Engine Backfire and poor performance

gearshift46

I recently repowered (Nov 2016) my 1992 Rinker Fiesta V 260 with a 5.8 325 HP Bobcat (drop in)  Mercruiser. (all new) During 2017 season I had no issues. This season however, since August of this year I have experienced backfiring through the carb and poor performance. This engine has an ECM and when the "poor performance" is present it seems the engine is in "Limp mode". When this happens, WOT is approx 5-10 MPH and won't come up on plane. When this occurs, I can normally correct the situation by backing off the throttle all the way and slowly kick up the throttle  (real slow).....at the same time putting the trim tabs down until I come up on plane. Once I'm up on plane, the boat will do 4000-4200 RPM's @ 32 MPH. It should be noted that most backfiring occurs after a long idle (leaving a harbor) or a period of trolling at 1000 RPM's.

Any feedback is much appreciated.

Black_Diamond

You should be able to pull the fault codes from the ECM. 

gearshift

Thanks for that suggestion.

212rowboat

this sounds like a TPS (throttle position sensor) issue, but we're talking about a carb'd motor, correct? 

being 'new' folding into the equation..... and 'good performance' prior also involved... 

..... get a timing light on that thing.  make certain and connect the stunts that lay vacant along your carb on starboard side to force the thunderbolt ignition into base timing mode.  it is supposed to be set to 10* advanced, but you can likely squeeze it to 12* for better performance- you do this by loosening the hold down and twisting the dizzy clockwise for advance, counter for retarding... 10* advance on base added to the thunderbolt advance of 24* gives you a total advance of 34* and where you safely want to reside... the reason i suggest this is timing chains on new engines like to stretch and wear, and what was good timing when the nylon sleeves that keep the chain quite allow for as much as a few degrees 'out' when they wear off...  

if that isn't the issue, then it's time to look at crank position sensors next. 

gearshift46

Thanks for your posting. As the season is winding down, I doubt I'll get to this problem this year. I was hoping it might be something simple like a clogged fuel filter. When this issue happens, I can usually get her running great by throttling back and advancing the throttle very slowly......(I mean slow) until the boat planes up and I'm doing 32 MPH at 4200 RPM's I can go miles once she planes out and she runs great.  ????????

gearshift46

Oh yes, it is a carb engine

212rowboat

checking and setting timing is literally a 5 minute job... 

gearshift46

Boats coming out of the water in a few days. My mechanic is going to check it out.