1993 mercruizer 4.3lx no power after half throttle

[Greenbean79]

First off. I'm new to this I'll do my best but I want to thank you guys for all your post. I've got a 1993 bayliner trophy with a mercruizer 4.3lx alpha 1 that I purchased about 3 months ago. It was on salt water several years ago but has been a fresh water boat for about the last 8 years. Boat has 305hrs on it. It had been sitting for a year. When I went to look at it we took it to the lake started right up ran good had it on the lakes here at home a couple times done great then started to get sluggish after half throttle didn't want to plane out as easy it had about a half tank of fuel when I purchased I put stabilizer in and filled up.ive ran about 40 gallons of fuel through it since I've owned it. So I decided to give it a tune up hoping that would help. Changed fuel and oil filters new plugs mr43t wires rotor button and got a cap and ignition sensor but the cap wouldn't fit and the ignition sensor looked different it is all plastic but I put the sensor on any way bolted right up boat ran honorable so I started chasing what I had done swapped out new sensor for old one boat ran great on muffs so I took to the lake for a load test starts good alittle slow but idles great got out of no wake and no power past half throttle runs 3200rpm @22mph and will climb to 4k rpm about 25mph but that's it can back off throttle to half way doesn't change anything go back into the throttle you can feel it bog down. I dont know what else to check any help would be appreciated thank you

 

[achris]

Hello Greenbean79, and :welcome: aboard.

The very first thing (before even taking it on the water) anyone should do when they buy a secondhand boat (or car, or truck, or house!) is a full service. That way you start to get to know the machine. Yes, machines will talk to you, if you know how to listen.

For your problem, do a service and take it from there.
Compression test,
Replace spark plugs and check leads, cap and rotor.
check timing.
Replace fuel filters (check contents after you removed the old one)
Do an oil and oil filter change.
Pull the drive, grease the gimbal bearing, and unis
Check condition of bellows and shift cable
drain oil from the drive and split it and replace the water pump impeller, housing if required.
check engine alignment
tighten gimbal ring bolts
Check condition of and grease steering system
general check of the entire engine and drive system.

As you have what appears to be a fuel delivery problem, I would add a strip and clean out of the carburetor.

Chris..........

 

[Greenbean79]

I forgot to add the boat was running decent before the tune up just didn't plane out like it use to. But after the tune up it wont plane at all even with just me in the boat. Feels strong out of the hole. Wondering what I done wrong gaped plugs st .035 acts to me like the timing is not advancing properly

 

[achris]

Ah, 1993... This engine will not have the distributor ground wire connected. That would be my first check. There's a 'spare' ground wire in the harness near the distributor, just pull it up and connect it to the distributor body. I did my with one of the TB-IV module screws on the outside of the distributor....

 

[alldodge]

Agree fuel filter and check contents, and if all clean check fuel pressure between pump and carb, should be 3 to 7 psi from idle to WOT

 

[Rick Stephens]

I read

"back off throttle to half way doesn't change anything go back into the throttle you can feel it bog down"

and can't help but jump on the secondary circuit being plugged up in the carburetor. Yeah, it could be the ground wire on the ignition, it could be the advance is not advancing, it could be plugged up fuel lines or poor pump output, but I'd start at the carburetor and do a very careful clean and kit on it.

Being a new to you boat, EVERYTHING needs going through. Chris is 100% on target with that. It will cost you a lot less to make sure of the serviceability before you break something and have to walk back to the ramp.

 

[Greenbean79]

Thank you guys I am going to check fuel pressure and compression and change the fuel filter again this evening when I get off work i will let you know the results. Yes your right I need to step back and check everything good. Just get excited getting a boat I've wanted for ever and I jumped the gun alittle

 

[achris]

It was 6 weeks after I bought my boat before it hit the water.

 

[Greenbean79]

Update I checked the compression #1 130lb #3 115lb #5 130lb #2 130lb #4 125lb #6 125lb. Dumped contents of fuel filter in a clean dry clear container let set for about 2hours no debris or water. Attempted to check fuel pressure but gauge I got wouldn't tighten up on fuel lines after filter I used the inline fitting and put on before the pump not sure if that would even work but got no reading gauge is in 2lb increments. Also checked anti siphon valve looked clean very hard to blow through. Thanks

 

[Greenbean79]

Something else I should have mentioned it backfires through the carb when you try to really get in the throttle like going from 1000 to 4000 rpms it only does it around 1k-2500rpm

 

[achris]

... I used the inline fitting and put on before the pump ...

That will read the pressure drop on the suction side. Not the pumps OUTPUT pressure.

Chris.......

 

[achris]

Something else I should have mentioned it backfires through the carb when you try to really get in the throttle like going from 1000 to 4000 rpms it only does it around 1k-2500rpm

That sounds like a lean condition. Could be caused by the carb secondary jets being blocked, or low fuel pump pressure. Most likely the carb needs pulling off, stripping and cleaning.

 

[alldodge]

Also checked anti siphon valve looked clean very hard to blow through.

If you were blowing from the barb fitting (one going to the filter) back toward the tank, then it should be hard to blow thru. If you were blowing from the threaded side going toward the filter, then it should be easy

 

[Greenbean79]

It was from threads to barb that was very difficult small amount of air the other way barb to threads I was not able to move any air

 

[alldodge]

It was from threads to barb that was very difficult small amount of air the other way barb to threads I was not able to move any air

Then the valve needs replaced, it should be easy to blow thru in that direction.

https://www.iboats.com/shop/catalogsearch/result/?q=anti+siphon+valve

 

[kulle]

Ah, 1993... This engine will not have the distributor ground wire connected. That would be my first check. There's a 'spare' ground wire in the harness near the distributor, just pull it up and connect it to the distributor body. I did my with one of the TB-IV module screws on the outside of the distributor....

funny i found my ground from the ignition tucked up in the boot!..i ran it to the ground bolt on engine block, not to the back of distributor. weird thing is it ran fine disconnected. 93 merc 4.3

 

[Greenbean79]

Last night I replaced the anti siphon valve with a normal fitting (without check valve) no improvement but I'm going to order and replace anyways. Took carb apart to inspect and clean. Wasn't very dirty found some debris in the bowl someone has had this carb apart before because they have damaged the top of 2 of the jets and I'm not able to remove them one of the metering rods is sticking when pushed all the way down I believe it's because of the top of the jet being messed up.I got it moving as smooth as I could. Nothing is stopped up so I'm going to take to the lake today and try out I'm going to need a new carb ether way I guess

 

[achris]

Hopefully it fixes the problem, but if you need a new carb, look at an Edlebrock 1409...

Chris....

 

[achris]

funny i found my ground from the ignition tucked up in the boot!..i ran it to the ground bolt on engine block, not to the back of distributor. weird thing is it ran fine disconnected. 93 merc 4.3

Most will. But after I reported a problem with a bad distributor ground in 1995 (that took me 6 months to find :facepalm, all Merc delivered engines since have had a ground wire on the distributor body.

Chris....

 

[Rick Stephens]

Nothing is stopped up so I'm going to take to the lake today and try out I'm going to need a new carb ether way I guess

How was float height? After cleaning all the passages, float height most important.

You're right about damage to jets possibly causing flow problems. Could even be the jets were mixed up and leaner ones that are supposed to be in the primaries got swapped over to secondaries.