Setup for 750DP on this 408

-
Mattax said:
Tuning for Performance.
This requires systematic trial and error to improve performance. Performance defined as power, torque and engine efficiency.

Start at the idle and work up.
Idle and off idle are your foundations. If its crappy at idle then the engine is going to have to clear up before it can respond to additional throttle.
Performance here is going to be demonstrated by running fairly clean exhaust output and how little throttle is needed
(manual) to get into first gear without stalling.
(automatic) lose the least rpm when placed in drive. Alternatively have the best vacuum at a given rpm in drive.
Work with initial timing, idle speed screw (transfer slots), and idle mixture screws.

Next confirm off-idle has good performance by very slowly accelerating from stop with no hesitation. This test minimizes the role of the pump shot and focus on the idle circuit's begining of transition.
If that's good, then test more normal acceleration (for public streets) from stop to make sure the pump shot is not too much or too little for this basic job.

Then you can test out steady driving at various speeds. The fuel ratio at steady interstate highway speeds is determined by main jet and its air bleeds. Local highways is partial or completely idle circuit (transition slot, idle feed restriction and idle aiir blleed).
If its too lean, it will surge. Slow down and take it back to garage. Increase primary jets.

If you have access to the strip or dyno you can test wide open throttle in 3 or 4th. AFR is controlled by the jets plus power valve channel restrictions. Change jets for the best mph through the lights.
If primary jetting for best power is different than jetting for best interstate highway mpg, adjust the power valve restrictions.

All the above tests are effected by timing. If you change timing at a given rpm and condition, then retest before change fuel mixture for that condition.

Once all that is tuned, then concentrate on transition conditions. They are:
opening the throttle quickly from low rpm (pump shot).
power valve opening point.

Reality is you may have to go out of sequence. You will almost certainly have to make repeats. How many repeats depends on how much interest and persistance you have.

Good luck!

A little more about AFR for different conditions
Power Valve Selection: The Definitive Answer
Click to expand...


I am familiar with the excessive testing for best results. I know only a little about Holleys. I think the jets are part of the idle circuit, are they? I know the cams increase, decrease or lengthen and shorten the amount and timing of the shot of the acc pumps. The squirters effect the amount of fuel dumped by the selected amount of pump shot. And the power valves, I have gotten conflicting information. Some say go with 1/2 the idle vacuum amount for the numbered size of the power valve. But Others say, for more power increase the PV by like 2x one half of the vacuum, which would be at the vacuum full number. I am still not sure what the booster is or how it works, what it effects, or if it can be changed. And the air bleeds are also part of the idle circuit but I'm not sure if they can be changed or not, Not sure if they bleed the amount of air to mix with fuel at all rpm's or not.

I'm going to start with 73 primary, 84 secondary jets, that sounds about right, then 33 or 34 squirters, which ever my acc pump rebuild kit has. I will run 2 orange pump cams with the 30cc pumps for now, and I happen to have 2 6.5 power valves which I will try.

I'm going to soak the metering block and bowls as long as it says to, maybe a little less. and do a complete rebuild of all parts. No sense in sparing a part to get a clog again. It will take a few days, but I will post what I end up with.

I have an AFR Gauge, EGT Gauge, Plan on adding Boost/Vacc gauge to test on highway. It took 10 months to get it built by a pro, that's why the gas is funky. Already ordered the 6AL And a Blaster2 coil. Already have the MSD Ready to Run Dist.

I have a lot of persistence, maybe too much, lol, but I'll get it running right. If there is anything else I need to know about building the carb right, please let me know.

Thanks
 
70 Buster 340 said:
Rule of thumb ..from what I can still remember... lol
30cc - dual plane, longer stroke, higher vacuum and booster signal/airspeed works fine
50cc - single plane - kind of invert to above and if your convertor is too tight.
Click to expand...

I am always in favor of trial and experimenting. I will use the 30's I have now, then try the 50's. Seems to me the 50's would give you a lot longer shot with the same cam???
 
