I've been messing with my truck for a while and having carb issues but I just got a new carb on and found out the timing is still slightly off. I'm not too familiar with timing and I'm not sure what to set it to. I've read that 10° before TDC is what I want to be at for idle. But when it gets into total timing and timing curve it all gets confusing. It's a stock engine but I'm running a 750 holley and I know that's a little big but it's temporary. Is 10 before TDC a good place to start and what should I see for total timing. Also what about the timing curve I don't know much about that. I put a brand new OEM distributor in and would assume to just leave those springs in it

Edit: I've found that 36 degrees for the max total timing is around where it should be. Does this sound right and do I measure total timing with or without the vacuum advance

 

Ok lets sort the terminology first.

Base Timing or Initial timing: is what you set with a timing light on the damper
Total timing; Is the base timing PLUS all the mechanical advance. It does not include vacuum advance.

Ideally, you set the total timing and the base takes care of it's self. Usually, 30-34° is what mild SBF's like. So lets set your dist had 24° of mech advance and your engine ran best with 32° total timing that would make your initial or base timing 8° BTDC.

The timing curve is that rate at which your mech timing comes in See below example of a timing curve with Inital advance and Total. Generally, you want all your advance in no later than 3000RPM with 2500 being better.

 

Okay makes sense now. So would I just set the base timing around 10 degrees just to get it to run decent enough to rev it up so I can set the total timing? And how would I go about setting total timin? Would I have to set the base around 10 then check total and if it's not right then go down to 8 base and move in increments of 2 until the total is where it should be?

 

Here's another way to look at it.

Don't worry about 6° or 8° or 12° (or whatever) initial idle timing, because it doesn't really matter too much.

But the total timing is fixed inside the distributor, (unless you want to get in there and modify it) and what you really need to know is where it ends up on the high side, this is the important part.

32° to 36° BTDC is right in there where most every OHV V8 wants to be, so a lot of guys just spool up the engine RPM with the timing light, rotate the distributor housing till the pointer on the balancer is say 34°, and let the initial timing fall where it may. This would be without vacuum advance. The centrifugal or mechanical advance is always set first.

Then take it for a test drive. Without vacuum advance. Accelerate hard full throttle, add a little more timing maybe 1° or 2° if it doesn't ping or knock on hard acceleration. Back it off 1° or 2° if it does. The point just short of knock or ping is optimum for power and economy. Stock OEM distributors may have very stiff springs, or a slow and "lazy" curve. Make sure it's done advancing all the way when you check with a timing light. If you don't check the total timing, it may be excessively "late" or retarded. Everybody knows too much advance is bad, but so is retarded timing. It will tend to run hot, poor performance and economy. If your particular engine combo will run well at 37° total timing, that's what you want. Maybe it's high compression, and will only tolerate 32°. Then that's what you want.

When that's straightened out, then go ahead and re-connect the vacuum advance and take it out again for a quick spin. At this point in time don't mess with the distributor timing, adjust only the vacuum canister itself. Most aftermarket cans take a 3/32" or 2.5mm allen wrench. Listen carefully for rattle or ping again, but this time only at part throttle operation or steady cruise on flat ground. The knock will go away on acceleration when the vacuum advance is excessive.

Crane suggests starting out with the vacuum can at maximum advance (fully clockwise) test driving, and backing off 2 turns at a time as required. Again the general idea here is the same, to have the vacuum advance timing as far out as possible, though just short of any ping rattle. It's OK to have a slight momentary part throttle vacuum advance rattle every now and then.

 

Thanks a lot that cleared it up a lot for me. I can't take it down the road because I'm in the middle of doing a body restoration but can I listen for pings when it's in park or does it have to be under load? Also with the canister do you mean loosen and turn the whole vacuum canister on the side of the distributor?

 

Nope, it has to be under load, on the road. But if you set it up ahead of time it should be pretty close. Might see close to 50° (Total + vacuum advance) with a timing light, in neutral. No load, so it reads a little high in the garage but that's factory timing.

Most aftermarket distributor vacuum advance cans are adjustable by removing the vacuum hose and inserting the allen wrench. It might take ten full turns or more till it hits the stop. It increases the spring tension and needs a higher vacuum to move the breaker plate and advance the timing.

What you can do before getting it on the road is kind of get an idea of the engine idle vacuum. A stock OEM engine with a factory camshaft will (or should) have a high steady manifold vacuum. If you've a performance camshaft, the manifold vacuum will be reduced and a factory vacuum advance canister may not work correctly. It can get a little involved messing with this but if you take the time to get it right you'll thank yourself everytime you drive it. It's all done with computers and crank position sensors and gee-gaws today of course, but back in the day they only had mechanical weights & springs, and engine vacuum.

