1941 dual main Carters

[valk]

Selfishly, I thought I would start a new thread consolidating info regarding my effort to use 2 main Carters for a dual carb set up for my '41 Roadmaster so I can find and refer to this info again should I need it. While most conversions I've read about entail using 2 smaller Carter 528s, I am rolling the dice and am going to try using my existing '41 490S with what I think is a '42 533S, both main carbs for a 320, and hope it works out. The carbs appear identical, with matching venturi, jets and metering rods, with the exception of the accelerator pump. The 533S pump appears more advanced, which makes sense if it is a '42. Below are informative quotes from other threads relating to this:

(CarbKing)

The larger engine (320) SINGLE carbs has 1 3/16 inch venturi and are 4-bolt flange.

 

Compound carburetion carburetors 320 cid 3-bolt with 1 1/16 inch venturii

Compound carburetion carburetors 248 cid 3-bolt with 7/8 inch venturii

 

Single carb 320 cid 4-bolt with 1 3/16 inch venturii

Single carb 248 cid 3-bolt with 1 1/16 inch venturii

 

Note the venturii for compound carburetion are SMALLER than for a single carburetor for the same size engine.

 

And two fronts from a 248 will work well on a 248; two fronts from a 320 will work well on a 320. The problem in using the fronts is availability and price. The carbs from the engines with single carbs are much more readily available, and significantly cheaper.

 

(Lawrence)

The 1941 509S is the same as a 1942 528S both fronts for the 248 for 1941 and 1942. I dont see any problem having one of each if thats what you come up with. The 1941 509S should be a lot more common as 42 production was significantly less. If you still want to try the 320 spec setup and cannot find a 490S look for a 533S which is the front Carter for the 1942 60 70 80 series and the same as a 490S

 

(me)

Carb #	Jet Size	Metering Rod	Optional Metering Rod
486S	.089	75-526	75-550, 75-551
487S	.082	75-459	75-488, 75-489
490S	.086	75-473	75-490, 75-491, 75-492
491S	.0531
509S	.086	75-473	75-490, 75-491, 75-492
510S	.0492
549S	.082	75-553	75-557, 75-558
551S	.082	75-553	75-557, 75-558

 

 

(Matt) 

I believe this is the linkage Lawrence and I used. You'll need two, and the one between the carbs will need to be cut to length and re-tapped on one end to accept the heim joint. Be sure to cut the correct end, since one end is reverse-threaded so the overall length can be adjusted simply by turning the rod itself (this is how I was able to get both carbs synchronized properly). I seem to recall that threads are 1/4-28. The longer rod from the pedal to the front carb just barely works as-is. It's 33 inches long and that's exactly how long you need with enough threads in the rod to be secure.

 

https://www.summitracing.com/parts/aaf-all54158/overview/

 

(Lawrence) 

The hardware for the rear choke is available from the auto parts store. Just tubing and nut to screw on choke assembly. Also I soldered on a copper 3/4 inch plumbing cap with a hole drilled on top to the end of the pipe which projects into the cap and ends just shy of the caps end. I cross drilled several holes into the projecting section of the tube for heat transfer of hot air and bent it up to sit on the exhaust manifold right on top of an exhaust port. This provides plenty of heat to actuate the choke and with just a small adjustment to the spring mechanism it works in total sync with the front choke. It is very important to employ both chokes with the setup or the rear carb will not operate correctly. 

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I hope it's ok to consolidate all this. At this point, I have the carbs but need to order the linkages. Matt, why do I need 2 Summit linkage rods?? 

 

Peter

 

 

 

[Matt Harwood]

6 minutes ago, valk said:

Matt, why do I need 2 Summit linkage rods?? 

 

 

You need one from the pedal to the front carb, and a second from the front carb to the rear carb. I tried to adapt the stock linkage somehow, but without cutting and splicing it wasn't possible and I figured the stock linkage was far more valuable in stock configuration. I also looked at other old-looking linkages to help it blend in, but none were satisfactory. Going with the arrangement Lawrence discovered was easy, fast, and works perfectly. It also doesn't stand out in the engine bay like I expected--very few people will notice that it's not stock.

 

   

Edited December 16, 2019 by Matt Harwood (see edit history)

[valk]

Hope everyone had a great holiday.  I have received my second main carb (believed to be 533S) from pont35cpe and it looks great - thank you Tom!  I intend to swap it out for the one I have to see how well it works and test it before diving into the dual carb mod. Should know something in a day or two if I don't get distracted...

