You bumped compression to 10.29:1 and changed cam and used stock carb??? What cam??? Stock carb easily could be leaning out
BTW, that is way too much compression for boat engine. I doubt it'll live 10 hours without a race fuel mixture.

https://www.youtube.com/results?sear...and+backfiring

[QUOTE=Griff;4801204]You bumped compression to 10.29:1 and changed cam and used stock carb??? What cam??? Stock carb easily could be leaning out
BTW, that is way too much compression for boat engine. I doubt it'll live 10 hours without a race fuel mixture.

Stock Quadrajet had 68 jets, I put 70 in before I ever started it. I have a small tub full of hangers and metering rods ready to go. I CANT TUNE IT IF IT DOESNT RUN!! Plugs are black at idle.
I don’t have the cam part number memorized. I’ll get it tomorrow.
What jetting should I run? I have called and asked people for a month now and nobody can give me an answer on jetting. They say run it and check plugs but it won’t rev up.

I have a picture of the plug at backfire condition but this website won’t let me post it until I have 10 posts?!? Dark tan

I'm going w/ broken valve spring

Reg,

The fact that the backfiring starts at secondary opening is very telling. Can you get a look into the carb and see if you are getting any fuel through the secondaries?

Is it a vacuum advance? It could also be timing advance issue related to the vacuum drop when the secondaries open.

Either way, it would appear to be linked to the secondary actuation.

Does it change behavior when not under load?

Thanks. Brad.
Model Machine and Precision LLC
(937)545-8991

Do you have thru hull exhaust? I would have assumed so, but by the looks of your video it looks like it may be through the prop. If so, see if either (or both) of the exhaust shutters are in the top of the Y pipe are missing. Twice I have seen these lodged in the drive and it will give symptoms like you are having. In both cases I saw, the flappers dropped out when the drive was separated from the transom assembly.

I really think you have a valve train issue. To test my theory, advance the timing another 10* and see if it will rev higher without backfiring.

It is not vacuum advance, thunderbolt IV.
camshaft is Comp cam #11-232-3
One exhaust shudder was bent, but I removed both trying to fix the issue. Once resolved I’ll put them back in.
Exhaust through drive, bravo 2.
Backfire condition is almost non existent in neutral but bad under load.
Checked valve clearances once already, nothing was loose so no lost cam lobes, didn’t see any broken springs but wasn’t really looking for that either.

Getting back to checking fuel press, Having a metal line from pump to carb does make it more challenging. There's a couple of different options.
1. Cut out section for a "T" and install a gauge there. This is a PITA since you would need flaring tools and fittings. Very easy to screw things up.
2. Remove fuel line from carb and rig up some hose/ fittings to connect end of fuel line to carb with a "T".
Both of these options will most likely mean disturbing the alignment of the fuel line to the carb. Even a small amount of mis-alignment will make threading the fuel line back into carb inlet a PITA and possibly lead to cross-threading the fuel line fitting.
3. Get another metal line, cut out a large section in the middle and use hose to attach the 2 ends. Install a "T" and gauge and make this your "test line". This would be the best option since your original line would remain un-molested and would be easy to re-install once troubleshooting is done.

As a rudimentary test, you could do what my father (auto mech) showed me when I was a kid. Disconnect fuel line at carb and slip hose over flared end of fuel line ( add hose clamp). Place other end of fuel hose in a 16oz Coke bottle (glass) and have someone start eng. Coke bottle should fill rapidly at idle speed (don't rev eng), like less than ~30 secs. Can't determine fuel press this way but it will be obvious if fuel flow is restricted. You should see strong surges of fuel into the bottle.

In regards to option 2, loosening the fitting at fuel pump may allow the metal fuel line to be rotated slightly to help avoid bending the the fuel line too much.

The cam is very close to a stock 454 mag except it has a 112 lob sep.
Because of the 112 lob sep it will most likely have water reversion issues with the stock thru prop exhaust.