Smaller carb and vacuum signal ?

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rustytoolss

rustytoolss

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Two questions : 1 will using a smaller carb increase your engine vacuum ? ( I don't see that being possible) : 2 Will using a smaller carb increase your vacuum signal and make your engine more responsive, improve idle, mileage, at lower RPM (say under 2600rpm)etc ?

I have an EDDY 650 thunder(AVS) on my 273, with small cam, dual exhaust, electronic ignition, 3.23 , automatic, cruiser, driver, looking for MPG. And have been thinking of going to an Edelbrock 500 thunder AVS.

Also can some one tell me how to tune my carb (idle mixture ,idle speed) with my vacuum gauge. I tried to do it as the Edelbrock manual says. But I think the vacuum gauge way may be better ?????
 
It increases your vacuum because you are trying to suck more from a smaller opening.

Try running a mile and then breath through a straw. the air will have to move much faster to compensate for the smaller area.
 
Adjust the carb by turning the screws in, or out to highest rpm or highest vacuum. Then reset idle speed, but set timing first.
 
krazykuda said:
It increases your vacuum because you are trying to suck more from a smaller opening.

Try running a mile and then breath through a straw. the air will have to move much faster to compensate for the smaller area.
Click to expand...

So a smaller carb will increase my total engine vacuum. Right
 
NO

Engine vacuum, IE "manifold vacuum" or maybe you might say "idle vacuum" has nothing to do with the size of the carb

VACUUM SIGNAL is what makes a carb WORK. This is caused by the airflow through the throat and venturies and is what makes the whole thing operate "above idle" speed.

The big big problem with a "too big" carb is that the engine can only pump so much air. When you open the throttle of "that big carb" open a long ways, the "little" engine can only move so much air through the oversized carb. So the air SLOWS DOWN. The vacuum cause by the "venturi effect" no longer causes the fuel metering to operate, and the thing goes very lean

Maybe you try and compensate by installing bigger jets.

NOW what happens is that maybe, at part throttle, the damn thing IS marginally flowing enough air, the venturi is "OK" with that, and the metering starts to work. So now the thing goes incredibly RICH

See, here's the deal. There has been huge thick books written on this. There's been entire engineering classes taught on this subject. The TV car guys make this sound "easy." "Well, we bought a Hollowood 2000 carb, here, and look, we just bolt it on and.................."
 
rustytoolss said:
So a smaller carb will increase my total engine vacuum. Right
Click to expand...

Yes, to a degree. But I don't think that will do what you are looking for.

If you properly tune the carb, then it shouldn't make that much difference in vacuum.


The camshaft has more influence on vacuum than carb size on engine vacuum. (Rhoades lifters can help that also).


If you want more MPG, increased vacuum, and improved throttle response, then order a set of Rhoades lifters part # 2018


Read the second article here to understand how they work:

http://www.rhoadslifters.com/Pages/Articles.html


You can get a set here:

http://www.summitracing.com/search?...tOrder=Ascending&keyword=rhoades lifters 2018



You should also be running a good dual plane intake manifold.

The best for your application would be to find a used Edelbrock LD4B if your 273 is 66 or 67. Edelbrock D4B for the 64 & 65 273 as it has the odd intake bolt angle for the older 273 heads. If not, find a good dual plane intake, not single plane.


You may get a small increase if you change carbs, but I don't think that is your solution. I feel you will get more of what you are looking for with MPG, increased vacuum, and throttle response with rhoades lifters. I've been running them for over 25 years and they work. I've ran a set over 250,000 miles on a daily driver. They bleed down at idle and take out about .025" lift and 15° duration, then "pump up" and give full lift and duration over 3500 RPM. It's like having an adjustable cam. You get your cake and eat it too. You will definitely get more manifold vacuum with rhoades lifters.


I built a 318 many years back with 10.5 compression, used stock 340 cam, Rhoades lifters, stock 318 heads and intake, stock exhaust manifolds, 2 1/4" dual exhaust, and a 2.76 gear in a 68 Barracuda daily driver. It idled at 24" hg, but had problems with detonation causing overheating in warm weather.

I swapped the stock 318 heads and 2 bbl for a set of 77 360 heads (1.88" valves), a 69 340 dual plane intake, and a Holley 600 vac secondary. My vacuum went from 24" to 22.5" at idle. That is with 9.2 compression, and larger port heads, I only lost 1.5" of vacuum at idle. That is also going from a 2 bbl to a 4 bbl. I don't think that the increased carb size (even from going from 2 bbl to 4 bbl) had as much affect on the idle vacuum as the compression and port size did.

With a 2.76 gear, I was getting 17.75 MPG on the highway.


Run a dual plane intake, get a set of Rhoades lifters, and then tune in your carb. I think that would be your best approach. Then if you wish, you may try another carb, but I would try tuning what you have before throwing it aside.
 
krazykuda said:
Yes, to a degree. But I don't think that will do what you are looking for.

If you properly tune the carb, then it shouldn't make that much difference in vacuum.


The camshaft has more influence on vacuum than carb size on engine vacuum. (Rhoades lifters can help that also).


If you want more MPG, increased vacuum, and improved throttle response, then order a set of Rhoades lifters part # 2018


Read the second article here to understand how they work:

http://www.rhoadslifters.com/Pages/Articles.html


You can get a set here:

http://www.summitracing.com/search?...tOrder=Ascending&keyword=rhoades lifters 2018



You should also be running a good dual plane intake manifold.

The best for your application would be to find a used Edelbrock LD4B if your 273 is 66 or 67. Edelbrock D4B for the 64 & 65 273 as it has the odd intake bolt angle for the older 273 heads. If not, find a good dual plane intake, not single plane.


