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DT3 (also DT2) Carb rebuild: Clogged Air Passageway? VM26SH

  • Yamadude
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Hey guys,I'm restoring a 1973 DT3 with the original Mikuni VM26SH (which is also found on the DT2) which I am rebuilding.I've cleared all the passageways except for one, for which I tried carb spray, compressed air, and soaking for 20 hours in Berryman CarbDip.But now I'm thinking that maybe it is not suppposed to be a clear passageway. Can anyone tell me?It is an air inlet, labelled "A" in picture  one. It appears to travel down to the Pilot jet tube "C". It is clear all the way to where the pilot jet tube is, then deadends a few mm from the tube.I don't see "light at the end of the tunnel (passageway)" when I put light in the pilot jet tube.Is this passageway supposed to be clear or blocked? In pic two, I labelled the passageway "B".  
Current bikes: 1973 DT3, 1974 TY250, 1969 Triumph Bonneville, 1975 XS650, 1976 XS650, 1982 XS650, 1983 XS650, 2006 FJR1300
Last edit: 22 Feb 2021 10:39 by Yamadude. Reason: change picture
22 Feb 2021 10:27 #1

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The center passage is the "air bleed" to the needle jet.
The passages to each side are for the pilot circuit...  which one is used depends on which side of carb has the idle air screw.  On your carb, the idle air screw should be on the left?  So "A" would be blanked internally.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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22 Feb 2021 12:38 #2

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I looked at your picture closer and it looks like the left passage is blocked by a brass ball? 

That probably means Yamaha/Mikuni tied the right side idle air passage to the center air bleed.  This was done on some models as a form of "fine tuning".

(Some models had the idle air passage connected to an external fitting...  but that was rare and I don't think I've seen in on an enduro.)
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
22 Feb 2021 14:39 #3

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  • Yamadude
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Thanks for your replies Mark,  You seem to be clarifying some of the head scratching I've been doing in regards to the configuration of this carb, which my Clymer manual does a poor job of. Yes, the air screw is on the left side, see diagram below, #36.  And yes, the left air inlet is blocked from the factory with a brass ball bearing.  And when I screw in the air screw,  the tip of the screw can be seen entering the center passageway.  So, am I correct that when I spray carb cleaner into the right inlet "A", the liquid should squirt out into the Pilot tube "C"?  At the moment, it doesn't squirt anywhere, just builds up in the passageway. 
Current bikes: 1973 DT3, 1974 TY250, 1969 Triumph Bonneville, 1975 XS650, 1976 XS650, 1982 XS650, 1983 XS650, 2006 FJR1300
Last edit: 22 Feb 2021 16:40 by Yamadude.
22 Feb 2021 15:02 #4

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Note that I corrected my last entry by saying that : " And yes, the left air inlet is blocked from the factory with a brass ball bearing."
Current bikes: 1973 DT3, 1974 TY250, 1969 Triumph Bonneville, 1975 XS650, 1976 XS650, 1982 XS650, 1983 XS650, 2006 FJR1300
22 Feb 2021 15:07 #5

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Yamadude wrote: .............................................  So, am I correct that when I spray carb cleaner into the right inlet "A", the liquid should squirt out into the Pilot tube "C"?  At the moment, it doesn't squirt anywhere, just builds up in the passageway. 
 

No, you are not correct.  As I said above, the right inlet should be blocked as air screw is on left side.
Left inlet should connect to center passage very near back of carb.  That left passage goes past the air screw and connects to the pilot tube "C".

The brass ball makes it harder to test the left passage... I've known people to remove the ball to make sure the left passage is cleaned out. Leaving the ball out doesn't seem to make a significant difference in running according to someone I know that "does it all the time".  I haven't tried it.
 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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22 Feb 2021 17:58 #6

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Maybe this helps...  if the air screw was on right side of carb, then the right side passage would be open to the pilot well and the left side passage would be blanked.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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22 Feb 2021 18:00 #7

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OK, cool, I'm getting it;  the right passage should be open at the intake side (as it is)  and blocked at the front of the passage (as it is).    Therefore, when I've shot cleaner into and it just fills up with fluid, this is as it should be, yes? 
Current bikes: 1973 DT3, 1974 TY250, 1969 Triumph Bonneville, 1975 XS650, 1976 XS650, 1982 XS650, 1983 XS650, 2006 FJR1300
22 Feb 2021 18:13 #8

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Yamadude wrote: OK, cool, I'm getting it;  the right passage should be open at the intake side (as it is)  and blocked at the front of the passage (as it is).    Therefore, when I've shot cleaner into and it just fills up with fluid, this is as it should be, yes?

 

Yes, exactly.  It's not in use. 

The blank carbs have the passages for either a left or right air screw..  which passage is machined to connect to the pilot well is determined by which side of the carb is machined for the air screw.  The other passage is a dead end.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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Last edit: 22 Feb 2021 18:21 by MarkT.
22 Feb 2021 18:19 #9

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Mark, you have been exceptionally helpful !    Thank you.  I have printed out your responses to add to my file on this bike. I can't wait for the snow to melt to  do the next road test. 

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Current bikes: 1973 DT3, 1974 TY250, 1969 Triumph Bonneville, 1975 XS650, 1976 XS650, 1982 XS650, 1983 XS650, 2006 FJR1300
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22 Feb 2021 18:26 #10

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