Cam Timing Questions......

[StealBlueZ]

OK, guys, here's the deal; This year, I've been battling backfire problems and never found any clues as to what the prob could have been. I replaced the complete Mallory 6 system with an MSD6AL, Blaster coil and Pro-Billit, carbs have been rebuilt, once by me, the other time by a "carb guy" hera, fuel pressure is at 5# (at the regulator, 4-port) and steady, and the valve lash and cam timing was checked more than once. Still backfired and caused damage, as discussed in another thread. So the result caused me to pull the motor and look for internal problems. Here's what I found;
NOTHING.
Block is good with no cracks, pistons/rings are good and new, O-rings (on the deck) are intact, no sign of leaks or probs at the manifold/head flange, all the cam lobes are there (no flat lobes), pushrods are straight, and the NEW heads (AFR357) were checked as well as the valves at the machine shop. So I think I covered everything, but I'm just not convinced that I've eliminated the problem.
So here's my next step; I think I'll try a different regulator (or put a gauge on the line instead of the body, as it is now). Now here's a dumb question.... I've never degreed a cam before, so I've never really had the question. I've always just lined up the dots (Pete Jackson gear drive) and all has been well. But on a big cam with lots of overlap, is a valve still open at TDC? This cam is .637 and *280/*290, but not sure of the overlap.
OK, the thought process has begun. I sure would like to find SOMETHING that was the prob before I hit the start button. Anything for me to think about would be appreciated....
Thanks
Randy

[blowngas]

that is not enough cam being either advanced or retarded to get into trouble with----since I haven't read the other threads, I'm gona go out and ask what kinda cam is it?---hydraulic, solid, roller??----just for grins, has anyone checked the valve spring pressures, installed heights, ect??----I have heard motors make a continuous pop, back thru the carbs when a valve spring was weak under WOT----have you replaced the emergency kill switch??---have had muttiple problems with them in race boats!!!----with all different types!!!----just throwing out ideas---guess I better go and read the first thread!!!

[StealBlueZ]

thanks blown... the cam is solid and the valves were checked when I had the heads off. height/pressure is set to Comp Cams specs by my machinest. also, it's not a continuous pop, but a random backfire. keep in mind that I haven't cranked it since this last rebuild and check out. plan on that this weekend to see what happens. Someone else asked about ignition timing. I moved it all around and no matter where it was it backired at random.... Kill switch???? Hmmmm.... I'lll check that out.... thanks for the ideas!
randy

[BUSBY]

I too have not read the other thread ... but you speak of a "pop" through the carb or out the exhaust? Under load or at idle?
If under load & through the carb ... you might have a flat spot on a lobe ...
If at idle or random ... obviously it's firing while a valve is open. If you have set your cam straight up ... the valve should be closed at TDC ... I would look into learning how to degree a cam ... it's really basic. George from Clay Smith even sells a video on how to do it ...
http://www.claysmithcams.com/index.a...ROD&ProdID=178
good luck ...
Brian

[ColeTR2]

Backfire 90% of the time on a blown motor is from fuel delivery. Did you buy your carbs and not have them set-up to be run on a blower? You can’t just bolt Carbs from a Normal Aspirated motor on to a Blower, it will back fire like crazy!!! There are many things that need to be done to the Carbs so they work right with a blower.

[obnoxious001]

"fuel pressure is at 5# (at the regulator, 4-port) and steady"
I think 5 lbs of pressure is marginal at best. Doesn't leave much room for error, and I would suspect you can't watch the fuel gauge constantly while you run hard? I think most guys are going to agree that 7 to 7.5 lbs fuel pressure might be a little better deal.

[sweet-v]

"fuel pressure is at 5# (at the regulator, 4-port) and steady"
I think 5 lbs of pressure is marginal at best. Doesn't leave much room for error, and I would suspect you can't watch the fuel gauge constantly while you run hard? I think most guys are going to agree that 7 to 7.5 lbs fuel pressure might be a little better deal.
I have to agree, sounds like a fuel problem. I always thought marginal pressure was 7 pounds all this time!!
Thanks for the info.

[Infomaniac]

not enough "initial" ignition timing can cause that also