Dyno'd my 67S today.. What the heck!!
 

So I've been annoyed by this flat spot in the 3-5K range for quite some time on my stock 67S. Been fiddling with this that and the other to no avail. Decided to Dyno it to get a good A/F ratio and see I could figure anything out.

Wow, this thing's a mess! Look how lean it goes at 3.7K, off the chart. Power falls a bit from 3k-4.5K (there's my big flat dead spot!). Power's down quite a bit too over a stock 160HP 2.0S. This motor has 5K on a fresh rebuild, good compression, good leakdown numbers. Just installed a NOS dizzy too, valves adjusted good, etc.. I'm thinking it's all about going totally lean, gotta be a fuel problem.

My IDS Webers are all on the stock jets and such for a 2.0S. 3.5PSI of fuel on my PMO gauge. Floats set in the middle of the adjustment. Mains are 125's, and I'd suspect that by 4K it's transitioned fully to the mains, maybe not?? If so, do I just need to check those and maybe go to 135's or something?

Suggestions, thoughts?

Bill

http://forums.pelicanparts.com/uploa...1275593492.jpg

I think that something is wrong with your fuel delivery setup. Even if the jets are slightly off there is no reason to flatline above AFR20:1.

Is your fuel pump dying? Have you cleaned the delivery screens? Have you checked fuel volume delivery? Have you removed the carbs and made sure all jets and passages are clean?

I don't think you have the wrong jets. I think your current system is not working properly.

Are the carbs fresh? As vacuum increases, if our shafts are worn, you can drawn in unwanted air, but your numbers seem to high for this. I'd agree with Jamie: check your fuel system from tank to fuel rails for clogs or failure.

What kind of fuel pump and where is it located? If you have a rotary style pump (Holley) it has to be below the fuel supply and as close to it as possible. They won't pull fuel, only push. If the pump is in the rear left wheel well it won't work. Had the same problem myself.

Lindy

http://forums.pelicanparts.com/support/smileys/wat6.gif Good luck and hope you get it right.

911st, I think the AFR of 20 means it's super lean, correct?

I'm with Jamie thinking fuel delivery problem. I wish I would've looked at my PMO gauge while he was doing the pulls to see if fuel pressure dropped, didn't think of it at the time.

My fuel pump is a factory Bendix unit. Last time I tested it I'd say it flowed 1-2 GPM's, filled my 3 gallon can pretty quick anyway, that was several years ago though.

The Webers are very fresh IDS's with the tightest shafts I've felt, virtually no play. My old one's had a bunch of slop and replacing them with these really cured my idle issues.

Advance is set to 29 at 6000RPM.

Long time since I fooled with Webers. Is there an air correction jet involved? Looks like the transition from the idle jets to the main jets is very wrong. I had some dirty idle jets that would have made that chart. ON the Webers I had, the carbs used the idle jets well into the RPM range.

Factory Bendix pump, really? M12x1.5 inlets with banjo fittings, wire between the fittings with a faded German sticker on the side? That would make it the original pump from 1967, right? Hard to believe that thing could still be putting out enough fuel, when's the last time you cleaned the filter screen in the bottom or the magnet? The Bendix pumps (and the modern Purolator/ Facet) have a filter in the base and a magnet, these can easily get clogged up with gunk. Don't forget the filter sock in the fuel tank, this can also fill up.

OK, what am I missing? He has full on fuel flow to the extent of an overly rich condition at high RPM. How do you figure this is a starving fuel situation?

Or are you saying the fuel is not properly regulated and/or the floats are not right?

with my 2.0s and stock carbs, i ran into the same hole in the power band. We ever so slightly increased the size of the idle jets and it made a big difference. I can't remember the exact size we took them out to, but it was small, i did it by hand by just twisting the bit into the jet. I'll look in the am at work and see if I can find my notes.. good luck..

Where do you guys see rich? The AFR is >20. That is severely lean!

There is one condition when you are so rich the fuel quenches the spark and does not burn. In this case there is leftover air in the unburned mixture that will cause a misleading lean read on the wideband O2. In this case you would know if it was a mis-read lean as there would be raw gas pouring out of the tailpipe.

Given the rpm of lean transition corresponds with the overlap from idle to main jet circuit I think there is a fuel delivery problem. It also looks like the idle circuit is misadjusted to compensate for the transition. This explains the 10-11 AFR and general trend to overly rich on the idle circuit.

Yes, my understanding is the higher the AFR reading the leaner it is, so I'm very lean here.

I'm going to tear into the fuel delivery system this week, clean out the Bendix screen and then bypass the Holley fuel pressure regulator (Not sure why that's in there anyway).

If I test flow the Bendix pump into a gas can what should I expect flow wise?

Stock specs from the factory for the '67 911S were 160 DIN HP at the flywheel, which converts to about 158 SAE net HP. Deducting 15% for drivetrain losses, and you are left with about 134 RWHP. Some people might use 17-20% for these losses. The measured 123 RWHP here is not all that far off. I think I got 128 RWHP on my tired '67S engine w/100K miles on it on a Dynojet 248C.

http://forums.pelicanparts.com/uploa...1275663417.jpg

The measured A/F ratio is ridiculous, though--it is pegged between 4-6.5K RPM it appears. 20 is probably as high as it can display. The actual measurement may have been higher! This makes no sense at all, as that is so lean, any reasonable dyno operator would have aborted the run to prevent damage to the engine. You are into piston-melting, ring-breaking, detonation ranges with that lean a mixture. Something must have been wrong with the equipment or the technique used for obtaining those measurements, IMHO. Perhaps an exhaust leak is introducing high 02 levels into the exhaust that are registering incorrect A/F levels, or the wideband sensor used was whacked? I don't know, but something is definitely wrong with those numbers.

