Hoping someone can give me some advice here. I have read literally hundreds of threads about timing and I am confused. I am getting a little ping at WOT and considered that my stock HEI distributor may be worn and giving way too much advance. I got out the timing light and have 12 deg initial, 36 deg w. vacuum, and 24 deg with vacuum unhooked all by about 2800rpm. I have been reading alot about ported vs. full vacuum and car seems to respond a little better to full manifold vacuum. What I do not understand is why my vacuum advance does not seem to be dropping out as I rev the motor and why so much advance from vacuum. Does this seem normal? From reading threads seems normal would be about 15deg from vacuum and 20deg from mechanical with vacuum dropping out so with 12 initial + 20mech = 32 total? Any help would be appreciated.Craig
The 1st thing to do is unhook the vac advance and don't worry about it til after you get the mechanical advance fixed. When working properly, the mechanical should give between 20 and 24 degrees advance. But there are all sorts of reasons why they sometimes don't. They may just be rusted so that the weights do not swing out. Or the pivot pins could be worn or the holes in the weights wallowed out, or the springs are too strong. Find out why the weights are not moving far enuff to give full advance. Most agree that aftermarket weights are no good. If your weights are bad, try to get some good factory weights. AFTER you get the weights moving freely to total advance, then hook up some springs. DON't use the light springs in the aftermarket kits. They are too light and will not pull the weights back completely. As long as your total is in by 3000-3500, that's OK for street use with pump gas. I'd start with 32 degrees total. See if you have pinging there. 36 degrees may be a little too high for our bad gas. It is said that the dist shaft and bushings can wear out, causing lots of timing fluctuations. So check for side to side excess clearance and slop.Now, if you get the weights advancing fully, then there is a good mod that will help with performance. You need to make a positive stop for the mechanical advance. For example: you can rig the stop for a max of 20 degrees mechanical advance. Then set your initial on 12 degrees. That will give you a total of 32 degrees at full mechanical advance. There are several ways to make this positive stop. Basically, you just screw a self tapping screw into one of the existing holes in the bar that the rotor attaches to. You can grind flats on different sides of the screw head so it will be adjustable, just by turning the screw slightly. Here's a thread discussing this mod, which shows pics of two different ways this stop can be made.http://forums.performanceyears.com/forums/showthread.php?t=755792
hi;ok1. PLEEEASE do NOT read anymore other than right here, we have some of the best bad advice on the net, lol.2 disconnect your vac advance as instructed and do NOT connect it again until we tell you to, lol.3. test drive your car with it disconnected. if it pings at all, you have too much adv too soon and/or your octane is too low and/or your engine is hot. floor the gas pedal hard from a dead stop until around 25 - 30 mph and see if it pings. tell us the exact conditions under which it pings.4. you should be getting around 32 degrees total without your vacuum advance. 24 is NOT enough. you can easily modify your distributor to do this.check your adv at the lowest rpm your eng will run. turn rpm up until your adv just starts to move and tell us what rpm this occurs at. if it is below around 800 rpm it is starting too low. this can be corrected with springs.determine as close as possible what rpm it reaches full advance and tell us.5. loosen distributor slightlyset engine to round 18000 rpm.if eng runs smoothly, turn dist counter clockwise until it just starts to stumble/run slightly erratic and write this number down.retard dist a little and advance it 5 degrees at a time. stop when it no longer makes big increases in engine rpm and write it down.do the same thing at around 2600 rpm.we will subtract 3 from one of these sets of numbers and use that for your "ideal" timing then it is simply a matter of installing the right parts to get it close to these numbers. piece of cake, done it a zillion times.VACUUM ADVANCE - Above around 1/4 throttle, ported vacuum and manifold vac become the same, therefore it cannot really respond better with ported vacuum. Your car is not under load in the driveway, therefore, your engine vacuum is not dropping enough to disable the vacuum advance. Bet ya didn't read that anywhere, lol.15 degrees from the vac can is common but often too much. you may need a $25.00 adjustable can AND a vacuum advance limiter plate for around $6.00.what is your elevation?what is your EXACT dynamic compression or the compression with a good/accurate compression gauge?aluminum or iron cylinder heads?gear ratio?what gas exactly are you using?is your eng high perf?If your weights are sloppy on the pins and/or your shaft is loose, you might consider sending it out for repair or buying an accell or mallory etc.. much easier deal..