You may get a small increase if you change carbs, but I don't think that is your solution. I feel you will get more of what you are looking for with MPG, increased vacuum, and throttle response with rhoades lifters. I've been running them for over 25 years and they work. I've ran a set over 250,000 miles on a daily driver. They bleed down at idle and take out about .025" lift and 15° duration, then "pump up" and give full lift and duration over 3500 RPM. It's like having an adjustable cam. You get your cake and eat it too. You will definitely get more manifold vacuum with rhoades lifters.


I built a 318 many years back with 10.5 compression, used stock 340 cam, Rhoades lifters, stock 318 heads and intake, stock exhaust manifolds, 2 1/4" dual exhaust, and a 2.76 gear in a 68 Barracuda daily driver. It idled at 24" hg, but had problems with detonation causing overheating in warm weather.

I swapped the stock 318 heads and 2 bbl for a set of 77 360 heads (1.88" valves), a 69 340 dual plane intake, and a Holley 600 vac secondary. My vacuum went from 24" to 22.5" at idle. That is with 9.2 compression, and larger port heads, I only lost 1.5" of vacuum at idle. That is also going from a 2 bbl to a 4 bbl. I don't think that the increased carb size (even from going from 2 bbl to 4 bbl) had as much affect on the idle vacuum as the compression and port size did.

With a 2.76 gear, I was getting 17.75 MPG on the highway.


Run a dual plane intake, get a set of Rhoades lifters, and then tune in your carb. I think that would be your best approach. Then if you wish, you may try another carb, but I would try tuning what you have before throwing it aside.
Click to expand...

I have an Edelbrock performer intake dual plane 2176
 
do you have the adapter plate on the intake between the eddy carb and top flange? your carb is not too big but the thunder carbs are set to run richer going to a 500 thunder won't solve you milage problem but jetting it down can help.
 
rustytoolss said:
Also can some one tell me how to tune my carb (idle mixture ,idle speed) with my vacuum gauge. I tried to do it as the Edelbrock manual says. But I think the vacuum gauge way may be better ?????
Click to expand...

So far as "tuning" that is "the things you can change"

The things that affect idle vacuum are

idle speed

timing

idle mixture

Warm the car up completely.

Adjust the distributor timing with vacuum pot disconnected for highest vacuum reading. Sometimes you may need to retard slightly from this setting, if you get "ping" and if the starter "kicks" on starting. You also much keep in mind that as you advance INITIAL timing, the "all in" mechanical timing will ALSO be advanced. This is a function of your distributor


Then "rough" adjust idle speed Adjust the idle mixture screws for highest RPM / highest idle vacuum. Go back / forth between idle speed and mixture. You want the mixture "just a touch" CW from the peek which is towards the "lean" side of the peak. Turn the screws until RPM or vacuum 'just starts' to drop
 
rustytoolss said:
I have an Edelbrock performer intake dual plane 2176
Click to expand...

That should be good.

any spacers/adapters between the intake and carb?


I say try to tune it.

Rhoades lifters if you wish, but then tune it in better.


Find your timing first, then dial in the carb.
 
teringer said:
do you have the adapter plate on the intake between the eddy carb and top flange? your carb is not too big but the thunder carbs are set to run richer going to a 500 thunder won't solve you milage problem but jetting it down can help.
Click to expand...

Yeah I bought the Edelbrock required plate. And I also have a 1" perbolic 4 hole spacer that fits square bore carbs.
 
IMO, your 650 may be a tad large for best economy on your 273 but the 500 would likely be a bit on the small side for any power. With the spreadbore 650 (smaller primaries) and vacuum secondaries, you have the right type of carb for your objectives.

After the tune, I would personally look into a different cam that empahsizes torque; those typically give the best mileage; it sounds like you have cast manifolds and not headers, so I suspect the breathing is not there to allow a high RPM cam to work any way. Never tried the Rhodes lifters, but the reduced duration is in the right direction for improved mileage....but I would not expect the reduced lift to make any difference.....

What is your MPG presently?
 
nm9stheham said:
IMO, your 650 may be a tad large for best economy on your 273 but the 500 would likely be a bit on the small side for any power. With the spreadbore 650 (smaller primaries) and vacuum secondaries, you have the right type of carb for your objectives.

After the tune, I would personally look into a different cam that empahsizes torque; those typically give the best mileage; it sounds like you have cast manifolds and not headers, so I suspect the breathing is not there to allow a high RPM cam to work any way. Never tried the Rhodes lifters, but the reduced duration is in the right direction for improved mileage....but I would not expect the reduced lift to make any difference.....

What is your MPG presently?
Click to expand...

My Summit cam is for low end torque/good fuel mileage 204/214 .420/.442 112 LSA. (1200-4800rpm) I do have a fresh set of 302 heads.
As for mileage , I have not driven it enough yet. I was just thinking that the 650 Edelbrock Thunder (AVS) was a bit much for the 273. It was bought for a 383 chevy stroker a while back. Since it was sitting around, and I need a carb, I put it on.
As far as high RPM usage, I rarely see 4000 rpm, well maybe 3800. I just cruise to some shows . After I spent a lot of $$$, and broke a lot of parts (long ago dragracing) . I learned you don't have to go fast, to have a good time.
So maybe the 500cfm is just what I'm looking for.?
 
One more time. The 650 AVS will be fine. Use it. Anyone who says it's too big does not understand the fundamental principle of how the carburetor works. IT WILL WORK FINE..
 
I am just amazed at the excellent advice I keep seeing on FABO. One thing only I would like to add; Cruise timing. If you start the fire really early, I have found that you can often take a lot of fuel out. Be adventurous, be persistent and above all; be careful.
 
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