I'm looking forward to responses as well I'm new to SUs. Sure my 64 Midget has them but that things still in the same boxes I bought it in.

I would like to know how much oil do you put in these things. I read the manual and didn't see a level may have just missed it.

 

http://www.mgbexperience.com/article/images/su-hs4-section.gif does a good job showing up to where the oil level should be. from the picture, it also looks like if the oil is at the proper level, there is nowhere it can go. but my oil is disappearing. i use atf type FA which according to the local napa shop clerk has superceded type A. not that this should matter in this case because this is happening in only one carb.
i've even done this: after making sure both dashpots are at the same level, i'll do a piston drop test, and for the first one or two tries they feel pretty much the same, but after that, the rear carb (the one that has disappearing oil) starts loosing resistance.

 

Time for y'all to do some reading...

http://www.vclassics.com/contents.htm

http://www.vclassics.com/archive/index.html

http://www.vclassics.com/archive/stoichio.htm

http://www.sw-em.com/

http://www.sw-em.com/su_carbs.htm

http://www.sw-em.com/SU Carb Service.htm

http://www.sw-em.com/SU Carb Service.htm#SU Carb Service - Expanded

George Dill

 

i will get on that. thanks!

While its good to keep an eye on all these things, I think that the oil level is not that critical to the operation of the SU's. The oil dampening system is important only a fraction of a second after you opened your throttle. At a fixed throttle position, when the front piston is not moving much anyway, the dampening has little or no effect in the mixture strength. So, in other words, the oil level may affect throttle response, but will have no effect on a constant throttle acceleration or high speed highway driving.

will the difference in piston movement have an effect on my upcoming attempt at tuning the carbs?

 

While its good to keep an eye on all these things, I think that the oil level is not that critical to the operation of the SU's. The oil dampening system is important only a fraction of a second after you opened your throttle. At a fixed throttle position, when the front piston is not moving much anyway, the dampening has little or no effect in the mixture strength. So, in other words, the oil level may affect throttle response, but will have no effect on a constant throttle acceleration or high speed highway driving.

 

The dashpot can not leak.

The only escape for the oil is through that tiny hole in the cap.

Refilling the dampers and then finding them near-empty in just a few trips indicates that the piston is travelling to at or near its highest possible physical position and pushing the oil out that tiny hole.

Many factors can cause this including the wrong fine-wire springs, worn piston oil chamber, worn/incorrect damper cap, etc.

Try this - with the car at its best state of tune overall take a trip with the dampers at proper oil level (see below) then repeat with both dampers completely dry then report your findings here.

Thanks.

George Dill

http://www.sw-em.com/SU_HS6_explode.gif

http://www.roversd1.info/2600/carburetor-drw.jpg

From: http://www.vclassics.com/archive/stoichio.htm
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Dashpot out of oil: Oil is essential for damping and lubricating the pistons, so keep the dashpots filled to the proper level. I prefer to use a light oil (hydraulic jack oil is nice and comes in handy small bottles; many others swear by ATF), but regular motor oil is adequate. Here's my scheme for putting in the correct amount: Pour in some oil, refit the plunger and tighten the cap. Hold a rag around the cap and lift the piston all the way up, hopefully pumping a bit of excess oil out the cap's vent hole and into the rag. If not, repeat. An overfilled dashpot will squirt oil clear across the engine compartment when the throttle is opened -- I'd rather just get it into the rag right at the start.
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The dashpot can not leak.

The only escape for the oil is through that tiny hole in the cap.

Refilling the dampers and then finding them near-empty in just a few trips indicates that the piston is travelling to at or near its highest possible physical position and pushing the oil out that tiny hole.

Many factors can cause this including the wrong fine-wire springs, worn piston oil chamber, worn/incorrect damper cap, etc.

Try this - with the car at its best state of tune overall take a trip with the dampers at proper oil level (see below) then repeat with both dampers completely dry then report your findings here.

Thanks.

George Dill

-

\the funny thing is that that only happened once, when i overfilled the rear dashpot. atf sprayed all over the engine bay. that's not what's happening, the atf is going somewhere else. i have some time today, i'll take the dashpot off and have a looksee.

