IDF tuning, glowing pipes.

[carlmb]

I am in the process of tuning my 48 IDF's on a small block ford. At idle it appears I am getting a lean misfire as some of the pipes are actually glowing red and the engine will not run smoothly. If I adjust the idle mixtures out beyond 1 full turn it does not seem to make a difference. Any thoughts? Thanks.

[Bryan Wilson]

Check engine timing may be to far retarded. I have had this problem and running more advance cured it.
Cheers,
Bryan

[carlmb]

I was wondering if it was timing but my balancer is showing 10 degrees advanced at idle. Maybe my balancer slipped. I'll have to check that out. Thanks.

[ENTDOC]

assuming the timing is ok, I would think your idle jet is too small. Also make sure you have the carbs balanced against each other, if you do not, everything else will be wrong

[68gt500]

Depending on your camshaft you could need much more advance, I would try up to 16 degrees.

Keep advancing it until the idle smooths out.

Just make sure not to exceed 36 degrees at full advance.

Alternatively, if you have a vacuum advance hook it up to manifold vaccum, that will give you more advance at idle.

Spotting the faulty cylinder is pretty easy if you use an infrared thermometer to check exh. temperatures.

The rule of thumb is opening the idle screws pretty close to 1 turn - if you need more or less that´ll indicate too small or too big an idle jet.

[carlmb]

I was able to balance the carbs and they are all matched up exactly. I did also use an infra thermometer and some cylinders were at 400 degrees and others were as high as 600. I think the timing is the answer and I will know better after the weekend. My first step is to bring the engine to top dead and make sure my balancer is accurate. I will also try more advance. Thanks for all the input.

[Caprimaniac]

I post this here because it, simply said, fit under the threads theme....

As someone may have read elsewhere, I have been really frustrated on the behaviour of my 347 stroker with 48 IDF.

This weekend I went to a meeting some 200 km away. The night before I had done some adjustments to the engine. After readin a post in this forum I adjusted all my idel srews out 1 1/4" turn after listning to someone saying "1 turn maximum. If not enough, too small jets." Cruising along it drank 1,8 L/ 10 km and i noticed som popping at around 2- 3000 rpm....

As I started my car on saturday morning to go down to the display-erea, my carbs were popping alot.

This made me wonder: This cannot be haow it's supposed to be. Let's make another approach, as learned by reading Ceslow's writings:

http://www.gt40s.com/images/howto/Ly...carburetor.pdf

I used my fingers and tapped each header tube; 6 were warm (Ok, I got a small firewound at one of my fingers...) last two stone cold. Fiddled the two idle screws and whhops: One of themm needed yet one more turn out before things started to happen The other one needed 1/2 a turn inward....

What happened then? Popping stopped, drive "home" was pleasant.

"Home" this sunday was on Gardermoen Raceway & Autograss/ free riding.

When testing the car on full throttle, I was totally paralyzed......
Not in the engine's history have it run that well. I took it to 8200 RPM, and Mother Mary and Joseph; what a wonderful sound that motor made... Not to mention the power (although not that "physical" with non- sticky road tires...) Cannot wait to go racing this weekend.

I now will tune the idle with an IR thermometer and do one header tube/ cylinder/ carburator throat/ idle screw at a time.

As it's hard to get a correct A/F reading at idle, my question is like this: Is there such a thing as a temperature- area that will be correct for a "reasonable" (a little bit rich as it gives some cooling?) idle- AF? (Headers are 15/8" and wrapped.)

Any input greatly appreciated.

RuneS.

[ceslaw]

Aircraft often have EGT (Exhaust Gas Temp) indicators to help adjust the fuel mixture to compensate for changed in altitude. A simple gauge may have only one cylinder temp indicator while others have a series of 'bar graphs' giving an indication for each cylinder.

So it seems reasonable that using an infrared temp indicator would be useful in tuning the Webers. It won't be as accurate at an EGT, but may give a good relative indicator.

My modest 302 (375 hp) headers typically run around 250 degrees at idle. But when one cylinder was running 350 - 400 degrees I turned out the mixture screw a quarter turn to make it a bit more rich. The occasional popping immediately diminished. This is certainly not as accurate as O2 sensors, but it can be useful in my judgment.

