1969 25 hp Johnson, poor higher RPM performance.

[crappiechaser]

I'm new to this posting stuff, but have been gleaning alot of information from all of you for awhile.
I recently came into a 1969 25 hp Johnson, model# 25R69D, it's on a 1969 Glastron 147 AV. Motor has had electric starter added (no charging coil installed), and long shaft conversion. I am very disappointed with high RPM performance. It does not seem to develop any where near the prescribed 5500 RPM as stated in my Clymer manual, my best guess would be less than 4500 RPM, and probably does not exceed 10 - 12 MPH on the water. I have done a considerable amount of work on this boat/motor trying to fix it, or at least improve it. Stringers, flotation and floor are new and hull is watertight. New floor plan is wide open space, I have removed the casting decks, console steering, livewells, rod lockers and most storage compartments...no eccess baggage/weight here. Compression check is 93 and 95 PSI, a couple squirts of oil in cylinders bump it to 95 and 97 PSI. I have rebuilt, replaced or repaired the following: carburetor, points, plugs (champion J8C), fuel pump, all fuel lines and primer hose assy., fuel tank, impeller and lower unit bearings and seals. Cavitation plate of motor is about 1" below keel of boat. I replaced the original 9 1/4" X 11 prop with a 9" x 10 prop. I have quicker acceleration now, but motor and boat speed surges at WOT and motor still does not appear to be anywhere near 5500 RPM. Motor starts and runs well at lower to mid ranges, maybe a little uncooperative at idling speeds.
Questions:
What to check next or re-check?
How can I temporarily install a tach for accurate RPM reading?
Any suggestions on prop selection?
Is this maybe reed valves or low vacuum in crankcase?
Not looking for a water rocket here, but it would be nice to put a breeze in my hair while I still have some. Any input or suggestions are appreciated.

 

[Chinewalker]

Re: 1969 25 hp Johnson, poor higher RPM performance.

First thing I would check would be that you're running on both cylinders...

 

[nwcove]

Re: 1969 25 hp Johnson, poor higher RPM performance.

id check the timing and spark. you can install a tiny tach, some people like them, others ..not so much. jmo, but you should be able to get about 20mph out of it.

 

[crappiechaser]

Re: 1969 25 hp Johnson, poor higher RPM performance.

Throws a good blue spark from both coils, and I'm reading good compression on both cylinders. A little cold here to get on the water and pull plug cables running on the lake. I intend to do that as soon as weather permits.
I just checked static timing with an ohmmeter this past weekend. Both sets of points are opening right on the money.
Disassembled carb, today hoping to find a piece of crud in high speed jet or bad float setting...everything looked good.

 

[bonz_d]

Re: 1969 25 hp Johnson, poor higher RPM performance.

On these old motors there is a nylon pin that goes through the throttle control lever. the part number is 0304155 and is shown as part number 23 in the PRB parts breakdown. Check that along with the bushing it passes thru. On all the 18-25hp OMC's that I've worked on that pin has been either broken or worn out and it will prevent the throttle from openning all the way.

 

[TN-25]

Re: 1969 25 hp Johnson, poor higher RPM performance.

I am just throwing some ideas out there besides what others may post. How heavy is your boat? When I think of Glastron, it does not conjure up images of a lightweight boat. What is the boat?s horsepower rating? The lightweight 25 you have is a great motor, one of the best, but under a heavy load they perform like an 18. On the right hull where they can easily get up on a plane they can really wail, delivering 30-mph performance. All from a motor a guy can carry himself if he is in decent shape.

If your motor is a little tired or in need of rings and/or a complete tune-up (including link n sync), it might just be enough to make it sluggish and unwilling to get it ?over the hump? and onto a plane.

You mentioned that you recently came into this rig. Did the previous owner just pair up the motor onto the boat, or had it been rigged this way since it was new? If the latter I would expect the motor to be rigged and propped properly, but if it was recently paired, it is possible that the motor is ?over propped?. The motor will lug and be lazy, and may even overheat while struggling in vain to reach the proper full rpm operating range. In essence the 25 makes its power by being a high-revving 20 horse. It needs to be propped properly in order to perform.


Electric start was an accessory for your motor in 1969. There never was any provision for battery charging unfortunately, but if it starts without too much cranking you should be fine between charges. Just trickle charge it when you can, or consider one of those solar battery chargers (there?s an idea!)

 

[crappiechaser]

Re: 1969 25 hp Johnson, poor higher RPM performance.

TN-25,
I haven't wieghed the boat, so I don't know exact weight. I do know it is a heckuva lot lighter than my buddies 16' Fabuglass with a 25 hp Merc, and he'll run circles around me.
The boat is rated for 50 hp maximum. The original owner died and boat was given to me by his son after sitting for a few years, so I have no information on prior performance. I assumed it is original set up. I have replaced rotted flooring, stringers and water-logged floatation and boat is at a minimal weight.
I suspected the propeller to be the main culpret, but did not notice much change when switching to the 9" X 10, I am open for suggestions. The 10 pitch helped the acceleration, but this is when the suging started. Just before the boat gets on plane, the RPM's drop off, then start to build again, then fall off again...almost makes me sea sick.
Compression is around 95 PSI for both clinders, not so bad. The throttle butterfly is 100% open when throttle cam follower is on high lobe of throttle cam and control is fully turned.
Could this be a reed vavle problem or crancase vacuum problem? How to test/troubleshoot for that?

