Vintage Dirt Bike Q & A

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 Post subject: Jetting issues
PostPosted: Mon Mar 06, 2017 4:45 pm 
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Joined: Thu Nov 03, 2016 3:54 am
Posts: 28
At least I think its jetting issue on this old bike. Any info would be greatly appreciated....here's the info:

Reconditioning:

Head/piston de-carbonized, exhaust cooked out, new magneto and points, carb ultrasonically cleaned and rebuilt (keyster mt250 kit #122, #60), fuel tank rust removed, 30 micron fuel filter, new aftermarket air filter, leak down tested, compression tested, new left crank seal, trans oil flush and refill. Using Bel-Ray synthetic injector oil (using the stock pump).

Other pertinent info:

Tested the intake boots for leaks: none found
Snorkel has been removed from airbox (tended to collapse a bit when engine revved)
93 Octane fuel
Needle set at lowest setting (Since everything I’ve read is these bikes run too rich)
Bike starts (cold) on first or second kick even at 35 or 40 degrees outside temp

Symptoms:

Idles steady and revs to red-line when NOT under load
Breaks up/sputters at about half throttle when under load

I did a couple “burn outs” in the field and immediately thereafter (after pulling in the clutch) the engine nearly died; I kept easily blipping the throttle to keep it running and then it recovered, but was idling low thereafter. I then rode it back to the barn and let it idle low and erratically until it died; then it wouldn’t re-start after several kicks.

Any ideas???


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 Post subject: Re: Jetting issues
PostPosted: Tue Mar 07, 2017 8:31 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
These sound like classic symptoms of very low compression. Try checking it with the engine warm & throttle closed.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric


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 Post subject: Re: Jetting issues
PostPosted: Wed Mar 08, 2017 6:50 pm 
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Joined: Sat May 24, 2014 7:19 am
Posts: 20
How's the sparkplug lookin?

Attachment:
sparkplug.jpg
sparkplug.jpg [ 95.34 KiB | Viewed 867 times ]


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 Post subject: Re: Jetting issues
PostPosted: Fri Mar 10, 2017 4:10 pm 
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Joined: Thu Nov 03, 2016 3:54 am
Posts: 28
Thanks Gentlemen.


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 Post subject: Re: Jetting issues
PostPosted: Fri May 12, 2017 7:30 am 
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Joined: Sat May 24, 2014 7:19 am
Posts: 20
rayivers wrote:
These sound like classic symptoms of very low compression. Try checking it with the engine warm & throttle closed.

Ray


Shouldn't compression be checked with the throttle wide open?

Mark


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 Post subject: Re: Jetting issues
PostPosted: Fri May 12, 2017 2:40 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
Quote:
Shouldn't compression be checked with the throttle wide open?

Normally, yes. In this situation I'd prefer to see the worst-case number, but any reasonably-accurate compression check is better than none.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric


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