Author Topic: 2 Stroke Woes  (Read 607 times)

Offline Lobo

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2 Stroke Woes
« on: February 03, 2021, 08:08:56 AM »
Hope ya don’t mind Steve.....!

Not esp familiar with 2 strokes - any help much appreciated. Small twin Yam 9.9HP outboard. Carb thoroughly cleaned; no rubber intake manifold... seals directly to the Reed Valves / head. Fresh fuel. New plugs & correctly gapped.
The thing starts on full choke, and idles there - at a high speed. When you start to back the choke off it pops; looses momentum / rpm and dies, during which there’ll be the odd backfire. I’ve not had it in the water / under load, but all indications are it accelerates / revs freely on the throttle. Bottom line, it won’t idle off choke at a nice rpm.
Engine look good / unmolested / never opened up. Nonetheless it’s prob 20 yrs old, and possibly worn. All thoughts appreciated - You Tube not been enlightening so far ...
What to do lah?
Ta, Simon

Offline Lynx

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Re: 2 Stroke Woes
« Reply #1 on: February 03, 2021, 08:36:36 AM »
Typical symptoms of an air leak or blocked pilot system. Could be leaking crank seals! Is the ignition timing adjustable?

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Yamaha RD500LC
Triumph Speed Triple RS.

Previous bikes include
1976 Honda CB400F Now sold :-(
CB350K3 x2
CB250G5
CX500C
VF500F2
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Offline ka-ja

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Re: 2 Stroke Woes
« Reply #2 on: February 03, 2021, 09:47:04 AM »
As per Lynx, loss of primary compression in the crankcase causes a lot of two stroke woes, the crank end and middle seals can harden with age loosing compression.
nice bike,nothing in the bank

Offline Rob62

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Re: 2 Stroke Woes
« Reply #3 on: February 03, 2021, 09:28:22 PM »
2 stroke very difficult to diagnose. Running worse off choke suggests it may be too lean but could be something else. My advice re 2 strokes is always to start with the basics before leaping to diagnose major problems such as crank seals.. Start with the basics, new plug, fresh fuel, strip and clean carbs and all passages thoroughly, make sure all settings are correct, jets clear, float height correct. Then you can move onto compression testing and then onto crank pressure drop testing. Do you have a manual for the carb settings? You’ll need one. Good luck with it, be careful cos if you get it even slightly out things can go wrong very quickly...melted pistons etc.

Offline Lobo

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Re: 2 Stroke Woes
« Reply #4 on: February 04, 2021, 07:21:00 AM »
Thanks gents. Verified / corrected the timing today (full advance was spot on, idle was at 5BTDC instead of 5ATDC), plus had the carb off again, float height checked, main & pilot jets checked, all galleries blown through with isopropyl. Reed valves checked ok with Mk 1 Fingers & eyeballs.

The engine a bit smoother, but basically the same. The missing / popping as you back the choke off is so pronounced (engine shakes (250cc)) that I began to question the ignition. I put a strobe into each of the 2 HT lines, and noted the strobing failing iaw the missing. Seems odd that BOTH coils might be failing, so I disconnected the kill switch in case of some rogue grounding - no change.

‘Charge coil’ (ie magneto sys) resistance checked ok, though the 2 ignition coils (primaries) are out of spec. Will order new ones.
CDI unit... have no idea how to check.

Q - on magneto type ignition circuits, if the engine is running badly due mixture / crankcase leaks whatever, can you expect the spark to continue reliably all the way down to engine sputter & stall? (would have assumed so, but just verifying)

There’s a moral here - don’t bloody buy stuff off teenagers 😂😂😂

Offline Lynx

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Re: 2 Stroke Woes
« Reply #5 on: February 04, 2021, 07:39:31 AM »
Haha. We'll done on tracing it .

Sent from my HTC Desire 530 using Tapatalk

Yamaha RD500LC
Triumph Speed Triple RS.

Previous bikes include
1976 Honda CB400F Now sold :-(
CB350K3 x2
CB250G5
CX500C
VF500F2
VF1000FE

 

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