Carb Mixture Screw Tuning

Also I am running the wrong jets.  2276 with Panchito heads 40x35.5.  44 idf with 36 venturis,135 main, F11 emulsion, 200 air correction.  The car seem like it should pull harder.

Bobby are you running a single 44idf or duel?

either way you should have an idle screw per corroborator barrel. I personally like to set the idle as recommended by the the Bob Book. Then back out an additional 1/4 turn. this will help with idle stability at different operating temps.  if you keep it at Lean best/fast idle it will change significantly based on the weather and running temp of the engine.

you should be able to hear the RPMs, one key thing to remember is tun a little then wait for the engine to stabilize.. 5-10 sec then turn some more. go slow .... letting the adjustments impacts take effect before turning more. do this for each throat. then start over. you will find that after a couple of cycles you have it nailed down.. 

Thanks GomerP

That car does not idle at first startup.  It needs to warm up.  I will follow the Bob T book.  Hopefully it will fix the idle.

I used to hook up a tach/dwell on low range to easily see the rpm changes, but now just listen to the engine.

Lean best idle means highest speed but not lean/not rich. Think Goldilocks. I used to richen a bit but have evolved into a crispy fall/early spring setting OR a hot summer setting at lean best idle.

If you set HOT idle at just under 1000(950 is good), the car will idle when cold after about 20-30 seconds.

Danny

Thanks

I agree with both Danny and Gomer, but you'll be idling on the idle jets and all you've mentioned for sizes installed were the mains (which, at 135s sound small for a 2276 engine - I would think something closer to a 155 or 160 main jet for that).  

I usually just listen to the engine, but if you have a stable ignition system and a decent tach, then using Gomer's/Bob's method of watching idle speed should be just fine.

Remember, too, that any "pulling" you do under 60mph or 3K rpm or so will be on the idle jets, NOT the mains, which begin to come on around 3K - 3.2K (you should be able to feel it).  Getting your idle jets and mixture straight first is paramount, then mess with the main jets.

Gordon

I will adjust the idle mixture screws using a timing light with a built in RPM setting.  When I said pull,I meant when driving spirited and pushing up to 6K RPM.  There doesn't seem to be any popping under hard acceleration.   But from everything I read, it seem like I should have a larger Main jet than 135.  Maybe the 135 is a good match with the 200 air correction. 

I am running with a Engle 120 cam with 1.25 rockers

I don't what to leave better performance on the table.

Regarding the "best lean idle", the car has to be up to operating temperature or you're just going to be chasing your tail.

Turn the adjustment screws in until the engine starts to stumble, then back out at least 1/2 turn. If it bucks and farts and sneezes etc. when cold, I give them all 1/4 turn out more. I'll keep going until it'll idle after a couple of minutes of warming up - but no more than a turn or a turn and a quarter from the stumble. If it takes more, I move up a size (or a half-size) in the idle jets.

Regarding the state of tune with your engine: 95% of all carburation problems are ignition. What's your total timing? Is the advance curve stable? Do you have scatter?

Every engine is different. The BEST way to know what it needs is a wideband A/F gauge. My 2165 is very happy with 1.40 main jets. If the heads are really efficient, and they are, a 1.35 might be perfect.

IMHO Panchitos are too small for a 2276. I'm quite sure you have plenty of low end torque, those heads are great for that. But they are a choke point, just like too small carbs or too small exhaust. For me, the max size would be a 2 liter. You're limiting power with those heads.

The engine isn't finished yet, but the CB heads I'm using on a 2276 build flow 30% more than Panchitos, yet retain a smallish intake tract to keep torque high. 30% is quite a bit more flow.

Stan

I am sitting at 32 degree max.

Danny what heads are you using fir 2276?  will you have a 44 or 48 IDFs?

Pat Downs seems to be using Panchito heads a lot on engines up to 2332. I’ve also noticed many of his combinations run a 1.25 rocker on aggressive cams that would normally use a 1.4 rocker.  Being that the Panchito’s don’t flow much more after .500” lift, there is no sense in stressing the valve springs.  These engines are making 170-180hp.

Most people running these type 1 motors in a true street car are not going to be turning the rpm to take advantage of a huge head.  Big valves are heavier, require heavier valve springs to control the valves, and stronger pushrods.

I have a Pat Downs 2165 CC motor...that is 6 cu inches smaller than you. I run your jetting, but I substitute a 175 air correction jet for your 200. My idle jet is a .50

The car, a Spyder, pulls from 1500 RPM (in 4th gear) on level ground to 6200 without any hiccups or drama !

ReV

Bobby, I'm using Super Pro 45 x 37.5. Chambers are flycut to 58cc. The cam and rockers(and dynamic compression ratio) I've chosen will work great with this combo. Webcam 86b has gradual ramps to work with 1.4 or 1.5 ratio rockers. The Berg rockers are 1.45:1, lift is 0.551" at the valve. The math works out and was also verified physically with a dial indicator on the valve retainer. Static CR is set to 9.8:1.  Also using CBs port-matched CNC manifolds. CNC porting ensures every cylinder has the exact same flow. Every part matters from carb all the way to the tailpipe, all must be chosen to work together.

