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Alcohol Carb Question

General engine tech -- Drag Racing to Circle Track

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n5ifi
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Alcohol Carb Question

Post by n5ifi »

I've been running a Gas Quick Fuel Carb that was modified many years ago to work on methanol. Pro Systems did the modification and with little exception and tuning its always worked well for me. I use an unusual yet effective method for changing the fuel pressure. I run a bypass regulator and use vacuum to increase and decrease fuel pressure. It works fine in my little motor.
My son bought a quick fuel methanol carb recently and (he didn't know) it has no provision for doing this little vacuum trick.
I took mine off the car and sure enough it was modified to work this way. Pro Systems slotted a spot on the main throttle shaft plate on the passenger side and added a pressed in nipple to the outside of the base plate. It looks like they drilled a hole in the throttle shaft so that it picked up full manifold vacuum at idle and then gradually shut off vacuum as the shaft was opened. Anyone seen this? Anyone know of a better way to do this? I cant tell if the hole goes all the way through the shaft. I guess we are going to mod his carb to work like this.
A belt drive fuel pump is not in the cards right now.
Tnx in advance
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Re: Alcohol Carb Question

Post by panic »

That's great, thanks for sharing.
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Re: Alcohol Carb Question

Post by n5ifi »

I was actually asking someone who knew something about carburetors like Mark or Tuner for example.

See the above ? for reference.
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Re: Alcohol Carb Question

Post by n5ifi »

If my post is in the wrong section will the administrator please move it.
Thank you
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Re: Alcohol Carb Question

Post by jmarkaudio »

There are a lot of ways to make things work, what size carb, engine, etc...?
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Re: Alcohol Carb Question

Post by miniv8 »

Its a throttle positioned vacuum control basically.

No I haven't seen it before so that question is answered, and no I'm not a carb expert.

If the carb is really small for the application, this setup will hinder the manifold vacuum at wot on the top end, from lowering the regulator's pressure.
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Re: Alcohol Carb Question

Post by n5ifi »

jmarkaudio wrote: Tue Nov 03, 2020 1:46 pm There are a lot of ways to make things work, what size carb, engine, etc...?
Mark
13 to 1 408 with a 950 cfm non modified Quickfuel alchy carb.

Runs 6 teens and twentys in the 1/8th

Can I post pictures here if I need too?

What I'm looking for is high vac at idle and lowering vac as i open the throttle.

I 'm using a return regulator and vacuum is attached at the regulator. It gives me about 4 lbs at idle and 8 or more going down track.

I do it on my little 347 with no problem using a modified pro systems Quick fuel carb and it looks like they did this throttle blade mod to do what I was talking about with the vacuum.
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Re: Alcohol Carb Question

Post by ProPower engines »

n5ifi wrote: Sat Oct 31, 2020 6:47 pm I've been running a Gas Quick Fuel Carb that was modified many years ago to work on methanol. Pro Systems did the modification and with little exception and tuning its always worked well for me. I use an unusual yet effective method for changing the fuel pressure. I run a bypass regulator and use vacuum to increase and decrease fuel pressure. It works fine in my little motor.
My son bought a quick fuel methanol carb recently and (he didn't know) it has no provision for doing this little vacuum trick.
I took mine off the car and sure enough it was modified to work this way. Pro Systems slotted a spot on the main throttle shaft plate on the passenger side and added a pressed in nipple to the outside of the base plate. It looks like they drilled a hole in the throttle shaft so that it picked up full manifold vacuum at idle and then gradually shut off vacuum as the shaft was opened. Anyone seen this? Anyone know of a better way to do this? I cant tell if the hole goes all the way through the shaft. I guess we are going to mod his carb to work like this.
A belt drive fuel pump is not in the cards right now.
Tnx in advance
We do alky carb builds all the time and the 2 things that are needed in your case is to get the fuel pressure right at idle and WTO. All you need is an adjustable bypass regulator. It means running a return line but they are stupid simple to use and set up. If your running a vac. type regulator system you can just duplicate the same mod to the new carb or use a different regulator system.
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Re: Alcohol Carb Question

Post by JDR Performance »

