Originally Posted by v2racing

Good read and it is right on. Norval has done his homework and it pays off. Unfortunately, not to insult anyone, but I think it is beyond most enthusiast skills.

Originally Posted by v2racing

The idle problem is not uncommon with low vacuum motors. You may have your throttle blades open far enough that you are into the crossover circuit (cruise) which is beyond the idle circuit. You may need to drill holes in your throttle blades as gkull mentioned was done to his. Mine also had to have this done to it for my 406.

Set your idle mixture in gear as this is where you drive the car, unless you are putting it in neutral every time you stop.

To get your cruise rpm a little richer I would try going up one jet on the primaries and going down one on the secondaries to compensate for the wide open throttle.

It is odd you need such a large accelerator pump, 50cc, on the front. Are you adjusting it properly. I would also double check your ignition timing. If it does not have enough initial lead and is retarded in the lower rpm's it will cause a stumble. You may need to limit the total advance of the distributor so you can set the initial higher. You may also need to put lighter springs in to come onto the advance earlier.
Use a timing light and tach to check your initial timing and map out the timing through the rpm in 500 rpm intervals up to your maximum advance. Also, are you running a vacuum advance and is it adjustable?

What is your cam specs and your compression ratio? What do you have the squish set at? What heads do you have and what size ports and valves do you have? What do you have for an intake manifold? Do you have headers and what do you have behind them for exhaust. What kind of engine temps is you engine running at? All of this ties together. It all needs to match to run properly.

Originally Posted by MotorHead

I would like a little more info on the crossover circuit. I know there is one in Webber carbs but didn't know there was a separate crossover circuit in a Holley type carb.

In Webber carbs the idle circuit and crossover inter-connected, meaning the idle jet is responsible for both circuits. Meaning that for cruise up to approx. 2500RPM the carb is working from both circuits.

This is same for Holley type carbs however the transition circuit is the second circuit after the idle circuit. The idle circuit is adjusted by the idle feed restrictions, idle air bleeds, and the idle mixture screws. The transitions circuit come into play when the throttle blades open slightly exposing the transition slots and get fuel through the idle feed restrictors.

At this point the engine is running off both circuits the idle circuit and the transition circuit. Depending upon the size of your engine and the rpm's it is running at, you can have flow from the idle and transition circuits well into the higher rpms.

But lets just say 2500RPM and call it a day as the main circuit will take over after that. But most of us are cruising below 2500RPM, I know I am and therefore the idle mixture screws play an important part in determining your cruise A/F ratio

Originally Posted by v2racing

The idle problem is not uncommon with low vacuum motors. You may have your throttle blades open far enough that you are into the crossover circuit (cruise) which is beyond the idle circuit. You may need to drill holes in your throttle blades as gkull mentioned was done to his. Mine also had to have this done to it for my 406.

Set your idle mixture in gear as this is where you drive the car, unless you are putting it in neutral every time you stop.

To get your cruise rpm a little richer I would try going up one jet on the primaries and going down one on the secondaries to compensate for the wide open throttle.

It is odd you need such a large accelerator pump, 50cc, on the front. Are you adjusting it properly. I would also double check your ignition timing. If it does not have enough initial lead and is retarded in the lower rpm's it will cause a stumble. You may need to limit the total advance of the distributor so you can set the initial higher. You may also need to put lighter springs in to come onto the advance earlier.
Use a timing light and tach to check your initial timing and map out the timing through the rpm in 500 rpm intervals up to your maximum advance. Also, are you running a vacuum advance and is it adjustable?

What is your cam specs and your compression ratio? What do you have the squish set at? What heads do you have and what size ports and valves do you have? What do you have for an intake manifold? Do you have headers and what do you have behind them for exhaust. What kind of engine temps is you engine running at? All of this ties together. It all needs to match to run properly.

Originally Posted by noonie

You must go beyond basic jetting.

Here is a link to a guy that did extensive testing with some surprising results, like 20 mpg with a supercharged 540. He seemed to get it perfect throughout the whole rpm range.
His research is very detailed, so either search for more results or pm/email him. Nice guy, sure he would be glad to help.

http://forums.corvetteforum.com/c3-t...fer-ports.html

Originally Posted by C3BB4SPD

Dave,

When you say 'big blower cam'... ...how big? And which transmission? What is your timing (...intial and total) set at? The reason I ask is that's not much converter, with a fairly tall gear. And you can't look at one piece of the puzzle without considering all the other peices. Also, has the off-idle stumble been completely resolved? If not, and you're running a bunch of camshaft, you might be fighting a battle you can't win with your current gearing.

