Sudden drop in rpm’s

[dougs-outboards]

So, I have a 1966 15′ fiberglass Glastron boat, and running a nice 1957 Johnson 18 hp long shaft on it. It’s a 20" transom boat, so takes a long shaft. My motor has a whopping 140 lbs compression, and looked almost new when I got it from a dearly loved aomci member. Had the original cracked coils etc.

I did a full meal deal on it, new ignition, carb rebuild, water pump, the works. Starts and runs great on one partial pull in the bucket. ( I always told customers after a ‘one partial pull start’ that "looks like we got dennis"… They say "dennis?" And I say "yep dnes"… "That’s Damn Near Electric Start" har! Made it up myself, gets laughs every time…).

Anyway, took it out for the maiden voyage (this was a couple of years ago), I was alone, and boy the boat took off and actually pretty much planed out, (it’s a little underpowered with the 18). I was zipping along at certain higher rpm, and suddenly the top speed went from 100% to about 80-85% speed- same throttle position. I checked and it was still getting full throttle position, motor was hitting the stop.

So I rebuilt the carb again, checked and re-checked the ignition system and settings, changed gas, (non-eth gas) and did everything there is to do concerning basic parts install and tune up proceedures. I have done 100’s of motors in my time, and humbly consider myself to be very good at what I do, if I do say so myself.

So can anyone think of anything else why the drop in rpm’s? I have readjusted and re-did the carb, link and sync and everything a few times over the last couple of years, never to get that higher original rpm back. With the underpoweredness, it was a great speed at first for 2-3 minutes, and I was very satisfied with the performance. Now I am stuck with what I know to be a motor not living up to it’s potential as I had known it. Still runs great, is getting full throttle and still at the lower rpms when I cruise.

Point of interest- It had a 9×9 prop to get upper rpm’s out of it, (9×10 was too high a pitch) so it’s the proper pitch on my boat. Have even tried a spare prop- same results.

Thanks for any imput, I am still scratching my head here…

[chris-p]

Well, assuming all the normal things are as they should be… Have you tried running with inline neon testers, to ensure good fire while underway on both cylinders?

When you pulled the carb, did you have a peek at the reeds? stuck open, stuck closed, etc…

Did you pull exhaust cover, and ensure nothing blocked, or partially blocked the exhaust route, limiting power?

[rudderless]

Is that 140psi now or a couple of years ago? I’m not sure of the ignition configuration, maybe a partially sheared wheel key..It did happen suddenly so Id look for a sudden change in timing or fuel delivery.

[dave-bernard]

how about a air leak fuel issue

[billw]

I guess I would be thinking reeds, too……but maybe more along the lines of something getting sucked in there at WOT and partially blocking the inlet hole on one or two of them?

I take it that it still idles very well?

Long live American manufacturing!

[fleetwin]

I am a little confused, maybe you can clarify for me. "….Is getting full throttle….", does that mean that at WOT throttle it is running properly, or just that the sync/link is correct? In other words, is this loss of power at just 3/4 throttle, or at WOT as well?

[dougs-outboards]

Thanks for all the responses guys…

Chris_P- I looked at the reeds and they are perfect, I don’t have neon testers but always wanted some- will get a pair, and I did not pull the exhaust cover yet. I did decarb it with sea foam when I first got it running.

The thing starts and idles so well, as normal except for the slight loss of top speed, which I would never have known about had I not been cruising and notice it happening. It was just one smooth instant loss of power and sound, didn’t skip a beat, kept on tunning as tho the new lower speed was the top speed it was supposed to go, only I know better having been thrilled with the original speed for 2-3 minutes or so.

Rudderless: That was then and now on the 140 psi. I have inspected the flywheel key the 5 or so times I have re-checked the ignition, so it’s good.

Bill and Betty, thanks for your comments too.

Fleetwin: Getting full throttle in this case I meant the lever is in full forward, the motor is hitting the full speed stops, I am going as fast as the boat will go, and suddenly it’s as if after 2-3 minutes of fast run time, it’s as if I had grabbed the throttle (which I did not) and suddenly pulled it back to say 80% (or about 3/4 position), even tho the throttle was still totally in forward position, After that, I came to a stop, turned off the motor, put in in full forward, (I am running it from a control box up by the steering wheel) checked the motor and everything on the motor was in full forward position, and the notch on the mag plate was butted up against the high speed stop as it should be. I guess ‘loss of power’ isn’t the best description, it would be ‘loss of top speed’, I guess. I just mentioned that the link and sync is correct as I checked all the other settings too, carb and ignition.

I do have another identical carb, I could try that I guess.

I just wanted to pick you guy’s brains a little. I will look into all the things you said as I go along here.

Thanks all!

[fleetwin]

One more question, sorry…Does this process repeat? In other words, will the engine reach proper WOT RPM now, then back off after 2-3minutes? Or, did it just happen that once, and the engine has never reached proper WOT RPM since that first time?

[chris-p]

Im still wondering about an exhaust blockage. Perhaps the seafoam dis lodged a chunk of carbon, which is now blocking the flow, limiting power?

[dougs-outboards]

Fleetwin: It just happened the first time out, and it’s been running at the lower speed wot ever since, never could get the rpm’s back up again to the original higher rpm’s.

Chris_P: That could very well be. In the past I have broken screws/bolts on the exhaust cover, so kind of gun shy here… Perhaps you could toss me a couple of veteran tricks that you have learned in your experience, please. Then I will try it.

On a side note, thinking about the carb, I can’t remember which way the spacer goes on the low speed needle packing. I know there is a spacer with one end cupped, then the plastic and 2 packing washers. Which way does the cupped end go into the carb?

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