Stumbling

If you have a banjo fitting on the 3-4 carb which then passes fuel to the 1-2 carb, the fuel is not filtered for 1-2 in the 3-4 banjo - it just passes through.

There is a tiny cylindrical filter in the 3-4 banjo fitting that filters fuel into the 3-4 carb.  It is a VERY fine screen and could be crude up or easily distorted during assembly and choke off fuel (ask me how I know), thereby starving just the 3-4 carb while 1-2 runs fine.  I would start there.  Sounds a bit like fuel starvation to me. Unusual on just one side, so I would start looking at the fuel inlet on the 3-4 and probe inward from there.  Pop the top off of the carb and see if there isn't some flakes of crud in the bowl.  Whatever it is, it's not affecting the idle circuit - only when it calls for lots of fuel.

Check to see if your gas tank is venting correctly.  Run with out the gas cap and see if it still does it.  Second, check to see if the fuel pump is working correctly.  We had one do the same thing and it was a bad fuel pump.  Another one we had it was a cap that was not venting. 

Carlos, this may be stupid, but check your firing order is 1-4-3-2. It's possible you swapped 3 and 4, it will run, but run like $hit.

And pop the distributor cap and make certain the ignition module screw is TIGHT and very clean underneath(as in making good electrical contact).

Carlos-This will be very interesting to hear what you find out.

Carlos...I keep a handy note book at my computer dedicated to information from this forum. It's comprised of hints, tips, addresses, part numbers, and most importantly, diagnosed and 'resolved solutions' to problems and questions from members.  In the back I've penciled in this comforting, philosophical, piece of wisdom  from a member (I've forgotten who, but it's dated "11/29/14")

"There will be uncertainty, then doubt, then quiet determination to find the cause...but that all serves a purpose. It makes truly rare and memorable those perfect days on a deserted country road when the sun is shining and the birds are singing, and when the temperature gauge never drifts above half way...It's conquering the demons that lets us better savor the good times."   

Oh there's lots of uncertainty and doubt. When I get some time here  soon, I'm going to have to open a 55 gallon drum of determination and get this thing figured out.

I did check the firing order, Danny. All good. Sometimes it's those simple things.

Carl:  Sounds suspiciously like the musing of Sacto Mitch.......

So I got some results today. I went for a drive and everything worked awesome.

This is what I did. I usually only change one variable at once to pin point what exactly was wrong, but I just did it all. What the hell, it can't run worse....

-I checked the valves the other day.

-I rechecked the floats, did some minor tweaking. Set at 10mm and 25mm

-Checked the carb filters. Minimal debris, not enough to restrict.

-Replaced the 65 idle jets with 50s. I had them in stock and the plugs were all sooty, so it was running a tad rich.

-Cleaned the new plugs and rechecked the gap .70mm.

-Refastened the ignition module

-Cleaned up the rotor and cap contacts with some fine sandpaper. It is a non resistored rotor BTW.

-Pulled out a copy of Danny's Carb Write up and went through it step by step.

Now, when I drive up the hill in 2nd gear running about 2000-2500 on the tach, to get to my place, there is no stumble at all, like there used to be even when I thought it was running good. There is an ever so slight hesitation going through the transition when I give it some gas, but it's so much smoother, I hate to mess with it. The timing at about 28 degrees is even less sporadic looking with the timing light.

I figured the Idles were too rich fouling the plugs, and my linkage was out of whack making the carbs unbalanced when the pedal was moved.

I'll try and dial it in some more, but it's running damn good. I'll start it up in a day or so to see if this just wasn't some kind of dream. I will check the plugs too and see if I can dial in the idles more with that info.

Oh happy freakin day

Carlos

Congrats! 

Good news, Carlos.

Drive it like you stole it.....

Good for you, Carlos, I hope it stays running well. To get rid of that transition stumble, I would try .55, .57(they are actually .575), and .60 idle jets. They are cheap and you can always sell the ones you don't need after you dial it in. I've got a set of .60s, and I think I gave away my .55s. I run .57 in my 2165.

Sorry for the very late reply.

I totally agree with you Danny. I was going to try a 55 next. Here is a shot of my plugs day before yesterday.

