Hello everyone,

My name is Lee Bohon, but most of my friends call me "Leebo". Some of you may know me from the engine conversion section of the other forum. I just stumbled across this from a link on miataturbo.net.

Anyway, here's the "short" history of myself... My wife bought a new, base model 1991 miata in May of 1991 when we were dating. We married in '94 and she continued to drive it daily as she had been up until around '95 or '96 when I discovered a little thing call the internet. I started reading about folks putting superchargers and turbos on their miatas, racing them at autocrosses and getting together for drives and meets. Up until this time, it was just a fun GIRLS car. I think that's when I woke up and realized how much fun this car is and could be. I would drive it on the weekends for fun, did the basic maintenance that I was comfortable with doing (i.e. oil changes). The info on the internet struck again and soon I was tinkering here and there with some mods. First big thing... cheap-o walmart fog lights! Then some basic subframe braces and bigger sway bars. Then the big power increase... bumped the ignition timing from 10deg BTDC to 14! Whoo hoo! I was having fun for sure.

A year or so later, I saved up and bought a basic 6psi Aerodyne based turbo charger system designed by BEGI. I tweaked on the system a bit by adding a intercooler, bumping up the boost and adding in a FM/Link ECU. That got me to around 185whp @ 10psi. After several turbo failures in one form or another over a years time, I got out of it and converted to Garrett T28 based system that was mostly in the form of a FM3 kit sold by FM. Of course, I played with the system a bit to improve reliability, performance, etc... and enjoyed it for almost 9 years. It ran pretty solid around town, autocrosses and occasion lapping days at 15psi and 240whp. Over that time, I blew 1 differential (original tiny '91 diff; swapped for '94 Torsen), and 2 trannies. The original tranny died after about 4 years of FM3 use, and the second tranny went about the same length of time. When it died, I had a choice: (A) replace the tranny with the spare I had in the shed, or (B) replace the tranny... and engine and diff and, well everything with a complete different drivetrain. I chose option B.

Starting in November 2007, my miata went up on jack stands. In August 2009, it came off the jack stands for the first time with a carbureted Ford 302 stroked to 331. The engine dyno'd at 412hp/399ft-lbs. I got several parts of the Monster Miata (Martin's) kit from a friend that had blown his engine and decided to go a different direction with his car. Aside from going with a carb, I did a few things differently with the kit mostly involved with the engine accessories and cooling system. The accessories are a '94-'95 Mustang design to open up room behind the radiator. I converted the kit pusher fans to puller fans, modified the radiator for a better upper hose path, had a shroud made to help draw air from the radiator through the fans.

My current exhaust uses two Magnaflow mufflers but has a significant drone at 1700-2300 rpm. I have some ideas to try and hope to tinker with it more soon.

-Leebo

 

Welcome Leebo!!

It's funny how similar our Miata evolution experiences were, except my Miata was always My Toy Car, not the Wife's since I couldn't find a Firebird Formula reasonable. The major moment moving me into the V8 direction came after one of the best track days of my life at Laguna seca. The liitle Blue Miata gave her last full measure that day in Monterey in HPDE4, she was the only Miata and the next slowest car in the 28 car group was a ZO6. When we got back to Arizona, she backed off the trailer under her own power, drove about 50' forward and then wouldn't move and the engine wouldn't run anymore. We took her apart and when I pulled the tranny back, clutch parts cascaded onto the shop floor, the compression was gone in the motor and ME Very Thankful, the Car had'nt ruined my Track Day (The car on street tires, turned Spec Miata qualifying lap times at Laguna!) All while it was quitely dying inside, thrashed gasping it's last Fast Breath!!

Hey Leebo, don't forget to add a Photo Album of your Car!

Everytime I see your car it gets more beautiful, NICE Work and welcome over here, a small but mighty forum!!!

- Lee

 

Great track story Lee! Litt'l engine that could... and DID. Definitely sounds like that car deserved the LS2 reward it got. That reminds me of a friend that had acquired a '95 Miata that had a very big turbo, built 4cyl engine, was known to be quite a fast car in it's previous ownership (in the high 300hp range; I've watched it on the track before), when it went out for a lapping day running hard but limping home. Some how, it ended up w/ a bad tune and the engine basically had a melt down by the end of the day. Rings were shot, very little compression, sad ending.

Thanks for the comments!

 

You build quality is really something to be proud of Lee, we gotta get you away from the carb and get you into a state of the art injection system.

