1991 4.3 not reaching WOT RPM's

[CHARGER2]

All in the title. I have had this boat (18.5 open bow) for 2 seasons, and the problem has always been there. The engine sounds just fine. It idles smooth and sounds fine at throttle. However, it is sluggish in the hole (takes a LONG time to plane, at times it does not plane). It maxes out at 3800 rpm and 35mph with a full tank and no passengers. It takes 75% throttle to maintain a plane. It runs at 3100rpm when wakeboarding people at 15mph. I currently have a 19 pitch 3 blade prop in good condition which seems to be what everyone else has. Boat was tuned up last summer. I have not yet done a compression check.

How do I manually check the gear ratio? What else could be bogging down this boat?

Thanks in advance.

 

[Scott Danforth]

Re: 1991 4.3 not reaching WOT RPM's

what kind of drive?
what kind of boat?
if the drive is a mercruiser, it should have a tag on it
are you using the trim properly?

when was the last time a tuneup was done? plugs, wires, cap, rotor, etc.
do you have trim tabs?

 

[Don S]

Re: 1991 4.3 not reaching WOT RPM's

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

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

 

[choppywaters]

Re: 1991 4.3 not reaching WOT RPM's

Well if it was tuned up I'm sure they replaced the fuel filter, maybe check to see if your throttle is adjusted right. As far as terrible hole shots...been there,done that. I just installed a new 17pitch prop. And just installed smart tabs tonight. The prop seem to help a bit but nothing great. Ours is a 20ft cc and with about 700-900lbs of people and gear it took about 10 seconds to get on plane,with the new prop. It took about 7seconds, now with these tabs I'm hoping it will be instant or close to that. Has this boat always been like this or has this developed over the past couple seasons?

 

[CHARGER2]

Re: 1991 4.3 not reaching WOT RPM's

Scott: Alpha one gen one. 18.5 openbow fiberglass runabout. I am using the trim properly (I am not new to boating, just this boat). Fresh tuneup this season. I have no trim tabs.

Dan: Through troubleshoot and inspection, the only possible scenarios are 6,8 and 9. I will check those soon.

Choppy: The boat has acted this way as long as I have had it (2 years). A tuneup did not change anything.

Thanks all again for help.

 

[CHARGER2]

Re: 1991 4.3 not reaching WOT RPM's

Spent some time addressing this issue. I did a full carb rebuild, replaced plug wires, dist cap and rotor (thunderbolt iv). Now the engine will not start. It cranks away and tons of fuel goes in (can see the accel pump squirting in, fuel is coming out of the exhaust). It is getting spark (confirmed via timing gun). I had a couple of backfires with throttle and choke wide open.

Any ideas why this is not starting?

Thanks again for any help.

 

[CHARGER2]

Re: 1991 4.3 not reaching WOT RPM's

Still need some help here.

I figured out why it wasn't starting... I was dumb and I was at TDC on the wrong stroke (exaust not power). I fixed this, put it back together and now I have no spark. Everything went back together the way it came off. I have 12V on each side of the coil, 0V on the grn/wht terminal of the dist and 9V on the grn/wht red/wht terminal. Are these all the right voltages? Am I supposed to have 12V on each side of the coil?

Thanks for any help.

 

[litman252]

Re: 1991 4.3 not reaching WOT RPM's

Positive side of coil should be near battery volts while cranking. Negative side should be fluctuating as the distributor points or electronic pickup open and close.
I'm new to boats but above is from other experience.
best,
Tony

 

[Scott Danforth]

Re: 1991 4.3 not reaching WOT RPM's

a couple of things after going thru my buddies boat with a 4.3 and similar issues.

check the exhaust flappers - his dislodged and plugged the outdrive (#8 on Don's list). also, do a plug pull test once its running. he had a bad plug as well.

why did you pull the distributor?

 

[CHARGER2]

Re: 1991 4.3 not reaching WOT RPM's

Exhaust flappers are good already checked them. I pulled the dist because I couldnt get the rotor off while it was in the boat. It was really frozen on there.

Ok so it is ok to have voltage on both sides of the coil. I guess the next step is to replace the sensor in the distributor.

 

[John3]

Re: 1991 4.3 not reaching WOT RPM's

how do i manually check the gear ratio? What else could be bogging down this boat?

Thanks in advance.

If everything is correct & you're still not getting full RPM at WOT it could be the gear ratio.
The upper gear set controls the ratio. To count the teeth remove the top cover on the OD.
To determine the ratio of the set you need to count the teeth on the two gears, a 1.98:1 would have 20/24 teeth for #1 mercruiser and alpha from 1971 to 1997. 17/19 is 1.84:1, 24/24 is 1.65:1 & 22/20 is 1.5:1.
A high ratio means the engine turns faster for a given boat speed. So going to a higher ratio gets you further up on the power curve, therefore greater speed at wot. Check your owners manual for max rpm at wot, probably about 4400 to 4800.

 

[CHARGER2]

Re: 1991 4.3 not reaching WOT RPM's

OK problem solved: here are some closing statements.

It was not getting spark because I was dumb and put one of the GRN/WHITE wires on the RED/WHITE post on the dist. Started right up.

All in all I did a carb rebuild, new cap, rotor, and wires, set the timing, checked gap on 6 month old plugs, confirmed the tach was accurate, visually verified the flapper valves were not stuck closed and ensured the carb linkages were in good order and operating correctly.

All these things got the boat to 4300 RPM at WOT with a 19 pitch 3 blade, semi beat up aluminum prop.

Re-proped to a new 3 blade 17 pitch Merc Black Max and WOT RPM'S increased to 5,000. I will be sure not to go above 4,800 for obvious reasons. The 17P seems not to have changed the hole shot and dropped the top speed by a couple MPH, however the mid-range performance has greatly increased (2000-4000 RPM). Cruise RPM is also about 400 RPM faster.

Thanks for all the help. Hope this comment can help someone in the future.