Jetting for Dual Carbs

Bobby:  I tried wading back through your (and other's) posts but got lost.  So, backing up, if it pops through the carb coming off idle to a slightly faster speed it is lean.  Try turning your mixture screws in (one at a time) til it stumbles, then backing your mixture screws out 1/4 - 1/2 turn and see if that cures it - I would go as much as 1 to 1-1/2 turns if it needs it.  I can't remember engine or carb size, but I've seen old Solex carbs that needed to go out 2-ish turns above stumble to run right.  If that doesn't help, go up one size on your idle jets and start over with the mixture screws.  Increase your jets, say, from 45 to 50 - BTW:  a 45 idle jet, if that's what's in there now, sounds kinda small and may be the culprit).

Next step after that if, it doesn't improve, is to check the output of the accelerator pumps.  On Dells, the pump adjustment is two-fold:  Changing the accel. pump jets or adjusting the pump actuator lever.  I assume Webers are the same.  Adjust the pump actuator for more stroke/more fuel and see if that helps.  If not, try going up 1-2 jet sizes for the accel. pumps

I do not set my Magnaspark advance at idle - with a centrifugal advance disti the idle setting is sort-of meaningless.  Remember that centrifugal advance distributors were designed for commercial engines running at a single speed, usually 3000 rpm, all the time, like running a pump or generator.  That's why most of them have a curve that looks like a step function.  To set advance, I run the engine at 3,000 rpm and set the maximum advance to just below the detonation level for the gas I use, then back off 2-4 degrees if I'm planning a trip and don't know what gas I'll find.  I went out and checked (just for the hell of it) and this is what I'm running:

Engine RPM       Degrees Advance

1,000                            20º

1,500                             20º

1,750                              28º

2,000                              30º

2,250                              36º

2,500                              38º

3,000 and above           38º

Before everyone goes nuts about the bottom and top limits, it runs fine for me and my situation - Running on "89" Octane pump gas, it never knocks on hard pulls so I feel it is OK - if it knocked I could either run "high test" or simply run it back 2-4 degrees.  PLUS I have never mechanically checked #1 top dead center with a probe through the plug hole to see if my degree wheel crankshaft pulley is even remotely accurate.  The aftermarket ones are notorious for being weird.  If it is off, God knows what I'm really running........  I suppose that I could keep advancing it til it does knock on acceleration and back off from there - THAT might prove interesting, too, as at least I'll find the knock limit for THIS engine with THIS degree wheel - other engine combos (compression ratio, cam, etc.) will probably be different.

BTW:  IIRC, Tomlinson's book has a section on trouble-shooting different scenarios.  Might be worth a look.  He'll probably mention possible vacuum leaks, but if it idles smoothly without hunting (up and down rpms around a mean) then I wouldn't look for leaks.

I'll say it again, because it bears repeating.

On Weber carbs or copies, if you are more than 1.5 turns out on idle mixture volume screws, you have the wrong jets (too small). If less than one turn, too big a jet.

Bobby, if you very, very slowly open the throttle, does it still spit and sputter?

If you stab it, you're dealing with Accel pumps, the very last adjustment when tuning carbs.

FYI, you will never be able to get a final jetting on an engine stand. Sorry.

Also, on the wideband rental/borrow thing.

I don't see this working at all. You need a bung in the exhaust, or some sniffer tube setup that allows getting into the exhaust flow. It can't be too close to the end.

And you need to know how to interpret the readings. Not simple if you've not done it before.

Not enough timing can also cause the spit/sputter off idle you're experiencing. Maybe not the carbs.

I messed with the mixture screws last night and may have solved the popping.  I turned the screws out a little more and got the popping in the carbs to stop.  So I might have been too lean.  I will go back out today and see if the popping comes back.  I should get new 150 main jets on Thursday to replace the 155's that are in there now.  I know overall the car was running rich by the looks of the cylinder heads and spark plugs.  I hope going down one step in the mains will fix it.  If not, I have 145's in a set of 48 IDF Weber's sitting on a shelf.  Am I still good running 28 to 30 degrees of total advance at 9 to 1 compression?

Re timing, yes, total advance is good. My motor tolerates 32 at 3000 without detonation. You may need more advance right off idle. I know you're not running an 009, that's a notorious fault of that particular distributor that is almost always attributed to the carbs.

As far as tweaking the carbs, the motor needs to be fully warmed up by driving, an engine stand doesn't really do it. 

Another pet peeve of mine is an arbitrary number of turns with respect to mixture setting. Each cylinder requires it's own setting. Mine vary by as much as 3/8 turn from lowest to highest. Best speed each cylinder, engine HOT.

Thanks Danny

I know I will have to tweak things once the motor is in the car.  But I want to be close with the jettings and adjustments before hand.  As I have mentioned before, we had an overhead issue which resulted in breaking the motor all the way down.  So one thing I am testing is that the heads and valves are ok.  I don't want to get in the car and find out there is something deeper problem other than just carb adjustments.  Although my son and I are getting pretty good at pulling the motor out.  I can get it out in 45 minutes, but it is a PITA.  Putting it back in seems to take significantly longer.

Popping could also be the linkage not opening both carbs at the same time.

Success!   Finally got it dialed in on the stand with no carb or exhaust popping.  I took both carbs off and cleaned them.  The driver side carb had some junk in the bowl.  John at Aircooled.net recommended 55 idle jets and 145 mains.  I Went through all steps again and she seems to be good.  I have started it up twice now.  After letting it warm up, I gave it some revs up to 3,500 RPM and all seems good.  It is idling a little high, around 850, but I can adjust that again.  I am sure there will be some tweaks needed after I drive it.  My wife can't wait to get her little speedster back.  I am going to try to get the motor in the car Friday so she can drive it a little bit before we leave for Saturday afternoon.

Thank you guys for all your help.  I appreciated everyone's input.

An idle of 850 is not high for these cars.  I have mine set at 1000.  Congrats on getting things working properly.

Mine is around there as well, I think 900 once warm. 800 to 1000 is good.

Over on TheSamba, there's a near 100 page long thread about tuning carbs. It's as painful a read as it sounds, and the further in you go, the worse it gets. Mark Harney posted there before he moved on to other things, as did John Connolly. Both strongly advocated (as I did here) for a wideband.

There's a guy I've referenced before named Glen Urban who has a way of putting things succinctly that I wish I had. He summed up a billion page thread into one very easy to follow statement on carb tuning. He said (bold highlights are mine), 

"You can make the idle jets as small as your engine will tolerate, no problem with that. 

How LEAN it can handle......really varies, depending on the engine, compression, heads, distributor, carbs....even the muffler.  Some people want to go lean and find out their heap won't do it, well, tough luck. Others probably could, but they don't want to. Grass is always greener. 

Once on the mains, then you want to be 12.5-13 

Widebands are great, and all that.... but for choosing idle jets, and idle mix, it's does not change what is possible. You still basically just try leaner until it falters then go back a little, which has been the working procedure for over a thousand years.  

The idle jet should be called the "cruise jet", it would make more sense if it was."

... and there it is, in a nutshell...

Uh oh, the price of admission is even lower! No excuses left, I would say.

https://www.wide-band.com/product-p/apsx_d2n.htm

No excuses?  

Well, ummmmm.......  I'm saving for a Disney Cruise?