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TOPIC: Im a little confused here on specs for 75 400

Im a little confused here on specs for 75 400 13 Jul 2019 18:33 #1

  • Don Dieago
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Ok first the book says main and pilot are 160 and 40 but the parts list says 170 and 50. Which should I believe? Then secondly it says float arm set to 17.3 + - 2,5 No way! what is correct setting?

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Im a little confused here on specs for 75 400 13 Jul 2019 20:30 #2

  • RT325
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Just to help confuse you--& me, this pic from Partzilla says 180 & 170 if you expand the pic up a bit--goes blurry though. Might be different markets like european & oceania & different elevations. Float arm looks like it has floats on stalks so how does your arm measure up depending how you measure it. Square on is a pretty good starting & finishing point if you have nothing else to go by. Floats need to go the right way up of course, which is with the lifting pin on float at the bottom--depending if ya standing on your head, bottom being bottom of float bowl. Sorry--i'm not helping an i. Tell us more please.
www.partzilla.com/catalog/yamaha/motorcycle/1976/dt400c
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Im a little confused here on specs for 75 400 14 Jul 2019 05:14 #3

  • Don Dieago
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I understand how it works Suzuki used same style mikuni VM . I just don't know what setting measurement to go with or what jet size it came with as a starting point. On float level I just set it until with bowl removed I had fuel 1/4'' below top of bowl. I did see the chart below parts list but never noticed it showed 23mm setting Until now. Thanks think I go with that.
That's 6mm more than manual says that close to 1/4'' difference. I've seen so many odd things in the 1970's service manuals didn't The Japanese proof read anything before printing????

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Im a little confused here on specs for 75 400 14 Jul 2019 06:04 #4

  • MarkT
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Unfortunately there are a lot of errors in the specs. Float level is likely one of them, jet specs might not be.

Setting the float arm parallel to the gasket surface with needle closed (but spring loaded plunger on needle not compressed, if equipped) like RT325 said works very well on all Mikuni's...

The EPA was starting to regulate emissions... noise and smog... which resulted in more restrictive "muffled" air filter systems (a surprising amount of noise comes from the intake on a two stroke) and attempts to reduce exhaust gas pollution.

Then add in the "surging" that seems to plague the 360/400's and it shouldn't be surprising that Yamaha may have updated jet specs a few times. My experience is to check ALL the jet specs as sometimes they changed the slide needle, needle jet, slide cutaway, etc. For example, it's not a good idea to run a main jet from one table of specs when that table specs a different slide needle than your carb has installed.

Finally, fuel has changed a lot in 40 years... and altitude and ambient temperature can change the required jets considerably. If you're not riding at sea level and 68 degrees F adjustments to the stock jetting might be needed... but I'm sure you know that.

Good luck and I hope you get it running perfectly without too much fiddling!

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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

Im a little confused here on specs for 75 400 14 Jul 2019 06:21 #5

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Thats what I did Mark It actually runs fine but a little rich side with the fiber reeds I put in. with new needle n seat arm at level it overflowed. I'm assuming the jet needle and needle seat that in it is right so I ordered new genuine same from jetsrus and a new genuine needle n seat along with main 2 above and 2 below size in it now. we will see if assuming bites me

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Im a little confused here on specs for 75 400 14 Jul 2019 12:21 #6

  • JayB
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Don Dieago wrote: Ok first the book says main and pilot are 160 and 40 but the parts list says 170 and 50. Which should I believe? Then secondly it says float arm set to 17.3 + - 2,5 No way! what is correct setting?


I was so confused, I did a spreadsheet months ago:



When you sort it out and get it running well, please share your settings!
Thanks,
Jay B.
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Im a little confused here on specs for 75 400 15 Jul 2019 06:31 #7

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Thanks Jay! i did some looking and sure enough the number on my needle came back as one that came in one of those cheep ebay kits. I have everything on the way genuine mikuni So I can put it back to OEM and will go from there. have enough genuine jets coming to go 140 to 180 and pilot 35 to 50 so Im covered also genuine needle and seat and air screw as well.

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