Rejetting carbs

a stumble at 1/3 throttle at all revs. haven't had a chance to get the gas analyzer on it yet. it drives perfectly well below 1/3 throttle....perfectly well above (15:1 a/f at wot is very very bad!) .....but i just have to drive it above or below that 1/3 throttle point. i'm busting to see how the mixtures are now...either way, at least i know how to fix it now. i think i'm understanding carbys!

 

i'm with you on that. i can't see why it can't run economically AND get power. That's why there are two circuits (idle and main) and that's why there are air correctors.

 

http://books.google.com/books?id=ze_w0i3xZmAC&pg=PA21&lpg=PA21&dq=best+exhaust+mixtures&source=web&ots=mY1E5aZscU&sig=qvUWCR1d35054Yt1GfknWP0L_4c

I'm going to try for around 15:1 on light cruise and 12:1 WOT AND have the car run well. There's simply no point doing it by the seat of the pants. are you using more fuel than you need to? are you getting as much hp as your car is capable of? that's the bueaty of modern fuel injection...you can get both. i'm going to try and do the same with my carbies if it's at all physically possible.

 

Hoi Hans
The idle jet is #25 on the diagram. Look for a brass fitting with screw slot on that side of the carb. Hiding underneath is the idle jet. On the highway you spend most of your time running on these so they are important to fuel economy.

Gerrit

 

But there are two, one on each side of the carb, its just not shown, just like only one jet assembly is shown.

 

I remember reading something eons ago, way back in the mid 70's I suppose. It was an article relating to Ferrari mechanics tuning the old V-12 cars for different race tracks. They had boxes of all kinds of various jets, whatever they needed. They kept records each year, noting temperture, altitude, humidity, barometric pressure, and notes how the car performed. To reach optimal performance they were always altering jetting, probably ignition and valve timing too.

I suppose thats one factor with electronic FI, it kind of does all that for you. But you can get a very reasonable optimum that will work pretty good all the time, and if you hit it close the engine will run really good. I just think its amazing how those old guys could tune a V-12 with nothing more than listening to the engine and looking at sparkplugs, and here we are struggling to figure it out with handheld electronic AF meters.

 

well, i don't know about you...but i'm not a genius....those guys were..hence me needing the AF meter.

you're right about temp/altitude etc. that's why you aim for something but accept something close with a carby i spose.

 

my mechanic just made an adapter that slides into the exhaust and hangs on with a nut. works a treat...quick on and off.

100mph!!! awesome.

 

no pic at this stage...but next time we use it i'll take a photo.

imagine a pipe about 300mm long. 1/2 way along the lambda sensor is installed. the pipe goes into the exhaust pipe and is clamped in place. the exhaust comes out the exhaust and, obviously, also through the pipe that the lambda sensor is in. no burnt wires before they're not touching anything hot.

a picture will tell the story much better.

 

Hmmm, it just occurs to me that the choke is not hooked up (just pump the accelerator pumps once and the engine is primed too). Could it be that the choke is simply ON all the time, rather than off???[/QUOTE]

My car was the same when I got it.
Choke cable disconnected and no linkages on the carbs for choke.
Looks like you may have the problem resolved, but I was going to suggest checking things more on the air side of things.

My theory being are you getting too much fuel? or too little air?
Is there any restriction in the air duct into the airbox? plastic bag ingested or the like?

What is the condition of your air filter? Is it clogged and restricting air flow?

And the last, are the chokes really disconnected?

I had a problem just like this at the track in my Alfa 105.
It had been working just fine.
On the track flat out, fine, into the corners and then pulling out (2nd gear typically) it would bog down and cough and splutter.
The pull away. I couldn't work it out.
Then after much stuffing around a mechanic friend said check the choke.
The week beforehand I had some work done on the starter motor, which is directly underneath the carbs on the 105.
Yep, they had been working in this area, and had disrupted the choke linkages/cable and it was tensioned up and was running on about 25% choke all the time.
Simple but frustrating.
Thats why I suggest the same to you. Your jetting seems more lean than other cars. Maybe it is being strangled by lack of air or the chokes are active.
Anyway just a thought.

Steve.

 

give me a couple of days. will do. you'll be amazed what you find, btw. there's no reason why this stuff should be done by ear or the seat of the pants!

 

after many years of doing this sort of stuff, i've perfected holding a car at full throttle with the brakes on and maintaining the same speed. remember with a lambda sensor, 2 seconds is enough. i also do light throttle/cruise readings.


anyway...i bought an air flow/sync tool. i brought the revs up to 2000rpm and imagine my surprise to see a constant flow of fuel from two of the carbs (4 throats) from the accelerator pumps! doesn't happen at idle...and only happened with two of them. can't be normal. anyone got ideas about what could cause that?

 

The linkages between the carbs may be lose. Those ball fittings that allow things to slide/rotate could be lose or old. Sometimes they will have a lot of play in them resulting in sloppy connections between the front and rear bank.

Clogged ports in the carbs would also restrict flow.

 

I had the same problem when I got my car.
I noticed that @ idle two carbs and one in particular was dribbling fuel from the pump jet. Nearly a constant flow.
Seeing that I have just bought it I was keen to go all over the car to see where I really stood.
That included stripping all the carbs and putting a kit through them. Somewhat hoping that this would fix the pump accel dribbling.
Lucky for me it did. Read all the doc's, bought a flow/sync tool and did it all for the first time.
Very satisfying.
As it stands, mine is 140, F36, 170a/c and 55 idle. I have a suspicion that it is too rich, plus also reckon that my fuel pressure is too high and may be causing flooding that is adding to the woes.
Once it gets past 2500 it goes like a bat out of hell.
I was going to get a fuel pressure gauge this weekend but my supplier was closed, I am going to get it this week, plus I want to get an A/F ratio meter and take it step by step to sort it out. I reckon I will end up with 200's instead of 170's.

BTW, is there any rule of thumb/ratio that helps you choose where to go for A/C jets.
eg, if your A/F ratio is one or two units out, how much does a change from 170 to 180 or 190 typically change the A/F ratio?

 

i found 55s to be too rich using an A/F analyser...so now trying 50s but haven't had the analyser back on yet.

140/170...that sounds SO rich! but....i know a guy in perth had that...so maybe it's right. i've gone, as it turns out, from 125/220 to 135/200.

the 125-135 because it was too lean at WOT and 200-200 because the higher the revs the more it leaned out.

i have a set of 140/170 waiting to go in if needed when i can get my hands on that analyser again.

amazingly, the fuel from the accel jets stopped. maybe it just needed a drive. very odd...i'll be keeping my eye on it.

 

here are those photos i promised you, Hans.

http://www.fcavic.com.au/afr/afr.htm

a bit crude looking, but does the job perfectly.

 

50 Idles
135 mains
200 air correctors
F36 emulsion tubes

Throttle Position/RPM Air/Fuel Ratio

Light Throttle

3000 17.0
5000 18.0

Cruise

3000 16.0


Full Throttle

3000 13.2
4000 12.4
5000 13.2
7000 13.9


Going to try 52 idle jets and 170 a/c next.

I'm aiming for as close as possible to 14.5:1 on light cruise and 13:1 at WOT

 

That sounds good. Always err on the rich side.