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 Post subject: Nikki Modification finally
PostPosted: Thu Apr 29, 2010 8:24 pm 
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ok here we go something that im sure many people have been waiting for.
this is a bit of a guide on moding your stock carbi to flow more like a weber but with more control. (well i think anyway) no fights please.

bit of an intro, my name is Troy im from the gold coast 19 years of age.
i have a 79 td wagon with a pretty heavily modded 1600. and i needed a decent carbi to suit so i thought i would give the nikki a second chance at life.

i had a spare so i didn't care it was all trial and error. but i had a fair bit of success.

ok lets get started shall we.

so here is the list of mods i wanted to achieve
general reshaping of venturis and tidy up.
improve flow
re-jetting
removal of choke (optional) haha
mechanical secondaries
larger volume accelerator pump
probably some others that ill remember half way through writing this.

here is a list of what ive done to my "poer house" i mean power
Engine Type: G161z

Engine Mods: Bored 020thou
G161 Pre emission head (TX big valve)
Port and polished head
Ported inlet manifold and carbi
High lift cam very lumpy idle
Flat top pistons
Shaved head
Shaped pistons to suit head clearance
hardened Double valve springs
4-2-1 extractors
polished valves
rejetted nikki carbi, with mechanical secondaries and full reshaping with disabled vacums. bigger ac pump
g200 crank
bosch fuel pump
fwd gemini/rodeo hall effect dissy, bosch ignition module and twin gt40r coils
and a 2.25 inch exhaust single muffler straight tip.

so first i started off with a stock 1600 carburetor. all g series carburetors are the same except jets and the secondary aux venturi is taller.
see photo. 1600 and 1800 are the same height but different sizes.
Image
i stole this from Lok15 hope he dont mind ill just buy more stuff from him

first off i strongly recommend getting a carbi rebuild kit, i got mine from super cheap, cost like 12bucks i think they are getting harder to find but ebay has them for 45 i think. anyway you need this so fork out and buy.

so first up we want to take the hole thing apart and clean everything we can with carbi cleaner or kero. i used petrol i think works the best just try to keep all rubbers and diaphrams dry.
we want everything spotless and remember what you took off and where it came from, write it down if you have to.
and unless you are totally re-jetting we want to clean these to, fine peice of wire and carbi cleaner will do. any blockages and your car could run like dump. some stock jets will still be used. write down what went where so you know what jet is what. and watch out for the ball bearing and rod in the AP (accelerator pump) jet.

Once this is done its time to start cutting.

This is a aux venturi there are two of these one for the primary and one for the secondaries.
Image
and this is the modified ones,Image
what I did was file down the extra lug that supported the other side, that isnt needed. Also the supported side I reshaped rounded on the top and sharpish on the bottom aswell as just giving them a general polish maybe a reshape for sharper edges on the bottom.
The main thing to remember is that a bigger hole is not better, the purpose of a venturi is to increase the airs velocity to suck the fuel through.

Next I reshaped the butterfly shafts to increase flow. This is my modified one I simply filed the rounded shaft down thinner and gave it smooth edges. One shaft had support on both sides of the butterfly so I completely removed that and kept the side with screw thread side. I also filled the extra thread on the screws to sit flush with the now much thinner shaft.
Image
and here a butterfly fitter to shaft. (secondary)
Image

next we want to ensure that both butterflys are actually opening to full, this is a big problem with nikkis. Place the butterflys and rods back in the body and file these lugs that work as stoppers. We want to just file the ends off so the butterfly is able to rotate to completely straight.
Image

so I wanted to have mechanical secondaries like a weber so when I put my foot down it does what I want, the usual vacuum system needs the revs to operate before it will completely open.
So what I did was first I had to disable the vacuum system. In the vacuum diaphragm there is a spring you need to remove. you need to peel the diaphragm off the rest of the unit and take out the arm that is connected to the diaphragm and remove all these components. Leaving us with the inside of the secondary lever looking like this.
Image
we also need to block the vacuum tube that operates this diaphragm, I used a piece of spare gasket and covered the hole and screwed the top cover back over the top sealing the hole. If not done correctly this will cause a vacuam leak and idle may be compromised. Other options could be maybe an epoxy filler or plug. Be creative.
Finally I used a small amount of wire to tie the levers together. Hard to explain but you will see what needs to be done. There is a tab that you can bend to time the secondary's opening time this is located where the diphragm lever was attached. We want to time it so that the secondary reaches full opening at the same time as the primary butterfly. Trial and error you will figure it out. the secondaries should be begining to open at arounf 1/3 throttle and reach full at the same time as the primaries.
This shows the linkages but doesn't show the wiring.
Image
and this is the butterflys at full throtle
Image

