1981 TT250 dies with throttle blip

Airbox, Fuelpump, Carburettor
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scottieh
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Posts: 484
Joined: Wed Mar 30, 2011 3:58 am
What Bike or Bikes Do You Own?: 1988 XT600Z 3AJ
1985 XT 600
1980 TT500
1981 TT250
Previously Owned Bikes: 1982 XT550
1989 Yamaha Virago 750
1994 DR650
1980 Suzuki GR650
1997 KLR 250
1986 Honda Spree
Location: Salt Lake City

1981 TT250 dies with throttle blip

Post by scottieh » Fri Jan 18, 2019 5:30 am

I’ve been rebuilding a TT250 carb and while it runs better than before it has an issue. When i smoothly open the throttle the bike revs evenly and sounds great. However when i blip the throttle the bike dies. My initial thought is a lightly clogged jet but I’d like to hear what y’all think.

https://youtu.be/s_YM-WkEy-Q

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scottieh
Financial
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Posts: 484
Joined: Wed Mar 30, 2011 3:58 am
What Bike or Bikes Do You Own?: 1988 XT600Z 3AJ
1985 XT 600
1980 TT500
1981 TT250
Previously Owned Bikes: 1982 XT550
1989 Yamaha Virago 750
1994 DR650
1980 Suzuki GR650
1997 KLR 250
1986 Honda Spree
Location: Salt Lake City

Re: 1981 TT250 dies with throttle blip

Post by scottieh » Sat Jan 19, 2019 12:43 am

So I'm learning more about the carburetor on this old TT250 and it turns out it has an accelerator pump on the side. I'm not that well versed in carbs other than the XT600 series so the pumper carb was a slight mystery at first. Some blogs I've read state that the pump action only affects throttle twists at idle. Others state they work throughout the whole throttle range. Either way when I cleaned it I wasn't as thorough as with the other components and so I suspect a stuck check valve may be my culprit. Removing this carb is a hassle that requires unbolting the cylinder intake boot. Not a lot of space between the airbox and cylinder on these bikes.

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scottieh
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Posts: 484
Joined: Wed Mar 30, 2011 3:58 am
What Bike or Bikes Do You Own?: 1988 XT600Z 3AJ
1985 XT 600
1980 TT500
1981 TT250
Previously Owned Bikes: 1982 XT550
1989 Yamaha Virago 750
1994 DR650
1980 Suzuki GR650
1997 KLR 250
1986 Honda Spree
Location: Salt Lake City

Re: 1981 TT250 dies with throttle blip

Post by scottieh » Mon Jan 21, 2019 4:21 am

Took the carb apart today to investigate the accelerator pump and encountered some curious perplexities.

In the first photo is a small jet that leads from the throat of the carb down into the accelerator pump. Inside the jet appears to be a small ball which I've read functions as a check valve. Spraying carb cleaner in one side I could get fluid to pass to the other, but spraying in the other direction I could not see the carb cleaner spraying through. Check valve works. Check.

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Here's the side of the carb intact

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From below you can see the lever action of the rod. By twisting the throttle the rod is depressed. The long spring to the left pushes down on the rod which pushes the lever up on the bottom of the carb in the middle of the phillips head screws.

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Here you can see the diaphragm disassembled. The rod pushes up on the center piece of the diaphragm which is resisted by the spring.

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Now here's the odd part to me. Inside the accelerator pump body I can find only one fuel hole which goes up to the carb throat past the check valve jet. What I can't find is a fuel hole to drain fresh fuel into the accelerator pump.

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Here's a photo from another guy's moto repair website that shows where fuel would drain from the bowl down into the accelerator pump.

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Any ideas about what to look for? I'm not going to put the carb back on the bike until I can see small jets of fuel coming through the accelerator pump orifice in the carb throat.

jibber
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What Bike or Bikes Do You Own?: 2 1vj's, 2 3aj's none complete and a mountain of parts
Previously Owned Bikes: Quite a few
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Re: 1981 TT250 dies with throttle blip

Post by jibber » Mon Jan 21, 2019 7:18 am

Nope, but good luck

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scottieh
Financial
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Posts: 484
Joined: Wed Mar 30, 2011 3:58 am
What Bike or Bikes Do You Own?: 1988 XT600Z 3AJ
1985 XT 600
1980 TT500
1981 TT250
Previously Owned Bikes: 1982 XT550
1989 Yamaha Virago 750
1994 DR650
1980 Suzuki GR650
1997 KLR 250
1986 Honda Spree
Location: Salt Lake City

Re: 1981 TT250 dies with throttle blip

Post by scottieh » Mon Jan 21, 2019 5:59 pm

Took another stab at the accelerator pump and found the hidden hole at the bottom of the carb bowl that drains into the pump. (hole to the left-center of photo)

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I was able to spray carb cleaner down into the hole and have it spray through to the disassembled diaphragm reservoir with a somewhat steady stream, but when the carb bowl is filled with gas the gas doesn't flow down. :foot_tap:

I've tried priming the pump with several actuations of the lever hoping to see some bubbles pop up to bleed the pump, but no luck, so I've ordered a replacement OEM diaphragm which is miraculously still available on partzilla and will have to try again when new parts arrive.

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