427390

I put a 502/502 in a 67 coupe. I swapped out the HEI distrib. that came on the engine for an MSD pro billet tach drive distrib. (w/6AL box). I am running the 25 degree advance stop bushing, 10 degrees initial and one blue and one silver spring (silver is the lightest, blue is second lightest). This set up is supposed to produce 35 degrees of total timing, all in by about 2,500 - 2,600 rpm - how do I know if the MSD vacuum cannister is correct for my set up?

The limited paperwork that came fom GM with the engine said to set it up with 10 degrees initial and 32 degrees total (that seemed a little conservative to me).

The engine has the 850 Holley (which came with the engine) and I am using the timed port on the metering block - ANY TUNING TIPS ARE APPRECIATED -

I am having a hard time getting this to idle and run nicely - its popping through the carb (with a flame here and there)...Thanks - Joe

midyear

in my experiances the 502/502 engines run great timed around how you have it, silly question but did you check to make sure firing order is correct?, its mighty easy to swap 5 and 7 and have a rough idle and pop thru the carb.

427390

midyear - great call - initially i had the #7 and #2 backwards (don't ask) - reversed them and things are better, but still not perfect - looking for any other ideas - while I was waiting for Forum responses I looked at some other sites on the Internet and found GM specs which say 8 degress initial (800 RPM) and 36 degress total (5,000 RPM) - these are different than the sheet that came with the motor, but I don't think the difference would be all that great..........ANY OTHER IDEAS? what causes the F I R E through the carb? too advanced or too retarded? too rich or too lean? I know I am close, but it kills me to run it at all like this, so when I go back at it on Friday I want to be able to clear it up quickly - Joe

midyear

pop thru the carb generaly means lean mixture or retarded timing. another thought, a fuel fouled plug can cause the same symptoms, ac delco plugs are known easy foulers and often never clean themselves back up.

SWCDuke

What are the specs? My advice on vacuum can selection is the same today as it has always been. You should select a vacuum can that provides full vacuum advance at least 2" below the normal idle vacuum.

As far as how much total vacuum advance you should have, I'm not sure as I do not know 502s that well, but I understand they are similar to open chamber 427s, and the recommended initial and centrifugal specs you've quoted appear pretty close to a SB, in which case total vacuum advance should be in the range of 12-16 crank degrees, and if the engine idles at 14" it should be all in at 10" or less, and, of course, the vacuum can should be driven off full manifold vacuum so full vacuum advance is "locked in" at idle. If the MSD dist. uses off-the-shelf single point or HEI vacuum cans you should have no trouble finding one that is well matched to the engine. E-mail me if you want Lars vacuum can list and specs.

If the engine is backfiring at steady speed and load it is probably lean, and it will probably be surging, and richer main jets should be the fix. If it's backfiring when you increase throttle, the pump shot is probably insufficient.

If the idle mixture is okay ( you can adjust it for max idle speed/vacuum before running out of adjustment sensitivity.) and it's backfiring when you blip the throttle it probably needs more pump shot, but the first thing to do is come up with what seems to be a reasonalble ignition advance map and then work any fuel mixture issues.

I recently when through this exercise after I altered the cam indexing on my Cosworth Vega to take out most of the overlap. Since it has an adjustable MAP sensor and a potentiometer to set the idle/off idle mixture it was just a matter of turning screws. The OEM HEI did not have a vacuum can. The OEM recommended idle speed was 1600, and it pulled 16". The high idle speed was required to maintain a stable manifold vacuum to keep the EFI from hunting and screwing up the emissions. Years ago I added a vacuum can (max advance 10 deg @ 8") and it would idle okay at 1200/12" with about 25 degrees total idle timing.

Reindexing the cams for much less overlap allows a butter smooth idle at 900/18", but it was so lean it would barely start. (The higher vacuum read by the MAP sensor leaned it out at idle and off idle.) After some experimentation with different MAP/idle pot combinations, I ended up with the MAP sensor at the OEM setting, but the idle pot is at full rich, versus the OEM position of about halfway between full rich and full lean. If I lean out the idle pot it will backfire when I blip the throttle! I kept the same vacuum can. The engine runs so cool now I could probaby drive it around without the fan and just let the Redondo Beach sea breeze keep it cool at idle and low speed driving!

My current set up is 14 degrees initial, centrifugal starts at 1000 and is all in (18 degrees) at 2000, so total WOT timing is 32 degrees from 2000-up. The vacuum can actually measures 11 degrees total advance, so my total idle timing is 25 degrees at 900/18". The idle is very smooth and stable, and I can drag it down with the brakes to 200 revs in first gear, then floor the throttle and it pulls to 7000 plus without missing a beat. That's flexibility, and it makes more torque all the way across the operating range.

I went through this story just to give you an illustrative example of how to approach dialing in your fuel and ignition timing map. The first thing to do is come up with a reasonable ignition advance map, and then work out any fuel mixture issues.

Duke




[Modified by SWCDuke, 4:49 PM 5/21/2003]

richscorvettes

Hope you have been able to make further progress based on the great input you have received already. As has been suggested I'd pull the plugs to check their condition particularly on the two that were out of sequence.

