[SOLVED] Wrong upper gear ratio causing low WOT?

spartenos

Seaman Apprentice
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Sep 7, 2014
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33
Good Morning!

I have a Alpha One Gen Two with a 3.0 inline 4 (1992).

I have done a ton of work on this boat, and in May\June, I had the upper replaced with a new SEI upper as the gears were shot and I was just done with it.

Fast forward to now. After a busy year, we finally finished break-in. I go to open her up, smoothly accelerates until about 2/3 into the throttle and then the RPM just sits there at about 2500 RPM. I can watch the throttle cable moving throughout the entire arc of the throttle swing.

Thought, it could be the carb, even though it was rebuilt 2 years ago - and I rebuilt the carb again (mind you, had ZERO issues accelerating BEFORE the upper swap). Took the boat out this weekend, same thing. Gets to 2500 RPM and SITS there. Cannot plane, do not pass go, do not collect $200. I also replaced the spark plugs, engine oil and fuel\water separator at this time.

I had previously popped open the cover after reading about ratio issues and I swear that the drive gear looks like the teeth spacing is different than what my old one was. I have done a lot of reading about ratios and I wanted to ask this.....

IF - (And I understand this is a big IF) the mechanic installed a drive with say, the 1.54 ratio instead of the 1.98 (I know the ratios were updated, but bear with me here), could this explain the sudden RPM loss? I hesitate to say it's the fuel system, or something went south in the engine, when this worked just fine before the SEI upper was tossed on.

If you think I should start looking at all of the cylinders, etc., I will definitely do so - but I am in SE Michigan and boating season is rapidly dwindling for me to figure this out.
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,092
you do need the 1.98-1 ratio . that rpm can be caused by a 1.50 ratio and cannot be fixed by a prop
what ratio did you order?.
 

DeepBlue2010

Lieutenant
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Aug 19, 2010
Messages
1,304
After changing the gear ratios, did you (your mechanic) do any calculations to see if you need different prop?!

Wrong prop pitch would limit your RPM.
 

spartenos

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Sep 7, 2014
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33
That's the thing - I left it in his hands, he ordered the upper and replaced it. I am now going to get in touch with them and ask about this.... I don't think they would have done the wrong one on purpose, they are a reputable shop, but, mistakes do happen... I will reach out to them.

To answer the pitch question - They didn't say anything to me about needing to reprop or anything along those lines.
 

Alumarine

Captain
Joined
Feb 22, 2005
Messages
3,684
You can put a mark on the prop and then turn the motor over by hand to determine your ratio.
If it takes 2 revolutions of the motor to rotate the prop 1 turn then you have a 1.98 gear ratio.
 

Rick Stephens

Admiral
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Aug 13, 2013
Messages
6,118
SEI will be using a 1.98/1.94 ratio gear set for the 3L. Should be stamped on the upper.

That said, a 1.94 should have 20/24 teeth on the gearset. Pop the top off and count.
 

spartenos

Seaman Apprentice
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Sep 7, 2014
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33
Well - Intriguing update!

On the good news side (kind of?) It appears that the mechanic and SEI probably both shipped and installed the right drive. Used an iPad to film the prop (put a piece of tape on one blade) and then jumped in the boat and turned the engine over (using a wax mark on one of the bolts to count my turns). Looks like 2 turns is just over 1 full prop turn. Good news there!

So I decide to start moving along to other tests. Yank all the plugs (I replaced them right when I got the boat back) and note that plugs from cylinders 1 and 4 are cleaner than 2 and 3. Since these were NGK plugs instead of the MR43T, I decide to cross reference the plug number myself, JUST to be certain (as the guys at the parts store where I bought them sold me the NGK plugs). Well, they sold me NGK UR4's, which, according to a cross reference sheet, are closest to a MR44TS (not even an MR43TS). Now, I would have to assume that using a longer\shorter plug in a hole designed for a different plug would definitely correspond to power loss and not being able to achieve WOT RPM.

Thoughts on this?

Edit - Ack - On researching, it sounds like, while definitely not the right plug, probably not my main issue. I will still replace the plugs and do a compression test first.
 
