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Ryo's carb'd ZX


Ryoskatekov

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http://community.ratsun.net/topic/52606-bought-a-zx-and-forgot-about-the-permit/

^ Moved the thread from here since The Z section seemed more appropriate. 
Basically I'm a highschooler kid wasting all the money on what used to be beautifully stock ZX and turning it into a confused mix of S30, S130 and Z31 pile. 

Here's overview from the last thread. 

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So I installed the round valve flat tops.

My current problem 

-its a 36mm x 2 compared to my previous 46mm x 2. But mathmatically both are still bigger in Surface area than the Stock EFI throttle body.
-I don't know how it runs. Throttle shaft seems better than my 46mm
-The 36mm butterfly works in a opposite direction than the 46mm off of the S30. 

-^ So my only choice is to go Cable link. currently waiting on parts. 

-Haven't fired it up yet.

-my wallet is empty

-Have to make my own airfilter because no one makes 36mm SU filter that supports 2" air funnel. 

The good

- someone rebuilt the carbs previousely and everything is plated and shiny <3 
- it looks cool 
- It will sound cool 
 

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Those 38 2 bolt(?) SUs are to small. Replace them with some early 240Z 46mm 4 bolt round tops  and buy a wide band to 'profile' the needles to suit the engine

Also......your fuel lines (tees) looks a little suspect????

Not sure which hose over the valve cover is the return line...?...but...if it's the one in the center, reduce the size of the tee so the rear carb does not lean out.... relative to the front.

Or... locate a stock fuel rail

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Those 38 2 bolt(?) SUs are to small. Replace them with some early 240Z 46mm 4 bolt round tops  and buy a wide band to 'profile' the needles to suit the engine

Also......your fuel lines (tees) looks a little suspect????

Not sure which hose over the valve cover is the return line...?...but...if it's the one in the center, reduce the size of the tee so the rear carb does not lean out.... relative to the front.

Or... locate a stock fuel rail

Yes they are indeed 38. I've had the fuel line setup on the previous 46mm but the throttle shaft was too loose and undrivable with choke on start up / unstable idle. so 1. I kinda had to settle with the 36 (shaft is good) I have for now and 2. couldn't verify the effectiveness of the fuel rail but it ran "fine" with all the spark plugs looking the same brownish white ish. .I will indeed consider investing in a proper fuel rail.  I may eventually save up my money and get some big bore SUs. But considering I only daily my car I'm thinking 36 would be fine with the needle and jet I have on.   Let me know what you think now. 

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Well, um the 46s were stock on the L24... like .4L smaller motor than you have... And the 38s were used on a 1.6L... and often run out of breath on anything above a 2.0L above 5k rpm...

 

honestly I have to wait for my throttle cable and test it out.  There's no better way than actually trying it than going theoretical right? keep in mind there's 6 cyl instead of 4 too. each cyl only reqs 458.9cc, thats almost 1800cc if it was a 4 cyl.  only 1~2 at most cylinders are sucking through 1 carb too. 

Like I said, its a daily. not some track car always doing 5000rpm+, my shift at most is 4600 rpm.

And its a good time to see how much of things people said in the past is true or not.  But yes I am aware and concerned that 38 is in theory too small for 2800 6 cyl considering avg shift change is 5000rpm

 

even though I don't like doing things theoretically, an official formula given by Mikuni for ideal carb diamter is .82xsqrt(cc per cyl x peak hp rpm x 0.001) 

Since I usually max at about 4900 at most, 0.82sqrt(458.9x4900x0.001) yields 38.884mm. 

 

 

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SUs are nothing like Mikunis... Each cylinder gets its own half of the carb... Why there are 3 Carbs vs 2...

 

The other way to think of it. On a 2.0L(1952CC L20b) 4 cylinder with SUs. At 5k RPM, that is 83.3333RPS (Revs per second). Per revolution, the carb will see the draw of 2 cylinders, or 976CC/Rev. That equals out to  81333.33  CC/Second

 

On a 2.8L (458.9CC per Cyl) 6 cylinder with SUs, at 5k RPM, same Revs Per Second. Per Revolution, each carb will see the draw of 3 cylinders, or 1376.7CC/Rev. That equals out to 114725 CC/Second. Considerably more air...

