Vac. advance VS. no vac. advance?

[hav24wheel]

I did a carb swap on my 83 LTD, n used a stock distributor for a 79 with a 302, and duraspark. i had the timing set at 10* with the vac. advance unhooked at idle. But when i would hit WOT to pass someone, (above 3500 rpm) it would make a rattle sound. So i set the timing to 12* and unhooked the vac abvance(mainly because at a steady 3000 rpm without a load on the engine it would miss fire a bit, doesnt do it with it unhooked) it made the rattle go away when its in second gear, but when i get up above 3500 rpm in 3rd gear it starts to rattle again. i havnt had time to bump up the timing another 2*s yet, but I'm starting to think the rattle could be from something else.. Any ideas? It seems to run just fine without the vac. advance hooked up.

[1987cp]

It sounds like you may need to look at the total advance curve. Things to consider include how much vacuum advance you're running, what vacuum source you're running it from, how much centrifugal advance is being allowed, and what centrifugal advance springs you're using.

Is the engine still stock mechanically?

[hav24wheel]

It sounds like you may need to look at the total advance curve. Things to consider include how much vacuum advance you're running, what vacuum source you're running it from, how much centrifugal advance is being allowed, and what centrifugal advance springs you're using.

Is the engine still stock mechanically?

Yes all internals are stock. Even the timing chain(i should change it out soon)
I had the vac advance hooked to a manifold vac port, not a timed port..could that be part of it?
The springs are the whatever was in the distributor when i bought it..

here is the distributor i bought.

http://www.oreillyauto.com/site/c/detail/A1C0/302899.oap?year=1979&make=Ford&model=LTD&vi=1131051&keyword=distributor&partType=02780&parentPartType=C0334

[1987cp]

Hm. Well, if it's sold as a stock replacement for a '79 LTD, it's probably configured with 20 degrees centrifugal advance and fairly heavy springs.

You can easily test how much vacuum advance you have by checking the spark timing at idle with the vacuum advance first disconnected, then connected. Being connected to manifold vacuum will give you the greatest amount of vacuum advance at idle, when manifold vacuum is greatest.

My particular vacuum advance canister give me about 14 degrees at idle when connected to manifold vacuum, so that means if I set the initial timing to 10 degrees and then connect the vacuum advance, the effective advance at idle is around 24 degrees. This is good for crisp throttle response so long as your tip-in enrichment is sufficient to let you pull away without either popping or hesitating. Keep in mind that this doesn't necessarily make for crazy advance at high RPM, because the vacuum advance will in this case pull off quickly as the throttle is opened.

Once you get your new engine, you may want to contact a guy Tom knows of that claims to sell you a distributor already tailored to your combination. Alternatively, Pirate likes to suggest getting Mr. Gasket's advance spring assortment designed for HEI distributors. It has bushings and stuff you won't need, but it at least has a selection of springs.

[hav24wheel]

Well i guess I'll check that. I will be pulling the motor out of my 86, doing a basic over haul, putting a Lunati cam and lifters in it(61002LK) along with matching springs. than a intake off a truck that i will port match the gasket(on heads and intake) I'll worry about getting the distributor set perfect with the new motor.....

Speakin of that, some people have told me that the 86 302 in the vic could be a roller block, is that true?

[gadget73]

86 Vic motor is indeed a roller block, and will need a different distributor drive gear to work with the Dspark distributor.

+1 on checking the total timing curve and maybe switching to a ported vacuum source. Seems the fix for these a lot of times is less mechanical advance, and allowing it to come on earlier.

[hav24wheel]

Will I be able to just swap the gear off the distributor I have with the one that's in the 86. Also, do I need to buy a special cam in order to use it with the rollers? Because the cam I wanted is a flat tapet.

I'll try a timed port to see it thAt helps.

[1987cp]

The Voodoo cam you indicate is a flat-tappet, in which case you could leave your distributor alone except for a tune. The roller block just makes it easy to run a stock-style hydraulic roller setup ... and with luck, the old roller lifters will still be OK, meaning you could cut $120 or so off the cost of tossing in a roller cam if you choose.

Annoyingly, it's difficult to buy just a steel gear by itself, though the discount parts stores such as AutoZone are more than happy to provide you with a distributor that already has a steel gear on it. Alternatively, if you send your core to the distributor-tuning guy, he could easily install a steel gear ... actually, I wonder if you asked him if he could send you a steel gear ... hm.

I have read that some people's engines just seem to like ported vacuum for the vacuum advance ... no idea why; all three engines I've messed with all seemed to prefer manifold vacuum. Ported vacuum behaves very differently, as you'll note by hooking a vacuum gauge to that port and driving: it's zero at idle, climbs during tip-in, and then goes back to zero at heavier throttle applications.

Definitely consider the Mr. Gasket HEI advance-spring kit, and go ahead and pull your inductor to make sure which advance-limiter slot is currently engaged ... for the dizzy on which I spec'd an LTD, it was on "10" (20 degrees centrifugal advance), but the one where I spec'd an '85 Mustang, it was on "15" (30 degrees centrifugal).












