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71 chevy c-10 08-11-2002 06:42 PM

Well, I'm new to this site, but I've tried everything that I can think of to fix my prob short of yanking the heads off my motor and starting over. I've got a 71 chev pickup with a 78 chev rebuilt 4 bolt with smog heads. The guy I bought the truck from bought the long block and slapped it in about 1 year age. My problem is the thing detonates like crazy and sounds like a glass jar full of marbles. It doesn't seem too though when you put your foot all the way in it. Only at part throttle and mild acceleration does it occur. It's got an Edelbrock Performer, HEI, headers ,and a Carter AFB carb. Anyone got any ideas?

HotRodMan 08-11-2002 07:10 PM

I have an idea that few people know about. Since you bought the truck from someone else there is always the posiblily there are differenct than stock cam and other parts. If you have a timing light I would check your current timing, and check to see that the timing is advancing on the vibration damper. Then buy a vacuum gauge and try this.

If you have points, set them to 30 degrees, with a dwell meter.

Adjust the idle screws on the carb so that it runs the best and adjust to the factory idle speed.

Connect vacuum gauge to an outlet on the intake manifold.

Turn the distributor until you get the highest reading.

Then turn the distributor back the other way so that it reads one less. If your max reading was 20, turn the distributor until the vacuum gauge reads 19.

I have 4 cars that I have timed this way and it works great as long as your advance is working like it should.

Let us know if it works for you.

71 chevy c-10 08-11-2002 07:22 PM

I'll try it and see if it works. Just to let you know, I have retarded the timing enough to keep the engine from detonating, but it obviously has noe bottom end and idles like I have a monster cam. My ignition is pointless as I have a stock HEI. Shouldn't it run on 93 octane? I can't imagine th C/R being any higher than 9.5:1 with stock 76cc smog heads.

4 Jaw Chuck 08-11-2002 10:24 PM

The vacuum advance mechanism should be getting be getting ported vacuum from above the throttle plates, if it is hooked up to manifold vacuum it will cause part throttle detonation. Check your vacuum source at idle, if it is getting vacuum you have found the problem. There should be a hook up on the carb for ported vacuum.

71 chevy c-10 08-12-2002 03:29 PM

Already done the ported vacuum thing. It is hooked up to ported vacuum. Any other suggestions?

4 Jaw Chuck 08-12-2002 03:46 PM

Check your mechanical advance mechanism under the advance plate for broken springs or corrosion that might cause sticking. Any of these might cause the problem. Worn distributor shaft will scatter the spark and not help things any either, if you can move the shaft at all (even a little) it has had it.

71 chevy c-10 08-12-2002 07:15 PM

Thanks 4jaw. You got me to really thinking that this may be the prob. I have tried everthing from gallons of octane booster, toluene, retarded ignition, and carb. adjustments from lean to rich. Nothing seems to work so it must be mechanical. I'll let you know how she turns out. 08-12-2002 07:28 PM

4-jaw; You sound like the guy who can answer a question I have had for years. I have called radio 'expert' mechanics and they him-haw around and have no answer. When do you use manifold vacuum and when do you use ported vacuum to power vacuum advance mechanisms? They do their job at very different times in engine operation.

[ August 12, 2002: Message edited by: ]</p>

4 Jaw Chuck 08-12-2002 08:00 PM

Ported vacuum for advance,

manifold vacuum for retard.

Of course you only need retarded spark in a high performance application when forced induction is in the picture...or emission controls-yuck! :p

There are all kinds of things you can do with mechanical systems like bleeds and different diameter diaphrams as well as preload springs for the diaphram(s). I experimented with all these things at one time by cobbling together various types of diaphrams together (what am I nuts?) and I always came back to a plain old adjustable vacuum advance canister like they had on early sixties fords. They are nice to have so you can adjust not only the preload but also the total vacuum advance, great for mileage running pump gas.

Then electronics came along and all that adjustabilty went out the window, thank God for the aftermarket black boxes that are adjustable with a laptop. Honestly trying to keep the facory non adjustable stuff is a waste of time, if you can't play with the spark advance, a plain old mechanical distributor (with electronic pickups) is better than the non-adjustable stuff for power. 08-12-2002 08:15 PM

So you're saying I could move my vacuum source from the manifold to the port on my Holley 3-bbl, 9:1, headers, 3/4 race cam, 354 hemi and get more power w/o detonation?

