Su Carb Fuel Leek

[Tom Bryant]

On 3/25/2021 at 6:31 PM, alanwcoote said:

This does confuse me.

I also had leaks from same area on my 2, so spoke to burlen and found out about the updated viton kit since purchased sorted done.

Now why do suppliers continue to supply not fit for purpose cork seal kits ?

More important why are people trying to save pennies on multiple use of o rings and different washers when the correct kit is available . Quote:you have to buy the whole thing and it’s quite expensive, Bloody hell guys your talking about £25 k + cars and spending what £50 on item that keeps petrol away from your sparking electrics.

Al

 

What confuses me is why anyone would buy this kit from Burlen, which contains rubber (their description) seals, which are inherently sticky, for 37.08 British Pounds, when they could easily just plop in some far superior Teflon seals for about a buck (if they buy them from the right source).  Although I would certainly be interested in the field experience of those who have purchased this rather expensive kit, my guess is that the long-term results will be considerably less positive than using the Teflon o-rings.

Tom

[NCS_TR3A]

On 3/1/2021 at 11:08 AM, Tom Bryant said:

A few comments:

1. Lots of luck with the Viton.  They don't work.  Well, they might seal the leak, but they have too much friction, and the jet won't return.

2. PTFE (Teflon) does work, but you need to install two of them in place of each original cork jet seal.  And, of course, the two dished washers must be installed the proper way around.

3. Neil's problem (with leaking Teflon seals) is not the usual outcome.  I've installed thousands of the Teflon o-rings, and although there have been a few leakers, all of them were due to some sort of fixable problem, sometimes a mistake of my own, never the PTFE o-rings themselves.  I suggest he try again, and if there is a leak, look for the source of the problem, and not give up until he finds it, because it will be fixable.  And, once properly fixed, those PTFE o-rings will not fail in your lifetime.  The holes in the sides of the jet should be chamfered with a file to remove the sharp edges so they don't cut the PTFE seals as they slide over the holes.

5. Back to the OP (Chipperman52) and the loose conical ring above the gland nut:  Sounds to me as if the cork ring itself is too thin.  I always toss those in the trash and replace them with a 1/8" X 3/4" ID ("Dash 210") Buna-N o-ring.  That should do the trick.

6.  Don't even think about tossing that copper washer above the upper jet bearing.  It's absolutely necessary for two reasons:

a. It seals the jet bearing so that fuel doesn't leak around it and past the jet orifice, upsetting the air/fuel mixture.  Leakage here will not come out the bottom of the jet where it would be visible, but it could make it difficult (or impossible) to get the proper air/fuel mixture.

b. It must be present (and of the proper thickness) to lock the whole jet bearing assembly in place so that the jet adjustment nut can be turned without also turning the male-threaded jet bearing itself.  "Proper thickness" is 1/32nd inch (0.031") no more and no less.  Many repair kits come with a much thinner (typically 1/64th inch) copper washer.  That should be either thrown in the trash, or if necessary, doubled up to get the proper 1/32" thickness.  Replacing that 1/32" copper washer with fiber is, well, just plain silly.  Don't do it.  (Even if that copper washer did leak a smidgen, you'd never know it by examination of the bottom of the jet.  Any leakage would be upward, into the throat of the carburetor.  Small amounts of leakage past that copper washer, if it should occur (unlikely), would not be enough to upset the air/fuel mixture.  So... there's just no reason to ever even think of using a fiber washer there.)

7. In answer to Neil's comment:

"I'm struggling to think why the bottom nut would be tight before you got enough pressure on the cork seal and washer to stop it spinning."

It's not the pressure on the cork seal that prevents that male-threaded piece from turning; rather, it's that 1/32" copper washer that keeps the thing in place and prevents both turning and leakage.  It's also quite possible for the cork seal (or Buna-N o-ring) to be "wicked tight" and the upper jet bearing to be loose and leaky, particularly if that 1/32" copper washer is missing or if it's been replaced by a 1/64" copper washer.  But again, I must clarify, any leakage past that copper washer and upper jet bearing would be upward, into the throat of the carburetor, and would never be visible at the lower end of the jet.

8. Sometimes fuel leaks past the screw that holds the yoke onto the bottom of the jet.  Such leaks can be fixed with Loctite.

