RabidStreetRacer

iTrader: (1)

Hey all, I am in need of some tuning help. I just converted my truck from the 700R4 to a T-56. After doing this I have a bad off idle to about 2,000 rpm lean out. It idles at around 13.4:1 A/F ratio and when given any part throttle acceleration it will jump to around 17-20:1 A/F ratio. The carb is a Holly 4150. I added a 50cc accelerator pump and a different pump cam that engages the accelerator pump earlier. I even enlarged the accelerator pump nozzle holes. Under full throttle the A/F ratio goes to around 11.5-12:1. I will be reducing the jets after I figure out this off idle lean out. I have played with the idle air adjustment and that also hasn't helped.
Any ideas?

Pop N Wood

iTrader: (1)

Did it run alright before you swapped the trans?

I'm no expert but I will give advice like I am.

2000 RPM is about the range where the main jets take over. In between idle and there the idle feed restrictor do the job of the main jets. You need to go up a size or two on the idle feed restrictor just like you would on main jets. Not sure if you have adjustable IFR's on your carb. If not you can drill them out and make your own.

Make sure nothing is blocked in the carb, float levels are good, etc. Once you open up the IFR you can probably go back to the 30 cc pump

RabidStreetRacer

iTrader: (1)

Thanks for the advise. Yeah it ran fine with the auto but I had a 2,800 stall so once the engine was under load, the RPM's were above 2,000. What confuses me is I am already at about 12.5:1 AFR ratio at idle. So if I fatten it up even more then my idle will be even richer and it stinks bad enough with the 12.5:1, lol.

Pop N Wood

iTrader: (1)

What you are saying about the stall makes sense. Guess you don't do much part throttle driving.

The idle screws are for adjusting the mixture at idle only. Come even a little off idle and you transition to the idle feed restrictors. Completely different thing. Here is a picture of where they are in the metering block. You need to drill them out a couple of steps. Get this right and not only will you be able to go back to a 30 cc pump, but you will be able to adjust your idle screws to get closer to 15:1 at idle

What power valve do you have and what is your cruise vacuum?

topbrent

iTrader: (1)

Speedtigger has a few threads on here that offer some really solid carb tuning data backed up by real world driving and track results.

https://ls1tech.com/forums/carburete...p-problem.html

https://ls1tech.com/forums/carburete...cd-tuners.html

RabidStreetRacer

iTrader: (1)

Power valve is a 6.5 and i am about to order a vacuum gauge. Have not measured it yet. Thanks for the info, i will look into it trying it.

RabidStreetRacer

iTrader: (1)

Thanks for the info

3pedals

iTrader: (3)

stick shift combo is VERY sensitive to the accelerator pump and powervalve.
check what vacuum the engine pulls at a steady state cruise - then set the powervalve accordingly.
what size shooters?, what pump cam and setting?

Timing can really help low load and transition performance once you cure the lean O2 readings.
Auto trans guys dont have to get the carb anywhere near as close as a stick shift combo needs it for good street performance

RabidStreetRacer

iTrader: (1)

Yeah it seems to be vary sensitive now with the 6-speed. I believe my shooter was around a 28, i drilled it out very slightly and it helped a bit. I used the pump cam from the 50cc pump kit that had the biggest rise off immediate throttle to increase the size off the shot off idle.

Pop N Wood

iTrader: (1)

The MSD box will show your manifold pressure on the lap top display. Pretty simple matter to google a table converting the pressure to inches of vac.

I run a 6.5 PV. Seems to be near perfect. Car cruises at AFR of 14.5. Get into the throttle a bit and it drops down to 13 or so. I have a stock LS2 cam and pull 23 inches of vac at idle, manual trans.

3pedals

iTrader: (3)

good move on the pump cam.
28 shooter is too small no matter what combo you are running!, lol. I had 40's in my 6.0 with less of a cam than your "thumper"cam. My first instinct is that you should go to a 35 shooter in the primary to start off.
I think the 6.5 pv is probably close enough, but Id like to know what your vacuum reading at cruise speed and rpm is?
which thumper cam do you have?

3pedals

iTrader: (3)

I think the thumper cam he is running is the biggest challenge to tune for.

RabidStreetRacer

iTrader: (1)

I am going to try to get some vacuum numbers today, been working a lot which makes it a pain to get out and play with the truck. I will try to post some vacuum numbers and maybe try to get a video of the wideband so you can see what it is doing.

I have the smallest Thumper cam.

Basic Operating RPM Range:2,000-6,400

Intake Duration at 050 inch Lift:219

Exhaust Duration at 050 inch Lift:233

Duration at 050 inch Lift:219 int./233 exh.

Advertised Intake Duration:275

Advertised Exhaust Duration:295

Advertised Duration:275 int./295 exh.

Intake Valve Lift with Factory Rocker Arm Ratio:0.553 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio:0.536 in.

Valve Lift with Factory Rocker Arm Ratio:0.553 int./0.536 exh.

Lobe Separation (degrees):109