Nikki carb modding
08-10-11, 01:28 AM
#51
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Well in that case I'll stick with the j ported 12a. I don't mean to sound confused, I'm just trying to choose my build around my carb since this is a budget build. :-/
08-10-11, 03:19 AM
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That is true. But I'm on ausrotory.com also and I'm thinking about using me 12a because I won't have to make an adaptor nor will I have to get a new oil pan etc etc... I'll just have to get some one piece seals of some sort. I wish they made one piece steelies. Lol!
08-12-11, 10:48 AM
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Update, I will be using my 12a instead of the 13b because im finding the replacement parts i need and I kinda wanted to use it anyway. With the plates im getting im going to do a standard bridge to it and get the jets from mazdatrix. Im thinking about going with 115 jets for primaries and im thinking maybe 155-185's on the secondaries. I dont know about the air bleeds though. :-/
08-13-11, 02:08 AM
#55
Update, I will be using my 12a instead of the 13b because im finding the replacement parts i need and I kinda wanted to use it anyway. With the plates im getting im going to do a standard bridge to it and get the jets from mazdatrix. Im thinking about going with 115 jets for primaries and im thinking maybe 155-185's on the secondaries. I dont know about the air bleeds though. :-/
Have a look at Sterling's site for jetting recommendations. I can say from experience that running on primaries only on a stock port 12A, 100s got me to 5kRPM before it started losing power. Just to give you an indication of how far those sized jets will get you before they run out of flow. So the stock secondary jets are quite large.
Air bleeds are tough because they are built in to the top of the emulsion tubes so unless you want to tap threads like Sterling does, the only way to change them is by drilling or finding different model Nikki's with different sized air bleeds.
08-14-11, 03:00 AM
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I doubt you will need to go that big on the secondaries. Fuel jets don't really control the mixture so much as the total amount of fuel that can flow through the jet. So if you are running out of fuel on the primaries or secondaries then you should enlarge the respective fuel jets but if not there is no point making them bigger. The air bleeds are what controls the mixture across the RPM range.
Have a look at Sterling's site for jetting recommendations. I can say from experience that running on primaries only on a stock port 12A, 100s got me to 5kRPM before it started losing power. Just to give you an indication of how far those sized jets will get you before they run out of flow. So the stock secondary jets are quite large.
Air bleeds are tough because they are built in to the top of the emulsion tubes so unless you want to tap threads like Sterling does, the only way to change them is by drilling or finding different model Nikki's with different sized air bleeds.
Have a look at Sterling's site for jetting recommendations. I can say from experience that running on primaries only on a stock port 12A, 100s got me to 5kRPM before it started losing power. Just to give you an indication of how far those sized jets will get you before they run out of flow. So the stock secondary jets are quite large.
Air bleeds are tough because they are built in to the top of the emulsion tubes so unless you want to tap threads like Sterling does, the only way to change them is by drilling or finding different model Nikki's with different sized air bleeds.
08-14-11, 03:38 AM
#57
Ahh... Ok. So should i just start off with drilling my primary and secondary jets out and my air bleeds? And what size bit should I use? Also, I know its off topic but can you use stock apex seals on a bridged keg like mine or do I absolutely have to have carbons? Thanks.
08-14-11, 09:22 AM
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08-14-11, 11:03 PM
#61
Here's a great article by Paul Yaw that describes the purpose of each type of jet when it comes to tuning: http://www.rhinoracing.com/yaw/carb_tuning.htm
I managed to hit good AFR's by only enlarging my primary fuel jets, but I think I got lucky. Not to mention that it's the middle of summer, so my numbers will be a lot different in a couple months when cold air rolls in.
I managed to hit good AFR's by only enlarging my primary fuel jets, but I think I got lucky. Not to mention that it's the middle of summer, so my numbers will be a lot different in a couple months when cold air rolls in.
08-18-11, 01:16 PM
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Sorry i havent been on this thread lately. I have been working like crazy. Ive been fighting with this carb trying to get the screws out and its like pulling teeth from a camel. Once I get it apart ill drill out me jets and I may just have the venturies lathed at a shop because i dont wanna f-em up ya know. lol!
08-19-11, 10:50 AM
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Lizard, hard left handers can be an issue with a nikki. Some threads lower fuel pressure to solve the problem. I have an ITA 83' I race and the carb has corners blocks. That also solves the problem.
08-19-11, 04:22 PM
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08-20-11, 01:17 PM
#65
Tri-angles, that's Amurrrrican for "tight left turns."
08-20-11, 11:26 PM
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Just did this to the rx7 today. I tore the carb apart and smoothed out the venturies. I then drilled out the primaries and primary air bleeds out with a 1.2mm drill bit. I also cleaned up the intake manifold by sealing off all the ports for the emissions stuff. Took the car out for a drive, doesn't feel fast. It just makes power all the way to 7k now. I also did the venturies by hand with a dremal. There was no lathe used like Sterling used. But I did do all 4 barrels.
