Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

1991 Larson 20ft ski/fish SEI with a Mercruiser MCM 5.7L V8/350 Thunderbolt IV /Alpha one. Stalls when jumping into full throttle and does not start back up right away. Example, yesterday I tried to pull a skier out of the water, went from idle quickly up to full throttle and she stalled. I tried to restart after it stalled and would not turn over. This time I unpluged the 2 wires from the sensor switch and the wires from a switch that is located on the throttle plate and reconnected them and she started right up. This has been going on for 2 seasons, I have replaced the coil, distributor cap, sensor switch, plugs and wires. Some times I can go to the lake and the boat runs like a champ, it's a strange proplem. Is there a reset or a switch that turns off the engine if its at high RPMS and will not start back up till it is reset????

Re: Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

Is there a reset or a switch that turns off the engine if its at high RPMS and will not start back up till it is reset????

Click to expand...

Ayuh,.... Welcome Aboard,.... The stalling problem, sounds like the Carb...

The not starting,.... Is it turning over,..?? clickin',..?? turnin' over, 'n not startin',..??

Re: Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

Thanks..Its turning over and not starting..I was stuck on the lake last week and was waiting for a tow. While waiting I sprayed out the carb and tried starting fluid, it would turn over but not start. I then made sure all wire connections at the carb and distrubutor were all secure. I disconected each one (thunderbolt IV) and reconnected them, tried again and she started right up. I canceled the tow and boated back to the dock, all was fine. I'm a little scared to take back on the lake and have the same problem, I hate this tunderbolt ignition, I'm not sure how I can test to see if it is the expensive circuit board on the distributor or just a kill switch...

Re: Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

What wires did you disconnect clean and reinstall that let it run, or did you do a bunch of them without testing? Were any of the terminals/connections loose or corroded?'
There are test procedures, but you sure can find anything if it runs..


View attachment TB4 Troubleshooting.pdf

Re: Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

If you go up to full throttle a bit slower, will it do the same thing?

What about if you go quickly to 3/4 throttle?

Re: Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

tpenfield said:

If you go up to full throttle a bit slower, will it do the same thing?

What about if you go quickly to 3/4 throttle?

Click to expand...

Ayuh,.... How 'bout when it's shifted into forward,..??

Thinkin' out loud, a sticky controller, 'n a bad lower shift cable, could possibly be a possibility...

Re: Thunderbolt IV 5.7L V8 Stalls when jumping to full throttle

The thunderbolt IV system is one of the best and most reliable systems on the market. Your problem sounds like it's outside the TB-IV. You need to be more explicit in your fault descriptions. I don't know about the rest of the guys here, but I am having trouble understanding exactly what's going on.

So, a few things you need to verify for us to help you...

1. When it won't restart, are you getting spark? (by the way, throw that can of starting fluid away, unless you really want to kill your engine!)
2. What exactly do you mean buy 'jumping into full throttle'? Are you going from idle in neutral to full throttle, or are you accelerating the boat from idle in forward to full throttle?
3. Be more exact about wires you're removing. Colours and where they run to and from.
4. When does it stall? Does it run up to a certain engine speed and then stall or does it just cough and die?
5. Can you accelerate slowly up to full speed?

As much as we would all love to come and see the problem with our own eyes (and ears) we can only rely on what you put down here. You have to be our eyes and ears and describe everything as fully and accurately as possible if we are to give you the best help....

Chris.........