9. Frankly, I don't know why everyone doesn't just give up on cork jet seals and move on to the far superior Teflon.  There is now a far better material than cork, so why Burlen, Moss, and everyone else, just doesn't enter the modern age and toss the cork in the dustbin of history is beyond me.

Tom Bryant

 

 

Right, I've revisited this on my car now the engine is back in. Tom you are a Saint!!!!!!!!!!!!  I got the carbs on the bench and really thought through what was going on. The key information you shared, for me, is that the jet bearings should be locked in place by the bottom nut. I had never registered that in my mind before assuming the internal spring would have the stregth to push the seals to make good. 

   So on stripping down my carbs I found the top jet washer was only 10 thou. I did order 2 new washers from Moss and they were 40 thou. Just my luck. I have quite a selection of old washers (which were all 10 thou.). This allowed me to do some testing to seel how th I the washers needed to be to truly clamp jet bearings in place. On carb 1 (nearest the fan) a 30 thou washer wasn't thick enough, but the 40 thou from Moss did it. On carb 2 a 30 thou would have done but I decided that given I have a 40 thou it was better than using three old ones until I could find one of the right thickness. 

   So all back on the car using Teflon o rings and I don't have a leak, Great................... But I couldn't get the mixture weak enough even withe the jets fully up. Felt like I was back were I started to be honest. With the 10 thou washers I could get the mixture right but had a leak. 

     When I looked at the top jet when it's placed in the carb it's flush with the chamber base when there is no washer on it. So when you add a 10 thou washer it's 10 thou lower and with the 40 thou washer it's 40 tou lower and that's enough to remover the tolerance to tune the mixture. 

   So I need to find a way to raise the jets. I guess reducing the top washer thickness and increasing the bottom washer thickness would theoretically work, but I didn't have the washers to try that. Because I have many spares I took a plug and filed a bottom jet bearing down slightly at the end the nut attaches. This allowing the nut to travel further up (there is plenty of thread). I filed 40 thou, about 1 mm off it, which allowed the jet itself to rais until it is level with the bottom of the throttle chamber when fully wound up. Refitted and I now have no leaks and the ability to adjust the mixture from way to weak to way to rich. I know I'll have offended purists with this but I couldn't see another way for. The jet bearings to be higher up in the chamber without having a small leak. 

    I haven't tried the viton washers in the end so cannot comment on these. But the Teflon (PTFE) ones are working and have no restriction on the choke jets going back up. Just wanted to report back so others in the future so the issues were not Teflon washer related. 

Thanks,  

    Neil

Edited May 27, 2021 by NCS_TR3A

[Tom Bryant]

14 hours ago, NCS_TR3A said:

Right, I've revisited this on my car now the engine is back in. Tom you are a Saint!!!!!!!!!!!!  I got the carbs on the bench and really thought through what was going on. The key information you shared, for me, is that the jet bearings should be locked in place by the bottom nut. I had never registered that in my mind before assuming the internal spring would have the stregth to push the seals to make good. 

   So on stripping down my carbs I found the top jet washer was only 10 thou. I did order 2 new washers from Moss and they were 40 thou. Just my luck. I have quite a selection of old washers (which were all 10 thou.). This allowed me to do some testing to seel how th I the washers needed to be to truly clamp jet bearings in place. On carb 1 (nearest the fan) a 30 thou washer wasn't thick enough, but the 40 thou from Moss did it. On carb 2 a 30 thou would have done but I decided that given I have a 40 thou it was better than using three old ones until I could find one of the right thickness. 

   So all back on the car using Teflon o rings and I don't have a leak, Great................... But I couldn't get the mixture weak enough even withe the jets fully up. Felt like I was back were I started to be honest. With the 10 thou washers I could get the mixture right but had a leak. 

     When I looked at the top jet when it's placed in the carb it's flush with the chamber base when there is no washer on it. So when you add a 10 thou washer it's 10 thou lower and with the 40 thou washer it's 40 tou lower and that's enough to remover the tolerance to tune the mixture. 

   So I need to find a way to raise the jets. I guess reducing the top washer thickness and increasing the bottom washer thickness would theoretically work, but I didn't have the washers to try that. Because I have many spares I took a plug and filed a bottom jet bearing down slightly at the end the nut attaches. This allowing the nut to travel further up (there is plenty of thread). I filed 40 thou, about 1 mm off it, which allowed the jet itself to rais until it is level with the bottom of the throttle chamber when fully wound up. Refitted and I now have no leaks and the ability to adjust the mixture from way to weak to way to rich. I know I'll have offended purists with this but I couldn't see another way for. The jet bearings to be higher up in the chamber without having a small leak. 