[valk]

What did you guys do with the extra vacuum advance port on your second "main" carb, plug it somehow?

[Lawrence Helfand]

Yes just put a bolt in it. If you have a coffee can of assorted bolts you likely have one and if not your auto parts store or even the fastener aisle of your home depot. Its nothing exotic

[pont35cpe]

It`s actually 1/8" NPT

[valk]

Thanks boys.  Got a brass 1/8 plug at the store today, works great. 

I installed Tom's 533S (above) on my car just to see how it worked and she fired right up and ran great! I used the same setting as on my carb, idle screws a little short of 1 full turn out, and I can't ask for a better result.  On to the linkage...

As pont35cpe pointed out some time ago, the round choke housings on  Carter 320 main carbs (both 490S and 533S) foul the air cleaner preventing it from seating properly on the carb throats. The factory solution was to "dent" the main carb air cleaner wing to accomodate the housing, which can be seen on all 320 air cleaners on the main carb side. Long story short, I need to put a similar "dent" on the secondary-carb side of the air cleaner so it sits squarely on the second main carb. I think this is an issue unique to Carter 320 main carbs. 

Unrelated (I think), my car belches an inordinant amount of sooty condensation at initial start up when cold.  It clears up fast and I certainly expect some condensation, but this appears to be a lot of water to me.  Is this normal for these engines? 

Thanks,

Peter

[valk]

Just to clarify, I installed the 533S as the main carb on the original set up (in tandem with the secondary carb). Haven't tried both main carbs together yet... 

[pont35cpe]

Peter, I`ve always had the air/gas screws backed off 1 1/2 turns for initial fire-up, then once the engine is running, warmed up and idling, then I screw in (one at a time) the air/gas screw until engine flutters a little, then back it off until it smooths out, usually 1/4 to 1/2 turn.  If your idle speed increases, reset the idle, and repeat the air/gas screw adjustment. If you have started the car several times without driving it, it may have built up condensation in the muffler. Taking it out for a drive should clear it up..

[valk]

Hi Tom, I've done it that way too but I find it takes a lot of movement of the idle screws for the engine to respond one way or another, or for me to detect it anyway, and therefore hard to get a precise setting. I start where you suggest but have had better results running the engine and then looking at the plugs to determine if its rich or lean, and then adjust both screws in or out the same  amount, clean the plugs and check again until I get a light dusting around the rim.   Front carb idle screws effect the front 4 cylinders while adjusting the rear carb idle screws effects the rear 4 cylinders.  I may, of course,  be full of it but this is what I have observed and it has worked well for me. 

Yea, I guess the sooty crud is normal. It definitly clears up but I have to mop up my garage floor on every cold start!

Peter

[Lawrence Helfand]

Peter, I also use the method Pont35cpe uses and not just for Buicks but for every carb motorcycle and auto except for SU vacuum pressure type carbs. Reading the plugs at idle is not at all  accurate then in time engine rpm response. Frankly I have never seen a mechanic in my 70 years employ your method.  If you screw it in it will begin to flutter die quiet perceptibly and incrementally backing them out brings rpm back up and smooths out. If you cannot perceive any effect then something is awfully wrong. You can always use an rpm test indicator but that seems very unnecessary to me. I suggest you try this again as plug color at idle tells you nothing and will darken more and more the longer it idles. The only plug color reading that is accurate is for main jets and requires a long run at high cruising speed with a ignition chop and coasting to a stop and a looksee at plugs.  I dont believe You will ever achieve the correct air fuel ratio for your idle circuit by reading plug color at idle.

[valk]

Thanks much Lawrence. I laid this all out in the hopes some one more experienced than I would weigh in. I'll go over it again using the conventional method you and Tom recommend. In my weak defense, I drive the car around at various speeds/RPMs between pulling the plugs and tweaking the idle screws - not just at idle -  and she runs great, so I must be close. Maybe I just lucked out!

I'm puzzled about one thing, though.  Using the method you describe, the  2 idle screws on each carb will more than likely end up in different positions since they are adjusted independently.  So the 2 idle screws on each carb don't have to be synced, ie., turned out the same amount? 

Thanks man - nothin' like going back to school...

