Hey guys,

I have a 1981 motorhome on a f-250 1981 4.9L inline 6 300 with 93 000 miles on it.

I am in a big road trip right now and I had a lot of problems with the engine running bad.

The truck wasnt starting anymore so I changed the coil (cheap 30$ coil from Napa) and it fixed it, but before changing the coil I changed the distributor and spark plugs wires.

After changing those, the engine was running really rough and I found out that there was a big vacuum leak on the intake/exhaust manifold gasket and also the valve cover gasket was leaking oil. The cylinder #4 wasnt even firing anymore. Oil was getting out of the valve cover and entering in the manifold by the leak.

After changing both gaskets, the cylinder #4 was firing back and the engine was running fine again on idle.

But under acceleration, there is a miss and sometimes I hear what seems to be a vacuum leak, but I dont know if its the intake or the exhaust. But that leak sound was already there since a long time ago. So it might not be related to the miss.

I did plug a vacuum gauge and I am getting normal vacuum from the manifold. About 20hg.

I did try to play with the timing and here are the results:

-When I advance the timing, the idle gets too high (1k rpm) and the engine is pinging on initial load and at high speed on the highway, but the truck accelerate much better and I could say that the missing is gone.

-When I retard the timing until the pinging is gone, the rpm gets down, to about 600 rpm, but the miss under acceleration is back, and the vacuum gauge tells me that the timing is too late.

So the problem is, I can't get it right. The truck specs say that the engine must run at 600 rpm at idle. But there is no way I can get it to 600 rpm without having to retard the timing and missing under acceleration. And when I advance the timing, it accelerates better but does ping and idle too high.

My carburetor speed idle screw doesnt even touch the throttle plate so the idle speed is at the minimum on the carburetor.

These are the things that I also changed lately (the missing was already there before all those changes):

-PCV valve
-Air breather filter
-Spark plugs
-Power brake booster (was vacuum leaking)
-Brake master cylinder
-Installed a remanufactured carburetor
-The EGR valve gasket was leaking so I put a new gasket but it was still leaking so I put some epoxy to temporarily stop the leaks around the valve.

I did check the compression, it's really good on all cylinders, about 150 psi.

I have to mention that there is also a little miss when idling. And it's intermittent but always present.

What I suspect, (I am really not an expert) is that there is still an vacuum leak that I didnt find, maybe it's only present on acceleration? Or that the cheap coil that I did buy is too weak to light the mixture under acceleration? I did check for the spark and it seems strong on idle, it's gapping from 3/4 inch to the block and I hear a "tic" sound, it's the blue tho. It's a mix of orange/blue. But more orange.

Also, I did notice that the missing is absent or really low when the truck is cold, but I will have to confirm that. It did it yesterday, I thought the missing was gone but it came back after doing some road tests. (I did play with the timing tho so Im not sure it's temperature related)

Sorry for the long post, I did change so much stuff on that truck lately and I think it's good to mention those for a better troubleshooting.

So if you guys have any suggestions, I would greatly appreciate.

Thanks for your time.

 

I thinking you may still have a vacuum leak and/or possibly a plugged cat or muffler. Check the donut where the exhaust pipe connects to the manifold...the blow out over time especially with any inordinate back pressure in the exhaust system and can produce a "ticking" kinda sound.

Fueling up with non-ethanol gas when you can find it should improve performance.

 

Thanks for your awnser.

Actually, when I feel the miss, it's like the engine is trying to push air but something is blocking it, then when it stops I hear the leak sound and the engine then goes back normally.

So thinking of it, a block in the exhaust sounds logical!

I know that the end of the mufler at the back of the truck is a little bend so the pipe is not as big as it should be. Could it be that? It's still wide open but it's a little smaller then what it should.

So I guess to check that I would need to remove the exhaust manifold and the other exhaust parts? Or is there a way to check without removing the parts?

Thanks.

 

Well...the only other check I know of (there may be others) is to weld a bung in the exhaust pipe ahead of the cat/muffler, fix with fittings for about 18" of ss brake line, obtain a vacuum/pressure gauge, affix the hose from it to the ss line and see how much pressure is building in the system. More than about 3#-5# (any for that matter) is too much.

 

Alright I will see what I can do with that. I am on a road trip right now so I don't have all the big tools..

Thanks

 

the instruction booklet that came with my vacuum tester gives an exhaust restriction test. it goes like this. hook the guage up,start engine,increase rpm, and vacuum should increase as rpm goes up.

 

Reading your vacuum gauge

Here's a quick rundown of how to read a vacuum gauge. It lists how to tell if you have an exhaust restriction.

 

Thanks for your anwsers guys.

I did check the end exhaust pipe restriction and it was bigger than I thought, so I did unbend it to open it like it was before.

The engine did like it, it doesnt seems to be pushing air hard anymore. And that's a good feeling

However, the miss is still there. It feels like a fire miss but I don't think it's the fire because all my fire parts are recent..

My carburetor is also recent.. so the fuel is good.

The more I retard the timing, the more I feel the miss. It can't advance the timing more because it's pinging. (It's a motorhome so it's heavy load)

It mostly miss in 3rd gear, between some rpm, between some vacuum I guess.

If I open the throttle more, when it's starting to miss, it stops missing. That would be a vacuum leak I guess? Because if the throttle is wide open, it doesnt miss anymore because it doesnt need to get air from somewhere else. That's right?

Thanks.

 

Starting to sound like something in the vacuum advance. Remove at carb and plug the port and see what happens.

 

Yeah I did already try that. It's still pinging and the miss get worst.. I tried a lot of timing combinations, ajusting the vacuum advance, mechanical advance and distributor advance so I don't think the timing is the issue. But my timing scale is rusted so I can't really set it exactly on 6 btdc like it's saying on the truck specs. I will have to do the white paint trick and set it properly to see what happens..

 

Have you tried setting the timing with a vacuum gauge?

 

Yes I did, and when I do it, the rpm is too high and it's pinging on first gear load and on the highway at high rpm.

I took the timing to the highest vacuum reading then I back it off a little. That's how I should do it with a vacuum gauge right?

Thanks.

 

Correct. As long as the vacuum advance is disconnected and the RPMs are at idle (around 700).

Then, plug it back in and adjust the idle mixture screw until you get the highest vacuum reading.

Readjust idle RPMs.



Repeat whole process to fine tune.

 

Yup,that's how you do it...try backing it off until you only get pinging on a very heavy load like going uphill.

 

Well when I do that, the RPM get higher, around 1000 rpm, even if the idle speed screw is all the way counter clockwise. Is it normal?

For the idle mixture screw, I have a big play. I mean that when I get the highest vacuum reading, I can keep turning it and the vacuum will stay the same.

So Harte3, you're telling me that it's normal to ping on heavy load? Since it's a motorhome it's always a heavy load right? So when I start to advance on a hill on 2nd gear is it normal that it pings? Or should I retard the timing?

I say 2nd gear because my transmission is a total of 4 speed, but the first gear is just to pull at low speed I think? You guys know about that transmission?

Thanks.