TheGreat4.8

Alright i need some more opinions before i start taking things apart, in my 74 Nova with the 383. (750 DP)

i am having power issues at anything above what mixture screws control, from a dead stop it pulls hard off the line then all of sudden that torque feeling just goes away and it loses power the rest of the way up.

I know i have a mild cam and gears but this feels less powerful then stock really, i don't expect to run 10's or anything it should just be a quick street car.

I'm thinking it wants more fuel because when i richen up the idle screws the bottom end pulls but that's it only the bottom, maybe i higher power valve?

ill put all of the relevant specs below, let me know if I'm missing anything thanks

Lunati 00016 with locked out timing at 34*
holley 750dp 71primarys 82secondarys with 6.5 PV
9.5-1 CR
GM performance vortech heads with roller rocker
edelbrock rpm intake
th400 with 2500 stall
stock 308 gears (for now)

1 FMF

iTrader: (1)

because it's losing power at high rpm, can it be an airflow issue into the carb?
how is your air filter and intake setup?

if it has a vacuum secondary, you can buy a spring kit with different colored springs to adjust when the secondaries open up. You will also want to make sure that the secondaries are opening up fully, which is easy to do in say a boat rather than a car unless you're on a dyno. what you may be able to do is rig up a vacuum gauge to the intake and under full throttle you should see 0" vacuum, if you see some that may be because the secondaries aren't opening up due to a bad secondary diaphram which is easily replaceable. If you have mechanical secondaries, i am not that experienced with them and would have to think about it but in this case i don't think the problem would be caused at all by the secondaries, once they're fully open you may get a hesitation if they get open too soon but once airflow gets high enough the motor should pull hard and put out at high rpm.

when you say it pulls hard off the line, that leads me to believe it's not really a power valve issue. when you floor it and the primaries are opened all the way, the power valve will open, when it opens exactly is based on the power valve number which means inches in vacuum. What you may try is a two-stage power valve, if the 6.5 is opening and it's becoming too rich too soon. but on the other hand if the carb is dirty and in need of a cleaning and rebuild then simply replacing the power valve may fix your problem, I've seen that plenty of times where the power valve is shot and you don't get over 3k-3500 rpm in a boat when you should get 4500+. power valves are cheap and easy enough to replace. Other than that, an easy thing to always try is raising or lowering the fuel level in the bowl by adjusting the float to see if it helps, that is free to do. And one other fundamental is make sure you're not running out of fuel pressure under load and high rpm, you want at least 1psi to be sure it's not starving for fuel and running lean.

cambirdracing

iTrader: (4)

Sounds like a fuel starvation problem. What kind of tank do you have? If it's a stock tank. Top it off and see what happens. Maybe a weak fuel pump?

pancherj

That's a tiny cam (218@.050" and .458" lift)...more for torque and less for going fast. But, you still could have some underlying issue. Also, why are you running with locked out timing? Is this a race only car? There might be some low end power to gain with timing. Also, check to make sure the throttle is opening all the way when someone steps on the gas peddle. I had that get me before!

TheGreat4.8

@1 FMF well as far as air restriction it feels the same with the filter on or off so i think i can rule out air restriction, and I'm using mechanical secondaries with their out of the box setting (new carb) but i do feel a bog if i get into too fast but if i ease into it it the bog goes away, but still no power in the 2500-5000 range. isn't a grind like this supposed to sing at these mid range rpm's?

@cambirdracing Yes I'm using the stock tank and has been cleaned and ran new 3/8th fuel line to the pump, i have just changed the fuel pump 2 months ago due to a faulty summit chrome job with an oem one that will flood the carb easily without a regulator.

@pancherj To tell you the truth locked out timing doesn't sound right for this cam i know, but i have tried alot of different timing curves and it is boggy with any less then 16 initial so for the sake of easy carb tuning i have locked it out and it feels great at the bottom end(no pinging). Could it be that it wants more timing at the top end?

1 FMF

iTrader: (1)

[QUOTE=TheGreat4.8;15155443] i do feel a bog if i get into too fast but if i ease into it it the bog goes away, but still no power in the 2500-5000 range. isn't a grind like this supposed to sing at these mid range rpm's?

for the bog, getting into it fast = bog but getting into it slow = no bog is generally accelerator pump. Try adjusting the pump shot lever arms if possible, or get larger squirters.

when you say no power in 2500-5000 range, can you describe it in a little more detail? without experiencing it, maybe it's normal because that cam with intake is great for low-end torque and throttle response and 2500-5000 is not low-end. If you're lack of power is just when you stomp on it then i would look at the accelerator pump squirter sizing and cams. and for that rpm range, having the correct ignition timing is important so that's a whole other variable to work with.

TheGreat4.8

the lack of power is when i stomp it when I'm already moving and if i stomp it from a stop it takes off but quickly loses that momentum i had at launch, kinda like alot of noise but no go.

But lets say i just chose a wimpy cam, what kind of power range is this thing capable of?

