HottestZ28

I have this carb on my camaro...
1987 SC
Motor is a 350 small valve, cast heads, lightly hand ported
slightly larger then stock cam
performer intake
Hooker super comp longs
TH350 1000rpm stall
3.42 rear gears
27.5 tall tires
carb 4160 Holley quick change sec with lightest spring
looks like 31 discharge knozzle and 70 primary jet and 21 plate = 75 jet sec 65 power valve

ok now for the question....
car starts hard turns over fast and you need to pump the pedal a couple times and hold it open to get her fired up. When running idles ok no smoke after initial start up and revs great. The problem is when you stab it in gear just coughs and wants to stall. I had a friend power brake it and watched the squirt and seems to me like way to much gas and won't turn over the tires. I also have a Holley blue pump with 3 port Mallory 4309 bypass regulator and when its idling the fuel pressure gauge fluters from 5.5 to 6.5 lbs and I can visually watch the fuel filter pulse with the gauge. I have a carb sender and ran 3/8 from the tank to the regualtor and back to the tank. What are the possibilities?

pancherj

What's your base timing set up at? Sounds like it needs to be advanced. At idle, you should be a round 14* (with vacuum advance disconnected) and 36* total by 2800RPM or so. Get the timing squared away and then move onto the carb.

HottestZ28

I have been changing timing based on they way the car runs. I started with 18 at idle and 38-39 full mechanical advance and ran great above 2000rpm but still had a hard time smacking the gas and getting her rolling. So I backed it down to 14 and ran better lower but still stumbles just off idle when I get rolling and then hammer it she reacts good. With the timing set at 14 it also breaks up over 4800 rpm but at 18 runs strong to 6200. the problem is I know its a small cam but paperwork that came with the cam says 220/228 484 lift lunati cam and the cam grind is no longer available to acually look up much more then that. I have a hard time believing its got that much cam has great idle and vacuum and no lope at idle

I got the crate engine ignition kit from summit came with HEI dist and wires don't know if this cheap distributor is junk or is it something else?

been chasing down all kinds of things, engine came with a single plane and changed it to a performer
bought a 750 carb for it from specs of motor(think its to big)
though stock fuel pump was over riding the regulator so pulled tank modified sender ,pulled electric pump out, ran 3/8 line from tank to reg and back installed holley blue pump (thats another story...having pulsing problems with regulator but doesnt go over 6 lbs at gauge

HottestZ28

update...
played with the car today and pretty much started over
checked plugs and pass side plugs tan but driver side plugs black with carbon deposits cleaned regapped and installed
reset timing to 14 and reved it with a backfire so bumped it to 18 and reved it with a backfire. I then decided to elimintae vacuum advance and set timing to 26 degrees and no backfire and reved nice had a nice solid idle. I then re adjusted idle circuits and were within a 1/8 of a turn from original setting. Then felt pretty good with the setup at that point went for a ride and layed into it and smoked pretty good hard on the throttle pulled "ok" till sec hit second a lifted felt alot better but by 4300rpm started to fall down.
I really want to borrow a 600 and see if that wouldn't wake her up down low because this 750 def isn't helping the top end?

pancherj

If you set the base timing at 26 without vacuum advance...it was probably near 46* at full advance. That is WAY too much advance.

1) Unhook the vacuum advance and plug the port on the carb.
2) Set the timing to 18* with the idle AS LOW AS POSSIBLE (maybe 800RPM)
3) Rev the engine and see where the timing goes to. If no more than 38, you are good.
4) Hook up the vacuum advance. What is the timing advance now?
5) Get a vacuum gage on the carb and set the idle mixture screws for best vacuum signal. Keep turning the idle down so it stays around 900RPM.
6) Now, go for a ride and tell us how it acts. With the timing squared away (first) it will be time to start adjusting the carb. I like to tune WOT first because it is the easiest. Light cruise is always the trickiest (with bigger cams).

