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MG MGA - SU Damper Cap Unscrewing
|Three times now, in as many weeks, I have lifted the bonnet to find the front SU damper cap unscrewed. The threads on both the cap and damper pot seem ok - the cap screws up tight and does not 'jump' the threads. I use thin bicycle chain-type oil in the pots (it is called 3-in-One over here).|
The only point I would make is that I have horrible black plastic caps, not brass. Going to get brass caps now regardless.
What is going on? Do I have a midget in the engine compartment trying to sabotage me?
I would look for a sign that something has changed somewhere close to the front carb, causing a different vibration pattern, in turn causing the cap to unwind. Fan belt starting to wear? Bearing on Dynamo playing up? Engine mount losing its strength/elsaticity?
I just spoke to Bob West. He said to check that there is the washer in place. He also said to check for a hairline crack in the casing - has been known.
|Just spoken to Burlen. They can supply SU hexagonal brass caps that will need to be drilled for an air hole. Is it straight forward to transfer the existing rod and damper? They were talking about supplying matched sets that included the main vacuum piston assembly. They then went on about ensuring I ordered exact rod length and ball joint(?). Seemed a bit puzzling to me as I did not have the offending part in front of me. I was beginning to wonder if we were talking about SUs!|
My brass damper caps (washers in place) have always unscrewed themselves (have done this for years). It's a normal part of my routine to tighten them. Sometimes when I check the screw threads have completely worked themselves out of the carb body, so they're just floppin' loose - but no harm yet. This sometimes gets a little oil (3-in-1) on the carb.
In other words, it hasn't proven to be a problem for me. It's unlikely that they will fall out and just as unlikely for this to be a source of contamination in the dash pot oil.
I agree that the brass ones give a better appearance.
1960 MGA 1600
I have had at least one damper come loose - but they don't flop out and get lost.
I have read about the different dampers, some have a hole and some don't. There was also a change regarding the length of the piston (See MGAGuru article about that.)
Mine don't have the holes.
Also, I use 20w oil, isn't 3-in-1 pretty thin? Probably allows a leaner mixture on acceleration.
Good luck and always tighten up!
|Interesting that I am not the only one. This has only just started happening with me and I have been running the same carbs with various viscosity oils for the last 9 years. |
There has to be an engineering reason. It should not be acceptable as the norm as stated by Gary: "...have always unscrewed themselves (have done this for years). It's a normal part of my routine to tighten them."
My 3 instances are exactly as Gary describes: "..just floppin' loose, but no harm yet." The only reason I raised the lid to take a peak was a slightly high idle and reduced choke requirement from cold. Otherwise I had not picked up on any other performance changes.
|Steve I would get most haunted in if I were you|
|g c pugh|
|My tame aerospace engineer friend just dropped by. I explained the problem and, without hesitation, he said that the cause is the fibre washer. The washer provides a small preload on the thread. It will have lost its compliance or resiliance. Replace.|
|The advantage of the plastic topped dampers is that it isn't as easy for people to overtighten and split the top of the carb dashpot.|
There is a fiber washer under the damper. If you use a slightly resilient washer it will act as a sort of lockwasher and help retain the damper. If you are really worried about it put a dab of hardening Permatex gaskey goo on the threads (Loctite is overkill)
|Hi Steve. You might try a tiny dab of locktite on the threads. The light duty locktite will come free using just a wrench, but would likey put an end to the loose dampers. Cheers, Glenn|
|This happend to me while 1000 miles from home once. The threads were worn and I could not secure the damper at all. I secured the damper using duct tape across the cap and down to the chamber body. I ended up later just swapping the front damper with the rear. Problem went away. They are still secure to this day|
Any aircraft mechanic would probably safety wire it!
|B****r. Just got home from work and had a closer look. Thread on the plastic cap is ok. It's the thread on the aluminum chamber that is stripped! Not many options there, unless one of you guys can tell me a fix.|
They are reconditioned carbs that I bought 9 years ago. Maybe the damage was caused by a previous brass cap. Seems unlikely that plastic has stripped aluminum?
Swapped over the caps as described by Chuck. Seems a tighter fit.
Can I just buy a new chamber or are they matched sets with the piston?
Perhaps I can convince her indoors that I have to convert to a supercharger - has only one carb!
Glenn, I am glad you kept your Linux away from my thread. More than 2 damper caps would throw me!
|Steve, I've never used this but it may work.|
Permatex Stripped Thread Repair
Says it repairs threads in most metals. There isn't much stress on the threads in this application.
|Chambers are most definitely matched to pistons - very picky. There are enough threads that you can salvage by filing off about 1/8" from the top of the chamber; or get a clever lad to machine up some longer brass caps. I would try the gooey stuff on the threads first, without any washer, as that gives more reach into better threads. 3M yellow weatherstrip cement works well and lasts for years and many removals. Plastic can be extremely abrasive, especially if any grit has got embedded it.|
|Chuck and FRM|
Thanks. Some good ideas there. I will look in my local shop for the thread repair epoxy. Loctite products are the standard here.
