I/O backfires after trolling?

[outdoor ran]

After trolling at a speed between 2 to 3mph for several hours. When its time to cranker her up and head home I get stalling and backfiring for several min. If I run her at 8 -10mph for several min it clears up and I can get up and go. So it most be loading up.

I over heated my coil a couple weeks ago could this have anything to do with it? Bad plugs? Any advice would be great. Thanks

Almost forgot its a I/O 140hp merc crusier 4 cylinder.

[Capt. Don*]

You are having problems with misfires under load. Most likely is due to wrong carb air/fuel mix and cold water with a possible t-stat calibration issue. One thing I have done for this problem in the past is go to one range hotter spark plug and adjust carb. be sure to check and make sure plug wires are seated well in cap and on plugs and that there are no burn marks that would indicate arcing at the contacts. Years ago I ran a v-8 Chevolet motors w/sterndrives and had the same problem, I ended up using Champion Truck Plugs for similar year Chev truck and it solved it for good, I can't explain it and would never of put champions in anything but lawnmowers or Harleys, but a Napa rep gave me them to try and after several sets of A/C plugs the Champions worked best.

[Surface Tension]

Randy I have the same engine in my Browning.

What Capt Don suggests is right on track. I'd suspect your loading up. Whats your engine temp while trolling? Might be time for a new T-stat. I'd do what he suggested by changing the plugs and checking the wires then onto adjusting the air/fuel mix. Do you get any stumbling or hesitation when you give it throttle or is it only after trolling?

[outdoor ran]

Once in a great while it stumbles when I take off from the dock. But lately just after I've been trolling. I'll check the wires and plugs. Whats a normal engine temp while trolling? Start simple I guess. Thanks for the tips guys.

[Surface Tension]

I think the T-stat is 140 and thats what it should run at at all the time.

[Wishin2BFishin]

I have the same engine setup in my 1978 Penn Yan. Not sure if your's is freshwater cooled but mine is and it runs around 160-180 degrees. If your's is raw water cooled then it should be around 140. Mine also has the old points & condenser dizzy. Does yours? You say you overheated the coil? Have you replaced it? If not this could be your issue. After trolling for a bit, stop the engine and pull out one of the spark plugs. If it's black and carboned up then your running rich, if it's clean then it could be running too lean.

Best of luck finding the problem...

[outdoor ran]

I checked all the plug wires and put in a new coil for starters. Now i just need a break in the weather to get out and do some trolling and see if its better.