HottestZ28

first thanks for some help with this problem

Originally set timing with vac adv to 14* and showed 36 at 2600rpm
then had the backfire through the carb when stabbing the throttle
so I put it to 18* and was at 37-38 at 2500rpm but still had the backfire through the carb. thats when I decided to run macanical advance only and seemed to allow it to rev hard without backfire but I know thats to much advance but was told a long time ago old school sbc like advance and to just play with it till your happy so thats as far as I got but was still unhappy

I did hook up a vacuum gauge to the carb and the best I could get was 14-15. hg which to me seemed low concidering this cam acts like stock and can idle at 450rpm with no chop

pancherj

Lean mixtures burn real slow and can tolerate a ton of advance. Maybe your secondaries are opening real fast, and the jets are way too small. Seems unlikely, but you never know. If the timing is good, I would check for any vacuum leaks. If there are none, time to start working on the carb. Your front jets seem a little small, but they may be okay. Is the pump arm set correctly on the primaries? The big downside to a vacuum secondary on a performance setup is that you only get one pump shot and one squirter. If it isn't correct, you will get a big lean stumble. I would also up the PV to an 8.5 or 9.5. Make sure the secondaries don't have the lightest spring. Start out conservative and work your way up from there. Change one thing at a time and take notes as you go.

HottestZ28

well you are right I have the lightest spring in the secondaries thought because it was a light auto that it needed to open sooner . I actually put more lash in the pump shot and moved the cam back for a later hit. when I watched the pump shot it seemed like alot and completely covered the throttle blades. It just seemed like it was running rich and not that it was lean. Guess this is why I am asking for help LOL.
I only really get to screw with it on the weekends and I will put the heavy spring back in, adjust the pump shot and move the cam to factory spot

so you think it needs more jet what jets should I get don't really wanna buy a whole kit and you think the 31 nozzle isn't to big?

pancherj

I wouldn't worry about the jets right now. Reset the pump arm so it is just touching. Also, make sure that any movement of the throttle lever causes the nozzle to squirt some fuel. If it doesn't, the spring on the pump lever is probably worn and it is compressing instead of pushing down on the accelerator lever. Also, I am fairly certain that the size 31 squirter is not too big. It may be fine...but you may need to go up in size later.

I think returning everything in the carb to near stock is a good idea. It is always easier to start from a conservative setup and then slowly tune for more power. Tuning WOT is the easiest, so I like to start there. Warm up the car good, and then go to a secluded stretch of road. Go from a 20MPH roll or so and punch it, if the car stumbles and then takes off, up the squirter size. Try again. If okay, try the medium spring in the secondaries. Punch it again. If it stumbles, up the squirter again. Do this until you can get the lightest spring in the secondaries that still allows good acceleration. To get the jets right, you really need to go to the track because the gains may be so small you wont notice on the street.

Jets aren't the real solution to enriching WOT. You need to drill out the PVCR's. Playing with the jets will tell you how much to drill them out. After you get the PVCR's sorted, you want to lower the jets as much as possible so cruise is lean, but WOT doesn't suffer. It is tricky and takes time. An O2 sensor helps. I have a narrow band from my original TPI setup that I monitor, and that is good enough for me. I guess a wide band with datalogging would be nice, but it could also induce data overload!!

HottestZ28

stupid question but what is PVCR's?

pancherj

PVCR is the power valve channel restriction. When your PV opens, and lets in extra fuel, these are the ports that extra fuel goes through. This is the real way to tuneWOT. You can do it with just jets, but then you will be way too rich at cruise and partial throttle. If you take a PV out, you will see the very small PVCR ports in the metering block. When I was fine tuning my Demon, I had to jump 6 jet sizes to get WOT correct (use jets to show you where you need to go before you do any drilling). 6 jets is about .006" bigger on the PVCR's. I drilled them out and then dropped the jets back down to stock. WOT was spot on after that.

cuisinartvette

70 should be plenty of primary jet for what you got

stevek

I have to jump in here because I have a very similar problem and a very similar set-up. I have a stumble @ take-off and then wicked power(for me) to about 4500 rpm with a 4:10 rear gear set-up another stumble and backfire through the exhaust. Any thoughts would be helpful.

pancherj

stevek,

I think you have two separate issues. The stumble on launch is probably a lean stumble. You cover that up with the pump shot. It could be timing related too, so get the timing advance sorted out before you try any carb tuning..

