911SC lean hesitation issue

[porsche-1967]

Hey all, I have been searching the forums and have done just about all the digging I can do and been messing around with the car. I have an 80 RoW 911sc that I just did head studs on a replaced cams pistons with a set of 9.5:1 CPs and Web 20/21 cams. Replaced all of rubber vacuum hoses ect. Timing set correctly, idle mixture set correctly. It hot starts, cold starts great. Rebuilt 089 WUR.

Here is my concern, I have put a LM2 meter on it. And at just part throttle (cruising speed or or maintain 30-45mph) the car tends to have a hesitation as if it’s lean, which I have confirmed with my LM2. Once you tip to 1/4 or more throttle it goes away. It seems to be from 2-3k rpm range. My question is how do I richen up this small window with the CIS system. I understand that lower control pressure results in richer mixture. Would or could this be solved with adjusting the system pressure? If I were to add a shim to the FD to boost the system pressure that would that richen the whole range.

Anyway I am a skilled mechanic and do this for a living but mostly in 50s 60s Ferrari Mercedes Porsche so the CIS is a little new for me.

Any help would be greatly appreciated!

[911MANN]

I found a similar condition when I did my 3.2SS upsize and used Melissa M1 cams. Cruising was fine until I gave it just a touch of throttle. I had Tony (boyt911sc) richen up my WUR settings, and it is nearly gone now. Very recently I had Tony again richen them and convert my Warm Control Pressure stage to be adjustable, so that I could play with it a bit. This recent WUR modification is not yet installed but will be in the late Fall.

Yes, you should probably max out your Fuel Distributor's factory spec settings, at least as a trial. I actually went a tenth of a Bar higher, as mine is now a 3.2L.

My suspicion is that your cams are a bit thirsty (like mine), and transitioning off consistent cruise requires more fuel than previously. Wish I had more scientific evidence for you, but I think between the Control Pressure adjustment and WUR pressures I noted, you will reach your goals. I could share with you my WUR settings, but it won't be the same as your 3.0L requirements. Let us know how it goes.

[porsche-1967]

Originally Posted by 911MANN

I found a similar condition when I did my 3.2SS upsize and used Melissa M1 cams. Cruising was fine until I gave it just a touch of throttle. I had Tony (boyt911sc) richen up my WUR settings, and it is nearly gone now. Very recently I had Tony again richen them and convert my Warm Control Pressure stage to be adjustable, so that I could play with it a bit. This recent WUR modification is not yet installed but will be in the late Fall.

Yes, you should probably max out your Fuel Distributor's factory spec settings, at least as a trial. I actually went a tenth of a Bar higher, as mine is now a 3.2L.

My suspicion is that your cams are a bit thirsty (like mine), and transitioning off consistent cruise requires more fuel than previously. Wish I had more scientific evidence for you, but I think between the Control Pressure adjustment and WUR pressures I noted, you will reach your goals. I could share with you my WUR settings, but it won't be the same as your 3.0L requirements. Let us know how it goes.

As I understand with the 089WUR it has a vacuum enrichment on the side port, same as 069 and 045. I have disconnected this and plugged both sides of it to simulate it being at its richest setting. There is no change in symptoms of the part throttle hesitation or lean spot, but pulls right out of it and has no issues. I agree I might make
Mine adjustable and see if that helps or boost my system pressure as they should richen the whole fuel curve

[kltarga72]

Maybe just tweaking the idle air fuel mixture screw clockwise (rich) just a touch may solve your problem. screw is located just to the left of the fuel distributor and should have a rubber plug you will need to remove, 3 mm allen wrench required to turn the screw. Caution... be sure to note how much you turn the screw so if needed you can turn back to starting point. Adjust it carefully, just enough to know you moved it cw very little.
Than test to see if this helps with your issue. Test cold, hot, warm engine idle speed etc.
Check out this link
https://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html

[boyt911sc]

Originally Posted by porsche-1967

As I understand with the 045 WUR it has a vacuum enrichment on the side port, same as 069 and 089. I have disconnected this and plugged both sides of it to simulate it being at its richest setting. There is no change in symptoms of the part throttle hesitation or lean spot, but pulls right out of it and has no issues. I agree I might make
Mine adjustable and see if that helps or boost my system pressure as they should richen the whole fuel curve

First you need to do is confirm the absence of unmetered air going into the system. Using a AFM in a system without verifying the absence of air lea/vacuum leak sources is useless. Determine your fuel control and system pressures. Do you know how much enrichment your WUR is delivering?

Tony

[PeteKz]

^^What Tony said. Make sure you don't have air leaks. Most CIS light throttle running problems are related to air leaks into the intake side, especially off-idle or low throttle transitions.

If you enrich the CO screw a slight amount (1/8 turn), it may make the car run better, but it will probably end up too rich at higher throttle. At least that will let you know you have a lean spot. But fix the underlying issue, then reset the screw.

One other quick check: Make sure your distributor is set and advancing correctly. That can cause low RPM hesitation too. If it's advancing correctly, then back to CIS.