Lean at roughly 1/4 throttle

[mephi]

I'm having a weird issue with my 38. Every time I get to around 1/4 throttle, my car starts missing a little, and loosing power. I decided to get a cheap A/F gage to see what is going on, and in this area of the throttle, it goes way lean. It does not matter what the RPM are. Has anyone had similar problems?

Before I put the Weber on it did the something similar when going up hills, so I'm not really sure what is going on here. The only thing besides the carb that could be affected by throttle position is the distributor, and I think that is still in good shape.

[labeledsk8r]

when you say cheep a/f gauge how cheep?? did you just hook it up to your stock 02 sensor?? if so then the readings your getting out of it are completely rubish and might be leading you the rong way.. now if you got a wideband then your doing good but dont look into it to much counting on that gauge.

have you checked your timeing?? mayby your vacum advance is messing up..

[2oodoor]

I get a little of what sounds like a misfire but barely detectable, about the same place.
I give the dist a little more advance and reset everything and that seems to help a bit.
It would seem like some transistion stage is not jetted correctly. I suspect the power valve is the wild card there for transistion issues though.
Try some simple things like a little more advance esp if you are not using vacuum advance at all. Take out the idle jets, something you can do from the outside the carb. Clean them out and inspect them for a burr or distortion. Reset you throttle stop and do the best practice weber idle mix adjustment procedure that weber provides on the website.
I notice the idle air needs a little tweeking sometimes with fuel changes, and temperature changes. Just a quick thumb turn on both screws.

[itzdave]

yea my carb does the same thing...

[2ndGenGuy]

My 38 is doing that too. I think the problem is jetting like roodoo said.

The idle jets are providing enough fuel to make the car idle just fine, but as soon as you get into the progression stage (which you're still on the idle jets), the idle jet isn't big enough to feed both the idle screw and your progression orifices. If you have bigger idle jets, that would be the simplest thing to try after you do what Roodoo said and make sure everything else is in order first.

[mephi]

when you say cheep a/f gauge how cheep?? did you just hook it up to your stock 02 sensor?? if so then the readings your getting out of it are completely rubish and might be leading you the rong way.. now if you got a wideband then your doing good but dont look into it to much counting on that gauge.

have you checked your timeing?? mayby your vacum advance is messing up..

I know, the readings are probably crap, but for the most part they appear to be at least partly accurate. I am at least getting a signal similar to what my computer would be getting, and really, it's just a voltage reading between 0 and 1.

The thing about a lean reading on an O2 sensor, is that it isn't necessarily a lean reading. It just indicates incomplete combustion. Either I'm not getting enough fuel, hence a lean reading, or I am having ignition system issues--a high O2 count, but at the same time, a rich exhaust due to combustion failure.

I have the vacuum advance hooked up, and it seems to be working OK, but as I said before, it seemed like the car had this problem before I put the 38 on. It just was not as bad.

I was curious, is the distributor timing adjusted by the computer at all in these cars, or is it completely mechanical/vacuum controlled?

Oh...my current jetting setup

idles .57 (came with the carb, biggest ones I have)
Mains 1.45
air 2.10
pump .70

[turabaka]

That sounds way too big for idle jets. I'm running .45 idle jets with a 272 cam, and even they are a hair too big for me. Granted I'm at a really high altitude, but still. They mains sound about right though.

as for the dizzy. The computer does nothing, just mechanical and vacuum advance.

[mephi]

That sounds way too big for idle jets. I'm running .45 idle jets with a 272 cam, and even they are a hair too big for me. Granted I'm at a really high altitude, but still. They mains sound about right though.

as for the dizzy. The computer does nothing, just mechanical and vacuum advance.

I tried 50 and 55 idle jets, but I had to turn the screw out to far and just moved up to the next biggest I had. They also helped to minimize that dead spot. I'm on the verge of ordering up another 60. I have one, but my weber is one of the older ones that has 2 identical idle jets, so the jet kit I got does not work for it.

Agh...the thing drives me crazy. It's right were I need to have the throttle to do 70 on the highway or to go up a hill.

Has anyone tried messing with the emulsion tubes?

Does anyone know where I can find a weber manual cheap?

[2ndGenGuy]

Roodoo, do you know the basic operation of the power valve? None of my Weber books even touch on the power valve... I know that it's supposed to open up under low vacuum conditions. But is it held shut by manifold vacuum and then released when you open the throttle and vacuum drops, or does it stay shut all the time and gets opened up by venturi vacuum?