The miss at 4500RPM sounds like it is ignition related. Are you running an HEI? Could be the module if you are.

I'll throw this out there just because I am thinking about it...
1) Always start by getting the timing where it needs to be...initial, total mechanical and vacuum
2) Move onto the carb. Set idle and use a vacuum gage to set the idle screws. Chase WOT next and then cruise last.

Tools you need...timing light, vacuum gage and fuel pressure gage.

BiGgE

iTrader: (2)

I agree with everything you are saying, except this^

My Demon "stock" had #75's up front with a 0.059" PVCR, and #83's out back! That is 3 jet sizes lean out back, RIGHT OUT OF THE BOX!!! Holley is just as bad...

In my opinion it is always easier to get the transients right after you get your WOT jetting right (which like you said, comes after getting your timing right).

onebad82z

iTrader: (7)

Please explain this for the guys. I know what you are saying and totally agree. Not sure why they jet them this way... but.

BiGgE

iTrader: (2)

At WOT, the PV is open, so the main wells are being fed by the jet and the PVCR.

A #75 jet is 0.082" in diameter. That is an area of 0.00528 in^2.
One PVCR is 0.059" in diameter. That is an area of 0.00273 in^2.
All together up front you have an area of 0.01602 in^2 of area for the fuel to flow through.

A #83 jet is 0.094" in diameter. That is an area of 0.00694 in^2.
All together out back you have an area of 0.01388 in^2 for the fuel to flow through.

0.01388 < 0.01602 ...BOO!

A #86 jet is 0.101" in diameter. That is an area of 0.00801 in^2.
All together out back, that would give you 0.01602 in^2 of area for the fuel to flow through.

0.01602 = 0.01602 ...YAY!

Don't ask my why they ship them like this, I don't know...

onebad82z

iTrader: (7)

Good information! I just spent the last few weeks digging into how to correctly size PVCR's. Found this very helpful in understanding. Much the same as you provide, although it accounts for a 12% flow loss through the PVCR channels and routing.

http://e85forum.com/about989.html&highlight=pvcr

Talked with several tuners who use this formula to calculate and end up dead on.

BiGgE

iTrader: (2)

Interesting... I have never accounted for any losses accross the power valve channel, and the mass-flow rates between the bowls come out dead nuts on the dyno.

That channel must be less than 1/4" long-I wonder if there are higher losses accross the channel running E-85 as your actual flow rates (and thus velocity in the channel/well) are so much higher due to the lower BTU of the fuel?

onebad82z

iTrader: (7)

I myself wonder the same (somewhat) as far as E85 vs standard gasoline and the flow loss. I have no real data to back it up either way. For me, seems you have some real world experience with the formula with no flow loss as well as I have talked with other "gas" carb tuners who used the non flow loss formula and it was spot on. Seems the E85 tuners I have spoke with factor in the flow loss to bring the PVCR's in line. May just be coincidence or…

Myself on a "gas" tune as I use in my car I will be using the non flow loss formula when I get some chassis dyno time next spring to bring WOT A/F in line. I just thought it would be valuable information to share with others new to carb tuning to this extent. Thanks for sharing.

As it sits my Demon is jetted 72/84.

stevek

Timing is set and I believe correct.I have checked for vacuum leaks and none are apparent(I do have a leak at the header buddy to exhaust pipe)I am running true dual exhaust(Dynomax).I have 8# of fuel pressure@idle/ 750 rpm.#.31 squirters and the lightest spring for the secondaries.The cam is a Comp Cam extreme and when I bought it it was the biggest you could run w/o a stall( CC said) and then I bought a stall conv.Yes it is HEI