hi all, i know this is off topic as it's a 402 chevy big block from my buddies 69 chevelle.
first start a few weeks ago. today we timed her and played with the carb. 770 holley street avenger, MSD ignition
we hooked the vacuum to ported and tried everything between 6 and 12 degrees, but it would backfire (even with flame) through the carb when you open the throttle blades very quickly. it's classic sign of wrong ignition timing. So we tried and tried and it didn't happen with 30 degrees initial!!! he took it up and down the road in that setting and i could hear him shifting through the gears like there was no tomorrow. The car runs better than ever before.
So we checked the timing marks with a piston stop. i am quite sure it's #1, chevy has it in the front of driver side (not passenger side like ford) i believe. the marks where bang on. in our test the piston stopped at 10 past and 10 before. so the mark must be right and #1 cylinder as well

i really can't understand what's going on here ...

 

Could is be that something is funky with the timing chain from the crank to the cam?

 

Other possibilities:
sticky intake valve
incorrect firing order
bad wires cross-firing
Carbon tracked cap cross-firing
Bad cap (HEI's like to burn holes under the coil)

 

Could also be too lean of a pump shot. When the accelerator pump is too lean you can get carb backfires when you whack the throttle open.

 

um...don't chevy's use manifold vacuum?

 

That was the FIRST thing that came to my mind also.

I have always liked thos 402s I think they were only made 2 years, 70 and 71,, maybe 72 also.... (396 .030 over from factory!)

I had one of these from a 70 Camaro in my 39... I still have the engine. I was going to put it in a 62 Chevy truck but I sold the truck on Friday... I bought the Camaro in the early mid eighties for $600 just to strip the engine and tranny and then sold it for $500... Damn I should have kept that car...

Before we started working on my sons 66 Mustang, I did not give the Chevy in a Ford thing a second thought, after a while here in the classic section, that just does not work for me any more.

Also, I have come to BELIEVE that the 289/302/351W is a marginally better platform for performance than the SBC with all else being equal AND aftermarket heads are used. Not so much when we are talking big blocks though. (i.e. FE and or 385 series vs. BBC.)

Ramble on dude!!

 

all in all not good news. I hope it's somethig as simple as crossfiring or crappy cap.
I'll check on that. he had the engine out and the timing chain looked good when we had the cover off, so that's rather not like it. But I have not the foggiest idea if the cam is timed properly. I mean if it was installed correctly. he never changed anything on that matters after he bought it. the firing order itself is good but I'll go through it again.

 

Well its apple to oranges, but i am having the same damn problem at work

except rather than a big block 402, it is a 49cc Moped some one brought in to be worked on, i spent the good part of yesterday trying every dog gone timing setting i can imagine. and still it back fires or bogs down right at abotu 1/4 throttle or if you punch it.

i'll let you know if i find a solution to that, maybe we're dealing with the sameproblem

 

Remember, chevy cylinders are not numbered in a straight line. Even numbers on the passenger side, odd on the driver side. And the distributor goes clockwise.



8 7
6 5
4 3
2 1

1,8,4,3,6,5,7,2

BTW, if you were to start the order with #2, the firing order is the same as the early SBF... go figure.

 

yah. i know of the chevy order and distributor. but the cables are all correct. i checked every cable. pulled one cabl;e after another and everytime the rpms went down with that cable missing. I was thinking maybe the mechanical advance is dead
that would at least explain a lot of power going down the road when set to 30 ... as well as the misfire when revving .. we'll check tomorrow on all what was said (at least the ignition side of things)