4 Jaw Chuck 08-12-2002 10:13 PM

After tuning it for the change....yes. The ported vacuum source on any carbuerator can be anywhere above the throttle plate and depends on the design itself but basically if it you look at it this way you will understand;

Vacuum advance is for one thing...mileage and response at part throttle.

Race engines use only mechanical advance for this reason, if you introduce any vacuum advance into the equation you can have the vacuum advancing the timing when you least need coming into a corner and releasing the throttle (high manifold vacuum). Also at top end the engine will start to generate some manifold vacuum (due to restrictions) which could cause some advance. This is very dangerous as detonation at this rpm will destroy the engine quickly (6000+). Your ported source will be at atmospheric under the same conditions.

Ideally you want the best of both worlds, vacuum advance when your just cruising around when the engine can use it and pure mechanical advance when you are hard on the throttle....ported vacuum is as close to a linear response to engine load as you can get and is only active when the throttle is idle you get nothing.

Therefore you can run more initial advance in the distributor without the vacuum advance functioning and messing up your highly tuned mechanical advance setup. Usually what would happen is you would have to reduce initial timing to compensate...this kills off the line torque because as soon as you whack open the throttle the vacuum disappears and your left with a compromised timing for the engine.

I may not be explaining it all that well but here is a rule of thumb that applies to all high performance engines.

-Tune your engine using only mechanical advance on the dyno to get the maximum torque.

-once the engine is installed in the car you hook up the vacuum advance (to ported vacuum) and keep adding advance until it pings...then back off with a safety margin for changing weather conditions etc. Of course you need an adjustable vacuum advance canister. The major problem is load is what determines how much timing the engine can stand unless you are using really low compression )like 6-7:1 and big heavy cars can't handle as much vacuum advance as lighter cars under part thottle load.

Soooo....manifold vacuum will add advance in a non-linear fashion and the engine has to be tuned to compensate (reduced timing). You can see this if you hook up two vacuum gauges, one to ported and the other to manifold and then drive around. You'll see that anytime the load is removed vacuum skyrockets....but rpm stays the same and your mechanical advance mechanism is controlling that. this could cause detonation when you least expect it and least need it (high rpm, low load).

Am I making sense?

Really when you have it setup just right you gain a lot of "snap" from the engine since the timing is where it needs to be anywhere in the rpm range and load and the vacuum advance only kicks in when needed (cruise) and when it does, it does so more slowly and predictably. The mechanical advance mechanism is more in control of the timing that the engine sees.

Usually what happens is you end up dialing a lot more mechanical advance in and reducing vacuum advance. The term "put all your timing into the distributor" is very appropriate for power. Non-ported vacuum advance came about because manufacturers were already compromising the tune of the engine anyway by retarding the timing and the engine could handle that. But if you start trying to give the engine the timing it needs and retain the non-ported advance it will throw a wrench into the whole works. This scheme reduces emissions.

Try it and you will see, it helps to have access to a distributor machine or you can hook up a drill and digital tach to the distributor and tune it on the bench.

So what do you think?

66chevy2ss 08-13-2002 04:25 AM

4 Jaw Chuck:

You seem to know quite a bit about vacuum advance mechanisms. Here's a (likely painfully easy) question:

Should I be able to detect a broken vacuum advance can (diaphram...whatever) by sucking on the hose?

Some people have said I should be able to, others have said I shouldn't. I tried it out on my car, and I got no suction at all (i.e., I sucked until I was purple in the face!)

Any suggestions?

dwstevenson 08-13-2002 04:40 AM

Check your vacuum advance... It may be advancing the curve way too much.

Your vacuum advance port should not have suction at idle but have suction when throttle is applied.


4 Jaw Chuck 08-13-2002 05:46 AM

You should be able to make the advance plate at least move with mouth suction but.....the best way to check is with a suction pump because you need to check for slow leaks by maintaining a constant fixed vacuum and see if the diaphram holds the vacuum. I have been able to generate 18 inches of mercury by the the oral method if you use your tongue to block the hose during sucks.... :eek:

I detect the possibility of this conversation degrading with all this sucking and turning purple and such so I will just back away from the subject carefully. <img src="graemlins/sweat.gif" border="0" alt="[sweat]" /> 08-13-2002 05:52 AM

Thanx 4, all makes sense.

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