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Engine will not go over 2000RPM

1

1marc1

Contributing Member
I have a 4.3L throttle body injected Mercruiser (s/n M093527), we just installed a brand new long block. Ever since installation the boat runs good at idle and on muffs but under load it will not go over 2000rpm. We used some existing parts off the old engine like intake, distributor, coil, things like that but from that old engine everything was fine and working 100%. In between when i blew the first engine....... till now, the boat has sat for 2 years. Clearly whatever fuel that was in it was old but i did not have much left in it.

Before start up i filled the boat with a mid grade fuel. So fuel should be good enough in my mind to get me over 2000rpm. So far we have changed the electronic ignition, i still had the old factory green chip ignition....... and am now running a new Thunderbolt iv. I purchased a new distributor cap, a brand new coil, a brand new fuel line that was kinked and in which i thought was not allowing the proper amount of fuel to go WOT. Right now im at a loss!? I don't know what to do or change or look at next!?! Basically 96% of the parts on this engine are brand new?? and My prop i didn't change, i have a high 5 and its always worked! I also just replaced my fuel filter just to rule one more thing as well, so im not sure what to look at next??
 
MikDee said:
Check your timing, make sure it's right!
Click to expand...
I'll second that one!

All else can be near perfect... but if the engine is not seeing the correct progressive spark lead thoughout the RPM range...... it will not produce the power needed.

We're talking about the progressive and TA.... not BASE advance alone.




.
 
RicardoMarine said:
I'll second that one!

All else can be near perfect... but if the engine is not seeing the correct progressive spark lead thoughout the RPM range...... it will not produce the power needed.

We're talking about the progressive and TA.... not BASE advance alone.




.
Click to expand...



Motor came from Michigan Motorz. It's not a rebuilt.......it's a new GM marine. The timing should be correct!? We put it in base and went 8 degrees. I dumped my tank yesterday and am going to fill it with premium today to try again.....so if that doesn't work we we could rule out bad fuel.
 
All too often a blind eye is turned on Ignition timing.
BASE advance is BASE advance all day long.
The more important aspect of Ignition timing, is the Progressive and the Total Advance..... and at which RPM the Total Advance is "full in"!

This is very easy to verify.


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RicardoMarine said:
All too often a blind eye is turned on Ignition timing.
BASE advance is BASE advance all day long.
The more important aspect of Ignition timing, is the Progressive and the Total Advance..... and at which RPM the Total Advance is "full in"!

This is very easy to verify.


.
Click to expand...

So what do you suggest? Take it out on the water, loosen distributor and try to find timing that way?
 
1marc1 said:
So what do you suggest? Take it out on the water, loosen distributor and try to find timing that way?
Click to expand...
Absolutely NOT!
Ignition Timing is far too critical for any gasoline Marine Engine.

See your OEM service manual for ignition timing.
Look further... and you'll find the advance curve graph.

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RicardoMarine said:
Absolutely NOT!
Ignition Timing is far too critical for any gasoline Marine Engine.

See your OEM service manual for ignition timing.
Look further... and you'll find the advance curve graph.

.
Click to expand...

I don't have a service manual for my boat. And I think we did exactly that! It says For the thunderbolt iv on an efi engie it instructed us to put the boat in base timing and go 8 degrees. So im not sure what else there is we could do to time my engine.
 
1marc1 said:
I don't have a service manual for my boat. And I think we did exactly that! It says For the thunderbolt iv on an efi engie it instructed us to put the boat in base timing and go 8 degrees. So im not sure what else there is we could do to time my engine.
Click to expand...
You will strobe the timing marks as you increase RPM up to the Full-In RPM.
Plot this out in graph form, and compare this to the OEM specs shown in the correct model number advance curve graph.


This need not be done by a Marine mechanic if you can find an auto mechanic who is savvy to ignition advance.
Just make sure that you're not using automotive specs!


Ignition advance may not be your problem.... but this will at least eliminate it as a potential.



.
 
Last edited:
RicardoMarine said:
You will strobe the timing marks as you increase RPM up to the Full-In RPM.
Plot this out in graph form, and compare this to the OEM specs shown in the correct model number advance curve graph.


This need not be done by a Marine mechanic if you can find an auto mechanic who is savvy to ignition advance.
Just make sure that you're not using automotive specs!


Ignition advance may not be your problem.... but this will at least eliminate it as a potential.



