I'm a 60 year old dentist and a hopeless gearhead for 55 of those years and I need an explanation of the poor idle and medeocre performance of my '76 F250 4X4 360 C6. 108000 miles, decent compression in all 8, new plugs, coil, cap, rotor, wires. I rebuilt carb (Motorcraft 2100) and hade ecm module checked 3 times at auto parts store (I watched the guy do it). Distributor is in good condition, throttle shaft not loose. vacuum advance working, timing correct. float level correct. All plugs burning light brown. I don't think I've left anything out. I have a distributor magnetic pick-up that I have yet to install.

At idle, when I remove plug wires at the distributor cap from #1, #2, #5, #6, & #8, one at a time, the idle is very noticeably worse but when I remove plug wires from #3, #4 or #7 there is no perceptable deterioration of the idle.

When holding plug wire cap just above distributor terminal there is a good arcing spark at each position. When I remove each spark plug one at a time, attach the wire, lay it on a good ground and run engine each plug sparks well and about equally. When I swap plug wires, i.e. use #1 plug wire for # 4 and visa versa or swap plugs from one cylinder to another the same situation exists.

I feel that #3, #4 and #7 are not contributing to the idle as much as the other cylinders are and that is why it idles poorly and lacks power. I'll have to unplug #3, #4 and #7 all at the same time and see how it runs.

NE help would be appreciated.

 

It sounds as if you need to adjust the carb or have a vacuum leak. Check for proper operation of the EGR and make sure the EGR plate is not leaking. The carb may need some adjustment also. The broblem cylinders are all fed from near the back of the carb. You may have a leak in a power brake booster also. It could be a combination of things and as such is hard to isolate. Check your vacuum passages in the manifold also from the vacuum "tree".

Check the links section of my Club FTE website for information on how to read a vacuum gauge. Then get a vacuum gauge and learn how to use it. It is a very handy instrument to have around a vehicle much like a code reader is on the newer vehicles.

 

I have a different route for you. Has the inside of the motor ever been touched? You very well could could be looking at flat spot's on the camshaft.

 

Thanks Mr. Torque1st and ranger 429. This morning I determined that #3 plug wire, when detached from the distributor cap, does alter the idle a little (not nearly as much as the others) but #4 and #7 do not. When I detatch #4 and #7 at the same time there is no difference in how the engine runs at idle or at higher rpms, at the curb, even though both cause their spark plugs to spark when the plug is laying on a good ground. The plugs are light brown so they must be burning fuel at some time, i.e. probably during freeway driving.

The engine has never been apart and since it has similiar compression on all cylinders I wouldn't expect that the cam has any flat lobes although I guess that is a possibility. The truck seems to run well at freeway speeds.

I'm suspecting an intake manifold gasket leak on the affected cylinders although that seems difficult to check as it could be at the bottom of the gasket and not discernable from the top side. I'm not looking forward to pulling the manifold as I've done it on other FE engines and I know it weighs about 90#.

I could check for flat cam lobes by putting a dial on the rocker for the affected cylinders and comparing it to the strong ones.

I'll read your info on vacuum testing first but how that could affect only individual cylinders I don't quite understand.

Thanks again.

 

The flat spot on a cam lobe will have no affect on what you are reading for cylinder compression. You want the valve's to be closed when checking the compression. If you do have a dial indicator it would be very easy to check the cam lobe's as you stated.

 

Ranger429. I'm going with your idea first. It does make the most sense to me. If I do have a bad cam will you please come up to Gig Harbor, WA and replace it for me?

I think I'll just finish the 557 and install it before pulling the front end off of the truck to replace the cam on an old FE if that's the problem.

A vacuum leak would have to affect all cylinders the same wouldn't it? I suppose an intake manifold gasket leak at the weak cylinders could still be the culprit.

 

SURE!!! 80 buck's an hour plus a nominal travel charge!! You might be better off to have it done locally. 5 or 6 bolt's on the valve cover and you will know in minute's what kind of shape the cam is in. It could very well be a leak on the intake but it will be a little harder to determine that from a cam. Good Luck

 

I've done a little more testing. When driving up a long hill, stopping every so often and juggeling plug wires I find that under load every cylinder does obviously contribute to the power very noticeably even though at curb idle some cylinders seem to contribute not at all while others make a very noticeable difference.

I may measure the valve lift at each cylinder but I think I will play withthe carb a little first.

 

The vacuum "tree" feeds into the back cylinders. If you enrich the idle mixture by half a turn on both sides of the carb does it make a difference in whether the engine idles smooth? Depending on where the vacuum leak is, it can affect one or more cylinders more than the others.

 

Thanks for the info. I'll work on it more tomorrow.

 

How old is the distributor ?

I have seen distributor shafts that can cause this problem.

Have you tried spraying carb cleaner near possible manifold leak spots & listened for engine speed changes ?

 

Thanks for your response, Mil1ion. The distributor and its shaft are in good tight condition. I'll try the carb cleaner idea.

 

if u find the culprit tell me about it bc i have a 77 that has the same problem. not that im too worried cuz im going to get a new motor within 6 or 8 months but it would be good for future referance. i have also tried adjusting the carb and tried to find vacume leaks to no avail. if anyone has an idea please through it out there
thanx for the help

 

This has got to be the coolest dentist in the world. What Mil1ion said, usually when you find the vac leak the idle smooths out of the engine RPM will increase, I would use this method along with a vac gauge just for visual indications.

 

Hey,
i would do like Torque1st said and make ssure the EGR valve is working. I went through the same routine with my 73 and found that it ran a whole lot better with it off. Luckily mine is now smog exempt so I have the luxury of doing that, but that would be what I would suspect.
-Chris