High altitude timing

[redlaw]

I have a carbureted 5.0L G+ Alpha Mercruiser 220 HP 305 cid sterndrive engine. Thunderbolt ignition, ignition timing 10 degrees BTDC. Run at 6000 ft elevation. ​Stock main jet 1.65 mm changed out to 1.45 mm. Power valve stock jet .90 changed out to .74 Stock gear ratio of 1.62:1 changed out to 1.81:1.
Have two props, three blade 15Dx15P and 4 blade 14.5Dx17D. 15x15 wot rpm = 5300. 14.5x17 wot rpm 4600. Max WOT is supposed to be 4400 to 4800 rpm. Can't get over 32 mph in my 2400 Maxum SCR.
Does the timing need to be advanced? Help!

 

[NHGuy]

No that's the right timing. You have a big load of boat for a 220 horse (sea level) engine.
You are running at 6000 feet elevation. Of course your boat won't go that fast.
I'd inspect the plugs, if they are too white add some jet or metering spring. Who decided how to jet the carburetor?

 

[bruceb58]

Typically, high altitude fuel is lower octane so you should leave the timing.

 

[redlaw]

I chose the jetting after internet info and forum info was obtained. I also contacted Mercruiser. What is "jet or metering spring"?

 

[bruceb58]

Can't get over 32 mph in my 2400 Maxum SCR

Based on your prop pitch(17") your drive ratio and your RPM, your speed sounds about right. It is what it is. You have a very small motor in a big boat.

Too bad Bayliner/Maxum sold such small motors in those boats.

 

[NHGuy]

That's barely on plane. i'd recommend getting a boat with more engine. The cost to change to more power is too high.

 

[alldodge]

Howdy

With your timing being at 10 BTDC I'm going to assume your motor is between 1998 and 2001

Merc recommends at bot of your size with an Alpha drive, and that altitude be using a 1.84 ratio
http://www.boatfix.com/merc/bullet/93/93_07.pdf

Jetting and metering spring is to increase the jet size to give the motor more fuel, or changing the metering to allow more fuel. If you can tell us which carb you have we might be able to provide the info

 

[bruceb58]

Get some Bennett trim tabs too if you don't have them. Don't wast your time with Smart Tabs on a boat your size. You need as little drag as you can get.

 

[redlaw]

Carb # 86144 8N also has # 25 0107. Yeah, can't believe that such a small engine is in this boat. Same with all the Maxum 2400's I have seen. Ran great at near sea level when I bought it. Did not know about power loss at altitude. Bought the boat to keep at Lake Tahoe California. My ignorance, my problem. The boat gets up on plane reasonably well, planes at about 15 mph and I can cruise at 25 mph at about 4000 rpm. Seems high and I don't want to over stress the engine. Am looking for another boat, but would keep this one if I could get 4 or 5 more mph out of it. What about the stainless steel double props I see on some boats? Would such a prop add the mph I am looking for? The boat has trim tabs but not working, leak in hydro line. Plan on fixing if I keep the boat. Getting on plane is not the problem, cruising speed vs rpm and top speed are the issues. Thanks to every one for the information. Keep it coming. I want to see if I can take some reasonable action to cure the problem and keep this boat.

 

[stonyloam]

Yeah, with the 15" prop @ 5300 rpm, 12% slip, BIG BOAT, you are looking at 36 mph max. So the speed you are seeing is not unreasonable. The 17" @ 4600 would be the same 36 mph.

 

[alldodge]

You should have the Merc 2 bbl carb, original part number 861448A1 and new number 864942-A01

Came with 165 mm jets (sea level to 5000 ft), high altitude (5000 to 9000 ft) jets should be 150mm p/n 3302-811852

http://www.marinemechanic.com/merc/distributors/mercurymarine/sterndrive/mercarb.pdf

 

[redlaw]

I put in 145 mm per the recommendation from Mercruiser and info on internet. What will the 150mm do for the boat?

 

[bruceb58]

Bought the boat to keep at Lake Tahoe California.

I have a house on Lake Tahoe. 5% power loss per 1000' so you have over 30% loss since the lake is 6200' or so. With the 7.4L in my boat, I don't have to change props but I can tell there is a huge power drop off.

Too bad you have an Alpha because a bravo dual prop would help.

Where in Tahoe are you?

 

[alldodge]

I put in 145 mm per the recommendation from Mercruiser and info on internet. What will the 150mm do for the boat?

If Merc indicated 145 mm I would stick with that. My reference came from an older bulletin in link above

 

[redlaw]

Bruce, we have a house in the Keys on Venice, lagoon side. Where are you located?

 

[bruceb58]

Bruce, we have a house in the Keys on Venice, lagoon side. Where are you located?

Same....On Venice on the lake side. When you say lagoon, you mean no access to lake right?

 

[redlaw]

I contacted Merc again. Different tech. He now tells me my engine (I gave him the #) either came with a 185mm or 170mm main jet. Now says I should have a 175 main if stock was 185 or a 160 main if stock was 170. Now I am really confused. Merc told me before my stock main was a 165.. This guy says they didn't ship with 165's. I have the jet I took out of the carb in Tahoe. Going there this Friday. Will check and see what is what. Any other info or advice.

 

[alldodge]

Will be interested in what jet is in there

Two techs and both have different answers

 

[redlaw]

I have a 145mm in right now. Had a 165mm before.

 

[bruceb58]

Probably would be worthwhile doing a plug read. Run your boat at WOT out in the middle of the lake and then without letting it idle much at all, shut down and pull a few plugs.