Jetting time

[rd400canuck]

Yup.. replaced eveything in the carbs with all new  mikuni stuff. Did anothwr full throttle plug chop with the 130 mains.. bone white.. not even a hint of a ring at base of insulator.

[rd400canuck]

I havent replaced main jets yet but i did just take the bike on its first lengrhy-ish trip. 2 hours at a steady 65-70 mph to visit a friend. Bike ran like a freakin champ. Settles in at 5300 rpm or so.. the vibrations lessen and it isnt that loud at all with the stock exhausts. Bike seems to be pretty thirsty though. 7 liters of fuel to do 100kms. 40 mpg. Is that normal? Its only 11 liters before I have to switch to reserve and I burned 7 of them in 1 hour.

Anywho... i pulled the plugs when i got home. This is after 2 hours of steady hwy speeds. Looks good to me but there is no ring at bottom of insulator.. is that ok?

[1976RD400C]

It's not ok. You have to richen it. Both the main and raise the needle and see how it runs. Are you using a B8 spark plug?

[rd400canuck]

Yes, B8ES.

Damn.. that means needle on 5th and maybe a bigger pilot than the 27.5 - I tried enriching both last week and the bike didnt run as smoothly as it is now, it surged and chuffed just from the needle going from 4th to 5th alone.

What should be my next step to richen up 1/4 throttle hwy cruising? Should I consider the next size up needle jet to let more fuel past at part throttle hwy cruising? If I have to run 5th clip on needle shouldnt I get a fatter needle jet anyways?

I can't believe this thing is gonna get worse fuel mileage than it already does!

[m in sc]

40 mpg on modern fuel on these isnt bad at all. shit, my old s2 would get 15-18 mpg when stock. 

[Striker1423]

Yea I get 35 mpg on my bike if I'm easy on the dinosaur juice. Grab it and go a lot (which is always tempting) and that drips to 25 or so quick.

[rd400canuck]

haha ok... well I wont be taking this thing on any cross country trips then! Certainly thirsty little bastards.

Today I will try the 5th clip position again and play with the airscrew to see if I can get it to behave. I tried it a couple weeks ago just to see and although I felt it pulled harder in part throttle positions I hated the way it surged down low and the chuffing on the hwy at steady speed. I could feel the power shuddering while at a steady 65 mph.

I'm almost tempted to just sort out the fatter mains it needs and call it a day. I hate the idea of undoing how smooth and linear the power is now.. no weirdness, no surging and no chuffing and stuttering at steady throttle... I had it at 70 mph for 2 hours in coolish air and it didnt melt on me so...... I want to think it's safe to leave the pilot and needle as is !!!   Here's to hoping.

[rd400canuck]

Ok.. on 5th clip. And turned air screw from 1/2 to 1 out to kill some of the part steady throttle surging. The bike certainly pulls stronger at part throttle positions that's for sure. The plug is a more delicious chocolate brown than they caramel of the 4th clip colour and more of the insulator has it. Still no ring around the bottom but im on the 5th clip and cant go any richer.

I'm going to spend the day dialing in the air screw and see if I can make the bike smooth everywhere like it was before.

One thing I noticed was when I got on the hwy the bike got to 70 mph so effortlessly I didnt even notice until I looked at the speedo. It seems to seriously prefer 5th position on the hwy for sure. Kind of smoothed out more... seems to take less effort to get up to speed. I really like it.

Thoughts on plug colour?

[Striker1423]

Looking better. What main is in it again? Stock airbox  correct?

[rd400canuck]

Yes completely stock intake snorkel and all but I have a lightly oiled uni filter in it.

I am on 130 mains (stocks werre 115's) but I did a couple plug chops and they came out bone white. I have richer jets on order up to 150. Until then I dont pass 1/2 throttle.

[m in sc]

id get a cht gauge (the kind that uses the ring under  a plug lead)  to temporarily use to tune to verify for piece of mind.

[1976RD400C]

From the manual, how the needle and jet react to changes:

[rd400canuck]

Well... Ive changed my mind yet again, I dont like 5th clip position. Doesnt matter what I do with the air screw something suffers. After looking at some mikuni jetting guides I'm going to wait until I have some reasonable mains in. They show them as having some influence down as low as 1/4 throttle. Maybe I'll get lucky and some richer mains will allow me to keep the needle on 4th where it's almost perfect. I have 1/2 position washers I can try but I'll worry about those after I get the bigger main jets in.

Thanks for those jetting charts... explains the fatter idle and off idle from richening the needle.
I also have some CHT gauges coming :)

[rd400canuck]

I figured since my full throttle plug chops were coming out bone white that I'd go straight from the 130's to the 150's  and holy cow lol I got on the hwy near me for a 2km stretch and opened it up and noticed it wasnt quite smooth so I thought  "ok, too rich" I looked in the mirror and the entire stretch of hwy, and the cars on it, were completely smothered in smoke... looked like the fog had rolled in on them. I wish I had a picture... when I saw what I had done I started laughing. Everyone must have thought there was a car on fire up ahead but no... was just me.

I guess 140's are next up for a test...  Im just surprised 130's were white and just 4 steps up to 150's caused it to be insanely rich. Seems like with such big jumps in fuel delivery it would be hard to fine tune the main.

EDIT - 140's are way better. Bike pulls cleanly no matter what throttle setting or rpm. Only a small bit of smoke is visible after a full throttle blast. Now I'm wondering if I can lean the needle back to middle position, but... worry about that later. Plug chop to come this aft for the 140's

[rd400canuck]

Im starting to think I'm going nuts. Is it possible 140's are too lean and 150's cause it to billow smoke?

EDIT... just backnfrom plug chopping the 145's and they are white.  Im so confused... the 150's made it smoke the hwy up and it wasnt able to clwar its throat and now the 145's are coming out white.

Im assuming i did something wromg when i had the 150's in.. but what i dunno. Its just main jets.