1 was at 2 turns out on the idle screw and the rest were 1.75 turns out. I turned 1 in a quarter turn.

I went out for a long drive today and that slight hesitation was around 2000 rpms, but no back fires. It did give a burp through the carbs during startup this morning. At least that's what it sounded like.

I'll order up the next two sizes and tweak some more. I'll check the plugs before and after.

Thanks

CG

I have an almost identical engine to Carlos' 2276 and mine also suffers from a bit of a stumble.

With mine the engine runs fine when cold, but once it warms up I notice a slight stumble at part throttle.  At full throttle there isn't a stumble.

Any idea why it would run fine when cold, but have a slight stumble when warm?

Your original jetting is way off from what I have done on similar engines. You should be running a .60 idle, .200 air correction and a 140-145 main jet. Depending on the compression/cam/fuel being used, I would try bumping your max timing up to 30 and possibly 32 degrees. The .050 idle jet you tried is too small, you will have a hesitation in the transition area from idle to the main circuit with such a small idle jet

  Your issue to me sounds like the carb linkage is out of synch/one carb is leading the other. Also, Seeing how your jetting is so far off, I would throw a brand new set of plugs in it. I would also check voltage to your ignition coil. A weak spark will result from low voltage creating a loss in power and fouled plugs.

Also, To the guys having hesitation issues with dual carbs. A smaller venturi or "choke" will take care of the hesitation issue. Smaller venturies help to signal the booster venturi in the carburetor quicker  by creating more air speed through the carb at lower engine rpm's. It's a give and take though, smaller venturies speed up air through the carb but they also limit air through the carb resulting in a loss of power. The size of the venture will dictate how much power will be lost. On all of my 1915cc to 2110cc engines, I like to use a 32mm venturi. On bigger engines I will move up to a 34mm and in some situations, I will leave the stock 36mm venturi in the carb. This is all based on the 44 IDF Weber carb BTW, but the same philosophy goes for any dual throat carb.

Pat thanks for being part of this list and giving us guys good input. 

Ray 

Thanks for the reply Pat.

This is what my engine consists of:

2276 Type I, 44 Webers w/36mm vents, 50 idle (now) 150 mains 220 air F11 tube, 009 Distributer (clone?) w/compufire, NGK D7EA plugs set @ .70, 40mm Intake 35.5mm exhaust valves, FK 8 cam, 8.86/1 compression, 1.4/1 rocker ration, I run 93 octane corn free, and drive at between 2000'-3000'+ of elevation.

My linkage was out of whack, but that was one of the problems I fixed. I am going to order up some more jets and tweak some more.

Your engine specs are very typical to what I build for Vintage Spyders. Your jetting sounds close but the .50 idle sounds too small. I would move up to a .55 idle and drop the main down to a .140 or a .145. Also, at 2000-3000 feet, you should be running more ignition timing than you are. Bump your timing up to 33 total advance. This will richen up the AFR and allow for smaller jetting, making the engine more efficient.  I like the .220 air correction for your elevation.

I put in some .55s and took a few drives. No hesitation at all. I'm going to clean the plugs and recheck them after another run. I might order up a couple of mains just to tweak some more. I did bump the timing up to about 30, but haven't gone for a run yet. Once I decide that this is the setup, I'll go through the carb setup procedure from step one again to make sure everything is right. 

Carlos, don't bump your timing up beyond 30 if you're coming to Carlisle, much lower elevation......

Carry on

Good tip.

Unfortunately, I'm not going to Carlisle, but I'd love to meet all youz guys at some point in the future. I don't feel comfortable driving my car longer distances as of yet. I still need to sort a couple of things first. I do feel like I'm getting a good handle on the carbs though, thanks to you.

Happy car.

It takes a while to get the confidence up when you first venture out in these cars (any custom car, really).  I remember starting out driving around town, then a quick spurt into the next town an back, then, since nothing had fallen off or died in those short trips, I took a big leap and went four towns over to a cruise night.  I also alerted my son before I left so he had his trailer ready.  

Nothing happened.....In any of those trips, or many, many trips afterward, but it takes a while to believe that things are OK.

In the meantime, enjoy the rides through the mountains, Carlos, and keep stretching the miles out!