I understand everything you told me so far. i"m still trying to figure this out, with 13 a/f ratio across the graph (wot) one would think it"s right...... I"m thinking if i had a lm-1 i would see my cruise a/f ratio is way rich. Is that correct? Dave"s last question was:
is it ok to have the primaries jets bigger than the secondarys?
If i"m thinking in the right direction the answer would be "no" reason being with the larger jets in the primarys the cruise a/f ratio is going to be to rich.
By the way i have 79"s in the primary an 73"s in the secondary. Help from a carb shop is out of the question, I"m thinking i may be a good canadate for a 750hp main body with the adjustable air bleeds. Can i do that?

 

I had to go back and find out who posted first - CorvetteDave383 - right?

With a 112 lsa cam, I'd try jetting changes first - check stock jetting A/F ratios, and then tune with jet changes from those A/F readings. Don't mess with the air bleeds unless you are an expert, and then only after the jets are close.

I'm still thinking that 72/80 or 72/78 will run great.

 

My throttle plates are drilled too.

There is a lot of tuning you can do with power valves also. They come in standard and high flow, and come in different Hg ratings, 6.5,5.5,4.5, etc..., to match the vacuum of your motor.

I like to tune the primaries with the linkage to the secondaries disconnected. Use the power valve and jets to get 14 to 14.7 at low load and cruise and closer to 12.5 to 13 at wide open. It's not always possible, but get as close as you can. I then hook the secondaries back up and adjust the secondary jets to get between 12.5 and 13 at wide open throttle. A more efficient chamber with a tight squish will like the leaner jetting. Less efficient chambers will tend to make better power a little richer.

Set the idle with either a vacuum Gage or an O2 sensor (AFR meter). I prefer the O2 sensor. Set the AFR at idle between 13.6 to 13.9. Make sure all the idle mixture screws are out the same amount of turns when done.

Use only as much accelerator pump as it takes to give a crisp throttle response with no stumbling.

 

I changed the idle mixture airbleeds from 72 to 75, that has improved my idle mixture, it was quite rich, I turned the idle mixture right in, to under 1 turn, but still too rich and any less than that engine would start to die

I phoned my carb company(quickfuel), they advised me to change the idle air bleeds to a bigger size, to give more adjustment on my idle mixture screws

dave

 

Im going to change the jets tomorrow, I give 72/78 a go, I will report back with the a/f readings

dave

 

Both my last two motors 550HP 406ci and who knows 427ci don't like cruise at anywhere near 14:1 let alone 14.7:1. These are ideal numbers for some mystical low power 350ci out there somewhere.

Mine likes cruise as high as about 13.5:1 and down to 12 at WOT. On the Innovate website the guy who invented the LM-1 even mentions this about the older V8's

 

I have spent an enormous amount of time on the dyno and have 40 years of experience in performance street and racing. My profession is running a performance division for the company I work for. I not only do tuning but I teach it to people from all over the country and sometimes other countries. I design high performance parts and test them in our own dyno facility, so this is not something I have just read.

I've used these AFR settings on many a performance engine with excellent results. It doesn't matter if I am tuning a car, boat or Harley, they all respond well to these AFR's as long as everything else is correct.

 

Good for you, I have different results, with extesive tuning done with a wideband O2 sensor. My results seem to be backed up by some other very knowledgeable people. With the experience you have you should know a motor is going to behave different on the street than on a dyno.

I've been at this a while too

 

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.

 

Not trying to start anything either, I will agree with you about the new vehicles, my comments were about Gen 1 small block chevys specifically mine

 

Todays Report = changed the primary jets to 72 and backs to 78, at wot starts off good, you can see the shot of fuel going in, then goes lean(upto 15) for about 1000rpm, between 2000 rpm to 3000rpm, then fuels start coming back in again, and takes the a/f down to just above 12 for the rest of the run

when crusing, if I give it very light throttle, I can feel a slight stumble, and when you look at the a/f, it goes upto 14.5 to 15 at that point, to start with it will drop to below 12 with the shot of fuel, but then climbs upto 15 very quickly

when crusing and staying at a steady throttle, it starts off at below 12, then steady climbs to 15, maybe taking 10 secs to get to 15

any thoughts?

cheers

dave

 

The slight stumble is a lean miss and the reason I don't run mine at 14.7:1 A/F. Tune the carb so you get about 13.5 at idle.

The idle circuit basically controls the cruise up to a certain RPM so you will be in the 13-14 range while at part throttle and the slight stumble will go away

 

Now for going lean at WOT, I had this problem and if you can't get rid of it then you need to drill out Power Channel Valve Restrictors, I had to do that on my 406ci with a Holley 750 dp 4779 and believe it or not I had to drill out the PVCR's on my 825 Race Demon

 

this is where we have a problem , the idle at present is running 12 to 12.5 a/f, (in gear it is around 13 to 13.5)if I lean it up at ilde with the idle mixture screws, I end up affecting idle in gear, the engine goes really lean in gear, and whats to die all the time, how do I solve this issue??

what ever the idle mixture is set too, I still get this stumble at light throttle

dave

 

what about bigger front jets, or changing the high speed airbleeds at the front??

dave

 

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

 

That's Norval, I told him to buy the LM-1

corvettedave383, like stated above you are going to need to go beyond basic adjustments, if you have a wideband go to the Innovate site and on to their forums they have some real knowledgeable people there

 

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.

 

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.

 

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