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alldodge

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Edit - Ack - On researching, it sounds like, while definitely not the right plug, probably not my main issue. I will still replace the plugs and do a compression test first.

Sounds like a good plan :thumb:
 

spartenos

Seaman Apprentice
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Sep 7, 2014
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Compression is a solid 150 PSI in all 4 cylinders.
New plugs (BR6FS instead of the incorrect UR4 - Both NGK) seem to be much more pleasant in terms of engine stability at low RPM\Idle - at least on muffs.

Is it possible that simply changing from the incorrect plugs I had been given to those would fix the RPM at WOT issues?

Going to try and load test in the water in the next day or two..... Hard with the early night times!
 

Benny67

Chief Petty Officer
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Apr 21, 2015
Messages
571
What did the plugs look like? Were they tan in color,white,black with soot or wet?

I doubt it's your plugs.

You got the same prop on that new drive?

I'd think the prop has too much pitch before going the route of engine issues.

I'd assume if they changed it you would see it on the bill unless the new drive came equipped with a new prop and they just kept your old one cause they could...

Oh also, the plugs might be a bit on the dark/wet side if the motor is being loaded down by a prop that has too much pitch so you are kinda chasing your tail on that theory.

Check your manual and see what your drive ratio/prop diameter,pitch is suppose to be.
 

spartenos

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Sep 7, 2014
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Upper was the only thing that was changed on the drive, lower is the same, engine itself is the same. Ratio is correct based on manually turning the engine (Roughly 2 turns for roughly 1 prop turn) at the 1.94 or 1.98 this engine should be. Same prop as what was on here before these issues started.

Brand new plugs were black with soot, but that is likely because they haven't gotten hot enough to burn off the carbon.

Plus, we are talking about almost 2000 RPM here....
 
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Benny67

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Apr 21, 2015
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Plugs are like that cause the motor is getting loaded down...or the wrong heat range
 

spartenos

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Engine has only been on muffs at roughly idle since the plugs were replaced - Likely not getting hot enough to burn the carbon off.
 

Benny67

Chief Petty Officer
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Apr 21, 2015
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571
Or...carb issues....the list can go on and on but I would think something changed as far as that ratio relationship from old to new.

Go back to the marina and explain the problem and see if they will let you try a lower pitch prop without charging you for it.

Seeing it's a 4 cylinder I think the ratio issue would be more sensetive due to the fact alone they don't deliver a whole Lotta torque at lower rpms (as compared to a v6 or v8)

I had a similar issue once and the place actually lent me a prop to test out. You never know what they will do unless you complain about the repair not being done properly.
 

spartenos

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Sep 7, 2014
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Yeah - I think I will try and call them tomorrow. I just want to water test it one more time before calling them up.... I MIGHT be able to tomorrow.
 

spartenos

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Sep 7, 2014
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Well... Both good and bad news. I redid the test of turning the crankshaft and carefully watching the engine with my wife watching and being very careful to remove any slop.... Turns out - It WASN'T a 1.98 ratio.... I separated the upper and lower and shot Justin @ SEI the serial number. Turns out, they ordered a 1.64 instead of a 1.98..... Trying to see now if SEI will work directly with me or if I will need to contact the mechanic. In all honesty, at this point, I just need an upper mated and then it will basically be time to winterize the boat....

Thanks again all. I had a sneaking suspicion from the very beginning that this was the case. But hey, at least now the boat will be in tip-top shape for next year...... Right?!??!!?
 

alldodge

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Good to hear you found your issue

at least now the boat will be in tip-top shape for next year...... Right?!??!!?

Well, kind a sort a maybe, it's a Boat and we all know what that stands for. :rolleyes:
 

Benny67

Chief Petty Officer
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Apr 21, 2015
Messages
571
Fyi,

The plugs, even if you are running with no load on an idle they should burn tan and dry...anything else and you need to do some troubleshooting, wrong heat range, running to rich, etc

A engine that is running properly should have clean dry tan plugs.

After you fix your ratio issue you should check them again
 
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