Tinbender59

I think your power valve may be wrong, get a vacuume guage and monitor your vacuume at the time of the "bogg" you should have the power valve at least .5 below that vacuume pressure. 6'5 is ok for stock engines or in the front on spread bores, but not anthing modified I am assuming that you have 2 power valves, this one is for the rear

after getting the PV right, I would start moving up from stock in 2 size jumps until you platoe out, next thing get the ----- I think it is either white or green accelerator cam, it is the biggest and quickest ramp that you can get and put it in the back. I am assuming that you have a 50cc pump in the back. leave the back squirters alone for now it sounds like when you run out of squirt it falls flat. when the squirters are done the power circuite should have or is begining to take over. And It sounds like your secondary power circuite is not tuned corectly for your setup.
???? I have been out of the Holly sceen for at least 20 years, I am really working my bean on this.

lets start simple, first thing, buy a "Jet pack Large size" cheaper in the long run. first thing jump up two jets in the back, and report back the results. it is your butt in the seat so pay attention. maybe even jump once more then report back????

quick help note. buy a tube of teflon greese, greese both sides of your bowl, and metering plate gaskets (EVERY TIME YOU OPEN IT UP) and your gaskets wont stick and be destroyd every time you work on it.

also Always remember make only 1 change at a time, it is slow and teadious, but damn well worth it in the end. you know exactly where you have been and where you are going "Tinbender"

Tinbender59

information in a carb you have three stages: an Idle circuite, a power / enrichment circuite. and a secondary or cruse circuite.

sweet-94z28

iTrader: (30)

To me, I think your carb may be too rich. I would try going a little smaller on the jets. What LSA is the cam and what intake do you have (needed to determine power valves and idle vacuum level)? I would also probably install a GM HEI distributor with a nice coil and allow the timing to move around. That will really help your low end/mid range response.

Just for a reference in similar engine size and carburetor, my friend has a 71 Camaro with an 11:1 383, 230/230 .511/.511 cam, AFR 210's, Vic Jr, and 1" carb spacer with a 750 DP. He runs 72 jets front, and 76 jets rear with a 6.5 PV in the front and a 9.5 PV in the rear. No bogs, no hesitations in his. I know his would need a lot more fuel than yours because of the heads/cam, but it runs perfect at this smaller jet/higher power valve combination. It gets decent gas mileage too.

1dirtybird

iTrader: (7)

ok.... start from 0

1... set idle if your carb has 4 cornner idle bleeds then start by screwing in each one till the motor starts to die then back it a 1/4 turn.
OR use a vacume gauge and do the same process till it get the highst vacume reading.

IDLE SET

2. set power valve
Take a vacume reading at idle, cut that in half and thats the power valve you will need for the front. Get a delete plug for the back and up the back jetting ive found a 7-10 size split from front to back this way.


3 set curse.... with out an afr reading this is shooting by the hip but 74-78 jets sound about right just gonna have to play. It it surges its lean if it fleels flat and people behind you die of carbon monoxide poisning its too rich.

4 tip in.
If all the above is good and it still boggs when mash it from a dead stop start with bigger squriters, then gonna have to start swapping pump cams. the kit is like 30 bucks.

Wot... with out a dyno and afr reading again shooting from the hip.

... as an idea

My 355 w/10.2:1 and stock vortecs a much larger cam (238/246..flat tappet 490/495 on a 112) took 78's in the front with a 2.5 power valve... and 88's in the back with no power valve. through a holly hp 750 dp that was a 14.7 at curse and 12.9-13.0 at wot. all my timing is in by 2500 (36*) but my converter is much larger then yours, though it is tight.

Hope this helps man good luck!
Dirty

TheGreat4.8

sorry i have not posted back to this thread in awhile as i am wrestling a 14 bolt into a half ton truck (that"s another story), i appreciate the info on the power valve and secondaries although I'm not sure whether or not my secondaries have a power valve?

but at any rate i think my problem is within the secondaries as the bottom end is very responsive and snappy but then the top end is less impressive.

Ill post back when i figure out what this thing wants, ill start tinkering with the secondaries first.

Intimidator02SS

Im with sweet94z28. If your bottom end power is great, don't mess with it. If you stomp the gas and it accelerates until your mid range rpm - Its not your accelerator pump adjustment. (They've already shot the gas, and won't shoot anymore as long as your foot stays on the floor) ... I would start with changing the jets in the secondary side. Hard to say if your rich or lean, but pick which way to try and move 2 sizes (up/down) and see if it gets better - if not go the other way. Once you feel like you can't get any better - you will most likely have to play with the primary jets.
Like the other guy stated - do one thing at a time - it WILL take a while, but it will be worth it. Keep it easy.
Good luck

93 LT1 Vette

Back your rockers off 1/4 turn and see if it pulls harder through the mid range and revs quicker to redline/top end. Try bumping timing up to 18 then 20 after loosening up the rockers a little. The cam you're running is tiny for the 383 but check for valve spring binding at full lift on each cylinder.

LilJayV10

iTrader: (39)

Have you checked your float level on the secondaries? As mentioned check to make sure you are getting WOT.

With the car off and you open the carb by hand are you getting fuel out of the back squirters?

It sounds like you are running out of fuel in the secondaries. I think the jets you have are big enough for what you have but it's been a while since I've messed with a carb.

With a 383 and that small of cam there is really no reason you should have to lock out the timing unless your compression is really low or the cam is a tooth off.

How much manifold vacuum are you pulling at idle? It should be a bunch w/ that setup. If the throttle blades are open to far exposing the idle circuit slots may be the reason you have to run so much timing. The good thing about that carb is you can open the secondaries if you need to let more air in at idle. You can back down the idle screw to cover up the idle slots to get a good signal to them and open the secondaries some to give the engine more air if it wants it. That will help stablize engine RPM which makes setting the timing easier. As I'm sure you know there is a relationship between initial timing/engine rpm/manifold vacuum.

As mentioned start simple first.

Keep us updated. I'm all excited now. I might have to go find something with a carb and mess with it.

Big Al

iTrader: (1)

There are a number of good books on tuning Holleys. Your local library can help.

Or: http://www.holley.com/TechService/Library.asp

Or: http://www.nastyz28.com/~ericf/tech/htune.pdf

Al