1990 4.3 2bbl losing power on the return trip home

[Badgerfan79]

I have a 20 foot Rinker with the 4.3 2bbl with the Thunderbolt IV ignition. It starts great, idles beautifully and runs like a top after starting. To get to our "spot" it's about a 45 minute cruise, some wide open, some trolling. I'll get there with no issue. We will either beach it or drop anchor and hang out for anywhere from 90 minutes to a few hours. On the trip home, I have power loss above 3000 RPM or so. If I punch it it stutters but if I give it steady throttle I can get up to about 24 mph (typically I can get 35 mph at top end with a few passengers and the towing prop I have on currently) and if I push the throttle further it loses power. It will also stutter if I hit oncoming wake. I have cleaned out all the filters and checked the tank for "gunk" and it was crystal clear and the gas in the bowl looked good. Oil also looks good.

I'm sure this is connected somehow, but the first time this happened was after I got a "high voltage warning" on my fish finder and my volt gauge read 16. I shut down, gave it a minute, switched batteries and on the way home this power loss above 3000 RPM started so I'm assuming it's electrical in nature. I have ordered a new distributor cap, rotor, sensor, and ignition coil. (It was all from Amazon with Free Returns so I can return anything I don't need).

FYI, I have had it out several times since the "high voltage warning" and that has never happened again.

Anything else I should try? Thanks in advance. You guys are awesome.

 

[Rick Stephens]

Personally I'd be inclined to kit the carburetor and make sure there is no crud, the float height is correct, and all passageways run to make sure they are clear. Other possibility is get a pressure/vacuum gauge and make sure you are maintaining 6-7 PSI between fuel pump and carburetor.

 

[Badgerfan79]

Personally I'd be inclined to kit the carburetor and make sure there is no crud, the float height is correct, and all passageways run to make sure they are clear. Other possibility is get a pressure/vacuum gauge and make sure you are maintaining 6-7 PSI between fuel pump and carburetor.

So, everything looked good in the carb and all three filters, so I took off the distributor cap and rotor. Lots of burning and corrosion at the contact points and the sensor module was very corroded. So, I replaced all three components. I took it back out and the problem went away! HOWEVER, changing out all those ignition components must have changed something because now it seems like I lost horsepower. It no longer sputters or misses at higher RPM but it feels like I'm not getting enough throttle. I can only get up to about 3400 RPM and 27 mph when I could get close to 4000 RPM and 35 mph prior (before the "missing" symptoms started, that is). I also have to give it MUCH more gas upon starting. Even when it's warm. I used to only have to put the throttle at about 2/3rds and crank it. Now I have to pump the throttle a few times for a warm start, which is new. I'm going to install new plugs and wires. A neighbor thinks it could be the timing. Does that make sense? By improving the ignition system with new parts (and in the case of the sensor module, a much updated part as the OEM had a completely explosed board while the updated version is encased in plastic and has a separate ground while the old one didn't) is it possible that actually affected the timing in some way? Thanks!

 

[Rick Stephens]

Easy enough to check timing, It is spec'd right on the engine intake if I am not mistaken. However, the Merc #13 manual has it as well. If replacing the module also included the cam follower in the distributor, then yes, changing that will change timing.

Your motor is either always been sickly or you are propped too high. At wide open throttle you should be above 4400 RPM and best near 4800 RPM. When everything in the motor is running right and you only get 4000, your prop is too big in pitch.

RIck

 

[Scott06]

Check timing and firing order

 

[Badgerfan79]

Great, thanks fellas. I'll do that and report back. I did label and take a picture of the spark plug wires before removing anything to ensure that I at least didn't screw up the firing order but I can see that being a common mistake.