To big or to small?

[76Duster]

On a 71' 340 with a stock bottom end end the stock J heads, mopar 284. 484 cam, LD340 intake and headers, I can run the current 750 Holley -I think its a double pumper- or an 1850 Holley which I belive is 600cfm? Woul I be going from to big to to small? I'm not dead set on Holley just curious if anyone else has this setup and what they run

[plydus340]

I got a 71 340 in my duster. Same set up. I run the edelbrock 750 on mine with no problems. The 340 came stock with a 750 carb. .

[6 packin]

Post the list numbers located on the front choke tower! Holley made so many carbs for so many applications.

Is it a 4150 series carb? (does it have a metering block in the back like in the front). If so a 4160 series is what you have. Both are good.

Does it have duel line going from bowl to bowl?

A double pump carb is easy to tell at fisrt glance. It wont have a big plastic vacum housing on the rear passenger side of the carb.


Its hard to say about to big or to small. Fact is small blocks are very easy to over carb, easy when using a Holley. Most people are over carbed.

The 600 will definatly work better on the street, but when you mash on it you can feel the diff.

If you can find one Holley does make a 650 Duel line carb which will be perfect.

[76Duster]

Problem is I dont have the car right now, the guy is just getting it ready tieing up a few loose ends as part of the deal and the 750 which has duel feed lines and he said was a double pumper started leaking. So He has an 1850-4 on a GTX he recently bought that runs great and would swap it on. I had a Holley 650 but it was that one peice 9010 -??- Think that was the model number. Wasn;t overly impressed as it didnt have the Holley qualities. Basically what I am wanting to know is for a street car if I jet the 1850-4 fat enough it shouldn't have problems on the highway should it?

[rumblefish360]

The 600 will limit top end. It will do fine. The 750 is better for top end performance. The stock T-Q carb comes in @ 800 cfm. The reason this carb works so well is that it is a spreadbore carb with tiny primarys and HUGE secondarys. A great street/strip performer.
You should have no problemswith the 600. Just as you think, just jet corectly.

[76Duster]

Alright thanks for the help! Since I've got a few of the "regulars" attention how do you think this thing will perform? thebasic setup as mentioned in my first post with headers, an msd ignition and stock 3.21 gears, 3000rpm stall in a 727. Is anything really mismatched? IE: Cam

[dwc43]

Just remeber the 340 used an 800 cfm thermoquad. The 750 will be your best bet.

[Doug Wilson]

800 or 850 ThermoQuad.

It will run high 13s, low 14s, with either carb (600 or 750).

[76Duster]

I'd love a thermoquad but The LD-340 is a square bore correct? Where the thermo-quad is not.... I have a read a few threads that the adaptor plates ruin the intakes characteristics... I also read about opening the intake up to make it fit as well.

[skankweirdall]

I've done alot of experimenting with carb sizes and configurations on my car. All in all though it's a crap shoot at best. This is what I've found.

Smaller carbs and vacuum secondary carbs have better throttle response. Larger carbs tend to offer better top end performance and low speed driveability. There are formulas out there to figure a cfm capability for your engine, but they are just to get you in the ball park. I've discussed this at length with the Demon people, for instance where is the formula that says a 358 cid engine would run best with 2, 1250 Dominators on it like the Pro Stock trucks were using? Fact is, there is none. You have to experiment to get optimum performance. I kept going bigger and bigger on my car, from 750 to 1000 cfm's. I've finally settled at 925, a modified 750 main body mated to a 850 base plate. all highly modified and flowed.

The difference in ET from small to large was .30 but the big difference was in the launch and overall driveability of the car. The small carb launched hard but was down on power. The car didn't idle consistently well or run good at light throttle, like driving though the pits, etc. The large carb sofented the leave but ran strong on the top end and brought the idle and low speed, light throttle applications well into line. In other words the car is well mannered in the pits with the big carb. The 925 brought back a little better launch and didn't lose any MPH over the 1000, so that's what I kept.

Keep in mind though that this was what worked on my combo, I'm not saying your results would be similiar, only that you need to experiment on yours to get the optimum performance.

[dwc43]

Originally posted by 76Duster
I'd love a thermoquad but The LD-340 is a square bore correct? Where the thermo-quad is not.... I have a read a few threads that the adaptor plates ruin the intakes characteristics... I also read about opening the intake up to make it fit as well.

Yes the ld can be machined to take the tq. And an adapter can be massaged as well and work just as good.

[rumblefish360]

Look here

http://www.moparchat.com/forums/show...threadid=63965
The intake was modified, before me, and accepts the pictured 850 T-Q. Look before you cut/grind. Measure twice, cut once.

[76Duster]

might look into cutting it to fit the thermoquad next winter, my dads friend is a machinest so I imagine he'd know how to do it correctly. What would you do for a choke though?

[dwc43]

They make an electric joke for these. See Demon Sizzler. I think he's the one that rebuilds them on here as a bussiness. He would have one.