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grannyknot

I Need Some Carb Jet Recomendations Please

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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

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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/

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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

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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.

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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?-LOL

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.

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

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.

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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.

post-25243-0-17003000-1429310615_thumb.j

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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 by grannyknot

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

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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

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April 19th  I turned the mixture screws back out to where they were last year, 1.5 turns.

I soldered up the 55F8 idle jets and re drilled to 45, undrivable!, way too lean, then drilled them out to

50, driveable but lots of lean pops on slow acceleration and light cruise on a flat was between 14-17 AFR

So today I drilled the idle jets again back to the 55 they were before and no surprise I'm back to 11-12 light cruise on a flat.

It seems impossible that .05 mm would make such a big difference in AFR, but I'm starting to think the mixture screws are playing a part in this that I don't understand yet. The fact is I just don't know how to adjust the mixture screws properly, I have read all the tutorials saying turn them until you find the fastest idle but turning one screw at a time doesn't make that much difference and really, for me it's just sort of guessing.

There has to be some other way to verify that the mixture screw is where it should be other than just by ear?

Chris

Edited by grannyknot

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Great exploratory work!

 

When you look a weber idle jet you will note the number and size of air holes and the size of the cavity where the air and fuel mixes/exits vary.

 

Later idle jets have a new 3mm mixing cavity/exit added. I'm not sure what the larger cavity effect is but it is something I wanted to explore.  I have a feeling Weber changed all later jets to 3mm for a reason.

 

You now have 55F8's  running rich at cruise so there are two more areas to tweak:

 

1. You may want to drill out the single 1.2mm air hole on the side of the jet to ~1.5mm first. Then, if needed, drill to  ~1.7mm to lean the cruise.

 

2. You may want to enlarge the mixing cavity to 3mm to see what happens (very exploratory).

 

 

Finally, since the mixed fuel and air out of the idle jet has an awkward trip through the holder, then through passages , then through the progression holes and around the idle enrichment valve, it is difficult to imagine non-orifice changes in the jet will make a big differences when the fuel eventually reaches the manifold.  Changes to the holder (with 4 holes) are also worth exploring.... drilling 8 holes or enlarging the existing 4 holes are possible exploratory items. 

 

 

 

 

post-7641-14150820069169.jpg

 

post-7641-14150828787592.jpg

Edited by Blue

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I measured the side air hole on the F8 idle jets and they were all 1.1mm, it is suppose to be 1.2.

Anyway, I drilled those out to 1.5mm and the idleing at 900rpm is was great at 13.5AFR but light cruise was 15-17 so soldered them up and went down to 1.3mm and the readings were very similar to 1.5.

I realized later, after going back to 1.1mm air holes that the air temp was cooler the day I tried 1.3 so that is where I'll probably end up.

 

So, at the moment, MJ 135, ACJ 210, Idle jet 55F8 with 1.1mm air hole

Idle at 900rpm is 11.5-12.5AFR

WOT 9-12AFR and FANTASTIC

Light cruise 10.5-11.5 but with a little more throttle it comes up to 13.5-14 AFR

for example, 55mph, 2500rpm in 5th gear is 10.5-11.5 AFR but add a bit of gas bringing the rpm up to 2600-2700 and the AFR goes to 13.5-14. I can't figure this last bit out at all.

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Glad the air hole exploration helped a bit in understanding the interactions.

 

I have a theory that small chokes (thick) come in proximity of the partially opened throttle valve causing a venturi effect in that region (also where the accelerator pump sticks out) and pull fuel from the accelerator circuit.  You may wish to close the 3 spill jets with bolts to see if the cruise will be tamed.

 

You could also try lowering the fuel level to lower the rich cruise then address the jets.

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The fuel outlets at the bottom of the fuel bowl that feed the accelerator circuit. You have to go fishing to get them our. Just replace with bolts and the accelerator circuit is taken out of the picture.

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Gottcha, yes, I have those bolts put aside and ready for that test. For anyone following along those are

M6x1.0

But if I determine the accelerator circuit is the culprit is anything really do about it?

 

I'll start with lowering the float level from 26mm to SK's recommended 29mm and go from there.

Thanks for all this help Philip,

Chris

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Glad to help. We are SK brothers!

 

I think it is key to get the cruise first then work out the WOT and idle.

 

For cruise, the progression holes and top of the emulsion tube are the active parts.  Fuel level changes will affect these for "tuning".  It will be good to rule out the accelerator just in case.

Edited by Blue

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