Peter

[carbking]

In the for what its worth category:

 

The below applies to downdraft carburetors where the idle mixture control screw meters a mixture of fuel and air. It does NOT apply to those carburetors (such as a Zenith type 23) where the idle mixture screw meters only air.

 

Most carburetors have a range for adjustment of the idle mixture control screws specified by the factory.

 

The method I have used for some 60 years is simple:

 

(1) Look up the range. WHERE? In your service manual

(2) Divide the range into three equal parts

(3) Set the initial setting of the idle mixture control screws (see below)

(4) Turn the throttle positioner screw to give a fast idle

(5) Start the engine, and warm to normal operating temperature.

(6) Set the idle RPM by backing down the throttle positioner screw.

(7) At this point you are going to be very close, PLUS you are going to minimize or eliminate hesitation from a stop from having too rich an idle mixture.

 

Setting the idle mixture screws:

 

I use the following:

 

(A) Divide the idle mixture screw range into 3 equal parts.

(B) If the engine is freshly overhauled to about 1500 miles set the screws at two thirds of the range

(C) If the engine has 1500 miles or so, and in good to excellent condition, set the screws at one third of the range

(D) If the engine is burning oil badly, and you are just trying to prolong the inevitable rebuild, set the screws at the maximum of the range.

 

Example:

 

Lots of carbs use a range from three quarters of a turn to one and one half turns. So the range is three quarters of a turn. Note the beginning of the range is also three quarter of a turn. Each third would then be 1/4 turn

 

So with this example: B above beginning of range 3/4 turn plus 2/3 of the range (2 times 1/4 or 1/2) so the setting would be 1 1/4 turns

C above beginning of the range 3/4 of a turn plus 1/3 of the range (1/4 turn) so the setting would be 1 turn.

D above the maximum (from our factory range is 1 1/2 turns) so that is what I would use.

 

Caviat: Do not automatically assume that ALL carburetors use this range!

 

A large majority of the carbs 1967 and before used an idle mixture control screw with a large angle and short taper. The example range is much more likely with this type of screw.

 

In 1968, Federal smog emission became effective. To acquire a finer idle mixture setting, the idle mixture screws (for the most part) suddenly were redesigned with a smaller angle and much longer taper. It is not unusual for the range on these screws to be 1 1/2 turn to 3 1/2 turn or even more.

 

Comments:

 

Setting the idle mixture screws out further than the range will have no effect on the mixture above that at the maximum other than in the mind of the adjusting individual.

 

Setting (and leaving this setting) for the highest vacuum will virtually guarantee a hesitation from a stop sign if the vehicle is equipped with an automatic transmission. WHY? Vacuum is measured beneath the throttle valve. The highest vacuum will be acquired when the throttle plate is completely closed (no signal to the idle transition circuit) and 100 percent of the idle fuel is coming through the lower idle port. As there is no air velocity past the throttle plate(s), puddling occurs in the intake manifold. When the throttle is opened to accelerate, the air velocity sweeps the puddles into the cylinders causing a RICH hesitation, followed immediately by a LEAN hesitation because there is no fuel available from the idle transition circuit, and the fuel from the accelerator pump is a few milli-seconds from being available.

 

If you wish to "tweak" from this setting go ahead, but you are going to be very close on pre-1968 vehicles.

 

Multiple carburetor setups (solid linkage) will have a range which is closer to zero as the beginning point of the range.

 

Multiple carburetor setups with progressive linkage - use the factory settings. I really cannot fathom why anyone would consider using progressive linkage on anything but a numbers-matching show car, so no need to discuss trying to make an aftermarket setup work.

 

Jon.

Edited January 2, 2020 by carbking (see edit history)

[valk]

Wow. Thanks Jon.  This will go into my reference "library". Thanks so much for your thoughtful response. 

Peter

 

 

[valk]

Here's a pic of my just painted intake. Now on to installing the carbs...

 

I'm still curious whether or not the idle screws on each carb should be turned out the same amount. Jon's method suggests they should be...

 

 

 

[pont35cpe]

Peter, once you get both primary carbs on, you may need a Uni-Sync tool to get the throttle valves set. Maybe someone will speak on this procedure and if it`s necessary.

[Matt Harwood]

I used the Uni-Syn on my carbs and found it very useful. What my ear told me was pretty close turned out to be WAY off. The device is cheap, easy to use, and ingenious. It really made a difference.

