I am used to aftermarket distributors and have a pretty good handle on how to tune them and what to expect. That being said, I think we are seeing a problem with the stock vac advance distributor in the 65 283 in my brothers hot rod. If you set the timing at idle properly and then rev it to 1500-2000 rpm slowly, the timing actually RETARDS by 5 degrees or so. Once you wing the throttle beyond 2500-3000 rpm, the full ADVANCE appears to work. We shouldn't see timing RETARDING correct? Seems like a problem with the advance plates, weights or maybe even shaft play up/down? Thoughts???
I have been told that problem is caused by the walking of a camshaft if there is no cam button. I have seen it on some of my engines but not on my roller cam ones as the cam is held in place by a plate in the front of the engine.
Vacuum is hooked up at Holley carb vac port on front bowl. Well try removing the vacuum and see what happens. Had to stop for the evening.
Been doing a little more reading. I guess the situation makes sense if the advance can is adding timing at idle but going away as the vacuum drops and you slightly accelerate. And then, of course, the mechanical advance then takes over and adds all of the advance required as the engine passes over towards zero vacuum (higher rpm to full throttle). I just read this link which is pretty interesting... http://rockridgefarm.com/vettdoc/Timing_101.pdf PS: The reason for the concern is a mid throttle mid range miss that does not seem to be apparent when winging the throttle or during idle. Though with a choppy cam it is difficult to tell.
Wow, I hate to be saying this, but try the ported vac line thats on the side of the carb. A cam that dosen't provide a lot of vac sometime creates a problem that this can solve. This shuts of vac to the can at idle and turns it on as soon as you touch the throttle. This alows a little more static advance for idling, and than boosts the advance a little more as soon as you tip into the throttle, and compensates for the droop you are seeing. There is also a vac can for GM high performance engines that pulls in full vac by 10 or 12 inches, rather than a stock engine vac can. You may also want to play with adding a spacer sleeve around the pin on the advance arm. This will limit the total amount of travel of the vac mechanism. As a final tuning measure to adjust the advance to the load and get the best driveability. Always an interesting compromise to get all this tuned in right. frank
^^^^^ Spam??? WTF.... Frank, thank you. Yes, we are using ported vacuum (above the throttle blades), NOT full manifold vacuum. Perhaps I worded that improperly before. I agree that we may need a high performance can or possibly an adjustable can. The engines I am used to have run mechanical advance only so this is a bit of a learning curve for me. Thanks for the response. We will keep at it and get it figured out!
Always disconect the vac. advance to set the timing. And depending where the vac. advance is connected with it connected you may experance exactly what you are seeing.
Send the distributor to "Bubbas" it will come back clean , painted and advance adjusted per the engine specs. I can also install a new vacuum can ( adjustable) if ya want.....
You got that wrong actually it should be, "Is this hell or am I getting old." Two things come to mind, one is cam walk which can be remidied with a cam button. The second is that the old distributer is just real loose and that can be remidied by a trip to Bubbas if you are not aprt to do that one for yourself.
All the Chevy's that I ever set the timing on retarded instantly as soon as the throttle plates were opened as the vacuum drops. Usually you shut he engine off to connect the vacuum back up and put away the timing light. I used to keep the timing light hooked up after reconnecting the vacuum just to tell if the vac adv. is working properly. With the vacuum hooked up to manifold vacuum as (GM designed it) the light jumps up off of the timing tag. As you rev the engine you can watch the light move back and forth. This has always been normal dating back to the 60s. In the 80s? they came out with ported vacuum for emission purposes and confused all the hotrodders ever since. Yes the timing is far advanced at idle helping the engine to run cooler in traffic. The universal carb makers must have both types of vacuum because they must work with both types of systems. The problem is that most of the modern guys grew up with emission equipped cars. I'm an old fart and did not. Try and find a ported vacuum port on a Rochester carb.... there ain't none. Don't exist. Yes this is very normal for factory cars of that era. Ported vacuum is a product of the emission era. (not counting the Fords of 48 -56) Don't ask me about ported vacuum. I have no experience with it. Ask a young guy.
