It's just got a single vacuum port. I can't figure it out either. With the advance hooked up it'll advance probably 30*, but runs like garbage. Retards without advance.
Did I maybe put the new dist. In and get a tooth off or something?????

 

Just pulled the cap off with the timing at TDC and the rotor is between #5 and #1 cylinders on the cap. Looks like I'm off a tooth for sure.

 

Move the dist around till the engine runs good, then check to see where the timing is.

How many miles on the engine? Helped a friend having timing issues on a 351, messed with timing several times and finally turned the dist until engine ran good (idle and driving), checked it with a timing light and it was at 35 degrees, timing chain was off a tooth.

 

I have no idea how many miles are on this engine. Previous owner claimed that it was gone through 49k miles ago.I'll start just moving the dist around and see what I can get, but it seems like when I advance too much I get a miss and if I retard it I get a miss.
The original dist. would mechanically advance fine just fall on its face with vacuum advance hooked up. The new dist. is a brand new duralast and even after pulling it again and setting it a tooth different, the same old stuff. No mechanical advance, advance with vacuum but runs like garbage and backfires. Timing is still around 10* where it runs the cleanest. Could the new dist. be faulty? Just unsure what to do next?

 

Also curious if this could be a result of faulty wiring? I had to re wire the brain box when I bought the truck because it was wrong and wouldn't produce spark. The plug going to the dist has been cut and spliced as well if one of the wires was mixed up could it cause all of these headaches?

 

I like to use the "divide & conquer" or process of elimination. I ain't the brightest crayon in the box either, tho working smarter is better than working harder and usually faster in the long run.

Double check, verify, and confirm that wires, and firing order, are correct, #1 Piston @ TDC compression stroke.

Then observe where rotor is pointing. Should be a little shy of the # 1 plug terminal location. Since the cap has to be removed to see this, mark the location on the body of the distributor with a sharpie or paint marker or something. Just make sure it's pointing where it needs to.

If we've confirmed everything is mechanically lined up, the rest is gravy. Now just so we're clear, and future reference to anyone else, when we remove the the vacuum advance for setting the timing it has to be disconnected and the carb port plugged. The distributor itself won't draw any air or vacuum, but the source of vacuum will. That's what needs to be plugged.

An excellent engine diagnostic and troubleshooting tool, suitable for carb tuning too, is the vacuum gauge. Timing lights can cause trouble. The gauge is not a substitute for a timing light but if there's an issue (like now) it's real handy to use.

 

I'm no expert but I think I read that you had the vacuum advance hose connected to the carburetor vacuum port. Mine is connected directly to the intake manifold. Try routing the hose to the manifold, plug it, set the timing somewhere around 10*, reconnect vac adv, tune your carb. Drive it, if you get some spark knock back the timing off a little
If I'm an idiot please let me know

 

Had this exact same problem. Is your vacuum advance adjustable? Typically a small size Allen wrench can fit inside the vacuum advance nipple. Turning this left/right can vary the "rate" of vacuum advance (or maybe it when vacuum advance initiates ).

The PO had turned it to the right about 4 turns.....that's why it ran like crap with the vacuum advance hooked up. I turned it all the way to the left until it was just barley seated and backed it off a half a turn.

Hope this helps.

 

Connecting to manifold vacuum gives full advance at idle and retards the timing when you give it gas, this is backwards from what is needed.

 

The only difference is at idle. It's useful to run manifold vacuum because it will run a lot cooler at idle or stop and go. It causes problems when running automatic transmissions and all the other vacuum solenoid gee gaws they hang on engines today, erratic idle RPM mainly.

Engine vacuum itself, always goes bye bye when you first give it gas, it matters not where it's hooked up. As the speed levels off, the vacuum comes back up and so will the engine vacuum, and the vacuum advance too.

Anyway Mg390 try this. Make sure you will be connected to a source of "spark" or ported engine vacuum. You can try manifold vacuum later if you want, but let's not confuse things further. This connection will be on the carburetor, above the throttle plates somewhere.

The way to tell that, there won't be any vacuum present at factory idle RPM. Still, disconnect and plug that port on the carb when setting the timing. Ignore vacuum advance completely for now.

It's also important to tune the engine while at factory idle RPM, for one thing, so the weights and springs in the distributor aren't tipping in. There's a method to the madness.

Next put a timing light on the #1 cylinder plug wire. Being careful to stay clear of the plane of the fan, run the engine up in neutral through 500, 1000, 1500 RPM etc., all the way up and watch the timing advance on the balancer. Hopefully your balancer outer ring has not slipped due to age, or the numbers will be useless.

See what the timing stops at. It might be 25, or 45 degrees, or 50 or whatever. Increase RPM until you are certain it is done advancing.

Adjust the distributor as required, so that the total mechanical advance never exceeds 36 to 38 degrees. If it's a higher compression engine, maybe 34 to 36 degrees. Then bring the RPM back down to factory idle. Observe timing. Should be between 10 and 20 degrees typically. Lock the distributor holdown.

Check for ping on acceleration. When you are satisfied with the mechanical timing, THEN and ONLY then reconnect the vacuum advance to ported or spark timed port on the carburetor. Check (again) for ping or detonation at steady cruise, part throttle acceleration this time. WHEN ping occurs means different things, from a dead stop - initial timing. Steady cruise or part throttle - vacuum advance.

The manufacturer of the aftermarket vac advance cans suggests starting at full clockwise and backing off two full turns at a time, until ping goes away.

All of this is tedious but will give you an engine that starts easy, idles well, has decent snap and acceleration and gets the best fuel mileage possible. This means about 50 degrees BTDC cruising down the highway, maybe more. All V8s are setup this way, basically.

 

[QUOTE=Tedster9;16215188]The only difference is at idle. It's useful to run manifold vacuum because it will run a lot cooler at idle or stop and go. It causes problems when running automatic transmissions and all the other vacuum solenoid gee gaws they hang on engines today, erratic idle RPM mainly.

Engine vacuum itself, always goes bye bye when you first give it gas, it matters not where it's hooked up. As the speed levels off, the vacuum comes back up and so will the engine vacuum, and the vacuum advance too. [QUOTE]


It's a little more involved than that ^^ and as a mechanic in the late 60's and at Ford in the 70's all early engines had the vacuum advance connected to ported vacuum at the carburetor until EPA emission laws made tuning an engine for best performance and mpg illegal



This I completely agree with ^^

 

Everyone thinks ported vacuum is an emissions era invention. It isn't, it's been around since the 1930s at least.

The reason is because as it was explained at the time, it allows for a smooth steady idle. And this truck was hooked to ported or spark timing afaik, so for now he should do the same till he gets a handle on things. Less complications during troubleshooting is a good thing too.