I was wondering about shortenening the chamber by 3mm (the depth of the cap thread). However, this would also mean taking the same amount off the piston barrel as it comes to the top of the chamber. The rod would also have to be shortened. Any comments?
|Some but not all carbs would need to have the piston hollow shaft shortened, same with the damper rod. Check to see what limits travel on yours. Shortening either or both will not generally cause any functional difference, aside from reserve oil capacity. Some carbs in odd apps came that way for bonnet clearance, etc. |
|A comment about drilling a vent hole. The original carbs as fitted to MGA needed a hole in the the brass caps (1/16" I think) to equalise the pressure on the piston. Without it they do not work correctly. |
|And another thing - my brass caps always come loose if I forget to nip them tight with a spanner. But not too tight Steve!!|
|Some carbs are "dustproof". These have a hole drilled through the diagonal rib at the junction of the neck with the main vac chamber; these use damper caps without holes. The space in the damper area is vented to the main vac chamber, so dust does not get in. |
Carbs without the diagonal bit or with it not drilled, use damper caps with vent holes. These vent to the outside, and do get dirt and water in the damper oil.
If you fit vented caps to chambers with drilled holes, then you have created an air leak into the vac chamber, with resulting unpredictable and rich mixture, especially at low throttle openings. Compensating this by leaning the idle mix adjustment will give lean mixture throughout the range = not good.
I have the diagonal ribs. I don't think they are drilled, but I will check. The hole in the cap could always be for the aircraft mechanic to safety wire!!
I swapped over the caps between the carbs last night. Both were still in place when I got to work. However, rather puzzingly, idle has now risen by 700rpm from the original position. I cannot fathom it out. Any views?
|Can anyone tell me the thread size at the top of the piston chamber?|
|My front one undoes itself as well and it is brass - as did the plastic one on my GT - could be the washer - will have to try it - don't thnk the threads are stripped - cheers Cam|
I'm pretty sure that all A used the dustproof carbs, so the diagonals should be drilled. If you have vented caps, it will screw up the mixture - rich- and this may go back to your problem of poor mileage. Remember that carb response to fuel flow is non-linear, so predictions on the effects of non-standard compensating adjustments can go surprisingly awry. But switching the dampers around should not do anything to idle, assuming both are drilled or not drilled. This sounds like loose carbs or similar, unless the dampers are binding somehow.
The threads are something special - British Standard Brass 26TPI or an optical standard.
|The undrilled caps were also used on the downdraught SUs which were I think only on some pre-war cars. With the dash pot on its side and a hole in the cap the oil would obviously have run out. To re fill with oil the diagonal was drilled and a small flip top oiler fitted.|
Where abouts on the diagonals are they drilled? It is possible that mine may have been filled as there is some rough material in the corner of both diagonals.
|From inside the damper chamber, connecting to the main vac chamber. About a 3/32 hole right through the length of the rib.|
Thanks. Yes, just found some diagrams of the 3 types of chambers on the H-type (the third is a shortened version). Mine is definately one of the undrilled chambers, so the drilled cap is correct.
Amazing what you learn about your car on this site.
Off into the garage now to decide what action to take.
|My understanding is that the 1500 had dustproof carbs (thus there were NO drill hole in the brass cap) and the 1600 did not (thus there were drill holes in the brass cap). If you look at a Moss catalogue, you will see that they have different caps listed for the different cars. Of course, with restorations over the years, you may have later carbs on an earlier engine, so by all means check the chamber as FR suggests.|
FWIW, I only finger-tighten my brass caps and they never get loose. They also have the fibre washers in place.
Any standard is possible with my car. When I bought it as a wrecked 1500 I knew virtually nothing about MGAs, just the old pre and post war T-Types. I sourced both reconditioned and new parts during the rebuild as needed. I exchanged the existing carbs for a reconditioned set etc, not really understanding the different modification standards of the MGA during its production cycle. It is, therefore, quite possible that I have 1600 carbs, or maybe not. It is a fascinating journey on this forum finding out all the detail. Don't get me wrong, I am not a concours buff, but I do enjoy the knowledge of originality, that is the difference.
FWIW, I have just received two new brass caps. The depth of thread from the underside of the head is 5.16mm compared to 4.38mm on the plastic cap, so I have a smidge more thread to grip on. The cap is also a tighter fit so there should be no more mystery 'undoings'! In fact I need a spanner to tighten, whereas before it was fingertip as you describe.
This thread was discussed between 03/04/2007 and 05/04/2007
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