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madkaw

Triple Mikuni Help

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Well I actually got to drive the damn thing today

 

I left the 200 main airs in and ran with 60's on the pilot side, but still had to run 2 turns out.

Cruise was dead on AFR-14.5

Idle is less conclusive since the wacky O2 sensor-but 13+ I will guess

WOT is where it needs work, with 140 main jets I was at 16+ AFR

It wasn't very strong with those AFR numbers, but straight cruise wasa delight

Yes there was a slight stumble, but I believe the 62.5's might get me there.

 

Jon, doesn't look like a big difference in pilot size despite the vent deal. Of course it doesn't die now and I never had it out on the road until now.

 

The airbox seems to be working out, but I have work to do on my vacuum log

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I should have gone to page two first!

 

 

D'OH!

 

 

I'd never even considered someone making a box without the bowl vents incorporated. It's a must-do, learned way back in the 50's! The box is fine for ITB's but for carbs it's a definate NoNO!

 

I would also be curious to see what depression you have in the box under WOT at speed. If it's more than 4" W.C. sans-filter, you are undersized on the feed line. If you are there or lower (2-3" W.C.) then everything is hunky dory.

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On a dyno with a water manometer.

On the road most easily with a Magnahelic Gauge that reads in Inches H2O and has been compared against a water manometer to verify there is no drift or skew.

 

Did it all the time in my youth!

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You can get them at WW Grainger and McMaster Carr if no place else. I have a Dwyer Slack-Tube Manometer as well--but a water manometer don't work on a bumpy road.

 

2000_pic.jpg

 

Wanna See How Carbs Work? Cut 'em apart and make a technical article like in G-Works Magazine in Japan:

 

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For ONCE I'm on an API-Style job with me in the office a bit more, and on the equipment a bit less, so I can actually run across this stuff!!!

 

FACTORY CAI BOX:

 

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RE: The missing holes... don't feel bad, I just ran across these photos I took at Nostalgia 2Days in Yokohama last year (Same as where I got the above carb cutaway shot!).... The "In the Flesh" version of G's Works Garage Project FRP Airbox (though admittedly they converted their Mikunis to EFI ITB's!:

 

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Just Rubbin' it in! LOL:

 

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Edited by Tony D

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Well Todd made me deal I couldn't refuse on main jets- I think he feels sorry for me.

I've got 145,150,155,160's coming.

 

I'm running 62.5's on the pilots, still not enough - I think.

Too small main jets alters my driving, don't want to run lean and my buggers mains aren't here yet.

I'm thinking I'll be running big pilots with a smaller accelerator pumps- got 47's in there now and it seems to bog the motor .

But I have more driving and tinkering to do.

Thanks for the info Tony.

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I'm thinking I'll be running big pilots with a smaller accelerator pumps- got 47's in there now and it seems to bog the motor .

This matches what I found. Bigger pilot and normal size pump jet I think I ended up with 40s. I tried HUGE pump jets trying to get rid of the 2500 rpm stumble and found that the bigger I went and the more movement I tried to get out of the linkage the worse it got. Bigger pilots helped a lot though, and they work way up into the rpm range. They affect more than just idle and low speed.

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Thats what I'm feeling when I drive. My stumble, if you want to call it that, feels like the pumps are drowning the mixture. Now that I have had the carbs off and have witnessed first hand how much fuel the pumps put out, I can see this happening.

Jets came in today, so tomorrow us R&D day!

 

Jon, did you wind up leaving the plunger rod in te middle hole?

I also considered moving it to the lowest setting, but it's easier to change the pump jet from on top.

Edited by madkaw

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The accel pumps move HUGE fuel. I'm on board with JM's comments 100%. It follows with everything I've seen through the years. People always undersize that idle pilot jet. You can make the mains larger, but they still don't 'tip in' until around 3K putting this 2500 stumble directly still on the pilot jet.

 

The Japanese guys were like mantra "main jet over 3000, pilot below"....

 

Remember these are OEM Street-Driveable Carbs. To get any kind of fuel economy (like EFI with closed loop) you run on the smallest jet you can and the lowest throttle opening you can. If your cam is 'coming on' at a low rpm and your idle vacuum goes UP at off-idle speeds then you start drawing VERY HARD on that pilot jet---and while you get more fuel, you get a LOT more air through that fixed bleed. The only adjustment you have for that is to make the gasoline jet bigger to compensate.

 

You could try to tweak your float level, but at this point, with you making progress and being able now to see changes in the direction you want to go with the jetting, I'd 'stay the course' and keep going in that direction.

