Random dying at full throttle

[steveclv]

Went out on the lake today and had an interesting experience

After all my repairs and maintenance, my Chrysler 125 runs sweetly - get's up on plane quickly and the boat cruises at 35mph @ 3800 and seems happy to do that constantly.

If I push it to full throttle I'm doing 42mph @ 4200 rpm (which is still too low). However, every so often the engine just cuts out. It restarts rightaway and continues as if nothing happened. Only seems to happen when running flat out.

What do you think? Fuel blockage from something floating in the tank? It has new fuel hose and filters from end to end and a new pump diaphragm. Haven't had a chance to look into the tank to see if it's full of crap but I did put a bottle of dri-fuel in just in case. There is nothing obvious in the clear fuel filter I added.

The prop is the 'standard' 13*19 AL so I'm a little concerned that the boat isn't pulling at least the 4500 minimum for WOT. The foam is dry and the compression is 150# on all 4 cylinders.

 

[dkonrai]

Re: Random dying at full throttle

better check your fuel mix, sounds like it could be running a little lean. fyi, better to be a little rich, or you will have the same problem i have... dont ask.
also make sure your timing is correct. how is the fuel pump?

 

[foodfisher]

Re: Random dying at full throttle

Check all fuel connections. Might be sucking a spot of air.

 

[jerryjerry05]

Re: Random dying at full throttle

Any inline connectors?Vent clogged?Diaphram?Does the squeezie go flat when it quits?
If it goes flat then there is an obstruction.Not flat possible air leak.
Your right about the RPM's Try a 17p maybe a 15p Your 1200 below what you should be.
You sure the tach is reading right.J

 

[steveclv]

Re: Random dying at full throttle

The fuel flow is unrestricted - I pulled the hose off the fuel pump into a container and pumped plenty of fuel with each squeeze so that's not an issue unless something is in the tank that get's caught in the pickup randomly and only at full throttle.

Mixture is right insomuch as it smokes at idle (as they all do) so there is plenty of oil. Fuel is a mixture of old fuel and new (Premium) fuel which I am gradually burning off but the engine seems to run fine up to the point that it suddenly dies and then it restarts just fine.

Fuel pump has new diaphragm and gaskets.

I did think of a 17p but then the prop is the original so you would think that the motor should be able to spin it ok.

Timing is 32 degrees at WOT checked with a timing light

Have no way of testing the tach but the needle is rock solid and it 'sounds about right'.

I guess I will have to do some more investigative work - might borrow a carry can fuel tank and try that next so I can isolate the problem area.

 

[SwampThing]

Re: Random dying at full throttle

Steve, did you try looking for a partially broken / chaffed wire that only opens or grounds out when the timing / advance or throttle linkage is at WOT? I also know those coil primary wires have a tendency to get rubbed through over time because they're all squashed behind the backside of the plate the coils and CDI's are mounted to.

I did have a problem once where the engine would cut out under the right vibration conditions. I finally traced it to loose wires at the back of the ignition switch. I still have the original OEM switch and the electrical spade type connection tabs are held to the switch with tiny little screws. All the screws were loose as a goose.

I'm also of the opinion that replacing a prop every time the max RPM's isn't in spec, isn't necessarily the best advice. The engine was manufactured to run at spec with the OEM components. Although the octane rating of gasoline has declined in the time since these motors were produced. I don't know if that has any effect though.