Hey all, I'm going to apologize before hand since this is going to probably be a long post and I'm on mobile so my grammar might suck.

I got a 1986 f-150, it's got the 4.9l in her, mated to the m50d-r2 transmission. The engine is as basic as you can, got no computer, not even eec.

The engine has got the Carter yf carb, an msd ignition coil, new spark plugs and wires, the detached icm on the fender, with the feedback going from the carb into the distributer, and the one vacuum line going to the brake booster. Mechanical fuel pump and clear filter going into the fuel pump, just the metal one attached to the carb after the pump.

​​​​​​I've had a myriad of issues with this engine, most I've gotten a grasp of but the big ones I'm struggling with are severe lack of power, backfiring like crazy, hesitates and backfires when you hit the gas going at low speeds and sometimes when you shift 4-5th and 5-4th. (she shoots flames). And now the engine temp is sky rocketing to max, on my way to town (bout 25 minutes) it'll be fine for the first probably 10 then spike to max, then go back down (even when accelerating) it doesn't do that every time, just the one I've noticed the most. Then it'll be fine when I turn it off, turn it back on about 5 minutes later and it'll be back up at max temperature. Boggles my mind.

Regarding the weird sputtering and loss of power, I think a possible cause is my accelerator pump, I already have rebuilt the carb twice. When I did it the first time I put a new accelerator pump/ seals and straighted out the weird bronze pin inside, worked better but still sucked. The second time I pulled it all apart again to clean it and accidentally dried out the accelerator pump. So I have a new one I haven't put in yet so that is an option, but would that one piece really be the main reason for all those power/ backfiring issues you think?

Any and all help is greatly appreciated, I really tried to provide as much info as I could that might pertain to the issues, but I could add some pictures if anyone needs any.

~Vod~

 

If the truck is new to you and you've not already done so, check the distributor position, to me it sounds like it might be aligned wrong.

 

I've never actually checked that rotor is pointing to #1 at TDC but I have checked that the cap doesn't move on its own and it's buttoned up nice n tight. However what makes me think it's not that, is that the losing power just gradually gets worse.

There was one day my truck used to be unbelievably loud which I was used to at that point (even though it drove me mental) then all of a sudden as I was driving down the highway it totally changed how it sounded. I did lose a bit of power when it happened but it felt as though as a few days went by it was as though nothing happened and I had my power back, noise level never went back to it's ear piercing howl though, thankfully. The weird inconsistency leads me to believe it's carb related, hopefully not head gasket related.

~Vod~

 

It sounds like you have gone through the Carb enough times to eliminate Carburation as an issue so I would be looking at ignition. Both systems can produce similar appearing issues.

Based on your description I would start with a basic ignition investigation. By that I mean the following.
1 - Start with static timing. Ensure that Cyl 1 on the compression stroke and then set the timing pointer so it is at 10* BEFORE TDC. No look at the rotor and ensure it is pointing to the number 1 Cyl plug wire.
2 - Hook up a timing light to the number 1 Cyl start the engine and see if the Light shows the timing at idle to be at or about the 10* mark, (A dob of paint on the 10* BTDC line help with this) It should be stable, not bouncing around. If it is bouncing around you may be dealing with worn distributor bushings.
3 - Rev up the engine the timing should advance. If it does not you are looking at a failed ignition advance, either mechanical in the distributor or electronically if using a digital timing system.

To me, your description of the engine suddenly changing tone etc. sounds like the ignition advance stopped working or is working sporadically..

 

It is good to check the timing but if you are still running the feed back dist. and carb you will never get it to run right as it sounds like some of the feed back system has been removed.

Lets see a picture of the dist. dose it have a vacuum line going to it, it sounds like no so not good.
Picture of the carb also. dose it have wires going to it other than the choke? If so also not good.

Because the feed back system is not working timing would be lock at what ever it is set to.
Not good for power or MPG.
The carb is then set to run rich. Also not good for power and NOT good for MPG.

My guess time for a non-feed back dist & carb that we and not a computer can make adjustments to.
Dave ----

 

I agree with Dave...

I bought one of these Chinese HEI distributors to troubleshooting my genuine DUI one and the quality honestly seemed pretty close to me at $80 vs my $470 one.

Ignition curve was not exactly what I needed, but HEI so easily changed.

 

I just went back over the OP first post and would still like to see a picture of the dist. and carb to see just what he has as it sounds like he may have some but not all of the feed back system changed out and if so it will run like crap.

But one of the big things was he said the truck was loud EXH? and got quite and lost power?
He also said the exh was not a normal sound.
This leads me to think the exh system may be plugged.
Either the cat or the muffler or both could be chocking the exh from leaving the motor.
This will kill power and make the motor run hot also.

A member in the 80 - 86 area had a truck that would just start and idle ruff but any throttle it would cut off.
After checking everything he cut the exh open and found the cat plugged solid.
Dave ----