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Carb Question

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jjbunn

jjbunn

Jedi Knight
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I have a question about the carbs. On one of my carbs the piece arrowed in red moves up and down as the engine is revved (presumably to adjust the flow of air).
293401629_5db50e6644_o.jpg


On the other carb, it does not move. I assume there is something wrong with this carb. I'm a bit reluctant to take it apart, due to the complexity of these things, but perhaps I will have to. What might the problem be?

The engine idles roughly and misfires a bit. I have new plugs and wires.
 
Wow Jack! I have been searching for ages for some PDF versions of manuals like these! Many thanks ... I'll take a gander and come back if I have questions.
 
You're correct the piston should move with engine speed and based upon the picture that you provided there is a good chance that it caused by nothing more than a build up of crud. Note the dark buildup in the slot on the side of the piston.

The SU carb is a surprisingly simple device - its almost lawn-mower simple. Except for rebushing the throttle shafts, repair is not that difficult and a good cleaning may be all that you need. The only word of extreme caution when handling the piston is to be careful with the metering rod attched to the bottom of it. You don't want to bend it.

There are several online pictorial sources about tear down and rebuild of SU's. Otherwise Haynes and Bentley manuals do a good job covering the subject.

GJ Oakes
 
The part you are pointing at is the piston. The fix may be very simple. Remove the damper rod by unscrewing the very top knob from the aluminum dashpot. Remove the three screws holding the dashpot on. Lift the piston out of the carb. Be aware that if you tip the piston over, a small amount of oil will pour out.

Carefully clean the piston until there is no grime left on the sides, bottom or on the brass needle. Don't bend the needle and don't hit it on anything! IF there is so much as a scratch on it you may have problems.

Once clean, reverse the process to reassemble. Don't forget to refill the piston tube with oil. Fill with 20-weight oil if possible. Moss sells a nifty little bottle that should last many years for a few dollars. Or in a pinch use 20W-50 engine oil or ATF fluid. Fill to 3/8" from the top. Once reassembled, it should work properly. If not, you may need to recenter the jets.

As a check, with the damper removed, you should be able to lift and drop the piston and hear a soft "cluck" as it hits bottom. Also, if you remove both pistons, do not mix them up! They are precisely fit to their respective carb bodies at the factory and will not fit properly if mixed around.

Good luck!
 
All you need is a screw driver and several cans of spray carb cleaner Julian. These are the simplest carburetors imaginable. Cleaning is a snap too. Don't be mislead though they are very well thought out instruments and can be a challenge to get set correctly but in this case you need to start with just a simple clean up to get things back on track and to build your confidence in dealing with them, so here goes.

Put a fender cover over the paint so the carb cleaner doesn't ruin it. Take the three small cap screws out from around the base of the bell shaped thing (vacuum chamber) and lift it away. Most probably the piston and spring inside will come out with it because it is stuck. Take the bits over to the bench and carefully work the piston out of the bell taking care to not bend the brass needle. Lay the bits out and spray/wipe them clean. Take the plastic cap off the top of the bell and lift it out. It is a damper for the action of the piston.

Go back to the car and take the other bell/piston assembly off and do the same thing to them.

Now take each assembly and turn it up side down without the spring being inside. Hold the piston "up" and allow it to drop of it's own weight down into the bell. It should do so without hesitation or "catching". If it does you haven't gotten things clean enough so go back a spray/wipe the insides again.

Now use up a whole can of carb cleaner spraying off the rest of the carb assemblies inside and out. Use two cans if you have to.

Now take the three cap screws off the top of the little chamber hanging beside the main carb body. That is the float chamber. Disconnect the rubber fuel line and lift the top up and away. There will be a plastic thing (the float) dangling under the bottom. Take the top/float assembly over to the bench and spray clean it inside and out.

Go back to the car and squirt a bit of carb cleaner down into the float bowl. The use a narrow screw driver with a cloth over the end to swab out the compacted brown gunk that you see down in the two bottom recesses. Repeat the spray and wipe routine until you get all of the gunk out.

Repeat the same effort on the other float bowl.

Now you have two bell/piston assemblies and two float bowl tops all clean and tidy on the bench.

Take each assembly back to the car and re-install them just as they came off and re-install the fuel lines.

Once the bells are on and secured use your finger to once again raise the piston nearly to it's limits and allow it to drop. It should do so with a smooth movement and a definite "thunk" when it hits bottom.

Squirt enough engine oil down into the open tops of the bells to cover the top ends of the pistons that you see down in the hole and re-install the plastic headed dampers.

Turn the ignition on and watch the carbs to see that there is no gasoline being forced out of the float bowl vents. Don't worry. If you see a leak you will know what a vent is.

Now try to fire up the car. If you did all of this as directed I guarantee the engine will run much better.

This is in no way an "overhaul" and I have skipped over LOTS of little details and adjustments, but if you get it done this time you will feel much more confident in dealing with the carbs and tuning in general. All you need now is several books on the appropriate tuning subjects and a good deal of time teaching yourself what to do and how to do it.

The rest is fun! Jack
 
Jack, you aren't suggesting completely filling the dashpot covers with oil are you? I assume I've misunderstood!
 
