Carter carbureator

[Timothy B.]

Re: Carter carbureator

John,

The 350hp cam with 15" idle vacuum at 750RPM is excellent, the best I could do with a L-79 cam is 14". I am surprised the cranking pressure is 160 with flat top pistons, are you sure about the cam that's used.

That being said, try to increase the inital timing to 10-12* for a test. The engine will idle even better and should make more vacuum.

My thought is that when the throttle blades are opened like after the clutch is out, the engine goes under load and vacuum drops, the fuel drops out of the emulsion in the manifold and creates a lean condition. The accelerator pump is there to cover this transistion with a good shot of raw fuel at just the right time. I am not sure the nozzle size (diameter)but on Holleys this can be changed for more fuel initally.

I guess the first thing is to make sure the pump is working and adjusted properly. That includes the check valve in the float chamber and the check valve (needle seat) under the shooter to keep air and vacuum from sucking fuel.

I would think the idle jet at .038 would be large enough so A/F ratio is probably good. Make sure the rubber vacuum line at the vacuum advance is not egged causing a leak.

[John F.]

Re: Carter carbureator

Timothy,

Thanks for your response.

I'll double check the accelerator pump. I can tell you that the pump shoots fuel as soon as I move the accelerator linkage. I thought that the timing may be too retarded, I'll advance it also.

I'm beginning to think that the solution to this problem is to simply to open the throttle further when starting off.

[Stuart F.]

Re: Carter carbureator

John;

You never said which intake manifold you are using, i.e. cast iron or aluminum? and I don't recall where you are located in the country. Another quick and easy test is to unwire your heat riser butterfly and check then to make sure it works properly. I used to believe it necessary to wire it open, but have changed my mind due to further testing and changes on my L-76. As I have noted in other threads on the subject, I have eliminated my hesitation problem altogether and part of that includes using the heat riser butterfly valve as originally intended. Now that's my application with a number of other changes including jetting. Others, including yours, may differ.

I intend to do a write up on my changes after the first of the year and will be happy to share the fruits of this old "backyard mechanic's" successes in this area.

[John F.]

Re: Carter carbureator

Stuart,

Thanks. The manifold is the stock cast iron piece. I live in New York, but the car is only driven in warm weather above 45 degrees.

I would be interested in seeing your write up regarding the changes that worked for you.

Today during my search for a solution I found a Carter troubleshooting document where the author claimed that a fix for the stumble at take off is to increase idle speed to uncover a portion of the idle transfer slot. This sounds like it could work but I think my idle speed will end up at 900 - 1000 rpm.

Here is the link I found.

http://www.thecarburetorshop.com/Troubleshooting.htm

[Timothy B.]

Re: Carter carbureator

John,

Warm the car very well to operating temperature (go for a couple mile ride) then let it idle. Take the air cleaner off and turn the idle screw slowly to increase engine speed from normal idle speed to approx. 2000 RPM. If there is a problem going from one circuit to another, (curb idle to transfer circuit) you will not hear the RPM's increase and the engine will sputter. I bet that's not a problem with the Carter.

Make sure the two emulsion screws are correctly adjusted with the engine warmed so the manifold is hot, you mentioned 1 3/4 turns. Open them until the engine vacuum drops off or RPM"s start to fall and keep both sides about the same.

One other thing to consider is the age of the Carter. Is there play in the throttle shaft. The bore is aluminum and if the engine sucks air instead of emulsion you get a lean mixture.

[John F.]

Re: Carter carbureator

Timothy,

Thanks, I'll check the throttle shaft and the change from the idle to transfer circut as you suggested.

I was looking at the Edlebrock performer carb # 1405 and 1406. Both are 600 cfm carbs. The 1405 is calibrated for performance and the 1406 for economy. I've noticed that these carbs use very different metering rods and jets compared to the stock AFB. Considering the Edlebrock is a modern carb presumably designed for today's fuel, I am thinking of re jetting to the Edlebrock 1405 specs. The cost for the jets and rods is minimal. I think it's worth a try.

[Timothy B.]

Re: Carter carbureator

John,
In my opinion it's not that much different than the old Carter.

The metering rods and jetting you talk about with the Edlebrock needs to be checked by the sq.in. (area) of the opening through the jet. You need to find the area of the advertised jet size and subtract the area of the metering rod on the economy or enrichment step. This will give the effective area of opening.

Compare this to the area of the Carter jets and rods then you will know the calibration differences.

Also, I think the diameter of the currect Edlebrock is larger at the top where the air cleaner sits so your air cleaner base will not work.

[John F.]

Re: Carter carbureator

Tim,

Thanks. I did calculations using the Edlebrock settings. The effective areas are similar in the economy mode. The Edlebrock settings are richer in the power mode, but the Edlebrock uses a soft metering rod spring so the power mode activates later at a lower vacuum level.

