Boat bogs down after 30-90 minutes

[Why did you buy that Boat]

1990 25' sylvan cabin cruiser stern drive, Alpha I 350, 260hp. Heard boat run before purchasing, sounded good, Aug. 2022. First time out it overheated quickly under full throttle and smoked like it was on fire.

Since then........new, fuel filter, water seperator, thermostat, coil, distributer cap and ignition module, rotor, impeller, shutter valves, spark plugs, spark plug wires, checked cylinder compression(wet and dry), gas pressure at carb(4bl quadrajet), emptied gas tank twice, very clean, new gas.

The shutter valve replacement in the exhaust manifolds seemed to take care of the overheating, so now in will open up and come up on plane and run well for 1-2 hours and then it bogs down to less than half power then eventually died if you back completely off the throttle and won't start until it cools down. It will idle good setting in my driveway with water hooked to it.

Is there some type of module getting hot???? Any thoughts are welcome!

 

[Why did you buy that Boat]

Sorry, just saw that engine questions aren't allowed.

 

[briangcc]

Just ask a mod to move it to the appropriate section.

In regards to your issue, if I didn't know better I'd say you have some sort of fuel issue. In Hotrodding, I'd call it heat soak (of the starter) when you can't restart it until it cools down.

IF this is an HEI distributor, you might look at the module under the rotor for replacement as they are a normal replacement item. Use the thermal grease when reinstalling.

 

[Why did you buy that Boat]

1990 25' sylvan cabin cruiser stern drive, Alpha I 350, 260hp. Heard boat run before purchasing, sounded good, Aug. 2022. First time out it overheated quickly under full throttle and smoked like it was on fire.

Since then........new, fuel filter, water seperator, thermostat, coil, distributer cap and ignition module, rotor, impeller, shutter valves, spark plugs, spark plug wires, checked cylinder compression(wet and dry), gas pressure at carb(4bl quadrajet), emptied gas tank twice, very clean, new gas.

The shutter valve replacement in the exhaust manifolds seemed to take care of the overheating, so now in will open up and come up on plane and run well for 1-2 hours and then it bogs down to less than half power then eventually died if you back completely off the throttle and won't start until it cools down. It will idle good setting in my driveway with water hooked to it.

Is there some type of module getting hot???? Any thoughts are welcome!

 

[jbcurt00]

Merged

 

[Why did you buy that Boat]

I took the boat in to a marina and they found no compression in number 3 cylinder. I pulled the head and have a bent push rod and bent valve stem. What are some possible causes for this?

 

[Scott06]

I took the boat in to a marina and they found no compression in number 3 cylinder. I pulled the head and have a bent push rod and bent valve stem. What are some possible causes for this?

Rust on valve stem due to leaking exhaust manifolds or risers is one possibility.

Would tear the head down and inspect the exhaust for signs of water intrusion. Did the valve hit the piston? What does spark plug look like ?

Was engine previously rebuilt and improperly machined?

 

[Why did you buy that Boat]

So, bent valve and push rod @ #3 cylinder exhaust side. Had both heads flattened and bent parts replaced. Got it put back together and started it today but need to set the timing, with a light. I have an older basic timing light.

This is a Thunderbolt IV and timing should be set at 8deg BTDC. But do I need to do something with grounding wires to lock it into base timing mode? Or, do I just connect the timing light "normally" at set the distributor at 8 deg?

 

[Scott06]

So, bent valve and push rod @ #3 cylinder exhaust side. Had both heads flattened and bent parts replaced. Got it put back together and started it today but need to set the timing, with a light. I have an older basic timing light.

This is a Thunderbolt IV and timing should be set at 8deg BTDC. But do I need to do something with grounding wires to lock it into base timing mode? Or, do I just connect the timing light "normally" at set the distributor at 8 deg?

With thunderbolt 4 you do not put in base mode set to 8 deg at idle as ‘normal’.

Basic timing light is fine

I use a timing light and tach/dwell my dad bought in the late 60s …

 

[Why did you buy that Boat]

Thank you very much!!

 

[Why did you buy that Boat]

Got the boat back in the water and it runs really well. I still have engine temperature concerns, however.
It has a new 142 degree thermostat and when running at around 4200 rpm, that is where it stays. But at trolling speed or 5500 rpm, it runs at 220 degrees. I have verified these temps with an infrared gun. The impeller and shutter valves are new also.

It doesn't go past 220 but that seems high to me.

Any feedback related to normal operating temps on a 1990 mercruiser 5.7 thunderbolt IV would be appreciated.

 

[Scott06]

Got the boat back in the water and it runs really well. I still have engine temperature concerns, however.
It has a new 142 degree thermostat and when running at around 4200 rpm, that is where it stays. But at trolling speed or 5500 rpm, it runs at 220 degrees. I have verified these temps with an infrared gun. The impeller and shutter valves are new also.

It doesn't go past 220 but that seems high to me.

Any feedback related to normal operating temps on a 1990 mercruiser 5.7 thunderbolt IV would be appreciated.

If you get above 170 with a 140 thermostat its too high. You may be sucking air or bypassing too much raw water if you have the check ball system

Would not use until you figure this out.