08-21-11, 01:33 AM
#67
Just did this to the rx7 today. I tore the carb apart and smoothed out the venturies. I then drilled out the primaries and primary air bleeds out with a 1.2mm drill bit. I also cleaned up the intake manifold by sealing off all the ports for the emissions stuff. Took the car out for a drive, doesn't feel fast. It just makes power all the way to 7k now. I also did the venturies by hand with a dremal. There was no lathe used like Sterling used. But I did do all 4 barrels.
Also, you've done two things that counteract each other. Upping the primary fuel jets will enrich the mixture, while upping the primary air bleeds will lean it out. I'm not sure if there's any benefit at all to enlarging both at the same time. You also made pretty huge jumps on both. So there's no telling what your engine is really getting.
08-21-11, 02:33 AM
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Yeah. I had to think about that after I posted it. I was sleepy as hell when I posted that. Lol! I got my carb apart. But here is my question, where are the primary main jets and air bleeds/correctors? I have no clue what they look like. :-/ I'll shoot someone my cell # if they are willing to walk me through this or just the big parts of it because I really don't know much about carbs at all. Also. While taking it apart I broke one of the bowl floats. I think that's what its called. It's kinda like a hard sponge with a square metal pin on it.
08-21-11, 07:47 AM
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I luckily do have a wideband, I just need to calibrate it because it's reading 5:1 and 7:1wot. This was pre mods and post mods. The car won't be driven until it's solved. It's an innovative LC1 with the XD-15 gauge. I don't see the point in just enlarging the jets so you have a really rich mixture? If you open up the Venturies and the manifold wouldn't you want to enlarge the jets and the air bleeds to get your mixture back to normal? I could see a rich mixture help in a forced induction set up. But not NA. This is just me speculating though.
I'd like to stress that without a wideband o2 or at least an EGT, there's not much point in changing jet sizes. You can guess and maybe you'll get lucky and hit something that works, but you're still shooting in the dark.
Also, you've done two things that counteract each other. Upping the primary fuel jets will enrich the mixture, while upping the primary air bleeds will lean it out. I'm not sure if there's any benefit at all to enlarging both at the same time. You also made pretty huge jumps on both. So there's no telling what your engine is really getting.
Also, you've done two things that counteract each other. Upping the primary fuel jets will enrich the mixture, while upping the primary air bleeds will lean it out. I'm not sure if there's any benefit at all to enlarging both at the same time. You also made pretty huge jumps on both. So there's no telling what your engine is really getting.
08-22-11, 06:46 AM
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No pictures but cornering blocks are aluminum blocks, maybe an inch long by 3/4" tall and they occupy the space above the float. They keep fuel out of that area. On very hard left hand turns, the fuel can spill out of the float chamber and flood your combustion chamber and make your engine sputter. The block is either glued to the top of the chamber or in my case, drilled and tapped. My block was epoxied but it started to loosen and drop down into the float which caused the flooding. It only happened on "very" hard and prolonged left handed turns. I run 2 1/2 lbs. fuel pressure. I'm told turning down your pressure to as low as 1 1/2 lbs will also cure this problem. I'm not sure if a Sterling would have the blocks but I would think so. Race Nikki's seem to be prepared pretty much the same.
08-22-11, 10:30 PM
#71
No pictures but cornering blocks are aluminum blocks, maybe an inch long by 3/4" tall and they occupy the space above the float. They keep fuel out of that area. On very hard left hand turns, the fuel can spill out of the float chamber and flood your combustion chamber and make your engine sputter. The block is either glued to the top of the chamber or in my case, drilled and tapped. My block was epoxied but it started to loosen and drop down into the float which caused the flooding. It only happened on "very" hard and prolonged left handed turns. I run 2 1/2 lbs. fuel pressure. I'm told turning down your pressure to as low as 1 1/2 lbs will also cure this problem. I'm not sure if a Sterling would have the blocks but I would think so. Race Nikki's seem to be prepared pretty much the same.
After all is said and done, the car is much faster and drives wonderfully around town as well. The only thing I might do differently in the future is to go a little smaller on the primary jets and make up the difference with the secondaries so my gas mileage doesn't take such a hit while cruising. For stage 2, I will be boost prepping it. But I have a lot of money to throw away and supporting mods to do before then so that one will have to wait
Also, I found more pics of the build process on another memory card, so I'll share a few more.
