Ok so i had my Camaro on a chassis dyno today and made 330 hp and 390 torque. my air fuel through out the run was 12:1 at 3000 rpm to 12.5:1 at 6300. My carb is a Holley 750 street HP double pumper, manual secondary. What suggestions does any body have to lean it out? Change jets on the primary? , secondary? or both? how big a swing should i take?

 

What jets are in it now and what fuel pump/pressure? Did the dyno only do a WOT run?

 

yes the dyno just did a WOT pull. I asked the guy to look at my idle A/F but he must have forgotten. The car will burn your eyes at idle. I know it would help to know whats in the carb but its been worked on at a shop before so im not sure what the jet sizes are. The guy tuned the carb on a engine dyno so im sure thats why the numbers are so consistent around 12:1 but the motor he was tuning on wasnt mine and the motor was running 110 leaded race gas, so im thinking thats why its a little rich now.

 

WAtching on an afr with carb is good but to a point give the car what it wants, as far as feel goes. If its burning your eyes at idle you may want to look closer at idle circuits, air bleeds and the setting for your transfer slot.

how does it run/act in general?


Whats the list # and the #s underneath it?
Its on the front (right) of your choke airhorn

 

If I were you, I'd start at the basics...
1) What is your timing curve? (Base, mechanical, vacuum)
2) How much vacuum is it generating at idle and light cruise?
3) Set the throttle blades with the carb off (to get the transition slots right).
4) Play with jets to get WOT correct. Drill out PVCR's to keep that fuel at WOT and start leaning the jets out as much as possible.

Your carb might not be off by much. Go slow and take notes. Change one thing at a time, but don't touch the carb until the timing curve is set pretty good. Your cam is small, so there may be a lot of siganl to a carb (vacuum) that was designed for a little signal (think race cam)...that would cause it to be rich.

 

Cam Update - Comp HR288 Duration 288/294, Lift .520/.540
Timing Mechanical. Locked down at 36*
12" at idle, ??? at light cruse.
PVCR??

 

That is similar to the cam in my 350. I generate about the same vacuum signal. My car really like a ton of vacuum advance at idle and cruise (around 50*). You have more cubes, so the locked out timing might work.

PVCR = Power Valve Channel Restriction. When the PV opens, this is the passeage that allows more fuel into the carb. Most people tune WOT with jets. Jets are more for the intermediate areas. For just racing, you can get away with just tuning the jets, but then part throttle will be WAY rich. I dialed in WOT at the dragstrip using the main and secondary jets. Then I calculated the open area of the jets versus the open area of the PVCR's. I drilled out the PVCR's to match the jet area (approximately) and then dropped my jets way down so I could cruise a lot leaner. I am still a little rich at idle. I think my idle air bleeds could use a little more work.

Follow the steps I posted above. First timing. Second, idle and throttle blades (with carb off). Then WOT using the jets to figure out where the PVCR's need to be (and what PV to put in). Then move on to jets so you can lean out the cruise. Your stinky idle is probably due to low timing advance (36 is probably not enough) and primary throttle blades opened way up and the secondary screw not opened at all.

 

http://www.carcraft.com/techarticles...eds/index.html

When MR changes with RPM, changing jets won't fix it.

 

Is it a steady climb from 12 to 12.5 or is it a lean "hook" at the top end of the RPM range or a rich "hole" right at the point you go wide open? Those are VERY different situations. If you've gone this far, looking at the entire AFR curve is the next step.

Little dips or peaks right after you go wide open are often accelerator pump related and not much of a concern from a jetting perspective. What you want to see is a steady A/F ratio after that point all the way up the RPM range. Weird things happening suddenly at the top of the RPM range are usually fuel system inadequacy, ignition system inadequacy or floating the valves.

 

There was a rich "hole" right where i went WOT, probably the accelerator pump and the power valve opening, but the a/f went right back to 12:1 and it was a steady clime from 12:1 @ 3000 to 12.5:1 at 6300. No sign of valve float or fuel pressure loss. just a steady slight incline. Since it was such a steady clime i figured it would be an easy adjustment.

 

any more ideas?

 

Was fuel pressure being monitored during the dyno runs?

 

No, it was just a mobile chassis dyno.

 

Just saw this post. I thought the mixing ratio only changing .5 was pretty steady.if I was tuning at the dyno shop I probably would have gone down 2 jet sizes front and back. But not being able to have immediate feed back I'm a little nervous. .

 

Well I guess you could take 1 jet out on all 4 corners or try bumping the timing another 2 degrees. More time to burn means leaner AFR. Does it run good? I felt happy with my car running 12.5:1 on motor and the plugs were beautiful. I would also think in cold weather, it would be closer to 13:1 AFR.

How do the plugs look? Black? Brown? Tan? Grey? Yellow? Speckled? Where is the timing mark on the ground strap?