From my experience with the 502 the 850 is usually jetted on the rich side but that could have changed since I got mine. My timing is set at 8 degrees initial and 36 total coming in by 2,500 RPM. The curb idle is set at 800 RPM. These things can "diesel" if the idle is set too high and can also backfire through the carb when shutting it down.

I am also using the MSD billet dist. and 6A box. One thing that often goes unnoticed on these units is the need to use a "mellonized" dist. gear. The HEI unit that came with your engine has one and it's needed to avoid premature wear of the gear because of the roller cam. That wouldn't have anything to do with your back-firing problem but I wanted to make sure you knew of this to prevent future problems. I had never heard of this until I was reading some literature on the engine and realized that the MSD ignition system didn't have it. I called MSD and they didn't offer the mellonized gear but were aware of it. They may offer one now but you could do what we did and take the HEI gear and open it up to the .500 shaft size on the MSD unit. The stock HEI is .492 so it doesn't take much to get it to the correct size.

Hope you find the source of the problem. Good luck.

Rich Lagasse

cbernhardt

I don't think you mentioned it, but did you run the engine with the HEI before you swapped? If so, were you having the same problem?

427390

First - Duke, thank you for the in-depth explanation - I will read this several times and as you recommend, address the overall timing map first, then carb issues, including the "shooters" - I already have Lars' vacuum canister listing (I was just hoping I never had to use it - one of those "black magic" things that you kinda don't understand until you are forced to)

Midyear - I will pull all the plugs and have a look see - you are right, maybe the #2 and #7 are way fouled as a result of my initial mistake - good call
(and very basic)

Rich - you are on the $$ - the thing does diesel on shut down - how did you stop this? With respect to the distributor gear, I called MSD and they told me all of the gears on their pro-billet models are hardened - I tend to believe them... should I not?

cbernhardt - I never ran it with the HEI - it looks like a great distributor (probably made by MSD) - its billet, with built in coil, two wire setup - real nice, but not tach drive (and the large cap could interfere with the firewall) - anyone want to make an offer?

The motor looks sweet sitting in the engine bay all trimmed out - just wanna get it running right so I can finally realize the benefits of a lot of work - I will post a picture of it later on tonight.

Thanks for all the input thus far - any other suggestions are, as always, appreciated - Joe

427390

Here's a picture of the engine installed at initial start up....

Denney

Joe, pardon me for asking but, did you install the dist with the timing mark lined up on TDC with the firing location lined up to #1? In other words, have you ruled out being off one tooth on the dist timing? :confused:

427390

When the engine was brand new, I pulled the HEI distributor out -- the timing mark (scribed in the balancer) was lined up with the mark on the timing chain cover. The position of the rotor was nearly straight ahead (towards the front of car) and the vacuum can was near the passenger valve cover (as you can see from the photo). I noted the position of the #1 plug wire (where it is in the photo above is the same place GM had it when it was shipped).

While the HEI was out, and before the MSD went in, the engine got bumped slightly. I moved the balancer back to what was the TDC position (mark on balancer lined up with mark on timing chain cover). I then dropped the new distributor in place to get the rotor and vacuum can positions correct (as they were). I then put on the plug wires. The first time I tried to fire it was N/G. We moved the plug wires one position counter clockwise (which as we looked at it seemed more like the position on the HEI when shipped - a little hard to tell because of the different sized distr. caps) and it fired pretty quickly. However, in haste while moving the wires, we reversed #7 and #2 - ran lousy when you tried to rev it - but it ran ok at 2,200 rpm, so we let the break in proceed.

Finally saw the #2 and #7 switched and fixed that which helped a lot, but while it was running poor I set the timing three or four times, messed with idle screws....you know the drill... i just don't want to run a brand new motor that much longer like this and of course it can't be taken on the street....


[Modified by 427390, 10:19 PM 5/21/2003]

SWCDuke

It didn't occur to me that there might be an issue with wire and rotor indexing, but a problem here that throws the rotor tip out of alignment with with the cap terminals could be a cause of backfiring due to a weak spark.

Doesn't the MSD come with an instruction sheet that tells you how to index the wires and install the distributor? Dont' assume it should look just like the HEI. If you don't have some instructions I think the vendor owes you an explanation and some consideration.

Duke

6T7L71CPE

427/390, Engine Looks Great!
You may have changed it since the picture, but as Duke pointed out,
"and, of course, the vacuum can should be driven off full manifold vacuum so full vacuum advance is "locked in" at idle", yours appears to be ported. What have you got comming off the manifold port? Seems like a short line, maybe plugged. Are you maybe getting a vacuum leak on other carb fittings and can't hear it over loud pipes? Been There :yesnod:

Allcoupedup

I saw a tech tip on a TV show that mentioned this problem. To solve it they driled a dome size hole in the top of the distributor cap so that a timing light could be used to verify the index.

Someone check my logic here but I assume you would drill a hole near the terminal of the wire the timing light uses as a trigger. If everything is set up correctly the metal part of th erotor should point to the terminal.

The show mentioned that indexing often causes weird symptoms and is overlooked as the usual culprits (timing, carburetion, etc..) are checked first.

