Revisiting carb issues from last summer...

[livinginthepast]

It's Spring and time to reopen old wounds! Yay.

Okay, now that I've got the ignition and charging issues all addressed (new distributor, new coil, new generator, spark plug wires, plugs)... Timing and dwell set...

...I've returned, with reticence, to the carbs. I'm attempting, yes again, to adjust the mixture. Ugh.

Recap - Stock 948 SUs. Carbs were rebuilt last summer, oversized throttle shafts installed. I've checked for leaks by spraying carb cleaner around throttle shafts and around all carb/manifold connection points. No change in idle detected. However, both carbs run really lean (the rear even more than the front).

In the worst case, the rear carb, I can lower the adjusting nut all the way to the final thread and the idle will stumble and die when lifting the pistons via the lift pins (whatever they are called) - which I keep reading indicates a lean condition. My solution has been just to adjust until I get the smoothest idle and drive the car. However, I really do want to get this corrected.

Should I just order the rich needles for these carbs and install or is there something more fundamental that I need to be looking for?

 

[jlaird]

Check to see, eyeball if the jets in bottom of carbs are level with the floor of the carb, the jets are brass, carb body is alum.

Sheck that floats are adjusted properly that controls the level of the fuel at the jet/carb body, critical.

If ok check that needles lip is flush with the piston where it comes out, should be a lip on needle there.

Now follow the book for proper adjusting.

 

[Billm]

Bill
One of these days when it isn't raining like crazy (middle of August??) bring your car over and we can check it out.
BillM

 

[livinginthepast]

Jack, Bill:

Did a superficial check of the rear-most carb. The needle lip was about .014 out (sticking out of the piston) from flush. Corrected.

I also eyeballed the jet and it is not flush. It's recessed about 1/32" (I should clarify this: the jet itself was lowered relative to to the adjusting nut. The top of the brass bearing(??? I guess that's what it is) through which the jet travels was recessed about 1/32" below the aluminum carb housing. If I turn the jet upwards, it will run up flush to the bearing but not the aluminum housing). Hope this makes sense!

BillM: Thank you so much for the offer. I hope the next clear day comes before August, but judging by the maelstrom that's currently blowing through, I have my doubts! When that clear day does come, I'm pointing the car in your direction, crossing my fingers, and stepping on the gas pedal!

 

[Billm]

As soon as the flood backs-off head on over!

 

[jlaird]

Close enough, now how about the floats. Check and adjust as needed.

 

[Sarastro]

One thing to keep in mind: if things aren't working right with the stock setup, the thing to do is fix what's wrong. Don't just put in richer needles to compensate. That's fixing a problem by introducing another problem.

Second thing to keep in mind: when everything is set up right, it really does work. That's how they came from the factory; they worked then and they'll work now.

As for my best guess, I agree with Jack: the fuel level is critical to getting the mixture right; it's not just the jets, needles, and such. So, if it's running lean and all the other things check out, low fuel level in the bowls is a prime suspect. This can be caused by the floats being set wrong or perhaps some fuel restriction. Or, maybe something assembled wrong in the carb?

Good luck--I'm sure you'll get it.

 

[jlaird]

I got a couple of bucks that says the floats are incorrect.

Please please keep in mind that most of the repair manuals are incorrect. This is an H1 carb therefor the curved part of the needle valve should be checked with a 7/16 dia. drill bit.

NOT THE 1/8 TO 3/16 FOR THE H2.

However, just for example the picture in Hynes on page 85 top right is correct for methoid.

My info is taken from an orginal shop manual printed when Miss Agatha was new.

 

[livinginthepast]

Okay, I haven't run it and tried adjusting the mixture yet (I'm tired of working on it tonight), but I did find that the rear carb float was way off - supposed to be 7/16 - was more like 9/16" maybe almost 5/8"). Figuring that out was a chore as my set of drill bits is conspicuously missing a 7/16" bit). So I took the mic and started measuring everything in the toolbox - finally found a socket that was precisely 7/16" and used that as my gauge. The front carb was a little off so I corrected it (at first making it worse with my brutish, ham-handed attempts to bend the fork, but in the end winning the battle...)

Put it all back together, started the car...noticed the tank was on empty and said to myself, I'd better wait until there's more gas in the tank before moving forward...

So I'll run down for gas in the morning and report back when I've made another attempt.

Wish me luck!

 

[jlaird]

Excellent, sounds like you are almost there.

