Hot Rods Blown BBC backfire gremlins

Hi Everybody!! I decided it was time to pick some brains that are working better than mine. I'm working on a 32 3window street rod project that will be powered by a big block Chevy. First I'll describe what I'm working with, then on to my problem. The engine is based on a 454HO GM Crate motor. It is running 8.75-1 compression. Comp Cams beehive valve springs, roller rockers, Mallory mechanical magnetic distributor with 16 degrees mechanical advance, running 16 degrees initial, and 32 degrees total, all in by 2500 RPM's. It has a BDS 8-71 blower, underdriven 6%, for approx 6-7 PSI boost. It's running 2 650 Mighty Demon blower carbs.
Now for my problem. The engine has been running in the ch***is, on jack stands several times for initial setup, testing, and tuning. It has proven to start and run very well. No noticable blower roll. runs clean, exceptional off-idle response, no backfire. While running a couple of days ago I dropped it back to idle from about 2500 RPM and it back fired through the carbs, and the pop-off valve. After this happened I couldnt get it to start and run without backfiring, or firing out the exhaust. I pulled the plugs and ran a compression check. (all cylinders within 2 PSI). I checked the timing and found it was at 2% initial. Removed distributor and inspected all internals, advance plates Cap, rotor, and drive gear. Checked distributor prior to removal and found it locked down solid and still on my reference marks. After putting distrutor back in, and setting timing back to 16%, engine started, and ran perfectly, with no more problems. Tried to get it to repeat the problem, and it won't cooperate. My problem is, this thing is going to make me crazy untill I come up with a reason for the timing change in the first place!!!! ( I also checked TDC to verify my drive hub didn't move) I Know this is really long winded, but any help would be greatly appreciated.

 

My guess is when the distributor was installed and the timing was set the first time, one of the advance weighs was stuck and not allowing the advance plate return to position.I have seen it before when the distributor was in storage laying on its side. One weigh will fall out of position and of course after while stick there. Just a thought. Let us know if you come up with something better.

 

Just a guess as to what the problem is....

 

Good guess Squirrel ... plenty of junk there.

 

What kind of timing light are you using?

I got some strange results from a Sears Penske dial-back.

It showed about 20* off - I forget which direction.

The old light showed 8* initial and 32* all in on my 462" Buick.

The engine runs an MSD 6al and Mallory Unilite.

I understand the MSD will throw some timing lights off.

When I wrote up this info several years back, most guys were reporting their timing lights with dial-back were reading correctly.

Took the light back to Sears....

 

paulweldit; Sorta thought about the advance weight thing, but it's a new distributor and I had just changed the springs and advance profile, and tested for proper advance operation prior to installation. It still could have been hung up, just not sure! C9; The timing light I'm using is an older Sun conventional with no dial. Running a standard mallory coil and no box. (MSD etc.)Good thoughts so far, Thanks.

 

You guys do understand about timing light accuracy? There is not a timimg light made that is dead on. A timing light is responing to what has happened so it can't be dead on. Very good timing lights are good to maybe 1 degree and that is per 1000 RPM. so that means that if the timing is read with the motor at 4000 RPM the light is going to be 4 degrees late. Cheaper and older lights are off even more than that.

 

I might add that the closer the timing light spark plug wire sensor is placed to the Distributor cap the more accurate it is.

 

This may sound odd, but we had the same problem on the engine dyno. We checked everything we could timing, plugs, and even rechecked the cam end play. What we found was the cap, it had a high build up of carbon deposits and it was causing misfire inside the cap. we put a pertronix unit just to check it out and the motor ran fine. After talking to some people mallory distributor caps tend to have this problem.

 

fasttimes; Checked the cap for carbon tracking and buildup, and found nothing.(new cap). I don't think if this was the problem, that taking the timing back to the original setting would have stopped the misfire and backfire. (could be wrong, have been before!) The only thing I have found since my last post was a very loose negative terminal connection on the coil, which could have wiggled around at idle rpms, where the engine is a little lopey and would cause the terminal to move. I'm begining to believe that I had a combination of problems. It sounds reasonable to me that I may have had a weight hung up in the distributor as paulweldit suggested. When the weight broke loose it dropped my timing back to 2% btdc, which caused the engine idle to slow down to where it was idling rough, which in turn caused the electrical connection to vibrate and create a misfire problem. Whatever the problem may have been it appears to have been resolved, as the engine is now running perfect. Does any of this sound reasonable, or is this just a fantasy thing on my part?

 

bttt

 

oldeuce: I'm not sure about your idea, but I do know now why I can't get you on the phone!!

 

Sounds to me like the dist holddown was not quite tight. When you winged it it retarded and at idle it was then retarded. You checked it, it said 2 deg. You pulled it, put it back in re-set it and locked it down. Presto back at 16.

 

lippy; Good thought, but the distributor was locked down solid, and was reference marked on the manifold. It didn't show any movement of any kind. To re-time it back to the original setting required moving it from the original reference "punch marks".

 

Sorry, missed that! then I would go with something in the advance moved like the guys previously said , or was stuck. Something in dist had to have done it.

 

It sounds like you solved your problem with paulweldit help. The loose wire is a big problem, also remember on a big block chevy that the distributor likes to pull back the cam towards the back of the block so make everything is nice and tight.

 

I'm going with the advance sticking and the loose coil connection theory untill
something better pops up. I think I've pretty well eliminated all the other possibilities. At least I'll be able to sleep untill it does it again!! ( These things make me crazy )

 

with the distributor in teh car, take off the cap and grab the rotor and give it a twist, see if it travels beyond the advance. What you are checking for is a busted weld on the advance/rotor mount.

Had one do the same thing, couldnt get it to keep time, sure enough the rotor had a mind of its own.

 

Da Tinman; Tried that. Rotor stops solid at advance weight limit. mechanical advance adjustment screws locked down solid, no visible problems. Thanks for the thoughts.

 

cool, just throwing it out there, pretty sure I'm not the only one blessed with odd mechanical problems.

 

I want to thank everyone who posted ideas to help solve my problem. I'm posting a few pics of my project so you've got an idea what you are helping with. Probably not everyones cup of tea, but "it is what it is".

 

Damn That did really nice, Keep up the good Post and GOOD WORK!!!!

Ol Deuce

 

Thats is my cup of tea. I seen a blower blow apart on youtube. Makes me gun shy every time im near mine now

 

I don't worry too much about a "Mild Street Setup". Blown full compe***ion, high boost, high horsepower, exotic fuel setups, are a whole different story for me. I saw that u-tube also. Does make you think doesn't it?

 

What is the link?>>>>>>>>.Ol Deuce

 

Blower's no big deal. Now, repeat after me....I will not run the engine lean, I will not run the engine lean. There, all fixed.

 

Most of the time a backfire in a boosted motor is an intake or blower base gasket issue. From past experience

 

Here's a link to a blower explosion. Scarry!

http://www.youtube.com/watch?v=HGcIJWnClz4&NR=1

 

Bttt

 

You describe your mallory as being magnetic, do you mean a unilite module? We had issues with that as well, the turbulance or pressure would affect the spark. I wound up sticking an msd 6al up under the dash and the motor just loved it. Cleaned it right up, has never coughed back since. The timing change is a conumdrum for sure, i ***ume you are using the same timing light and procedure? I have never even seen the advance plates in a unilite, they're 'down under', but that seems to be an explanation. It's gonna take some pondering.
I like your blown bbc in a '32. Just my style.