RD-17 bottom plug fouling – no high speed power

[The red boat]

I recently converted my 1955 Johnson RD-17 to electric start. This necessitated several parts swap-outs including using a different carburetor with electric choke.
The previous set-up ran perfectly before this conversion but I preferred electric start.

I bought a used carb and rebuilt it and put everything back together.
I tested it in a barrel at home and it started right up and idled perfectly and had smooth low-range operation.
So I took it to the lake yesterday to get high-speed dialed in and do a fine-tune on the carb.

Once again, it started and idled perfectly and slow-speed was flawless leaving the dock. Out on the open water though it would barely run when trying to go high-speed. It smoked like a chimney and sounded like it was bogged down (flooded) and the engine rpm’s would never increase. No amount of turning on the high-speed needle made any difference at all. It was leaving quite an oil slick.
Keep in mind that it ran perfectly on the original carb, the cam is indexed, all ignition components are perfect.

So I said a few words my Grandma wouldn’t approve, drug it out of the water and took it back home for a tear-down.
I discovered that the bottom spark plug was completely black and fouled. It was perfect before I started using this current carb. Upper plug looked perfect.
I tore down the carb and everything checked out but the high-speed needle end was pretty blunt looking so I switched it out for another that had a finer point to it.

I put it back together and into a barrel. Started right up and idled as good as before. This time I revved it up a bit and it seems to increase the RPM’s as it should. It didn’t smoke nearly as much and I could get the motor to much higher RPM (as much as I could in neutral anyway).
The lake is an hour’s drive away so I ran out of weekend before I could get it back on the water but I’m pretty confident the high-sped needle was the problem here.
I checked the (new) plugs after running it in a barrel for a good 10-15 minutes and they looked good.

Do you all think it was as simple as the high-speed needle not seating properly and flooding the motor and fouling the plug? The newer high speed needle was only slightly more pointed – its hard to see that it would make THAT much of a difference.
Why would only the bottom plug be fouled?
Am I missing something obvious?

thoughts?

[fleetwin]

Well, there are a few conflicting symptoms you point out here….
If the carb is at fault, why is just one plug so messed up? I don’t know where you got the replacement carb, did it have a nozzle gasket? Once again, a carburetor issue is not going to affect just one cylinder. Are you running this engine on its pressure tank, or have you converted it to single line/fuel pump? If you converted it, perhaps the fuel pump diaphragm is leaking into the combustion chamber.
You report that the engine seems to idle fine, but it sure seems like it is running on just one cylinder at high speeds. Reed issues, or internal damage would surely affect the idle quality first.
So, if everything you report is accurate, I’m thinking you are losing spark on that cylinder when the throttle is advanced. Perhaps the vacuum switch isn’t working correctly knocking out one of the cylinders….
Are you sure it is idling on two cylinders?

[The red boat]

thank you for the reply. That was very helpful.
I’ll verify the things you mentioned.

– the rebuild kit i used on the carb included a nozzle gasket. I verified that it was seating properly when i opened the carb back up.
– i’m running it on the pressure tank
– how would one check the vacuum switch? is there a way to do that? I’m still leaning toward the carb as the culprit since it ran perfectly before I changed the carb out for a different one. But I would like to check that vacuum switch too!

I am almost positive it is idling on 2 cylinders but I will verify. I’ve pulled the plug wires before to check it and remember how poorly it idled. It isn’t doing that currently but it would be good to check it again.

When i swapped out he high speed needle and ran it in the tank, it seemed to be able to achieve the proper higher rpm so it would appear that whatever was ailing it is working now. Both plugs look fine after 15 – 20 minutes in the tank at various rpm speeds.

the one plug being fouled and the other one being merely “wet” is what stymied me.

I’m taking it back out on the water this weekend to see how it goes.

[fleetwin]

It just doesn’t make sense that a carb problem would affect just one cylinder on a single carburetor engine….
I know the plugs “look bad”, or at least one of them does, but is it running on two cylinders in gear at high speeds? Sounds like you are running it in a test tank only, do you have a test wheel, or are you running it with the prop? Trying to run that big engine in a tank with the prop while in gear is near impossible, it will just throw the water out of the tank and ventilate…So, if this is what you are doing, seems unlikely you are getting the high speed mixture correctly set. I think the best bet is to take it to the water this weekend….
Just for grins though, check how many turns the high speed needle is set out from gently seated now. If the needle is set a couple of turns out, then I am confident the test tank is confusing things and you will get the needles set properly once on the boat. Now, back to the nozzle gasket…Is this carburetor the exact replacement for the 1955 RD? Or is it from a newer engine that had two needle valves instead of a fixed high speed jet? The reason I ask is that many of the older carbs with an adjustable high speed needle did not use a nozzle gasket. The bowl nozzle tube fits pretty tight against the carb body boss, I guess there is so much fuel going through there, the little that might leak by is negligible. So, my concern is that adding a nozzle gasket where none is called for might either damage the nozzle gasket and have it fall apart….Or, perhaps the nozzle gasket might prevent the bowl for positioning properly on the body….I guess the only way to know is to pull the carb bowl and see how the nozzle gasket looks.
The vacuum cut out switch is wired into one set of points to kill spark to that cylinder if the engine “runs away” in neutral, which these engines had a tendency to do if over revved in neutral…The vacuum switch closes, kills spark to one cylinder, allowing the engine to settle down, the vacuum switch to open, then spark returns to that cylinder.
At this point, I think it is best to take the boat to the water to determine if it is running on two cylinders are high speed….This could just be a case of attempting to run and adjust this big engine in a tank, which is near impossible without a test wheel…

