Uhoh, missing suddenly. Help?

[Anonymous]

Ok, I've been hearing some knocking lately and seemed to cure it with premium gas ($$). But on my way home from a 60 or 70 mile trip, it started missing a bit on the highway at about 70 MPH. When I pulled off to check it out it was running worse and worse the slower I went. It sounds like a diesel!

The plugs look ok. # 3 and 4 are a little fouled, but not seriously. I've just now put the points back in thinking it was the Pertronix going bad, but there is no change. Timing is good at about 12º BTDC. Changing the timing didn't seem to help the idle at all.

Any ideas?? The car is a 65 Mustang 200 with a stock 1100 and distributor.

Thanks,
Scott

 

[ski4evr]

Sounds like overheating, but since you swapped out the Pertronix for the old points, the engine would have cooled off. Unless your super fast.

Timing chain slipped a tooth (a tooth broke on the timing gears)? But, I think you could compensate for that by readjusting the timing with the distributor.

Clogged idle circuit (junk from the fuel line / gas tank)? Try some heathy doses of carb cleaner when the car is idling and while reving it. With the engine shut off, fully back out the idle mix screw and squirt the carb cleaner directly into the idle circuit with the cans straw attachment. Before pulling the idle mix screw out, count the turns inward to lightly seat it so you know where to set it when you put it back.

Vacuum leak? Again, with the car idling, squirt starting fluid (CAREFULLY) around the base of the carb and other suspect areas. If the engine revs up, you found a leak.

Probably a clog or vacuum leak. Timing chain scenario not likely.

Any other thoughts out there?

ski

 

[addo]

Sounds like a vacuum-related issue to me. Burned valve?

 

[Junk-Falcon]

ppp

 

[Anonymous]

First, thanks for the tips.

Ok, I sprayed carb cleaner in the throat and in the idle circuit, both had no effect. Just to see, I also swapped out the coil - still rough. Ski, I did put the points in while the engine was hot, but it has since cooled and there is no difference in the idle. I tried to spray carb cleaner around the base and the vac lines, but couldn't tell any difference, either.

Addo, how do I know if a valve/valves are burnt? What would I look for?

Thanks,
Scott

 

[ski4evr]

To check for burnt valves, use a compression tester. You can pick one up at most auto parts stores. $25? It screws into the spark plug hole. Do one cylinder at a time (all plugs installed except the one you're connected to). Pull the coil wire so you don't actually start the engine. Have someone crank the engine for you a few times and see what the gauge reads. If a cylinder does not hold pressure or is 20% (or more) lower than the rest of the cylinders, then that cylinder is suspect. There are a laundry list of things that could cause a cylinder to be low, but a compression test is a good way to rule out a lot of possibilities

ski.

 

[addo]

First is check compression, like Charlie said. That'll pinpoint any inconsistent cylinders. A vacuum test is worth doing, too. Then inspect the motor running with the valve cover off. The reason is to check for even valve action on all cylinders. Get this far and consistency means the compression is "leaking" elsewhere.

At that point having eliminated other options, you'd be pulling the head, but at least knowing which cylinder to pay closest attention to.

Regards, Adam.

 

[Junk-Falcon]

yep thats

 

[Asa]

thought the percentage was supposed to be lower than that? something like 10%

 

[Anonymous]

Alrighty then, will do a compression test tomorrow. I was really hoping it was something simple. And by simple, I mean cheap!

Thanks for the help.

Scott

 

[Junk-Falcon]

ppp

 

[Anonymous]

Ok, did a comp test; below is the average of two tests:
#1 140 psi
#2 135
#3 160
#4 136
#5 50
#6 45

So #5 and 6 suggest a head gasket, right? Would a blown gasket cause the symtoms I described? Also, what about #3? Is that too high?

Thanks for all your help.

Scott

 

[ski4evr]

Ouch. Looks like a head gasket leak to me. Head Gasket - $45.

ski

 

[addo]

Better than burnt valves. I'd just hate to say "head gasket" then see you find out it was the valves.

Factor in a maybe need to skim the head's exhaust face/gasket surface (depending on present setup).

As a positive - you now have another useful tool!

 

[Junk-Falcon]

ppp

 

[Anonymous]

Now I know what a blown head gasket looks like! There was a 1/4" or more missing b/w #5 and 6.

I had the head machined a bit and the exhaust edge machined. The exhaust manifold is on w/o a gasket now and is holding up well.

I took it to a shop to see what caused the blow and they think it was because of the pre-ignition/knocking. So, to keep it from knocking, they reduced the timing considerably! It is SLOW, now. It cruises nicely, but accelerates poorly from a start.

So my question is: what causes pre-ignition if the timing is set to factory specification? The timing was at about 12º BTDC and is now, I'd guess, about 0-5º. Is it the vacuum advance or a carb problem?

Thanks,
Scott

65 Mustang 200, bone stock except Pertronix Ignition.