Choosing the right carb for my car (340 4-speed)

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73SwingerBuild

73SwingerBuild

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Looking to see if this makes sense to everybody else.

When I was building my car, I bought a mystery 340 which turned out to be a pretty hot motor. It's got a pretty aggressive solid cam and a single plane Holley Strip Dominator intake. I have headers on it and a nice free-flowing exhaust. The transmission is a 4-speed with 3.55 gears in the rear.

When I bought the motor, it came with a Holley model 3310 carb, rated at 750cfm with vacuum secondaries (no choke). I had the carb rebuilt by my friend's dad, who said that it was a mess of parts but he said it should run alright.

Anyways, I've been driving the car for a few months and there was this really noticeable hesitation if I put my foot into it (even half throttle) under 3,000-3,500 RPM. After that point, it screamed and there were no issues. It also seemed to be running just a tad rich, though the plugs looked fine.

Anyways, I went to a buddy's shop last night to tune the carb a bit (it wouldn't idle well below 1,200 RPM and it was hesitating) and we ended up putting on a new Holley model Holley model 4776c 600cfm carb with mechanical secondaries (manual choke). We did a quick tune and the car now runs awesome. It burns tire through second gear and it pulls hard at low RPM, which it definitely wasn't doing before. I think the car being a 4-speed with minimal vacuum is the reason it responded so well to the mechanical secondaries.

I know I could gain more low-end by putting on a dual-plane (like the Edelbrock air gap) but I feel like this carb might be sized perfect for the motor on the street.

Does this seem to jive with what everybody else has seen? The carb and fuel line were over $600, but it's a local shop with a guy trying to compete against Summit and Jegs, so I don't mind giving him the money if the carb is right for the application. I mostly drive it on the street, so I think the 600 is sized better. I know the 750 might outperform it on the strip, but those times are few and far between.

Just looking to see if this makes sense. Thanks all for looking.

-Mike
 
In my opinion a 600 cfm carb is not enough for that motor. I run a 750 on my 340 which is also not stock and it runs great. There is no stumble when I punch it and it pulls strong. If the 600 runds good go with it, but I feel that your are leaving some performace on the table with that small a carb! JMO
 
You hit the nail on the head there buddy! All soooooo to often, the thinking pattern is to run with the larger carb on top. The second you say, I have a 340/360, the voice of opinion is almost always use a 750.

Now the main thing with any engine is what ever carb is on top, the unit must be well tuned a d gone over and over. If the carb is a bit large, a slow mushy feeling is what you'll get. That is the indication that the carb is to large and or the carb is opening up to much for the engine to use it all. Other factors can be involved such as bad/poor timing, too big of a tire, gear ratio being numerically to low, cam to big and the wrong intake manifold.

The beauty of a carb being slightly small is crazy crisp response, stronger signal to the carb is the reason.

Unless the build is a crazy one, your not looking at a big HP difference between the two carbs. I'd bet less the 15 HP on the big end.

Try the dual plane, you might like it even better with the added torque it gives. It may very well help since you have 3.55's.
 
rumblefish360 said:
You hit the nail on the head there buddy! All soooooo to often, the thinking pattern is to run with the larger carb on top. The second you say, I have a 340/360, the voice of opinion is almost always use a 750.

Now the main thing with any engine is what ever carb is on top, the unit must be well tuned a d gone over and over. If the carb is a bit large, a slow mushy feeling is what you'll get. That is the indication that the carb is to large and or the carb is opening up to much for the engine to use it all. Other factors can be involved such as bad/poor timing, too big of a tire, gear ratio being numerically to low, cam to big and the wrong intake manifold.

The beauty of a carb being slightly small is crazy crisp response, stronger signal to the carb is the reason.

Unless the build is a crazy one, your not looking at a big HP difference between the two carbs. I'd bet less the 15 HP on the big end.

Try the dual plane, you might like it even better with the added torque it gives. It may very well help since you have 3.55's.
Click to expand...

