89 4.3 Mercruiser Low RPM at WOT

[tbeaze24]

Just got a 89 Caravelle with a 4.3. Has 17 pitch prop, was not started for awhile, will run at only 3100 rpm's. I changed the cap, rotor, coil, wires, and plugs. It is supposed to run at 44-4800 rpm wot. Could just bad gas be the cause of not getting the right rpm's? I have done all I know of, it will rev to the correct rpm, with a flush kit on it with no load, the compression is fine. I also changed the fuel filters on it,,I am just wondering if the old gas can really cause all this, and the low rpm?>?Thanks for any help, Tim

 

[Reel Poor]

Re: 89 4.3 Mercruiser Low RPM at WOT

Here's a list of possible problems. Compliments of Don S

 

[Gary H NC]

Re: 89 4.3 Mercruiser Low RPM at WOT

Verify the carb is opening all the way then go down Don's checklist above.
Yes,bad gas can do it along with a ton of other things.

 

[180shabah]

Re: 89 4.3 Mercruiser Low RPM at WOT

You've got a great check list above but one more thing to answer. Is this a new problem(if so did it come on gradually or all at once) or was this problem present, but unknown, when you purchased the boat?

 

[Silvertip]

Re: 89 4.3 Mercruiser Low RPM at WOT

An engine running on half its cylinders will rev to its upper limit with no load. Means absolutely nothing other than the engine is running.

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

The boat had actually sat for about 2 years, a mechanic got it running, took it out twice, it sat for about 7 months, then I just bought it a few weeks ago. I know the gas was somewhat bad when I got it. He said it took him a while to get it going. I have taken it out 3 time, and I can say that after each time, I have done a little more to it, that it has ran better each time. Another ?,, and idea how to get to the fuel pickup. It is 89 Caravelle, 190ES, it has the built in tank, there is an access cover in the front floorboard, but when you take it off, it is not centered over the screws for the pickup. Another thing, the fuel line is routed going back around the left side of the boat, but where it comes out of the tank, it goes up, and is about a foot and a half above the fuel tank until it gets back to the engine, where it goes back behind it, the to the front of the engine to the fuel pump. I also notice that it is a straight line, and there is not a fuel/water seperator any where in the line????? Thanks again for all your help, Tim

 

[Gary H NC]

Re: 89 4.3 Mercruiser Low RPM at WOT

I would try to get all the old gas out before doing anything else.put in fresh gas,a can of seafoam,and add a water seperater.
two year old gas is no good,no matter what you add to it.
That old gas you can use in your lawn mower.

As far as running it on the muffs,mine had a blown headgasket but sounded fine on the muffs with no load on the engine.

 

[Gary H NC]

Re: 89 4.3 Mercruiser Low RPM at WOT

I would try to get all the old gas out before doing anything else.put in fresh gas,a can of seafoam,and add a water seperater.
two year old gas is no good,no matter what you add to it.
That old gas you can use in your lawn mower.

As far as running it on the muffs,mine had a blown headgasket but sounded fine on the muffs with no load on the engine.

Welcome to Iboats!!!

 

[HT32BSX115]

Re: 89 4.3 Mercruiser Low RPM at WOT

After I checked all the items on the below list, I found that there's an additional item that's not on this list.......(the problem I had with mine)

one or more flat lobes on the camshaft!


Cheers,



Rick

Here's a list of possible problems. Compliments of Don S

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

Not sure about when it started, just know the problem was there when I bought the boat. I had never heard of sea foam, but a friend of mine told me about before I saw the post on here, I had already went and bought a bottle of it,,,,hope it works like he swears by it. Just hoping the old gas will burn out and it will run like it should. Thanks again, for all the help. Will let you all know after I take it out again

 

[180shabah]

Re: 89 4.3 Mercruiser Low RPM at WOT

If the gas is that old, I would get several fuel filter and change them often, and probably plan on a carb rebuild in the near future. As Ethanol in the new gas you put in the tank will dislodge all of the sludge and varnish.