What I'm saying is if you want to change the heads on your motor you may need to do more than just change the heads. A cam, lifter, rocker, and pushrod change may be needed along with a serious look at carbs and exhaust, and yes transmission gearing.

This motor will be running Spanish Weber 44 IDFs. No idea what jets it will want. Yet.

BobbyD: first get the idle mixture correct. Then you set WOT correct above 3000(some motors aren't fully on the mains until 3500) for the main jets. The air corrector jets, float height, fuel pressure, and emulsion tubes control the transition from idles to mains.

Danny

Thank you.  On your new motor what do you mean by "Dynamic compression Ratio"?  I assume it means the ratio changes.  How is that accomplished?

@Bobby D posted:

Danny

Thank you.  On your new motor what do you mean by "Dynamic compression Ratio"?  I assume it means the ratio changes.  How is that accomplished?

Valve overlap on the cam.

overlap between the intake and exhaust values, I assume.

Correct - and at what point relative to crankshaft rotation they open.

Stan

Thanks.  I learn something new every day.  The problem is I forget about 5  other things everyday.

On the IDF carb there is a lot that can be done with both the pump jet and the pump stroke !

ReV

On a 44 Weber IDF, the nut on the accelerator pump rod, what is the relation between how far in or out that nut is to the amount of gas pumped when hitting the accelerator pedal???

IIRC,  as you screw the nut in you get more fuel, sooner.   Ultimately, it is governed by the jet size.

Ideally you should take the carb off the car and test it on a bench. You need to first check the pump diaphragm to make sure it is not ripped/cracked etc. Assuming it is functional you need to have the float bowl full of some liquid and determine the time interval of the stroke i.e. the time period that fuel comes out of the pump orifice usually 1- 1.5 seconds.

The stroke is not linear !

Once you have it back on the car, I would adjust the nut gradually either in or out, being mindful of the time (1-1.5 sec) for reference.

The accelerator pump should add sufficient fuel to the mixture to eliminate flat spots during throttle roll-in.

It may be adding too much fuel so you need to be mindful of that as well...

ReV

I don’t know about the Weber side, but I suspect it is the same as with Dellorto carbs in that if you get a suitable syringe from your local pharmacy (tell them what you’re up to and they’ll give you one, gratis) and make a hanger for it from a piece of light gauge wire, you can hang it down in the throat of the carb and actually catch and measure how much fuel is coming out of the accelerator pump jet on each pedal stroke.  (Thank you @MusbJim for coming up with this terrific idea)

The Thomlinson books give you a good baseline for both Weber and Dellorto carbs (from different books) and you can adjust up or down from there.

Happy Tuning!

Gordon is correct, you can easily measure the volume without removing the carbs on Webers just like he does on Dells. Just be careful if the syringe is cheap plastic, the gasoline/alcohol will melt it.

Webers used to come with 50 pump valves, which bypass 50% of the gas back to the float bowl. Today, most come with 0 bypass, which means ALL the fuel gets pumped through the pump jets. I couldn't get the 0 bypass to work right, the 50% were the only ones that I could dial in.

You really don't need to measure it though. I start out with the minimum possible, and screw the nut in about 2 turns and retry. Keep going until it bogs, then back off until it stops the bog. Then leave them alone.

@Carlos G posted:

On a 44 Weber IDF, the nut on the accelerator pump rod, what is the relation between how far in or out that nut is to the amount of gas pumped when hitting the accelerator pedal???

When you screw the nut in, the stroke of the pump increases.

Check this link out for calculating the dynamic compression:

http://www.wallaceracing.com/dynamic-cr.php

A 2276 is 3.70 bore and 3.23 stroke in inches

Boost will be zero. 7.5 to 9.5 is a good range for dynamic compression, but I'd think the lower end is probably better, because of the air cooling and flat top pistons(both less tolerant of high compression).

My 2165 comes out at around 8.5:1 dynamic(it's either 8.49 or 8.52, can't remember my exact rod length), while having a static of 10.2:1.

As @Gordon Nichols mentioned, I would adjust the stroke volume of the accelerator pump using a 3cc syringe.

Long story short (no engineering data or molecular structure explanations), I needed to adjust the accelerator pump stroke-volume on my '57 1600 Super to the suggested 0.5cc (this was back in the early '70s). I also wanted to do this without removing the carburetor.

For this task, the book called for Porsche tool P25A, part # 000-721-025-10....

Not having access to a P25A, I dug through the Musbjim toolbox and fashioned a device similar to this...

Hung that bad boy down the carb under the accelerator pump outlet and made the appropriate adjustments for 0.5cc.....and VOILA, back on the road!

Great information.  Thank you