Post Hibernation issues...SUs

[Baz]

Pushed her out of the garage last night, fluid changes all round, new stuff for the ignition, adjust valves, timing light, all that good stuff. She fired up 1st turn of the key and wanted to play, oil pressure good, idle good.
Carbs are still too rich.
For the life of me, I can't lean them out, seems they're leaned all the way by the screw. The car didn't get that warm. I balanced them, red ball half way on both, adjusted the throttle body, everything good there.
The only thing I can think of here is the float level; sometime in it's history, Grosse jets were installed if that's relevant, but I'm pretty stuck as to lean them out further....
Any suggestions would be appreciated.

 

[Trevor Jessie]

Asking the obvious... the choke was fully off and the jets fully return?

 

[Baz]

I had no choke at all, which is a blessing in itself. (I've learned that lesson!)
What do you mean by the 'jets fully return'?, this could be my trouble.

 

[Trevor Jessie]

The black plastic shoulder needs to be against the adjustment nut. You not see the metal jet tube exposed.

Why do you have no choke?

 

[Baz]

Oh, right, yes, the nut is against the plastic shoulder.

I have a choke, I just didn't use it all. The last time I tuned the motor I did so with the choke 1/3 out and that messed me up.
Thanks Trevor.

 

[zimmy]

are u setting air OR gas intake?
theres adjustments for both....
im not having much better luck though...
ive got a little popetty pop when coasting
in gear, and my tailpipe is sooty....z

 

[regularman]

If you think the carb is too rich when it is running at high speed then the main jet is probably too big. Never messed with SUs very much my midget had the weber when I bought it. I had a 72 mercedes that had them and I never had to touch them other than to add some oil to the dashpot and go,go,go. That car was fast and I found myself running over 100 all the time. I need to read up on these SU carbs, seems a problem for many folks. I have had a lot of experience with mtorcycle carbs and these might be the same. If they are then you have different parts controlling the fuel/air mixture at different throttle positions. You have the air and fuel screws for idle up to about 1/8 throttle, then the slide from aobut 1/8 to 1/4, then the needle from 1/4 to maybe 3/4, then the main jet from 3/4 on to full throttle. These are not exact because as one fades out the other fades in as you increase the throttle.

 

[PeterC]

Baz... I had a similar issue last year. Is the gas fresh, new, recent? If it is a month or more old, replace or freshen. I, like you, had the symptoms of rich running... black smoke, sooty plugs, etc. etc. It seems as though the gas wont ignite. After fiddling for the remainder of my 1/2 empty tank, I had to refill with fresh gas, and bingo... all good, totally different. The car just seemed to smile! Had to re-readjust a little afterwards and it was great all year. This year, started with fresh gas, and all is well.

I've talked with others in our industry and related ones who have the same experience. Don't even try to re-use the modern fuels the next year, even 2 months. Lawn mower shops love it, if that helps.

Honest, try that first. Fresh gas.

Peter

 

[Hap Waldrop]

Peter is shooting you straight, the shelf life on unleaded gas is like nothing, sometimes it will stay good six months, other times it will go bad in 30-60 days. I had regular unleaded go bad in my ATV after only sitting 45 days, had to take the carb off and clean out, starting using unleaded premium in it and never had the problem again. You may already be using premium in your MG, but if not it's worth the extra dough just because it will last longer without going bad. It's seems the octane level of gas has alot to do with it's life, leaded race gas for example will remain good a couple of years in storage.

 

[regularman]

They also make this stuff called "stabil" that you can put in gas to keep it from going bad when you store somethng. I use it in my generator because it goes for a long time between power outages and it seems to work great. Beats pulling a carb off and fooling with it in the dark. Hap, you might want use it in that ATV if it sets a lot, I put in in my motorcycle tanks over the winter.

 

[Baz]

Yes, my gas is 'old' I think my last fill up was late October, I ran the tank dry last night with all the fiddling. The symptoms you describe Peter are spot on; I guess I had more faith in Stabil than I should have.
I did put stabil in it, but 5 months is 5 months, I wouldn't drink 5 month old beer without being upset about it, so why should my car?
Thanks!!

 

[Bugeye58]

I need to read up on these SU carbs, .....
You have the air and fuel screws for idle up to about 1/8 throttle, then the slide from aobut 1/8 to 1/4, then the needle from 1/4 to maybe 3/4, then the main jet from 3/4 on to full throttle.

Yeah Kim, you need to read up on SU's. /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
No air/fuel screws. It's all done based on the position of the needle in the jet. If the jet or the needle is worn, it can cause rich running. The jet size remains the same, and changing the mixture is done by changing the fixed needle to one of a different profile.

Jeff

 

[Hap Waldrop]

I've had stabil gas crap out on me too, I'm a big fan of anyhting I used now and then gets premium.

 

[regularman]

I need to read up on these SU carbs, .....
You have the air and fuel screws for idle up to about 1/8 throttle, then the slide from aobut 1/8 to 1/4, then the needle from 1/4 to maybe 3/4, then the main jet from 3/4 on to full throttle.

Yeah Kim, you need to read up on SU's. /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
No air/fuel screws. It's all done based on the position of the needle in the jet. If the jet or the needle is worn, it can cause rich running. The jet size remains the same, and changing the mixture is done by changing the fixed needle to one of a different profile.

Jeff

Wow, that is how they work? Now that does sound interesting.

 

[Bugeye58]

Yup. A tapered needle rides centrally in a fixed orifice.(The jet) The needle is attached to a piston that rises with throttle application, increasing the annulus for the fuel to flow through. The taper of the needle determines the mixture. Different profiles will change the mixture at different points in the power band. Pretty much self compensating for atmospheric changes, and simple as a stone.
Synchronizing a pair of them can be a fiddle, but a very basic carburetor.
Jeff

 

[regularman]

So the pistons don't have any effect on the fuel other than contolling the depth of the needle? Are there any adjustments to raise and lower the needle with respect to the piston?

 

[Bugeye58]

The needle is held into the piston by a set screw, and there is a limited range of movement there. Standard procedure is to mount the shoulder of the needle flush with the bottom of the piston.
The piston blocks airflow, so the higer the piston, the greater the airflow. Coupled with the taper of the needle, it balances things out.
I can send you a PDF file on SU's if you want. It's about 1.6 Meg.
Jeff

 

[DrEntropy]

Other factors like "fuzz" and deposits from sitting can keep the needle from fully dropping "home". Jet mis-alignment, goop in the "chamber pots"... many things can cause 'em to cludge, but usually a good cleaning and setting to a "base" adjustment with fresh fuel will sort the problem(s).

Old gas is a terrible issue. As noted above: premium will outlast most everything else.

 

[DrEntropy]

So the pistons don't have any effect on the fuel other than contolling the depth of the needle? Are there any adjustments to raise and lower the needle with respect to the piston?

Only marginally, and only on the older pre-emissions units. The jet height is the only real mix "adjustment".


EDIT: Zed-S units apres-smog are a completely different animal: THEY use the needle height as the mixture adjustment.

 

[regularman]

The needle is held into the piston by a set screw, and there is a limited range of movement there. Standard procedure is to mount the shoulder of the needle flush with the bottom of the piston.
The piston blocks airflow, so the higer the piston, the greater the airflow. Coupled with the taper of the needle, it balances things out.
I can send you a PDF file on SU's if you want. It's about 1.6 Meg.
Jeff

Sure, send it to my e-mail when you get a chance. I'm sure I will be fooling with a set sooner or later. Sounds like they are much easier than motorbike carbs.