Triple Mikuni thread

[madkaw]

Mitch,

Using bpr6 and running T bleed pipe. I get no back fire on decel and never have with all my settings. You are not going to get any better AFRs going with smaller main jets. Your 390 gearing is going to put you in the 13 range because of rpm .

I see you have bumped up the pilots. Surprised your not up with me in the 70 range with your L 28. Bigger pilots will tan those plugs up and might get rid of your back fire.

Timing might also help with decel? What rpm is the backfire?

My MS has timing way high at low load- maybe 45 degrees, but I didn't backfire with the ZX dizzy either

[Travel'n Man]

What the heck is another $24.00 + shipping. I'll shoot Todd an email - I'll grab a 67.5 and a 70 pilot jet. That will give me 57.5 / 60 / 62.5 / 67.5 / 70....................hell, you can't take it with you! Thanks for the direction. It's worth a shot.

Timing is about 35 degree's @ 3000 RPM

RPM backfire is any decel downshift or when the engine is holding you back..............no backfire on cruising or acceleration

Edited June 14, 2014 by Travel'n Man

[madkaw]

Your running a " stage 2" cam also right ?

I have a complete set of jets too

Did a long drive with the 70's in there today. With the bleed holes plugged and the 70's I noticed my AFRs get near 12ish at 75mph - eek.

So I went in an 1/8 turn and that bought me a whole point. Now a steady 13- 13.2 at high speed cruise. With the 67.5's I achieved 15 at that speed, but put up with the "bobble" in transition.

Carbs=compromise

[Travel'n Man]

I went on a long drive as well to a local brewery - I only had 2 - 4oz pours (don't drink and drive). But pleasantly when I pulled in the garage I looked at my AFR gauge and she was reading 14.2............Now need to go back to the brewery with my wife driving!

[240260280z]

With the bleed holes plugged and the 70's I noticed my AFRs get near 12ish at 75mph - eek.

So I went in an 1/8 turn and that bought me a whole point. Now a steady 13- 13.2 at high speed cruise.

With the 67.5's I achieved 15 at that speed, but put up with the "bobble" in transition.

Carbs=compromise

Fuel level is a tuning parameter in DCOE's to set where the mains come in during progression. It may help optimize the two options you state above.

[madkaw]

Been the float route already. The Mikunis can be adjusted with a screw from outside the carb . It didn't help the lean spot on transition no matter how high the level. I haven't revisited that option now that I have done the bleed pipes. It might add a fine tune option.

I think there is a limit to how precise the tune is with carbs. If I didn't have a wideband to stare at I would probably be happy with a powerful rich mixture. Now I'm obsessed with my AFRs and my Z :0

[madkaw]

So I have the 70's pilots in there now and I watch my AFR's hang right in the beefy section on 13eens. At 75mph I was probably hanging around 13.2-13-5 at best. I was also watching my gas gauge thinking it was going to plumit at anytime. What happened was un eventful as the engine purred as good as it ever has and the gas gauge didn't move much different from before.

I made a round trip to work which is 50 miles each way 80% highway. I tweeked the pilots to 1&3/4 turns out. That gives me 14.5-15 AFR idle. I'm telling ya-the engine/carbs love fuel. It just sounds better and runs better. Putting around town the engine responds quickly and decisively.

Now look at my plugs. I have done nothing with these plugs since I installed my megasquirt. These plugs have seen everything from flooding carb to len mixtures. So after last nights run a pulled number 3. It is way better then I anticipated. The blackness at the top of the threads is dry soot and I always seem to have that. My tan line goes all the way down the plug!

I might still turn in the pilots another 1/8.

[zKars]

Steve, with all the discussion and testing, perhaps you have forgotten the role the accel pump has in transitions? Perhaps you need to move that teensie weensie cotter pin one hole up to get some more squirt or up the size of the old pump jets?

Just so's you know and as way to say thanks, all this discussion has helped me to finally go deal with my 44's little "annoyances". Seems I only had 2/6 pump jets squirting anything at all... Plugged knozzles on the jet outlets. Had to ream them out with a fine SS wire strand.

While I had them out, of course there were other little lessons learned and points to mention that I'd thought I'd share.

1. I discovered that it's possible, (not that I did this, well, ok, once during troubleshooting...) to assemble the pump block with the cover that faces the carb body reversed 180, which would render its output useless. Check the gasket and body to ensure you align the outlet with the gasket and the gasket to the outlet hole in the body so the squirt has someplace to go.

2. The condition of the pilot screw taper and its seat can vary all the way across and drastically affect just how many turns is "right" for each cylinder. While I had the carbs off, I was checking the throttle blades for consistency and noticed that one side had the pilot screw tip just poking through, while the other was no where to be seen. I checked that both were the same 1.5 turns from "closed". what a difference. Who knows what the flow differences are. My 44's are oldies, they are the "Q" roadster type, and were old when I got them, so even replacing the needles might not help all that much if the seats are worn as well. How to adjust them now? At least I know the relative positions of the tips on each so I can compensate some, and the rest is by ear. What matters here is the available cross sectional area at the hole into the throat, and how much needle is filling it at just that position.

3. Everyone knows how nice and stinky these things are, fuel stink seems the norm. The bottoms of my carbs were quite fuel-ie and stained, almost to the point of being wettish. Can't imagine the stink that I will have now with 6 really strong accel pump shots every time I touch the gas pedal! I think I have a bit of a solution though to keeping gas on the inside. The base of the carbs have four locking bolts/nuts/bend-over tabs that retain the chokes and venturis. Being on the bottom, they naturally will let the inevitable puddling fuel seep out the bottom past these threads. Just a touch of thread sealant (rtv?) on the threads when you put them back in will help alot. I hope... I love the "turn off the pump exactly one fuel bowl's worth from home" approach. Nice.