a few more paragraphs of advice..you will need a few devices.. a timing light with adjustable advance. they are cheep.. 29 at HF.at least ONE vacuum gauge.. a hand vacuum pump will really help.. a tachometer is handy.. this is the MSD 8428 HEI curve kiti forgot who makes this kit.. notice the shell shaped device.. it goes under the vacuum advance mounting screw.. and limits the full retraction of the vacuum advance.. if you limit the retraction. you reduce total advance movement possible by the vacuum advance.. you will want to read and flip thru the individual images in this POINT TYPE ADVANCE article. http://www.highperformancepontiac.com/tech/1308_how_to_optimize_the_centrifugal_advance_in_pontiacs/with the above stuff.. you should be able to figure out what is happening.. how much mechanical and vacuum advance you have.. hint.. TEE into the vacuum advance supply hose.. lay the gauge under a wiper and take a short test drive.. see what signal the vacuum advance is getting.. i worry that you might have a lean condition.. and its pinging as it is leaning out. do you have an oxygen sensor port in the exhaust system.. or room to have one welded in by a local muffler shop . they keep the weld in versions in stock..you can then even in a temp install with a gauge bracket upside down and a lighter plug to power it.. so you can easily remove it. use an 40 buck air fuel ratio display gauge.. and an oxygen sensor to monitor the air fuel ratio..can you also examine some of the internal components in the HEI that have been known to fail..please examine the sides of your coil for signs of high voltage leaks.. replace it if its leaking.. do you have the 5 and 7 spark plug wires separated so there is no inductive crossfiring happening.. i can explain this if you don't know what i am talking about..
OK I will get all my bad advice here! lol. Everything in distributor seems tight, no excessive play. Everything looks stock, weights/springs have surface rust but seem to snap ok. Bushings do not seem extremely worn but some maybe. Considering just buying a new GM HEI Dist. from Butler.1.- took it for a ride unhooked, did not notice it as bad but slight. Now that I read more I am not sure if it is ping or not. Power flattens out and then stumbles/rattles I back out(really sounds more like out of the exhaust than engine compartment to me. Starts at about 3500rpm under WOT in 1st, 2nd, especially if going uphill. I get a pop pop pop backfire out exhaust under downshift going downhill. This goes away with vacuum hooked up to manifold vacuum.4. Advance at idle set to 12* mechanical starts to come in at 950-1000rpm, revved to over 3300rpm or so I read 24*.5. loosen distributor slightlyaround 18000 rpm.~ 4* ATDC stumbles~ 32* BTDC no rpm gainaround 2600 rpm. ~ 6* ATDC stumbles (kinda guessing here it is off cover)~ 36* BTDC no rpm gainwhat is your elevation? ~800 to 1000ftwhat is your EXACT dynamic compression or the compression with a good/accurate compression gauge? - not certain.aluminum or iron cylinder heads? 6x-8 castgear ratio? - 293 getting ready to swap for 323 - Tremec TKO trannywhat gas exactly are you using? - 93 Octaneis your eng high perf? - It is not stock but not high performance. 73 block 40 over w. 6x-8 heads. Guy said it was supposed to be about 8.8:1cr. I have no clue. I have the cam specs but would have to dig them out. Running a Edlebrock performer intake with Holley 670 street avenger carb - 68 primary jets, 72 secondary jets.Thanks for all the advice guys!Craig
swap out the ignition coil.. you are in the range of the harmonic misfiring issues..