I tried breaking in my 502 this weekend and it was a disaster. It started right up and I brought it up to 2500 rpm's until I discovered a trans line leak. I shut it off,fixed the leak, started it back up and it started backfiring so I stopped. I then went over the timing and it seemed ok but I followed instructions and set the balancer at 8 degrees BTDC on the compression stroke and set the distributor so # plug wire was lined up with rotor and pointing at # 1 cylinder. It was pretty close already but I just double checked for piece of mind. This did not help but I then discovered I only had 2lbs of fuel pressure. I adjusted the pressure up to 8lbs and this didn't help. My plugs are black and it backfires everytime I step on the gas. Anyone got an opinion or suggestions ? It's a 502/502 w/850 vacuum secondary.

Does it idle fine? Does it do it if you open the throttle slowly, or only when you jerk it open?

I would go through and make sure all of your plug wires are on tight, and in the correct order. A crossed wire can do this.

When you brought it up to 2500 initially was it running good? Or was it still backfiring, or running rough?

Its possible you could have done something to the cam. If everything else checks out okay, ignition etc, pull the valve covers and run it w/o them to see if all of the rocker arms appear to be moving right. Also make sure the valves are adjusted right (unless it still has the netlash setup).

Wes.

Is this a hyd/roller cam? These roller cams are more forgiving. also , I think 8psi on the fuel press. is a tad high for street use. Let us know how you make out with it. good luck.

[This message has been edited by Joe454 (edited 09-12-99).]

When you took the spark plugs out and put them back in, you may have gotten some antiseize on the insulator. This will short them out. In any case, I also fouled my AC Rapidfire plugs in the first 200 miles of my 502/502. As a result, it backfired during acceleration. I put a new set of spark plugs in it to solve the problem. Of course, you should double check the basics as Wes mentioned. Good luck.

Tom

You don't want to here this, but I had the same problem shortly after first firing up a high quality rebuild. It turned out that one of the lobes on the new cam didn't break in right, (for the #1 exhaust valve) and was grinding itself off with the lifter! The exhaust from that cylinder had nowhere to go but backwards through the carb. I'd heard of this happening, but several engine builders told me this is very rare these days. Turning the engine over by hand, check the lift at all of the rocker arms. At least if this is the problem, the crate motors have a nice waranty.

I'm gonna put new plugs in it and recheck the ignition wiring and timing one more time. Actually the first time I tried to start it it backfired and would not start. I loosened the distributor and had someone adjust it while I turned it over. It then started and I brought it up to 2500 with no backfires until I had to shut it off. When I tried again and started having trouble and checked the distributor, It was almost dead on. The plugs are black and dry ? No real smoke out of the exaust. Nothing abnormal anyway. I'm really afraid the cam is gone. I had good oil pressure the whole time.

If this 502 has a roller cam, I don't see how this could be the problem. Roller cams and lifters don't need to wear to each other like a flat tappet does.

I would check the basics like spark plugs and timing. Any chance your coil or dist could have went south, maybe it's not getting full spark to the cylinders.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
www.athenet.net/~chevelle (http://www.athenet.net/~chevelle)

One thing that I'm hoping may be the problem is the plug wires. I purchased the engine with an MSD distributor that has no vacuum canister. The GM plug wires did not fit the cap so I purchased MSD wires. Is it possible that these wires do not fit the plugs properly? Could the terminals be to large to fit snug on the end of the plugs. The boots are so thick I can't feel the wires snap on. It almost feels like the plug ends are just sitting inside the terminals but not making secure connection. Is it possible these wires are the wrong size for the plugs ? Do they make different sizes ?

Ray,
I dont think chevy makes different diameter spark plug connectors,only they might be different length boots.If your car has been sitting for a long time,check to make sure the exhaust is not plugged with a mouse nest.
Buddy of mine pulled his hair out for a whole weekend trying to figure out his similar problem.He also removed timing chain cover to recheck the cam timing.If exhaust is plugged you might hear a funny wizzing sound at tail pipe. Good Luck Tony

Ray,

I had an experience with my MSD plug wires that the spark plug connectors were not seated in the spark plug boot all the way. When I was installing the plug wires, I expected to hear a click when the connector actually grabs the spark plug end. No click was heard so I took a look and the connector was not seated properly in the spark plug boot. I merely shove the wire more into the plug and then I can see that the connector seated correctly. I found more then one wire in this condition. After that I reinstalled the plug wires and then heard the famous click. You may want to check this if you did not hear that famous click. Even though the MSD plug wire boots are thick, you should still be able to hear the click or at least feel it.


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William Roche
San Diego, Ca
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67 Malibu
72 Malibu
ACES 130, Gold 26

If the boot will slide on the wire, what I do is push the connetor to the mouth of the boot, then ensure a nice solid fit onto the plug, then slid the boot over the plug.

You said you rechecked TDC. Are you positive you are not 180 out???

Good Luck,

Bill

Ray, I know this is after the fact, but I like to start new motors with the trans convertor unbolted to the flexplate so i don't have to deal with something going wrong with the trans. Sorry I can't help with your engine problem.
Philip

Sorry I haven't responded to all the advise but I've been working 16hrs a day and haven't been back to the car until today ( Not to mention this is making me sick to my stomach). A 70 SS flew by me on the way home Thursday night and I almost started to cry ! Anyway, I'll be the first to admit I'm no mechanical expert and I've made some pretty stupid mistakes so I have a few more questions to ask.
1.) There is a vacuum port on the carb (Holley 850 vacuum secondary w/electric choke). It is on the passenger side and I think it's on the fuel block. This port is not connected to anything and is not plugged. Could this be the problem ?
2.) I went to replace my plugs and using a 1995 GM performance parts catalog for reference asked for MR43T's. The parts guy looked kinda confused and brought what looked to be lawn mower plugs back to the counter. Does anyone know the correct plug for this motor ?
Thanks, for all the help guys ! I'm hoping this will turn out to be something simple and you'll all consider me to be a big idiot with a nice car ! Thanks again.

Plug the vacuum port! The lower port is full vacuum, while the upper one is ported. One of the ports should be connected to the distributor. My 502 idles a little better with full vacuum to the distributor advance. The correct spark plugs are AC RapidFire #4, GM part number 25164642.

Tom

My distributor has no vacuum advance. It is an MSD Billet dist. with a 6AL box and Blaster coil. The only things I have hooked up to vacuum ports are the brakes (ported), T350 trans and something else (manifold). Not really sure what the last thing is. I can't believe I put on the air cleaner and ignored this big open vacuum port on the carb.I'm gonna plug this port, put in fresh plugs, line up the timing marks one more time and see what happens. I'll let you know.
Thanks, Ray

Your power brakes really should get full manifold vacuum, not ported.