Backfire 454 problem??

[wet77]

Put a new holley marine type 750 carb on the 454 in my friends boat, trying to solve a backfire issue.
It runs great at full throttle and idles ok but at 3000 rpm it backfires??
We are putting on a water fuel seperator today to try to solve this problem :burningm:
It has a Mallory dist. in it with no vacume advance.
Anyone know the what the timing should be set at??
Also on the berk je pump when you twist the shaft back and forth there is some play, does this mean bad u-joints??
Thanks

[roostwear]

A litle background is necessary. Did you replace the carb to try to fix a backfire problem? Do you have a timing light, and if so, what is the total advance now? Have you plugged all the vacuum ports (including the one in the primary metering block)?
If you can twist the shaft and there is some play there's play in the coupling somewhere. If you grab the shaft thru the hand hole and move it up and down, is there any play?

[wet77]

NO play and the carb was replaced to solve the backfire issue
also the boat runs great at 5000 rpm but backfires and fires thru carb!!
HELP!!
timing was at 18 degrees when I checked it as is,hook up light and start motor 18 degrees before tdc

[wet77]

also this distr. has petronix in it.

[Fiat48]

First pull the valve covers and do a quick check for broken valve spring first. Look close...they break sneaky. We go from there.

[wet77]

I was hoping you were not going to say that :frown:
Will do!

[Fiat48]

Good place to start. We are firing with an intake valve open....so a valve spring is a possiblity. Better to catch now than later when the valve and the piston get together for a meeting.

[wet77]

Springs look ok did compresion test and 125psi in each one
BUT after each compresion test with valve covers off you hear a shhhh noise like air escaping somewere in the motor :eek:
HELP :eek:

[victorfb]

you might want to check the timing gear and chain. sounds like it may have jumped a tooth, or if a new install, installed incorrectly.

[Fiat48]

Springs look ok did compresion test and 125psi in each one
BUT after each compresion test with valve covers off you hear a shhhh noise like air escaping somewere in the motor :eek:
HELP :eek:
Now do leak down test..or rig up something to put air in the cylinder and see where it's going.
I know...you was hoping I wouldn't say that. :confused:

[roostwear]

Well, isn't hanging a valve, so I wouldn't screw around with ring/piston diag now. I'd be interested in seeing what your vacuum at idle is, and again what your total (at 3000 rpm) advance is. 125/cylinder would indicate a bit of a cam or valve timing issues. Do you have any cam specs?

[wet77]

Sorry no cam specs and the motor wont even idle anymore, we are starting to lean toward cam because it is just getting worse and worse.
The owner is taking it to a mechanic buddy of his to get his opinion also.