buddy of mine's got a 351C (maybe 351M. can't tell them apart) in a 1970 fastback.
We are having a hard time getting this engine to run properly.
it idles lopey/crappy. it revs quickly (as in the time it takes to actually rev up) _but_ here's the biggest problem:
when you rev to any certain rpm and hold it there (no change in throttle position) the engine shakes so bad that the fan its the shroud. it really is godawful to watch. again. this cqan be any rpm like 2000, 2200, 3000. this always happens.

the fella has put in a brand new holley (570 or 670 street avenger it seems) and I adjusted floats, curb idle and mixture screws

I don't like the distributors looks, but it seems to be fine (no play in axle shaft, the 'humps' for the points are even to each other. The contact breaker has seen better days but I adjusted to 0.02".
The plugleads are new (MSD 8.5mm) and the psark plugs are new (no idea if he got the right ones as in temperature and reach)

I have verified the TDC on balancer with piston stopper and adjusted the ignition.

Coil is getting 12V during run although it maybe supposed to run on 8V. it seems stock coil. distributor has D0xxx code, so I assume stock as well.

I'm totally out of ideas. We probably have to do a compression test next, but boy this is annoying as we can't get it right

Accellerating seems to work good (there is a lack of power as I expect more of a 351c), but no hesitation and a steady pull. I backfires badly into the exhaust sometimes. I think it happens mostly after letting off the throttle after accelleration.
It is just when I was a kid switching off the ignition in a tunnel while flooring it and switching it back on after 1 sec to blow the crap out of the exhaust (you've gotta be young ...) the same bang

i'm completely and utterly confused of what the problem could be. oh and fuel pump delivers 7psi, but I can't verify this during drive

On my very first look I would have said: wrong firing order or something similar. But we verified this. The fella even went out and checked the order against valve opening with pulled rocker cover to double make sure.

anyone any idea ?

 

I would say a compression test is in order. Sounds like a bad valve or maybe head gasket.

 

compression test is next on the list. If I'd be a betting man I'd have 25$ on ignition though.
my buddy says he suspects the plugs and I'm with him. I hope he's just bought the wrong plugs (to short reach, totally wrong heat range, or way to big gap for his stock ignition to handle).

Another thing I forgot to mention. she's dieseling when you switch her off. as in running on. whop .............. whoppphawhop ..... off

 

I had 351C in my '69 when I bought it. It too had a terrible shake that did not go away even after a new balancer, flywheel, and clutch. It had a cylinder that was way down on compression which was probably a valve. I have never tore it open to check what the guts looked like.

I would suggest verifying the balancer and flywheel weights. Outside of that, I would guess a compression test will tell us more.

 

urban: any chance you sold that engine? sounds like my buddy bought it with his car.
Such a shame. he stripp[ed the whole car. spent two years on it. looks fab but won't run for a dime

fella has bought accel 576S plugs. which might explain his dieseling (as they're the 2nd hottest in the range) , but reach is correct.

fella says it went worse after we changed the breaker gap to what it should be (0.02"), so i think he's trying to smallen the gap on plugs and smallen the gap on the breaker, both in the hope that it might help if the spark should be too weak.

I already have my compression kit holstered, so whenever we find a minute we'll have to check on that

Kalli