.
Click to expand...

Ok. Not sure what you mean by full in rpm? I have a timing light here and I can time it myself but I'm afraid you may need to dumb it down a bit more so I can understand. Sounds like may have to remove my back seat in order to do this as well.........that was one thing I wasn't looking forward to do.
 
If you put a timing tape on the vibration damper/ balancer, I believe you can do this with an average timing light. Otherwise, you need an advance timing light. Speed shops usually have this self stick timing tape.
 
MikDee said:
If you put a timing tape on the vibration damper/ balancer, I believe you can do this with an average timing light. Otherwise, you need an advance timing light. Speed shops usually have this self stick timing tape.
Click to expand...

Yup my damper already had the sticker on it. I found out the proper routine of timing my engine from a mercruiser spe******t. I was able to jump my engine into base timing, my engine automatically went to a higher RPM and I once again timed it to 8 degrees. Hopefully this time it works!
 
hystat said:
Start with a compression test.
Click to expand...

A compression test on a brand new engine with zero hours. I don't like the sound of that. My last engine that was In the boat before buying new...... had zero compression on 2 cylinders and I was still able to get it into full rpm range not to mention still be able to hit max speed as if the engine was still 100%. And the way my last engine was compared to this new engine, it's night and day! This motor runs beautifully at idle and lower rpm, if I was lacking compression I don't think it would run this good!?
 
Check your firing order, to make sure you don't have 2 spark plug wires crossed? I've seen these small block chevys run perfectly smooth with a certain two wires crossed.
 
MikDee said:
Check your firing order, to make sure you don't have 2 spark plug wires crossed? I've seen these small block chevys run perfectly smooth with a certain two wires crossed.
Click to expand...

Firing order was correct, and i have 150lbs on every hole. But upon pulling my plugs out i noticed cylinders number 1,3,2,4 are all burning and are black.....but cylinders 5 and 6 came out still looking like new.
 
1marc1 said:
Firing order was correct, and i have 150lbs on every hole. But upon pulling my plugs out i noticed cylinders number 1,3,2,4 are all burning and are black.....but cylinders 5 and 6 came out still looking like new.
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UhOh! Water intrusion from old manifolds, or gaskets?
 
1marc1 said:
Manifold and gaskets are both brand new.......doesn't mean it may not be them, but i would suspect my old plug cables being the problem before my manifolds.
Click to expand...

Well, if you think it's the plug wires, put a timing light, or cheap spark tester on them to see.
 
MikDee said:
Well, if you think it's the plug wires, put a timing light, or cheap spark tester on them to see.
Click to expand...

I just did a drop test......it seemed like on every cylinder i did my boat engine did bog down except for on the #1 cylinder, but again that doesn't make much sense because i have all brand new plugs and the plug that came out of number 1 was black. So it obviously must be burning.
 
W.A.G (wild eyed guess) here?, are your valves adjusted right? It could be some too tight (rich), and others too loose (lean)? But, I'd think you'd here some clacking?
 
Wait!, From what I know, the Thunderbolt IV does not need to be set with the base timing method! (That's just for the Thunderbolt V) It's just turn your Dizzy, & set your timing to specs, just the way it is,,, and it will automatically take care of the timing.

Go here to 4B-13 to see timing procedure,& verify:
http://www.boatfix.com/merc/Servmanl/16/16B4R2.PDF!
 
MikDee said:
Wait!, From what I know, the Thunderbolt IV does not need to be set with the base timing method! (That's just for the Thunderbolt V) It's just turn your Dizzy, & set your timing to specs, just the way it is,,, and it will automatically take care of the timing.

Go here to 4B-13 to see timing procedure,& verify:
http://www.boatfix.com/merc/Servmanl/16/16B4R2.PDF!
Click to expand...

A thunderbolt iv on an efi engine does have to go into base timing. We covered every base with a fine toothed comb on that one. right now the only thing left is insufficient amount of fuel at wot, so where going to squirt some gas in throttle body out on the water next weekend to see if I can pick up rpm that way.......and if it's not that then it can only leave my ecm not advancing timing when needed.
 
1marc1 said:
I don't have a service manual for my boat. And I think we did exactly that! It says For the thunderbolt iv on an efi engie it instructed us to put the boat in base timing and go 8 degrees. So im not sure what else there is we could do to time my engine.
Click to expand...

Your words??? Did you do something different then what you said here?
 
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