 

With that main jet selection you will be set for a variety of engine configurations!

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Can someone tell me how the main air jet and the main fuel jet interact with each other.

 

When I was expressing to Todd about richening the main side he suggested sizing up the main jet and dropping back on the main air to the 180's(from 200's).

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For anyone still following this;

 

Beautiful day here in Bloomington Indiana-high 70's and not much humidity

 

Current set up is;

pilots are 62.5's at 1.5 turns out. The engine is happiest at this point. NO hesitation or bobbles and AFR's hovering around 14. The engine idles best here and restarts the easiest. The 65's semed too rich and made my wideband go crazy-whic is usually an indication that my mixture is taking big swings.

 

Main jets are 155 and main air are 180. Since my O2 doesn't like big swings it is a bit more difficult to diagnose the main side. Plenty of power, but maybe more is available.

 

Accelerator pumps; went back with the 40's and the engine likes it. There is no bobble when bipping the throttle or heavy tip-in. With the 47's the engine sounded like it was drowning in fuel.

 

Back to more R&D

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Main jet controls operation where your foot is heavy on the gas or you're above a certain rpm. That certain rpm is determined by your air jet. The smaller the air jet (bleed), the sooner your mains "come on" (higher vacuum signal).

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Leon X2

 

That percentage is slightly beyond where you can see the transfer ports above the top of the throttle plates. Somewhere between 15 & 30% open. Open it past there and if you are on a dyno looking down the throat you can see the gas starting to come out of the booster venturi.

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This R&D is fun, but with a wacky O2 sensor makes for a bit of a guessing game-I'll be getting one on order shortly. I can see what's going on at cruise, which is a very steady 14-15 AFR, so I think I have the pilot's nailed. My only concern was the transitional stage around 3000 when all my timing was coming in-I thought I picked up the sound of detonation, but my loud exhaust makes it tough.

To be on the safe side I put the 180 main air jets back in to bring in more fuel in this transitional area-if I understand the relationship correctly. Since I can't see what AFR's are doing at WOT, I would rather have too much if anything. I pulled the plugs after about 200 miles or so of combined driving-drove it work, 50 miles one way highway- the plugs couldn't have looked any better except for the lack of color. Maybe not enough miles to put color on the plugs, but a very clean burn.

 

Engine runs very strong and maybe I can still run more fuel to it at the top end. I believe I have it running better than when I was running the SU's as far as mixture. I am over the top happy with the sound of my Triples+Duals= sounds better than any V-8 I have owned

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This R&D is fun, but with a wacky O2 sensor makes for a bit of a guessing game-I'll be getting one on order shortly. I can see what's going on at cruise, which is a very steady 14-15 AFR, so I think I have the pilot's nailed. My only concern was the transitional stage around 3000 when all my timing was coming in-I thought I picked up the sound of detonation, but my loud exhaust makes it tough.

To be on the safe side I put the 180 main air jets back in to bring in more fuel in this transitional area-if I understand the relationship correctly. Since I can't see what AFR's are doing at WOT, I would rather have too much if anything. I pulled the plugs after about 200 miles or so of combined driving-drove it work, 50 miles one way highway- the plugs couldn't have looked any better except for the lack of color. Maybe not enough miles to put color on the plugs, but a very clean burn.

 

Engine runs very strong and maybe I can still run more fuel to it at the top end. I believe I have it running better than when I was running the SU's as far as mixture. I am over the top happy with the sound of my Triples+Duals= sounds better than any V-8 I have owned

You're on the right track! I consider a working wideband critical, unless you can set up the carbs on a dyno. Even though you're running Mikunis, I'd still read through Keith Franck's "White Tuning Paper" on Sidedraft Central. Webers and Mikunis work on the same exact principles, just the jet sizes might be a little different because of geometric variations between the two types of carburetors.

 

Before we continue, we need a video... :D

 

I'll entice you, here's my twice pipe, triple carb Z (including short-shifting and a leaky intake/exhaust gasket!):

 

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Thanks for the video! My exhaust definetly sounds different then yours. I will posting up a video soon -well as soon as I get my video camera back-it's on loan,

 

Back to Mikuni tuning-After driving around with the 180 main airs for a while I noticed a slight stumble that I didn't have with the 200's, so I threw the 200's back in. I'm staying with this set-up until the O2 sensor arrives.

Hopefully next month I will get the car to the dyno or sometime after tuning with a WORKING wideband.

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