dellman83Z

iTrader: (9)

I have a momentary lean condition when going from part throttle (cruising) to wot. I finally got a wideband so I tune it accurately and get rid of it running crazy rich. What I got is a 383 with a roller cam, 242/248 .56/.56 110 lsa cam and 10.7/1 compression, dart 200cc heads, the carb is a 4150hp 750cfm carb, 74/84 jets, silver vacuum spring, orange cam, stock squirters and pump, and a 2.5 pv. vacuum at idle is around 6". Driving, my a/f ratio is 14 cruising and 12.5 at wot. The lean condition lasts only like a second and my a/f ratio goes up to around 16 and starts to detonate some, then drops very fast to 12.5 and stays there. Would a black cam fix the condition or should I try moving the position of the orange cam first. Tuning the cam is very new to me, so some help is greatly appreciated. Thanks.

pancherj

Your PV seems really low...and so does your idle vacuum of only 6". I have a bigger cam on a tighter LSA in a 350 and it pulls 12" at a 1000RPM idle. What does your timing curve look like?

It sounds like the vacuum secondaries are opening, but the PV is so low that it isn't (at least not right away). When the PV opens all the way and lets the fuel in, your AFR is going back to normal. A bigger pump shot may help, but I don't think it is the cure. A 10 jet split seems pretty big...however, I am not too familiar with the HP carbs.

dellman83Z

iTrader: (9)

I know the vacuum seems low, buts the best I have ever gotten it. I've readjusted the valve I dont remember how many times, but its always the same. My timing is at 17 initial, 36 total all in at around 2500, and 48 final with vacuum advance. I will try out a 4.5 pv that I have lying around and see what that does then. Thanks for the help.

pancherj

Why not make a drastic change...like a 6.5PV. Just to see if it makes the problem better? Of course, don't pay attention to the affect on idle AFR.

Blue pump cam in the #2 hole with a 37 or 41 shooter would be my next step after that. Lastly, I would put a stiffer secondary spring in to slow down the opening rate.

dellman83Z

iTrader: (9)

I am going to try these tomorrow to figure it out. I just needed a direction to go. Thanks again.

sofakingdom

iTrader: (1)

Looks to me like your ignition timing is about 20° retarded.

For a minute, ignore the "numbers". Advance it. No particular amount, just, some. See what happens. Ignore pinging for the moment; see if the flat spot is any better. If it is, you're on to something.

That would be the XR280R cam, no? I have one of those laying around I'll probably create a motor to contain some one of these days.

Most likely, your distributor has WAY too much centrifugal advance, which comes in WAY too late. Try setting it up like this: "total" timing 36°, vac adv 15° on top of that, "static" timing 20° (centrif adv = 16°), adv starting at 1200 RPM and "all in" by 2800, vac adv hooked to hard manifold vacuum instead of "ported". Most dists that you just buy, come with about 24° of vac adv; just altogether too much for a hot-running motor.

Once you get that curve, see what your vacuum is. Even with the little-inch motor you've got, should be around 12-13° at 1000 RPM. That should allow your PV to be around 7.5 or 8.5.

Right now, your vac is low because your timing is always so lazy; that forces your PV to be down in the mud, such that there's this LOOOOONNNNNNGGGGG flat spot between the point when you start to tip-in the throttles and the point hwen you start to get enriched mixture.

Having a WBO2 is only half the battle: now you have to understand (a) what it's telling you about how your engine is running, and (b) what to do about it.

Once you get the carb set up properly, yo'll probably find you almost don't need a pump AT ALL. All that a pump is, is a crutch to make up for the time lag between the movement of air into the motor, and the movement of gasoline which is heavier. If it's doing any more than that, it's only because all the other systems of the carb aren't delivering enough fuel; i.e., not doing their job. That's the responsibility of the person doing the tuning, to cure that. Thinking you need more pump is an almost 100% sure-fire ironclad guarantee that the ignition and/or carb are poorly tuned.

dellman83Z

iTrader: (9)

