High Altitude Timing for 25hp Johnson

Hey guys this is my first post here on iboats. I have been browsing the forums for a while and it has been a great resource for me when I have to work on my outboard. I brought my boat from sea level to Colorado this year and I have had to make some changes to make it run better. I have a 1999 Johnson 25hp. I ended up going from a 10x13 4 blade prop to a 11x9 3 blade. I also rebuilt the carb and leaned out the idle a little and left the main jet alone.

I have a question about advancing the timing on my motor. I do most of my boating at 7500 feet on blue mesa lake, and occasionally at 9500 ft. I have searched the internet and called dealers and brp, and I can't get a straight answer. Everybody says that people advance the timing but talk to somebody local about it. Today I went out and advanced the timing 2 degrees. I am running 91 octane ethanol free gas. I immediately noticed that it planed off faster, gained 1mph (gps), and gained about 50 rpms to 5350 according to my tiny tach. It also seemed to run smoother. I was hoping somebody on here with some knowledge of running at high alititude could help me out and if 2 degrees of advance is a good place to be.

Thanks,
Adam

Re: High Altitude Timing for 25hp Johnson

Timing adjustments at high altitude is not needed. The timing is a function of compression, fuel octane and rpm. The original timing is specified with low octane fuel, and just by running 91 you MAY technically advance it and gain, high altitude or sea level.
Check the color of your spark plugs. If they are turning black, lean out the main jet. At your altitudes you may loose up o 10% of rated HP.

Re: High Altitude Timing for 25hp Johnson

At 7500-9500' 2* advanced timing is about right for a 4 stroke engine, though I'm not sure if/how a 2 stroke would be any different. You have dropped pitch, so that's good. Your carb needs to be leaned out on the top end though, that alone is killing performance. Contact BRP or your local marine shop for jet recommendations, they should have a chart for elevation/jet changes.

On my 4 stroke I/O at 9000' I bump the timing 2*, drop 4" of prop pitch and lean the carb's main jets. I've done this in steps over the years and the prop made the biggest difference, next the leaner jets, least improvement was the timing advance.

Good luck and don't run it at low elevation without reversing the high altitude changes, or you could blow it up.

Re: High Altitude Timing for 25hp Johnson

The ONLY change needed is jet change in carb. A normally aspirated motor will lose ~ 2.8% per 1000 ft above sea level, so you will be down 20% or more w/o a jet change. Timing should NOT be changed.

Re: High Altitude Timing for 25hp Johnson

At 7500-9500 feet you have less oxygen, this is the same as lowering compression. That means you do not need 91 octane and will find 87 is fine, probably even better. I have a boat I run at 7000 feet elevation, along with chain saws, and various othe 2-cycle power equipment, and my kids two 50cc 2-stroke bikes, and a couple other 2-cycle motorcycles.
We had problems fouling plugs until I lowered the octane to 87, and now they run better. On my vintage outboards, you adjust the timing all the time while running as they have an arm you advance as you open the throttle, so you open the throttle, advance the timing, then adjust the main mixture for the smoothest running, and fine tune the timing, so I don't see why you could not do this with your engine, but it would be a bit more complicated as you have to do it with the engine not running, making it more difficult to find that sweet spot.

Re: High Altitude Timing for 25hp Johnson

I would like to see a shop manual except that recommends a timing change for altitude, because I have never seen it anywhere. BTW, I have more than a few hours flying time in normally aspirated engine aircraft, and the ONLY change ever made with an aircraft engine is to lean it when in excess of 5000' MSL. Never any other change, PERIOD. My family operated a boat business for many years at 6850' MSL (N of SF, NM) and we did a lot of jet replacements for sea level motors coming to our area, but never once did we change timing.

Re: High Altitude Timing for 25hp Johnson

Thanks for the input guys, I am just having a hard time getting information about this. I was running 85 octane with star-tron, but found a gas station in town that has ethonol free gas and the only grade they carry is 91. I have pretty much given up on changing the main jet because I have called brp several times and they wont give advice or a part number they just say talk to a local dealer. Well it is a small town and the closest dealer is 3 hours away. I have called dealers around the state and they all have told me not to change the main jet only adjust to adjust the idle. It doesn't make any sense to me. I even went to a local mecahic in town and he told me to leave it alone. I have lost about 8 mph off my top end and I am just trying to gain back any power I can. I'll drop my timing back to the factory setting for now. Thanks for the help again and I will have to dig a little deeper into this.

Re: High Altitude Timing for 25hp Johnson

The main jet should be changed. The only other change should be a lower pitch prop to get the rpm up, since your motor is producing ~ 20%% less HP, and even less with the SL main jet. An OEM shop manual would tell the jetting story, if you can get access to one. No matter what you do, you will not get all the sea level performance back, so don't try. You now have a 20 HP motor where you live.

Re: High Altitude Timing for 25hp Johnson

The only way to get the power back is with boost, or nitrous, or nitro-methane, non of which you can do practically. At high altitude, marina operators use the next bigger engine and prop it accordingly.