I then port matched the top ventury section of the carbi with the butterfly body. I used a small flappy wheel on a drill, the aim is to only alter the bottom section just to match where it lines up with the base. And give it a smooth polished finish, crack out the elbow grease and some wet and dry.
Image

in my carbi I totally removed the whole choke setup, and filled the holes with epoxy for better flow.
I find the car starts perfectly fine even on cold mornings just hold a bit of throttle until she starts to run smooth and let warm up, kinda gay for street driving. But still good.
Image

I tried a few things to make a larger Acelerator pump volume but im still working on that. We want a larger volume and travel so when you boot it we dont lean out and dig down, this symptom happens when the AP is blocked. But with the mech secondaries there is an excess of air flow for the stock pump. The extra travel works best say when ur sitting at like 80km/h and put your foot down there is still enough travel in the pump so leaning out doesnt occur and we get a smooth powerfull acceleration up to 110km/h(on the road boys).

Now putting it all back together, hopefully you have a good memory or wrote it down we want to put everything back with new gaskets seals and washers, from our rebuild kit. The kits come with a new needle and seat and adjustment of the fuel float may be needed, I tune my float to run on the line of the glass when the car is idling.

Jets..........
ok well here is the hard part that I have no idea. I'm currently running a set from a g181z carbi I have no idea if they are good or not. I'm hopefully going to chat to a carbi bloke about new jets. Ill start a thread on here see if anyone can help out on sizes. But I would recommend if you went to the trouble go see an expert and get them to jet it right, to your engine, its going to make a huge difference.

Here is my top section of the carbi together. You can notice there is a lot more clearance in there for better flow.
Image

Mine I put on 2 days ago and i've noticed quite a power increase and much smoother running. My car actually idles now. I seem to have more low and high up power. I've got to do some timing cause I haven't been able to get any revs out of the engine lately much before the carbi mods. I'm expecting some good power from this carburettor.
Image

so yer hope this helps everyone wanting to give their Nikki a bit of a kick.

I'm open to feed back to on any thing I haven't explained properly or anything I can add.

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 Post subject:
PostPosted: Thu Apr 29, 2010 10:12 pm 
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really nice write up!

also looks like you put in a lot of effort, and the carb looks really nice.

i have a few questions though.

is the secondry venturi height important?

make that one question.

cheers!


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 Post subject:
PostPosted: Thu Apr 29, 2010 10:39 pm 
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Location: gold coast RIP MANWELL we will always remember you
i think because of the size of the venturies or fuel system the 2litre may start to struggle for fuel when the secondaries start to open, this is when there is a low amount of air flow in the secondaries. The extra height and length of the secondary auxilary venturi may get a better flow to suck a good amount of fuel. I think it might just be just a mod that was put on the carbi to make it work. Cheaper than making a totally different setup for a rodeo. Its not going to hurt to use it on a smaller engine but i would probably try and source one if you have a 2L.

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 Post subject:
PostPosted: Fri Apr 30, 2010 9:30 pm 
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This is what I found between the G161 and the G200 carby's I have...

These pictures are to show the differences in...
  • Jet sizes
  • accelerator pumps
  • venturi sizes
  • Spring in the secondary vaccum section
  • Butterfly size
  • Choke sizes

The numbers in (**) are the G200 sizes...

The jet sizes are...
G161 Primary=111 Secondary= 175
G200 primary=118 Secondary= 170


The numbers inside the choke sections are the numbers
stamped onto the venturi's...



Image



The accelerator pumps are different...
G161 is softer and has a 3mm stroke of the spring...
G200 is harder and has a 4mm stroke of the spring...

