Mikuni Jetting

[lila725]

Hello all. I am asking for some jetting advisefor my triple 44s.

I have a 73 240 with a 3.0 kit,e88 big valve head ported, with a Isky cam, (360/500), all valves set to 010" cold, k&N filters, petronix ignition, holley fuel pump set to 3 1/2 psi no return, 7 dg advance timing @idle.

Went through mikunis and set all float levels to 13mm accel pump to 7.5 mm and used the highest, (Richest) hole for the cotter pin on the pump lever, all jets are tight and clean. The car currently has 22 main air with 180 main fuel, 57.5 pilot and 45 accel pump.

I am expierencing a flat spot around 2400 to 3400 rpm or at about 10% to 20% throttle, much more when engine reaches operating temp. Driving it hard its great and idles great.

I hooked up a air fuel ratio meter (Autometer) and have found that it is a very fast drop to lean when expierencing the hesitation. After driving it hard it does not seem to happen for a while but if im driving it normal or around in traffic it seems to get worse.

Anyone have the like situation or setup or have any ideas on a better jet configuration for me. Any advise would help.

[steve91tt]

I too am fighting a similar stumble with my Mikuni's 44's that shows up as a lean spot on my AFR meter so maybe we can learn in this thread together.

The lean condition you are seeing on your AFR meter can be caused by either a lean condition or extra oxygen being introduced via a misfire. In other words diagnosing transitions with an AFR can be tricky.

A few questions...

Are you running a heat shield?

What size chokes are you running?

Have you tried moving the accelerator pump to a different hole to see if it affects the transition stumble?

Have you tried the effect of other jet combinations on the stumble?

What are your AFR's at idle, WOT and steady state highway?

[esmit208]

BLUE posted a good illustration about side draft carburettors a few days ago. Check the archives. Something that caught my interest was the effect that emulsion tubes have on overall performance. The fact that they can be changed was what really got my attention. It might help if you look at the illustration and narrow down what parts may need to be swapped out for better performance.

[esmit208]

725 and Steve read this and study. It might help. Understanding a Weber Side Draft Carburetor Through a Fictional Supposition.

[lila725]

Hello. I am running a heat shield and I also just wraped my headers with that titanium wrap. I honestly dont know what size choke i have on there, and yes I have moved the accel pump rod to the third hole up (Top) for more fuel.

I just ordered accel pump nozzle (60s) from wolf creek as they are 45s in there now. Moving it to the third hole helped but it is still there. What is wierd is when it happens if i rev it up a few times it clears up which is why I originally thought it was loading up and to rich but that is not the case.

Watching the meter at idle is wierd because it actually fluctuates between rich and lean as a new car with an ecm would. It idles great and at wot it is right on the border from good to rich which is ok with me. At steady highway speed it is in good range and will every now and then hit rich.

I will let you know when i get the accel pump nozzles in, (Should be a day or 2) and then go from there. After that I also plan on doing basic ignition tune just because, (Cap rotor wires) plugs and coil are new and I have the petronix kit in there now

[steve91tt]

Your comment on reving it up a couple of times to clear it is interesting. I have found similar myself and had attributed it to the accel pump circuit being only partially full due to fuel boiling. The way the Mikuni accelerator pump system is setup somewhat different than the webber system. The volume of fuel is controlled by the position of the rod where the rate at which it is delivered is controlled by the nozzles. The larger nozzles should dump fuel into the system more quickly at transition. Hopefully that will help. You say that moving to the top hole helped. Have you tried extending the rod by turning the threaded portion a turn or two. This is a way of getting more fuel volume at transition.

My engine likes to idle a little on the rich side (11-12ish). Since the pilot system is also active at transition you might try backing off your pilot screws 0.25 turn to see if that helps. Keep us posted!

[lila725]

I will adjust the screws today after work. Right know they are 1 1/8 turn out from seat. Also I tried to adjust the rod a few threads but could not get it to move at all and i was afraid to get rough with it. Its almost as if they are jb welded or something. Ill try again after I get the nozzles and see what I come up with.

If the end result is that im always on the rich side ill be happy as then i have an excuse on dumping the cash on air horn's to even it out.

Do you have any expierence with the air horns and the steel mesh filters ot sock like filters from pipercross? Are they worth the money is what im wondering?

[steve91tt]

I'm running air horns with mesh filters but I have never run them any other way. I love the way they look and sound but I can't say what it does for performance.

[jmortensen]

Screens are really restrictive. You should run an air filter, preferably one that has filter element all the way around like the ITG, but even the chrome capped K&N style filter is a big step up from screens. You might note that K&N's have screen on them too, but they have a lot more surface area and they're pleated to increase it a lot more than just the size of the filter would suggest. Increasing the surface area of the filter/screen reduces the restriction. I wouldn't use socks for the same reason. Steve, if you remove the screens and go with a filter you'll probably need to re-jet.

Edited November 22, 2011 by jmortensen

[lila725]

Cool thanks. Im thinking then ill go with itgs in the near future, but do you think investing in the air horns is worth the $250 or should i just continuing running it without them and just with the filters.

I'd run open air horns but I live in Nevada and thing get quite dusty quite often here.

[steve91tt]

Jon,

When you are talking about screens are you referring to the following type of filter?

They are marketed as "Three layer stainless steel, filtering down to .009 with virtually no air flow restrictions"

I assume from what you are saying that this is not the case.

[lila725]

Those were the ones I was talking about. If they are restrictive then its ashame because they are damn sexy.