4. Transition holes and their location and idle settings. One thing that is not clearly covered in the Mikuni manual deals with mechanical idle settings. Webers have this issue too. It's mentioned briefly above. There is a terribly fine line between throttle blades closed (no idle possible), throttle blades open just enough to get some kind of low idle speed, and too much throttle blade open that exposes the first (or second!) transition holes. To get idle mix right with JUST the pilot screws you need to have enough idle to keep the car running, but NOT have the blades open enough to expose that first transition hole in the roof. With the carb in my hand, from closed blade to first trans hole 'just' peeking open is BARELY one full mechanical idle speed screw rotation. If you have a lumpy cam, or things aren't tuned close to right yet, you get VERY tempted to turn that idle screw up some more to get the dang thing to run while you twiddle, but DON'T DO IT! Exposing the transition holes while tuning idle will likely give you that flat spot in transition off idle, which will make you increase you pump shot or pilot screws turns out, but you're chasing your tail at this point. Take one carb off and determine how many turns you have until that first trans hole pokes out on the manifold side. It also shows you exactly where the mech idle screw starts to move the throttle blade. If you have to have a higher idle, I'd be plugging that first trans hole.

5. If the carbs aren't new, or you didn't get your jets from a known and reliable source, don't trust they are all the same size! Well mine were, but the mains were half 150's and half 80's! yup, 80's, at least that's what they said on them. But the 80's had been drilled to 150's.... Thanks to Todd for pointing out that possibility. Also later discovered my pilots were half 60 and half 65's, again, 60's were drilled to 65's. Drilled jets don't perform as consistently as factory jets. This thread pretty clearly indicates how small changes make big differences in performance. Don't need to have inconsistent jets too.

6. The short 5mm screws that hold the stock Mikuni linkage to the body. I discovered one I had substituted was just a hair too long. The choke in that one barrel now has a very slight extra 'wave' in it. Let's just call that a "venturi laminar flow disturbance" experiment.... Also the same holes exist on the other side of the body, and one at least empties right into the barrel behind the choke on that side. Does this cause a slight leak around the choke from the outside? Maybe, plug them (with SHORT 5mm bolts!)

7. Jetting for different displacements. It seems the "right" jetting for 2.4 and 2.8L engines discussed here are pretty much the same really. If I had to average what I've read above, would it be 190-200 air, 150 mains, 60-65 pilots?. I have a 3.1 stroker, but I'm using the jets as the rest of you. I have 34mm chokes. No, bigger engines do not simply need bigger jets or bigger chokes. Its about being able to supply enough fuel to supply the air flowing through to keep the APR's where you want them.

8. Get your timing, idle vacuum, engine temp in spec and carb/mani and mani/head gaskets sealed before proceeding with any carb tuning.

Can't imagine how I tuned these things before without a wideband and vacuum gauge to refer to. What does vacuum on one runner of six mean anyway? What if you have a lumpy cam? Retarded cam timing? A subject for another post.

And Mitchell, that plug reading chart is something we should ALL commit to knowing by heart. Thanks for posting it, as well as the other references. Great stuff.

[madkaw]

I haven't forgotten the accel pumps and I have chased those tiny cotter pins on the rods too many times. That is one of the hardest adjustments to do on these carbs while mounted on the engine. The accel pumps seem to be doing there job, and I wonder even about going a smaller size yet. but this doesn't have effect on the transition issue I am dealing with.

My issues comes in high vacuum, mid rpm range ,light tip-in, moderate loading. It goes beyond the expectations of accel pumps. Changing the bleed pipes had the most significant change, but not perfect. I am chasing perfection-which will not happen, but still fun yet.

These carbs were about brand new when I got them.

Idle is kept in the 750 range. I did check throttle plates by measuring the opening and setting all alike.

[Travel'n Man]

Everything I have read tells me to keep my idle at 1000 RPM's. That is where I am set. If I go lower than that my car want to bounce a little while at a redlight.

New setting as of last night (as you see this lower air jet made me a little leaner for my total mix)

Lowered my Air to 190 and below are my AFR's

• 1000rpm idle - 14.4
  
• WOT @ 5000 RPM - 13.1
  
• 35mph @ 3rd - 13.0
  
• 60 mph @ 4th - 12.9
  
• 60 mph @ 5th - 13.1
  

I did change to a hotter plug - BRP5ES-11

Backfire on decel did decrease with this setting

Still a little rich at cruise - going to decrease the air one more notch to a 185 this evening - they do say the magic spread is 40 points from the main fuel to the main air..............

Want to get my AFR cruise in the 13.5 - 15 range

Edited June 17, 2014 by Travel'n Man

[Travel'n Man]

Don't forget that your pump's should have a very small filter on them - make sure the jet holes can release the fuel and the holes are not blocked by the ribs in the filters.

[madkaw]

Mitch,

thanks for the filter advice.

Not sure how the main air jet change leaned you out. The smaller jet should have richened you up a bit at WOT and brought the main in sooner. The cruise is ALL pilot to a certain point.

I haven't read anything about the idle needing to be 1000rpm. I have been able to keep it around the 750 rpm range pretty consistantly. It gets a little lopey sometimes, but still idles well. I have noticed that hot starts are easier with a rich idle .