Mercruiser 4.3 lx WOT

[dozerII]

I'm having a few issues with obtaining WOT on my 4.3 OC519811 . Currently getting 4100rpm with a 21p or a 19p prop It has Thunderbolts ign, so no stuck advance springs, throttle plates are opening all the way. I'm not sure if the vacuum secondaries are opening though.
Any ideas where to check from here?

Thanks

 

[achris]

Re: Mercruiser 4.3 lx WOT

Getting enough fuel? Change ALL your fuel filters. Don't forget the one in the inlet of the carby.... No secondaries on your carby, it's a 2 bbl. And when was the last time you put new spark plugs in?

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Getting enough fuel? Change ALL your fuel filters. Don't forget the one in the inlet of the carby.... No secondaries on your carby, it's a 2 bbl. And when was the last time you put new spark plugs in?

Morning Chris thanks for the reply, all filters are new, just finished a complete rebuild on the boat, tanks cleaned and new fuel. I do have a Rochester 4 barrel. Compression is 145 on all cyl. Carb I rebuilt last fall.

 

[Don S]

Re: Mercruiser 4.3 lx WOT

You are going to have to verify the secondaries open under load. Just because it was rebuilt doesn't mean it works right.

List of possible causes of low WOT*/ 295070

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Thanks Don, will check it all.

 

[achris]

Re: Mercruiser 4.3 lx WOT

Morning Chris thanks for the reply, all filters are new, just finished a complete rebuild on the boat, tanks cleaned and new fuel. I do have a Rochester 4 barrel. Compression is 145 on all cyl. Carb I rebuilt last fall.

My bad... Missed a letter in the model number and it came up as a 2 bbl... :facepalm:

Chris........

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Went back to the 23P prop, checked the timing it was way out 6 deg or better, set it adjusted the carb and it runs a lot smoother but still won't go over 4300 rpm.

 

[achris]

Re: Mercruiser 4.3 lx WOT

Try it again with the 21" prop... You should make about 4700 with that one... And I'd say that is as close to perfect as you want...

 

[NHGuy]

Re: Mercruiser 4.3 lx WOT

ps. the secondaries are under the air flaps which only open when the secondaries create enough vacuum to suck them open. You can see them open with the motor off, just hold the flaps open and have someone push the throttle lever to wide open. They should start to open at around 50 or 60% throttle.

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Try it again with the 21" prop... You should make about 4700 with that one... And I'd say that is as close to perfect as you want...

ps. the secondaries are under the air flaps which only open when the secondaries create enough vacuum to suck them open. You can see them open with the motor off, just hold the flaps open and have someone push the throttle lever to wide open. They should start to open at around 50 or 60% throttle.

Thanks for the replys, the secondaries are opening, I screwed up in my last post I am running a 21p prop not a 23p.
Could the problem be in the Thunderbolt ignition? Not letting it go above 4300?

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Another question, how would you test the Thunderbolt ignition to see if I'm getting full advance?

 

[Bondo]

Re: Mercruiser 4.3 lx WOT

Another question, how would you test the Thunderbolt ignition to see if I'm getting full advance?

Ayuh,.... Easiest way is with a digital timin' light, 'n runnin' it....

or a regular timin' light, 'n a degree tape on the crankshaft flange...

What are the Spark plugs tellin' ya is goin' on,..??
Try the ole Run it, stop, 'n check the plug Colors....
More fuel,..?? less fuel,..??
I kinda doubt it's the T-Bolt...

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Ayuh,.... Easiest way is with a digital timin' light, 'n runnin' it....

or a regular timin' light, 'n a degree tape on the crankshaft flange...

What are the Spark plugs tellin' ya is goin' on,..??
Try the ole Run it, stop, 'n check the plug Colors....
More fuel,..?? less fuel,..??
I kinda doubt it's the T-Bolt...

Thanks Bond-o I will have to see if I can borrow a digital timing light, I will check the plugs as well and report back.

 

[achris]

Re: Mercruiser 4.3 lx WOT

Do you have a timing curve graph?

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Do you have a timing curve graph?

No I don't Chris.

 

[achris]

Re: Mercruiser 4.3 lx WOT

Do now...

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Thanks Chris.

Okay a little more info, Here are a couple photos of the plugs, they were brand new and at the most an hour run time on them.





I know an hour is not much run time, but there is no color to the plugs what so ever. When I brought the SS home last April it had sat for a couple years after the engine was replaced and only run about two hours. I think the motor was a new replacement but can't confirm that. I had the vale covers off over the winter when I was doing the manifold and riser gaskets and the heads a spotless under the covers. The carb was in bad shape so I rebuilt it, but it still has some rust on the butterflys. Compression last April when I checked it was 145 on all six, I just checked it again and I'm at 125 on all six.

I'm thinking it is running pretty lean, although even on a full throttle launch there is no hesitation all the way to 4200.

 

[Bondo]

Re: Mercruiser 4.3 lx WOT

Thanks Chris.

Okay a little more info, Here are a couple photos of the plugs, they were brand new and at the most an hour run time on them.

I know an hour is not much run time, but there is no color to the plugs what so ever. When I brought the SS home last April it had sat for a couple years after the engine was replaced and only run about two hours. I think the motor was a new replacement but can't confirm that. I had the vale covers off over the winter when I was doing the manifold and riser gaskets and the heads a spotless under the covers. The carb was in bad shape so I rebuilt it, but it still has some rust on the butterflys. Compression last April when I checked it was 145 on all six, I just checked it again and I'm at 125 on all six.

I'm thinking it is running pretty lean, although even on a full throttle launch there is no hesitation all the way to 4200.

Ayuh,... I agree, the plugs are too clean, 'n White...

 

[dozerII]

Re: Mercruiser 4.3 lx WOT

Thanks Bond-o, so I know assume is not a good word but, because the motor runs good even on hard acceleration, the problem more than likely is a carburetor malfunction?

 

[achris]

Re: Mercruiser 4.3 lx WOT

Are you running at altitude? Or was the boat operated (and changed to suit) high altitude before you bought it?

Reason for lean at the top end... can be weak/low fuel pump pressure, low float level in the carb, partially blocked jets in the carb, power circuit in the carb not working, incorrect timing/timing advance, weak spark. Incorrect heat range of spark plug... On that, what plugs are they?

Chris...