Triple Mikuni thread

[madkaw]

Well after running the engine again and seeing #4 draw much less air then it's next door neighbor#3, I decided to rip them all off and pull the intake too. Of course the intake gasket was fine, which I thought it would be. The car didn't run it had vacuum leaks, but I'm getting desperate.

I did check the butterflies on 3&4 and they matched up beautifully.

Now I stare at my engine thinking what;s next. Not much in the way of evidence to justify pulling the head, but I think my carbs are okay. I did notice looking down in the intake runners of 3&4 that there was a lot of soot? on the inner walls of both cylinders. When I say inner, I mean in respect of each other. No other cylinders look like that.

Kenny,

I fought transition with these carbs and thought I won with the bleed pipe mods. I don't think those mods have anything to do with my issues now. I would try and go as big as possible on my pilots if I were you. I don't understand why my motor/cam wants such big pilots, but it works well.

I think it is mostly the Schneider grind, but I've run 70's on my motor. I run 67.5's now and it's just a L24!!

The bigger pilots might help you bridge the transition gap better, then maybe drop your main air's. I have smaller main airs if you need any. I bought all the way down to 160's to fight the transition issue. I'll never use them.

[Kennymonster]

^If the compression on the cylinders check out, I guess check the valve clearance? I feel like these carbs are pretty stout and hard to believe they'll start fouling all of a sudden for no reason, I wonder if your lashes went out of adjustment.

As for me, I have a set of 65 pilots but afraid that'll just enrich my cruise more than it already is, basically at a level of compromise at this point. I wonder if all that intercooler piping and whatnot is impeding off boost air flow and making my afr richer than it would be at cruise, i.e. compared to just open velocity stacks sucking in. My buddy just ordered a set of rebuilt 40's from Todd for his L24 on a smallish cam, and he received 62.5 and 65 for baseline, apparently that's where the starting point should be anyways.

I have 170 to 220 for air and mains ranging from 120's (don't ask ) to 165, I see that you're a frequent shopper with Todd as well haha. My current tune right now at 150 main /180 air / 62.5 pilot is actually pretty good at 10psi, I'll probably leave it unless I run into issues, haven't really done too many pulls at 4th gear yet since it hits 3 digits pretty quick now ;p

[Travel'n Man]

Ken -

Tell us about your build so we can keep up with your tuning. Pics are always welcome!

[Kennymonster]

Ken -

Tell us about your build so we can keep up with your tuning. Pics are always welcome!

Sure thing' date=' I'll try to be as brief & concise as I can, and attempt to avoid turning the thread about me / my setup. I figure some of this will be useful information for anyone going this route anyway, hybridZ has some scattered info on blow-through's but nothing comprehensive, I should really put my notes together and post the build log.

Current Mikuni settings, with 10 psi on the t3/t04b turbo that came with the Cartech kit (sorry not much details on the turbo itself, had it rebuilt due to it destroying itself during tuning)

Mikuni 44's

34mm venturis

150 main

180 air

62.5 pilot

45 pump

For those not familiar, the kit is basically an 80's Corky Bell / Bell Engineering creation, not too many around these days. I came across it randomly when someone offered it to me out of the blue, at a bad time to resist since I had a local friend/competitor who was about to turbo his Miata.

If you do a search for this online, you'll probably see a handful having ran it in the past, but switched over to EFI, as well as a bunch talking about wanting or running it, and never following through (in typical internet fashion of easier said than done). There are also tuning concerns stated, or the lack there of. I have to admit that I didn't know much about it, but still wanted the challenge based on the assumption that if the kit was being made and sold at some point, it had to have worked.

It took some time to really get it to run decent, not because of the jetting, but more to do with providing adequate fuel delivery and addressing the plethora of issues that came with pressurizing the carbs, against their intended purpose.

Here's a quick cliff notes to save headaches for those researching this route:

- Get an EFI fuel pump + bypass style regulator with boost reference + run a bigger return line. This will maintain 3.5 needed for the carbs, and with the fuel regulator upping the psi of fuel 1:1 ratio with the boost applied, it'll ensure that the float bowls stay full. i.e. Fuel pressure needs to be at 13.5 to have 3.5 psi at the carbs + 10 psi of compressed air coming in.

