Carb Tuning: AFR’s with gasoline containing ethanol

[edwardb]

I am in the process of starting to tune the Quick Fuel SS-750-AN double pumper with electric choke carb in my recently completed Mk4 build. I have installed an O2 bung in the collector of the PS pipe, and have a new Innovate LM-2 AFR wideband meter wired up and running. The only downside is the inductive pickup doesn't work with my MSD ignition. The Innovate instructions warned about this, and they're right. I could fish around in the wiring harness to find the tach wire, but really don't want to go there. For test purposes, I'll do without and carefully log each run using the tach on the dash.

This is all pretty new to me, so an interesting learning project. I’m using the excellent Carb Tuning 101 instructions from Wayne Presley, following a couple other tuning threads, and just reading and learning as much as I can.

First some basic info. The engine is the DART 347 in my Mk4 Roadster with AFR 185 heads, relatively mild COMP Cam custom cam, Air Gap intake, MSD ignition, BBK/FFR 4-into-4 headers and GAS-N side pipes. Compression ratio is approx 10.5, and I’m running premium pump gas. This will be nearly 100% street cruising, so want a nice drivable final product without leaving a lot of power behind of course. The carb came with Primary Jet: 72, Secondary Jet: 80, Idle Air Bleed: 71/71, Hi Speed Bleed: 33/33, Pri Nozzle: 31, Sec Nozzle: 31, Needle & Seat: 110, Power Valve: 6.5. Other than adjusting IMS, I have replaced the primary jets with 68’s when it was clearly way rich in cruise. I’ve driven it a couple hundred miles before installing the LM-2 and starting this tuning. It starts, runs and drives quite well. But it’s clearly rich based on exhaust smell, plugs, and the extra fast moving gas gauge.

So here’s where I’m at so far. I’ve confirmed the timing is 14 BTDC at idle, and all in at 34 at 3,000 rpm. The distributer is mechanical only. No vacuum. I went to lighter springs to get this curve. Didn’t have to change the bushing. The mechanical fuel pump pressure is indicating exactly 7 lbs. Fuel in both bowls is exactly splitting the sight windows. I set the T-slots exactly like Wayne’s directions. About .020 exposed on the primaries and just barely peeking through on the secondaries. It idles pretty OK at 750-800 RPM. I initially adjusted IMS with a vacuum gauge (before the AFR meter) and it was about 11 inches. All four screws were between 7/8 and one turn. With the AFR meter installed, found that it’s idling at 12.1 – 12.2 AFR. First recording session, per Wayne’s instructions (3rd gear, light accel from 1,500 – 3,000 RPM) showed slightly rising AFR from 12.1 to 13.0, average 12.82 over 35 seconds recording. Before changing any jets, went back to the IMS and adjusted some more. Found that the idle increased slightly, smoothed slightly, and vacuum now at 12 inches. All four screws now at about 5/8 turn. Any further didn’t do anything. AFR was slightly higher at idle, but still well below the target of 13.5 – 14.0. I ran another 3rd gear, light accel from 1,500 – 3,000 RPM session and the AFR is again slightly rising but now from 13.0 to 13.5, average 13.08. Going in the right direction with only changing IMS. Unfortunately now I do have an occasional hesitation coming off idle. Other than that, is does run a bit cleaner and crisper than before.

Sorry for all the detail and background, but needed to define the starting place. I have a ways to go to get to the AFR numbers in Wayne’s tuning post. Before I start chasing that, I’m reading in multiple places that target AFR numbers with ethanol based fuels are slightly lower. All regular pump gas in this area has ethanol. The only place to get without is at marinas, and I’m not going down that path. I typically run either Shell V-Power or the premium brand at Costco. Both say on the pump that they contain up to 10% ethanol. Per Wayne’s instructions, my AFR goals should be:

13.5 – 14.0 at idle
13.5 – 14.0 in light cruise
12.5 – 13.0 on the primaries
12.4 – 12.8 with the secondaries added

First question: Are these still valid with ethanol based fuels? I don’t want to throw parts and time at this thing trying to get those numbers if they aren’t the right ones.

Second question: Any words of wisdom regarding the initial tuning I’ve done? All the basic settings are checked and re-checked. All is tight, no vacuum leaks. I think I’m all in on the IMS setting. Reducing the IFR’s should raise the AFR in light cruise, but wouldn’t that make my hesitation off idle worse? Obviously there are more tuning steps to go, but just getting started in the proper sequence.

Thanks all.

[Bob Cowan]

First question: Are these still valid with ethanol based fuels? I don’t want to throw parts and time at this thing trying to get those numbers if they aren’t the right ones.

Thanks all.

Yes, they're still valid. The meter reads in Lambda. No matter what fuel you're using, Lambda reads the same. Then it converts lambda into an AFR number that we can understand. Even with E85, you'll still get the same numbers on the screen.

