds like a factor, is this a closed engine bay where the electronics are exposed? "New plugs gapped to .45 Plug" (gap at .045" right not .45). Thats still quite a bit of opening (I know maufacturers MSD etc etc say that's okay). Good luck.

 

Do you have a thermostatic clutch engine fan on it?

 

I had that problem once. I pulled the engine out, and replaced it with a Buick Nailhead. Works great!

 

Not sure if this is pertinent, but (I think) there are two different flywheel/flexplates for small block Ford, they changed the size of the weight throughout the years. I had a flexplate that had the weight missing from it. I caught it before installing the motor last year.

 

May sound dumb,but,do you have the firing order right for the cam?There are two firing orders for 302s,most cam manufacturers use the early firing order,15426378.Number two item is it an early 28oz imbalance engine[pre 85 I think] or a later 50oz imbalance engine,you can easily have the balancer or flywheel,or both,wrong!One more thing to check,requires intake removal,is the intake sealing on the heads,look at the gaskets for oil being drawn into the ports or an area that is not being crushed.Chuckles;how is that broken Nailhead crankshaft?JMO,ROY.

 

plugged exhaust, pre-ignition, timing, try .035" plug gap, separate #7,8 plug wires(cross fire)

 

DITTO on that cam..... check it out and see a 351 cam will work but requires changing the firing order,could be your problem.....

 

It's important that you find out what your "unknown" cam is, as the firing order is different for the HO302/351 windsor, and the standard 302. Without knowing what the cam is, there is no way of knowing what the firing order needs to be. ( Interesting problem) The vibration at a specific RPM does sound more like a harmonic balance problem, due to engine balance problems.

 

I think you said that you recently rebuilt the carb.......anyway since you state that it only has a problem above 200*F "but only above 200F." that eliminates a lot of things. Back to the carb....inspect the metering rod piston for smooth movement in the bore. usually the clearance here is adequate. However with a small burr on the piston and expansion by heat this may be enough to throw mixture off until you significantly change the vacuum signal by letting off gas or punching it as you describe

 

Lean in the mid range. Use smaller metering rods to richen the mid range

 

Thanks for all the tips! I'm still checking things out. I found a burn in a plug wire, so I put new wires on hoping that was it. Still have the problem. Here's some videos showing what is going on.

Here is the engine reving to about 2000rpm. The camera didn't catch the shaking very well. It's not dramatic movement, it's like a bunch of small random shakes. More than previously.

<iframe width="420" height="315" src="http://www.youtube.com/embed/P2wFe8XLW2s" frameborder="0" allowfullscreen></iframe>

Here's in car. I was hoping to catch the tach movement dropping, but at least you can hear the hesitation right before 2nd gear shift, and during and after 3rd gear shift.

<iframe width="420" height="315" src="http://www.youtube.com/embed/HlVIok58vTg" frameborder="0" allowfullscreen></iframe>

Here's readings from the vacuum gauge. At first it's in gear (sitting still), then in park freely reving to about 2000RPM. The readings are about normal for this engine, but the shaking needle started about 6 months ago. I've rebuilt the edelbrock afb since then, and nothing changed. The clicking noise is the needle bouncing very quickly. The stuttering hesitation issue came back just a few weeks ago. A lot of the background rattling noise is from the headers, they don't seal well and keep blowing gaskets on #2, 5, and 7.

<iframe width="420" height="315" src="http://www.youtube.com/embed/utfquJQL6UQ" frameborder="0" allowfullscreen></iframe>

 

It occurred to me, that maybe the ignition box is getting crappy voltage. Could inconsistent switched power to the ignition cause this?

Power comes in through a 12ga to the ignition switch. (it has a 25A breaker on it, at the battery). Then through the ignition switch, then is split 3 ways. One to the ignition box, one to the distributor, and one going to my electric fan controller. I replaced the ignition switch with a spare, but it didn't change anything. That 25a breaker has always been with the car (for at least 15yrs), and it's aircraft grade. Could it be going out?