I've never done this - use this advice at your own risk!
Brian

MassVette

Does sound like Indexing issue................
Beware of backfires- your power valve is probably junk now.
Also, make sure you have a hardened distributor gear on that MSD.
GM shipped me an engine with a NON-hardened gear, and the steel roller cam chewed the distributor gear up on the interstate, causing a massive engine fire! Thank God for fire extinguishers.
My 502/502 had similar tuning problems, and I wound up with a silly 2x4 setup- but it works REALLY GREAT! It even gets decent mileage (18-19 hwy).


Good Luck! It's a GREAT engine- TONS of torque and H.P.- You're gonna LOVE it!
:yesnod: :chevy :chevy :yesnod:

427390

I am taking off work tomorrow to dig in on this:

1) I will double check distributor installation/rotor phasing (I have Lars' instructions - anyone have additional "sure fire" distrib. install advice?) - which mark on the timing chain cover is 8 degrees BTDC? the one immediately to the left of the TDC line when looking at the engine from the front? (its not marked)

2) Duke - do you mean that I should run the engine at all times with the vacuum line hooked up to full vacuum? or just when checking the timing? Holley says to use the ported vacuum source, but I've heard conflicting stories here. The other vacuum lines coming from the carb - there are two, one "regular" sized, the other small -- were connected with a T supplied with the carb and capped - I assumed this was for easy vacuum gauge attachment.

3) I will call MSD again today and ask about the vacuum can specs AND make sure that the gear is hardened (Mass Vette - sorry to hear your story - all looks well there now)

4) I hope the power valve isn't blown - I don't think I am getting the super rich symptom that I think you get when its blown - also, the Holley propaganda says their power valves don't blow anymore...

Thanks for all the thoughtful input to date - it is causing me to re-consider everything....Joe

427390

The following is from the GM Service Parts Operations bulletin that came with the engine (and assumes that the user has the HEI that came with the engine installed):

Set initial timing at 10 degrees before top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32 degrees of total advance at wide-open throttle (WOT). The HEI vacuum advance canister should be connected to a ported vacuum source (no vacuum at idle). The HEI distributor supplied with the 502/502 engine has mechanical cetrifugal and vacuum spark advance with the following curves:

Mecanical advance:
RPM Advance
1100 0.0
1600 12.0
2400 16.0
4600 22.0


Vacuum advance:
Vacuum (in Hg) Advance
3.00" 0.0
7.5" 20.0

Remember the above assumes that you are running the HEI, I am using a MSD pro billet tach distrib. When I call MSD today, I will ask them what the specs are on their canister. I also thought these settings looked a little "lazy" thats why I went to 10 degress initial and 35 total...As I posted yesterday, I also found GM specs which say 8 degrees BTDC initial (800 rpm) and 36 degrees total (5,000 rpm)......

Subfixer

Duke has posted several explanations of vacuum advance, ported vs manifold vacuum. You should be able to search the archives for those. Ported vacuum was introduced by GM to take care of emissions issues. I think you should be running manifold vacuum to the can for your application. Time the engine with the vacuum line disconnected and plugged. Once initial timing is complete, then hook up the vacuum line to a manifold vacuum source (at the base of the carb, drawing vacuum from beneath the butterflies). I believe the timing marks for BTDC should be to the left of TDC when viewing the engine from the front, if you are standing on the driver's side of the car, those marks will be away from you (towards the water pump or center of the engine). I think they are marked in 2 degree increments, but I could be wrong.


[Modified by Subfixer, 10:48 AM 5/22/2003]

SWCDuke

Yes. From the other posts is sound like GM set up this engine with ported vacum advance. I have no idea why unless it is some feeble attempt to reduce emissions. I recommend you reconfigure the vacuum advance to provide vacuum advance all the time including idle.

GM's initial and centrifugal curve are probably conservative to keep the engine out of detonation. As with any engine finding the optimum total WOT timing is a matter of playing with different initial settings and springs to get the right total and get it in as quickly as possible without detonation. The HEI vacuum can probably has too much advance, but it comes in quickly.

Can I assume that MSD in their infinite wisdom actually provided you with the centrifugal and vacuum specs for you distributor? If not, find out. It would help if we knew what they are. Also, can off-the-shelf vacuum cans be installed in the MSD?

Initial timing should be set with the vacuum can disconnected. Then with the vacuum can connected you check total idle timing and it should be in the range of 25-30 degrees - the sum of initial and full vacuum advance.

It would also help to know your current idle speed and vacuum.

Duke

richscorvettes

"Rich - you are on the $$ - the thing does diesel on shut down - how did you stop this? With respect to the distributor gear, I called MSD and they told me all of the gears on their pro-billet models are hardened - I tend to believe them... should I not?"

Joe,

MSD may have changed the gears they use since I bought mine. I don't know if there is a difference between "hardened" vs. "mellonized" but you do want to make sure that they confirm that it will work with a steel roller cam. I know that Jegs now has a gear they say will work with a roller cam that fits the MSD size shaft.

On the dieseling all it took was lowering the curb idle to specs and setting the initial advance to 8 degrees.

Good luck in tracking down the source of the problem.

Rich Lagasse