Engine runs rough after 3500 rpm

[Monte Cryts]

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???

 

[horndog]

Re: Engine runs rough after 3500 rpm

hmm... electric fuel pump? whats the pressure on it? should be 3-7 psi. I just think its odd to see an electric fuel pump on a carb motor, but I have seen it on cars..<br /><br />Anyways, if its backfiring then more likely its a carb/timing adjustment. Did you set TDC or was that a factory mark?<br /><br />Also was the carb rebuilt recently? Whch carb is it? Mercarb 2-bbl? I solved my backfiring problem by setting the proper accelerator pump rod length. The pump needs to be somewhat compressed even at idle otherwise it will fall on its face

 

[akriverrat]

Re: Engine runs rough after 3500 rpm

what horndog said. gummed up carb. and...1)did you disable electronic advance to set base timing? 2)put in oil pressure safety switch on electric fuel pump as per abyc and uscg? 3)check fuel pickup tube in tank and function of anti-syphon valve? could be fuel starvation problem. pump pressure should be 3-7 lb at wot. also check for intake vacumn leaks and uncapped vacumn ports. and really off the wall...you didn't do a really neat job of looming plug wires, all neatly lined up and not crossing each other? can cause crossfire from induced voltage.

 

[Bondo]

Re: Engine runs rough after 3500 rpm

Another Thought....<br />What color are the plugs running... Could you be to lean at WOT ??? You May need to richen the high-speed circuit...<br />At 12* timing is Way to high... 7* or 8* Should be about right... Advance Should be all in way before 3000rpms... This can be checked with a timing light...<br />Your WOT Should be up around 4200/4600rpms... You're lugging the motor....

 

[Monte Cryts]

Re: Engine runs rough after 3500 rpm

Thanks for the responding! I'll do some checking and get back to you.<br />(horndog) I used the timing cover from the old motor and the damper. I set the tdc acording to it when I set the distributer in and timed to it. I have since taken No.1 plug out and with a piece of wire found aproximate tdc and the indicators seem to be very close.

 

[Monte Cryts]

Re: Engine runs rough after 3500 rpm

Well here it is!!! Can you believe the choke lever and collar assembly on the carb was put together wrong by the preveous owner not alowing the choke valve to open all the way? What a jdflkanbvnvo! I found it at the parts counter when right before I was about to buy a new carb. I was comparing the carbs and there it was. Thanks to all that tried to help!!! Just one more hard lesson never to be forgotten...