Skip to main content

Hey Guys

I have a question as to which method of setting the Carb idle mixture screw is correct.

Most of the things I have seen say to set at best lean setting. Turn screw in until the engine stumbles then turn out 1/4 to 1/2 turn.

The Bob Tomlin Weber book says something a little different.  Turn the screw in until the engine speed drops off.  Then rotate screw out until the picks up speed and runs at its maximum best.  I assume I would use a timing light to monitor the rpm increase with this method.

These are two totally different approaches.

Any thoughts?

Original Post

Replies sorted oldest to newest

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.. 

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.

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.

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.

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.

Last edited by DannyP

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

Attachments

Images (1)
  • 40 IDF Bottom

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.

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.

Last edited by DannyP

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.... Screen Shot 2021-05-11 at 7.44.14 PM

Not having access to a P25A, I dug through the Musbjim toolbox and fashioned a device similar to this...DSC02864

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!

Attachments

Images (2)
  • Screen Shot 2021-05-11 at 7.44.14 PM
  • DSC02864

Add Reply

Post Content
×
×
×
×
Link copied to your clipboard.
×
×