I can't get into the 13s at idle. I don't care. I adjust for best idle (either lean best idle or, infrequently, by vacuum) and the number is what it is. Long term I think it is important to get the best idle possible. There will be times when you idle for a LOT longer than you really want to and it is a nice feeling knowing that it can do that all day long. OTOH, you can mess w/ the idle screws as a test (as you have been) of whether the idle system is affecting something you are chasing, but, at some point, you need to come back and tune for best idle. So your adjusting the IMS, and seeing changes in the light throttle 1500-3000 test, has told you that you can improve things by changing idle feed restrictors or idle air bleeds. As you start changing those, you should be able to get the light throttle 1500-3000 test better and then come back and get your idle better all at the same time. I haven't messed w/ my carb for over a year but I really enjoy the process you are starting.
Edit to add: Have you seen this thread?
http://www.ffcars.com/forums/17-factory-five-roadsters/496561-331-timing-carb-setup-thread.html
His engine may be different than yours so just look at the changes and the results, and not so much the absolute numbers.

 

Finally something on this forum I can offer advice on! I've been running engine dyno testing for quite a few years. When the switch was made from non ethanol to 10% I found that the ideal A/F ratio for maximum HP richened up by 3 to 4 tenths. It used to be pretty consistently about 12.8:1. With ethanol it's more like 12.5 or even a little richer (naturally aspirated). Don't let the exhaust fumes fool you at idle. As the cam duration and overlap increase the unburned fuel increases at idle so you get more fumes. Leaning the idle to get rid of it quite often will create a stumble when it transitions off idle. Also don't just target a lean cruise A/F ratio. It may be worth putting a vacuum gauge on it and see what jet changes do. You may get it leaner, but require more throttle for the same load and end up burning more fuel.

Bob Lloyd

 

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

 

You asked for suggestions.... Change to a distributor with vacuum advance. You will feel the difference.
Frank

 

I agree with Frank. Didn't know what ignition you were running. One of the programmable ignition boxes might be a good choice. Getting the timing way up there at cruise will really help economy.

 

Come on guys. This is a brand new engine in a brand new build and I'm not going to be changing out parts. It has an MSD ignition box. Said so in the initial post. The new MSD distributor stays.

I asked for suggestions for carb tuning based on my first test runs and the main question was whether ethanol based fuels affected the target AFR's. That's it. Just trying to get the carb dialed in. It runs good now, I just want it to be as good as it can be. I've got some smaller size IFR's on the way, and that's the next step. I'll put them in tomorrow and keep testing. But thanks.

 

(13.3) – (13.8) at idle
13.5 – (14.5) in light cruise (16" to 7" vacuum)
12.5 – 13.0 on the primaries (7" to 4")
12.4 – 12.8 with the secondaries added (4" to 0")

Those are very close to the numbers I'm getting on a Holley 600 DP. The only change I made was drop the main jet one size and restrict the power valve fuel channel slightly by inserting a 0.015 wire into both holes. Without the wire, I was getting 12:1 below 6.5" when the valve opened.

I tuned on 100% gas and the number AFR difference burning the corn squeeze blend are basically unmeasurable on the wide band. Seat of the pants is the same after the engine warms up. Before it's fully warmed, it feels a little sluggish.

Greg

 

Tuning Work Completed For Now

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. :001_rolleyes: 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.

 

Re; possible leaks. I always let the car idle for a while which gives me time to wipe around under the bowls and all fittings w/ an old t-shirt or towel or whatever. Then, and I think this is key, wipe your finger tip under each those same points one at a time. Don't try to feel if it is wet because a little seep won't be felt as wet. Instead look at the skin on your finger tip. If it's damp, keep drying until you can come away w/ a dry finger. Also, coming from a guy who stripped a main body (good excuse for a new better carb but embarassing), if the bowl gaskets show a seep, be real careful about tightening the screws further. Instead, take it apart and put in new gaskets.

 

Eddie,

Unless you have a power valve in your secondary metering block to enrich the mixture you should have the secondary MJ 8 to 10 sizes larger than your primaries. You really don't want to run lean at WOT and end up with wrist pin inspection windows in your pistons!

 

CraigS: Good comments, and certainly agree to check for things you've mentioned. I've been intentionally very careful not to over-tighten the carb bowl screws. But the main point of my comments was leaving something not assembled. Just starting the connection for the fuel feed by hand, and never putting a wrench on it, could have been a major disaster. Trying to encourage people to be careful and thorough while doing all the carb dis-assembly and assembly. Still scares me to think what might have happened...

bingo2: Interesting. No, the QF SS-750 does not have a PV in the secondary circuit. But 8-10 jet size differences between primary and secondary is a lot. I could see the PV coming in and going out on the AFR curves for the primary jet testing. Happens pretty quickly. The engine was pig rich with the 72's in the primary. I can't imagine it being too lean with 72's in the secondary even if some enrichment is needed. If anything, they may need to be a size or two smaller. But I don't have a good way to test as I described. Quote from Wayne Presley, our resident tuner guru, on using larger jets in the secondary vs. the primary, "No, the front bores and rear bores are the same size so why would they flow a different amount of air?" I'm leaving it as is for now.

Thanks for your feedback guys.