Flummoxed - C40TLRX, 40 HP 2-Stroke, '99 Model - Harsh "Knock" at Idle

[a387673]

Hard to know where to start, so I’m putting the end-of-the story here at the front, but following with the build-up for anyone who prefers a novel to a short story:

Engine is idling terribly, with a hard knock that almost stops it before it tries to catch back up, and repeat hard knock. *Pulling plug wires for cylinders 1 or 3 will smooth the engine out. Pulling plug wire for number 2, it sounds the same (bad). I think I would have rather have two vs one the other way, with one cylinder omitted that would smooth it out. Any suggestions as to what this is telling me?

Long Version:

About a month ago I was having problems with rough idle, excessive fuel consumption, and excessive carbon build up. After replacing a faulty fuel pump, a leaky fuel connection, clearing an obstructed lo-speed jet in one carb, and lots of tweaking, it was running much better except for a high-speed intermittent miss.

Symptoms of this miss included that it was prompted more than a few times by the bump of a little rough wave or me touching the trim button. It would often clear out with “goosing” the throttle, but always came back.

Pulled carbs again yesterday, everything looked clean, but I cleaned again. I was working toward testing stator/pulser coils and CDI, but during synching, adjusting, it became apparent that something is now worse. The idle is terrible, with some sort of recurring hard knock that almost stops the engine. Adjustments including timing (per marks on CDI, and stop screw adjustments) are way out of whack to make it run at all. This is most apparent (worse) after it’s warmed up.

I have tested for spark (jumps 7/16), ignition coils resistance, measured pressure/vacuum in fuel lines, cracked a rib while laying over the seat back with timing light while operating the boat at full throttle, etc., etc., some before, some after the recent carb rebuild. Plugs had gone from showing rich, to virtually perfect.... and now back to slightly rich. At this point, the most interesting observation may be that (Go back to “*Pulling plug wires…”, 2^nd sentence in 2^nd paragraph).

Thanks for any suggestions.

[99yam40]

the knock sounds like a lean sneeze.

set the link and sync according to manual.
if it will not run when set to factory spec then the carb is not clean

[TownsendsFJR1300]

Can you post a "You Tube" of the engine when it's acting up?

[a387673]

I will when I can Scott. I broke off one of the carb bowl drain screws, and waiting on the delivery of another. It seems almost like a backfire, it is not rhythmic. It will do it every two or three seconds after it gets warm.

I must've screwed up something in the carbs, which may have been perfectly fine. The problem before that was a miss that made me worry I could lean out a cylinder. I've wondered if an ignition problem, but I thought I would eliminate the carbs as a possibility before doing a lot more along the lines of ignition. I have verified spark on all three. At 7/16, it was blue, and made the snapping noise, but it appeared as a very thin line, as opposed to bold and bright. I wondered if that could signify weak spark, or if it's "got it or you don't" deal"....

[a387673]

(Light bulb comes on…)

When I pressured up the fuel after reassembly, I observed a leak from a hole in the "pipe" hose that feeds the bottom (#3) carb. I see why they call them pipes, as they are not flexible. I cut off the last 3/8" or so, but attaching it that way may well have crimped the hose enough to impede fuel flow. The pipe on the middle carb was too long and firm to allow the now-shortened pipe on # 3 to attach, so I cut off maybe 3/8" of it. It's a straight pipe, so this would not impede flow to the middle carb, but the new angle it introduced to the line that goes up to #1 may have created too much of an angle there as well.

I did not think it would have affected it that much at idle, but I’ll know after the parts arrive (bowl screw and new OEM pipes). I guess I'm not supposed to run it with a plug wire pulled (temporary insanity), but evidence from that exercise would support this theory: pulling #1 or #3 smoothed it out (somewhat), pulling #2 allowed it to continue running terrible (made no difference).

carbhose.jpg

[a387673]

I wasn't lucky enough that the new hoses fixed it. The carbs had to come off again, but I got the idle back. There was some sort of blockage at the needle valve going into the # 3 carb. I did not even see it, but could tell it was there, and it apparently came out with the spray. Thanks again, 99, you were on the money.

Two other items of note that came from older posts on this forum, that I will remember in the future: 1) Scott, it may have been you, but someone said that they set up a small test fuel tank to evaluate carb function after cleaning and before re-installing. 2) Rodbolt had mentioned that he does not always change everything that can be changed when cleaning carbs, but one thing he said he does always change is the needle valve seat.

Following either of these suggestions would have saved me some time and anxiety. I hope now I'm back to chasing the hi-speed miss.......

[a387673]

Here is a video link showing what it was doing,..... begins coughing and stalls.

https://youtu.be/SsHWg9CiBmM

[TownsendsFJR1300]

Here is a video link showing what it was doing,..... begins coughing and stalls.

https://youtu.be/SsHWg9CiBmM

Yup, lean sneeze....Glad your making progress!