WOT RPM Issue

[maxumized]

1997 Maxum 1750SR.

New GM 3.0L.

Rebuilt carb.

New Fuel pump and fuel system test.

New EST ignition set. Checked base timing (1' BTDC) multiple times.

14.5x19 prop (OEM was 13.75x21)

Anyone who has kept up with my posts has noticed a trend... constantly chasing a WOT RPM issue. I've been through and through the WOT RPM tips on the site.

The boat runs better than it ever has, however, it still won't reach operating RPM of 4400-4800.

In fact, I'm starting to believe that conflicting Merc info related to carb jetting may be the issue. The jets in my carb (#807504) are 1.55, as the parts manuals suggest. However, the service manual references sizes of 1.60 and 1.65.

Current symptoms are fully trimmed at WOT, I'm only getting about 3600rpm. If anything, the aftermarket prop should be over revving the boat.

However, 3400 rpm is reached somewhere between 1/2 to 3/4 throttle, and going to WOT only gets about 200 more rpm.

It's also worth noting that anything past 3400rpm and the engine begins to sound much different, a good bit of noise, similar to what sounds like an aluminum can rolling around in the exhaust. Because of this, I rarely run anything over around 3/4 throttle, where the noise doesn't exist. Now, this could be the exhaust baffle, although it's new too, so I'm not sure why it would be making the noise.

Also - I've been pulling the plugs and they are as clean as the day they went in. This makes me think I'm running lean. Which leads me back to my theory on the jets. Could having 1.55 jets instead of 1.60 or 1.65 really make nearly 1000rpm difference?

 

[Don S]

Re: WOT RPM Issue

Your carb (807504) shows that it should have the 1.55 jets.
Here is from the merc manual (#26 1998 and up)
What is the SN of your engine, cause it doesn't seem to be a 97 carb.
What WOT rpm are you actually getting?

 

[maxumized]

Re: WOT RPM Issue

Keeping in mind I've replaced the long block with a brand new GM marine 3.0, the original serial is 0K029578.

Also, as I mentioned in the original post, WOT, fully trimmed is about 3600. However, I don't keep it there for long being that since something isn't right, I'm not quite sure if I'm running it lean, etc...

 

[Don S]

Re: WOT RPM Issue

Have you actually done the fuel system checks in this thread and everything came out within specs no air bubbles in the clear hose?

http://forums.iboats.com/showthread.php?t=167561

 

[maxumized]

Re: WOT RPM Issue

Yes, in fact, even when the vacuum test and pressure test came back normal, I even ran the boat off a fuel cell and replaced the fuel pump. No change.

 

[Don S]

Re: WOT RPM Issue

Are you sure your tach is reading right?

 

[maxumized]

Re: WOT RPM Issue

I may get flamed for admitting this, but I've run it up to about 4800 in neutral while in the water, just to make sure the tach was hitting it. And it did.

 

[Don S]

Re: WOT RPM Issue

I'm not asking if it gets to 4800 or not, what I am asking is your 3600 rpm you are seeing on the tach is actually 3600 and not 4600.
Get a good shop tach and verify it. at WOT and see if they are the same.

 

[maxumized]

Re: WOT RPM Issue

Are you suggesting that the tach reading 4800 without load could possibly read different under load?

I will verify with a shop tach next, but keep the ideas coming. I have a feeling I'll be marking this step off my list as well.

Thanks for the help!

 

[Don S]

Re: WOT RPM Issue

Are you suggesting that the tach reading 4800 without load could possibly read different under load?

Good grief man, no that is not what I am suggesting.
What I am suggesting is that when your engine is actually running at 4600 rpm, it may be showing only 3600 rpm.

It means your tachometer could be wrong, set wrong, defective, who knows. Check it.

 

[maxumized]

Re: WOT RPM Issue

I will check it. I guess I was just confused as to how the tach will run a full sweep from idle to 4800 rpm when in neutral, as if there is no tach problem.

 

[wil7483]

Re: WOT RPM Issue

What Don is saying is that your tach could be defective (broke) and therefore is incorrectly showing a reading of 3600 rpms when you are in fact doing 4600 or some other number. He is suggesting that you run the tests with a "known" shop tach to verify that your tach is not the culprit behind your problems.

 

[Maclin]

Re: WOT RPM Issue

Maybe 4,800 in neutral is more like 6,000


And no kidding, it is a good idea to verify the rpm's.

I would want to check the total timing and not just the initial. Your problem kind of sounds like too much advance, just my first blush read on it.

 

[maxumized]

Re: WOT RPM Issue

Maclin, when you explain it that way, I understand what everyone is getting at. However, if it was all relative, I would be idling at like 1800rpm... but right is right, the only way I'll know is with a known good tach. Will be checking this week.

Now the timing, the EST set is brand new. And I've triple checked the base timing to be 1' BTDC. This is based on the service manual 13, for my 1997. Now that the long block is a new GM marine, has the spec changed? I see that service manual 26 mentions later 3.0s have incrementally retarded timing based on serial (1' BTDC, 1' ATDC, 2' ATDC). That being said, I'm not quite sure which number to use. If the manufacturers timing is based on mechanical characteristics, I guess I should be using the latest available. However, if it's based on fuel delivery and ignition, I guess I should be using 1997 information.

Furthermore, when you have a timing tab that reaches only 12BTDC, how can you accurately measure if the ignition set is advancing over the rpm range to anywhere from 23' - 26' BTDC?

 

[Maclin]

Re: WOT RPM Issue

A different type of timing light can do the trick. It is called a dial-back, has a dial on it that when you turn it the timing mark comes "back". Whatever the setting on the dial is when the timing mark hits zero is the total advance.

Example of how to use:
Make sure the dail on the gun is at zero then point the gun at the timing marks, rev engine to 3000 or so and hold, turn the dial on the gun until you see the mark moving towards zero on the timing marks, then work the dial on the gun until the mark stays at zero. Rev the engine up a bit just to make sure the advance is all the way in from the distributor, if the mark moves then hold the rpm's higher enough to where it is steady and turn the dial again to get the mark back to zero. Let the engine return to idle then shut it off. The dial will show the total advance. I am guessing that it should be between 26 and 32 somewhere. Hotrods like it at closer to 36-38 total, boats not so much. Anything over 40 introduces kick-back and detonation, can limit revs.

 

[fat fanny]

Re: WOT RPM Issue

Are you ensuring the shift interputer switch is by passed and the two leads from the 3 pin connector are jumped? Also after setting base timing to 1degree BTDC are you reconnecting the shift inturputer switch and removing the jumpers from the 3 pin connector leads ? If so @that point your timing should be @ between 10-14 degrees BTDC after that w/ thew rpms @2,400-2,800 the total advance should range from 25-29 degreesBTDC if not your ignition mod is bad. I had the same issue but when I ignored jumping out the 3 pin connector wires I suffered poor performance and if need be never buy after market modules only AC Delco the cheap ones heat up and do not advance correctly. Good luck

 

[maxumized]

Re: WOT RPM Issue

Yes, I followed the timing procedure to the letter. And yes, under normal operation at idle, advance is right at 12'BTDC. However, I'm only estimating that its reaching total advance of around 27'BTDC since I don't have an adjustable timing light. I am going to borrow one this week, hopefully, and then also run a shop tach on it to confirm the boat tach readings.

 

[nailem]

Re: WOT RPM Issue

to reinforce checking the tach. i had my tach working when pulled from the water last year and this year tach was off. still worked and seem to sweep normal. when i checked it is was off maybe a couple hundred at idle and a couple thousand at wot. good luck