[The red boat]

i probably am not being very clear so I’m sorry about that but I appreciate your replies.
Let me see if i can clear it up.
When I ran it on the lake last weekend, it seemed to only run on one cylinder and only at high speed. Idle and slow speed seemed perfect. It has never done this before with the previous original RD-17 carb.
When I pulled the plugs, the bottom one was fouled but it was fine before the lake run.
the carb I rebuilt was an original from an electric start RDE-17. I bought a carb rebuild kit that included the cork seal where the nozzle seals up against the bowl and it is installed properly. That is what I think you are referring to when you say bowl nozzle tube. The carb had one when i dissembled it (as have all the RD-17 carbs I’ve rebuilt) so I replaced it with a new seal from the rebuild kit.
The high speed needle is approximately half a turn out from seated at the moment (adjusted in the tank). when I was on the lake, it didn’t matter what I did to the high speed needle since I think it was running on one cylinder. More on that at the end of this response in terms of the cut-out switch.
I only put it back in the tank to see if I could achieve any kind of higher rpm at all since it wasn’t happening at the lake. So I put it back in the tank after replacing the high speed needle to see if it made a difference (it did).In the tank I WAS able to get some higher revs in neutral so I agree – need to get on the water again for a real test. I only use the tank to make sure it starts but i always set the needles while on the lake.

cut-out switch question:
I tested it last night by removing it from the boat, and hooking it up to a multi-meter.
as it sat on the bench, there was no continuity between the wiring post and the body of the switch. When I applied vacuum, the resistance measured approximately 1 ohm.
when i reinstalled it on the boat and hooked up the lead wire to the post, I checked it again and saw that I had resistance though the entire assembly.
SO my question is: when it is installed on the boat, should I have resistance like that and does that mean the points are being grounded? I would assume that the post would still show no continuity with the switch housing until vacuum flexed the diaphragm and competed the grounding and cutting out the set of points. Maybe I have a bad switch that is constantly grounding out the points?
Like I said – it tested properly when disconnected but seems to be grounded when I connect the wire (not running motor of course).

sorry for the long response – i do appreciate the help

[frankr]

You answered your own question. You are reading continuity to ground through the points (if closed) AND through the coil primary, even if the points are open.

• This reply was modified 4 years, 11 months ago by frankr.
• This reply was modified 4 years, 11 months ago by frankr.

[The red boat]

thanks Frankr

so with the wire connected to the cut-out switch, it should read continuity. and that’s correct?
Are you saying that it is acting properly then?

to break it down further: when I have the switch installed on the boat and everything hooked up properly, what should my ohm meter show when i touch the post with a lead and touch the switch housing with the other lead?

In my mind I would think there would be no continuity until the vacuum is introduced. then the switch would create a ground for the points – but maybe i’m thinking about it backward.

[frankr]

Now you are getting complicated. The system was changed in 1956. But your question concerns a 1955, so let’s stick with the question at hand.

1. Yes it is as expected.

2. Touching your meter to the switch terminal, it will show continuity to ground, via the closed points and/or coil primary, as previously discussed. But, AHA! The switch housing is also grounded through it’s mounting bolt. Therefor you have continuity as discussed and also to the switch housing, because the switch housing and ground are the same thing.

[The red boat]

perfect – thank you

so what changes when the vacuum is introduced since it is all continuously grounded anyway?
Does the vacuum-actuated diaphragm “break ground”?

– I may be a slow learner but I’ll eventually get there!

[frankr]

You gotta stop and consider how the magneto works. Here is the simple, non-engineering-degree version. For the time being, forget about the vacuum switch. As the flywheel magnets pass by the coil laminations, electricity is generated in the coil’s primary winding. That electricity flows from one end of the coil to the other end. Called a “circuit”. Electricity leaves one end of the coil, goes to the points, and they being closed, the electricity goes to ground. Then it flows through ground to the ground lead on the other end of the coil (remember—a “circuit”). At a certain time, the points open, and that breaks the circuit, setting off a chain of events that results in spark voltage sent to the spark plugs.

Now, suppose that coil lead going to the points was shorted to ground. The points could open and close all day and not break the circuit to ground–thus no spark. That is exactly what the vacuum switch does. In event of high manifold vacuum, the vacuum switch closes, which shorts the points lead to ground, and the cylinder stops getting spark.

That’s about as simple as I can say it. Next, you get the written test.

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