Altitude Issues

[Cariboo Milkman]

I am new to boating and have recently become the proud owner of a 1969 merc 65hp 4 cyl mounted on a 14 ft fibreglass runabout of probably the same vintage.
The motor seemed to run reasonably well out of water on muffs when I purchased it at sea level. I took it up to our cabin at 3600 ft and it has a lot of trouble getting boat to plane even with one person but when it does eventually get over the hump, it has no trouble getting up to what sounds like max. revs. I had a mechanic check it out and he said the compression was good straight across and carbs were not leaking (apparently a common problem with these engines when not looked after) He was actually surprised at the good condition the engine seemed to be in. He made what adjustments he could to timing, idle speed and mixture. I have a replacement prop (15 pitch vs the 17 that was on it) coming in a couple days and have been told this is pretty much a requirement for any boat taken up there from the coast. Should I also be looking at rejetting? Are replacement jets even available and how difficult are they to change? I am fairly mechanically inclined. Thanks in advance for any advice.

 

[fishrdan]

Re: Altitude Issues

Try the lower pitched prop and see how it does, that's the first thing to do anyway. Unless you are going to use the boat exclusively at that elevation I wouldn't swap around the jets. And, I don't think it's recommended to replace jets until you get up to around 5000', at least that's my perception.

Power loss at elevation is about 3%/1000', so your 65HP is only down 6-7HP, small difference. If you were going to 9000' that would be about 25% power loss, huge difference. I wouldn't do the jets for 3600'.

 

[Gomer50]

Re: Altitude Issues

Yes the jets can be replaced,but if I were you I would wait for that 15 pitch prop to show up and run it, you will have a better holeshot and end speed might drop a few MPH not much.If your holeshot suffers you might be running a little lean so with a screwdriver on each carb open up the idle mix screws another 1/8 of a turn and give her a shot.repeat if needed.Now if all fails maybe a rejet is in order.

 

[Cariboo Milkman]

Re: Altitude Issues

Thanks for both of your replies. The boat will pretty much spend it's life up there as long as I own it so if the prop doesn't give the hoped for results, I may look at rejetting in off season. Are jets still available and how do I figure out what size to change to?

 

[Gomer50]

Re: Altitude Issues

I would suggest you get yourself a genuine mercury repair manual for your model and year worth it's money in gold.Will tell you everything you want to know.

 

[rivermouse]

Re: Altitude Issues

The octane of the gas that engine was made to run on and that which you are using most likely is not the same ( cant get leaded gas now you know) and this might be part of the problem.

 

[fishrdan]

Re: Altitude Issues

I wouldn't worry about the jet issue until you see how it runs with the lower pitch prop. I don't even worry about leaning things up until I'm over 5000', that's where I notice power dying off with sea level jetting. Being a 69 jets could be hard to come by, you could check Crowley Marine and see if anything is available in their parts breakdown. If they say NLA, you are probably SOL on the jets. Only luck from there would be calling around and doing a bunch of internet searching with the P/N (P/N from Crowley if they have it). I've found some NLA parts by doing extensive searching.

If your holeshot suffers you might be running a little lean so with a screwdriver on each carb open up the idle mix screws another 1/8 of a turn

At altitude he would be running richer, and would need to close the idle screws a bit, if needed.

 

[Gomer50]

Re: Altitude Issues

You know what Fisherdan is right.I got to thinking about that after a bit but before I could get back to edit my post fisherdan got me.

 

[Cariboo Milkman]

Re: Altitude Issues

I tried the new prop on thee weekend. Only had a few minutes available for test and it seemed to make some difference but still takes a lot longer to get out of the hole than I would expect. When I have more time I'll have to try the idle jet adjustment. I think I'll also try to locate smaller jets but I guess I'll need to get the others out first to see what size is in there now?

 

[Cariboo Milkman]

Re: Altitude Issues

I had time to run the boat for over an hour on the weekend. It starts and idles fine. When I give it throttle, either gradually or abruptly, it seems to just "bog down" and not get over about 1/2 max rpm for up to a minute or so and then gradually gets to planing stage and then accellerates at what seems to be a normal rate. Top speed seems fine. With more than myself, my wife, and the dog in the boat I have my doubts it would have ever got to planing. Once it gets to that stage throttling seems to operate normally as long as it doesn't slip below the planing, otherwise it takes awhile to get up to speed again. Does it sound like rejetting is in order or something else? I wasn't able to locate the idle mixture screws to try that. I haven't got a manual yet but probably will when i bring the boat home for winter. Thanks for any help.