........... of course, all this is a moot thought experiment if your tip-in enrichment is simply not calibrated appropriately. What brand and model carburetor are you running, and what have you done to it since bolting it on?

[hav24wheel]

I have a holley 500 cfm 2 barrel. I've only adjusted the idle mixture screws.

Also if I can find a roller cam to use with the same specs as the one I already picked out, I would get it. Havnt done too much research on the roller stuff yet.

[gadget73]

well, if you want to put a flat cam in the motor, you just put flat lifters and the proper pushrods. If you want to go roller (and I would strongly suggest that from a longevity standpoint), you just plunk it in with the stock roller lifters and change the distributor gear. Possible you'll need to change the pushrods to get the right preload, but thats a possible with any cam or head swap. Not 100% sure, but I'd be willing to wager the TFI distributor steel gear will go on a Dspark distributor. Theres a roll pin involved, and you have to use a press to swap the gears.

[1987cp]

Hm.... I sense a red flag going up ......

It sounds to me at this point like you need to get cranking on that carb before you waste too much time worrying about the details of the ignition timing. You'll ultimately need to calibrate the part-throttle (jets), make sure WOT is OK (jets plus enrichment passages) and power valve is opening at the correct time (6.5 inches Hg is probably stock), and of course select the correct cam for the accelerator pump. I'm sure you already know this, but one of the surest ways to guarantee unsatisfactory performace is to install a carb that isn't already tuned for your particular application and then look everywhere else for the resultant driveability problems, instead of just knuckling down and adjusting the carb to suit your needs!

If this popping is really only on tip-in, the first thing I'd do is dial your initial timing back to like 6 or 8 degrees, leave the vacuum canister connected to manifold vacuum, and get a set of Holley accelerator pump cams. One of the neatest things about your Holley is the fact that with these cams you can adjust not only the size of the pump shot but fine-tune the rate of accelerator pump actuation as well, and all without changing the squirter or getting the huge (and basically race-only, from what I've read) 50cc accelerator pump. This appears to be one of the typical genuine Holley kits, though off-brand kits can be had a little cheaper: http://www.summitracing.com/parts/HLY-20-12/

[hav24wheel]

The carb was pulled off my 74 van with a 302. On that motor I never had any noises, that's why I was thinkin timming. I don't drive the van very often anymore, so I picked up a used 500 holley and put it on there and used the one that was on it, because it was a lot newer and I knew the carb worked good. Could it really be that much out of tune from one stock 302 with headers, to another stock 302 with headers? Only difference is the van is a points style distributor, n the car is duraspark....

[1987cp]

Fuel demands can vary even if it's the exact same engine but in a different vehicle. Ask me how I know. :smirk: Exhaust upgrades can require more fuel overall, less weight or more gear can require a leaner tip-in and possibly a lower rated power valve, etc. etc. And yes, typical symptoms of an incorrectly calibrated tip-in include popping or stumbling off idle, regardless if how much spark timing you're giving it.

So yeah, don't go assuming that a carb that was correctly calibrated for one engine in one vehicle will automatically be perfect for a different engine in a different vehicle just because it's still 302 cubic inches, because the carb doesn't know and it doesn't care. :p It doesn't give a crap what style ignition system you're running, either - all that really matters at the carb is that it gives you the correct amount of fuel under as many driving conditions as possible. I don't pretend to be any sort of carb expert, but my rule of thumb is that unless it's the factory original carb with its factory original calibration installed on its factory original application with the factory exhaust, factory gears, and factory ignition timing and whatever else you can think of, it'll usually need at least a little tweaking before it really works right.

Sorry if I sound lecturey, I just get really aggravated at the number of fairly intelligent people who seem to think the idle mixture is the only adjustment a carb should need when bolted on something it wasn't originally calibrated for!




Anway ... right, if it were my car, I would do as I mentioned before: Dial the initial timing back to 6 or 8 degrees, leave the vacuum canister connected to manifold vacuum, and check how much vacuum advance (by measuring the total advance at idle) and centrifugal advance (by looking at the limiter slots) you're getting. Then get yourself a pump cam kit and get rid of your off-idle pops or stumbles, and only then will you have a solid starting point (or, best case, a driveable combination that will work OK until it's time for the engine swap).

[hav24wheel]

It doesn't have any off idle issues, or idle issues. Just a WOT rattle above 3500 rpm. At a free rev its fine, just under load it rattles, let off the throttle a tad and it goes away. Without the vac advance hooked up there is no miss fire from idle to 4000 rpm without a load on the motor(that's with a slow steady climb from idle to 4000.) I'll try to figure it out this weekend after I replace the front seal on the trans and install a shift kit.
So maybe the rattle could be a whole diff issue than tuning??

[1987cp]

So part-throttle is OK, no pops or stumbles under normal city driving? Is this rattling always RPM related, or does it seem to be affected by throttle opening as well?

Vacuum advance or no vacuum advance should have no effect on WOT performance, especially if using manifold vacuum. Should still check on the amounts of advance, just so you know for sure what you're working with (if for some bizarre reason you have very light springs combined with a 36 degree advance limiter giving you near 50 degrees total advance at the RPM indicated, that could be bad :p).