1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

[MacDan06]

Hello,

I have a 1986 Mercruiser 4.3 205HP w/ an alpha one outdrive. Had a problem with the engine stalling out above 3000 RPMs, and replaced everything trying to figure out what was wrong. Finally got it going so that its not stalling out, but its still not running at its best. At WOT, it bounces between 3000 and 3500 RPMs. It will hit 3500, and then seemingly lose power, drop down to 3000, and then race back up to 3500. This happens every 10-15 seconds, even while its planed out and trimmed (takes a while to get out of the hole). Its definitely below the recommended WOT RPM, and it wont hold steady. Below WOT, runs fine... idles like champ. Heres whats been replaced in the last few months...

New carb (rochester quad)
new Fuel pump
gas tank cleaned out and refilled fresh
new fuel filter
new shift cable
new plugs, plug wires, and distrib. cap

I've been trying to eliminate things, but I'm running out of things to check. Could it be overpropped? It has a 21" on there now. I'm thinking of trying a 19" or 17". Any other ideas?

 

[nailem]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

they may move this to the prop forum but i'll try to offer my advice. i have a 86' 5.0l and it had a 21 pitch prop, ran bad could hardly get on plane. long story short it was way over propped. looked up the correct prop on this site and it called for a 17. installed the 17 and it plane out and ran around 4300 rpm. i would say look up you motor and i bet it is way over propped. i guess for trolling and fishing.

i also read that being that over propped and "lugging" the motor that hard isvery bad for the motor. so i would suggest not running that prop.

 

[BAYLINER185]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Well I dont know about being over proped. I have a 23 Pitch on my BAYLINER 185 with 4.3. Granted Im talking a 2007 carbed engine rated at 190 Hp not 205.

What kind of boat are we talking about for 1 thing?

2nd thing, is the tachometer operatign correctly? as in set correctly.

The 1st day I took my BRAND NEW bayliner out it would seem to me to max out at 3500 RPM. I thougt OH dealer put on wrong prop.

I bought a new prop and while it was in shipping stage I took the boat back to the dealer. Sure enough the tach wasn not set for the 4.3 V6 it was set for a V8.

Went back to the lake with the stock 23 pitch and hit 4600RPM and did 50MPH......SWEEET put on a SS 23 pitch doing 55MPh at 4200RPM.....




Turns out the 21p prop I bought I used on a boat I just bought so it wasnt a loss at all!



CHECK THE TACH before you start messing with prop pitches.

 

[180shabah]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Ignore te prop until it is running properly. Using the wrong prop will not cause the surging he has.

DonS has a great check list for "won't reach WOT" but here area a few.
Vacuum leak?
Partially obstructed vent line???
Carb properly tuned for your exact engine??
Proper float adjustment??
Throttle valves opening fully???
Anti-Siphon valve gummed up(restriction)??

 

[MacDan06]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Thanks for the replies.

Its on 20' Wellcraft Elite Bowrider.

I just had the brand new carb installed by a good mechanic, so I don't think its the carb tuning or float, and I know the throttle valves are opening fully. I'll check the vent line and anti-siphon valve. Theres no easy way to check for a vacuum leak, is there? Also, is there any way to check the tach is operating correctly, aside from switching it out and seeing if it gives the same reading?

A friend of mine has a spare 17 so if everything else looks ok, I'll toss that on and see if I get any improvement at all. It seemed to me that the surging had to do with something other than the prop, but could be a combination.

180shabah- I searched around, but couldnt find DonS's post of checks for WOT problems. Do you have a link? Thanks

 

[jerryjerry05]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

To check for vacume leaks use a can of starting fluid and start the motor then spray around the base of the carb,then spray around the intake manifold.If the motor surges or starts to stall or any difference is noted then there is a vacum problem.Jerry

 

[Don S]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

If the engine used to run ok and get the proper rpm, I don't see how you could think changing the prop pitch would help. It might cover up the problem, but it fixes nothing. Besides, 1" of prop pitch change is only going to be between 100 and 200 rpm.

Here is that list of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[twag4]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Don't forget timing. My 99 4.3l just went through some problems and was running very sluggishly after new cap, rotor, plug wires and plugs. Turned out it was timing. Very quick and simple fix and now it runs harder and faster than it ever has since I've owned it.

 

[180shabah]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Timing is number 14 on the list above.

 

[MacDan06]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Thanks for everyone's help. After trying out a few things, I replaced the ignition coil, and it fixed the problem. Weak spark did us in. Runs great now. Thanks again.

 

[Ion66]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

This is exactly the same post I was going to make as I am having exactly the same issue with the same engine and have so far done exactly the same repairs (minus the shift cable) in an attempt to fix it. My boat has run well up to now, so I know it's not a prop issue. I am hoping that the fix is the same too, as that was next on my list. It's great to find that an old post can be as helpful as a new one....fingers crossed.

 

[Don S]

Re: 1986 Mercruiser 4.3 Running 3-3.5K RPM @WOT

Welcome to iboats lon66, best to start a new thread of your own, and include all of your engine information and what you have done to it so far.