My 79 f250 has a 400 with a weiand intake, a two inch spacer, and a 4 barrel edelbrock. All done by the previous owner, I'd do it all back to factory if I had the parts. When I first got the truck it was very underpowered and would diesel everytime I'd turn it off. I turned the distributor a little a couple months ago and it's got more power and now only diesels occasionally. I'd don't remember what it was timed to. I also replaced all the vacuum lines. I can't ever tell when it's going to run on. Hot, cold, ran hard, going easy on it, driven for a while or just to the store. It just randomly runs on for about 10 seconds about 20% of the time. Some googling says this could be caused by three main reasons, a bad fuel to air ratio heating valves which causes premature detonation even when the keys off, carb not closing all the way, or to high of an idle speed.

The truck does have a high idle, around 1000-1100 rpm pretty much all the time. I've tuned the carb to the best of my abilities. I got the idle down to about 900 when I timed it and replaced all the hoses but now it's come back up again.

The truck will also stumble off the line if I give it really any gas at all. It's only right off the line though. I have to really take my time and baby it forward until I can get the clutch all the way out and then around 8-10 mph I can stand on it and it won't stumble. Only when I first take off. I can't get it moving with any quickness, which really sucks when I'm stuck in the turn lane waiting for a mile wide hole to open so I can beg the truck to accelerate from a stop.

The truck also runs rough, whether idling or going down the road it's got this constant vibration to it that I can feel through the seat, the shifter, the clutch pedal and the gas pedal. Sometimes I can see the hood vibrating at stoplight. It also sounds like it's running rough.

The trucks a dog too. I know it's not supposed to be a power house or anything but Christ it's slow. I go out of my way to stay on 65 mph and below zones because I can't keep up with traffic at 75 in 5th. Even on pretty flat ground it lags. I can put the pedal down and nothing will happen. Last week I was getting passed on slight grades by loaded flatbed semi trucks at 65. Its abismally slow and underpowered. There's a hill I go up on my way back home everyday that I either have to be going 50 in a 35 to top (even then I start lugging really close to the top and barely make it) or downshift to third and slow down to 25. I also get 8.75 mpg on average with a 5 speed od, 3.54 gears, 31" tires, and me driving like a little granny.

What does this seem like to you guys? I'm going to try to retime it within the next couple of days. I don't have a vacuum gauge but I'll pick one up soon. I did unplug the vacuum adavance the first time. I guess I'll go back through the vacuum lines to see if they are the culprit to my high idle. I'm really sick of the engine in this. I understand that no amount of timing will turn it into a powerhouse or get it great mileage but I really think somethings wrong with it. The running rough, high idle, and mileage are my biggest gripes. I've done some googling and it seems like a lot of people are getting closer to 11-12 mpg with their 400's. And that's with no overdrive 4 speeds.

Any help or knowledge would be greatly appreciated.

 

You do have a serious problem. My truck Rusty, with the little brother to your 400, pulled our 5000 lb boat at 65 up some serious hills, and towed a '50 Chevy pickup on a trailer at 65 as well. And that's with a similar intake and carb.

Given the vibration I think you have a mechanical problem in the engine, and probably a bad cam or burned valves. So, I highly recommend a leak-down test. It won't tell you if you have a bad cam, but it will rule out a lot of things that will also cause a bad miss, like bad valves, leaking head gasket, worn out rings, etc. Then, if the leak-down test says all of those things are good you need to pull the valve covers and measure the amount of lift on each push rod, which will tell you if you have a bad cam.

i could comment on the dieseling, but that is caused by the high idle, which is probably due to the mechanical problem. So, you need to find and fix that first.

 

Ok, leak down test. I can do that. What exactly could go wrong with the cam to make it "bad"? I think the truck has about 358,000 miles on it. A Google search says that a tool to perform the leakdown test with will run me about $100. Any way to get one any cheaper? What should I time this engine to? When I did it the first time I just got my timing light out and turned it until it started running a little better. Should i get a vacuum to time it?

 

I see leak-down testers from $46 and up on Amazon.

What can make a cam go bad? Well, in 358K miles a myriad of things could have gone wrong. Bad oil is one of the problems, and today's oils don't have the additives they used to have, so some say that flat-tappet cams need special additives. On the other hand, at that number of miles it could be just worn out.

I had a 351W that shook pretty badly at idle and ran poorly. A leak-down test showed that it had several burned valves, so I had the heads re-done. The power came up some and the idle was a bit better, but I then learned it also had a bad cam. Several lobes had gone flat.

So, run the leak-down test and follow the instructions very closely. And, make sure you listen for blow-by on the rings/pistons. You may find that you have burned valves, but you may also find you have a worn-out engine. And, I'd expect that if it really has 358K miles on it. If that's true, a cam won't "fix" it. Only a rebuild will.

 

With 358K miles on it, I agree with Gary's assessment. Old and tired, flat lobes on the cam, concave lifter feet, weak valve springs and maybe a burnt valve or two, leaky/stiff/cracked gaskets, etc.

Pull a couple lifters and put them together such that they look like a 'T'. If you can see a gaposis in the middle of the lifter foot, time for a rebuild. ....You don't have to pull the whole engine apart for this - just the valve cover on the right side and pull lifters 2 and 3. They' the easiest to get out and back in.

Just remember the zinc/phosphorous (zzdp) additive when yo put it back together.

 

Flat tappet cams don't last 358,000 miles! Wow! Many tired parts in that engine as Filty Beast stated I'm sure!

Honestly, I'm no expert, and I don't know the lifespan of a typical flat tappet cam, but I can promise you the performance of one with 385,000 miles on it is "significantly reduced" compared to new! LOL! It would be interesting to hook up a dial indicator and check valve lift compared to a new, stock cam.

 

You can get a cheapo compression tester from Harbor Freight. That will tell you a lot right there. The stumble with the Edelbrock carburetor is a common problem. I just put one on my truck and had to do extensive tuning and finally ended up drilling out the squirter nozzels and viola, no stumble. My truck has been in the family since new 1968, I just rebuilt it 2 years ago and it had close to 300,000 miles and had the stock cam in it. I rebuilt it but the crank is still standard as are the pistons, just put in new rings and bearings, so it is possible to get that much mileage "IF" it has been cared for.

 

Yetiman,

Did you re-use that 300,000 mile camshaft? If not did you notice a significant performance gain after you rebuilt it? That's pretty cool to have the truck in the family its entire 47 year life!

 

No. I ditched it for a performance cam. Put on headers and aluminum intake etc. Bought some rebuilt heads cause mine had non hardened exhaust seats. Bet it won't last 100,000 with my foot in it. But yes, it runs like an animal now. I am running an FE motor. Just saw the stumbling problem posted and since I just went through that I thought I'd chime in.

 

Thank for the help and suggestions guys, I'll look into what you guys said. I can't rebuild this engine anytime soon. Trans rebuild comes first and then I'll have it find another week off work to rebuild the engine. After I aquire the funds. It's always been my dream to swap my cab onto a newer diesel trucks frame. So I might not ever rebuild this tired dog. Lol. Thanks again and happy Thanksgiving to all!

 

Run the leak-down test. It will pinpoint bad cylinders via the loss showing on the gauge and the sound of where the air is escaping. It will tell you if the cylinder has:

• Burned exhaust valves: You will hear air escaping in the exhaust
• Burned intake valves: You will hear air escaping through the carb
• Headgasket leak: You will hear air escaping through an adjacent cylinder's spark plug hole or see it in the coolant
• Worn valves: You will hear air escaping into the sump

You may not need to rebuild it.