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engine seems to bounce back and forth from 1 to 2 cylinders

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slimpanama

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i have a early 90's 18 hp Nissan. it runs fine once its spinning fast. it idles decently for the most part, but does stall sometimes. on occasion while idling it makes a clank noise which shakes the whole motor...which sometimes kills it, not always. when applying the throttle it boggs then instantly surges into power. trying to hold a low rpm, fast idle, in a low wake situation the rpm surges up and down as if one cylinder is turning on and off. i was thinking that the stator may not be producing enough at low rpms, but figured someone on this forum knows how to test.
 
You did not mention model, which is critical for understanding which parts your motor uses. You might have an NS18C2, D, or E2. Likely the 18D. None of those motors have stators. Your symptoms are classic carburetor symptoms. The C2 and D are very obsolete, and few parts are available. The clank could be ignition popping, slamming the reed valve shut, or it could be mechanical. Best to do a complete check of all the mechanicals, a thorough carb cleaning (not just a spray) and also grab a good analog (not digital) ohm meter to test the ignition system. The Factory service manual (not an aftermarket one) has all the specifics, and is available from your favorite Tohatsu dealer.
 
sorry, it is the ns18c2. it only has 1 carb for the 2 cylinders. it seems like its only running on 1 cylinder at low rpms and instantly pops into 2 cylinder with a major torque thrust. that is why i am thinking that i'm having intermittent ignition problems at low rpm. what supplies the coils since there isn't a stator? charge coils??
 
That motor may well be 30 years old. Quite obsolete. No carb parts available. The CD is charged with an exciter coil, and the spark is released with a trigger coil, neither are available any more. It uses a single CD, available for about $223, and a siamese coil, which costs about $50.

Check or change plugs, freshen up the plug wire caps, clean the carb, verify timing, and if that doesn't do it, you are better off replacing it, since parts aren't available.
 
Use a timing light and lay it on the deck and watch it at different rpms that way you can see if it is spark related just switch it to the different plug wires.
 
kimcrwbr1 said:
Use a timing light and lay it on the deck and watch it at different rpms that way you can see if it is spark related just switch it to the different plug wires.
Click to expand...
Indeed. But since it is in fact a single, siamese coil, unless there is a bad plug wire or cap, that will not be the problem.
 
it has 2 seperate coils on it i believe. i have a timing light i can hook up to it. is there a ohm test that i can do to test the coils or trigger or whatever? i have replaced both plugs and caps. reset the timing since the last owner changed it. i believe it is set right. when i turn the throttle, i have to turn it about 1/4 of the way before it will start to get power. it instantly revs up knocking over any drinks and people not holding on. it could be a timing issue maybe since the timing gets advanced as you turn the throttle??
 
i screwed around with the engine yesterday. i checked the timing, and it was ok. i used a timing light to check that the coils were firing, and they were. i sprayed fuel into carb and it almost stalled it, so its not starving for fuel, i also adjusted the all the linkages. i did all of this on the trailer. i can't tell if its acting up much with no load. i'm going to creek tomorrow prob and i will put the timing light on it and spray fuel into it while its actively messing up.
 
i finally got a chance to get the boat in the water today. it is still doing the crazy low speed surge. i was in the salt water, so i didn't take the cowling off and get to use the timing light. i didn't want to get salt water on the engine...understandably. any ideas??
 
Is the carb correctly synced with the ignition? Usually, the ignition is advanced quite a bit before a lot of carb is advanced. If the carb is opening early, that might do it.
 
i believe it is timed right. i looked it up and came up with around 5 degrees idle and 25 degrees wide open. what is the procedure to sync the carb with the ignition? the ignition components seem to be keeping the carb from opening up all the way. it bottoms out around 25 degress leaving the carb just a little more room to open up. if i advance the timing at idle, it will prob keep me from going to wide open, which isn't that big of a deal since i don't go full throttle much
 
You need the carb synced correctly. The carb should be wide open at full throttle. That may be part of the issue.
 
what is the proper steps to sync the carb? when i put the tiller handle at full throttle and adjust the linkage to make the carb go to full throttle, i seem to have binding issues trying to get the timing to go to 25 degrees. i can't get the 3-5 degrees at idle and the 25 degrees at wide open throttle at the same time. should i try to advance or retard the idle timing in order to make it run well at low rpms and hope that it runs good at high rpms?
 
The cam (on the breaker plate assembly) or roller (on the carb) may be shot, or you may have an incorrect cam. When the handle is fully at WOT, the timing and carb should both be at maximum.
 
i put the boat in the water today. i took the timing linkage off and turned the throttle to about the position where it seems to give me trouble. then manually pushed the timing until it began to run right. it seemed that i had to advance it a lot more than the the linkage would allow for. i adjusted the timing to about 6 degrees at idle instead of 3. it seems to run a little closer to how it should. i turned the ball connectors as far as they would go. is there any type of adjustments under the flywheel? also, how many turns should the idle mixture screw by out? 1.5-2?
 
No timing on the breaker plate under the flywheel; You may an incorrect link rod, or incorrect end piece. If it works well at maximum extension, go with that. As for idle mix pilot screw, you must adjust that by ear and Good shop tach at warm idle, both in N and in gear (in the water, not just in a tank) for best adjustment. About 1-3/4 turns is a good starting point. Then get fully warmed and idle, adjust mix for highest RPM, then slow idle stop back to factory spec. Repeat a few times, and then test in gear, and repeat the adjustment a few times. Then check in N again.
 
Factory service manuals are copyrighted, so not available free. The Tohatsu 1 and 2 cylinder service manual retails for about $50.
 
the engine seemed to run better after some tinkering, but now it doesn't run at all. it has no spark. i did some digging and think that the spark plugs aren't correct. my research suggests that i should be using B7HS-10 plugs with a 1 mm gap. the plugs that are in it doesn't even have a prong for the spark to jump to. it just goes sideways. also, the plug caps are resistor type with 5000 ohms of resistance. the coils are supposed to have roughly 2000 ohms and it was around 2100 without the caps on, 7500 with the caps. the trigger coil has 20.6 instead of 36 to 46, which i believe to be the culprit for not spark at all. the exciter coil tested in spec. i bought this engine used and figured all the parts were correct, guess i was wrong. any thoughts about the trigger coil?
 
Sheesh. You want NGK B or BP 7 HS plugs. Sounds like you have champion-style plugs as used in Mercury motors. Gap is 35 to 39 thou, or a tight 1mm. Coil impedance tolerance is wide; if a trigger is really bad, it's usually shorted or open. It is likely OK at 20+ ohms. Really does not matter, as the coil is NLA. If it's shot, the motor has had it.
 
UPDATE! i acquired a DVA an retested the exciter and trigger coil. they both were performing well. i was going to order a new CDI from CDI Electronics for $180 in the next couple of days since it is the only part left. for some reason i decided to test the spark again, but this time the plug caps were OFF. boom...we got fire! i put the new plugs and caps on and i fired right up. it turns out that the reason that i was having timing issues is because of the resistor type caps. the timing had to be advanced to get the spark to come out at the right time, which was all done by the previous owner. advancing the timing was causing it to bottom out before the carb was wide open. i haven't test driven it, but i'm feeling pretty good about it! thanks to everyone who helped out!!!!!!!!!!
 
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