Stalling mystery

[8ender]

I have a 1972 45hp Chrysler franken motor. After much head scratching it appears to be a 72' motor that someone has swapped an older late sixties powerhead onto. Aside from this weirdness I've been able to get it running quite well until today.

The main issue is that it doesn't like to run anywhere in the 2000-3000rpm range and tends to stall if I keep it there. I can coax it past this range with some quick throttle and it's happy above and below it. It idles just fine and starts on the first turn of the key.

Some clues (maybe?)

- I recently replaced a condenser that caused a misfire because it's grounding clip snapped off
- When going WOT there's a weird spot at the top of the throttle range where the motor seems to slow down. 95% throttle gets me the most speed and more than that less speed.
- Because my motor is a franken motor my leg didn't have the port and splitter for the two output barbs on each reed. I tapped a new hole where the blank spot was on the leg and ran the splitter and new hoses like my older parts motors. The previous owner just had a single hose going from reed to reed which I assumed was wrong?
- I relocated my gas tank to the front of the boat and have about 8-10 feet of 5/16 line running to it. I'd have figured if this was causing fuel starvation I'd see it across the whole RPM range.

Any ideas here?

 

[jerryjerry05]

I had a 18gal. tank installed under the front seat, along with the 100 gal. built in, it had 10+ ft. of 1/4" hose and it ran twin 85hp. motors, your 5/16" hose is more than enough to supply fuel.
The first thing I'd do is check/remove all the inline hose connectors, then check the tank vent and check the hose for Ethanol damage.
Ethanol tends to melt the inside of the hose and cause blockages???

 

[8ender]

Hoses are all about a year old at best. The hose for the relocated gas tank is brand new. I was worried about ethanol as well. Good news is that I figured out the second problem. Throttle linkage was out so the butterfly opened past wide open on max throttle.

I feel like fuel isn't the issue here. If I can get WOT with no issue then it can deliver the fuel the motor needs. Something else is up that the engine would get so grumpy in that particular range.

 

[dingbat]

Hoses are all about a year old at best. The hose for the relocated gas tank is brand new. I was worried about ethanol as well.

Unless you used some cheap, off brand fuel line you’ll have no problems with ethanol.

Use a SAE certified (stamped on jacket along with date code) fuel hose and don’t worry about it

 

[8ender]

Definitely used the good stuff for fuel lines. Anyone here know enough about old Chrysler outboards to say if my fix to drain the reeds into the leg is the right thing to do? Previously it was just a line connecting the two barbs on the reeds which seemed wrong. On the more modern 45hp motors those hoses go to back into the powerhead. On the older into the leg.

 

[Nordin]

Yes your hoses for the draining of the unburned fuel are right.
Earlier Chrysler 35-55Hp had the two hoses from crank case manifold to a T-coupling and down in to a barb fitting down the leg.
Later had a hose from top cylinder crank case down to bottom cylinder crank case.

 

[8ender]

Any ideas why it might fall flat in that 2000-3000rpm range? It's the only issue left on this motor that otherwise runs amazing.

 

[The Force power]

Any ideas why it might fall flat in that 2000-3000rpm range? It's the only issue left on this motor that otherwise runs amazing.

Check the trigger-wires & connectors, the wires move around as the trigger advances.
These wires are very thin and tend to break under the shrink-tube.
(maybe at that particular RPM is the spot where it looses ignition??)

 

[Nordin]

Have you checked if it is a electrical or fuel issue?
Do you loose spark at actual RPMs or do you have spark at both plugs.
Connect a timing light at the plug wires and check if you loose spark, if not I suggest you to clean the carb well as it would be a fuel issue.

 

[oldboat1]

Any chance it's a hull issue? Water under the floor/soaked flotation foam?