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Mikuni jetting question


Cakebreak

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I am curious as to what jets and emulsion tubes i should use as a sort of base tune for my setup, or if anyone is running a similar setup, i am running an L20b with a closed chamber A87 head, stock L20 dished pistons, an isky stage 2 cam 480 lift and 280 dur, and dual mikuni/solex 40mm carbs, the type T's from the 70's celicas to be exact.

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This is the baseline for Mikuni PHH40 with a stock L20B

 

Carburetors: Twin Mikuni PHH 40

Venturis: 32 mm

pilot jets: 57.5

main air jets: 180

main bleed pipes: T

main jets: 140

pump nozzles: 40

starter jets: 130

pump nozzles: 40

 

But with a high-RPM (7000 RPM) L20B youll want the largest venturis available: 32mm? PHH44 would be even better with 40mm venturis. The jets used will be proportional to the venturi sizes.

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Good luck on a long post, but there isn't much information elsewhere on the web regarding tuning Mikunis.

 

I have nearly the exact same setup as you do.  L20B bored out .75mm with stock, oversized pistons; closed A87 head with what seems to be a .030'' shave; unknown regrind cam with .480'' lift and unknown duration; and type R Mikuni PHH40 carbs with stock 32mm venturis.  I'm also running an early single-point distributor with Pertronix, and for the time being (as a test), I am using BR7ES plugs.  My Mikunis are set up for vacuum advance, so I'm running with the vacuum hose hooked up.  It runs like doodoo without it.

 

Regarding the carbs, I am currently running 67.5 pilot jets, 135 main jets, 165 main air corrector jets, and 40 pump nozzles.  I think my idle mixture screws are 2.25 turns out right now?  I'll figure that out next time I have the hood open.

 

To answer your question as what you should use for a base, I would vote for starting with 62.5 pilots.  Mains are very forgiving, and I have both heard and experienced that it almost doesn't matter very much what you run.  Just run what's in there.  If I had a wideband A/F gauge, I would mess with the mains, but they seem to be running fine as is.

 

Tuning my setup has been a long saga field with tales of woe.  Full open throttle is really happy most of the time.  Partial throttle is hit and miss, and I have been fighting engine noise and detonation for a long time now.  My guess is that everyone's going to tell me that I'm running my pilots way too rich.  My experience has been that richening up the pilot system has made a HUGE difference in both performance and fighting off detonation.  The car is happiest now that the pilots are where they are.  With leaner pilot jets, I was getting really bad stumble between idle and 2000 RPMs.  I have heard that you want the idle mixture screws to be 2 to 2.5 turns out, and that if you have to turn them farther out than that, your pilots are too lean, and vice versa if the screws are too far in.  My car initially had 57.5 pilots, and I had to have the idle mixture screws out at least 3.5 turns, if not more.  As I understand it, the idle mixture is going all of the time, not just idle, so if they're turned too far out, it's going to make everything else too rich, including the mains.  I could be wrong.

 

I have some theories on my setup, but nothing is concrete, and I'm still trying to find that sweet spot.  Theory 1 is that my cam has WAY too much duration, and thus overlap.  The power band is too high for street use, and I think the lack of manifold vacuum at idle and low RPMs is messing up tuning my carbs.  Launches are no good, and I can't put my foot all the way down until I hit 2500 RPMs.  Even then, it still doesn't really "go" until 3000 or 3500.  2500 is just where it won't bog down on me from putting my foot into it too early.  As far as what this does to the idle and low RPM mixture?  I have talked to some guys, and the thinking is that because of the lack of manifold vacuum, there isn't the appropriate velocity of air running through the carbs, and the air isn't pulling the petrol as much as it should, thus leaning out.  If this is hair-brained nonsense, please chime in.  As a result, I have had to richen up the pilots to compensate?  Another effect of a high duration cam is that the throttle plates have to be opened up more at idle.  I've also read about guys drilling holes in their throttle plates for high duration cams.  I'm NOT going to do that.  My idle adjustment screws are nearly binding on the springs, that is, they're screwed in almost all the way.  And my idle is usually around 650 - 700 when warm.

 

Another bummer about my setup is my compression ratio.  Mike roughly calculated mine at at least 9.3/1 which is getting into ping territory.  I'm using stock L20B dished pistons, so I don't get any help from the "quench area" of the closed head.  Trying to find the right static timing has been a total bearcat, and I think I'm getting closer to the sweet spot now.  I have been trying to find a compromised spot where I don't get any detonation and performance doesn't totally suck.  But the problem I have been running into is trying to find out where the ping is coming from in the first place.  Lean fuel?  Too advanced?  Spark plugs too hot and acting like glow plugs?  I think the lean fuel thing is taken care of, as right now I am running too rich if anything.  But it doesn't take much to lean it out too far.  If I screw the idle mixture screws in .5 turn, it starts pinging on me real bad.  I'm also running cooler plugs right now, so I don't think I can blame it on them.  This week my timing is set around 5 degrees BTDC, and I think that's pretty close.  I'm also running 91 octane.  With this setup, I'm not getting much detonation, but I could be happier.  It only acts up if I'm loading it up at low RPMs, so I stay clear of those.  My timing might be lower, but the funny thing is that it doesn't backfire much at all.  