 

[Chinewalker]

Re: 1969 25 hp Johnson, poor higher RPM performance.

TN-25,
Could this be a reed vavle problem or crancase vacuum problem? How to test/troubleshoot for that?

Probably not. Reed issues would result in hard starting and fuel blowing back out the carburetor. Bad crankcase seals would result in hard starting and poor idle.

When the cold weather breaks, do the check to verify that you're running on both cylinders and go from there before spending anything on props.

 

[jbjennings]

Re: 1969 25 hp Johnson, poor higher RPM performance.

I personally think that the boat is still just too heavy to get up to the proper rpm's, or that you're running on one cylinder.
Even though the boat may be lighter, it is probably pretty darned wide and therefore creates lots of friction on the water. That's just one of the reasons it probably came with a 50hp motor. If it's a trihull, even moreso on the friction thing.

"The 10 pitch helped the acceleration, but this is when the suRging started. Just before the boat gets on plane, the RPM's drop off, then start to build again, then fall off again...almost makes me sea sick." I reread your post again and noticed this. Surging is either a coil going in and out, a vacuum leak, or a weak fuel pump.

I would do the cylinder drop test, check all fuel line connections for air leaks, squeeze the primer bulb when it starts surging and see if anything improves. The surging thing is definitely a signal your motor has a problem(running lean) and that very well could be why it's not getting full rpm's. 95psi isn't just super for that motor, either, but I doubt it's your problem. That motor probably had about 125psi when brand new.
I also think that a newer merc 25hp is quite a bit faster normally than a '69 25hp OMC---probably as much as 3-5mph faster.

 

[crappiechaser]

Re: 1969 25 hp Johnson, poor higher RPM performance.

Here's the latest information if anyone is still following this. The boat is heavier than I thought. Boat, trailer and full compliment of gear weighs in at 1200 lbs (best guess at trailer weight is about 300 lbs). It is an old tri hull design, it's 14'6" bow to transom and has a 60" beam. I know this 25 hp won't be winning any races, but I think it should do better than it is now.
Me and my son went to the lake today to do some troubleshooting. We put the 11P prop back on because the motor started the surging problem when I put the 10P on. Started with adjusting the carburetor and I am pretty satisfied with low and mid range performance, idles and accelerates fairly well.I got a wireless handheld tach and it showed a max RPM of 3200 and GPS clocked us at a blistering 11 MPH. I pulled the plug wires off at WOT and had the same effect dropping each cylinder, so I believe both cylinders are sharing the load. I used an automotive timing light and at WOT both cylinders were firing within the 2 timing marks. I also pushed the carb throttle lever by hand to see if it was fully open, it moved ever so slightly and did not increase RPM, so I think all is good with throttle linkage. Adjusting trim angle of motor did not significantly affect motor or boat speed. I squeezed primer bulb several different times and that didn't help and it was always firm.
We put the 10P back on for comparison and the surging did not occur. Motor came up to a little over 3400RPM and best speed was only 10 MPH. The motor never missed a beat with either prop at WOT, it just won't develop the RPM's. I have a little bit of mis fire accelerating from mid range to WOT, but everything else is smooth.
Questions:
Is the weight /hp ratio causing the low RPM problem?
Is there anything else to adjust on carb beside low speed screw?
Can I up-size my non adjustable high speed jet?
Would 95 PSI compression on both cylinders warrant an overhaul?
I have only seen a 10 and 11 pitch offered for this motor. I have both of these, but neither one is in very good shape...nicks, chips and dings. Any suggestions?

 

[bonz_d]

Re: 1969 25 hp Johnson, poor higher RPM performance.

Sounds now like you are underpowered and over proped. Check here in the iboats store they have 4 props listed for the 1969 25hp.
Smallest pitch prop is a 9.25"X7 pitch, next is a 9X9, next is 9X10, last is 9.25X11

You already know you are not getting to RPM range with either the 10 or the 11 pitch. My guess would be the 7 with the amount of weight you have. I run an 11 pitch on a 35hp Evinrude pushing a boat half your weight.

 

[Chinewalker]

Re: 1969 25 hp Johnson, poor higher RPM performance.

Go WAY down in pitch. As Bonz_d recommended, go with the 7 for starters as you're only spinning a little over half the recommended RPM as it is with the 11. Regardless of where you end up, you're pushing a lot of boat with a relatively small motor. Best bet might be to sell the 25 and put the proceeds towards a 40 or 50...

 

[crappiechaser]

Re: 1969 25 hp Johnson, poor higher RPM performance.

I think if I could get 15 mph out of the boat, I would make do. The lakes I fish aren't that big. I'm not ready to buy a bigger motor (yet), I've got my eye on a side scan sonar outfit.
I guess I was searching a specific manufacturer earlier when looking at props. What are the chances of increasing boat speed by reducing pitch and increasing RPM's?
Anybody have any info about changing orifice size in high speed jet?

 

[bonz_d]

Re: 1969 25 hp Johnson, poor higher RPM performance.

The high speed jet in that carb is fixed, cannot be changed.

On the prop you have look around on the hub and see if there are extra numbers stamped in it. That would indicate that at some point it has been reworked and may not even be the size it is supposed to be. Slipping even more than it should.