351C general questions

kalli · 09-22-2009, 04:51 AM

Hi all,

currently helping a buddy that can't get his 351C to run right. occassionally it flies and at other times it runs like crap and worse.
He installed a 670 street avenger about 2 months ago (new) to help with iut, but it's about the same.
I checked on his ignition:
- TDC verified with piston stop
- no vacuum leaks (i sprayed like a lunatic but no changes)
- i plugged vacuum advance (at carb side) and tried to time it to 8 degrees, but the car wouldn't go near that. it would run so bad it was unreal. i then tried to time it off vacuum, it pulled about 9" at some point, but the biggest miracle is that if I just turn the distributor by one degree this would decide if the car would stay running (crappy) or not at all.
- checked on points and they were something to laugh at. The car would run better when the gap was nearly closed. Which indicates to me the spark might be weak

told him he might either get new points/condenser (cap/rotor were ok) or ignition module, or full distributor with ignition module.

So the fella bought the pertronix II billet distri with module for his 351C

He said he puit it in right, but can't start the car, backfiring. He'll try once more, if he can't get it right I'll call over and have a look myself what's happening. probably wet plugs or 180 out or something.

Anyway, that car is cursed. whatever new part you throw at it makes it worse ...
Some might remember an old post of mine with a car that revs higher when you pull a specific spark plug fr0om distributor ...

I have a feeling that it is firing all over the place. whenever it wants and not when i tell it to fire. He's getting a new set of sparkers as well I believe (new brand). long story short:

I wanted to check on his valves but they seem non-adjustable. Are they done via different pushrod length? or what would be the way to adjust valves in thos clevelands (seems to be 1970)

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dodgestang · 09-22-2009, 08:21 AM

What firing order is it wired up with?

kalli · 09-22-2009, 10:20 AM

he's using the stock firing order. I told him to remove both valve covers and double check the firing order (rotate the engine manually and doublecheck starting with #1 on compression stroke, which one is next in the order). it's 351C stock firing order (1-3-7-2-6-5-4-8). He promised me he did that and I believe him to the extend as on the day I tried to adjust ignition I could actually rev the engine once or twice and it sounded pretty sweet but would then die again in idle. and we never got it back in this stage with turning the distributor ... BUT

I forgot the most important info:
The scariest thing I ever saw. Twice, when he switched off the engine, the engine actually ran _backwards_. I'm not joking.
I was looking at the fan, it stopped and then revolved the other way around and after 2 full turns or so the engine fully stopped. It scared the crap out of me and I was fit to call an excorcist.
this is why I believe the engine fires when it feels like. i have no other explanation to that

[in any case I'll probably with the car again on thursday or friday and the first thing I'll do is to check the firing order against the valve timing)

Kalli

THUMPIN455 · 09-22-2009, 10:28 AM

When it runs backwards with the ignition off that is dieseling, probably carbon buildup on the valves, pistons, and everything else inside the cylinders. Also if its a 70 4v it needs some good fuel, it wont run well on 93 octane due to the CR it has.

With vacuum that low it must have a bit of a cam in it, so 8 degrees initial isnt going to cut it. You will need more advance to keep it running, usually I would run around 14 initial with engines that have larger cams. It wants what it wants and the book doesnt apply anymore, actually the book is a good starting point but the be all end all for a 40 year old car.

kalli · 09-22-2009, 11:06 AM

thanks for info.
I usually get 95 octane at my petrol station (advertised ...) but most of the ones around are 92. So I advised him already to use octane booster (does that really help). he uses a bottle that has lead additive (not lead, but some replacement) as well as octane booster in the same thing. If that really cuts it I don't know.

I'm quite sure that it's either compression or wrong plugs that's causing him a lot of grief.