The air cleaner won't matter since I was intending to use the new Edlebrock pieces in my old AFB Carb.

Do you know what the stock metering rod spring rate was on the 3721SB? At what vacuum level did it allow the metering rod to rise?

I have a set of the replacement Edlebrock springs that I have not yet tried. The metering rod springs currently installed in my carb are comparable to the strongest spring in the Edlebrock set.

Could my problem be that using the strongest spring is activating my AFB power circut too early (at a higher vacuum level), thereby creating a rich mixture causing a bog or stumble on take off? I think that the springs I am currently using may be allowing the power circut to come on at around 8-9" of vacuum.

Thanks again,
John

[Timothy B.]

Re: Carter carbureator

John,

I believe there are three springs tensions available (if you could even find them) but you can shorten or install washers to change tension on the spring if you are at the point of fine tuning the power enrichment circuit.

The proper way to do this is to drive the car in high gear at different speeds in a normal way, 25mph-35mph etc. (cruise) and monitor the engine vacuum. Taylor the spring tension so the metering rod raises up approx 1 to 2" vacuum below the lowest vacuum reading so you do not have enrichment at cruise. If your metering rods raise at 9"vacuum you are probably fine.

I don't think the power enrichment is your problem. Even if the rods raised at the momentary drop in vacuum when you let the clutch out there is no air flow through the booster venturies to create a signal for fuel to flow.

The idle circuit is what is controlling mixture at this low speed. The idle jet feeds the curb idle ports and the transfer slot. Are you experencing a bog or hesitation..

[John F.]

Re: Carter carbureator

Tim,

Thanks again, What I am experiencing is a lack of power when starting off. From your response, I can see that the springs would not be the issue because of the lack of airflow through the venturies. I have ordered the jets and rods so that I can set up my carb to the Edlebrock Performer 1405 or 1406 specs. I hope that a combination of these jets and rods will improve my power when taking off from a stop.

John

[Stuart F.]

Re: Carter carbureator

You can also get a varity of springs from Edelbrock as well or, for that matter, I believe they have a metering kit which contains jet and rod sets as well as springs. Look in their catalog.

As mentioned, the Edelbrock top is different from the old Carter including it's venting and possibly it's ports and air bleeds. I'd take a good comparison look at both before I'd sink too much money into Edelbrock parts thinking they are all interchangeable.

Stu Fox

[John F.]

Re: Carter carbureator

Stuart,

Thanks to you and all who have replied.

I 've done some comparison jet area calculations between the stock carb and the two of the 600cfm Eldebrock clones.

I've spent about $30 for two sets of rods and three sets of jets. I have a selecton of springs. I plan to experiment and see if I can improve the performance over what I have now. If I have any discoveries I'll pass them along to you all.



John

[Timothy B.]

Re: Carter carbureator

John,

One more thing to think about is what Duke and Stu mentioned about heat in the manifold. Make sure the heat riser is functioning properly. If it's wired open the heat will take the path of least resistance and go out the exhaust. The heat risers on my cars tend to spring closed even after warm up. The only time they look open is when it's really hot outside.

Also, the heat under the carburetor in the slots you plugged is there to keep ice from forming around the closed throttle blades so that may be something to think about. I know this hot slot has caused many problems to carburetor bottoms if the gasket arrangment is compromised and many here will tell you to plug them.

On my 67, after the cast manifold warms very good the engine idle speed will increase up to 100 RPM and I think it's because of better vaporization of the fuel caused buy the heat in the manifold.

I think going back to a stock arrangment is the best baseline to find a solution to the stumble. Mkae sure the distributor is working properly before getting crazy about the carburetor.

[John F.]

Re: Carter carbureator

Tim,

Yes I plan to do as you have suggested. I'll un wire the heat riser and set the carb up with the stock specs. From there I'll try a bit leaner and then richer set up and see if there is any improvement.

[Stuart F.]

Re: Carter carbureator

Tim;

It sounds like your choke may be full open before your engine is warm enough to idle normally. I recall, back in the day in the cold north, if I was able to kick off the choke before the engine was warm enough, the engine would idle too slow and sometimes die.

Speaking of those exhaust ports beneath the carb (on those so equipped), most any condition that contributed to excessive particulate matter in the exhaust would quickly plug the ports. Typically, cars driven mostly on short trips that seldom warmed the engine fully were prone to this. I had one 57 Chevy V8 that had the owners stumped. On his, the choke housing was getting all plugged with exhaust residue. It was the one and only car where I'd ever seen the choke air "X" tube burn through in the exhaust manifold. Boy! That will mess things up in a hurry!

Stu Fox