 

Another theory I have is on the use of air horns/velocity stacks.  For the longest time, I didn't have any.  Now I have 1" OEM Mikuni horns.  My theory has to do with turbulence of the air entering the carbs in relation to how far you can open the throttle and still get results.  Since I've had the car, there was a spot in the pedal where it didn't matter how far it was pushed down.  Past a certain point, you weren't going to get any more juice out of it.  With the horns, I can now go past that point and the engine will continue to pull harder.  My guess is that without the horns, too much turbulence or disruption of airlfow built up at a certain opening of the throttle so that the air could not move any faster and was limiting itself before the throttle could be fully opened.  With the horns directing the air, airflow and velocity can now increase past that point.  

 

Long story short:  I think my cam has messed up my tuning, and my compression ratio isn't helping.  I would love to get a U67 head with a stock cam and see how that changes things.  I might just swap in a stock cam with my existing head and see if that solves any problems first.

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You have put a high rpm cam.in your engine. So rev the engine up to 7000 rpm,.which is the factory redline. A 40 mm carb is too small to make.max HP at this speed fir a 2-liter engine.

 

Just make sure the 45 does not have too large venturis or as hainz found out it wont be good

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My theory has to do with turbulence of the air entering the carbs

not a theory, thats why they exist.

 

 

 

i have the same cam & 45's in my 2.22 :thumbup:

 

im guessing that tuning the 40's will be a lot more fun overall.

how often will you be full throtttle for more than a few seconds? ;)

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not a theory, thats why they exist.

 

Yeah, I must have misunderstood other people's analyses and things that I have read.  I always heard that "the horns don't do that much" or that they only improve breathing in the top end.  My experience is also that full open throttle is more open than it used to be, and not just in the top end.

 

On 40s vs. 44s, ggzilla must be right in that the Stage 2 Isky would be most optimized using 44s.  I think what everyone else is getting at is whether or not you really want to optimize that cam, at the cost of having poorer low end, and whether it matters all that much?  Is he racing or spending time out at the track, or is this a daily driver?

 

My cam, which has the same lift and probably similar duration, just isn't the best for street use.  I haven't ever reved my engine to 7000.  For street driving, I only ever get up to 4500 around town or 5500 - 6000 getting on the freeway.  Since I'm generally not using all the power available from this cam, it doesn't make any sense to have it in there.  I'm also not doing any track days or autocross.  FOR ME, I think 40s with the stock L20B cam would be awesome for around town.  Isn't the stock power band around 1000 - 5500?

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I Like 40S!!!!!!!!!!!!!!!!!

 

Hainz:  The only Ratsun member that can consistently get away with that many exclamation or question marks.   :rofl:  :frantics:  :rofl:   I always feel any thread hasn't reached its full potential until he has used extreme exclamation.

 

Your probably right about the rear end, but I also want to be able to cruise on the highway.  With a 3.9, I was hovering between 3500 and 4000 RPMs just to keep up with traffic on the interstates (70ish? average).  I swapped down to a 3.7 Roadster diff in my goon, and cruising on the highway seems a lot better.  I didn't notice a whole lot of difference in launches...

 

Another reason I don't want to be reving like crazy around town is because of my stupid loud exhaust.  EVERYONE knows when I'm getting into it.  I hope in the near future to go down to a 2'' pipe with a resonator welded in before the muffler.

 

I'm gonna stick with the 40's i have, she will be daily driver. They will even be a huge improvement, the stock hitachi that's on there runs like crap. I would definitely like throttle response and lower end torque more than high end power.

 

Sounds like you're in the same boat as me then.  With my cam, throttle response is better now than previously, but if I stab the accelerator before the power band (2500?) the engine still bogs down until the manifold vacuum can catch up.  I used to have the same setup on an L18, and it was even worse.  I've just had to get used to easing into the pedal a little.

 

I'll probably swap back to the stock cam in a few weeks.  I'll let you know what happens.

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I dont think 480 is a huge cam.

 

My Banzai car has a has a claimed 475 delta regrind cam and I got a 3.70 subie diff and even with the stock 4 speed would hit 70 ez and barely on the throttle once up there.

Now I got a 200sx 5 speed.

going to a 3.70 was a noticeable decrease in take off power. But I LIKE THE CRUISE SPEED MYSELF.

 

My yellow convert 510 has a L18 and 44s and you need to choke the shit out of it to get it to start . Pops and spits out gas all the time when cold sparying gas on the engine bay(cant put filter as its the long running Nismo manifold and big funnels.

 

Most production car if they had sidedrafts at the time alwasy had 40s. example the Toyota RG18 in the Celicas

 

My best running 510 I will say again back in the day was a built L20 with 40s and ran ike a Mother fucker. Whoever built the motor knew what he was doing.

 

 

if you dont have your sprocket on #3 yet try that first.

 

since you got a wagon run a resonator in the middle then a Turbo 3 or 3 chamber muffler in the stocl location and thay sould bring the noise down.