 

[fishrdan]

Re: Altitude Issues

It starts and idles fine. When I give it throttle, either gradually or abruptly, it seems to just "bog down" and not get over about 1/2 max rpm for up to a minute or so and then gradually gets to planing stage and then accellerates at what seems to be a normal rate.

Sounds very familiar, my old boat would do the same thing with too much prop, struggle to get on plane. I had to drop from a 19 P to a 15P, but that was going from 1000' to 9000'.

I think you need a prop with even less pitch, to get "over the hump".

 

[DavidAK]

Re: Altitude Issues

Don't waste your time or money replacing carb jets at 3500 ft. 15p prop should be correct. 14ft boats don't plane well at lower speeds, try one of those fins that attach to the cavitation plate. Your mechanic said that the compression was good on all cylinders? What was that reading. 100lbs is marginal. A good engine is 150lbs +/- 15lbs.

 

[Cariboo Milkman]

Re: Altitude Issues

If I recall correctly, he wrote 115 psi straight across. I don't have his note anymore. Although not great, should be acceptable since even all across?

 

[j_martin]

Re: Altitude Issues

Here's my advice. Get the factory service manual. Check the basics, like compression, hose routing, bleed line check valves, and maybe even go through the carbs. Then go through the linkage and timing adjustments, in the sequence in the book, and making sure you understand and make each adjustment correctly. It can be a bit intimidating to a freshman, but it's really not all that complicated.

Odds are good you'll find several things that are just not right.

If I were to guess, I'd say you have a fuel delivery problem, or a timing problem, or a main jet problem, or..........you get the idea.

Real even but low compression usually means you're looking at the wrong book, usually Seloc.

hope it helps
John

 

[CharlieB]

Re: Altitude Issues

Like John said, the VERY first thing is to get your hands on a FACTORY service manual.

Check these sites to see if yours is available,

http://1manual.com/Mercury manuals/Service_Manual_Index.html
http://forums.iboats.com/showthread.php?t=309079&p=2072211#post2072211

Next, get an inductive tach so you can check your WOT RPM, this is a critical # used to determine performance. Make NO CHANGES to your motor other than the spelled out adjustments in the service manual, carb link and sync, ignition timing.

Once Adjusted to factory specs AND you know the WOT RPM you can select the correct prop for your boat/motor combination.

Bogging, hesitation on acceleration is often a sign of a lean idle mixture. Idle mixture should be adjusted IN the water, idling in Forward gear, adjust to 'best' idle speed, PLUS almost 1/8 turn out (richer) to provide the additional fuel for acceleration.

If your mechanic made all adjustments 'on-the-hose' it is possible that the idle mixture is set too lean. Turn the idle mixture screws out in very small amounts and test holeshot, if it improves, continue making the adjustments until holeshot is perfected.

 

[Cariboo Milkman]

Re: Altitude Issues

Thanks for the help guys. I tried both of the links but neither list my motor-1969 4 cyl 65 hp. Any other ideas where I might look?
Thanks, Lorin

 

[Star]

Re: Altitude Issues

Why is it that all of the responders have not stated that a good running 65hp Mercury should make any 16' fibarglass stand on it's
head with any prop!!! Milk you are missing some of your cylinders!
Spark problems I would guess not altitude.

 

[CharlieB]

Re: Altitude Issues

lol
Star is right, usually that is the first thing John and I point out. John said check the BASICS, and spelled out most, just didn't mention spark this time.

Oops for both of us.

 

[Cariboo Milkman]

Re: Altitude Issues

Thanks everyone for the advice. I'm planning on bringing the boat back to the valley for the winter in a few weeks, but I'd like to try to find out if I should look at rejetting before next season. Would appearance of the spark plugs tell me? What should I be looking for?
My plan is to bring it home and over the winter get a manual and go through all the basic troubleshooting / repair and replacement proceedures recommended to get it back in good operating shape. It will be going back up next year and will always be used at that elevation.
Is there an easy way to check if I have spark problems in the meantime? I'm pretty sure it is firing on all 4 once it get's up to speed, but it makes sense that not all are firing, at least at full efficiency before it get's there. Any and all suggestions would be welcome.
Thanks.

 

[j_martin]

Re: Altitude Issues

Go back and re-read posts 14 and 15.