 

I haven't tried that yet. The chain was replaced about 6yrs ago with a Coyles double roller. Electronics are at the very front, but they do get hot.

It was a flex fan, but is now an electric.

It's a HO firing order. It's using a (has both 3 and 4 bolt holes) 50oz balancer on it, I installed it a couple of years ago to make it work with early model pulleys.

Something has changed on it, now it's doing it even when it's cold. Engine runs very rough cold, then smooths out as it warms up. Once around 160, it's ok until it starts hesitating and running rough. It's does it randomly... sometimes it's smooth through all the rpms. Then it goes back to hesitating.

 

sounds to me like its in the carburetor.

i'm not real big on carb tuning. i leave all that to my dad, but generally speaking when we get a deal like that it usually has something to do with the A/F mix when the car is transitioning between circuits.

grab an edelbrock metering rod kit and grab the phone. call the edelbrock tech line and check the website. extremely helpful.

i bet a simple metering rod change will go miles.

 

But why the sudden change though? It ran great for several years with that carb setup. When I had it apart, I specifically checked for issues, and didn't find any. It's the sudden change that's tripping me up.

 

I like the way you think sir!

Big fan of that car, really like it a lot.

 

This could be an issue since you have gone through the Carb and ignition,check the valve adjustment since the problem gets worse at a higher temp (heat expands) which would make the valve lash tighter and could cause your miss.

 

ck. ur rocker arms...sounds like ur pulling a stud

 

My old 302 did the same thing once. I used to run it hard. It pulled a couple of rocker arm studs ou. Ended up changing to later model heads, new valve springs and it was fixed.

 

I have a laser temp gun. If I take readings of the exhaust ports at the headers, would that help figure out which one has the problem? Would it be which ever ones are hottest, or coolest?

 

It might,you may want to do a compression check first and post the numbers here.

 

If you have a buddy with another carb, I would try that. It could be a little resitence in the cap and rotor, could be anything.

 

I was also thinking of a broken or cracked valve spring. I have had this happen twice on 2 of my SBFs. First time I ended up pulling every thing off down to the short block just to find a broken spring. Next time I just replaced the springs first.

 

I had same problem i my drag car 347 windser. check your distibutor gear for wear. if this is the issue your timing will keep changing.

 

I'm with the cam & firing order guys...... Easy 'nuff to re do the plug wires at the cap.....

4TTRUK

 

I took temp readings with a laser reading gun, at about 190F engine temp the header ports at about 2" from the flanges read around 515F. They were very consistent within about 5 degrees. The exceptions were the two front pipes, they were around 500F, I think from the air from the fan. I'm waiting until the weekend to do a compression check, and to look at the valve springs and rockers.

I'm not convinced it's the valvetrain though. It seems like it would be consistent. This hesitation stuff seems to be very intermittent... sometimes its not there at all. I had a SBC 350 that broke a valve spring once, it popped and rattled like crazy, and was dead rock consistent about it, and was worse at higher RPMs.

I replaced the wire from the ignition switch to the ignition box and distributor. It was butchered, it needed it anyway. But it made no difference.

Starting to wonder if it is the carb. I have a motorcraft gasoline fountain that used to be on the car, i'll put it on this weekend and see what happens.

 

I drove the car to work, and noticed that the hesitation isn't temp related anymore... it's vacuum related. It happens right around 7" of vacuum. If the vacuum is lower (punching the gas), or at 10" or higher (taking it easy on it), it's fine. I still have a slight miss at higher vacuum, but nowhere near as bad. Still trying to figure out what's causing it.

 

My 302 did that but I had points in it.Found dwell was way off.Do you have the correct voltage going to your ignition system?

 

I just went through some similar struggles with a 302 in my car. Similar situation, bought the motor used with an unknown performance cam. I looked at all of the things mentioned here, but what ended up fixing the problem was a distributor advance re-curve kit. The aftermarket distributor was putting too much mechanical advance in and causing a host of intermittent problems. Might be worth looking at your carb tune and timing, including total advance.