BF15a cutting out after chopping throttle

Carb recently cleaned. Idles like a kitten, runs like a top at all speeds. only issue is that if you chop the throttle from 1/4-full syraight to low speed or idle, it will stall and/or run as if on one cylinder or flooded if it doesn't die or when you restart. After 10-30seconds it snaps like the other cyl fired again and goes back to purring like a kitten.


any advise? New jets, seals, and needle/seat on order as prev maint.

UPDATE: bottom cylinder is in fact not firing after chopping throttle. also did it upon cold start this evening. took several minutes and several restarts and bottom cylinder kicked back in. repeatable by chopping throttle. verified by removing top plug wire while running - dies. removing bottom plug wire... continues to run w/no change.

now that its certified ignition, gonna start with plugs being the easiest.

Not sure what you mean by chop. Does it mean accelerating/decelerating quickly?

It is not necessarily a spark issue unless you actually see no spark. Your test just indicates something going on with the bottom cylinder. Plugs are a good place to start. You could just flip flop them to see if the trouble follows the bottom plug.

Besides ignition, it could also be compression, valve clearance, fuel flow blockage from the carburetor or idle mixture screw setting (initial setting is 1 5/8 turns out).

Mike

I'm signing on because I'm interested in what you discover. Mike! I thought you did a stint in the Air Force. Chop throttle is an old aviator's term for going to idle quickly so the bogie on your six throwing 30mm cannon at the back of your cockpit would overshoot and go by you when you suddenly slowed down. :~)

Anywho, I agree with swapping plugs first but also check valve lash no matter what.

This is definitely "different".

motor is on a little RIB... by chop, I mean you are cruising along on plane around 1/4+ throttle and approach a no-wake zone, you "chop" the throttle by throttling to idle and coasting to a near stop. can also be accomplished by gunning it, and "chopping" the throttle back to idle.

i'm pretty confident in the spark issue... either ignition or plug itself. when the bottom cyl comes alive, its as if a toggle switch is thrown on the plug wire... instantly purrs away.

good idea swapping plugs. unfortunately I will be out of town this weekend but will update progress next week.

UPDATE:
new plugs (could only find autolites stocked locally. it is behaving mostly the same. seems to possibly be timing... with new plugs removing a plug wire does not 100% duplicate the "bucking" that it does when acting up/missing after cutting throttle.

there is a little debris/rust on the timing cam tab, wondering if cleaning that up is suspicious.

tested easy to reach electrical...

pulsar coil 431ohm / 351-429ohm spec
primary ignition coil 3.4ohm / .35-.43ohm spec <--- suspicious?

going to check valve lash tomorrow as well.

should add that it was also misbehaving and not reaching top notch 1/3+ throttle performance after the plug change. could hear and feel a few hiccups and misses at speed. still accelerated fine... but sure seemed to be having ignition/timing issues all around this time out. weird.

hanging on to the old plugs for the moment.

Well I replied via my phone yesterday but I can see it didn't make here...again...
Anyway,

3.4 ohm primary coil resistance is WAAAAY out there...in my opinion. I don't like the spec you listed either. Working coils USUALLY present around .6 to 1.5 Ohm. What manual are you using?

I really don't trust that 3.4 Ohm reading and I don't believe the coil would even fire with that much resistance. BUT... I've been wrong before...a lot...so...grain O salt and all that.

I would, though, urge you to go back and re-check that while paying close attention to your lead connections and meter setting. Are you zeroing it out? Is the battery fresh? that sort of thing. If you still get 3.4 then I would pretty much call the coil and say your symptom is because it's breaking down trying to produce firing voltage at higher RPM.

I don't know about the rust around the tensioner. Might be a clue, might not. It IS an outboard and they DO get water up there. If the belt is tight and the tensioner is free to move, then it is likely ok. It will either be IN time or OUT. If you are working with the HELMINC shop manual then it should be easy enough to verify. If you are using a Seloc, Clymer Or Haynes or "other" then I don't know. I only trust the HELM info.

These engines only seem to do well when NGK plugs are installed. I don't know exactly why that is but it is. If you can find some, even if you need to use Amazon or Boats.net then you might want to try them. I've seen the correct NGK plugs solve quite a few "phantom" running issues.

Good luck.

update:
adjusted valve lash... .005" intake, .008" exhaust, per Honda manual. intakes were tight.

sourced and installed some NGK DR6HS plugs

installed new fuel pump wear items (plunger/diaphragm)

tested cold coil... 1.4-1.5ohm.

ran engine out... never run better. idled for 15min, started kicking/bucking for 10sec and resolved. ran 1/2 for another 10min, idled for 3 min, and started acting up for 5sec and resolved.

tested coil again, 1.4-1.5ohm.

long story short, still acting up. I tried popping off a plug wire, it is def not simply dropping to one cylinder... when its running good and a wire drops, it still runs better than when it acts up.

I know what you are thinking.....coil. Don't buy one yet.

I have see a few cases where the connector to the power pack has bad connections.
Timing is probably ok, since it ran good for a little while.

The carb is probably ok too, unless some debris got up into it.

Try unplugging it and cleaning the contacts. Before you put it back together, slightly twist the pins so that you will make better contact when you put it back together. Sometimes the connector, itself, on the CDI unit looses connections internally.

It is also possible that the CDI unit is crossfiring (if it is not the connection). It would be nice if you had a buddy with a 9.9 or 15 with the same CDI unit to try.

Mike

I am thinking coil. More accurately, I'm thinking ignition.

Quite honestly when it is running good, it runs good enough on one cylinder (until attempting to throttle up) that an unsuspecting bystander wouldn't notice that it is running on one. Sounds like my little one hole tohatsu 6hp. Doesn't die, just has no power and not quite as smooth.

When it is acting up, it is horribly knocking and banging but with good rhythm. Applying throttle above idle kills it. Sounds like the timing is off. But it will cure itself moments later, never longer than 15-45sec. Instantly purring again.

I have not thoroughly gone through and wiggle tested all the connections, that is on tomorrow's agenda (including wiggling said CDI harness).

When it is not sputtering and banging away at idle (which is only about 1% of the time), the thing runs absolutely amazing. Perfect idle control, smooth throttle, instant throttle response, quiet, runs like a watch. 100% certain at this point it is electrical... which just so happens to be the worst flavor of motor gremlins for me.

UPDATE:

Spot on Mike.

Ok! getting somewhere. I can replicate symptoms by wiggling connection on CDI. 75% positive its down to the 2-wire connector.

Can't quite tell if it is a loose pin in the male (CDI) side, or the female (harness) side.

I monkeyed around with it, made the problem considerably worse, and determined that when it is acting up I can simply wiggle the connectors on the CDI and it instantaneously resolves itself. Can't figure out how to disassemble connector to inspect/rebuild. Hoping harness side... other side seems to be a "replace CDI" situation.