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DT2 fuelling

12 Sep 2020 10:38
ie305mp3
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DT2 fuelling #1
I'm gradually working through my recently-acquired '72 DT2.

Current aggravation is fuelling. When I got it it would zing up to the red line in the gears, and then when cruising at around 45mph in 4th or 5th it would start to die before picking up again.

Tank breather is clear. I've cleaned the fuel system from end to end. The tank filter was actually pretty clean, but there was some minor gunge in the float bowl. Have blasted various carb orifices with compressed air. Air filter is an aftermarket cone thing I've inherited, but clean.

But since clearing the fuel system it now bogs in all gears from around 4000rpm, but will come back on power if I feather the throttle off and on, in which case I can coax it up to around 6000rpm.

The issue is that different manuals and online info state that the pilot jet/air screw should be 1 1/4 turns, or 1 1/2, or 1 1/34...

Also, the workshop manual says main jet of 160, although my classic Yamaha parts specialist says 165.

Other than that, the motor is crisp and sharp on the throttle, spark plug is grey-ish with a brown tinge. Starts immediately, ticks over, went on a couple of 20 mile trouble-free runs before the system clean, but limited to that 45mph-ish ceiling in 4th and 5th.

Doesn't feel like an electrical thing, and as its behaviour has changed while I've been messing with the fuel system I suspect it's in there somewhere.

Oh, and it's on fresh fuel (UK Shell V-power "super-unleaded"), and currently on 32:1 premix.

Any ideas?
.

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12 Sep 2020 11:24
apex
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DT2 fuelling #2
Air screw "specifications" for how far out are a start, and you tune the screw in/out to find the best/highest idle speed. I like to set the screw just a bit in from the highest idle speed, to fatten up the mixture a little bit at idle.

From your description, first thing I would do is see if the needle is under the spring plate. If the needle is set on top of the spring plate, it will rise artificially over fueling the engine.

If that's OK, I would adjust the needle clip, in either direction from the middle, in increments, ride it and see if you get a change. Lowering the clip/raising the needle from the middle slot richens the mixture moderate to WOT throttle, conversely raising the clip from middle leans out the mixture.

Pipe could also be carboned up at the spark arrestor. It's easy enough to remove, take the arrestor out, ride it and see if that changes anything.

Allrighty Then

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12 Sep 2020 12:04
MarkT
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DT2 fuelling #3
Welcome!

First thing I would check is the o-ring sealing the "jet setter" to the float bowl.

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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13 Sep 2020 00:01
ie305mp3
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DT2 fuelling #4
OK, further confusion... workshop manual says DT2 main jet is housed horizontally in float chamber, but online exploded diagrams and videos say it screws in to the the base of the mixing nozzle....

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13 Sep 2020 01:47
Gr8uncleal
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DT2 fuelling #5
Main jet is number 8 on the attached diagram.

Which manual are you looking at?

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13 Sep 2020 02:08
ie305mp3
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DT2 fuelling #6
That one. And I'm learning all the time.

Have researched further, though, and pulled carb apart again. Two things:

1) Needle jet o-ring is perished. They don't seem to be specifically available. What size is it? I assume they're available generally.

2. Looking into the float bowl, into the main jet fuel entry hole (I'm probably using the wrong terms) the hole in what I assume is the jet inside it is not centred with the hole in the float bowl. It also looks like the bolt holding it in has been glued, so possibly threaded.

All observations welcome.

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13 Sep 2020 02:31
Gr8uncleal
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DT2 fuelling #7
'O' ring (number 7 on the diagram) is part number 214-14147-00 and should still be available from Yamaha and Mikuni.

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13 Sep 2020 06:03
ie305mp3
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DT2 fuelling #8
Completely different looking carb. Are the o-rings common across a range of Mikuni carbs?

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13 Sep 2020 08:56
Pete-RT1
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DT2 fuelling #9

ie305mp3 wrote: Completely different looking carb. Are the o-rings common across a range of Mikuni carbs?

Hi Iain, are you saying your carb is different from the diagram above or the manual you are looking at?

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13 Sep 2020 09:03
MarkT
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DT2 fuelling #10
Posting a picture of what you have would be helpful.

Original carb should have tiny ID number stamped in it... usually on upper edge of mounting flange.

In the US, some have found Danco plumbing (faucet) o-rings that work... you want nitrile or something fuel resistant. Might be an "inch" size as the Mikuni is a copy of the Amal...

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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