RD-17 bottom plug fouling – no high speed power

[fleetwin]

Well, I guess you need to verify that loss of spark is the actual issue here. Using those neon inline testers or a timing light while the engine is attempting to run at a higher throttle setting to see if either of the cylinders is losing spark.. This test on the water is best done at dusk when the sun is going down so you can see the tester or light. .Have you changed the plugs? Just cuz they might be new doesn’t mean they are any good.
Let’s just say that one of the cylinders is losing spark, and it is the cylinder connected to the vacuum switch. I would simply remove the lead from the vacuum switch and tape it up so it doesn’t ground, then retest…If the engine runs on two cylinders now, then we need to check the entire vacuum switch circuit before condemning the vacuum switch itself. This system uses some delicate mercury switches in the throttle to keep the system from activating when the throttle is set up high…

[The red boat]

Frankr – Very clear and good explanation. I understood how the magneto functions but your explanation of how it interacts with the vacuum switch makes sense now.
Thank you

[crosbyman]

from the red book….

btw… VOM reading grounds across a wire with stuff connected at each end is dicy…

ever try to weld a copper pipe fitting after closing a main water valve …? . residual water …from the other end of the distribution will flow back and cool off any efforts to weld in a new fitting … same issue with electricity /grounds they flow both ways .

same for that vacum switch wire …. sometimes grounded by the vacum switch itself or…. at the other end …grounded by the points simply doing their job

or worst… grounded by intermittent grounding due to worn out insulation in some hidden nook or cranny between the switch and the point set for the lower cyl coil.

if Fleetwin’s excellent suggestion to disconnect the switch does not work …. disconnect the culprit wire at the points and retry… if the issue clears up I would closely inspect that wire between the switch and points

Joining AOMCI has priviledges 🙂

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

[need2fish]

Can you confirm the lower plug was inspected immediately after running the moter? As in, before any water in the combustion chamber had evaporated ?

Are the capscrews on the head all torqued down to spec ?

Is the compression in the lower cylinder more or less the same as the upper ? Sometimes when you have good fuel/air supply and spark, problems with one cylinder could be stuck or broken rings or mangled reeds (the latter is probably not the case as you wouldn’t get the carboning up of the plug.

[fleetwin]

need2fish wrote:

Can you confirm the lower plug was inspected immediately after running the moter? As in, before any water in the combustion chamber had evaporated ?

Are the capscrews on the head all torqued down to spec ?

Is the compression in the lower cylinder more or less the same as the upper ? Sometimes when you have good fuel/air supply and spark, problems with one cylinder could be stuck or broken rings or mangled reeds (the latter is probably not the case as you wouldn’t get the carboning up of the plug.

Good point, I thought I read that he had checked compression, but don’t see any reference to it looking back over the posts…

[The red boat]

thank for everyone’s help on this subject.
Other than swapping out the high-speed needle for a better one and replacing the spark plugs, I did nothing else to the motor other than some disassembly of the carb and re-assembly.
re-confirmed the cam timing, etc.

I took the boat out on the water this weekend and everything is in good order.
After I fine-tuned the hi and lo speed needles I got to enjoy an afternoon of boating and fishing!
Once the motor cooled down enough to touch i pulled both spark plugs and they looked great. dry; nice tan insulator and no fouling.

when revving in neutral, the vacuum cut-out worked as it should and i had no other problems.
whatever gremlin had take up residence the week before has apparently moved on.

thanks again!
great site!

[GARY SANDBERG]

I had this problem with my Mercury Mark 20. I converted from a Bendix to a Phelon ignition system. Did not fix the problem of the bottom cylinder flooding out badly. Turned out to be a hole in the fuel pump diaphragm. If you are using a fuel pump and not a pressurized fuel tank, it is a easy inspection.

[fleetwin]

Unfortunately, our test tanks can often be “the gremlins”…..

[Author]

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