Fresh 4.3 backfiring under load

[Hrobb11507]

1992 4.3 4 barrel bought the boat with busted block and did a fresh build with donor block. First test was yesterday got it cranked fine idles good put it in gear seems good until about 1/4 throttle and it acts like it loads up and eventually backfires through carb. I played around with the timing (didn't have a light) but no improvement. It's a new boat to me but I did notice the tach would get to about 1500 and quit climbing even though the engine was say 2500. I am unfamiliar with the thunderbolt iv and was curious if the tach was a clue that I'm possibly not advancing timing under load? Also even in neutral the throttle doesn't seem to want to rev to 3-4K but I didn't push anything being it was fresh rebuild. I haven't ruled out fuel problem either but the tach deal had me questioning the timing. Thanks any help is appreciated

 

[tlewis1]

Sounds to me like you need to find the manual for your engine and get a timing light. Read the distributor install and set timing sections of the manual so you have a better idea of what you are doing. No sense in just guessing at what the issue is when parts may never have been installed correctly to begin with.

 

[Hrobb11507]

The timing was set initially I believe at 10 degrees btdc when it wouldn't do right I advanced it a little and retarded it a little. I just didn't have a light to know if I was at 6 or 16 either way the boat doesn't like any of the timing settings

 

[alldodge]

fresh build with donor block

If you timing chain was installed correctly, and valves adjusted, then set timing at 8 BTDC
http://boatinfo.no/lib/mercruiser/manuals/mercruiser7.html#/24

 

[Hrobb11507]

It might have been set to 8 I looked it up but don't remember but I know at one point it was on 8 when it didn't act right I marked distributor and advanced and retarded in small increments from base setting with no change. I will check it tonight when I get home but assuming it's dead on 8 what's the first checks before I just start replacing parts? Cap, rotor and wires look good plugs are new could the pickup in distributor cause similar problems? Or should I just confirm base timing and start with fuel system?

 

[thumpar]

Double check your valve lash. I had mine too tight on a 4.3 in a vehicle and it did what you describe.

 

[Bondo]

I haven't ruled out fuel problem either but

Ayuh,.... Welcome Aboard,.... '92,..?? isn't that still the Quadra-jet carb, 'n has it still got the mechanical fuel pump, with the filter inside the housin' of it,..??

If so,.... Atleast plumb in a canister type fuel filter, 'tween the tank, 'n fuel pump,....
It's a much better filter, 'n a Great diagnostic tool,...

 

[harleyman1975]

If plug wires are not new then you have no idea if any good unless you ohm them out. it is possible that two wires are reversed so check the firing order again, could be a bad cap, could be a bad rotor, could be a lean condition on carb, could be water in the fuel.