Weber Help!

[JJ914]

Need help, just installed 40 IDFs on my 1.7. Cranks ups good revs up good sound great till all the loud popping(afterfire) starts. Checked points, timing tried running them lean and rich still lots of popping. And I am having hard time tuning for best lean. Any ideas. The carbs came off a 2.0 from what I was told, maybe jetting trouble??

Thanks

[HarveyH]

I've copied my reply from a previous thread:

I have a 2056 with a ¾ cam, Pertronix, Crane HI-6 and PS91 coil. I?m running 40 IDF?s with 28 mm venturies, 45 idle jets, 1.15 mains, f11 emulsion tubes and 2.00 air correction jets. There are several good Weber manuals around, read one or more of them and understand what is going on with carbs. Aircooled.net also has a tech write-up on tuning Webers. Remember that the jet recommendations in the manuals are just starting points, not cast-in-stone requirements.

First off, tuning carbs is all a very inexact science, and can take a lot of trial and error to get things exactly right. Seasonal (and sometimes daily) changes in the weather can affect your carb tuning. The generally accepted theory is that with a constant speed, load, and temperature situation, a carb will provide a better fuel/air mixture than FI. Obviously this does not define the operational characteristics of an automobile. If you change the speed, load, or temperature, a FI system can provide better control of the mixture. If you have a stock engine, it may be better to go back to the FI system and do what is needed to get it operating correctly.

Weber jet tuning for IDF?s breaks down into three basic ranges. Idle to about 3000 RPM is controlled by the idle jets. About 2600 RPM to maximum RPM is controlled by the main jets, and from 4000 RPM up the main jet circuit is modified by the air correction jets. NOTICE THAT THESE RANGES OVERLAP. This means that changing one set of jets MAY make it necessary to change either or both of the others. Changing the emulsion tubes will affect the overlap ranges of these three correction points. Another thing to note on idle jets is that the mixture screw adjusts mixture only at the static idle position. As you open the throttle, transfer ports in the carb are exposed to the airflow and more fuel flows into the airstream. This means that you have to size your idle jets for off-idle running through the transfer circuits and use the mixture screws to set the static idle mixture.

Adjust your synchronization, idle speed and static idle mixture. Operate the car for a while and evaluate what should be done to ONE adjustment point. Make that one change, resync and reset static idle parameters, and run the car some more. Has the change helped? Can it stand a little more (or less) adjustment in that one point? If so, make that change and evaluate the results. Once you?ve got one set of jets right, move on to the next. Remember that the effective ranges of the jets overlap. This means that when you change the mains, you will affect the upper range of the idle circuit operation and MAY need to go up or down a size in your idle jets. High RPM mixture is adjusted using the air correction jets, in reverse logic to the idle and main jet operation. To make the high RPM mixture richer, you must use a smaller air correction jet allowing less air to bleed into the circuit. Again, changing the air correction jets MAY affect the size of the main jets that you need, and that MAY affect the idle jets.

Good luck,
Harvey

[philinjax]

Popping and backfiring when snaping the throttle and when decelerating while the throttle is closed is frequently caused by an exhaust leak. Check for exhaust leaks around the exhaust gaskets and around the head exhaust ports to exchanger junctions. You can do this by listening through a length of tubing while probing around the gaskets and ports with the other end of the tubing. Using this method, you can quite easily detect leaks even at idle rpm.

There are a bunch of other causes of intake and exhaust popping and backfiring with carbs. Let me know and I will email a list of causes and fixes I got out of the Weber Tech Manual.

Phil

[914fan]

If you are popping out of the carbs you are lean. Check the gasket where the intake meats the head. On my car it is a pain to seal this good. At the 2 O-clock and 11 O-Clock positions between the two intake pipes, where the sheet metal comes down it rubs at the head. You may see that there is less area for a good seal here. I have so far had the best luck with the following.
Take your intake manifold anf use a long fine file to make the intale face uniformly flat. Use a new gasket and see how this does. If you have problems after this Try some copper RTV on the head side of a new gasket. If still problems Try copper rtv on both sides.
You did not say what jetting you are running. I have a 1.8 with hydro cam and 40's.
I am at 55 idle and 135 or 145 on the mains. Your problem is definatley in the idles now.

Have you adjusted all settings to the same on both sides? If not pull off both carbs and clean them out with some spray carb cleaner and compressed air. Then set all settings to the same.

My initial settings were as follows

2 turns out on idle volume screws (base of carb)
2 turns in from point of contack on idle speed screws (throtle position screws on side of carb)
1/2 or 3/4 turn out on air bypas screws (base of carb with lock nuts)

I ended up at 1 turn out on volume screws ans 1/4 turn in on idle speed screws. the air bypass was left as is.

It may take you a while to get it right. Just have some patients.

If by afterfire you meen out the exhaust check for exhause leaks. This may also be a sign that you are too rich or that your idle plates are not closing all the way when decelerating allowing fuel into the chamber.

Also check your lincage to be sure it is moving the plates the same amount at the same time.

Good luck

[JJ914]

Hey,

I worked on it last weekend and found that right carb was not completely closeing at idle. I put a spring on it and problem went away. It only pops when it first starts when cold then it run no problem. It runs smooths and pulls pretty good. I think it is need of rejetting had to turn mixture screws out pretty far. But things are going much better. Can some one tell me what jets theyt are using in there webers for a 1.7 motor with stock cam and a 009 dist. I dont know what mine are but I think there for a 2.0.

[philinjax]

Both carbs have an idle rpm stop. These stops must be set identically, otherwise you get the problem you describe. Remove your added on spring and set both trottlestops so that they bottom out identically at the idle rpm you want.