engine hesitating moving through 1500-1800 RPM range

[MikeDobbs]

This is a mercruiser 3.0lx engine

When we got to the water today the boat started up, idled, moved into and out of forward and reverse gear in the water all completely fine. When I went to move from idle to a 5mph no wake speed, the engine stalled out. Started right back up, and hesitated badly as I tried to bring the engine up through the 1500-1800 RPM range. After feathering the throttle I was able to get it "over the hump" without stalling, and once above approx 1800 RPM the engine ran perfectly fine. I was able to get up on plane, had plenty of power, and the engine responded well to both gentle and aggressive throttle movements. We went out and had a great day on the water. The boat ran fine at idle, and fine at higher RPMs, but continued to hesitate whenever moving through that 1500-1800 range.

I'm thinking something with the carb- maybe the idle/low speed jet circuit?

One other thing, possibly a coincidence: when getting gas I normally dose the tank with Startron enzyme treatment. The last time I filled up I did not because I had neglected to reorder when I previous bottle ran out. I didn't figure it would be an issue, since I was only adding gas to a tank that had treated fuel in it already (this was about a week ago). So, after the initial issue today, I went back to the dock the grab the new bottle, and gave the tank a dose of startron- hoping it would just kind of work itself out after running around for the day.

Any suggestions would be greatly appreciated. Thanks in advance!

 

[NHGuy]

It does sound like a carb transition point. But is everything else up to standard? That rpm is where many boat engines are under great load. Are the plugs, wires, cap & rotor good?
You may need a carb rebuild & cleanup. If you have a fuel/water separator pour out the contents into a clear container to check for junk. Check any fuel filters too.
If there is water it could have made it to the carb bowl, that would be a factor.

 

[MikeDobbs]

Plugs, wires, cap/rotor are new last year. I can swap out new plugs and see if it helps, but I don't think it's an issue of load because once I get over that small range (which is well before planing) I can give the boat full throttle which puts it under tremendous load as the boat climbs onto plane with absolutely no issues.

 

[R055]

Sounds like the carb to me.

Also seems like adding fuel treatment every tank is a bit of an overkill/waste of money unless you only take the boat out like once a year.

 

[MikeDobbs]

Sounds like the carb to me.

Also seems like adding fuel treatment every tank is a bit of an overkill/waste of money unless you only take the boat out like once a year.

so you're leaning towards coincidence on this being the trip that things went poorly?

 

[R055]

so you're leaning towards coincidence on this being the trip that things went poorly?

Yes, no fuel that you just filled up needs additives to run properly.

 

[NHGuy]

Yes, I don't run additive either. If the fuel is clean I'm ok. Is your environment dusty or dirty? Have you checked for junk in the filters?
Perhaps if you give us the year and carb someone who has had one open will chime in.

 

[MikeDobbs]

It's a mercarb- the boat is a 1999, so I think the carb is prob a 1998. It's the 2 barrel mercarb with the electronic choke (not TKS, the one with the spring that heats up when the ignition is turned on). I don't think my environment is dusty or abnormal in any way- I just run the fuel treatment as a precautionary measure

 

[Rick Stephens]

It will not have anything to do with idle mixture screws. Those literally only effect things when the throttle plate is closed enough to pull a vacuum over the idle fuel ports. Once you crack the throttle you are on the main jets on the Mercarb - and the throttle pump plays a large part there as well.

Sounds like carb kit time. Pay great deal of attention to every little passageway. Make them perfectly clean using air and cleaner, use wire probes, brushes and soakings. That is the key to a good carb rebuild.

Oh, not to detract from NHGuy's statement on everything else needing to be right. He's pretty sharp on this stuff. Just my opinion that carb is what needs attention.

Rick

 

[achris]

As well as the idle discharge holes, there are a series of holes above them, called progression holes. These progression holes supply fuel in the 'off idle' condition. That is when the throttle plate has opened enough to stop the draw of fuel from the idle holes, but not enough to start pulling fuel from the main venturi.

The sound of your problem is very much like the progression holes are blocked. I would pull the carb and clean it, paying particular attention to getting a decent amount of compressed air through all the passages.

Chris......

 

[Rick Stephens]

As well as the idle discharge holes, there are a series of holes above them, called progression holes. These progression holes supply fuel in the 'off idle' condition. That is when the throttle plate has opened enough to stop the draw of fuel from the idle holes, but not enough to start pulling fuel from the main venturi.
Chris......

Thaks for the deeper explanation Chris.

 

[achris]

No Title

I found a diagram in an old Merc training handbook....

 

[MikeDobbs]

achris thank you that is VERY helpful. Those "off idle" ports definitely sound like they could be my problem based on what I've experienced on the boat. I'll pull the carb tomorrow and see if I can get those cleaned out real well and see if it does the trick. I'll post back after I get her back out on the water.

Thanks so much!

 

[MikeDobbs]

achris Quick follow up if you don't mind. I'm not sure the picture you posted is of my carb. Looking at the engine from inside the boat, my carb has the fuel line coming in from the front (closest to you), and the idle mixture screw is on the right hand side. In the picture you posted it looks like the fuel and idle mixture are on the same side of the carb. Can you confirm whether this setup with the "secondary inlets" should correspond with my mercarb? I attached a link of the parts blow up diagram of my carb. Thanks!

 

[achris]

The picture I posted is from the Merc training handbook. It doesn't represent any particular carb, it's a generalization. It shows 'theory of operation', nothing more.

 

[MikeDobbs]

The picture I posted is from the Merc training handbook. It doesn't represent any particular carb, it's a generalization. It shows 'theory of operation', nothing more.

OK terrific. Thanks!