One secondary venturi bored with the dremel:
All venturis bored and polished:
Booster Venturis trimmed:
And here's some shots of my modified linkage. Cams for the cruise control and choke were all removed, as was everything else that wasn't needed to keep the spacing or serve some other purpose:
08-22-11, 10:54 PM
#72
I luckily do have a wideband, I just need to calibrate it because it's reading 5:1 and 7:1wot. This was pre mods and post mods. The car won't be driven until it's solved. It's an innovative LC1 with the XD-15 gauge. I don't see the point in just enlarging the jets so you have a really rich mixture? If you open up the Venturies and the manifold wouldn't you want to enlarge the jets and the air bleeds to get your mixture back to normal? I could see a rich mixture help in a forced induction set up. But not NA. This is just me speculating though.
The reason for enlarging the main fuel jets is that you are modding the carb to be able to flow more air, and the stock jet sizes are too small to flow enough fuel for an adequate mixture. As air velocity increases, there's more signal to pull fuel through the jets. But the size of the jet quickly becomes a limiting factor as the hole is too small to allow enough fuel to flow through. So you "rejet" by either putting in larger jets or enlarging the hole in the stock jets with a drill as I did.
When it comes to N/A setups, you're mostly just going to lose power and run hot by running too lean. There's not a terrible amount of danger in toying with a Nikki on a stock 12a. When it comes to boost, rejetting and careful tuning become absolutely vital, as a lean condition can quickly damage the motor when higher than atmospheric pressure is introduced.
Yeah. I had to think about that after I posted it. I was sleepy as hell when I posted that. Lol! I got my carb apart. But here is my question, where are the primary main jets and air bleeds/correctors? I have no clue what they look like. :-/ I'll shoot someone my cell # if they are willing to walk me through this or just the big parts of it because I really don't know much about carbs at all. Also. While taking it apart I broke one of the bowl floats. I think that's what its called. It's kinda like a hard sponge with a square metal pin on it.
You have to be really careful when removing the air horn... pull it straight up so you don't catch the floats on the side of the body and bend the tabs. Errrr... well, maybe too late for that. I know it's a drive but if you wanna come to Knoxville with it one weekend I'll help you put the thing together.
08-23-11, 02:07 AM
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Oh good, well then once you get that calibrated you should be out of the dark. anything below 10 is waaay rich... my 7 will usually sputter at that point. So yeah, definitely need to calibrate the gauge. I think it should read ~14.9 with the engine off, IIRC.
The reason for enlarging the main fuel jets is that you are modding the carb to be able to flow more air, and the stock jet sizes are too small to flow enough fuel for an adequate mixture. As air velocity increases, there's more signal to pull fuel through the jets. But the size of the jet quickly becomes a limiting factor as the hole is too small to allow enough fuel to flow through. So you "rejet" by either putting in larger jets or enlarging the hole in the stock jets with a drill as I did.
When it comes to N/A setups, you're mostly just going to lose power and run hot by running too lean. There's not a terrible amount of danger in toying with a Nikki on a stock 12a. When it comes to boost, rejetting and careful tuning become absolutely vital, as a lean condition can quickly damage the motor when higher than atmospheric pressure is introduced.
The main fuel jets are down in the bowls on the side, and the main air bleeds are threaded into where the booster venturi arms meet the carb body. You can see the size stamped on every jet. The primary/secondary air bleeds are stamped 70/140 respectively (may vary a bit depending on model year and auto/manual trans.) 70 means .70mm, and 140 means 1.40mm.
You have to be really careful when removing the air horn... pull it straight up so you don't catch the floats on the side of the body and bend the tabs. Errrr... well, maybe too late for that. I know it's a drive but if you wanna come to Knoxville with it one weekend I'll help you put the thing together.
The reason for enlarging the main fuel jets is that you are modding the carb to be able to flow more air, and the stock jet sizes are too small to flow enough fuel for an adequate mixture. As air velocity increases, there's more signal to pull fuel through the jets. But the size of the jet quickly becomes a limiting factor as the hole is too small to allow enough fuel to flow through. So you "rejet" by either putting in larger jets or enlarging the hole in the stock jets with a drill as I did.
When it comes to N/A setups, you're mostly just going to lose power and run hot by running too lean. There's not a terrible amount of danger in toying with a Nikki on a stock 12a. When it comes to boost, rejetting and careful tuning become absolutely vital, as a lean condition can quickly damage the motor when higher than atmospheric pressure is introduced.
The main fuel jets are down in the bowls on the side, and the main air bleeds are threaded into where the booster venturi arms meet the carb body. You can see the size stamped on every jet. The primary/secondary air bleeds are stamped 70/140 respectively (may vary a bit depending on model year and auto/manual trans.) 70 means .70mm, and 140 means 1.40mm.
You have to be really careful when removing the air horn... pull it straight up so you don't catch the floats on the side of the body and bend the tabs. Errrr... well, maybe too late for that. I know it's a drive but if you wanna come to Knoxville with it one weekend I'll help you put the thing together.