- Ignition timing, I think this was one of the advantages of trying this set up in 2014, as opposed to 1985. With the MSD 6AL unit, I was able to get an additional MSD module that connects to it, a 'boost timing master.' With the boost reference, it will retard the ignition timing between (1 to 3 degrees, adjustable), per each pound of boost, and you can also choose at what point the retarding kicks in, between 1 to 5 psi. This allows me to maintain 15 degrees base timing at idle, mid 30's at cruise when off boost, and total advance back down to 20 at full boost.

The kit out of Corky Bell's literature:

[img']http://i47.photobucket.com/albums/f180/Kennymonsterious/11cgqbp.jpg

Replaced that ridiculous 80's wastegate on the left with a newer model, same brand:

3 port Mallory regulator with boost reference: it actually has total of 4 ports for inlet/outlet, and the return which comes out the bottom. This worked out nicely for the triples, since I have 1 port as feed, the 3 remaining outlets have dedicated lines going into each carb to prevent any starvation.

Shot of my engine bay as of today, since there's no ECU, everything has to be referenced the old fashioned way, with vacuum/boost. Using a short runner Mikuni intake manifold, I'm using 4 out of 6 vac ports which most NA setups will retain the hex plugs/not use. Fuel pressure and ignition timing is referenced at the plenum, to achieve earlier and simultaneous signal for each. Only mods to the carbs at this point, are changes to the jetting above, and making slightly thicker gaskets out of cork material for the jet covers, to make sure there's no pressure leak under boost (could probably order new sets from Todd... but I'm afraid they'll be something like 20 bucks each)

[madkaw]

It's an accomplishment for sure to make the pressurized Mikuni's work. I think the programmable ignition is mandatory to keep this builds from blowing up.

I thought about pressurizing mine for a while, but since I am having issues with just NA, I think I will stick to FI for turbocharging:)

It definitely looks impressive under the hood Kenny

[Kennymonster]

^Yup detonation kills motors, not boost. I had the wastegate hooked up incorrectly on one run, accidentally hit 16 - 20 psi few times, but with the afr/ignition in check, it pulled like a space shuttle.

If I can get this to work after many frustrated nights, I know you can figure out your issue Steve Will be waiting on updates!

[Travel'n Man]

Ken-

Have you dyno'd?

[Kennymonster]

^I have not, but will soon. I always go to the same place so it should show a pretty accurate before/after comparison, I put down 150/150 as NA, expecting low 200's with the boost.

[madkaw]

No updates except frustration! On vacation in Irvine CA right now-so stepping away and also waiting for parts from Todd. I need to do more carb swapping- as far as position- to verify my issues.

[Kennymonster]

^You're only couple of hours from me, how long are you sticking around in so cal / any plans to attend ZCon next week? The San Diego Z club folks are going nuts getting everything set up, I'll be volunteering at the zcon AutoX on the 9th also.

[madkaw]

Not long enough

[Kennymonster]

Going to do some jetting this weekend.. at 150 main /180 air, 1st/2nd gear is dead on 11.5 but after getting hot (as in after 8 laps), it's leaning out too much in transition in any high load / high gears, 3rd/4th/5th.

I threw in 160's last night for the hell of it, pulled like a mother in 3rd/4th without leaning but saw the AFR dip down to 9's when main's kicked in and settled in 10's at WOT. I'm thinking of going back to 150 main, but trying 170 air to see how much effect it would have, hoping for lot earlier transition than the 180's to keep everything smooth.

Edit: Got impatient and just went out for a 4th gear pull with 150 fuel / 170 air... transition was ok with WOT at 4th around 12 AFR at 6000, this was 8 pm in the evening though. I'm sure it'll lean out in the day when it gets hotter. The 20 gap between main and air seems to be the trick?

Just ordered 155 main and 175 air's from Todd, hoping for rock solid 11 AFR

Edited August 15, 2014 by Kennymonster