Apoximately a year ago I purchased this truck with a 302. Fuel injection had been removed, and many aftermarket parts had been installed by the previous owner. All appear to be Edelbrock and MSD that are visable. Carburator, intake, headers, waterpump distributor, spark management (msd6a), blaster 2 coil, wires, electric fuel pump, roller rockers. The truck ran great then. Not super powerful but enough to keep interest.
Aproximately 2 months ago while I was on a 70 mile round trip to work I felt a change in power. Very noticeable. No noises, nothing to indicate catastrofic damage I got home and changed the spark plugs and distributor cap. No change. White deposits on the cap and all but one plug looked as if it were burning perfect. Black deposits on the ( I believe ) #4 cylinder plug. If my number of the cylinder is wrong, it is the one closest to the firewall on the passenger side please let me know.
Checked for spark, yes it was good as the others. My fuel mileage which was not good to begin with got severely worse. I have not checked compression because I don't know how to properly do this. If compression is weak or not there what are the possibilities of what it could be. Or am I jumping to far ahead. Please help. I need this truck to last a little longer.
It is a 1989 Ford F150, 4X4, no emission controls, delivers approximately 6-7 fuel psi. Any other information needed please let me know.

 

Buy a SCREW-IN compression gauge; not a press-in. I recommend Sears/Craftsman. Then read this caption.

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Check the timing with a timing light on #1 lead. You can also hook the timing light on each lead to see any mis-fires, sometimes the sparkplugs leads can burn up inside the distributor cap with the msd ignition. I had that happen to me a few years ago with my msd.

Also check the carb for any leaks or missing vaccum lines or plugs. You can also bypass the msd ignition to see if that might be the problem too.

 

vacuum leak maybe?

 

I will be checking the compression in a few minutes, just waiting for the truck to warm up. As for vacuum leaks I have checked the few lines visulaly and sprayed them down with carb cleaner. No change in idle better or worse. Thanks to everyone who replied. I didn't want to start throwing money away at the wrong things for lack of knowing how and what to check for. I will be back with my results in a while.

 

Use a 3' chunk of garden hose as a stethoscope to find vacuum leaks with the engine idling. Don't spray flammable chemicals on the engine - it's not very effective, anyway, because many of those small vacuum lines don't flow enough for that to affect the idle, even when they're active. Many aren't even active until the truck is moving.

 

So I did the compression check of all cylinders...
Here are my results.

Firewall
#4 - 165 #8 - 170
#3 - 165 #7 - 160
#2 - 165 #6 - 160
#1 - 160 #5 - 165

Radiator

# 4 sparkplug appeared black and wet, # 5 sparkplug appeared black and sooty. The rest were light brown/tan in color.
The distributor cap was replaced only two months ago or less along with the plugs. The rotor still looked in good shape. ( not replaced ) The distributor is frozen in the engine and will not move. Have not adjusted this since I purchased the truck, ran great until a couple months ago. The wires are suspect. No visable arcing while running at night but I don't know what else could go bad with them. They are MSD 8.5 wires with a sock around each boot at the head to protect from the headers. I have also checked the placement of #2 & #4 and they are not next to each other in the wire separators.

As for the vacuum lines...

I have 5 total.
1 from the front of the carb to what appears to be a vacuum canister
1 from the front of the carb to a tee
1 from the tee to something that is mounted on top of the heater box
1 from the tee that looks as if it goes in the heater box
1 from the carb to the vacuum booster
The smallest one looks like a hard plastic that goes from the tee to inside the heater box and is very small. Besides that and the vacuum booster hose they look to be new 1/4" hose and in good shape.

 

I'd swap 'em with some good 8-9mm silicones, and put in a new rotor. A) Where is it? How big? Any wires on it? How many vacuum ports total? Any open?
B) That's actually a 1-way check valve with 2 inlets.
C) That's the vacuum reservoir for the HVAC system.
D) It goes thru a grommet then thru the recirculate grille in front of the passenger footwell, thru the dash to the HVAC control panel to operate those vacuum motors.
E) Brake booster

So you have no vacuum advance, and with no EEC to advance the timing, you're running at base advance all the time. Assuming it's frozen at the stock position, that's only 10°BTDC. Sometimes, the engine needs over 30°.

Either put an EEC/MAP/PIP/etc. in it to advance the timing as needed, or swap in a carb distributor with a vacuum advance.

Personally, I'd trash the whole mess & put an EFI engine back in. It'll run better & be cheaper in the long run.

 

Personally, I'd trash the whole mess & put an EFI engine back in. It'll run better & be cheaper in the long run.[/QUOTE]

Oh I wish this truck was worth the trouble. It's rust with a hint of red.

I thought the MSD 6a Spark Managment system was in control of the timing advancement?
I will be buying new plug wires tomorrow and seeing if that improves it first. The rotor I have to order because it is specially fit for this MSD distributor.

I am still confused. Why would only two cylinders be troubled with a vacuum problem? Would they not all be troubled? My questions mean nothing. writing thinking.

Anyway, the "vacuum canister"(part A) is approximately 6" tall, 4" wide and 2" thick. It is located below the right hand side of the battery standing at the grill looking at the windshield. No wires. 2 ports. and what looks like two round ball caps on top. None open.

 

Well I replaced the wires only on #4 and #5 cylinder and it seems to have cured my poor running condition. I'd like to thank everyone who helped. I was assuming the worst and ready to scrap it. Thanks for giving me more time with this truck. Runs super smooth again. Much appreciated.

Thanks-

 

i will be replaceing the rest of the wires later today. just have to go to work to pay off the $100.00 set that I purchased.

 

I have no idea how or if MSD advances timing. IMO, the stock ignition is superior, so I don't mess with aftermarket.

 

The msd 6 box does not advance the spark. You need to have a mech advance distributor - vaccum port or a Unilite one. I got the msd dual point distributor that's been running strong since 1980

 

How can I tell which distributor I have? It's definately a MSD. I am so sad I know nothing of this truck. I can read the labels of course but don't know what everything does. The engine sure looks and sounds cool though.

 

Check here

http://www.msdignition.com/

You should be able to click on product tab and find it. At least you can get the instruction on how to set max timing, may just get you a few more mpg's