Monte Cryts
Cadet
- Joined
- May 17, 2003
- Messages
- 16
1991 boat 1992 motor 4.3l dbl barrel carb. Thonderbolt IV Ignition. Swaped short block only(same intake etc except whats new). Engine runs great at Idle,on takeoffand planes out. Like clockwork the engine goes to missing kind of a backfire sound at 3400 RPM's. It cleans up alot but not completely at full throtle around 3900 rpm. Ran 3) 20 gal. tanks of gas after dumping the leftover from last season. <br />This is what all I have done...<br />New distributer cap, New button with metal sensor tabs, new coil, new plugs, new 7mm.wires, new fuel filter at carb and at the fuel pump, new electric fuel pump (old engine had a mechanical fuel pump newer engine didn't), new fuel line, cleaned conections at pickup and at ecu, used a current meter to the ecu ground and the distributer ground to the neg post on batery (all 000.000004 ma.,Completely cleaned the carb, compression checked the engine (160 to 170 checks ok), Checked all conections, timed to 8 deg. btdc @ idle 700-650 rpm (as stated in the SELOC repair manual)I also tried 4,6,10,&12deg.(seems to be worse at 12 deg.not sure) Water temp never gets above 120 deg., oil presure aprox 30 to 50, 19" prop will get 40 mph @ 3900 rpm with slight intermiten backfire. After each testrun the dipstick would be pushed out so I wathed it the last time and it stayed in untill the backfire pused it out. <br /><br />The only thing I can tink of is the ecu is not anvancing properly or the pickup is not working properly. Also engine sounds great untill I barely push the throtle from 3300 to 3400 rpm, then it starts to sound dogish and starts the backfire then right at full throtle it begins to try running right but don't.<br /> How do you test the ecu and pickup? My manual does not cover the thunderbolt IV for some reason, though it states it covers up to 1992. Also what in the world is wrong with this thing???