With injection, that sweet motor will last longer, cause you're not gas washing the cylinders. You'd finally be able to see some gas mileage and with big injectors and a good tune, you could feed that Beast all the fuel she wants and then some, on hard acceleration!

A Really Big Turbo, you mean like this one? My own personal opinion (sure to offend someone) is that; if you're looking at adding Turbos, Super-Chargers or Nitrous, you may not get what you want, but you're sure gonna get what you deserve. Build a solid N/A motor, the results and longevity are better.

Like they say; "To Win, first you must Finish".

 

I have a couple friends that have a running bet on how long I'll go until switching over to EFI. I hate to say it, but I know they are right and at some point I'll let go of the carb. The washing of the cylinders is definitely something I worry about a bit. My friend that had that monster turbo engine lost his rings in a large part due to an extreme rich state. They ran it on a dyno after the lapping day to see why it was down on power; the WBO2 readings were off the scale (under 8:1 A:F ratio; way rich). Big time washing.

I still like the idea of the Pro-Flo EFI system but the expense doesn't make a lot of sense. http://www.summitracing.com/parts.aspx?sku=EDL-35210 A 1000cfm flow rate probably isn't too great for low end torque either. There's also a Mass-Flo system (http://www.massfloefi.com/mass-flo-s...fi-system-p-17)

Something to think about, and realize I can't afford for quite a while now.

 

Both are WAY overpriced! A Victor Jr. intake with Ford red top injectors, a modded stock harness, stock PCM, and a PMS to tune it all would do the trick really well (minus fuel lines). Selling the carb stuff will bring the price of the swap WAY down

Info/insight/opinions on the PMScan be seen here-http://forums.corral.nets/showthread.php?t=608845

Ya know you want to

 

Yeah....... I know, I know. Way over priced. You're right, they are expensive and there are cheaper ways to do it. It's a matter of want vs what makes sense. And to add to this situation, the buddy of mine that de-V8'd his miata still has the stock harness, stock PCM, injectors, fuel rail, trick-flo intake and a few other items all of which he used on his Monster setup that he would sell to me. Selling off the carb stuff, I could probably almost break even w/ his EFI stuff. That's probably the route I'll ultimately go, but I'll give the carb some more time through the spring/summer.

 

I know what's Really goin' on Leebo, You're enjoying the fact that your Miata is Running and running good and you Like Driving it!!

The perfectionist in you will kick in, and rationale will go out the window and you'll be working fast & furious to improve that beast!!

It's OK, many of us Really Do understand

 

Lee,

There is no reason you need to run pig rich on a carbed setup. Unfortunately, most people still tune carbs 60s style..if the plugs look good after a WOT pass they leave the carb alone! Put a wideband on your car and start driving it around at light cruise...you will be shocked at how rich a standard untweaked 4150 Holley runs at these conditions. Not unusual to see 12-13:1 AFRs at light cruise. Fixing the problem is not as easy as changing the fuel map in an ECU (or running closed loop at 14.7) but can be done:

1. Make sure the floats are set properly and make sure your fuel pump (elec or mechanical) isn't supplying too much pressure to the carb inlet(s)
2. Adjust throttle plates properly on the idle transfer slots
3. Monitor your vacuum at cruise/light load and make sure your power valve is sized appropriately
4. Carefully examine your stock Holley metering blocks and make sure the emulsion tube holes aren't plugged. If you have the budget, crap can the Holley blocks and get Quick Fuels billet blocks which are much better and fully "tunable"
5. Look at the Innovate site or do a Google on "tuning Holley air bleeds"
6. Verify primary & secondary main jetting with WOT passes monitored by wideband
7. Lean tip-in or rich tip-in is often caused by improperly adjusted or sized accelerator pump(s).
8. Vac secondary opening is timed by changing the spring.

A wideband will find the "fat spots" in the fuel curve for you; you can fix them once you understand how and when a Holley carb meters fuel.

 

Thanks for the tips. Actually, I already have a WBO2 on board (Innovate). In all fairness, it needs a little more time w/ tuning. I had it fairly decent at around 12-13:1 at WOT but have had trouble getting consistent 14-15:1 at cruise and light load. Much like you mention... at light cruise, 12-13 (if not worse at times). I also tend to get a lean-tip in, even with the engine fully warmed up.