[mephi]

I contacted the guys over at redline. Will let you know what comes of this.

[2oodoor]

link it baby
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0305em_carbureted/index.html

very nice reference to explain fun-duh-mental z

Rochesters, Carters and Edlebrock style carbs do not use power valves but use vacuum step pistion and metering rods.
Webers are unique to themselves in regards to transistion from pulling fuel from bowl thru idle jets (not idle air jet) up until mains. The power valve pulls fuel during tip in. Disableing the PV makes rejetting more effective and predictable.
There is better info about this mod. on the opel site.
http://www.opelgt.com/forums/archive/index.php/f-1177.html
http://www.opelgt.com/forums/aftermarket-down-draft-carbs/
Here is another one I have marked as good info

http://fordsix.com/forum/viewtopic.php?t=23153

[2ndGenGuy]

Interesting reading how the Power Valve works...


Enrichment by a power valve or metering rods references engine load through vacuum, and offers additional fuel beyond the main metering circuit. It is evoked when the throttle angle is steady, but the load on the engine increases.

That might explain why my engine starts getting richer and richer and richer at cruise. Though load stays the same, if it's too sensitive, it would result in a rich condition.

Though it doesn't sound like it addresses the issue of being lean at part throttle. What you're saying is disabling the power valve causes a more consistent tune. But if the power valve can only enrich the mixture, then it seems like you'd need it there to help reduce that uphill stumble, and disabling it might only make it worse... ??

[2oodoor]

I dont have this stumble, but then I have the pv functional at this time. I have no idea if it is the ideal part however.
You use the wideband to jet the carb appropriatley without the powervalve as to not show the mixture flucuate "illogically" as with the powervalve in place.
It sounds to me like a few of those with 38 webers could use some tweeking on the base timing. You can go over spec, but be carefull and do not let detonation occur.
Mephi, if you can access a timing light, find what your timing is at now and increase it in 2 degree incriments until you start getting preignition and go down from there. Also hold the light on it while you increase the throttle (out of gear of course) and see if your centrifugal advance is coming in. It should go over the base timing so around I think its like 32-42 degrees btc at over 2K rpm. If it is just staying put you need to free up those wieghts so the "breaker/pick up coil plate" turns to advance the timing as they pitch out from rpm force.
Of course IDEALY, converting to megajolt stand alone ign, even on these webered cars would be the best route. Use the EDIS crank trigger, cygnusx1 has a thread here (at PP for sure) somewhere with good info.
Like i was saying with mine on the A20 and the same carb on the B20, it still has a spot where under a decent load without flooring it, I detect a hint of misfiring ( no stumble) like it may be too raw of un atomized fuel going in the CC. Im thinking of trying the F66 emulsion tubes for that myself. I also feel that a deeper 272 cam could help the vacuum end of things with these.
You also need to consider if you modified the intake manifold too. They were designed for progressive type carbs, you have two diffent size "plenums" going into the main runner. Cutting out those (making it an open plenum) evens out the distribution but also can cause other issues that would change the jetting needed.

[cygnus x-1]

I agree about it being a transition problem. That's one of the hardest things to tune on these carbs. Bigger idle jets will probably help but if you go too big the idle won't be right. If your mid to heavy throttle and higher RPM mixture is not too rich you could also try bumping up the mains. But that's a pretty big hammer and it will probably go too rich overall.

And then there's emulsion tubes. Those things are the blackest of the black arts as far as Webers go. If you can figure out how to tune the emul tubes you should write a book. Seriously. I tried a different type once and the character of the carb completely changed. With the different tubes, changing the mains and idles seemed to produce very different effects compared to what I had been seeing with the original tubes. It was really weird. I probably could have figured it out eventually, but at that point I had already started down the dark path to fuel injection. And once you start down the dark path, forever will it dominate your destiny.


And yeah, a little more timing might help. Check your plug gap too.

C|

[mephi]

Are the weber power valves susceptible to backfiring like those in Holley carbs?

As far as the distributor goes, I know the weights are free, I did a quick disassembly and relubed the thing a little while ago.

I'm also curious how you determine the advance. There are only a couple of marks on the flywheel, and they really aren't all that easy to see.