I Need Some Carb Jet Recomendations Please

[grannyknot]

Triple SK/OER carbs and I'm about to pull the old style SK e-tubes with air corrector jet and main jet and install Weber style e-tubes and jets so I'm speaking the same language as everyone else.

 

Triple 40s

36mm main venturi

40 (.4mm) pump jet

70 (.7mm) starter jet

55F8 Idle jet

 

So one of the formulas for choosing MJ and ACJ puts me at 144 for MJ and 194 for ACJ

Does that sound like a good place to start?

 

Also, L28 engine, head work done, header, all carbs balanced, 900rpm

idle, fuel level at 26mm

Thanks,

Chris

[240260280z]

36mm Chokes will work great at WOT but may not promote air velocity at lower throttle driving and may delay the main circuit from starting.

2.8 litre main jets seem to range from ~125 as a starting point.

I will be tuning a 2.8 liter DCOE40 for a friend this weekend and will have some data for you. It has 30mm chokes.

Here is a great L28 triple carb read to get you going:

http://datsunzgarage.com/weber/

[grannyknot]

Yeah, I wasn't too pleased about the size of the chokes when I finally got around to measuring them,

but in order to change them you have to remove the butterflies right?

 

I thought I would do that test where you pull all the e-tubes and just drive the car with the idle jets,

I had no idea the idle jets and progression holes were responsible for so much of regular driving.

If I punched it the engine fell on its face, chugged and coughed, but if I was smooth with the accelleration

3500 rpm was easy to get to and at one point in 2nd gear I got up to 4200 rpm before I hit the wall.

Chris

[240260280z]

Nope the chokes come out the front where the air horns are.

I now have a lathe and plan to make some special chokes in the summer so I may be able to make you a set then if the experiments go well.

[madkaw]

Lot's of guys are looking for the bigger chokes it seems. I'm running the 32's that original came with the mikuni kit. So you don't speak the Mikuni language, just the Weber?-

So what help do you need fo jetting? Sounds like you have the pilot jet pretty close and yes it covers a lot of the driving spectrum. I would go with the numbers you came up with. 145 main jet and 190 or 200 main air should be pretty close.

[grannyknot]

And then there is Madkaw language... idle jet = pilot jet.

It took me many pages of your big carb jetting thread to figure that one out.

 

The idle jet does seem to be pretty good and I bought 6x 115 main jets and 6x 160 air corrector jets and thought I would drill up to larger sizes by steps and record my impressions.

I tried the 115MJ/160ACJ combo last night and couldn't get the engine over 4000 rpm.

I am also thinking about going down to 32mm chokes cause I think I'm missing all kinds of low end torque

and I don't spent a lot of time at WOT at high rpms.

[madkaw]

What jets were in the carbs when you got them?

[grannyknot]

MJ 100, ACJ 210

 

And this is the reason I wanted Weber jets,  this is a 150 MJ that has been soldered up and re-drilled to 100 (1mm) but gas is also flowing through the sides of the slot and filling the e-tube chamber, so I have no idea how much gas is coming through. So the cone shaped Weber jet will allow me to be much more accurate.

[grannyknot]

The SK/OER tech manual says when calculating the air corrector jet add .60mm to the main jet,

ie, 140 MJ +.60 = 200 ACJ

 

Frank Honsowetz of How To Modify Your Datsun OHC Engine says to add .20 mm.

I know these are just estimates to get you in the ballpark but that seems like a big difference to me.

Can anyone explain why two knowledgeable sources would be so far apart?

Thanks,

Chris

Edited April 18, 2015 by grannyknot

[240260280z]

I have read +60 to +80.  It depends a bit on the emulsion tube.

[madkaw]

I'd rather believe the carb maker myself. Plus too many variables - cam, etc...

[grannyknot]

Made some progress today, realized I couldn't get the engine over 4000 rpm cause I didn't synchronize the gas pedal stop and the butterflies, I was missing a 1/3rd of the throttle. Talk about a rev limiter!

 

I worked my way up in jet sizes to 135 MJ 210 ACJ. There is a small stumble between 1200 -1600rpm but more pedal pushes through it it easily, smooth acceleration all the way up to 6000rpm

AFR at 13-13.5 on cruise up a slight hill, 12.5 on a flat. I think I can tweak that a bit more to lean it out but for now I'll leave it and try and fix what I did to the idle.

 

Idle and progression sounded good and performed well but were in the 11-12 AFR.

Mixture screws were out to 1.5-1.75 turns so I turned those in .25-.4 of a turn which brought the idle up to 13 AFR but made the cruising much richer at 11-12 on a flat road.

 

Frank H. recommends trying to get the mixture screws down to .75 - 1 turn so I could go up to 60F8 idle jet from the 55F8 which will help the mixture screws but will probably make things richer when I'm trying

to lean things up.

I'll play with it again tomorrow.

Chris