I tried the bigger 4.5 pv, which seemed to help, but the problem is still there. After I get my tire repaired today to fix the hole in it from a rock in the burn out box at the track that decided to find its way home inside the tire, I am going to try advancing the timing a few degrees and see what happens. The dizzy is just a stock hei with lighter springs and a vacuum advance that only allows 12 degrees timing and a msd 6al-2 box. I found a dyno graph for the cam from comp, its a xr294hr and on that sheet, it says it produced only 8" vacuum at 1000 rpm, so maybe I am not that far off. If it helps, my best time a the track was 12.06 @ 113.2 with a crappy 1.8 60' but a great r/t of .012. I run nitto 555r 245/50/16 in the rear and no matter how hot I got them, thats the best 60' time I could get. It was about 60 outside and I was running 110 gas this time. The other thing I have a concern with, what kind of valve lash should I be using, right now its at 1/8 turn past zero lash. I 've read people use anywhere between 1/8 to 1/2, even full turn on roller lifters which seems like way too much.

pancherj

If the extra timing doesn't help, do a cranking pressure test. If it is reading real low (less than 180), you may need to advance the camshaft some to get the best performance.

The amount of preload depends on the type of lifter. Stock type usually need 1/2 to 3/4 turn. Limited travel plunger lifters (Comp R and other ones geared for racing) need only 1/8 turn or so.

dellman83Z

iTrader: (9)

The lifters are crane retro rollers, so I would think 1/8 turn is correct then. This is going to sound like a rookie, but what would be the difference between a cranking pressure test and compression test. I know the last time I did a compression test, it was around 215 psi on average. What do you think about the times I ran? Do you feel the motor overall is running overall for what you expect for the build or am I truly missing something thats holding it back, besides the fact I need to get a driver mod or better tires to improve my 60' times.

pancherj

Cranking pressure (my term) = compression test. 215psi sounds good (that is right about where mine reads). Your 1/4 times look pretty good. If you can get the 60' a little better, 11.70's should be no problem. I would say the car is performing well at the strip. Seems like you just need to clean up a few minor driveability issues.

dellman83Z

iTrader: (9)

Thanks again for the help. Do you think my lashsetting is correct or should I try a half turn to see what it does. Figured I would ask before doing it because it does take time to do it right.

pancherj

I would leave it as is at 1/8 turn. What does the rest of your time slip look like (from a good run)? What was the DA at time of run? If you are at higher altitude or crappy weather, that can make a huge difference in 1/4 mile time.

dellman83Z

iTrader: (9)

The DA is no where on the slips, but I do the altitude is around 900ft and they were nice days the last two times going, low humidity and in the mid 70's.

whatever84

I know some people will disagree but I can't believe your running a VC carb on a 12.0 car. I have less motor then you and I have a 750dp and no flat spots anywhere in the rev range. Just my . Good luck.

Mark.

dellman83Z

iTrader: (9)

I wanted a vc carb because I drive the car quite a bit, not just at the track, I agree though, if it was more of a track car, I would actually prefer a mech carb. At the time I bought the carb, I got it from a friend who had only four dyno pulls on it and it turned out it was not enough for his motor and sold it to me for 150 bucks. It was too good of a deal to pass up. Thinking of that brings up another dilemma I have, which is to stick with the th350 or just bite the bullet and rebuild my grenaded 700r4 because I really need to have 3.73 gears in it to get more out of it, but because I drive it a lot, and if I put 3.73 in it with the th350, that poor thing would just scream on the highway, more so than it does right now with 3.23 in it.

pancherj

I would go with the 700R4...and a double pumper! You can get decent gas mileage with a double pumper (and overdrive). The secondaries don't activate right away because of the progressive linkage. I like my Might Demon 750 with annular boosters. It took some work, but I have idle/cruise leaned out okay (still needs some work) and good AFR under full throttle.

To find out your DA, go to the www.dragtimes.com website (DA calculator) and pick your track, day and time. It will give you the DA.

dellman83Z

iTrader: (9)

Been busy last few days to try anything new, but finally got around to doing something. I bumped the timing ahead two degrees to 38 total and that helped out the idle a bit, but nothing for vacuum, so I left it there for now. For testing purposes, I am borrowing a friends 750 dp mech carb that he had laying around not using. I jetted it the same as what I have and the same pv just to see what would happen. The a/f ratio is very close. What I noticed is the primary side still goes lean, but no where near as bad before, like maybe a fraction of a second. When I punch it and the secondaries open, then I get the lean condition again like before, however, it does not last as long as it did with the old carb, like almost half a second. I would say the old carb was not what I needed. I did check out the DA when I raced, and after the conversion, my times remained the same overall.