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TOPIC: DT175MX 1980 mikuni carb float heights

DT175MX 1980 mikuni carb float heights 19 May 2019 06:09 #1

  • ajhuxley
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Hi
So I've been having pain with my carb, all started off with a cracked brass overflow in the float bowl. Got that sorted with a used float bowl. During the times I had the carb on and off trying to sort this, I got different cold starting behaviour. It was a right pain taking many kicks, then after having the carb off and cleaning it up and discovering the cracked brass overflow, I put it back with the external plastic overflow pipe looped above the fuel level stopping it draining out and the bike then always started first time.
Now I've put a new floatbowl with uncracked brass pipe on, ITs back to taking several kicks to get going. The idle also doesnt seem too good. IT seems like its idling rough on pilot system and only starts to idle and pick up nicely when the throttle slide kicks in (I have to have it idling at 2K RPM to avoid stall. I visually checked the rubber connections between the reeds and the airbox and all look ok.
Now I've just noticed that when I lean the bike over 30 degrees violently, I get a bit of fuel drip out the overflow. I did check the float heights when I had the carb off and they were within tolerance (I think 21mm) So I have 2 questions -

1 / fuel coming out overflow when I shake the bike - does this suggest float heights are wrong and the fuel level in the bowl is too high?
2/ would too much fuel in the float bowl affect cold starting and the idle / pilot system?

As always, appreciate peoples experience on this. Looks like Im going to have the carb off again for about the 6th time

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DT175MX 1980 mikuni carb float heights 19 May 2019 08:54 #2

  • MarkT
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There is a well in the the float that a brass tube from the carb body goes into. In the bottom of that well is a tiny jet... the starter jet... it's where the starting device ("choke") knob draws the fuel from. If that tiny passage is restricted or blocked, the bike will be very difficult to start cold.

It's normal and common to puke some fuel out the overflow when the bike is leaned way over and/or shaken violently side to side... that's why it's there.

Check and re-clean your pilot jet... I go up one or two sizes from 22.5 to 25 or even 27.5 on the later 175 and they run a lot better. They came terribly lean from the factory in the attempt to meet emissions and today's oxygenated fuel makes them run even leaner... a slightly bigger pilot helps the starting and the idling... 25 is usually enough. Use original Mikuni jets.

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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

DT175MX 1980 mikuni carb float heights 19 May 2019 10:05 #3

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I forgot to add I set the floats on Mikuni's with the float arm parallel to the gasket surface when the needle just closes.... if the needle valve has a spring loaded end you want to make sure the spring isn't being compressed...

Has always worked well for me.

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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

DT175MX 1980 mikuni carb float heights 19 May 2019 12:19 #4

  • ajhuxley
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thanks markt, as ever you are the fountain of knowledge :-)
Maybe the jet pick up some crap in all the on and offs but I'll take your advice and increase the jet if I can easily locate one here in the UK and at the same time I'll see what the floats look like compared to your tip of parallel as the fuel puking seems just a little too sensitive.
out of interest where do you source jets from?

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DT175MX 1980 mikuni carb float heights 19 May 2019 12:37 #5

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Niche Cycle Supply in Florida USA is where I like to buy Mikuni parts... they also sell on ebay.

The 1980 DT175 should have the small "Banshee" ATV type pilot jet... In fact I've bought the standard pilot for a Banshee from a Yamaha dealer to use in a DT175 as it's a #25. Yamaha part number 260-14142-25-00. Probably fits many other bikes so a good bike shop or Yamaha dealer in the UK might have them.

Here's a link with a picture just in case the UK model uses a different style pilot jet.

www.partzilla.com/product/yamaha/260-14142-25-00

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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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