Vintage Dirt Bike Q & A

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PostPosted: Sun Jul 03, 2016 7:16 pm 
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Joined: Fri May 29, 2015 7:34 pm
Posts: 34
After a winter engine rebuild I've been having a blast on the MT.
The only snag is that I have to prime the cylinder when cold as the enricher circuit doesn't seem to have any effect.
I found and soldered the obligatory crack in the feed tube; air jet to plunger passage is clear; plunger to carb throat is clear; plunger to choke standpipe is clear.
When the bike is idling at operating temp, flipping down the enricher lever has ZERO effect.
The tiny starter jet in the float bowl is screw driver marked but is definitely allowing fuel to pass.
I can't think of anything else to do.
Has anyone any insight on this issue?
Thanks.


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PostPosted: Sun Jul 03, 2016 7:57 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
I'd definitely check the starter jet again - I bet the screwdriver damage happened for a reason. This jet has to supply a lot of fuel (at very low vacuum) with little restriction and lives down in 'sediment city', so - even if it is passing fuel - IME you have to make sure it's completely clear from both the top and bottom (float bowl) directions, and that any crap loosened up down there doesn't get sucked right back into the jet orifice (there's a round area directly under the jet that collects dirt and debris, which can be hard to get completely clean with the jet in). By 'air jet', I assume you mean the choke air inlet on the center RH side of the venturi area, and not the idle air screw inlet at the lower RH side of the carb bellmouth?

I've had some success getting these jets out if the jet slot is in good shape, but if not I wouldn't bother trying.

Ray

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'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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PostPosted: Mon Jul 04, 2016 7:42 pm 
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Joined: Fri May 29, 2015 7:34 pm
Posts: 34
Thanks Ray!
I've just finished checking out the well in the bowl and the jet.
The jet has no screwdriver slot left but the hole is intact and has a 32 thou opening. Maybe it's been drilled bigger?
Fuel fills the well to the same height as in the bowl.
I can't find anything wrong.
I'll check to see that the plunger is lifting high enough to clear the passages in the body; other than that, I'm stumped.
I'd def like to not have to carry a plug wrench around with me.
Does the rubber cap that slips over the top of the plunger assembly also act as an air seal?


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PostPosted: Mon Jul 04, 2016 8:13 pm 
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Joined: Fri May 29, 2015 7:34 pm
Posts: 34
I put my finger over the end of the pickup tube and sprayed cleaner into the plunger side and voila!
My solder repair of the crack wasn't very good.
Update tomorrow.


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PostPosted: Tue Jul 05, 2016 8:41 pm 
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Joined: Fri May 29, 2015 7:34 pm
Posts: 34
I inserted an aerosol straw into the outlet hole in the carb throat; applied compressed air and the remains of a shelled insect blew out.
Now the starter circuit works. :shock:


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PostPosted: Wed Jul 13, 2016 3:41 pm 
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Joined: Tue Jan 27, 2004 8:51 pm
Posts: 136
Location: Gilbert, AZ
Thanks OP for the great updates and glad you and Ray figured this out.
I will have to take my carb apart again as IMO the choke does not work as good as it should. Sometimes I have to lean the bike waaayyyy over to get it to start.
I have 2 carbs so I will play with the spare first to learn what does what before I risk tearing a rubber boot removing my carb from my bike.

Rick


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PostPosted: Wed Jul 13, 2016 8:59 pm 
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Joined: Fri May 29, 2015 7:34 pm
Posts: 34
I understand that the carbs on these aren't very highly regarded but she's a two kick starter now. :D


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