4.3LX Bogging above 4K RPM

[Kevin S]

Hi,

New here and hope that I can get some direction or symptoms diagnosed.

I have a 1992 4.3LX w/ 4BBL Thunderbolt ignition. This is what is occuring consistantly (3 times out in a row) - When we first start out with the boat, she will start fine (ok), runs really well all the way up to WOT. I bring up on plane, cruise at about 3200RPM, go to WOT throttle, no problems. I can do this for say up to 30mins-1hr. All is good until we shut down the boat, drift for say 15-30 minutes, at this time when I restart, she has a little trouble getting up on plane, not much, but a little, however, I can now no longer go to WOT because it will bog. Also, she runs fine up to 4000RPM's. If I tap the throttle to get her above 4K, she will surge up and down a little, not much but it is noticable. Again, if I try to go to WOT, it just bogs like it wants to shut down, pull back the throttle and she runs fine around 4K RPMs and below.....no issues up to the 4k RPM's. I can cruise the rest of the day as long as I don't go above that mark.

Any help with this would be very much appreciated. I'm not a mechanic and at best a "rookie shade-tree" mechanic. However, I'm not afraid to try or work at fixing this myself, if I can.

Kevin S.

 

[Don S]

Re: 4.3LX Bogging above 4K RPM

You could try this,
http://forums.iboats.com/showthread.php?t=167561

Or an easier way to isolate the fuel system is to get a remote tank (6 gal ouboard tank works great) and hook it up directly to the fuel pump and see if it works.

 

[Kevin S]

Re: 4.3LX Bogging above 4K RPM

Thanks Don!

The "easier" way would likely be best for me. Just to confirm, your saying to get an "outboard fuel tank" with bulb or no bulb?, hook this up to the fuel pump, go out on lake and see if it still acts up? If the motor still acts up, can I pump the bulb while it's acting up to see if it's a Fuel Pump issue? I would assume that if it is the pump, the symptoms would go away during "pumping" of the bulb, or , is that a bad assumption?

By the way, my motor is a mercruiser, forgot to say that.

Again, thanks!

Kevin S.

 

[Don S]

Re: 4.3LX Bogging above 4K RPM

Doesn't make any difference with or without the bulb. But it should have 3/8" fuel line so there is plenty of fuel.
I guessed it was a Mercruser, since they are the only ones that used the Thunderbolt ignition and the LX designation.

 

[Bondo]

Re: 4.3LX Bogging above 4K RPM

Ayuh,.....

Don is asking you to eliminate the Fuel Tank,+ supply it with Fresh Gas from a remote tank,....

But your Right,..... The Primer Bulb can be used to eliminate the Fuel Pump......

Btw,......

Welcome Aboard to the forums here at iboats.com..........

 

[Kevin S]

Re: 4.3LX Bogging above 4K RPM

Thanks for the clarification guys! I will go ahead and give that a try. I don't mind eliminating a piece at a time.

BTW...just figuring out what motor this is has been a little bit of an issue, but it's the Mercruiser 4.3LX 4BBL (GEN II). Ordering a manual tommorow.

Kevin

 

[imported_TheMan]

Re: 4.3LX Bogging above 4K RPM

If you go here and click on Service Manuals:
http://www.boatfix.com/merc/mainmenu.pdf

You can download the manual for the engine.. #18 I think. There is another link to service manuals somewhere if that isn't the right one, and I'm sure someone can link it for you..

 

[Don S]

Re: 4.3LX Bogging above 4K RPM

You can download the manual for the engine.. #18 I think.

He needs manual #7 for his engine, #18 is for engines with the balance shaft.

The other link (that had all the manuals) doesn't exist anymore ........... Which is a bummer for everyone.

 

[Kevin S]

Re: 4.3LX Bogging above 4K RPM

I just ordered the Service Manual - Mercury Manual.

I was wondering how to get any service bulletins that may help, any resource online that actually has them downloadable?? I haven't had any luck with that yet.

Tech. Question: Since this problem only surfaces once the engine area gets warm, wouldn't that most likely eliminate the Carb?

Kevin

 

[soler]

Re: 4.3LX Bogging above 4K RPM

Try a new ignition coil. It's cheap, and if it's bad, your engine might run, but the coil won't produce enough volts to produce very much power. They might work fine cold, but act funny when they get a little warm.

SOLER

 

[Kevin S]

Re: 4.3LX Bogging above 4K RPM

Update:

I went ahead and replaced the ignition coil as the first step. We took the boat out last Sunday and she ran great, no bogging, no surging and she got up on plane a lot easier. The decision to change the coil first was based on a "hunch" and some input from here, the problem only surfaces when the engine is warm, so from there, I took the "risk" on the Coil.

We duplicated the same operation as before and she showed no signs of problems.

Thanks for all your help! I will continue the tune-up process once I get the Service Manual in this week.

Kevin S.