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Old 08-27-2013, 07:21 PM   #1
Joey SC
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Default Alky Carb questions

I just went to a 1250 Quick fuel carb with dual needle and seat bowls for my 638 motor, - with Sniper X heads.

A couple months ago I was able to run 7.58 in 4300ft air with my 1150 quick fuel gas carb and now I can only run 7.72 with my new 1250 alky carb at 3000ft.

During my tuning this past weekend at the track, I first went leaner and it helped a bunch (196 jets to 192), but it wasn't enough so I went to 188 jets and it didn't help

Needing to run closer to my 7.60 index I went from 6.25 fuel psi to 7psi and it helped more.

After reading various threads on here I find that higher fuel pressure seems to be required for alky. - why is that?

At the track I found cars with alky carbs having 3-4 psi at idle and 9 psi down track - I've read some even have 13psi I also saw a B-gas car with a very similar alky 632 motor run very well with a consistent 7psi.

Does higher fuel pressure help a gas carb too? - why not? Why would it help alky and not gas?

I changed to alky to be more consistent, but it also seems that Top Dragster and Top Sportsmen cars mostly run gas carbs, - why is that? They should want to be more consistent too.

I'm considering going back to gas and putting my energy into reading the weather better.

Thanks,
Joey
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Old 08-27-2013, 08:17 PM   #2
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Default Re: Alky Carb questions

What are the main air bleed sizes, and how many emulsion bleeds are there, and how many and which ones are plugged?
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Old 08-27-2013, 09:49 PM   #3
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Default Re: Alky Carb questions

Not sure how a BigBlock will respond but alky usually likes less timing over race gas.
I run 36* on race gas and 32* on alky.

With alky it's all about volume.....I'm running an electric pump so I have it set at 25psi at the pump and 8psi at the regulator.
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Old 08-27-2013, 11:51 PM   #4
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Default Re: Alky Carb questions

Mine was a SBC with a belt drive pump but I idled at 3-4 psi and crossed the traps at 12psi. That's where mine seemed to work best. I also ran 5 degrees less timing on alky than gas.
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Old 08-28-2013, 12:15 AM   #5
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Default Re: Alky Carb questions

a 638 would like more than one carb with alky,IMO. the volume of fuel is a factor at some point.
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Old 08-28-2013, 12:45 AM   #6
RayJE Carburetors
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Default Re: Alky Carb questions

With 4 x 150 thou needle and seats at 7 psi with correct amount of float drop you should have 4 x 350LB/ hr = 1400 lb/hr which should by rule of thumb be good for around 1400HP on alcohol

your issue could be the following

1: your floats are screwed down too far to hold 7psi with the nitrophy floats... and your not getting enough float drop therefor not flowing the fuel...
2: your igniition is letting you down.... going from big compression with gas to big compression with alcohol i would run a pro mag or at least a MSD 10 if you could find one
3 your chamber /piston combo is wrong for alcohol
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Old 08-28-2013, 01:53 PM   #7
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Default Re: Alky Carb questions

2 circuit
Idle bleed 36 / 36
High speed bleed 25 / 25
squirters 67
needle & seats .150
There are four emulsion bleeds on each side of the power valve plug, - only the top and third down have a holes.


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Old 08-28-2013, 02:07 PM   #8
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Default Re: Alky Carb questions

Quote:
Originally Posted by RayJE Carburetors View Post
With 4 x 150 thou needle and seats at 7 psi with correct amount of float drop you should have 4 x 350LB/ hr = 1400 lb/hr which should by rule of thumb be good for around 1400HP on alcohol

your issue could be the following

1: your floats are screwed down too far to hold 7psi with the nitrophy floats... and your not getting enough float drop therefor not flowing the fuel...
2: your igniition is letting you down.... going from big compression with gas to big compression with alcohol i would run a pro mag or at least a MSD 10 if you could find one
3 your chamber /piston combo is wrong for alcohol

My Compression is 14.9, my ignition is a MSD 7al2, I run a NGK fine wire plug NGK Iridium BKR9EIX with a .035 gap

The Sniper X head has been angle milled to 23 degrees, The dome is 32cc and chamber is 109.6 with a .060 gasket, and my piston to valve clearence is really tight - I had to advance the cam 4-5 degrees to get better EX clearance.

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Joey
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Old 08-28-2013, 04:27 PM   #9
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Default Re: Alky Carb questions

I'm not sure I would run a fine wire plug, try a standard -9 NGK and bring the gap to .025. Fuel pressure is more of an issue with methanol, getting enough through the N&S. With dual N&S bowls it should not be an issue as long as the pump supplies the volume. Gas you want to keep the fuel pressure lower to minimize foaming the fuel.

Are there skirted banjos holding the booster inserts?
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Old 08-28-2013, 05:18 PM   #10
Joey SC
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Default Re: Alky Carb questions

Quote:
Originally Posted by jmarkaudio View Post
I'm not sure I would run a fine wire plug, try a standard -9 NGK and bring the gap to .025. Fuel pressure is more of an issue with methanol, getting enough through the N&S. With dual N&S bowls it should not be an issue as long as the pump supplies the volume. Gas you want to keep the fuel pressure lower to minimize foaming the fuel.

Are there skirted banjos holding the booster inserts?

The boosters are skirted - but they're not that big of a skirt.
My fuel pump is a Magna Fuel 500 and I use a BG 4 port regulator, using just two of the ports.

I just spoke to Reher Morrrison - where i bought my Sniper heads and cam from - they said that methanol works fine for their smaller motors (565 & 58 but with my 638 - my heads may not allow for methanol to work as well. They recomended I go back to gas.

It appears that my tune is not that great, but not that far off either. It doesn't look to me like I'm going to get similar performance as i did with my gas carb - I'm seriously considering going back to gas.

Unless you can see how my carb may be way off

Maybe I need a much larger cfm alky carb to match the performance of my 1150 gas carb - but then again my Sniper X heads may be too small for alky and methanol with my 638.

With my gas carb I ran 7.623 at 179mph at 7550 rpm at 4280ft
With my Alky carb I ran 7.724 at 175mph at 7200 rpm at 3050ft


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Joey
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Old 08-28-2013, 05:38 PM   #11
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Default Re: Alky Carb questions

One of the issues is the available passage size in the booster pins and banjos when you get to a 1250 and above. The 2 circuit E85 1250's use around a .170 jet, with methanol and booster pins limited to about .220 or under it's tough to get enough fuel at the top and the bigger the engine the worse it gets. You should be running less timing with methanol as well than you would with gas, I'd start about 5˚ less.
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Old 08-28-2013, 06:00 PM   #12
Joey SC
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Default Re: Alky Carb questions

Quote:
Originally Posted by jmarkaudio View Post
One of the issues is the available passage size in the booster pins and banjos when you get to a 1250 and above. The 2 circuit E85 1250's use around a .170 jet, with methanol and booster pins limited to about .220 or under it's tough to get enough fuel at the top and the bigger the engine the worse it gets. You should be running less timing with methanol as well than you would with gas, I'd start about 5˚ less.
If I drop timing from 36 to 31 or so - with my combination - is it likely that I'll match the performance of my old 1150 gas carb? I'm not even sure I had the gas carb right. I've been told I should go with a bigger gas carb anyways.

I just don't want to go through a bunch of trouble tuning the alky carb only to find that with my particular combination I should stick with gas - as Larry at Reher Morrison recomended. I don't get to the track very often to test.

But your the carb expert and I trust your opinion - do you think I'd be better off with a 1250 gas carb?

Thanks,
Joey
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