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Why is my 3cyl 25hp 2-stroke bogging down???

B

bigtime_mcalpine

Regular Contributor
Running out of ideas here...
Symptoms are:
- Idles nicely in neutral and in gear
- very limited acceleration capabilities (I can increase speed by about 10% only)
- if I start increasing throttle be it quickly or progressively, engine speed increases about 10% and then I bog down i.e. it flutters and doesn't gain any more speed or RPM. I'm able to turn to WOT but the reaction remains the same, fluttering with no increase in power or RPM.
- I'm able to reduce back down to idle and it will behave normally again
- spark "seems" to be good. A visual confirmed each of 3 cylinders is getting spark
- compression is good
- carbs appear clean (I took them apart and inspected by eye ensuring that jets were clear of obstructions)
- I've switched gas tanks and gas
- fuel lines look good

I'm going to take vacuum pumps apart next to ensure there are no tears in diaphragm. Beyond that I'm running out of ideas.
 
The pumps operate with crankcase PRESSURE and not so much vacuum..-----If the oil injection unit is still on there you should take it off and throw it away as far as you can.
 
racerone said:
The pumps operate with crankcase PRESSURE and not so much vacuum..-----If the oil injection unit is still on there you should take it off and throw it away as far as you can.
Click to expand...

Got it. Alright, vacuum pumps inspected - bloody hell, those are fun to put back on if one is too cheap to replace the hose clips!!! Racerone, you are the second person to mention this oil injection thing, I don't think mine has it...I'll take a closer look at parts diagrams to verify. Is it possible that one of those parts may have been left on and is creating havoc?

I will go take a few pics and share.
 
bigtime_mcalpine said:
Got it. Alright, vacuum pumps inspected - bloody hell, those are fun to put back on if one is too cheap to replace the hose clips!!! Racerone, you are the second person to mention this oil injection thing, I don't think mine has it...I'll take a closer look at parts diagrams to verify. Is it possible that one of those parts may have been left on and is creating havoc?

I will go take a few pics and share.
Click to expand...

Video below with running demo. As an aside, should there be an empty bolt hole on RHS of head (you'll see what I mean)...actually thinking about this further, I think it may be the mounting hole for the aforementioned oil injection pump.

https://www.youtube.com/watch?v=0AMdwbSL-Mg
 
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Oil injection unit is gone !----And no those fittings / holes had nothing to do with mounting the oil injection.----Water pressure and temperature gauge sender come to my mind.
 
racerone said:
Oil injection unit is gone !----And no those fittings / holes had nothing to do with mounting the oil injection.----Water pressure and temperature gauge sender come to my mind.
Click to expand...

Thanks! Starting to wonder whether CDI box may not be adjusting timing the way it is supposed to...is that possible you think?
 
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alright, went outside and reset the timing pointer to make sure it was correctly aligned for further idle timing test. I did it twice and got the same outcome which was a few degrees off from where it was before. Timing light arrives tomorrow so I'll check that next. We'll see.
 
Update: problem persists but it did have some idle timing issues that are now sorted. The timing process did have me pushing the linkage adjustment arguably past its limit though which I'm a bit concerned about. See video below. The motor now idles nicely at 4deg ATDC with RPM's in the 700-800 range which was the goal.

https://www.youtube.com/watch?v=EdQo_SkxK2Q

It may be reving up ever so slightly better but it really does seem like there is a governor installed at this point. I feel like I've exhausted all the easy stuff and have got a used CDI and two spare coils on order (thank you ebay). Not sure I feel great about a "used" part to trouble shoot another "used part" but these CDI units are really not cheap.

Here's something interesting... I got curious and kept the tachometer on as I rev'd and I repeated this for each of three cylinders. Motor begins by reving nicely and then at anywhere between 1700 and 2400 rpm I can hear the motor start to lose it's synch and RPM's suddenly dropped to 700 on the cylinder being monitored even though overall, it didn't sound like motor was back down at idle. Overall it sounded more like motor was struggling to generate any more power. I'm thinking this might point to an electronic component vs. carb or fuel (could be wrong of course).

So far I've:
- checked sparkplugs and gaps (almost new)
- check coils vs. specs in operating manual - ohms static check (not ideal)
- inspected all fuel lines and replaced one older fuel line that was waaaaay past its prime
- took carbs apart 3x and inspected as best I could paying close attention to needle valves and jets (on this one, it doesn't appear that orifice that sticks into venturi is removable so can't quite inspect that one
- preliminary check of reed valves with finger gently pushing on each to confirm they close - they seem good (I know, this is not a thorough check)
- checked compression, didn't try that hard and got 105 off each, probably could have pulled a few more times for 110 but they are all even, good enough for now
- took vacuum pumps apart, closely inspected diaphragms (they looked almost new)
- manually moved the optical timing sensor to verify reaction from engine while running (it reacts therefore I believe optical sensor to be good)
- popped flywheel off as well as stator and closely inspected wiring (looked great)
- replaced gear oil (yeah, not related but figured I might as well)
- visually inspected powerhead for signs of any exhaust or gas/oil leaks, none found
- changed tank and fuel and connector lines
- re-synched carbs and did full retiming at idle as per manual
- re-synched carb follower

Up next...wait for CDI in the mail and pray that's the issue. Now I'm really running out of ideas short of replacing a few powerhead gaskets that honestly look perfect.
 
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Well F me. I continued my reading while waiting for my used CDI to arrive. In doing so I came across a fair amount of literature regarding "S.L.O.W" which refers to engine being limited to roughly 2000-2500 rpms under certain fault conditions. I then thought, wait a minute...that's what mine is maxing out at too.

So just for the heck of it, I went down to the backyard in between meetings, filled up the test barrel and started up the outboard. While it was running I unplugged the head temp sensor...no difference. Went upstairs dejected. Then I thought, let's try again but I'll disconnect BEFORE starting the engine.

YES SIR, THAT ENGINE TOOK THE WATER IN THAT BARREL AND THREW IT ALL OVER THE PLACE. FULL THROTTLE BABY!!!

So, I guess this means sensor is bad?
 
racerone said:
Oil injection unit is gone !----And no those fittings / holes had nothing to do with mounting the oil injection.----Water pressure and temperature gauge sender come to my mind.
Click to expand...

Just read through this string again…racerone, were you biting your tongue here? You called the issue out immediately. I now remember reading it and thinking, temp sensor, what the? And then promptly ignoring the comment. Anyhow, prize goes to you on this one, my friend.
 
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