What carb for my 340 build?

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340MPR

340MPR

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Hey guys, I'm moving along with my 340 build and I'm starting to think about carbs. The car's got a holley 750 VS that's in need of a rebuild but with switching over to a 4 speed a few years ago and the new build I'm thinking of going with a DP. Here's a quick run down of the build so far;
- 340 .030 over
- 10.5-1 compression
- W2 heads w/ valve job & bowl blend
- MP single plane intake manifold
- Headers
- 3.91 gears
It's a street car that I'd like to get into taking it to the track once a year but 95% of the time it'll be driven hard on the street. I'm thinking a 650DP would be a good fit? Input's appreciated, thanks.
Mike N.
 
I’d do a 750 myself but the 650 will be OK.
 
I would rebuild the 750 VS and stick the short yellow spring in the secondary pod, then tune from there....
 
After reading through old threads I've decided to go with a 750DP, sounds like the best fit for my build. I've narrowed down to either a Holley or Quick Fuel, what I can't figure out is weather to stick with an electric choke (what I'm used to) or go without a choke, input?? I should add that I will only have the car on the road May thru September, so no true cold weather starts.
Thanks guys, Mike N.
 
Last edited:
Bump, I'd like some input from the guys on the board about going with a choke or not, appreciate any help.
Thanks, Mike N.
 
Since you have true winter in BC, I'd go with the electric choke. Down in south Texas, a choke isn't needed because I doesn't get that cold.
 
340MPR said:
Bump, I'd like some input from the guys on the board about going with a choke or not, appreciate any help.
Thanks, Mike N.
Click to expand...

I’d buy the QF Brawler if you can get one and I’m not sure you need a 750. I live where it gets cold in the winter and I never, ever use a choke.

I just looked at your OP and I’m on the fence about the 750. I don’t size carbs based on CFM rating. I size a carb based on venturi and throttle blade size.

If you didn’t have W2 heads I’d say to run a 650 based carb. So the heads shove me to the 750 based carb. But...the single plan intake shoves me back to the 650 based carb. I’ll post a link to the QF carb sizes and CFM ratings.

Even with the 3.90 gears I’m still leaning towards the 650 carb.

The QF Brawler 650 is a 1.250 venturi and a 1.687 throttle bore. The 750 is a 1.390 venturi and a 1.687 throttle bore.

The difference in venturi diameter is a big deal, especially for a street or street/strip deal. I run a 1.375 venturi and a 1.750 throttle bore on my 340 but I’m 11.77:1 and I run a 4.56 gear so I can pull that bigger carb. Plus I have a stick. Converter is a big deal relative to carb sizing.

Well, for now I’ve convinced myself you should stick with the 650 based carb. On a flow bench the 650 will easily flow 750 CFM plus at 20.4 inches of water.

EDIT: forgot the link...the Brawler carbs are on the last page. Study the diameters of the venturi and throttle bores before making a decision.

http://www.quickfueltechnology.com.au/content/Quick Fuel Carb Size with details-AUST.pdf
 
I was thinking of going with Quick Fuel's HR carb;
Quick Fuel HR-750 HR-Series Carburetor 750CFM
I was told the Brawler line was made in China which is why they're a bit cheaper than QF's other lines. That lead me to stay away from them but now you've got me re-thinking, just when I thought I'd narrowed things down. I should have the last missing bits of info on my build this week, my Machinist will have flow numbers for me by Wednesday and from there I'll talk again with Jim at Racer Brown and sort out what cam I'm going with.
Holley has all their lines on sale till the 31st so I'd like to pick something up while they're on sale.
 
340MPR said:
Hey guys, I'm moving along with my 340 build and I'm starting to think about carbs. The car's got a holley 750 VS that's in need of a rebuild but with switching over to a 4 speed a few years ago and the new build I'm thinking of going with a DP. Here's a quick run down of the build so far;
- 340 .030 over
- 10.5-1 compression
- W2 heads w/ valve job & bowl blend
- MP single plane intake manifold
- Headers
- 3.91 gears
It's a street car that I'd like to get into taking it to the track once a year but 95% of the time it'll be driven hard on the street. I'm thinking a 650DP would be a good fit? Input's appreciated, thanks.
Mike N.
Click to expand...

Why not go EFI and eliminate some carb headaches at the same time. I heard a lot of good about them.
 
I'd stick a 1050 Dominator on that thing so fast, it'd make you do a double take.
 
