Too Much Carb?

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pagilman

pagilman

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Hi, Have run out of money and a friend gave me an 850 holley with a rebuild kit. I want to put this on my almost finished 340. Everyone tells me to much air!!!!!!!!! I say with an XE268 H cam which was much more cam than the 340 6barrel came with and I believe the 6 barrel was around 900cmf? What do you guys think ? {street car with headers and X heads} Can I change the jets down to work if it is to much? Thanks your mopar brother Walt :shaking2:
 
It is probably too big. Is it a double pumper or vacumn secondaries? You could try it, then go with smaller jets. It should work till you find what you need. It will probably be a real gas hog and cost you more money, especially if you are buying parts for the 850.
 
It is to big, it will have to low of a signal to meter fuel correctly. One thing to remember is on a 6 barrel the outer carbs were vacuum operated so they only opened if the motor needed it.

If you decide to not use it I just did a trade with a guy for one of my Q-jets for his Carter AFB. Maybe we could work out a trade for the AFB, I was thinking about taking the tunnel ram off my drag car and going to a single four and that 850 is about what I need. The number on the carb should be #4781 if it is an 850.


Chuck
 
The carb is in good shape and has vacuum secondaries. I guess I can rebuild it and try it untill I can afford a new carb. Why didn't you guys tell me I would need thousands to build an engine!!!!!! I'm 59 and was thinking 1970's figures when I started, what a surprise. Will i get at least the money back I have in this duster? I mean if I get sick, I would never sell the most beautiful car in the world unless myn health fails, You MOPAR brother Walt.

PS You guys eat your hearts out I'm going to Bristol to smell some nitro and burning rubber, my first sniff of the year!!!!!!!!! :love4:
 
One thing to consider when comparing the six packs is they are progressive and come in as rpm and fuel demands increase,you dont really have that option with a 4v carb-especially if its a double pumper.I was in a pickle and ran a 850 d.p. on a pontiac 400 with a stout cam,9.1 compression and ported heads/headers and it would barely idle and was a real bear at any rpm under 2k.You can jet it down considerably but then you will create other problems because your getting to far out of the carbs original setup/calibration.Im not trying to sound negative but it would be reall challenge to get it to run right or even decent.If it were me I would sell or trade it for a 650-750 carb and save some headache.
 
I wasnt as fast as 340mopar lol,by the way I have a nice 4781 that I may get rid of.
 
Vacuum secondary 850?, it must be a spreadbore, if it is then the driveability thing would be better because the primaries are small and the secondaries will only open as much as your motor will need. Thermoquads for the most part are 800cfm. Get the number off the choke horn.

Chuck
 
I will send chuck some pics when I can get back to the shed this weekend. If he doesn't want it with the rebuild kit, I will list it on the parts site here. I will send the numbers to chuck it has a duel chrome inlet on it. It might be a double pump. thanks to all who responded, I learned from you guys that if you don't have the proper vacuum the carb will not meter properly even if rejetted? Walt
 
i run a 625 cfm on my 340 with same cam and set up you have....

850 would be overkill for the street
 
340mopar said:
It is to big, it will have to low of a signal to meter fuel correctly. One thing to remember is on a 6 barrel the outer carbs were vacuum operated so they only opened if the motor needed it.

Chuck
Click to expand...
Exactly, and this is the important point. If the carb is too big the velocity of the air moving through it will never reach the proper level and the carb will not meter correctly. The two most common errors are too much carb and too much cam. Everything has to work together.
 
I have a 360 that is at 10.6:1 compression and the XE268 cam. I have run a Holley 600 vacuum secondary, a BG 725 vacuum secondary and a Holley 750 vacuum secondary. While the 725 and 750 give me noticable increase in power above 3500 rpm the throttle response in the 1000-3500 rpm of the 600 is much much more satisfying than either the 725 or 750. Plus the car is no slouch with the 600 running a best of 13.7 @ 102. I have all three carbs on the shelf and will from time to time put the bigger ones on but I keep going back to the 600. NOTE: the BG725 was much much worse in regards to throttle repsonse below 2000 rpm than even the 750 Holley.

The 850 is going to be all together too much carb for a car with a relatively mild cam like the XE268. The throttle response for street driving will not be good and the engine will never beable to use all the cfm capability of the carb. If you are extremely talented at tuning carbs and are willing to switch boosters and bleeds you may be able to get it to work ok.

You can't compare the 6-pak or even the spread bore carb cfm to a square bore carb. The 6-pak ran most of the time on 2-bbls and the primaries of the spread bore carbs were tining and the secondaries never fully opened on these cars in stock form.

Take the 850 and see if you can trade it for a 650-700 cfm carb, you will be much happier in the long run.
 
Cant hurt to try it. Also FYI 2bbls and 4bbls are rated differently so you can't really compare CFM ratings.
 
Definitely too big. You are correct in surmising that a too-large carb can be made to run by jetting the fuel way down, but you will suffer in the torque, driveability and gas mileage departments due to the low vacuum metering signal. The Six pack setup used vacuum operated outboards which only opened on demand by a high vacuum signal from the plenum. Also, these carbs were rated differently as previously mentioned and actually flow only 250cfm. The Carter ThermoQuad, while rated at 800cfm, had an airflow operated air valve on the secondaries, and tiny primaries for good low end torque, driveabilty and gas mileage. Stick with something in the 600-650 cfm range and you will be much more happy with the results.
 
Captainkirk said:
Definitely too big. You are correct in surmising that a too-large carb can be made to run by jetting the fuel way down.
Click to expand...

Actually with a carb that is too big you will need to go up in main jet sizes to get it to run. The reason being is the air velocity is low and the vacuum signal in the boosters is not high enough to pull adequate fuel for the air that is going through the carb.

To make the 850 perform adequately he is going to have to swap to high gain boosters and likely play with air bleeds to get adequate part throttle performance.
 
dgc333 said:
Actually with a carb that is too big you will need to go up in main jet sizes to get it to run. The reason being is the air velocity is low and the vacuum signal in the boosters is not high enough to pull adequate fuel for the air that is going through the carb.

To make the 850 perform adequately he is going to have to swap to high gain boosters and likely play with air bleeds to get adequate part throttle performance.
Click to expand...


I stand corrected. Regardless, I think most of us agree the carb is too big for a 340 and not worth the hassle of trying to tailor it to your needs. Go smaller, be happy! :love4:
 
pagilman, try it out. I think you'll be able to get it to work. You may not actually be able to use all the cfm there, but it can be jetted to work OK. Another secerct...<<Eyes look both ways>> is not to mash the long pedal, but to slowly tip it in.
 
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