First of all id like to say Im a new member and Hi everyone.

Ok now with the problem. I just got my license a few months ago and my Grandpa gave me a beautiful Short wheel base 1979 Ford F-100 for fixing the motor in it...... well for getting it running. Anyway i swaped out the old 2-barrel carb to a 4-barrel. And i put an oversized cam in it. Now it runs like crap. At first i figured it was the timing, but i checked the timing on it and it said 6 degrees BTDC and it was dead on it. It hesitates when you give it gas. And when you start it in the morning it backfires like crazy and wont pull itself till it warms up. I dont know much about these trucks but im trying to learn. Anyone got any suggestions on getting her to run right? Oh its a 351m by the way.

 

Welcome to the forum.
I have moved your post to the 351M forum for better replies to your question.

I would look into vacuum leaks, carb tuning and try bumping the timing to 10btdc.
Also make sure that the vacuum advance is working and hooked up properly. It should be hooked to ported vacuum, not direct vacuum.

 

Thanks ive did carb tuning i even put a brand new Edelbrock carb on it but it made no difference. I will check for cracked vacuum lines though

 

Carb and choke tuning can be difficult for a newbie. You may want to get someone with experience to check it out. There are guys in the carb forum that may be able to help.

Did you change out the intake manifold or did you use an adapter plate? Without a lot of work those adapter plates can leak badly. That should have been evident when the carb was tuned tho. An improper manifold install can also cause leaks.

 

also, how big of cam. these engines don't have the compression to run much. If you placed a 4bbl on top of a 2 bbl intake and placed a big cam in it you may never get it to run very well cause you would have dumped the operating compression below what a motor can take and with a 2bbl intake it isn't going to breath enough to the run the cam.

Great news eh? Its good to learn this way- I'm a much older near newbie and this site has offered a ton of info- too bad you didn't find it before your adventure- but it's all part of learning. keep us informed and let these folks help you out...they know their chizzle.

 

I switched the intake manifold. I bought the intake and carb used it came off of an old Ford Bronco with a 351m. I dont know much about cams i had someone put it in for me it has a 484 valve lift and a 510 exaust lift.

 

I would guess (by reading up on cams) that you have a cam with lower duration- like an rv cam- I bet it's around 208/215 @050 or so with that lift ( you should find out). Just a barely educated guess- enough to almost be dangerous...so thats good news if true.

The motor has a serious compression problem even when freshly rebuilt to stock. So even a smaller cam like yours will hurt some IMO as you didn't have enough compression to adequately drive the stock cam.

How fresh is the bottom end? ie rings, bearings etc.

The best option may be to get this running satisfactorily- it will never be a great motor- and look for a donor 400 to build the right way and swap when done- that way you can have a truly awesome motor in a great truck and get better mileage as well.

 

Cmon fellas lets give this guy some things to try here. First off i would find out exactly what cam you have. Was it broken in properly, was the lifter preload done properly.

If the new manifold has egr, possibly hooked to manifold vacuum.?

Get yourself a vacuum gauge, possibly the timing chain was not installed correctly. Do a compression test, get a baseline of the state of the motor.


On a side note roger i have run some seriously large cams on both 351m and 400's . 310 deg adv duration on one of them. The 351m i had in my 79 had a comp 268h,stock pistons and i pulled a 32ft fifth wheel all over hell and back. The cam is not his problem short of not being broke in correctly and flat lobes. If the engine ran fine before this work, something was done wrong during the change over.
Do a comp test and vacuum gauge let us know the results. and some cam specs

 

"On a side note roger i have run some seriously large cams on both 351m and 400's . 310 deg adv duration on one of them. ..."
I didn't say anything to the contrary

 

Quote"The motor has a serious compression problem even when freshly rebuilt to stock. So even a smaller cam like yours will hurt some IMO as you didn't have enough compression to adequately drive the stock cam."


I merely suggesting that there is no way a cam with 206@.050 is goin to cause a rough idle or rough running for that matter

 

When i get paid i do need to buy new vacuum hoses. The truck moter had about 35,000 miles off of a rebuild when i started work on it and it had sit in a garage for about 3 years. Now the rear seal is leaking. I got the paper on the cam but like i said i dont know much about it. But ill tell you whats written on it. Advertised duration for the intake is 262, for the exhaust its 272. Then it got another section saying cam timing and starts off. @.050. Then it says intake and opens at 5 degrees ATDC closes at 29 degrees ABDC, Max lift 107 degrees ATDC, Duration is 204 degrees.
For the Exaust it opens 44 degrees BBDC, closes 10 degrees BTDC, Max lift 117 degrees BTDC, Durationg is 214 degrees. Sorry if thats confusing but its even worse for me.

 

crunching some #'s. had to edit as i already had a response going when you posted....

update:
done: compression should not be a huge issue...if you choose to rebuild at some point you can make tremendous gains in performance while adding a touch of economy (mpg) to you truck by increasing compression. I like your cam- i have a special grind going in mine that is very similar.

any idea why it was sitting- any problems? 35k on a rebuild sounds decent- especially if it was running well when it was stored- drained the gas i imagion- and changed the gas filter. cap, rotor yada yada yada

also...when idling pull the plug on each cylinder - one at a time- and put it back before going to the next. There should be an even and small drop in rpm when each is pulled and then back to normal when replaced. If you have a cylinder where it doesn't decrease then you may have found the problem- ck for spark at that cylinder and if you have decent spark ck the compression- if not get fire then ck again.

Compression ck would be huge to rule out any problems with rings/valves etc. any knocking, ticking or other noises?

is it 2 or 4wd- beautiful truck eh?

socketman is right about the compression...differing thoughts on "adequate".

 

Thanks ill check the plugs on it. Its a 2wd. It was sitting because my grandfater bought a new toy to mess around with. It also had a couple warped pushrods and the old cam was tired it had wore off the bottom of the lifters.Ill get checking on that it may be tomorrow before i reply.

 

So we have a bit of negative history going- good to know. let us know how the plugs turn out.

 

It sounds lean to me. It may be a vacuum leak. Get a vacuum gauge and measure the vacuum at idle.

Spray some carb cleaner around the carb base and around the manifold mating surfaces. Also all vacuum connections and hoses. If the idle picks up and smooths out, when you spray an area, that is where the leak is.

What kind of carb is it? If it is a Holley, it may have a bad Power Valve.