From what I've learned about carbs so far, I still am not quite getting the results I expect from the changes I've made. No doubt, I'm a noob w/ carbs so it's probably something I'm doing, or not doing. I've played w/ accelerator pump cams to try to get earlier squirt on tip in, and have stepped up the squirter size a couple points. Still not really helping much. I can see it briefly go lean on tip in, at the same time I get the brief hesitation. I may simply need to step up the squirter size again (and/or the

I checked the vacuum at cruise and the power valve seemed appropriately sized. I think I was getting around 13-14" at cruise; it has a 6.5 power valve.

Fuel pressure seemed right at around 6psi at idle, but I really should check it remotely while driving the car. Haven't done that yet.

I played around with the transition air bleeds (the outer 4 corners) to try to lean out cruise but didn't make any major strides.

Overall, it's been a few months since I've messed with it due to the weather changes. I haven't driven it much either in the cold. Once spring rolls around, I'll start tuning it again.

One basic question though, if I'm running too rich at cruise, could this be caused by multiple things or just a few? I'm thinking it could be caused by fuel pressure (over loading the pintles), too large primary main jet (btw, this is a mechanical secondary carb), too small trans air bleeds. What else?

Thanks for the help.

 

Lee, are you running a Holley or a Barry Grant? Tuning a carb is very similar to EFI tuning, make only 1 change at a time and then go out and see if helps or hurts the current problem.

1. Is your initial ignition timing properly set at 10-14 degrees with the vacuum advance disconnected and plugged? Have you verified the ignition advance curve with a dial back timing light and verified vacuum advance function with a dial back timing light. If you change timing, reset idle speed.

2. Once ignition is set,you need to start from scratch on carb adjustments. Make sure that fuel pressure remains in the 6-8 psi range at idle, cruise and WOT. Duct tape a mechanical gauge to the outside of the passenger side windshield and then go out and test (Don't laugh, this is how I have to set/check the BEGI ARRFP regulator on my "bandaided" turbo Miata). If you make any fuel pressure changes, test drive and note any improvement/decline in performance.

3. Once you have verified fuel pressure, get the car on level ground and set the primary and secondary floats exactly as the Holley or BG manual advises.
If you end up changing float level(s), test drive and note any improvement or decline in performance.

4. Look at the orientation of both the primary and secondary throttle blades on the fuel transfer slots in the primary and secondary venturis. Make sure they are within Holley/BG specs. BG refers to this as adjusting the closed butterfly positon. If you make adjustments here, you'll need to reset idle before test driving.


5. Idle mixture can't be adjusted until fuel pressure, floats, closed butterfly position and ignition timing are properly set. To set idle, have the car warmed up. Check and record what the current idle mixture screw settings are. Connect a vacuum gauge to an intake manifold source (not a carb fitting!) and hook-up an accurate digital tach and then start the engine. Start adjusting the idle mixture screws, turn 1 screw 1/8 of a turn in and see if vac and/or idle speed increase. Tweak the first idle screw to get peak vacuum and peak idle RPM. Go to the next screw and repeat. If idle speed is above spec, reset to spec. Keep working around the carb until you have optimized vac and idle speed with all of the screws; it's an iterative process but you should get it perfect with 2 or 3 trips around the carb. Test drive and note any improvement/decline in performance.

After you've done all of the above, see if your cruise and or lean tip-in are better or worse. Ideally, you would do this with the accelerator pump settings and idle air bleeds in the stock as-delivered settings (Did you record what these were before changing? If not BG or Holley should be able to give you the stock as shipped settings).

You are pulling 13-14" of vac at cruise, am I correct in assuming that your are pulling about 15-16" of vac at idle?? If your idle is in the 15-16" range, BG suggests running a 7.5 or 8.5 PV for that range rather than the stock 6.5. Holly's old rule of thumb was .5 X idle vac = power valve for a manual trans. If the 6.5 is too low for your idle vac, going to a 7.5 or 8.5 may help your lean tip-in.

If your lean tip-in persists, especially under rapid throttle opening, your accelerator pump shot most likely needs to be increased (assuming the linkage has already been properly set).

If your AFR at WOT is still within your targets, your primary and secondary jetting should still be OK.

If your cruise and light throttle AFRs are still too rich after all of the above, you need to carefully start tweaking your IFRs, they control the absolute amount of fuel "admitted" to the idle and cruise circuits. Tightening up IFRs will raise off/idle and cruise AFRs to get you to 14-15 AFR. The air bleeds may also need to be tweaked if you downsize the IFRs.

That should be enough to get you started, PM me or email me if you need more help.

 

Thanks! Email sent.

 

Welcome to the site Leebo