I'm back to working on the Bronco for a bit. Here's the deal 1969 Bronco, 302 motor low compression but even, Edelbroke Carb freshly cleaned out, rebuilt distributor, new plugs and wires, and new coil. I had it running fairly well but then like a light switch it seems to drop one or two cylinders. I live in the hills so I put it in low range to limp home, maybe after a mile or so again like a light switch it will through you back in the seat as it starts running on all eight cylinders again.

I put it in the shop this last week and found I was not getting the fuel I thought I should get. Checked fuel line for blockage seemed ok to me so I replaced the fuel pump, same problem not enough fuel to the carborator. Found out that my Bronco has or should I say had a plastic fuel line that was very brittle next to the headers and a **** more connection to a fuel valve that was not being used anymore and sucking air, I replaced the line with a rubber line from tank to pump.

OK fuel pressure gauge installed looks good, vacuum gauge hooked up readjusted points things are looking good engine running good. Checked initial timing 10 degrees advanced ran RPMs up to 2200 or 2300, 52 degrees advanced WOW that seems high. Pulled vacuum line checked manual advance 31 degrees that seems ok reinstalled vacuum line and readjusted initial down to 6 degrees. Now when I went to check full advance the thing starts running bad again, let it come back to ideal 450 rpm maybe 6 or 7 cylinders and only 8 to 10 lbs vacuum. At the same time when it is running bad I can here what sounds like a tappet rattling or an exhaust manifold leak. Pulling plug wires #8 cylinder is dead and #5 is all but dead very slight change in rpm and sound.

Next step exploratory surgery, pull the valve covers and look around I guess.

Any ideas?

Tim in Chelan

 

Well with low compression you may never get it to run well. You may have wiped a cam lobe or two. I would pull the valve covers.

 

Of course it ran like hell. Six degrees of timing is way too little timing. A 302 on the street runs best with 14 degrees of advance. Rule of thumb..... don't fool with the advance unless the engine is pinging when you are driving around. Pull the vaccum line from the distributor and plug it. Us the timing light to set the timing with no advance on the distributor to 14 degrees. Re-install the vaccum line.
See if the engine runs better. Try it with no vaccum advance, see if it runs well like that.
Check the plugs in the cylinders that go dead and see what the look like.
I think you could also have a bad pulg wire or two. Never trust that a simple problem like an improperly crimped spark plug wire can't cause you a major headache. This is the kind of stuff that causes people to panic and then before the know it they have their engine in a million pieces.
Hopefully you saved the old wires you had on the thing. Next time it loses cylinders install the old plug wire on the cylinders it loses and then see it it come back to life. Electrical gremlins are a real bear to deal with.
Also, you might very well have a bad lifter that sometimes loses pressure and rattles.
Also, get that rubber hose from the tank to the pum replaced with a good stainless solid line set unit like you can get from a Bronco place, NPD, of Classic Tube.
I don't think you have a major problem because it runs well sometimes. A major problem like a wiped cam lobe will cause you ending to run badly all the time.

 

Would a bad plug wire cause the low vacuum? or is that just a side effect of not running on all eight cylinders?

 

It could very well be a side effect of not running on all 8. It could also be a side effect of a poorly tuned carb. Or it could be the indication of a vaccum leak.
Do you have a stock cam or aftermarket cam?
First thing you need to do with any carb is make sure that the power valve is funtioning properly. I know how to verify that on a Holley, not on a Carter/Edelbrock. You need to call Edelbrock and ask them how to verify that on their carbs.
First verify the power valve on that carb is functioning properly.
Once you get the engine running on all 8, pull the vaccum advance from the distributor and bump the timing up to 14. Make sure you either have the hole plugged of the vaccum gauge hooked to it.
Now if you have an aftermarket cam, your idle vaccum may very well be low 10 to 15 in hg vac. Stock cams usually 18 - 20 in hg vac on the guage.
Now with the timing set start looking at the vaccum guage.
Flick the throttle a bit watch the guage.
Now start working with the idle mixture screws on the carb and set for either highest vaccum if you have a stock Ford cam, or turn the screws in until the engine starts to run rough and stumbles (to lean) and back them out a little bit until is runs smoothly if the cam if aftermarket.
With a Holley/Autolite carb if you have a blow power valve when you turn the idle mixture screws in all the way the engine continues to operate. I don't know if it the same for Carter/Edelbrock on that. Newer turn the idle mixture screws into the seats hard, turn them in gently until they seat. You can ruin a seat if you run them into the seats hard.

If the vaccum is totally erratic, or low (0-5 in hg Vac) you could very well have a vaccum leak.

If the engine looses a cylinder, a tester that shows if a plug wire is conducting spark would be helpful, but just try by install a new wire there on that cylinder and is it magically comes back to life.

What do the plugs in those cylinders look like?