What is the ideal temperature? I doubt there is a single answer. There are too many variables based on engine design. But as a relative indicator to let one know if a cylinder is too far off it may be useful.

I see not point in trying to get the temps closer than 50 - 75 degrees. It is just not accurate enough. But if one cylinder is running 30 % higher than the others, it is likely too lean.

I am curious as to the experience of others using this technique.

[Caprimaniac]

Hi, Ceslaw.

Are you talking degrees Celcius or Farenheit??
If C, we are very close.

1 x EGT or 02/ A/F- cencors in each runner would be the ultimate......

These are the results this far:

4- 5 cylinders showing 260- 270 deg C (5. has idle/CO screw out by 4 turns..)
1 running 165 deg C
2last show 80 deg C and clearly not firing.

For the two cold ones I have done all kind of testing; cleaning carb filter & idle jets. Checked fuel delivery and changed spark plugs.
The spark wires or terminals in dist. cap has to be dead? (I took it for granted this was 100% since everything worked so well before parking in the garage...)

The 5. with all those rounds out + the 165 C will be checked for leaks, theese are the same cylinders that showed vacum readings out of range, as well.

( To rule out another possible fault, I will check the valvetrain....)

So far, and brain boiling from frustration!

[Excaliber]

10 degrees aint squat, I bet you need at least 16 (as stated above), or more!

John Bishop’s Hot Rod Tuning Shop:

The basic rule of carburetor tuning is Ignition First.
Once the ignition advancing mechanism is correct, the air/fuelmixture can be tuned for improved power or fuel efficiency.

[ceslaw]

Caprimaniac:

Temps are F, not C. But use the temps as a relative indication, not an absolute. Different engines will have substantial differences in 'normal' operating temps for the exhaust gasses.

I assume timing is right, getting spark to all the plugs, and only issue is carbs.

Are the two non-firing cylinders on the same carb? If so check the air flow to make sure it is flowing the same as the other carbs. You might try to open the idle stop screw on the problem carb just slightly more than the other three to see if that will get it to fire. That would tell you it is indeed a carb issue.

Of course you need to do all the preliminary checks: linkage working properly, floats properly set, clean idle circuit, etc.

Experiment with the mixture screws. There is a 'sweat spot.' As you back it out you may find it suddenly firing - back it out a bit more and then fine tune it.

I don't suscribe to the notion that all eight mixture screws must be exactly the same and less than a single turn out. Get it running right, and if one is out a half turn or three fourths of a turn more than another don't worry about it. Of course if all the mixture are more than a two or three turns out you lilely need a larger idle jet.

At this stage the temp indicator will tell you whether it is firing or not. Don't rely upon the temp readings as you primary means of adjustment.

Good luck. Patience . . . . .

[Caprimaniac]

Ceslaw, I agree on everything you say here.

One more questions on your numbers: are the headers coated in some way? Chrome?

I use only one idle screw, and the link to regulate the ones on the pass. side. Have read a post or two about using two of those, might try this...

After looking at the problem yesterday, I cleaned out the idle passages, changed some O-rings, checked the wires , cap & rotor. Will do more testing today, and hopefully get it running better & ready for the weekend.

[Caprimaniac]

Quick update!

Got all 8 cylinders up to temp (180- 240 C, wrapped 15/8" primaries). 2 years since I last cleaned out the idle- runners in the carbs? Clearly, this is something one has to take care of every now and then.

Will try to get same temp on all 8, but might be reading differences due to varying thickness of wrapping.

Excaliber: I'm not the one running 10 at idle; 14 at idle, 38 at 3000 or something. Mech. only; 24 deg.

RS

[Excaliber]

14 and 38 sound about right.

[Caprimaniac]

Hi.

Have been going from 10, via 12 to 14 deg. Haven't noticed any detonation yet...

RS

[ceslaw]

Caprimaniac:

My headers are stainless, no covering / coating.

Try using two idle stop screws on the two primary carb connected to the bell crank. It may help with the left / right syncronization. Linkage, linkage, linkage.

Sounds like you are making progress. Good luck.