The pistons are only supposed to have resistance on the way up, they're supposed to drop freely. Not sure what a drop test tells you about the oil level. Or is that what you mean, it's resisting movement at first but then it's gone and after a few strokes the piston lifts easily?

If things are heavily worn out in the piston and dashpot assembly, you can suck the oil out with vacuum, maybe.

Are you having accelleration problems? Or are you just worried that the oil is going away? If the car runs fine, maybe it's not something to worry about.

that's what i mean. no acceleration problems, just don't like disappearing oil. who does?

 

The pistons are only supposed to have resistance on the way up, they're supposed to drop freely. Not sure what a drop test tells you about the oil level. Or is that what you mean, it's resisting movement at first but then it's gone and after a few strokes the piston lifts easily?

If things are heavily worn out in the piston and dashpot assembly, you can suck the oil out with vacuum, maybe.

Are you having accelleration problems? Or are you just worried that the oil is going away? If the car runs fine, maybe it's not something to worry about.

 

... If things are heavily worn out in the piston and dashpot assembly, you can suck the oil out with vacuum, maybe. ...

+1 IIRC, and that was about 50 years ago, I had that problem with a well used A-H 100.

 

George, are you implying that his dashpot may be going past the position it holds at WOT and high rpm? That would cause lean operation at high loads, probably well outside the range that the mixture adjustments can provide (the mixture adjustments are relevant for small piston openings, yes?

 

If you have a bad exhaust cam lobe in one of the cylinders served by that carb it will spray out when the intake valve opens - that is the only way it can get out if the exhaust doesn't open. Wiped cams are fairly common on these engines, especially if someone tried to reuse a worn lifter.

 

The engine would probably not run so good.... But easy to confirm. just remove the valve cover and observe the rocker arm with the engine running. If Warlus' guess is right, you will notice a much reduced movement of rocker arm of one of the exhaust valve. If that turns out to be the case, it would be time for a new cam. Plus, replace oil filter frequently until the cam is replaced, because all the metal particles will be in the oil. Once the hard layer of the cam wears off, the rest of the cam will wear rapidly.

 

okay so what was coming out of the rear intake was gasoline, not atf. does this indicate my needing to check the float bowl level?

the engine runs pretty well ( i have no real frame of reference, but i get smooth acceleration, deceleration, no backfiring). i have even compression too, if that matters. and at the last oil change the magnetic drain plug didn't have a ton of sludge on it.

 

In my experience an exhaust cam lobe completely GONE will not affect compression.
I say again, measure the LIFT of the valves! It is a little tedious but will tell you if your cam is bad. Blowback of fuel vapor from a carb is a very good indication however.
The only thing the tiny guard does is to force the fuel down over the lid rather than it shooting straight out the hole when the float valve sticks open.
The only thing you can do if you have a bad cam is to replace it. While you are at it get new lifters and maybe a D cam rather than C.

 

Compression test is done at very low speed, 300 or 400 rpm, so it is more than likely that even a tiny lift on the exhaust valve will accomplish emptying of that cylinder.

But before you get too concerned, just remove the valve cover and start the engine. Observe the motion of the rocker arms. If one of them appears to be moving much less than the rest, then you may have something to worry about. If they all move about the same, don't worry about it, and/or check again in a few months.

The consequences of a worn out exhaust lobe would be loss of power on one of the cylinders at the high rpms. Given that the B18 has only 4 cylinders, you should be able to notice a loss of power. But of course, you just got the car, so you may not have a good feel of how it should be. But if you fell a loss of power from now won, do this simple test again. If one of the exhaust valves appears to be moving less, then get a dial gauge and try to measure exhaust valve lift.

 

thank you gentlemen, as always.

just finished rebuilding the clutch slave cylinder, so i've had my fill for today. correction: my dear beautiful wife thinks i've fondled the amazon's parts enough for one day (i couldn't help it). tomorrow morning i shall take a look at the rocker arms.

as far as power goes, the only frame of reference is my last 4 banger: 1999 1.8L passat, 5 speed manual. just from memory, it feels like the 122 has similar power especially at high rpms. i could be biased... i am in love...