The 10" vacuum is measured when we tried to adjust the initial ignition using vacuum. it might have been on a different spot than the 8degrees. But all that with the engine running crappy. so i'm sure once we get it to idle (fire) right, the vacuum will probably increase

I'll get back as soon as I have more info. But has anyone ever heard of a car running backwards ... it was unbelievable. that has to do with selfigniting (or dieseling as you say)

Any way to figure if it's a 4V (i cheked on compression ratios and they seem to be 11:1 ...)
he has 4bbl intake, but that doesn't mean anything on a 351C I learned. But I always felt like that engine has a high compression ratio

[PS: the mustang in sig has great shape to it]

Kall

my77stang · 09-22-2009, 11:21 AM

i think the 4v heads had a 4 cast in the top corner right past the valve cover mounting surface.... if i remember correctly.

jrrhd73must · 09-22-2009, 11:54 AM

I built up a 351C to be a Boss 351C, using higher compression, 70 closed port heads, aggressive cam ect.

I ran into quite a few problems during the process. Car would run like crap on 93 gas. I did not want a race gas car, I wanted a driver. If you ever got bad gas it was even worse.

I put a pertronix and she ran a little better. Then I installed an MSD 6A and coil better yet. But I blew the shear pin in the stock distributor - twice. Now I have an MSD distributor and I can run on pump 93 octane gas. She diesels because of the compression, so I shut the car off in reverse, this loads the engine a little and it won't diesel.

Troubleshooting:

Check the plugs - temperature rating and gap.
Check the coil - may not have good power or may not have good coil.
- if no power to coil, check ignition switch and wiring
Check plug order following every wire to cap and to plug.
Drain old gas and install new gas.
Don't Time the car with a light, time it by feel. You are going to have to be pretty advance if you have the CAM i am thinking that you do. Twist the distributor until she idles smooth, let her warm up, then drive it, making very small adjustments for any stumbling during acceleration.

With the pertronix you should be able to run on 93 plus boost with little or no pinging.

One note, my car ping and knocked unless I had the right gas, but it fired fine. With the MSD and coil, I can run on 93 octane - no boost

1971mach1 · 09-22-2009, 01:41 PM

Dieseling often happens when the RPMs are too high when ignition shut off. It
could be as simple as the idle set too high. High compression engines and/or carbon build-up can make it alot more prone to diesel though. Yes it does run backwards and sometimes even backfires out the carb. Mine was dieseling with the old Autolite (crappy) carb because the fast idle would be stuck on, even when hot. It needed to be kicked down to around 700rpm or lower to not deisel. A new carb (Edelbrock) fixed that problem, and several other issues.
FYI I have the same engine 351C-4v but the PO rebuilt it (possibly with somewhat lower compression) because it runs strong with 91 pump gas.

The 351C-4v has hydrolic lifters that don't require adjustment. Only the Boss 351 has mechanical lifters that need adjustment. How old is the timing chain? Maybe it slipped a gear.

2+2GT · 09-22-2009, 01:50 PM

Quote:

Originally Posted by kalli

- i plugged vacuum advance (at carb side) and tried to time it to 8 degrees, but the car wouldn't go near that. it would run so bad it was unreal. i then tried to time it off vacuum, it pulled about 9" at some point, but the biggest miracle is that if I just turn the distributor by one degree this would decide if the car would stay running (crappy) or not at all.

I wanted to check on his valves but they seem non-adjustable. Are they done via different pushrod length? or what would be the way to adjust valves in thos clevelands (seems to be 1970)

I don't believe your timing numbers, there is something wrong with the marks. You'll need to verify where TDC actually is by mechanical means. The most foolproof way is to examine the front of the crankshaft. The key slot on the crank should be exactly at the top.

The valves should be tightened to the stop. If this isn't correct, under or over size pushrods are required.

boogerschnot · 09-22-2009, 01:55 PM

Ohh,, Timing chain could definately be a culprit!

I also thought about the new distributor making it backfire. He probably did drop it in 180 deg out.

I think the guys hit everything else I was thinking of. Sounds like a doosy.

Good luck Kalli, the guy is lucky to have a good friend who knows his stuff.