Manifold vacuum is going to be higher at idle with no load than it is with the engine loaded and throttle open. All you need is manifold vacuum to make a vacuum assisted bypass work.
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Re: Alcohol Carb Question

Post by jmarkaudio »

The biggest issue with alcohols is getting enough fuel thru the regulator. A vacuum controlled regulator works, using pressure changes to push more fuel in the carb. I prefer a regulator that will pass sufficient fuel, and a N&S that will handle the volume. You can get a N&S up to .170, and a Magnafuel 9833 regulator will handle the volume of methanol for about 800 HP. Above that a belt drive pump with a bypass is ideal.
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Re: Alcohol Carb Question

Post by JDR Performance »

If it's an actual bypass valve the carburetor would be fed by the pump and the bypass is downstream creating a "dam" to build pressure. The vacuum helps to lift the diaphragm in the bypass so fp is lower at idle (high vacuum, no engine load) and the diaphragm is pushed down harder increasing pressure when the vacuum is low (throttle open, engine loaded). This makes it easier on the float and n&s to control fuel level in the bowl. You don't need high pressure at idle, so if you have more than you need it works against you and you have to lower the float to try to get a "normal" fuel level setting. People have mixed experiences with the vacuum assisted bypass valves. The pressure differential between idle and open throttle can also be accomplished with a mechanical "ball valve" throttle set up. Belt pumps are better yet.
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Re: Alcohol Carb Question

Post by n5ifi »

This setup works great on my little 347 and my sons 408.
Never have any real problems with it.
I just assumed the vacuum was coming off of something set up by Holley but its a mod apparently done by Patrick himself.
Pro Systems slotted a spot on the main throttle shaft plate on the passenger side and added a pressed in nipple to the outside of the base plate. It looks like they drilled a hole in the throttle shaft so that it picked up full manifold vacuum at idle and then gradually shut off vacuum as the shaft was opened.

Can I post a picture?
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Re: Alcohol Carb Question

Post by n5ifi »

JDR Performance wrote: Fri Nov 06, 2020 6:16 am Manifold vacuum is going to be higher at idle with no load than it is with the engine loaded and throttle open. All you need is manifold vacuum to make a vacuum assisted bypass work.
I've tried that and it doesn't work.
It's not quick enough and you lose fuel pressure on the gear change.
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Re: Alcohol Carb Question

Post by n5ifi »

ProPower engines wrote: Thu Nov 05, 2020 6:36 pm
n5ifi wrote: Sat Oct 31, 2020 6:47 pm I've been running a Gas Quick Fuel Carb that was modified many years ago to work on methanol. Pro Systems did the modification and with little exception and tuning its always worked well for me. I use an unusual yet effective method for changing the fuel pressure. I run a bypass regulator and use vacuum to increase and decrease fuel pressure. It works fine in my little motor.
My son bought a quick fuel methanol carb recently and (he didn't know) it has no provision for doing this little vacuum trick.
I took mine off the car and sure enough it was modified to work this way. Pro Systems slotted a spot on the main throttle shaft plate on the passenger side and added a pressed in nipple to the outside of the base plate. It looks like they drilled a hole in the throttle shaft so that it picked up full manifold vacuum at idle and then gradually shut off vacuum as the shaft was opened. Anyone seen this? Anyone know of a better way to do this? I cant tell if the hole goes all the way through the shaft. I guess we are going to mod his carb to work like this.
A belt drive fuel pump is not in the cards right now.
Tnx in advance
We do alky carb builds all the time and the 2 things that are needed in your case is to get the fuel pressure right at idle and WTO. All you need is an adjustable bypass regulator. It means running a return line but they are stupid simple to use and set up. If your running a vac. type regulator system you can just duplicate the same mod to the new carb or use a different regulator system.
Will have to do some research on this. I understood that I was already running a bypass regulator.
May not be running it properly though.
My fuel pressure varies between 3-4 at idle to 8-9 lbs or so.
The only thing that you have to be aware of is to not turn the fuel pump on until the engine is running.
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Re: Alcohol Carb Question

Post by JDR Performance »

You might not have a vacuum assisted bypass valve like I described.
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