70DusterBob said:
I think the jets are part of the idle circuit, are they?
Click to expand...
Yes and no. :D
Yes in the sense that the fuel must go through the main jets before entering the idle circuit.
No because main jets are so big compared to the idle restrictions that changing the mains has little to no effect on the amount of fuel flowing through the idle circuit.
70DusterBob said:
I know the cams increase, decrease or lengthen and shorten the amount and timing of the shot of the acc pumps. The squirters effect the amount of fuel dumped by the selected amount of pump shot.
Click to expand...
thumbs_up-gif.gif

70DusterBob said:
And the power valves, I have gotten conflicting information. Some say go with 1/2 the idle vacuum amount for the numbered size of the power valve. But Others say, for more power increase the PV by like 2x one half of the vacuum, which would be at the vacuum full number.
Click to expand...
Click on the link. It will take you to a diagram from a real carburetor book (one written for commerical and military automotive and aeronautical engineering).
Power Valve Selection: The Definitive Answer
See if that helps.

70DusterBob said:
I am still not sure what the booster is or how it works, what it effects, or if it can be changed.
Click to expand...
The boosters are minitaure venturis. Venturis are sort of an hourglass shaped tube. The air speeds up through the narrow section. Having a second venturi within the big venturi increases the air velocity even more.
The air rushing past the booster pulls the fuel.
It's kindof like having an airplane wing that is held in place. The air rushing past the wing would lift it, if it could.
The booster is connected to the fuel supply in the bowl. Instead of the wing being lifted, the fuel is lifted.
An analogy Chrysler used in 1970 was a bug or paint sprayer.
Check it out here on this page.
Carburetion Fundamentals & Facts (Session 273) from the Master Technician's Service Conference
 
Mattax said:
Yes and no. :D
Yes in the sense that the fuel must go through the main jets before entering the idle circuit.
No because main jets are so big compared to the idle restrictions that changing the mains has little to no effect on the amount of fuel flowing through the idle circuit.

View attachment 1715491235

Click on the link. It will take you to a diagram from a real carburetor book (one written for commerical and military automotive and aeronautical engineering).
Power Valve Selection: The Definitive Answer
See if that helps.

The boosters are minitaure venturis. Venturis are sort of an hourglass shaped tube. The air speeds up through the narrow section. Having a second venturi within the big venturi increases the air velocity even more.
The air rushing past the booster pulls the fuel.
It's kindof like having an airplane wing that is held in place. The air rushing past the wing would lift it, if it could.
The booster is connected to the fuel supply in the bowl. Instead of the wing being lifted, the fuel is lifted.
An analogy Chrysler used in 1970 was a bug or paint sprayer.
Check it out here on this page.
Carburetion Fundamentals & Facts (Session 273) from the Master Technician's Service Conference
Click to expand...

Thank you!

20200314_123505.jpg
 
Air bleeds are used to correct the fuel curve. Generally you shouldn't have to mess with them too much, if at all.

More important, especially on a Holley 4150 and similar, is the transition slot - which acts as a variable airbleed and a fuel restriction. If its not set within the design range at idle, then the off-idle mixture will be too rich, or too lean (more common) and too slow. On an older traditional Holley - like a 4779 - the working range at idle is .020 to .040" showing beneath the primary throttle blades at idle.
upload_2020-3-21_21-18-51.png


Many newer cabs may have a tighter range. Definately will if the slots are long or wide. Some are so long or wide that its worth the effort to shorten the slot! I don't think you'll see a need to do that with a 4779, even a newer one.
 
Mattax said:
Air bleeds are used to correct the fuel curve. Generally you shouldn't have to mess with them too much, if at all.

More important, especially on a Holley 4150 and similar, is the transition slot - which acts as a variable airbleed and a fuel restriction. If its not set within the design range at idle, then the off-idle mixture will be too rich, or too lean (more common) and too slow. On an older traditional Holley - like a 4779 - the working range at idle is .020 to .040" showing beneath the primary throttle blades at idle.
View attachment 1715491243

Many newer cabs may have a tighter range. Definately will if the slots are long or wide. Some are so long or wide that its worth the effort to shorten the slot! I don't think you'll see a need to do that with a 4779, even a newer one.
Click to expand...