 

 

[valk]

Very cool. Thanks Tom and Matt, I'll get me one of those. 

I sync my Honda Valkyrie carbs, all 6 of them, with an octopus-like vacuum gauge...

[valk]

My linkage rods came today so I've been fiddling with them. You guys were right in that the long rod is barely long enough to work.  I maxed out the rod's adjustable length and I still have to move the accelerator linkage about an inch to make it work. Hope that's ok...Matt, I noticed on your front carb that you mounted both rods on the outside of the carb linkage. I was going to mount one rod on the outside, and the other on the inside as it seems to line up better that way. Am I off base??

 

[Lawrence Helfand]

FYI Peter I ran my bolt in from the backside and didnt use any washers as it didnt really need any. I must have buried less thread as I didnt need and inch to reach the arm hole. I will measure the length of my rod and let you know. As its a straight push action there is very little stress on the juncture. Looks like your about done! 

[valk]

I am a little worried about the length of the longer rod. As I said, I've maxed out the rod length and still need to push the accelerator pedal down about an inch to make it work. I might look for a longer rod and cut it to fit.  Folks have said it is 33' long which isn't even close. Might be 26'' but that''s it.  Everything else is going as planned..

[valk]

I cannot find any linkage rods over 24 inches long.  If the Summit Racing rod doesn't work, I may have to fashion my own linkage rod by using  1/4 aluminum stock and either threading the ends and using different hardware (if I can find it), or drill/tapping holes in the ends and using the Summit Racing hardware. The Summit Racing rod may work as it has for others, but , again, the accelerator pedal has to be pushed down some to make it work. I'd rather not do that if I can avoid it. 

[Lawrence Helfand]

39 minutes ago, valk said:

I cannot find any linkage rods over 24 inches long.  If the Summit Racing rod doesn't work, I may have to fashion my own linkage rod by using  1/4 aluminum stock and either threading the ends and using different hardware (if I can find it), or drill/tapping holes in the ends and using the Summit Racing hardware. The Summit Racing rod may work as it has for others, but , again, the accelerator pedal has to be pushed down some to make it work. I'd rather not do that if I can avoid it. 

I think you are worrying too much about the rod length and creating a problem that does not exist. I have been driving my Century with this setup for two years now without any issues. Matt too has not had any issue with it either. If you need to lengthen it to feel better just cut it in half  thread the ends and insert a length of threaded rod or bolt. The loss of an inch of pedal travel means absolutely nothing and undetectable when driving it. I think you could also back out of the ends a little as good purchase does not require much of an overlap.  Finish the installation and drive it and your worries will disappear. 

[Lawrence Helfand]

Peter I measured the length of my rod center to center and its 26 5/8 inch. I set it to that length over 2 years ago so you might find you can safety extend yours a bit more.   

[valk]

Thanks so much Lawrence. The range of my acceleration pedal is cut in half when I install the rod which might work but sure feels different.  I'll try extending the ends again and give it a try.  Otherwise I like your idea of cutting it in half, threading the ends and installing a spacer of some kind if I have to.  Wonder if the linkage arm on your carbs  are slightly different than mine.  The other rod has been cut, tapped and works great. 

[pont35cpe]

On 1/3/2020 at 12:06 PM, Matt Harwood said:

I used the Uni-Syn on my carbs and found it very useful. What my ear told me was pretty close turned out to be WAY off. The device is cheap, easy to use, and ingenious. It really made a difference.

 

 

Matt, do you have 2 Uni-Sync tools or just one to set the throttle valves. Also nice headers, homemade? And is that the original weighted flapper spacer under the rear carb?

[carbking]

One is all that is required, move it from carb to carb, and back again as necessary.

 

Jon.

[Lawrence Helfand]

No Peter our carb arms are identical. Just install it and see how it drives and if you still think its weird put lengthen it. You couldn't possibly be using up half the travel. When you drive it the linkage feels exactly the same. 

[Matt Harwood]

59 minutes ago, pont35cpe said:

Matt, do you have 2 Uni-Sync tools or just one to set the throttle valves. Also nice headers, homemade? And is that the original weighted flapper spacer under the rear carb?

 

Made them myself:

 

That's the original flapper, but the flapper has been removed. It's just a spacer now. There are details in that same thread, which is the entirety of the work I've done so far on my Limited.