 

Yeah I'm new to the timing and stuff so it's kind of confusing but I will thank myself later. The engine is stock except for the 750 holley I put on. How would I go about adjusting the canister when in the garage? Should I turn it all the way in then back it out a certain amount to start. Or just leave it where it is for now

 

Update: I'm in the garage now and got the initial timing at 10 BTDC just to get the thing running. I haven't had any backfires or misses when I rev it up but it does hesitate but that could be the massive Holley overfueling. The only problem I'm having is that it won't go back down to idle it just dies. I'll rev it up and when it comes back down it dies unless I stay on the gas. It'll start back up and idle after it dies but it's a low idle even when I mess with the idle screw unless I put it at a real high idle. I'll set a good idle but when I rev it up it goes back down lower than where I set it

 

What works really well as kind of a QC check on tuning, and gaining confidence that things are actually moving in the right direction, is the use of an inexpensive mechanic's vacuum gauge.

Just because the pointer says 10° BTDC on the balancer when setting the initial timing doesn't necessarily mean that's really where it's at. I haven't gone back through this thread, maybe you know this, and have already checked. TDC marks on the balancer can slip because of age for example. This is pretty common.

It all kind of comes back around to the engine basically being just a big air pump, and the carburetor expects and is calibrated for a strong vacuum signal through the idle circuit. Ignition timing has a direct bearing on manifold vacuum levels, if the timing is off spec, or there are vacuum leaks, late valve timing etc., the carburetor will not respond to adjustments and it won't run right.

A factory stock engine with a mild camshaft should pull 19" to 21" steady on a vacuum gauge at sea level, this means the ignition timing and carbutetor and valve timing and gaskets and valves etc etc are in good shape and tuned very close to optimum.

 

Thanks for that. The thing is being bipolar I just started it after it sat for a few hours and it runs great. Had one backfire and I think I have a vaccum leak somewhere which like you said would throw everything off so I need to check for that. Just now the rocker arms would clink around every 30 seconds or so and I need to check the oil level because that big 750 could've washed the rings and overfilled the oil

 

Reality check; I would not mess with timing curves or getting into the distributor for now. That is stuff for an advanced engine guy, if you just want a decent running truck leave the distributor at 10 BTDC like you have it. That will work on 100% of the different engine setups and get you going.

All the newer holleys have backfire protection for the power valve, I am guessing yours does not. One of those backfires may have blown the power valve, that will make it run rich. If it is blown, then it is rebuild time. And you know that carb is not ideal and has been messed with before. I think in one of your other posts you said you had a aftermarket holley 600 that you took off? If you are going to need to rebuild the 750, you might as well get a kit for the 600 which would be a better fit and rebuild it.

What will really ruin your day, if you get the other carb rebuilt and get it running good, and it's still a project in the yard and let it sit for a couple of months, and then you go out to start it and it will not run right again. The fuel you get at the gas station now is lousy and will ruin a carb if it sits around too long. So you need a long term plan if you get it straightened out but are still going to leave it stored. They make fuel treatments that help some.

 

Unless the carb is older than 92 it has backfire protection.

 

No no no ... I didn't suggest he curve the distributor, but it's really important to check for proper operation of both the mechanical and vacuum advance. Standard part of a Tune-Up. Ignition timing is critical, and it's very quick and easy to do once people understand what's going on. Most times when somebody thinks they have a Carburetor Problem what they have is an Ignition Problem. That's the reality.

 

Ok. I was trying to read between the lines and figure out his engine and mechanical knowledge. I didn't think he needed to get bogged down into timing and tearing distributors apart if he had other problems. From my experience the failure rate of the timing mechanisms in the Ford mechanical distributors is low. And he doesn't even need the vacuum advance to make it run right in the yard. Now the electronic part of the ignition may be a different story when it comes to failure rates.

 

The 600 was the factory one and I did rebuild it but I think there's something that can't be fixed inside of it. That's why I put the 750 on because I would rather not spend the money on a new 600 but that's probably what I'll have to end up doing. It's always had a backfire issue through the manifold even with both carbs. I found a vacuum leak when the 600 was still on and it ran a lot better but I also took the intake manifold off and repainted it so I'm wondering if it didn't get sealed back properly. I've only had one backfire with the 750 and that's when I just punch the gas from an idle. If it's already up to about 1500 I can punch it and it does great but from an idle it doesn't like to rev up very well. It runs better now than it ever has but the 750 just isn't ideal. I'm not sure if smaller jets would really get me there either. The timing sounds pretty solid at this point I think it's just a carb issue or a vacuum leak