 

[Pythias]

Don't forget to get the car properly warmed up before setting the carbs. That means running for at least 15 minutes.

I've never been able to use the lift pin method myself, as I can't seem to hear the increase in speed, or know how much to lift the darn pins, so I use a colortune if I'm doing it myself.

 

[livinginthepast]

Hey Bill L.

This is precisely why I am waiting until I get more fuel today - I have a feeling that if I were to try and warm the car to op-temp, I would run out of gas, suck all kinds of crap off the bottom of the tank and add yet another problem to may stack of existing ones! ;-)

The pins remain something of a mystery to me, too - largely due to the fact that they only make my idle stumble and die. I've never had the pleasure of hearing the overly rich increase in idle speed - my personal promised land.

 

[Pythias]

One option is to get (a friend let me one long ago) a portable tach perhaps we could see the increase in rpm speed on the tach that we can't detect with our ears. the other thing would be to actually look down the throat of the carbs while doing the process to SEE the piston rise. The covers need to be off in order to either -- A. Use a Unisyn, or B. listen thru a tube -- for balancing. ..

I've found that newer UNISYN's made in China are JUNK, or at least mine was. When I dialed it down enough to get the bead to rise, it choked out the car. I picked up a CARBALANCER made by GUNSON and it works like a charm tho.

YMMV.

OH, don't forget to separate the carbs before tuning. You'll NEVER get them right if you don't start there.

 

[livinginthepast]

Well - After some tuning it's better, but still not right.

BillM - look out, I may be heading in your direction sometime soon!
BillL: Thanks for the tip on the balancer. And it's inexpensive, too! I've got a crappy chinese unisync that I don't even bother with anymore and another tool (a german gadget) that was made for larger throats that I have managed to make work reasonably well.

The carbs sync up fine, I've got them idling at a leisurely 600rpm and both sucking/pulling identically. But nailing the mixture is killing me.

It occurred to me that maybe I'm using the lift pins to diagnose incorrectly. For instance, should I be lifting and holding until something happens, or is it simple a lift and release? The results are confusing my tired brain because they are inconsistent. For example, I've had this happen more than once - push in lift pin, RPMs rise, keep holding, idle stumbles and car nearly stalls out. Now how can that be if there are only three possible responses - rise in rpm, stumble, or little change? Stupid question? I never claimed to be all that bright, gentlemen.

Sorry to be so dense. I expected this to be much easier than it has turned out to be!

Yours truly,

Zippy the Pinhead

 

[DrEntropy]

My humble suggestion: disregard the lift pins entirely. Just listen at the exhaust and plug checks. Better yet, get a ColourTune.

 

[bigjones]

Doc,
Could you elaborate on "listening to the exhaust" a bit?
Cheers

 

[jlaird]

Only supose to lift the piston 1/32 of an inch in any case, ever note how high to pins push em.

 

[DrEntropy]

Doc,
Could you elaborate on "listening to the exhaust" a bit?
Cheers

The exhaust sound should be a steady, non-hiccuping sound. If it has a staccato beat you're likely off on the mixture. Called "splashy", too rich. "Snappy", too lean. You can actually smell the difference between rich/lean after a bit of practice. A ColourTune can give you the feedback to learn how to hear the difference and take a lot of the guesswork out of the process.

 

[Sarastro]

I've found that lifting the piston works fine. A couple suggestions;

1. Don't use that pin on the side. It's hard to control how far you lift the piston. Instead, use a SMALL screwdriver, off to the side so it doesn't affect the air flow over the jet.

2. As Jack says, lift the piston only a tiny bit. 1/32" is about right.

3. It helps a lot to start lean. This is much easier to see. When it's lean, even a little lift of the piston makes the engine stumble dramatically. Then, as you make it richer, that stumbling disappears, until lifting the piston has practically no effect. Frankly, I've never been able to see the engine speed up and settle back the way the books say. It does speed up, however, when too rich, but the speed change is not as dramatic as when it's lean.

4. Don't try to adjust one carb completely, then do the other. Adjust one a bit, then go to the other. Otherwise, it's easy to get one too rich and the other too lean. There's a lot of interaction between the carbs.

5. I have a colortune here, and got it with great expectations. The one time I used it, I ended up with carbs that were way too lean. Could have been just me. I haven't tried it again, as the carbs are now in good tune and, if it ain't broke, I don't fix it.

 

[Roger]

All the advice above, and more, can be found at the SU website.
It's free of folklore and rumour, too.