merc 233 not going above 3200 rpms at wot

[89cruisers]

my 78 wellcraft nova had a merc 188 engine (302) mated to a merc 233 drive which has a 1.3 drive ratio (previous owner went cheap when the factory 351 went bad) the boat would not go above 3200 rpms and above 32mph. I got rid of the 302 and put in a 351w with 4 barrel carb (marine engine) after a sea test with everything done, and it is like a repeat, the boat won't rev above 3200 rpms and any faster than 32mph with a 16" pitch prop, compression is good and all 4 barrels are opening up and dumping fuel in. Any ideas where to start

 

[haulnazz15]

Re: merc 233 not going above 3200 rpms at wot

Have you verified your tach is accurate? Does the engine rev above 3200rpms when in neutral? Have you verified your timing? Are you sure you have the correct prop for the drive/boat size? Checking these things may help narrow things a bit. I like how the previous owner cheaped out with a 302 when it doesn't really cost more to build a 351w.

 

[Bondo]

Re: merc 233 not going above 3200 rpms at wot

Have you verified your tach is accurate? Does the engine rev above 3200rpms when in neutral?

Ayuh,... Along with the tach, run a Compression test...
'n Read This,...


Revin' in Neutral is totally Irrelevant, 'n meaningless...

 

[89cruisers]

Re: merc 233 not going above 3200 rpms at wot

compression is good,timing is good and advances, tach does indeed go above 3200 on land, and even if the tach was wrong, it has a 16" pitch prop and only going 32mph according to my gps, were talking a 1k rpms difference which I don't think suck things like bottom paint would make that big of a difference

 

[Pete104]

Re: merc 233 not going above 3200 rpms at wot

Check the exhaust tube for broken flappers.

 

[haulnazz15]

Re: merc 233 not going above 3200 rpms at wot

Check the exhaust tube for broken flappers.

He better hope they aren't gone, cause replacing them on this model requires engine removal!

 

[haulnazz15]

Re: merc 233 not going above 3200 rpms at wot

Revin' in Neutral is totally Irrelevant, 'n meaningless...

Well if it doesn't rev there in neutral, he'd know it was an engine issue. If it revs freely above that, he knows that the problem is induced under load which normally points to an outdrive issue or a timing advance issue, right?

 

[Bondo]

Re: merc 233 not going above 3200 rpms at wot

Well if it doesn't rev there in neutral, he'd know it was an engine issue. If it revs freely above that, he knows that the problem is induced under load which normally points to an outdrive issue or a timing advance issue, right?

Ayuh,... A V8 motor, runnin' on 4 cylinders will rev to the Moon,...

Like I said, it's Totally Irrelevant, 'n Meaningless...

 

[89cruisers]

Re: merc 233 not going above 3200 rpms at wot

how exactly would one check to see if the flappers are working properly? Since this same problem was happening with the 302 aswell, what problems in the drive could cause this?

 

[Riegfrghtl]

Re: merc 233 not going above 3200 rpms at wot

I may be mistaken but the gear ratio should probably be 1.5 instead of 1.3

 

[89cruisers]

Re: merc 233 not going above 3200 rpms at wot

the merc 233 has a 1.3 gear ratio

 

[89cruisers]

Re: merc 233 not going above 3200 rpms at wot

I have my responses for that list that was linked, which did not seem to yield any ideas

1. Fuel condition. Type and Octane possibly old fuel - brand new gas
2. Propeller pitch or diameter, damaged blades - brand new 16" pitch prop
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test - looking down carb at WOT the fuel is pouring right down there
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse - oil checked, proper level
5. Marine growth on hull and outdrive - has bottom paint
6. Wrong gear ratio in outdrive - merc 233 has a 1.3 ratio
7. Restricted carburetor air intake (clogged flame arrestor) - tested without a flame arrestor even on it
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive - any way to test this?
9. Poor cylinder compression Compression Test - running around 150psi in all cylinders
10. Carburetor defective, or wrong type. - idles fine
11. Fuel pump pressure and vacuum - once again, can clearly see large amounts of fuel being dumped in
12. Boat overloaded, improperly loaded, or improperly trimmed. - test had no luggage, and rear seats not even in yet, as for trim, dont think that would cause a 1k rpm loss
13. Engine Overheating - was running at about 200
14. Engine timing and ignition system operation have timing at 10btc, and is advancing
15. Remote control cables and linkage for proper travel to open throttle plates fully. - plates opening all the way

The only things according to this list would be the exhaust, or trim, how would I check for exhaust flow? and would the trim being to far down really cause a 1k rpm loss?

 

[Quadruplecccc]

Re: merc 233 not going above 3200 rpms at wot

Are you sure you have a 1:50 drive in there?

Could be loosing power running at 200*. I'd change h20 pump in outdrive asap! Has it always run that hot?

 

[Bondo]

Re: merc 233 not going above 3200 rpms at wot

The only things according to this list would be the exhaust, or trim, how would I check for exhaust flow? and would the trim being to far down really cause a 1k rpm loss?

Ayuh,.... Check, 'n see if the shutters are where they belong,...
That's usually what causes exhaust flow issues...

Yes,... Trimmed under can Easily scrub off 1000 rpms...

 

[haulnazz15]

Re: merc 233 not going above 3200 rpms at wot

Unless you find shutter pieces in the exhaust passeges to the outdrive, you just have to assume the shutters are working properly or missing altogether. The shutters are located at the bottom of the y-tube behind/below the engine, there's no way to get to them short of pulling the engine.

 

[89cruisers]

Re: merc 233 not going above 3200 rpms at wot

Wish I could tell you if it has always run that hot, ive had it for almost a year and still have not gotten to enjoy it, it has a new thermostat. I'll try to look in there and see.

 

[F14CRAZY]

Re: merc 233 not going above 3200 rpms at wot

F14's been the engineer on this project...

We've swapped out the wires for new Sierra wires though we're having to re-use the coil to distributor wire since the one that came with it isn't long enough. The old one looks to be in good shape but I wrapped it with electrical tape just in case.

Based on another thread regarding a 233/351W, I've set the idle timing to 10 degrees BTDC and it advances to about 34 at 3k rpm on muffs. According to the thread I found that's correct.

I'm thinking it possibly could be due to improper trimming. It was planing fine but I can see how having the trim too far down and thus the bow too far down could create more drag. Being an OMC Series 400 owner, adjusting the trim didn't come to mind (lol). And the trashed out bottom paint isn't helping. Was planning on taking another sea trial but due to trailer issues it'll be a little while before that's possible.

At least it sure sounded nice at 3200 rpm...I probably shouldn't have looked down into the carb like I did but the fuel pouring down there is astonishing lol