Glad to see I wasn't too far off with my reasoning. However, I wasn't sure if it'd be worth pulling that carb and upgrading to a 650DP. I think I can find somebody to take the 600DP off my hands, since it was my cousin's carb that I took out of the box sitting on his car lol. (Maybe he won't notice :eek:ops:)

Then again, I can always play with the power valve and jets and tune the accelerator pumps... Just not sure what I'm leaving on the table, if anything.

Thanks again.
-Mike
 
I run a 600 on my 340 also. If you're not interested in strip performance or concerned about starving it at the top end, those simple online CFM calculators would probably put you at 650 or less.
 
A double pump works best with a manual Trans or a pretty well build auto. I'd run a vacuum secondary or the AVS tuneable secondary air door. The AFB/AVS are mechanic secondary carbs with the ( IMO) crummy counter weighted doors (AFB) or the spring tensioned air door (excellent!)
The 650 AVS will be a nicely sprited carb on top of a dual plane (RPM) use it with something like a K&N extreme top filter.

The mystery is what cam you have.
 
I reject the term "Staving it" at the top end. A correctly tuned carb will perform well. He'll be down a few HP at worst. He'll stack up on a high velocity snappy throttle response with possible mileage gains. The add velocity will add torque.
 
Timing first! Get that sorted out as MANY carb problems aren't really carb issues.

A 750 vac secondary is like having a 375 2 brl. Hesitations can come from lots of places, too light secondary spring, poor accel pump/squirter combo, too low PV rating.
 
crackedback said:
Timing first! Get that sorted out as MANY carb problems aren't really carb issues.

A 750 vac secondary is like having a 375 2 brl. Hesitations can come from lots of places, too light secondary spring, poor accel pump/squirter combo, too low PV rating.
Click to expand...

I definitely need to address timing since I've done it solely by listening and smelling so far, but the plugs have a perfect burnt marshmallow color and I've absolutely never had it backfire. I will be installing a Rev-n-ator soon and will be playing with the timing then.

I'm mostly wondering if I should return the 600DP and upgrade to the 650DP, or if I won't really notice any gains.

I'm inclined to keep the mechanical secondaries since I have minimal vacuum and a 4-speed. I've heard this is a good combination for mechanical secondaries.
 
The reason I haven't gotten the timing down is because my timing tape is wrong. Right now it reads ~43* at idle and ~57-59* at 3,000 RPM.
Something tells me that is wrong lol.
 
rumblefish360 said:
I reject the term "Staving it" at the top end. A correctly tuned carb will perform well. He'll be down a few HP at worst. He'll stack up on a high velocity snappy throttle response with possible mileage gains. The add velocity will add torque.
Click to expand...

Ok - I was just forwarding what I've heard others say. I have no experience to back it up. Haven't even got mine on the road yet.
 
Figure out what's going on with your timing. Something isn't right... verify the TDC timing mark on your balancer with a piston stop and go from there.

A decent starting baseline for an aggressive cam is about 20* initial. Sometimes really big cams and lower compression engine need more timing to run at idle. Your current reading are REALLY out of bounds for a total timing number.

Check for vacuum leaks.
 
Moving forward (not undermining the importance of timing, just progressing with the topic), do you guys think I should get the 650 DP ordered up and installed while I still have the option? I don't think the cost to upgrade from the 600 to the 650 will cost much, if anything at all.

All of the calculators that I have run have yielded a CFM range between 600 and 700. I have varied the V.E. between 0.8 and 0.9, and I limit the motor to 6,000-6,500 RPM. While I know the jets and power valves can be changed/adjusted, I don't want the carb to be a limiting factor down the line.

If you think the 600 will work fine, then I'm happy to just play with timing. I just need to figure this out while I still have the option to return the 600.

Thanks all.
 
I wouldn't buy anything new until you have the baseline tuned up sorted out.

Magentic pick up distributor? Make sure the polarity of your leads is correct. You could try flipping them and see if the car still runs except the timing marks are more in range.
 
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