Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: TMKIWI on October 05, 2011, 01:54:40 PM
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OK experts.
Recently I had a Yamaha 50Hp outboard in for tune up. 3 cylinder.
Compresson test motor , replace spark plugs , test spark.
Striped and cleaned carbs x 3. (Yamaha keihin carbs are known to block emmulsion tubes if run on dirty fuel.)
Reassemble and test run.Balance carbs and adjust idle mixture.
Motor idles perfect but when revs are increased motor runs on 2-2.5 cylinders.
Check fuel flow out of carb with timming light & torch. No. 2 carb running lean at part throttle.Motor runs good above 5k rpm.
Remove carbs.Strip & inspect.All seams fine.
Inspect reeds.Fine.
Reassemble and test run. Same problem.
Motor idles perfect & runs at WOT perfect. Not in between.
Whats wrong?.
Yes I found the problem. ;D
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The problem is it's a Yamaha... :D
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The problem is it's a Yamaha... :D
I work on outboards and i never had a 2-stroke Yamaha fail on me, yet. :D
@OP: i'm a 'see firsth then solve' kinda mechanic. :P So i can't help ya from here out.
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I work on outboards and i never had a 2-stroke Yamaha fail on me, yet. :D
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Oh yes they do. :)
This was the first time I came across this problem.
At the time was a real brain twister.
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You say when you strip the carbs "all seems fine",is it? Is there something causing the needle to hang up or slip then lift at wot?
What can I say,I know diddly fark about how multiple cyl outboards are set up.The closest thing I can use is my old RZ :D
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All seams fine means all jets,the needle and seat & emmusion tube have no blokages.
Don't think of it as a multi carb engine. Only 1 carb is playing up.
Here is an exploded view.
(http://i864.photobucket.com/albums/ab207/tmkiwi/Image.gif)
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I'm not familiar with that carb at all.Regardless,does one fuel pump run them all?If not is that an accel pump?I'll try it this way....what are part #'s 33,34 and 29-40?
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What happened was No. 23 is a rubber bung which plugs the idle casting so the fuel is drawn through the main circuit.
The bung had swollen ( not sure why but may have been ethanol ) and blocked the passage between the main and idle castings.There is a diagonal passage between the main & idle castings.
At idle, the idle jet was supplying enough fuel to run properly but once throttle was applied there was not enough fuel passing through the idle circuit.
Remembering the idle circuit operates from idle through to 1/2 throttle , the motor was starving for fuel until full throttle when the main supplies all the fuel.
Have never seen it before but now I trim the bung 1-2mm to make sure it can't happen. ;D
PS. The part No.s you asked about ford are for the choke/enrichment device.. Electric solenoid. It squirts fuel into the manifold.
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OK TMKIWI I have to ask... How in the hell did you figure that one out. I would of been pulliing what's left of my hair out
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What happened was No. 23 is a rubber bung which plugs the idle casting so the fuel is drawn through the main circuit.
The bung had swollen ( not sure why but may have been ethanol ) and blocked the passage between the main and idle castings.There is a diagonal passage between the main & idle castings.
At idle, the idle jet was supplying enough fuel to run properly but once throttle was applied there was not enough fuel passing through the idle circuit.
Remembering the idle circuit operates from idle through to 1/2 throttle , the motor was starving for fuel until full throttle when the main supplies all the fuel.
Have never seen it before but now I trim the bung 1-2mm to make sure it can't happen. ;D
PS. The part No.s you asked about ford are for the choke/enrichment device.. Electric solenoid. It squirts fuel into the manifold.
Hey, one of my old kawi's has a rubber ring that does the same sort of thing, but it's for the main jet. Took me forever to figure out why it bogged to death when the throttle was opened more than half!
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OK TMKIWI I have to ask... How in the hell did you figure that one out. I would of been pulliing what's left of my hair out
Well after pulling my hair out :D I took stock and just thought how a carb circuit works. I basically thought back to trade school.
Every thing was fine ( so I thought ) but something was restricting fuel flow.
I measured the depth of the cross drilling ( with a vernier ) and measured the rubber bung. :o
It was a bit hard to determine the exact depth in a hole only 6mm diameter but it measured up that there was no clearence. I trimmed all 3 bungs on all the carbs, reassembled and presto.
1 sweet running engine. ;D
I have found thoughout my life that if you get stuck on something, just walk away for awhile and genuinely the problem will come to you.
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I have found thoughout my life that if you get stuck on something, just walk away for awhile and genuinely the problem will come to you.
Good find.
Also,the above statement couldn't be more true.You often have the "aha" moment when thinking of,or doing something unrelated.
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I have found thoughout my life that if you get stuck on something, just walk away for awhile and genuinely the problem will come to you.
Good find.
Also,the above statement couldn't be more true.You often have the "aha" moment when thinking of,or doing something unrelated.
I can't agree more... I'm an accountant, and I have days when two plus two is five... yes I then take a stroll around the plant or just around the office. I need to get away from the problem let the brain matter (or what's left of it) cool off. TMKIWI your're right after you come back from cooling off, the answer is usually staring you in the face. :o