CD Test Procedure. Doing it wrong?

[FinNFethers]

I have a 2003 Tohatsu 4-stroke MFS25A that recently lost power under high RPMs. The condition started suddenly while underway at 3-4000 RPM. Basically, I get no additional power after applying about half throttle.

I replaced all fuel, purged lines, fresh gas, rebuilt (OEM parts) and cleaned carbs (which showed no varnish or gumming) and replaced the spark plugs (which not worn, burned, oil coated, etc..). After rebuild, I set the idle and balanced the carbs.

Now, the engine runs rougher than before with intermittent sputtering and it takes a minute of idling to drop to a normal low idle (below 1000).

I'm testing the CD with a multimeter according to the service manual. I find that almost nothing has any sort of resistance reading at all contrary to what the manual indicates. Starting out on the 24x24 grid of possible combinations of pin resistance readings, I get infinity until I hit pin 8 to pin 9. I've tried 3 different multimeters and probes. No change. If the CD was this far out of whack, the engine wouldn't run at all.

So, I must be doing something wrong. Any ideas?

 

[pvanv]

Re: CD Test Procedure. Doing it wrong?

You are likely using a run-of-the-mill, cheapie, digital meter. Won't work properly for most of these tests. You need a good analog meter. Are you sure that the ignition is to blame? Since the motor runs more poorly than before you worked on it, you likely missed something when you had the carbs off.

When you did the carbs, did you do a complete disassembly (including both jets and emulsion tube), and a 4-hour soak in real carb dip, such as Tyme? Did you reinstall the rubber plug that closes off the slow jet?

 

[FinNFethers]

Re: CD Test Procedure. Doing it wrong?

Sorry for the late reply. I've been waiting on parts. Trying to find fuel lines (inside cowling) is not an easy thing. Apparently, the ethanol in gas has caused a redesign of the fuel lines which makes finding OEM hose in the right OD impossible. OD matters because the fittings on the carbs are pressed in so close to the body of the carb that there isn't a mm to spare.

In testing the CD, I used 2 cheapie and a commercial digital multimeter. I'll borrow an analog meter to try it out.

Anyway, ALL fuel lines have been replaced. Fresh gas, new fuel filter, new plugs (gapped per specs), etc... as above. I disassembled everything on the carb and soaked for at least 4 hours each. Cleared each carb with air to all orfices, and jets. Replaced everything inside and outside the carbs exactly as it was. Set idle, balanced carbs, etc... Now, the sputtering occurs infrequently but it is still there. As it is warming up, it seems to idle smooth and has no misses, even when throttled up slightly. After a few minutes, it will start missing/sputtering every once in awhile. At throttle up, this continues. It is much better, but it is not purring like the kitten is once was.

I'll take it out on the water Saturday to see if I can get WOT or if the sputtering continues to prevent this.

 

[pvanv]

Re: CD Test Procedure. Doing it wrong?

Your ohm meter must use a battery of 3v or less. Any more and you risk damaging the CD by testing it.

Since your work on the carbs seems to be improving, if not fixing things, you are probably headed in the right direction. Considering that it seems to run better when richer, it may be that you are too lean once it is warmed up, which brings us back to carbs again. What did you use for carb dip?