[Wayne Presley]

The AFR's should be 12.4 at WOT with the ethanol (10%) in the gas. If you have your LM-2 set to display Lambda instead of AFR, the target of .82 doesn't change. So shoot for .4 AFR points lower than the above targets. Adjust the IMS to get the best idle and not any AFR target.

[Wayne Presley]

When I do the light 3rd gear acceleration on level ground, it just barely has enough gas to get to 3000 rpm. That way you really are looking at the IFR sizing. The check for the primary main is to open the throttle until you feel the step on the secondary and let it rev until it quits climbing

[edwardb]

Thanks for the replies. Much appreciated. From these, and also on the other forum, I have the direction I need. Let the tuning continue. :)

[edwardb]

Thought I would stick a fork in this thread and post the results of my tuning effort. This was very much a learning exercise for me. But that’s what I enjoy. I’ve tuned two other carbs in my life, and those with a vacuum gauge, reading plugs, watching the exhaust, and seat of the pants. Very scientific. Using an AFR gauge is obviously a totally different ball game. You see exactly what is needed and the results. I used the excellent Carb Tuning 101 thread posted by Wayne Presley. It explained the exact steps required, what the AFR curve should look like, and I’m very close to getting most of it exactly as he describes. More about that later though.

Here are my final results for the Quick Fuel SS-705-AN carb in my Mk4 Roadster:

Idle Mixture Screw (IMS): Just under 1 turn out, all four corners
Idle Feed Restrictors (IFR): .029 (stock size .033)
Idle Air Bleeds (IAB): 69/69 (stock size 71/71)
Primary Main Jet (PM): 68 (stock size 72)
Secondary Main Jet (SM): 72 (stock size 80)
Hi Speed Bleed (HSAB): 33/33 (unchanged)
Pri Nozzle: 31 (unchanged)
Sec Nozzle: 31 (unchanged)
Needle & Seat: 110 (unchanged)
Power Valve: 6.5 (unchanged, matched 12.5 – 13 inches vacuum at idle)

With these settings, the target AFR’s are all basically met with the exception of the idle. As Wayne suggested, mainly tune for best idle and transition. As you can see, the carb was overall very rich. It’s now running clean, very crisp, instant acceleration, starts easily, feels great. I’m going to remove the AFR gauge and drive it normally now. I’ll keep an eye on how it’s running and also the plugs. Can easily hook stuff back up and tweak some more if necessary.

Couple of things I learned in the process:

1. With previous carb tuning attempts, I never felt like I was really getting the feel of setting the idle mixture screws. I would twist at them with a vacuum gauge, think I had them where they should be, and then just drive the car. But never really felt like it was doing much. What I found in this exercise was as the other settings got where they needed to be, probably mainly the IFR’s and IAB’s (?) but I don’t know for sure, the IMS setting became quite sensitive and really made a big difference. Tells me my previous attempts probably weren’t very good.

2. There are definitely limits to this kind of tuning driving on the public roads, vs. a track or a dyno. I’m satisfied the idle, light cruise, and primaries are very close. Even then, checking the primaries by accelerating from 1,500 to 4,000 RPM in 3rd and the speedo is sweeping over 70-75 mph. I only did that one a few times. Fortunately, it came in quickly. My secondary jet is an educated guess. A full throttle run is just not going to happen on our local streets. Same with throttle stabs. I did the 1/3 stab to check the accelerator pump enrichment circuits. Instant response with no lag, and good AFR range. So no change required. A full throttle stab is just not going to happen on the street. Since these kind of things are not part of my normal street driving regimen, I’m OK with not measuring them for now. Some day on a dyno maybe.

3. Final comment, and this is just a warning. Couple days ago I scared myself to death. As most know, carb tuning often means tearing the bowls and metering blocks off the carb to change parts. No big deal. I got pretty good at it and doesn’t take long. Even got to where I wasn’t spilling gas all over everything each time I took the bowls off. But there are quite a few things to take apart and put back together. I tried to be really careful and make sure I didn’t forget anything. But after one test drive, where I actually went a little further than my normal loop, pulled back into the garage and noticed a pretty strong gas odor. Opened the hood, and there was wet fuel around the carb and even a little puddle down in the Air Gap intake. Sure enough, I had only finger tightend the –AN carb feed line from the fuel pump. Never put a wrench on it. Who knows how close I was to a fuel fire disaster? Still scares me to think about it. Made me slow down and be more careful. Others thinking about doing this kind of thing, be careful out there!

The Mk3 now has 330+ miles. Have I mentioned how much I like it? It's just awesome to drive. Needless to say, the DART 347 is crazy power and a real pleasure now that it's tuned better. I'm really taking it easy with that part. Let the fun really begin.