340MPR said:
Hey guys, I'm moving along with my 340 build and I'm starting to think about carbs. The car's got a holley 750 VS that's in need of a rebuild but with switching over to a 4 speed a few years ago and the new build I'm thinking of going with a DP. Here's a quick run down of the build so far;
- 340 .030 over
- 10.5-1 compression
- W2 heads w/ valve job & bowl blend
- MP single plane intake manifold
- Headers
- 3.91 gears
It's a street car that I'd like to get into taking it to the track once a year but 95% of the time it'll be driven hard on the street. I'm thinking a 650DP would be a good fit? Input's appreciated, thanks.
Mike N.
Click to expand...
What size cam and specs are we dealing with here?
 
That HR carb has made in China parts on it. If it says Quick Fuel on it, it’s China made.

If you look at the venturi sizes, QF’s CFM ratings don’t make sense.

The carb you are looking at has 1.375 X 1.687 sizing and they call that a 750. And that’s with a choke on it.

The Brawler is 1.390 X 1.687 and it’s called a 750. An .015 venturi size difference may not seem like much, but it’s a pretty big jump. And the Brawler doesn’t have a choke.

Like I said...with the single plane intake and your gearing I’d use the smaller venturi carb. You won’t lose anything on the top and driveability will be better.

Unless this is a street only car the choke is useless. The choke is the single biggest piston ring killer there is. These engines only need a very rich mixture for the first 10-20 seconds after fire up. Any longer than that and you are using excess fuel that does nothing but wash the oil off the cylinder walls and increase ring and bore wear.
 
Expensive to switch. I would not be worried over the little bit of HP loss on the street.
 
leblanc said:
Why not go EFI and eliminate some carb headaches at the same time. I heard a lot of good about them.
Click to expand...
I wanted to keep the build old school, Iron heads and stay with a carb so EFI isn't a direction I want to go with.

rumblefish360 said:
What size cam and specs are we dealing with here?
Click to expand...
I will have the flow numbers and Cam spec by Wednesday, I'll share once I have them.

Getting back to the choke the car is a driver, I'd like to hit the track once a year but for 95% of the time the car's going to be street driven.
 
340MPR said:
Getting back to the choke the car is a driver, I'd like to hit the track once a year but for 95% of the time the car's going to be street driven.
Click to expand...
Used a lot in the winter if the weather is OK?
I’d use an Electric choke for convenience.
A manual choke is fine as well.
 
rumblefish360 said:
Used a lot in the winter if the weather is OK?
I’d use an Electric choke for convenience.
A manual choke is fine as well.
Click to expand...

No I'll only have the car on the road from May through to the end of September. They don't clean the sand off the roads from winter until mid April and October is to hit and miss to keep the car insured so no the car will only be fair weather driven.
 
340MPR said:
No I'll only have the car on the road from May through to the end of September. They don't clean the sand off the roads from winter until mid April and October is to hit and miss to keep the car insured so no the car will only be fair weather driven.
Click to expand...


Then the choke is just a useless PITA.
 
yellow rose said:
I’d buy the QF Brawler if you can get one and I’m not sure you need a 750. I live where it gets cold in the winter and I never, ever use a choke.

I just looked at your OP and I’m on the fence about the 750. I don’t size carbs based on CFM rating. I size a carb based on venturi and throttle blade size.

If you didn’t have W2 heads I’d say to run a 650 based carb. So the heads shove me to the 750 based carb. But...the single plan intake shoves me back to the 650 based carb. I’ll post a link to the QF carb sizes and CFM ratings.

Even with the 3.90 gears I’m still leaning towards the 650 carb.

The QF Brawler 650 is a 1.250 venturi and a 1.687 throttle bore. The 750 is a 1.390 venturi and a 1.687 throttle bore.

The difference in venturi diameter is a big deal, especially for a street or street/strip deal. I run a 1.375 venturi and a 1.750 throttle bore on my 340 but I’m 11.77:1 and I run a 4.56 gear so I can pull that bigger carb. Plus I have a stick. Converter is a big deal relative to carb sizing.

Well, for now I’ve convinced myself you should stick with the 650 based carb. On a flow bench the 650 will easily flow 750 CFM plus at 20.4 inches of water.

EDIT: forgot the link...the Brawler carbs are on the last page. Study the diameters of the venturi and throttle bores before making a decision.

http://www.quickfueltechnology.com.au/content/Quick Fuel Carb Size with details-AUST.pdf
Click to expand...

I may not agree with you on somethings but your a very knowledge man. If I'm not in a hurry I always read up on your carb wisdom. I've never had it explained that way.
 
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