I have not done this for many years, but I don't recall having this problem before. I did the normal pre-adjustment on valves before I installed the engine. After prelube, I brought the engine to TDC and installed the distributor with the rotor to #1. When I cranked it, no start just got a backfire through the carb. So, I rotated the distributor 180 and tried again. Same thing, backfire through the carb.

Just to be sure, I pulled the valve cover and rotated the engine. When the #1 intake valve closed, I rolled on to the timing mark. Pulled the cap and the rotor was at the #1 tower. Tried to fire it up, same old backfire.

Equipment is MSD AL6 and MSD tach drive distributor, Holley 850 carb. What else would cause this? Am I just too nervous about damaging thing? Should I crank right on through the backfire and give it a chance to start?

Any help appreciated.

Dave



[Modified by Rolling Thunder, 9:34 PM 8/11/2001]

 

Just to be sure, the obvious things are ussually the most annoying.

Recheck the plug wires and make sure they are pointing to the correct cylinders.

Make sure none of the valves are being held open. You adjusted them, but I would double check them all now that the engine has turned over a few times.

If you continue to crank it and it still backfires, I hope you have a fire extenquisher lying around in a handy location. When I did mine two years ago and had the distributor off 180 it shot about a 3 foot flame out the carb. :mad

 

Thanks Joe, I just traced all the plug wires and they are correct. I guess I'll yank the valve covers again and reset the valves. I did it by the book (thumb roll plus 3/4 turn), but who knows, may have one intake valve not closing.

I'll check back in a few in case any other suggestions come in. I'm back to the garage.

Dave

 

You might want to re check the rotor position. Fire should be about 8-12 degrees before TDC. I have tried a start with what I thought was TDC-#1 and was actually retarded by one tooth.
bob

 

Bob - Good point. When I installed the distributor, I used TDC, then before I put the cap on I estimated the 8 degrees advance, rotating the base.

So far nothing found way off on the valve settings. :confused:

 

Do you have a timing light? I dont think the engine needs to be running to get a reading on the scale. Have someone crank the engine while you shoot the wheel to see where you are.

 

8 good, 8 to go on the valves. Man that 502 was a bear to get the passenger side valve cover off of. :smash: It sure was a lot easier to work on before I put it in the car.

Yep, got a timing light, although they are a little squirley with the multi spark ignition when you are not running. Besides, about the time it gets ready to flash, I flinch from the backfire. :D BTW, my helper is a remote starter and all you guys!

update

Alright, all valves re-adjusted. Found 2 intake valves just a tad (maybe 1/4 turn) tight. It's time to walk away and look again in the morning. I didn't try to start it, too late to wake the neighbors. ;)
:sleep:


[Modified by Rolling Thunder, 11:15 PM 8/11/2001]

 

Has it cranked enough to fill the carb. fuel bowls yet?

 

Dave

Which cam & lifters are you using? W/ Comp anti-pump up lifters you do not set by the book. I tighten until stops turning & will be on the money. The additional part turn is not needed & will hold valves open.

:cheers:

 

The best fire extinguisher in existance is vacuum. When it pops back thru the carb & ignites gas, you only have a few seconds to put it out before other things start igniting and melting. . Crank that motor over, suck that flame out with vacuum. Then grab the chemical squirter if you need to. This may sound spooky at first but look at it this way: It's already on fire! BTW, use a Halon type extinguisher. That dry chemical powder is corrosive and destructive.
Sorry. Don't have any suggestion to stop your backfire. Timing &/or a lean mixture are causing it.

 

Business got in the way this morning so I'm getting a late start (Pardon the pun).

Flareside - I don't know on the fuel bowls, but she is pumping a good stream.

Ganey - The adjusting method I used was on the assembly instructions that came with the engine from GMPP. The cam and lifters are stock GM for the 502.

Bubba - Got the Halon close by. So far, no flame out the carb.

 

Dave

Sounds good. If flame, you can put out w/ a rag.