 

FYI if you have a poorly crimped wire sometimes the spark indicator is unhelpful because the wire is conducting spark, but the terminal is not conducting the spark to the plug. If just depends what terminal is bad, the one at the distributor or the one at the plug.

 

Also, I think I mispoke. Usually the port on the carb where the distributor plugs in to is only going to pull vaccum under accceleration, not idle or WOT. You need to use a port that supplies manifold vaccum for the guage to tell you what you need to know.

 

Since it is an intermittent problem, I would check ignition system first as Boba Fett said.
You have isolated the dead cylinders to #5 and 8, I would try swapping plug wires and plugs with adjacent cylinders to see if the problem moves.

How low of compression do you have? Is either of the cylinders you are dropping lower than the others? Trying to tune in a really tired engine is frustrating. If it is truly worn out, plan on replacing or rebuilding it.

When it starts acting up, shut it off and pull the valve cover off and check the pushrods on those cylinders. If you have any lifters leaking down, you should have sloppy pushrods.

A vacuum gauge is the best tuning tool you can have. It can diagnose a lot of problems and help you to get the highest efficiency out of your engine.

I don't agree with BFett about the ignition timing. Since every engine is different, you can't really give an absolute number for base timing. What may work for a 302 at sea level or 2000 ft, may not run at all in Denver. Even identically built engines at the same altitudes need to be tuned independently. Specs are usually given for baselines anyways.
With that said, 52 degrees advance all in, is way too much for a typical street engine. And advancing the initial timing 4 degrees more is only going to make more problems. For most, 38-42 (maybe up to 46) degrees total advance should be plenty. I have ran up into the high 50's on a race engine, but that was running consistently between 4500-8000 RPM's.
If you are getting 31 degrees mechanical advance, that is too much when you factor in the vacuum. I'm not sure what the old time recurvers look for, but I shoot for 20/20 as a rule of thumb. Your rebuilt distributor may not have been curved properly or at all. I don't have much faith in rebuilt parts anymore. There is not any attention to detail in looking for worn parts when they are going down an assembly line. I have found more than a few Fords that had worn mechanicals, that were allowing too much advance. I have fixed a few of them by wrapping bare electrical wire around them and soldering it to the stops on the weights.
I don't think this has anything to do with your problem though.

Jason

 

When the load on the engine is low, and the vacuum is high, the vacuum advance is activated, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see approximately 50 degrees of timing.

 

quick question will more advance give u more power on the higher rpm range???

 

ooo take a look at your plugs u never know I used to burn plugs like crazy the automotive store gave me the wrong heat range but I don't thonk they would just come back to life though????????

 

Since vacuum advance is based on engine load, and mech advance is based on speed, you should have low vacuum to the dist (ported) when the engine load is low (at cruise speed on flat, level road). Your manifold vacuum will be higher with less load. You will have some advance from the mechanicals.

Under a hard, high RPM pull, you might see numbers into the 50's, but its nearly impossible to simulate that in your driveway unless you have a dyno.

Jason

 

I have not checked the compression sense just after I bought the rig last fall and I don't remimber which cyclinder was the lowest, but they were all between 105 and 110. When I said I had 31 degrees of mechanical advance that was the total advance including the 10 degrees of initial advance, so basicly I do have 20 mechanial advance. Right?

I have not had a chance to work on it this week and won't get a chance untell next week. If I had a lifter leaking down say on #8 cyclinder would it or could it effect the #5 cyclinder? It dosn't make sense that I would be loosing two cyclinders at the same time.

My plan is to replace the engine with a 351w, but it this point I would like to get by with this one. I know it is wore out but I should be able to make it run on eight cyclinders all the time I would think.

Thanks for all the tips, I will let you know next week what I find.

Tim

 

I'd say your compression numbers are on the low side, but not worn out at this point. Since they are all even, I think you can work with it.

You are correct about the mechanical advance. I misunderstood.

I'm trying to figure out what 5 and 8 have in common. Do you have the plug wires routed together somewhere? Could they be crossfiring through the cap or cross shorting on the wires? Did you replace the wires before this started, or because of it? Did you replace cap and rotor when you changed the distributor?

Jason

 

Back in the early 90s when my EB still had points and condenser, I had a similar intermittent miss from time to time. What I found was that the replacement breaker plate (the part that the points and condenser screws to and rotates with the vac. advance) in the distributor had worn out and at certain times the breaker plate would not return to the original rest position after advancing causing an idle problem. What I've learned was the better brand breaker plates used brass slider busings but my replacement had nylons which wore out causing the plate to not return to base timing at idle. I know you mentioned a rebuild distributor but I've seen some units with older breaker plates reused. This is easy to check by simply removing your dist. cap and try moving your b/plate with your fingers to check for side to side movement. It should only move with the direction of your vacuum advance arm but not side to side. This may not be your problem but just my input.