Image

The height of the G200 secondary venturi is heaps taller than the G161...
The secondary venturi centre hole is 10mm for both the G161 and
the G200...
The primary venturi centre hole in the G161 is 7mm and the G200 is 8mm...

Image

Secondary spring is shorter and thicker in the G200...
Image

The butterfly sizes are the same in both carby's...

Primary = 30mm
Secondary = 34mm

The chokes sizes are different....

G161...
Primary = 22mm
Secondary = 29mm

G200...
Primary = 24mm
Secondary = 29mm


Last edited by 155-low on Sat May 01, 2010 9:43 am, edited 1 time in total.

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 Post subject:
PostPosted: Sat May 01, 2010 8:36 am 
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Location: gold coast RIP MANWELL we will always remember you
now im confused. i have a couple carbys and heaps of bits i went through them all and found the same differences. on the side of the fuel bowl i think is a part number like G32 or something like that. i found a list of gemini carbi numbers and mine didnt match any. i had all the different peices except the chokes were the same. i tried the different accelerator pumps but found no difference, i figured the bottom spring only was so if there was a blockage there was a bit of play with out placing huge pressure on the system.

so yeh now i dno? did i maybe have a 2Litre carbi with the small ventury or is it just small differences between models?

hopefully someone can shed some light

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 Post subject:
PostPosted: Fri May 07, 2010 10:44 am 
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155-low wrote:


The jet sizes are...
G161 Primary=111 Secondary= 175
G200 primary=118 Secondary= 170



The Secondary jets are larger on the G161 than the G200??

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 Post subject:
PostPosted: Sat May 08, 2010 11:32 am 
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Here is a link to help everybody understand how a carby works...

http://sterlingmetalworks.com/carburetor_basics.htm

The secondary jet maybe smaller but the bigger engine has a stronger
induction pulse...


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PostPosted: Sat May 08, 2010 6:27 pm 
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Carby sizing calculator....


http://www.4secondsflat.com/Carb_CFM_Calculator.html


Cheers


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PostPosted: Thu May 13, 2010 9:25 am 
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on that sizing calculator it says 1CI = 2CC thats not right
an inch is 2.54 CM


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PostPosted: Thu May 13, 2010 11:15 am 
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i think that was option:1 cubic inches and option:2 cc

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 Post subject:
PostPosted: Thu May 13, 2010 4:27 pm 
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And with doing a couple checks, the stock nikki flows heaps of air for a G200...

Just have to sort out the feul side of things...


A pair of 40mm IDF flow 852 CFM...

That is 212 CFM per venturi...


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PostPosted: Thu May 13, 2010 7:15 pm 
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how much does a standard nikki flow?

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 Post subject:
PostPosted: Sat May 15, 2010 6:42 pm 
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The stock G161 Nikki has a combined venturi area of 2.01 sq. inch and will Flow 277 CFM.
This is just maths so in a real test it will be lower...

The diameter of the primary venturi/choke in the stock Nikki measures 22mm, or 0.87 inches,
The diameter of the secondary venturi/choke in the stock Nikki measures 29mm, or 1.14 inches.

mm to sq inches

22 / 25.4 = 0.87

29 / 25.4 = 1.14

0.87 +1.14 = 2.01

2.01 x 137.7 = 277 CFM... That is at 100% flow

This was using formulas off the net from carby websites...

This link gives the cfm rating for the 32/36 weber...

http://datsun1200.com/modules/mediawiki ... low.2C_CFM

The combined venturi area is multiplied by 137.7 cfm per sq. inch to give the maximum flow efficiency
through the carburetor. The venturi area is measured at the inlet/outlet diameter because the venturi
which we are comparing the real world carburetor venturis to flows at 100% efficiency even though the
smallest internal diameter is half the inlet/outlet (largest) diameter.

I know a guy with a flow bench and I might get a couple carby's together and test them....


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PostPosted: Sat May 15, 2010 7:34 pm 
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Location: gold coast RIP MANWELL we will always remember you
oh oh oh i want to test mine too

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 Post subject:
PostPosted: Mon May 17, 2010 11:04 pm 
ey chance of a pik of the wire you used for the secondery butterfly


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