:cheers:

 

Still chasing this ghost, the big bang is gone, but still no start. She's definately firing and at least the pop, more of a low rumble is coming from the headers rather than the intake. I'm getting a decent stream in the carb, but I wonder if it's not a fuel problem now. :confused:

 

Okay, I think the timing and fire can be eliminated. I pulled the #1 plug and grounded it. Bumped the remote starter keeping my hand on the #1 intake rocker. When the intake valve closed, I bumped it while watching the balance pulley and the plug. As best I could tell, it's firing at between 12 and 4 degrees BTDC. Looks like a good blue spark, too.

By process of elimination, I have compression, fire and timing. I think I'm okay on the valve adjustments, so all that leaves is fuel. Soooo, with a good stream in the carb, what am I missing?

Dave



[Modified by Rolling Thunder, 10:50 AM 8/12/2001]

 

The squirt is generated by the accerator pump, which is located at the bottom of the front float bowl. If there is any fuel in the bowl at all, the pump will squirt. This doesn't mean that the fuel level is high enough to run correctly. Can you see fuel through the sight hole on the side of the fuel bowl? If not, I would fill the bowls through the sight holes since you don't want to crank a fresh engine too much.

 

Fuel problems don't cause backfires so IMHO your problem still lies with the distributor (more specifiically timing & firing order). I had the same exact problem with my engine upon initial start after a complete rebuild. Ironically, I have an MAD 6AL and Billet Distributor as well. I went through the same exercise as you almost step for step. Valves, timing, fuel, spark, etc until I found my way back to the distributor. At this point I was almost totally frustrated because like you, I thought I had covered all the bases.

I finally broke down, threw out all the conclusion I had come to and went back to square one. I decided to swap the MSD dist for my stock one. Miraculously, it fired right up. My first reaction was, "Damn, how could that be? The MSD must be defective in some way". To verify it was the MSD, I shut the motor down and put the MSD back in. It fired right up. I'm like "What the ..."!!!

Apparently, after all the checks (timing, valve opening/closing, etc. etc.) I must have initally aligned the oil pump shaft pin off just enough that I had seated the distributor with the gear one tooth off. I know what your thinking...how was timing and firing order OK if the dist was off. My answer is hell if know but it just happened and I don't care as long as my engine starts!

Stay with the spark timing angle.

Mark




[Modified by mdsmith, 12:32 PM 8/12/2001]

 

Flareside - Front bowl fuel comes out the sight port. Back bowl does not. Looking for a squirt bottle to fill it with now.

Mark - I don't know if MAD 6AL (as opposed to MSD :lol: ) was a typo or tongue in cheek, but either way I have been suspect of it. I've had the distributor out and back in a dozen times in the past two days. I know I could be off, but if so, why do I get spark on the plug just before TDC? BTW most of the backfire is gone so I have to be close. I'm so desparate now, I'm actually reading the Haines Holley book to learn about this carb. :o I have a feeling a real mechanic could would have this thing running in 5 minutes. :rolleyes:

Dave

 

I just installed a ZZ4 in my 71 with the MSD 6AL box and tach drive distributor without any problems. Here is what I did. Turn engine over to TDC and leave it there. Remove Distributor and sparkplug wires. Stab Distributor and note which post the rotor is pointing to...I don't know what the firing order is on a Big Block (I like the mouse motors) but this will be the starting point for your firing order. Wire your cap in firing order. This is when you can use your remote starter switch to rotate the engine and get your advance set up. You should fire right up. Good luck
John

 

John has a good point and although I haven't used his method, I've heard of other's doing it with good success. As long as you get the rotor in line with one of the posts and the wires strung in the right firing order it will start. The real complication to your problem seems to be the ignition. MSD's are great devices, but nothing beats the simplicity of an HEI or even points when trying to get a car fired up for the first time.

 

Flareside - Both float bowls are now full. John, been there done that. Just did it again. No change in wires, all traced out right.

I did notice something on the carb. There is an open vacuum port (?) on the primary metering block. Should this be connected to manifold vacuum? Right now it is not connected to anything. This is the first carburated engine I've had since 84. :rolleyes: ...and why don't we have a smile with a dunce cap? :o

I'll try cranking it again in a little while. Recharging the battery now. Car hadn't been run since May.

Dave

Steve - I guess I missed your post. I have a brand new HEI distributor still in the box that came with the engine. If something doesn't "click" soon, the MSD is history. :smash:


[Modified by Rolling Thunder, 1:36 PM 8/12/2001]