I would look for intake and exhaust leaks for sure, but other than that, I found with my 2.0S that stock idle jets did not work well with modern fuel formulations. We reamed them out a bit and the engine performed better. No matter what we did, though, we could not get rid of the 3500 RPM flat spot. I think it is a function of the S cams and reversion pulses in the intake. The solution is to keep it above 4,000RPM when pushing the car hard, which is easily done with the 901/02 gearing.

TT

Flow rate for the factory electric pump is 110 liters/hr. You can convert the units as appropriate for your testing method, but this is slightly less than 2 liters/minute, obviously.

TT

Agreed Tom, That AF reading has me really stumped. When the Dyno operator first saw this he immediately shut it down. I told him I've ran the engine that way for 10K miles with no problems and couldn't imagine it could actually be that lean.. who knows, but I gave him to go ahead to do the pull.

What's confusing besides the AFR is the power curve. You'll notice I have more HP and torque at 3K than I do at 4.5K, power's gradually falling. In addition, peak HP is at 6K seems a little low in the rpm range.

I autox this car, and the majority of that driving takes place in second gear between 3-5K rpm's, so I'm effectively running a 65HP motor during my runs.. not much fun. :(

Your AFR after 4k is not really >20:1. Your car wouldn't even run at that AFR. Dynojets do this sometimes when there's something about the AFR from the exhaust that's to one extreme or another, occasionally causing the dyno computer to err like this. Likely your mixture has gone so rich by 3500 that it's fouling the plugs and causing lots of misfiring, which effectively makes your cylinders nothing more than air pumps pumping lots of O2 in your exhaust which will fool the wideband sensor of the dyno to read an excess lean condition.

Yea, I have also had a problem with a flat spot around 3500 rpm. I had a modified Dansk sport muffler where I added a second outlet. That and richer main jets got rid of the flat spot!
I just replaced the Dansk with a Bursch and my flat spot is back worse than ever!

jetting?
 

We have seen this condition before. Typically for this set-up we would recommend the following:
135 mains
110 corrections
F24 Emulsion tubes
32-34 venturis

Aside from the 125 mains, what are you currently running?

Early 911S per the FWM
Venturi 32
Pre-atomizer 4.5
Main 125
Air corrector 185
E Tube F3
Idle 55
Idle air bleed 110
Float needle valve 1.75
Float chamber vent 4.5

That changed from engine 4080773, (March 1968)
main jet 130
air corrector 180
enrichment jet 50
mixure outlet 5mm
idle mixture outlet 1.0mm
bypass orifices .8mm, 1.1mm, 1.35mm

Hmm..

So Steve, you think I've gone very rich? Wouldn't I be on the main circuit by that RPM, can't imagine how my 125 jets would allow it to be super rich like that, who know's though, I'm totally confused now.:confused:

Staylo, if a condition like Steve has mentioned above is true I'd be afraid to go 135 mains and make it even richer. Do you suspect it's actually as lean as the chart says it is?

My Current Weber setup is, best I can remember:

125 mains
32 venturies
55idles
180 air correctors
F3 emulsion tubes

Need to check Float levels again, been several years.

Assuming you have no other fuel restriction issues, I'd try the 110 air correctors.

I have seen this lean condition at higher rpm range before on the dyno. It turned out that one of the booster venturis was installed wrong. They will press down into the carb. main body either way you can install them, but the internal tube or horizontal wing inside the booster venturi has to face the direction of the air corrector jet. See photos below. You can look down each throat with a flashlight and mirror and you should get the idea.

http://forums.pelicanparts.com/uploa...1275695746.jpg

http://forums.pelicanparts.com/uploa...1275695823.jpg

I think it is best to separate your question into two parts:
1. Why am I off the scale lean above 3700rpm?
2. What can I do about the mid range flat spot?

1. Where was the AFR measured? If it was measured at the tail pipe with a probe, it is possible the probe came out of the pipe enough to be sucking some fresh air. I know from experience that it is tough to get a probe far enough into a stock 911 muffler to avoid that happening. Like someone else mentioned above, your car would barely run at above 18:1 air-fuel ratio.
My advice would be to completely go through the carbs, make sure everything is correct and clean, check your fuel pump for sufficient flow, etc. Have O2 sensor bungs welded into your headers/heat exchangers just after the collector and return to the dyno. The stock jetting should not be that far off.

2. I struggled with a mid range flat spot very much like you have for years with my '68 911L. I tried all kinds of jet/air corrector/exhaust changes to little avail. As mentioned above, running larger idle jets than would seem necessary did help. Maybe this is due to the differences in fuel today vs. the '60s? Eventually I looked to the ignition system and that is where where I found the best results. I suggest getting the distributor advance curve measured to make sure its correct. Make sure the ignition system is healthy overall.
What exhaust are you running?