Ok, I never heard of anyone changing the air bleeds, I guess that's why. I have heard a lot of people mention the air bleeds. Ordered some old Holley tech books a while back, I guess it's a good time to read them!
 
IMO, the 50cc pumps were a waste of money. Don't fall in love with a number with the A/F meter. Make the plugs happy and then look at the A/F numbers.

I don't even have a A/F meter because I don't like trying to watch a gauge. IMO, you need to data log it, and plot it against MAP, RPM and throttle postition.

If you do all that, you can get the tune up down to a gnats behind. Just watching the gauge is a very inaccurate way to get A/F numbers.
 
yellow rose said:
IMO, the 50cc pumps were a waste of money. Don't fall in love with a number with the A/F meter. Make the plugs happy and then look at the A/F numbers.

I don't even have a A/F meter because I don't like trying to watch a gauge. IMO, you need to data log it, and plot it against MAP, RPM and throttle postition.

If you do all that, you can get the tune up down to a gnats behind. Just watching the gauge is a very inaccurate way to get A/F numbers.
Click to expand...

You are right. Once I get the plugs light brown I need to see where my afr is. Then I will know what this engine needs.
 
I used to run points about 10 yrs ago. What should they be white or almost white?

I am new to the performance world. Also what should the MAP be, high as possible? Under normal conditions would the under carb pressure be higher, lower or the same?

Thanks
 
70DusterBob said:
I used to run points about 10 yrs ago. What should they be white or almost white?

I am new to the performance world. Also what should the MAP be, high as possible? Under normal conditions would the under carb pressure be higher, lower or the same?

Thanks
Click to expand...


Because electronic ignitions will keep the plugs clean. When the tune up is correct, the plugs should be a dull white color all the way down to the fuel ring.

I only care about MAP as a measure to see what engine load is compared to throttle position.
 
Okay, I think I'm good to go. I will spend as much time as it takes to get it right. I appreciate your time and advice. I will post the final carb build.

Bob
 
yellow rose said:
Because electronic ignitions will keep the plugs clean. When the tune up is correct, the plugs should be a dull white color all the way down to the fuel ring.

I only care about MAP as a measure to see what engine load is compared to throttle position.
Click to expand...

What/where is a fuel ring?
 
70 Buster 340 said:
What/where is a fuel ring?
Click to expand...

The fuel ring is what you look for to tune for WOT.

It should be .080-.100 wide (some guys can ring it under .080 but you have to be on top of the tune up to run it that close to the edge...a simple weather change can run you right into detonation) and you need to use a magnifyer of some sort and look down into the shell where the porcelain meets the shell. That's where you should see the fuel ring.

I think I have some ok pictures. I'll see if I can find them and post them.
 
image.jpeg
image.jpeg
image.jpeg


Ok, here is some plugs out of my junker. In the bottom picture you can see the fuel ring. It's way too wide. Also, if you look at the area just about the fuel ring to just up to the tip, you can see I'm fat at cruise too.

So the fix is to reduce the main jets until the cruise is correct, and then check again for WOT. If the cruise is happy and it's still fat at WOT, then I will reduce the power valve channel restrictions to get WOT in shape.
 
yellow rose said:
The fuel ring is what you look for to tune for WOT.

It should be .080-.100 wide (some guys can ring it under .080 but you have to be on top of the tune up to run it that close to the edge...a simple weather change can run you right into detonation) and you need to use a magnifyer of some sort and look down into the shell where the porcelain meets the shell. That's where you should see the fuel ring.

I think I have some ok pictures. I'll see if I can find them and post them.
Click to expand...

Thanks .. i'll be lookin' there

Duster plug.JPG
 
You must log in or register to reply here.
-

Back
Top