I compensated for 7000 feet by reducing the prop size, and added a 15 hp carb to my 1975 9.9, which helped it because as a 9.9 it had a smaller carb.

Re: High Altitude Timing for 25hp Johnson

wilde1j said:

I would like to see a shop manual except that recommends a timing change for altitude, because I have never seen it anywhere.

Click to expand...

If you really, really , really want to see one, I can dig up a Mercruiser manual that has recommendations for timing changes at high altitude. It recommends less prop, leaner jetting AND advanced timing. Yeah, it's not a 2 stroke manual, but it can be done. Advancing the timing probably isn't wide spread since it can be difficult and introduces the potential for serious engine problems. That and, the timing change doesn't give as big of benefit as the prop change or carb jetting.

I do agree that the jets need to be leaned out and that would be my #1 priority. Look on Ebay for a OEM Johnson service manual specific to that OB.

Re: High Altitude Timing for 25hp Johnson

This topic was about 2 stroke outboards, not four stroke I/O's. I've read loads of OMC service manuals, changed a lot of jets, and never ever saw a word about changing timing with any 2s outboard. Screwing with outboard timing has the potential to wreck an engine, but hey, it's not going to be my engine.

Re: High Altitude Timing for 25hp Johnson

The only thing I can say about going to a higher altitude with an outboard is you may have to lean out the jets and get a lower pitch prop. Never heard of changing the timing. OMC had a service bulletin out on this years ago giving leaner jet sizes for different altitudes.

Re: High Altitude Timing for 25hp Johnson

I'm not an BRP guy but with the Merc's typically the gear case is a lower ratio and jets are different. The prop you choose should allow you to reach max RPM without regards to speed, hole shot, etc. Its far more important to not lug/load the engine with a 2 stroke. Lower octane fuel will also help. I ran 92 at sea level in my hot rod V-6's but ran 87 at 6000ft and it did fine without a Jet change. I would estimate about a 15% power loss occured but I considered that to be normal. If you want to run like a 25 at 7000 ft, get a 40, there's not that much weight difference and I see a lot of boats a high altitude that have gone that route and don't see anyone giving them problems about overpowering because they're not.

Re: High Altitude Timing for 25hp Johnson

Cool thanks guys. I went out tonight and put the timing back to the factory setting. It did seem to run better but it isn't worth burning up a piston over. My new prop gets my rpms where they need to be, I know it will never have the power it did at sea level. Hopefully the smaller jet will improve performance a bit. I would love a new e-tec 40hp but i'll be sticking with the 25 for some time. If the factory service manual has the recommended jet sizes i'll pick one of those up. I could use one anyway I have a seloc one and it isn't very good.

Re: High Altitude Timing for 25hp Johnson

True, this thread is not about 4 stroke I/Os. But it's not about aircraft engines either. Ajusting the timing on a 2 stroke is relatively difficult, on a 4 stroke it's relatively easy, probably the reason OMC doesn't mention (advise) it for their OB's. Besides, huge improvements are not made by adjusting the timing, so as I said earlier, the small improvements are not worth the big risk.

Re: High Altitude Timing for 25hp Johnson

A lot of interesting comments and suggestions, and I do not think anybody is either completely right or completely wrong.
It is a combustion, Otto, motor, 2 or 4 stroke does not matter.
Due to less oxygen molecules/volume, the only way to easily balancing out is to reduce amount of carbon (fuel).
Advancing timing, will increase time to burn the mixture completely and might be done (within reason) depending on the carbon-oxygen mixture. However a risky business 2 or 4 stroke.
Reducing octane might have an effect, it all depends on amount of carbon pr volume unit. This might be slightly different in different octane segments due to additives.
However the factory recommendation is simply to re-jet (reduce fuel) and most likely a prop change. Even OMC had high altitude gearboxes on some of the bigger engines. In the E-tech however the barometric sensor takes care of the 'problem' as for the mix setting, but not the propeller change.
Re-jet, but carefully watch the color of the spark plugs. They will tell when you get it right.
To add to 'funny' suggestions, Ive even heard people ripping out the thermostats so the engine runs colder, which demands a richer fuel/air mix!

Re: High Altitude Timing for 25hp Johnson

Alright so I finally found the information I needed. I figured I would post it here so maybe it could help somebody else out in the future. It is a 1999 25hp Johnson, the main jet is a 51D. The part numbers for the smaller jets are 50D - 326526. 49D - 324020. 47D - 328542. I am going to order all of them and slowly step down, checking my plugs as I go. Thanks guys this thread made me motivated to look into it again.

Re: High Altitude Timing for 25hp Johnson

The shop manual will specify the proper main jet for a specific altitude range. I would follow that, rather than experimenting w/o knowing which is right. If you get too lean, the result will be a burned piston. You can't tell just by listening to the engine when you get the correct one.