 

So what RPM will have the same airflow of a 2.0L at 5k rpm?

 

81333.33 CC/Second  / 1376.7 CC/Rev = 59.07847Rev/Second      * 60second/minute =  3544.7RPM

 

Expect an early choke.

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Laecon, you forgot to factor in where 3 cylinders do not draw 100% of the carb. A 4-cycle engine only draws in a cylinder every other revolution.

 

Is why 6 mikunis only make a little more power than 2. The famous mikuni BS34 are similar to SUs.

 

 

38s will make less power than 46s on an L28.

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The mikuni formula  works for almost any type of side draft..   
If I set the rpm at 6000 and same L28 spec, I get 43mm  thats about damn right with the 240's 46mm.  

Anyways you guys are getting way too heated. I know that its not guna be efficient as the 46 on the higher end even with some racing needle it came with. Its just that I needed something that can get me where I need quick but still have the carb sound plus, uncracked stock L28 doesn't do anything above 5000+ anyways.. The 46 would have worked great (I even had the SM needle and .100 nozzle) if it wern't for the shaft bushings. 

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Fired right up.... but problem is its idling way too high (screws are maxed out) , It started without a choke in 60F. probably too rich .  

the nozzle is all the way lean but still idles too high and smoke. (ofcourse, if the engine warms up, the smoke goes away but then the idle's guna be crazy higher) 

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I really wish it ran as clean as it looks.

 

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Anyways, I'm pretty set on these 38mm carbs as they are so much better made than the stock 240z ones. 

 

- The jet needs no centering due to the fact that the guide is precisely pressed in unlike the 71' 
- clicking jet adjustment nut for equal and easy adjustment. I love it. 
- additional guide for bolting the dome. The dome doesn't move while screwing in and the needle stays securely in one place. 
 

So I decided to put some work in it. 

Started off by eliminating shaft leak. I forgot who's idea it was but I went fail safe by doing O-ring and a cap. 
 

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I don't have a lathe so I carefully used hacksaw and a file to make a U groove for the O-ring.

 

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With the o-ring on. shut-off'd the air so well that If I hold the hole that its going into, It won't go in because its air-tight. 

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With the cap and Neoprene washer just in case. 

I also changed the Jet/nozzle from .090 to .100
and Needle from unknown one for 4cyl to SM needle.  

Hopefully this and maybe a fuel rail in future will solve the problem. 

 

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  • 2 weeks later...
Ordered on sunday, came in mail today. Damn fast. 

 

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S13 Throttle body pulleys. Drilled holes and put a bolt through so I can attach the throttle cable for bikes.  Amazingly it fits the 30+ yr old Hitachi lol. 

 

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also changed the weird pull setup that half used mechanical linkage to full cable only linkage.  

 

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Custom Fuel level gauge for setup.

 

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I'm really hoping It'll finally run right. 
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  • 3 weeks later...
Finally going higher compression with N47 head off of a 280z

 

Cons-

Open chamber (guna run high octane)

Corroded coolant holes  (Fixed)

one broken exhaust mani stud  (forget it) 

#8 Spring collets are stuck  (will hammer about 100 more times)

 

Pros

It'll sound awesome 

It'll sound awesome

It'll sound awesome

 

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High compression sounds the same. Many high compression engines have other changes that make them sound different.

Oh darn.  I was hoping maybe atleast the 240z sound. I guess its more of the Bore x Stroke matter hun? 

Well I'm sure there are some difference in tone.... 

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When I first got the head for few bucks with everything on it. 
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Temporary fixed the coolant holes with JB weld. It'll hold long enough. 
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Took off the valve springs. Was pain in the ass with the collets / retainer lock seized. Had to hammer it with a deep socket a few hundred times. 
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2nd De-gunk wash 
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Grinded the valves' unimportant parts, polished the seat.  Smoothed the intake texture.
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New valve seals.
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Now All I have to do is clean the rest of the small parts and pull off the existing head on my Z. 

 

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