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

Okay Guys, here I am again. Took it out today, All the things I did, did make a difference. Last time I took it out, I got about 29-3100 rpms at wot, and about 34mph, today, between 33-3500 rpm at wot, and 40.5 mph on the gps. To get it there, I was at about 38 mph and 3200 rpm, and I could play with the throttle at wot, and each time(almost like pumping or reving a car engine) it would jump a little im rpm and mph. While at 3500 rpm, I could pump the throttle and it would stay there as long as I played with it, I checked all my linkage and it seems okay, but I was not the one who set it, I am just wondering if anyone thinks it may have some play that is not allowing the carb to say WOT? I am going back out there now to tinker with it some more. All help, info and ideas are greatly appreciated. Thanks again, TIm

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

One other thing, I did notice while sitting back there with the engine cowling off, when I gave it some gas real quick, I could here a sucking sound from the carb? Could this be a choke sticking or what??? Thanks again, TIM

 

[DJ]

Re: 89 4.3 Mercruiser Low RPM at WOT

sucking sound from the carb?

Most likely the accelerator pump.

The fuel tank pick up is not the area you see with the screws surrounding a plate. That is the guage access. The pick up is one lone fitting with the fuel hose attached to it. Be aware, there is a check valve there in the pick up that can stick. It is called an anti siphon valve. Remove it and free it up.

Do this.

1. Remove free up the anti siphon valve.

2. Replace the fuel filter.

3. Get rid of that old gas. You're going to be sorry, down the road, you didn't.

Wide open throttle is easy to check. With the engine OFF, remove the flame arrestor and look in the carb throat. Are the butterflies all the way open?

Don't rev it up so high, on the muffs. If you continue to do so, you'll have REAL trouble, REAL fast.

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

how difficult would it be for me to rebuild my own carb???I am sure I could do it, just not so sure on setting the float part? any hints would be appreciated

 

[Gary H NC]

Re: 89 4.3 Mercruiser Low RPM at WOT

If it is a 2 barrel its really simple,a 4 is just a little harder.
I have rebuilt a bunch of them.Most kits will have some instructions.The float setting is pretty simple with a small ruler.
Just rember how it all came apart and you should be fine.
Clean it real good,put in a new needle,seat,accel pump and some gaskets,set float and thats pretty much it.

If you have some auto background you can do it no problem.

I bet you find some trash in the bottom of the bowl when you take it apart..

 

[DJ]

Re: 89 4.3 Mercruiser Low RPM at WOT

You can probably rebuild it yourself. You'll need access to an air compressor for blowing out orifices.

Just take your time. A digital camera is a huge asset. Take pictures as you disassemble it.

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

Thanks Gary, I have a pretty good mechanical background. Work in auto parts for 15 years, and now I am an avionics tech on Apache helos. But ha ha, say you can teach a monkey to fly them, but takes a high school diploma to fix them,, Just never broke down a carb that for. It is a 2 brl, and just looking at the kit, does seem that bad, but I said that one time, and 8 hrs later, my buddy is standing over me,,,,,,"yea yea, how hard can it be"!!!!!!!!!!!!! But I do think I am going to give it a shot. Thanks for the info, Tim

 

[tbeaze24]

Re: 89 4.3 Mercruiser Low RPM at WOT

One more? Well maybe more than one later, but for now. Is there any in between for a mechanical fuel pump, in all my years working at a parts store, if a mechanical pump is bad, its bad, no it pumps, but just not enough. It there a possibility that it sat so long that the diaphram is weak and just not pumping enough gas. No doubt that the engine has the power, it is just starving for fuel. Please go back up and read all my post and all that I have done, and give me some more feedback. Thanks again, Tim

 

[Reel Poor]

Re: 89 4.3 Mercruiser Low RPM at WOT

It's in the list I posted for you. As well as checking the vacuum side of the fuel pump (between the pump and fuel tank).

You will need a fuel pressure guage to check the output pressure from the pump, and a vacuum guage to check the suction side.

Here's a link to Checking Your Fuel System