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1978 dt400 jet size with porting and chamber

  • DT400FRANK
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My dt400 has a #180 main jet right now and it's a bit lean after I gutted the oem exaust. 

I am in the the process of getting my third wiseco over bore to 86.50mm and I will be installing a new Jemco exaust and will be getting the cylinder ported. Also I'm going with the Boysen duel stage reeds and will be removing the inner reed cage bridges.

I will be using a stock 34mm carb and oem air box. 
what size main jet and pilot do you think I should order to compliment this set up. I will be breaking in the new bore job with this jet.

I was thinking a #210 main.
Any thoughts?
thanks
Last edit: 03 Jul 2022 21:56 by DT400FRANK.
03 Jul 2022 21:16 #1

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My advice is to buy a selection of jets.  Not that expensive if you get genuine Mikuni from someplace like Niche Cycle. 

Beyond that, the answer is "give it what it needs"
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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03 Jul 2022 21:23 #2

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For every change you're making I'd guess a size up. In that regard 210 is accurate. But then there is elevation and temp. I'd personally splurge and get 190-220 with a trail tech spark plug gauge and go off temp. No moto has this but I just love this chart my sled came with to some extent you can extrapolate from here; 
 
1972 Yamaha CT2 175
1972 Suzuki TS185 Sierra
1986 Yamaha IT200
2013 Yamaha XT250
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03 Jul 2022 22:40 #3

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and for what it's worth, for mechanical changes I have a sled with aftermarket twin expansion chambers instead of a single muffler, aftermarket head, and gutted airbox, and have main 430/440 jets for 6000+ elevation and 20-40f temp range, and stock for that elevation and temp is main 340 jets. (twin cylinder, exhaust runs by the right side so it gets a +1 jet for heat) It's a pretty similar size engine to your DT400 but water cooled so it needs to run cooler. If my needle is in the 3rd position it bogs over 8k elevation, really needs to be in the 2nd to pull clean up to 9k feet. Guessing you can massage those numbers into your needs to some extent, at least in their relation to each other.
1972 Yamaha CT2 175
1972 Suzuki TS185 Sierra
1986 Yamaha IT200
2013 Yamaha XT250
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03 Jul 2022 22:57 #4

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Yes, I've had to make changes of 100 or more on modified engines...  that's why I said "give it what it needs". 

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There just aren't any shortcuts with jetting..  Sometimes I've ended up same or even leaner than stock.  If engine is in good shape and I plan to ride it hard my go-to method is up on the main until it just blubbers....  engine must be well warmed up and ridden a while, you can't do this in 5 minutes...  then go down until it just cleans up at WOT..  Maybe one size leaner from there. 

Mikuni Tuning Manual (available on Internet or in Tech Library here) is a must-read IMHO. 

Jets are cheap. Pistons, boring, and possibly bearings are not. 

Most of the time with mild mods and stock carb size you're already going to be pretty close.
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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04 Jul 2022 05:49 #5

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Replied by RacerClam on topic 1978 dt400 jet size with porting and chamber

Im with Mark ,  Start richer than you think you need and drop sizes until it just cleans up. I would drop the Boyesen reeds also , V force works waaaaaay better but they arnt just a drop in, some mods required but well worth it . Maybe it would be worth your while to give me a call before you pull the trigger on porting and reed changes , A intelajet addition would solve your jetting dilema with a turn of a remote dial.  702-858-9177

Rich
racerclam
Last edit: 04 Jul 2022 11:33 by MarkT.
04 Jul 2022 11:21 #6

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  • AirborneSilva
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Replied by AirborneSilva on topic 1978 dt400 jet size with porting and chamber

racerclam what's this "V force" you speak of? 
1976 DT400C
1981 DT175H
1972 DT2 250
04 Jul 2022 13:35 #7

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Replied by RacerClam on topic 1978 dt400 jet size with porting and chamber

V-Force reeds , mototassinari, I use alot of them in my porting work , best reed set up , Normal reeds are set on one v cage , V force has two with eight carbon fiber pedals 
racerclam
04 Jul 2022 15:41 #8

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  • AirborneSilva
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Replied by AirborneSilva on topic 1978 dt400 jet size with porting and chamber

Interesting...  How  does one go about getting them?  Does it only make a difference if you're cylinder is ported? 
1976 DT400C
1981 DT175H
1972 DT2 250
04 Jul 2022 15:54 #9

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  • DT400FRANK
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Replied by DT400FRANK on topic 1978 dt400 jet size with porting and chamber

Ok Rich I was planning on you porting the bike but I'm just not sure if I want to put so much money in the bike with v force reeds.
I'll call.    Wiseco still has my old piston determining if the loose ring locating pin is coverd in warranty. Once I get piston its off to get bored.
05 Jul 2022 01:11 #10

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