[valk]

Ok I'll give it a go tommorrow. Everything is hooked back up, pre-flight check list completed and she's ready to fire up. I rigged a choke heat tube from a section of brake line that probably works but ultimately I want to come up with a more elegant solution. I also still have to "alter" the air cleaner slightly so it seats properly on the back carb.  I'll get back to ya....can't thank you guys enough for your help. 

[Larry Schramm]

On ‎1‎/‎3‎/‎2020 at 1:06 PM, Matt Harwood said:

I used the Uni-Syn on my carbs and found it very useful. What my ear told me was pretty close turned out to be WAY off. The device is cheap, easy to use, and ingenious. It really made a difference.

 

 

 

I have one of those.  I used it to balance the carb on my Corvairs years ago.  Works good.

[valk]

FFFuuuuudge. Ignition on, pedal depressed, no juice to the starter.  So I moved the wires from the front carb switch to the rear carb switch and she engaged so I guess the switch on the front carb, the one I just got, is bad. Of course, it worked last week when I stuck it on but not now.   Which is easier, rotating the carbs I just put on or swapping out the starter switches??  And on my birthday too....and its snowing like hell here...should have stayed in bed. 

[neil morse]

A visit from Murphy on your birthday!  I have only a rudimentary understanding of what you're doing here and the accelerator-activated starter on my car has been disabled, so this may be completely off base.  But is it possible that the linkage you have set up is no longer fully opening the front carb when the pedal is depressed and therefore not activating the starter switch on the front carb?  I'm only suggesting this because, despite Murphy's Law, it seems unlikely that the switch on the front carb would have failed after it was working last week.

[neil morse]

A visit from Murphy on your birthday!  I have only a rudimentary understanding of what you're doing here and the accelerator-activated starter on my car has been disabled, so this may be completely off base.  But is it possible that the linkage you have set up is no longer fully opening the front carb when the pedal is depressed and therefore not activating the starter switch on the front carb?  I'm only suggesting this because, despite Murphy's Law, it seems unlikely that the switch on the front carb would have failed after it was working last week.

[valk]

Thanks Neil. I agree it's very weird the switch would work one day and not the next.  I'll fiddle with it today and swap carbs if I can't get the front switch to work. I'm on my own here...

[Lawrence Helfand]

1 hour ago, valk said:

Thanks Neil. I agree it's very weird the switch would work one day and not the next.  I'll fiddle with it today and swap carbs if I can't get the front switch to work. I'm on my own here...

That switch is a simple device with a plunger spring and ball. I would give it a few taps first but it might be sticky or have some grime on the contacts. It is very easy to take apart and clean and does not use a diaphragm or any delicate parts that will be harmed. You can also swap the parts from on carb to the other. Neils advice regarding linkage is irrelevant as long as you can actuate the floor switch which Im sure you can. The Carter switch I find works better then the Stromberg switch and its not rocket science to open it up.   

[pont35cpe]

Peter, the copper contacts in the switch could be the problem. It would be easier to remove the one hold down screw of each switch and swap them. I can send one if you need it. Tom

[valk]

Thanks again Lawrence and Tom for helping me out. I'll try removing the switch and cleaning the contacts. If that doesn't work, I'll swap switches and take it from there. I'm so damn close...until some thing else pops up...

Peter

[Matt Harwood]

You can also use a jumper wire across the terminals on the switch to trigger the starter manually if you're just testing and tuning. I've done that in a pinch when the car wasn't behaving or I was alone.

Did you install the carburetor you bought on the front or back? It sounds like you put the new carb on the front and moved your original one to the rear position, correct? Perhaps swap them back and all will be well?

[valk]

Just removed both switchs (didn't know it was so easy), cleaned them up and reinstalled the front switch. Still no dice. I jumped the terminals and she fired. I then removed the switch and looked down the switch cavity and observed the ball in the bottom while activating the throttle and there was no movement. For the switch to work, something has to push the ball, which in turn pushes the contacts so they align and make a complete circuit but, again, nothing is pushing the ball when the gas pedal is depressed.  Nothing happens in the switch cylinder when the throttle is engaged.  I'm going to try the other switch to test this theory - the other switch will most likely not work either since nothing is pushing the contacts to align.  So I think both switches work fine. My problem is in the carb itself. The other option is to swap carbs so my working start circuit is in the front but it bugs me I can't get the switch to work. As always, I could be full of it but this is what I've observed.  Your ideas are most welcome.