351M Lacking Power
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351M Lacking Power
I picked up a 79 Bronco with a 351M. It is responsive, in park, off idle, but has no power. The engine looks recently rebuilt. I did the usual tune up scenario. Compression was 152 to 158. I have been playing with the timing, tried 6* up to 14*. Tried a Holley 670 and 750 as well as a Edelbrock 600. There was a cam card in the Broncos paperwork for a .484 lift Comp Cam. The Bronco has 456 gears with a C6 and cant even break the tires loose on a dirt road. I also have a 79 F350 with a mild built 400 that runs like a beast. I would think the 351M would be a lot more powerful than it is???? Any suggestions would be appreciated.
Post Fiend
Joined: May 2012
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From: Gods Country, Kansas
You may have a leak in a vacuum line as that cause your vehicle to lose power when you stomp on it and also when you are going and take your foot out of it, it would feel as if you are down shifting.
Also another alternative your gear ratio might have something to do with it.
Keep me posted if you figure something out.
Also another alternative your gear ratio might have something to do with it.
Keep me posted if you figure something out.
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Thanks for the reply. I will disconnect all vacuum lines for a test. I dont see where the gear ratio would be an issue. I would thing 456 gears would allow it to spin tires easier, 33's with a 456 gear and a c6? This would be almost like running in low range. I am having issues with the vacuum booster. I will plug that as well. Thanks again, I will post any findings when I can.
Post Fiend
Joined: May 2012
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From: Gods Country, Kansas
Oh well it would have nothing to do in your case because you are running 33's if you ran something higher it would cause less wheel spin of you don't have a built motor.
Since you are already having problems with the vacuum system I can just about guarantee that is the problem.
Since you are already having problems with the vacuum system I can just about guarantee that is the problem.
Junior User
Joined: Oct 2012
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From: Cape Town. South Africa.
Hello 1ford1,
I think I'd be crying like a girl if I couldn't wheel spin on 4.56 gears !
Your figures of 152 to 158 aren't compression. They're cylinder pressures. If you obtained them on a warm engine, and they're the lowest to highest, that would suggest a recently rebuilt engine as you stated.
The numbers are healthy !
With all of the vacuum lines connected, attach a vacuum gauge. The needle behaviour and pressure reading will speak volumes.
(Or just disconnect the lines and test that way as suggested)
Check that your engine is correctly earthed. A volt reading of 13.5-14.5 should be seen with the engine running.
Incorrect earthing creates major power loss.
I learned that one the hard way !
Is your ignition timing advancing freely up to +/-34 degrees (vacuum advance disconnected) at +/- 3,000rpm ?
I assume that your HT leads and plugs and air filter etc are in order.
I think I'd be crying like a girl if I couldn't wheel spin on 4.56 gears !
Your figures of 152 to 158 aren't compression. They're cylinder pressures. If you obtained them on a warm engine, and they're the lowest to highest, that would suggest a recently rebuilt engine as you stated.
The numbers are healthy !
With all of the vacuum lines connected, attach a vacuum gauge. The needle behaviour and pressure reading will speak volumes.
(Or just disconnect the lines and test that way as suggested)
Check that your engine is correctly earthed. A volt reading of 13.5-14.5 should be seen with the engine running.
Incorrect earthing creates major power loss.
I learned that one the hard way !
Is your ignition timing advancing freely up to +/-34 degrees (vacuum advance disconnected) at +/- 3,000rpm ?
I assume that your HT leads and plugs and air filter etc are in order.
FTE Legend
Joined: Jul 2010
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From: Northeast, OK
With those gears you should be able to turn the tires on pavement with a decent 351M. I have two of those engines, both 1981's. One is bone stock and is ..... lazy. The other is just a good rebuild with an RV cam, but it would run circles around the stocker.
I agree with Neek that you should check the vacuum at idle. I would expect at least 17 inches with either the vacuum advance pulled and plugged or with it connected to ported vacuum, which is where I would run it. If you can't get the vacuum at least to 17" you have a problem. And, while the cylinder pressure #'s look good, another test would be leak-down. I had a 351W that had great cylinder pressure but didn't run well and shook badly at idle. A leak-down test determined that I had two burned intake valves and one burned exhaust valve.
But, I would also check the balancer against TDC. If you have the original balancer there is a fair possibility that the ring has slipped and your timing is way off. Why do I think that? Out of the two 351M's I have one slipped balancer.
I agree with Neek that you should check the vacuum at idle. I would expect at least 17 inches with either the vacuum advance pulled and plugged or with it connected to ported vacuum, which is where I would run it. If you can't get the vacuum at least to 17" you have a problem. And, while the cylinder pressure #'s look good, another test would be leak-down. I had a 351W that had great cylinder pressure but didn't run well and shook badly at idle. A leak-down test determined that I had two burned intake valves and one burned exhaust valve.
But, I would also check the balancer against TDC. If you have the original balancer there is a fair possibility that the ring has slipped and your timing is way off. Why do I think that? Out of the two 351M's I have one slipped balancer.
New User
Joined: Oct 2012
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Well first off its a modified haha so your not gonna get much but you can wake them the hell up. Ditch the retarded timing put a healthy cam in her roller rockers bore her over put some air flow research or cobra jet heads if you can find them. Or Cleveland 2v heads that's what I have on mine. High rise and a good 650 and some header and there's nothing she won't pull down!!! Towed a 40000 lbs trailer with my dads old 77 with a 400m
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Thank you for the replies. I haven't had a lot of time since my last post. Today I took a chance on the timing and set it to 14* adv. by timing light. This made a huge difference. I also changed the power valve to an 8.5 and replaced the accelerator pump arm on the carb. It is running much better. The vacuum is a steady 17hg. I plan to go through the dizzy next and also verify the timing marks are at TDC. The motor runs smooth just weak. The cam card that was with the paperwork says its a .484 lift cam so I expected a lot more than I was getting. I will update asap. Thanks again for the help.
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335 Series- 5.8/351M, 6.6/400, 351 Cleveland
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Jan 6, 2002 03:44 AM