Carbd Procharger Part throttle lean problems

[Supervette]

The car cruises around town with light load fine, and runs great at wide open throttle but when you start to roll on the throttle and give the car more load in between light load, and WOT, the innovate wideband starts to show it going lean? This starts happening around 20 percent on up. Background information of the engine is that its a 383 chevy with a D1sc and CSU built carb w/ solid roller cam, single plane intake, running e85 with no intercooler, and a manual transmission. This carb has the 50cc acc pump kit with the big brown cams in the second hole. It came with the green ones in place but I had lean pops and the accelerator pump cam swap enabled me to fatten it up a little more but im still hitting lean AFR a little further up the transition to WOT. It also seems to read differently when im cruising around town. Sometimes at the same position of throttle at different times I will have leaner moments than others. Is my carb pump being drained?? It seems to reset itself kind of when I let off the throttle for a bit. Since my pump cams are maxed out Im wondering if this has to do with the shooters in the carb? I believe this is my transition circuit in the carb that is causing this. How do I fatten this up?

Any ideas to help me tune my way out of this????

Thanks in advance Mike

[Supervette]

This maybe a very dumb comment on my part but I was researching on the internet and came across this article in car craft.

Q: Why is it necessary for a blower carb to be boostreferenced?
Holley Performance responds: A carburetor does notsense the vacuum of the engine when there is a supercharger between thecarb and engine. This lack of vacuum causes the power valve to remainopen. Referencing the power valve to the lower intake allows the powervalve to operate properly, giving precise fuel metering as you wouldachieve without the supercharger. If the power valve is not referencedto the lower intake, it can be sucked shut from the high vacuum signalfrom the blower. This will cause the engine to run 6 to 8 jet sizes toolean, possibly causing engine damage.

With this stated my carb vacuum port is running directly to the vacuum advance port on the distributor and all others are blocked off. Could this be the problem... I also have the carb hat port running to the fuel pressure regulator, and the surge valve is being fed a signal from the intake 1 inch below the top of the intake where the carb is bolted on. Does my carb need to be referneced here as well????:idea:

[big_G]

Boost-referenced carbs are for Roots style superchargers, not centrifugal.. Before I went to fuel injection, I had a blow-thru carb. The only boost-referencing you need is on the fuel pump/regulator. Did CSU know you were going with E85? Your accelerator pump squirters may be too small. I had to up size mine, along with the needle/seats and the accelerator pump diaphram. You need a fuel pressure gauge mounted where you can watch it while you drive.

BTW....I was running a Holley 750 d/p'er and had none of the problems you are experiencing.

[Supervette]

I just got off the phone with Kevin, and we are going to switch my ignition system from a vacuum advance to a locked out style with a BTM this weekend. He said he would help me walk through it this weekend. I might have a problem with the boost stacking up in the hat. We also talked about upping the jets in the front for now and taking them down in the rear 3 sizes. IF this doesnt cure it we may need a boost activated front power valve. But for now I will have to get readings.

[Supervette]

Yes Kevin knew it was for E85 and the squirters have been questioned Big G thanks for the response any info is alwasy welcome.:)

[Supervette]

Well after talking with Kevin I went two jet sizes up in the front from 73s to 75s and kept the secondaries at 76, and instead of running 24degrees I ran 30 and it absolutley loved it. The car was ripping down the road. part load seems much much better however my 02 sensor crapped out on me so I cant see the readings anymore til I get a new sensor. I will test it more tommarrow but so far so good!!! I still have to put a BTM in as well thats next on the list.

[Supervette]

I bought the btm for the car, but now that I have it ive been wondering why I need it. If my timing is locked out at 29 degrees why do I want to retard timing per degree of boost? I run 12lbs on biggest pulley at 29 degrees of timing. Do I need the btm when I put a smaller pulley on?

[HILROD]

With the BTM you can run a bit more timing, but it will pull some back out under boost. My car loves the timing.

[Supervette]

So I can lockout the timing a little higher and have it pull out timing when I get into some bigger boost levels. What do you think about running a vacuum advance with this?

One other thing, If I'm cruising on the highway at 2200 rpms at a real light load condition, isn't this typically where you would want more timing than can be attained from being locked out? Is it correct in thinking that I can attain this from a higher lockout and bring the btm into play.

[OHCbird]

No on the vac advance; you're asking for even more tuning woes if you do that. Run a mechanical advance with the BTM or one of the next generation timing controls (they can be better tailored for driveability, whereas the BTM is a 1-for-1 deal).

IRT what you want at 2200 light load, your BTM won't be in play, as you won't be on the boost. If you go to a digital ignition setup, you will be able to tailor your cruise tables. You don't want excessive advance, as this will cause all kinds of cruise rpm overheating issues, as well as lowered part-throttle response.

[Supervette]

That is exactly what Im going to do. Im going to go digital with the lap top tuning capability. Thanks for the advice that is exactly what I was looking for. Thanks again