Hi Tommy, I don't disagree with the value of trying ported and manifold vacuum to see for myself which one my engine and I enjoy more. "In the 80s? they came out with ported vacuum for emission purposes and confused all the hotrodders ever since." I've read a similar statement to that too, in John Z's thorough and informative article. http://www.camaros.org/pdf/timing101.pdf He says “Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. ' and it's been repeated quite a few places since. But his characterization of ported vacuum as an emissions era invention does not sound right to me. In other threads I've mentioned a few shop manuals from the 40s (Chevy truck) and 50s (Edsel) that describe their vacuum advance systems as using ported vacuum. Attached should be a few pages from an Audel's New Automobile Guide from the 1940s, predating the emission era by few decades . It describes the function of one ported vacuum scheme ("carburetor side," ) , and 2 "engine side" (unported) . For those of us used to downdraft carbs the engine/carburetor side reference looks backward since an updraft carb (also popular once-upon-a-time) is shown. The text on Audel page 1242 says "carburetor side" (ported") is the most popular. Manufacturers back then were making their choice for reasons other than emissions I'm pretty sure. I'm thinking John Z may have started his career too late, and his interests have not lured him to look far enough back (into the 60s), so all his points of reference start relatively late, and he is "describing the elephant" from a limited perspecitive. That's not always a real good basis for building technical theories. http://en.wikipedia.org/wiki/Blind_men_and_an_elephant Here is a link to some folks who played with real cars with 455 CID engines and found for driveability and throttle response the choice of vac advance canister is at least as important which vac source to use. http://www.highperformancepontiac.com/tech/vacuum_advance_tech/viewall.html regards, Dan T
I won't argue theory. I'm admittedly not that smart. The original poster was talking about a 65 Chevy. No theory there. a Chevrolet dist. from 1965 used manifold vacuum. Part of the reason was to keep it cool in traffic. Retarded timing can cause the engine to heat up in heavy traffic. Not a problem on the race track. I am strictly talking about cars for the street. There is no place on a 65 Chevrolet factory carb to get ported vacuum. Experimentation is fine. My cars are closer to a daily drivers than to race cars. The factory engineers are a hell of a lot smarter than me.
A leaner fuel mixture takes longer to burn. When you first hit the gas the accerlerator pump puts in more fuel as well as more fuel coming out of the venturis. Since the richer mixture doesnt take as long to burn it does retard it at first.
Some more discussion , we use a old set of Sun Electric spec cards heres the one for a 1968 327 350 hp small block. Sorry guys i will reload a better picture tomorrow so thats the specs are readable ..
OK LETS LOOK AT THE NUMBERS AS THE SPEC CHART SHOWS ... 1= Intial timing = 8 degrees this number usually allows the engine to properly start and idle (with vac line disconnected) 2=Ignition Advance Cent and Vacuum @ 2500 or 5000 engine=39 degrees this is the maximum advance needed or wanted by the engine. Usually the absolute max based on engine design no matter what mods you put in it... 3=Mechanical advance = 15 or 30 degrees engine @ 4700 rpm This purely mechanical advance using weights and springs 4=Vacuum advance = 16 inches of vacuum = 15 degrees engine Now if you look at the 39 is what the engine wants by design there are many ways to get to this number. 1=with vac unhooked rev engine to 4750 and mech advance should be 30 degrees plus the intial on the balancer of 8 making the number 38 degrees ( pretty close to what we actually want... Mechanical advance would be lower at lets say 1400 rpm with a spec of 14 degrees and if the vacuum ws hooked up the vac would give me another 14 degrees making 14 mech plus 14 vac and the intial 8 degrees giving us 36 degrees (again close to what we are wanting. In actually the vac overlaps the mechanical advance as we cant have full manifold vac at 5000 rpm, so we just use a combo of both to keep the engine happy at 30-39 degrees at all speeds etc.... Questions ???
Lol! Head spinning, bubba! Gonna reread and let this soak in for a bit, but yea, it makes sense. Friday, We are gonna check for absolute top dead center, add an appropriate timing tape and double check the current status. A factory rebuilt distributor waits in the wings, but I really want to see where we are first. Bubba, I like the idea of an adjustable vac can and a finely tuned distributor. We will see what happens Friday and then may be giving you a call. Thank you do much to everyone for responding and providing solid details. Stay tuned
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