Yet another DT175MX question... (Mikuni carb)

Thanks to a certain Pip of this Parish, I now have a Hayne's BoL
for the DT. And I'm still puzzling over why the carburettor needs the
idle screw screwed in to the last micrometre for the bike to idle; the
needle clip is in the middle position, and removing the bodged knitted-
cloth air filter makes no difference, so that's not over-restrictive.
The slide is open by several mm, so it's no wonder that the idle mixture
screw has no effect.

But, I've blown out all the orifices, and poked fine copper wire
through 'em, so there is no obvious air or fuel blockage in the idle
circuits. It still won't run that far down (he says, realising that he
hasn't actually _tried_ it; it just dies as the screw is outwound).

Clutching at straws, what if the float level is wrong? Ah, the BoL
gives the float height spec -- but not how to measure it! I know this
varied between bike manufacturers, but I'm not sure if a Yam Mikuni uses
the same criterion as a Suzuki Mikuni -- and it's been so long that I'm
not sure exactly how Suzuki defined it anyway (upper limit of the float
body to the _alloy_ sealing surface of the float bowl?).

Anyone know?

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

ISTR you can download a complete Mikuni manual for free from their website.

Alternatively I have a copy here I can email over if you'd prefer.

We found it useful when fiddling with the TS50's micro-carb.

--
Dan L (Oldbloke)
My bike 1996 Kawasaki ZR1100 Zephyr
M'boy's bike 2003 Honda NSR125R (Going)
Spare Bike 1990 Suzuki TS50X (Patio Ornament)
BOTAFOT #140 (KotL 2005), X-FOT#000, DIAABTCOD #26, BOMB#18 (slow)

 

Ah, thanks! I hadn't thought of that route, focussing more on
"Yamaha" than "Mikuni". I've been a tad preoccupied with work this week...

Thanks, but I appear to have found it on the Web-site.
Wahay, the first illustration is a Yamaha carb -- Suzuki didn't have
the removable cap over the main jet.

Cheers.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Been wondering whether to ask... is the exhaust system free-flowing (or
at least as good as it can be)?

Surely float height wouldn't make any difference to it idling?

Regards,

Simonm.

 

Glad to have been of service.

Does this mean that I can no longer consider myself a newbie then? [1]

[1] Mr Blaney reckons you're the arbiter of newbiness.

--
Dan L (Oldbloke)
My bike 1996 Kawasaki ZR1100 Zephyr
M'boy's bike 2003 Honda NSR125R (Going)
Spare Bike 1990 Suzuki TS50X (Patio Ornament)
BOTAFOT #140 (KotL 2005), X-FOT#000, DIAABTCOD #26, BOMB#18 (slow)

 

At first sight, yes. I can feel individual puffs coming out the
spark-arrestor so I know there's not a _complete_ wasp's nest up there.

Well, I'm trying to work out why it appears to be getting no fuel
through the idle circuit. The Mikuni book should help (but it's generic
and doesn't give the method of measuring float height either...) but
I'm off tomorrow for a conference in Oxford[1] so further investigation will
have to wait until next weekend.

[1] Any recommended bike shops near the centre of Oxford, by the way?
It'd be a chance to buy a few bibs 'n' bobs.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Scratch that, it does tell how. I missed it scrolling through the
document on-screen.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

once had a blocked baffle thing in a H100 'zorst, and the main differnce was
that a slow bike became even slower. small bit of soot in a critical hole,
was all it were.

 

BTDTGTTS! I did get the carb off again last night -- and everything
is infuriatingly nominal. All the removable jets etc have the sizes given
in the BoL; float height is specced at 21 +/- 1.5 mm & I measure 21.04 ...
I think I'll put it back on with the idle screw where *I* think it should
be, and see if I can get it to run. It's been so high that I think the bypass
was not functioning in idle mode (where air is blocked by the slide and
enters the vertical hole to help drag fuel from the pilot jet and out the
downstream hole). I'm going to have to drag out my copy of the Two-Stroke
Tuner's Handbook, but I think Jennings may have taken a lot of his description
from this Mikuni handbook anyway.

If that fails, yes it's exhaust- and timing- time.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Large cork & caustic soda seems to work fairly well. But I don't know
what you do about the exhaust...

Regards,

Simonm.