Hi all, I’ve just stripped the carb on my 1975 DT400B (twin shock) to re-check all the sizes.

I’ve got a couple of queries…

1. The jet needle has 6F9 stamped on it. Is that standard? 
The reason I ask is that the Service Manual states 6DH2, but the only ones that I see for sale are 6F9. Is one the Yamaha part code and one the mikuni part code?

2. The service manual states ‘starter jet’ (ie, NOT pilot, air, needle or main) = #60. Is that referring to the starter plunger (choke)? The choke on my bike does not seem to have any markings on it. Is that a problem?

Thanks for taking the time to respond…..

Starter jet is pressed into a well in the bottom of float bowl and one of the only jets that doesn't have a size stamped in it.  Just make sure it's clean. 

 

Yamaha Service Data sheet I have says 6F9 is the correct needle..  Clip in groove 4.

Check Mikuni tuning guide in tech library for explanation of needle numbers and letters...  they denote different needle tapers and lengths.  6F9 is not the same as 6DH2.

Many thanks (again). I’m learning lots today…

I would be VERY interested to know what needle valve/clip setting (and other jets) others have in their DT400B twin shocks, especially if they have a Rex Speedshop 12v ignition system (as this may alter the standard carb settings?)
Many thanks…

If you go here:  yamaha-enduros.com/index.php/forum/74-76...b-float-level#124807 and scroll down to post number 4 you can see a chart showing conflicting info on the DT400 twin shock carb settings.

Like Mark said, you are good with 6F9.

Many thanks for that. It must be very confusing for everyone restoring a DT400B (twin shock). I am now happy that I have a ‘standard’ needle.
When I get a chance, I’m going to do a number of test runs:
Starting off with clip position 4 and 1.5 turns out on the carb air screw (standard), then all the remaining combinations of clip positions 4, 3 and 2, with 1.5, 1 and 0.5 turns out on the air screw.
I do hope that the Rex’s speed shop 12v ignition system isn’t making the ‘cruising/surging’ problem worse…
Anyway, see how I get on. I’ll post my results.

 

Hi there
for what it is worth, my DT360A had this cruising / surgine issue that others refer to as 4 stroking.  I reduced it a ton by going from a 50 to a 55 pilot jet (going on memory) and up one groove richer on the stock needed and needle jet

So, I assume a larger number for the pilot jet is a larger jet, therefore richer?
When you say up one groove on the needle jet, I assume you mean raising the needle, therefore making it richer?
I’m just trying to get a basic understanding of whether the jetting at the moment is too rich or too lean and I take it from what you have said that it’s too lean…
Sorry if I’ve misunderstood.
Thanks…

yes on both sir!
let us know what you end up doing and results
I have a DT400E that does same surging but to a much lessor degree
it runs so good, but I am tempted to increase pilot a size on this one too