No. Sorry for the hazyness Steve. :smile: What I was trying to say was that it was NOT necessary to completely fill the dash pots. Only fill them to the level of the tops of the piston's shafts visible down inside the openings at the top of the bells. I think that there is some sort of EXACT level but to try to get it perfectly filled every time would be frustrating to just about anyone, let alone a new comer like Julian. I was trying to keep things as simple as possible for him just as you were. No need to go into the rest of the carb BS at this point as it would only serve to confuse and frustrate him. LOL Jack
 
By the way Julian, aside from the Bentley manual, which you should have handy every time you do anything with the car for the time being, you might also want to buy a few books that address the subsystems of the car. One would be "How to Power Tune the MGB 4 Cylinder Engine" by Peter Burgess. There are others but between the Bentley manual and it you will have plenty of "learning capital" for the time being. Both are always on Amazon as new and used editions. Jack
 
Ok, gotcha. I was mistaking the damper rod assembly with the actual oil tube on the piston.

The proper measurement is 3/8" from the top of the tube as I know it. I've found it is easiest to fill the tubes with oil before putting them back in the carb body. You can get them exact that way and not risk spilling oil into the rest of the carb. Then just put the cover back on and the damper rod in.
 
And from real life ( two day old ) experience, make sure that the linkage is tightented down properly. I had disconnected the rear carb and while I thought I'd tightened it all up well enough....

And BTW, you should be able to operate the throttle on the problem carb independently by hand . If you do that, with the engine running, and it doesn't rise then you have a problem with the carb, but if it rises using just the throttle on the carb itself, then it means you most likely have a linkage problem., so check the linkage and tighten the nuts.

They'll actually run like that ( linkage not connected ), but the acceleration is really rotten past 1/4 throttle. Reminds me of the way VW beetles ran up hills when they needed a valve adjustment /ubbthreads/images/graemlins/devilgrin.gif
 
I am going to add a caveat that I didn't see in Jack's piece on cleaning the carbs. Don't be tempted to use any kind of abrasive (sand paper, steel wool or even a greenie) on the inside of the vacuum chamber Jack refers to it as the bell) or on the edges of the piston that mates withthe sides of the vacuum chamber when cleaning them. Doing so will destroy them, use only the carburetor cleaner and follow up with a lint free rag.
Good luck,
 
Julian, I asked this question a few weeks ago. I said intimidated instead of complex. Jack, along with others, gave me the same advice. Breaking it into step 1, step 2 etc made it a doable , even enjoyable job. My pistons looked about like yours also. It worked-and the car runs much better. Thanks again to these guys and good luck to you.
Paul
 
Sorry Steve. I think that the "precision" fill is a good thing but that it isn't absolutely necessary, at least I haven't noticed it being so in my experience. If you fill it as you say to about 3/8" below the top of the piston tube, when you insert the damper rod that metal displaces just about exactly enough fluid to bring the level up to the top of the tube. I suspect that the idea was to not waste any of that precious SU oil and to keep it from either being squirted out of the cap vent on the first hard acceleration or working it's way down the tube bore to contaminate the inside of the piston/chamber or into the intake where it might cause a little bit of smoke for a while, neither of which would be very high on a problem list. :smile:

And David's point is well taken. As a general rule any abrasive is a bad thing in dealing with SUs except maybe a gentle bead blast to clean some really unsightly stains. Having said that I have been known to use some paste paint rubbing compound on a cotton cloth to "finish up" the insides of the vacuum chamber rather than hit it with a bead blaster. Jack
 
Piston oil level is not critical in any case. Just fill almost to top and go for it. Just takes a thimble full or so, look at the tube when you have it apart.
 
In the dealer shops, we dump in some oil, screw in the cap, push up on the piston "smartly", and what shouldn't be there will squirt out of the little hole, and may hit you in the eye if you aren't careful. What's left should be there, and will be at the proper level.

Peter " we don't need to measure" C.
 
I agree, the "exact" level isn't important. But you don't want to overfill for reasons mentioned above and not putting enough can be a problem as well. Filling before reassembly is simply easier and cleaner.
 
Well, that was a blast. I removed the sticky one, and it was well and truly stuck. I had to use quite a bit of carb cleaner before it came free. After a good clean around the piston moved in and out very nicely: those things are machined to very close tolerance, because just the suction seems to prevent the piston from falling out if you turn the unit upright.

Then I did the other, for good measure. I also inspected both needles, and they were in very good condition.

After re-assembly the car fired right up and ran as sweet as a nut! It seems to idle a little high at about 1500rpm.

Also, a strange thing: the offside rear wheel slowly rotates when the engine is running (the car is up on four jacks).

Anyway, I'm well pleased. Thanks for all the input on this one.
 
[ QUOTE ]

Also, a strange thing: the offside rear wheel slowly rotates when the engine is running (the car is up on four jacks).



[/ QUOTE ]

That sounds normal. My TR3 does the same thing... kinda scared the crap outta me the first time it happened but my 5 year old thought it was hilarious. If you grab it with barely any force at all it will stop (be careful if you're feeling adventurous)

My theory is that with the clutch out and the tranny in neutral there's enough flow in the oil from the spinning input shaft to turn the driveshaft when all four wheels are off the ground.
 
[ QUOTE ]
[ QUOTE ]

Also, a strange thing: the offside rear wheel slowly rotates when the engine is running (the car is up on four jacks).



[/ QUOTE ]

Hi Jim,

Interesting! Any theory as to why only one wheel spins? Perhaps my brakes are binding a little on the other: I still need to examine them.



That sounds normal. My TR3 does the same thing... kinda scared the crap outta me the first time it happened but my 5 year old thought it was hilarious. If you grab it with barely any force at all it will stop (be careful if you're feeling adventurous)

My theory is that with the clutch out and the tranny in neutral there's enough flow in the oil from the spinning input shaft to turn the driveshaft when all four wheels are off the ground.

[/ QUOTE ]
 
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