Cherrybomb 2 Turbo, they have a 13 inch case(smallest I seen) 2 in that fits perfect. I got mine from JC whitey back in the day. I gave one to Datto510(Eric Long and he ran 40mm webers and it was alot nicer. Not that 510 sound you can hear in the distance , (so it gives me enough time to jump out of your girlfriends window)Loud Pipes saves lives! Esp my own.

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Hainz, do you have build threads on your cars?  I always read so much about your Banzai car and your yellow car, but I am oic starved for visual information.

 

I like my 3.7 too.  I've got an early 280zx five speed, so the 5th gear isn't as tall as it could be.  My car gets up to 70 (even 80 or more if I want) just fine.

 

Question on the sprocket:  How far is too far for the cam sprocket advance?  In my build, I installed cam tower shims to both correct valve train geometry and to compensate for the material taken off of the head (shave).  Now, with the sprocket set on No. 1, the V-notch shows that the cam sprocket is already bordering on advanced if anything.

 

DSCN1465.jpg

 

If I go No. 2 or 3, it looks like it'll start going off the hash mark.  I think I tried this with the sprocket on No. 3, and if I remember correctly, the V-notch was completely off the hash mark (to the right).  Is this acceptable, or should I keep it right on the money?

 

Thanks for the tips on the exhaust.  It'll prolly be a while 'til I get to it.  I'm also considering tuning down from my 2.25'' exhaust to 2'' from manifold to muffler.  Good idea?  Bad idea?

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I'm also considering tuning down from my 2.25'' exhaust to 2'' from manifold to muffler. Good idea? Bad idea?

 

I think its a good idea.

 

Just get timming chain tool and swap to # 2 or # 3 on the sprocket. it should stay with in the V and notch

I did on my truck motor and I didnt notice the V notch move really at all. This was between #1 and #2.

 

But I have the shorter stroke L16/18 motors. Being set to #1 you should get moreHP in the higher rpm s set on #1 sprocket.

 

if set to #2 or 3 youll get more torq sooner at lower RPM. So ask your self your driving habits.

 

I think going with the cam you got you should do 80mph pretty fucking EZ. Your distributor Ok?

 

 

as for my build ups. I just been lucky I bolt the shit together and it seems to work as with my banzai car. Not jetting noting just bolted the crap together and runs great.MY 521 also but EZer since I just use downdrafts carb on thrm and still going.

 

 

GGZilla

Hainz, tune your 44s to you engine, and you will like them much better.

 

Yes I put a Unisyc on the carbs and its better. But this was all from the privois owner and I only drove this car twice in the last 2 years and be honest Im done with it. If soembody has MONEY they can have it. Plus show me they have a garage and a steady job of 50K plus for 7 consecutive years I might sell it. Otherwise it will be like my last 2 510s I sold. Broke on side house and never seen again.

 

Bryon510 on the realm would be a good one to ask. PM him As he has done Cam swaps and has gone backwards thinking bigger is better and all it got was worse.

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if set to #2 or 3 youll get more torq sooner at lower RPM. So ask your self your driving habits.

 

I think going with the cam you got you should do 80mph pretty fucking EZ. Your distributor Ok?

 

Bryon510 on the realm would be a good one to ask. PM him As he has done Cam swaps and has gone backwards thinking bigger is better and all it got was worse.

 

Yes, I want torque, So I'm going to try Nos. 2 and 3 on the cam sprocket.

 

Yes, I was actually saying it does more than 80 just fine - no problems there.  I'd actually like to stretch it out a little farther in a safe place just to see what it can do.  With the way it feels, it seems like it would go all the way to 100 without any problems either.

 

What were you suggesting to ask Byron, and who were you suggesting it to?  I'm guessing me on getting advice on whether or not to go back to the stock cam?  I've never actually driven a stock cammed Datsun.  Is there that big of a difference?

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I can do 75 mph no problem with 4.11 gears.

 

 

> I've never actually driven a stock cammed Datsun. Is there that big of a difference?

 

Sure is. The stock L20B cam has a power curve about 2000-5000 RPM. The stage 2 cam is for 3000-7000 RPM. So it depends on how you want to drive it. If you don't want to rev the engine up, use the stock cam.

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What were you suggesting to ask Byron, and who were you suggesting it to? I'm guessing me on getting advice on whether or not to go back to the stock cam? I've never actually driven a stock cammed Datsun. Is there that big of a difference?

 

 

 

Yes Byron knows more than me on cam swaps but going to a stock cam??????

If you have 44s going to a stock cam dont make sense to me.

maybe go back to a 3.90 gear ratio

 

I think it be ezer to just swap out a a spare head than replace all the stuff on a head thats set up with all the correct lash pads retainer for that aftermarket cam.

This is all just my opinion.

 

maybe you find just bolting a another head and a downdraft might be best for around town.

 

On my Banzai car my 40mm carbs came more alive when the cam was installed.I too had ran the stock cam. But car pulled harder with the cam. Slower take offs as I needed to get the air flowing.

You can cheat the sytem by giving your lash pads more gap to make it thibk its a smaller cam.

 

I set mine t o .006 and .008 you could do yours .010 and .012 but might be noisey.

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