Weber spiting and popping. Ive been reading already

[Bmxrams80sc]

I just put a set of IDA's on my 80SC and i have almost got the carbs tuned right. The car pulls strong once warmed and has a steady idle. But while idling, the exhaust pops a little. Now from what i have read i understand that this is a sigh that the air mixture screws at the base of the carbs need to be turned out when this happens. I have turned them out a great amount and am starting to think i might have to just start over on the entire synch process. Another thing that happens is from 1000 to 3000 if i get on the gas lightly, it seems to spit back through the carbs. any ideas there? other problems are very rough cold starting (normally seems to be running on only for cylinders) I am going to change the plugs and see what that does plus i understand these are carbs and they take awhile to "warm up". The last issue i have is when i pop the throttle quickly it seems to bogg the engine. All ideas will be much appreciated guys!

[Grady Clay]

Treat this similar to the CMA procedure for the MFI. Check the basics first.

What are the cranking compression and cylinder leak measurements?
Are the valves properly adjusted and what is the cam timing (what cams)?
What is the condition (age) of the high voltage ignition parts?
What ignition type, timing and sparkplugs?
Are there any intake or exhaust leaks?
Fresh filters, oil and gasoline?

Exactly what carburetors?
Are all the passages, jets, nozzles, diaphragms, etc. clean and functioning?
What sizes are the venturi and jets?

Now you can start over again.

‘Popping' in the exhaust can be a lean condition but most often it can be an ever-so-slight exhaust leak. Even the slightest pinhole or not perfectly sealing gasket can cause this. Does it pop on trailing throttle when very hot exhaust?

In general ‘popping’ through the intake can be a lean condition. That said it can be from other very important causes – valves not sealing and others. Is the ‘popping’ worse cold, hot or stays the same?

Under light throttle and 1000 to 3000 rpm, the mixture is almost entirely controlled by the idle gas jets. Try the next size larger jets and see what happens.

Poor running cold is also a symptom of a lean condition. It also can be ignition related – sparkplug type, CDI type, sticky advance in the distributor, poor cap, wires, plug connectors,

Since the engine performs basically correctly, an extended high-speed ‘sporting’ drive may settle things down somewhat. Only do this after confirming that you have proper ignition timing, appropriate sparkplug heat range, valve clearance and fresh high octane fuel.


Best,
Grady

[1QuickS]

Check out the list of things to check in my posting dated 3Nov, titled "Weber remanufacturing". I use a Gunsen "ColorTune" to get idle mixtures spot-on after setting air flows correctly. Check for o-rings missing or cracked on your idle mixture screws and idle jet jolders.

The ColorTune allows you to watch the combustion in each cylinder so you can isolate where your issues are.

The IDT's have a leaner progression circuit than the IDAs do. In particular toward the transition from progression circuit to main circuit. Richer idle jets help at the expense of a rich lower end. Changing the size of the idle circuit air correction jet can fatten up the progression circuit at the top end but you need to do a little bench modification work. I can discuss this if you like.

[Bmxrams80sc]

Thanks guys, i have much to check now! haha. For starters, the carbs are 40IDA 3c's i believe, ill check. I am getting plugs for it and ill reset the timing. I havnt had the car very long so i will have to do a compression check and set the valves. The exhaust is solid just put it on. intake is also solid. So you think i should make it a little richer? if so what is the best method? and right now i currently have 60 idles in them. ill try bumping those up. I know that the ignition stuff is kinda old and i should probably replace it so i will try that also.

[1QuickS]

I strongly recommend buying a Colortune and if you don't have one get a synchrometer. Check fuel level first, engine must be warm and running to get a good reading. I think your idle jets are plenty big enough. Colortune allows visual verification of individual cylinder mixture at idle, no guessing, no adjust it until it changes routine.

[Davz912/911]

1quickS,

I have to ask.....how are you reading the colortune? The spark plugs a buried in the cam towers.....Just curious

Dave

[Bmxrams80sc]

Ok yeah ill give that a try. Where can i pick some of that up? And i have two different synchrometers the carbs are all almost exact. thanks for the suggestion 1Quicks

[Steve@Rennsport]

I use a Syncrometer for basic setup & balancing and an Innovate LM-1 to measure air-fuel ratios.

I think the Colortune is a neat device IF one understands what they are looking at and how to interpret what they see. It is rather difficult to view the combustion process when the plug connectors and seals are in place.

[1QuickS]

The ColorTune comes with a mirror that allows viewing down the spark plug recess, I've used mine for 30+ years on 911s. Besides, even if the absolute fuel mixtures are not right they are at least BALANCED! A CO meter or Oxygen sensor measures the average of the exhaust gasses, you cannot isolate individual cylinders. ColorTune is available through EBay stores for $70 or less.

As to balancing, I modified my intake manifolds to allow useage of mercury sticks, I can monitor 4 cylinders simultaneously, unfortunately mercury is a bad item environmentally. There are other devices on the market that measure multiple cylinders simultaneously but I have tried those I couls find and have my issues with them for various reasons. I have a design for a multibarrel syncrometer without mercury but haven't had time to develop it.

If there was an interest in such a device...

[Grady Clay]

Bmxrams80sc,

Don’t get distracted by all this discussion about Colortune, fancy manometers and such. While it might be useful to some, it is superfluous for your purpose.

You can fuss with carb adjustments and mixture devices ‘tell hell freezes over if you have a tight valve, an exhaust pinhole, intake vacuum leak or carboned-up cylinders.

You need to confirm the proper functioning of the basics of your engine first. Next you need some good miles on it. Only then can you ‘fine tune’ the carbs.


I agree that 60 idle jets are already reasonably large. Perhaps this is right on. Measure them and confirm the size (60 = 0.60 mm). Trying larger (and smaller) will give you an easy clue where to look farther.

In general I am in favor of slightly lean idle mixture and put up with some ‘popping’ and fussiness – given that everything else is correct. Slightly too rich runs great but tends to cover up a myriad of sins.

Best,
Grady

[Bmxrams80sc]

Thanks grady and all for you input. Ill check the basics first.