You've been given sum really good info so far, and I agree with most everything that's been said. However, in my opinion, tuning the way you're doing it (with the O2 sensor...in real world conditions) will ultimately provide better results than on the dyno. There's been plenty of money spent to have engines and/or cars 'dialed-in' on the dyno, only to have them run like crap on the track. Not to say dyno's don't have their place, just that sometimes they don't paint a complete picture.

A specific example: My '69 Camaro was running 10.60's in the 1/4 mile and having a fuel problem at the top end, so I decided to put it on a rear wheel dyno to sort it out. The tuner's recommendation included re-jetting (optimized), which we did. Back on the track the car slowed (consistently) to the high 10.70's. I went back to the original (pre-dyno) jetting and it went right back to the low 10.60's. (The problem ended up being a fuel pump relay chattering...which we couldn't get to replicate on the dyno either, so a complete waste of time and money.)

My point is, you've taken solid logical steps and done good work so far.

Just keep after it...and good luck!

...Jerry

Originally Posted by gkull

Get ride of the 3/8th PVC line and plug off that big hole inducing a vacuum leak and rerun you test with no other changes.

Then read this and consider the up one jet size primary or up the primary high speed dia.

IMO - if it takes 10 seconds to clear out the extra fuel from a WOT squirt you are inducing way to much gas. I drilled out my squirters .003 -.004 to give a bigger instant shot on the primary and left the 30cc.

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

Originally Posted by v2racing

Your are correct. I should have said the throttle blade could be open far enough to expose the transition slots at idle. You lose control of your part throttle and idle in this situation. That is why holes are drilled in the throttle blades so that the blades can be closed down below the transition slots and still get enough air to idle a modified motor.

Originally Posted by v2racing

I'm not trying to start a spat here, so forgive if it came off that way.

I will say that most of these vehicles I have tuned were used in real world use, not just on the dyno. This is not counting all the racing. I also tune my own cars on the road with DTT's WegoII.

All of the latest crop of performance cars and bikes run withing these AFR settings. 14.7 is the cruise AFR on almost every vehicle made in the world. ZR1, ZO6, Hyabusa Suzuki, the 650 HP 750 Tq V12 Twin Turbo Mercedes, it doesn't matter. They all have to be at this AFR at cruise to pass emissions. They all are very high horsepower and run very well.

Having said that there are times that combinations can show richer AFR's than they are actually at because of over scavenging at overlap.

Originally Posted by v2racing

Squish is how close your piston comes to the head at top dead center. The closer it comes to the heads flat area, the quench area, the more efficient your combustion will be, especially at low rpm's. To make use of squish you need to be .040" at the widest.

That is a pretty big cam for a 383 with only 9.2 to 1 compression. The ports are pretty good size and you have a single plane intake. All of this makes for an engine that would be somewhat weak in the bottom and more setup for high rpm. No vacuum advance is not helping street drivability either.

C3BB4SPD was correct in his assumption about having to much cam and not enough stall in your converter. Your gearing is pretty steep for that engine combo also.

I hate to say it, but you may not be able to tune your way out of this one.

What do you want this engine to do. Do you want good street manners with good bottom and mid range power or do you want a drag race engine?

Originally Posted by larrywalk

Your combination is too varied for good engine response. For a CR of 9.2:1, you should be running a dual plane manifold and a cam with a duration of 230 to 235 and a lobe separation angle of ~110.

Your initial timing is too much - it's not surprising to have a stumble off idle. Vacuum advance will help a lot for smooth running - try about 15 degrees of vac adv, and an initial setting of ~15 degrees. When you hit the throttle, the vac advance comes out and the motor should pull more smoothly with your initial setting of 15 degrees. Try it!

Originally Posted by MotorHead

I didn't see this before, you have some serious mismatched parts going on unless you were planning on putting a blower on it. Your DCR must be in the 6:1-7:1 range which is not good. You may still get it running OK with some tinkering but it is going to be lazy in the lower RPM's

Originally Posted by corvettedave383

yeah blower/turbo in near future, yeah the engine does feel lazy at low rpms, but once it hits 3000+ , it really starts to shift!

cheers

dave

Originally Posted by gkull

258 degrees of solid roller is more than I use in my 434 ci. How did you determine installing it?

Your up into racing motor cam timing. It can be made to run, but not smooth.

You did say no PVC line to the carb?

I use a 4 hole wood thermal spacer which also increases the vacuum signal to the carb on my single plane. It helps when you are on and off the gas.