    I haven't tried the viton washers in the end so cannot comment on these. But the Teflon (PTFE) ones are working and have no restriction on the choke jets going back up. Just wanted to report back so others in the future so the issues were not Teflon washer related. 

Thanks,  

    Neil

A few comments:

1. The fact that using a 0.030" (close enough to the proper 1/32") washer didn't allow you sufficient jet adjustment for a lean mixture indicates that, either:

a. Your jet orifice is worn, or

b. Your needle is worn, or

c. Your needle was inserted too deeply into the piston, or

d. Your jet might be a bit short; that is rare, but it does happen.

 

2. Your solution doesn't offend this purist, but I personally would have tried a couple other things first:

a. If the jet orifice is only slightly worn, that can be fixed by lightly tapping around the top of the jet with a hammer to collapse the hole, then reaming it to proper size.

b. A worn needle should be replaced, but I almost never see a needle worn enough to require replacement.

c. Rather than file the bottom of the jet bearing, how about dropping the needle down a smidgen?  No one will likely ever notice, and it's reversible.

d. A short jet should be replaced, but dropping the needle a bit would work, also.

Tom

[NCS_TR3A]

Thanks Tom, I have a collection of needles and jets, the needles are all SM although one does look a little different to the eye (I haven't measured it but fatter I would say about half way down) the other 4 all look the same. 2 jets and needles are new as in probably only done 2k miles. Changed at that time because I really couldn't see what else it could be even thought they didn't look bad. I've certainly seen bad jets on minis I used to play with (lack of being centralised) for fiends and family. 

     I had tried the dropping the needle previously and it helped but not much. Probably could have dropped it further though. 

     I do have some concerns that I'm using SM needles on a 87mm bore engine. It looks good now on tick over and actually OK throughout the range. I'm just thinking it's a fair change from the 83mm engine it should be so the needles should be of the richer type. I assume all the needles have pretty much the same idle profile though. 

     I have two standard lower jets so I do and did have the option to go back but I'm happy to leave it without the slight petrol leak and having a good mixture. If there was a big lesson it's the Jets need to be clamped in place by the tightening of the large nut, if they are lose after tightening this you will get a leak and you cannot rely on the internal spring to apply enough pressure to stop this leak. This was my initial working assumption and the clarity on this has made a big difference to solving the issue. 

Neil

[Tom Bryant]

19 hours ago, NCS_TR3A said:

Thanks Tom, I have a collection of needles and jets, the needles are all SM although one does look a little different to the eye (I haven't measured it but fatter I would say about half way down) the other 4 all look the same. 2 jets and needles are new as in probably only done 2k miles. Changed at that time because I really couldn't see what else it could be even thought they didn't look bad. I've certainly seen bad jets on minis I used to play with (lack of being centralised) for fiends and family. 

     I had tried the dropping the needle previously and it helped but not much. Probably could have dropped it further though. 

     I do have some concerns that I'm using SM needles on a 87mm bore engine. It looks good now on tick over and actually OK throughout the range. I'm just thinking it's a fair change from the 83mm engine it should be so the needles should be of the richer type. I assume all the needles have pretty much the same idle profile though. 

     I have two standard lower jets so I do and did have the option to go back but I'm happy to leave it without the slight petrol leak and having a good mixture. If there was a big lesson it's the Jets need to be clamped in place by the tightening of the large nut, if they are lose after tightening this you will get a leak and you cannot rely on the internal spring to apply enough pressure to stop this leak. This was my initial working assumption and the clarity on this has made a big difference to solving the issue. 

Neil

Some more thoughts:

1. To compare two needles, simply put the two of them in the jaws of a dial caliper and push them through.  If the large ends remain roughly aligned as you push them through, then they're close enough to the same size throughout.

2. One of the disadvantages to your solution of filing down the end of the jet bearing, rather than dropping the needle is that results in a higher position of the jet and of the jet adjustment nut.  And that could make it more difficult for the jet lever arm spring to pull the jet levers back up when you push the "choke" cable in.  All in all, I'd say it would be preferable to drop the needles, rather than file off the end of the jet bearings.

3. With your comment about engine size (i.e., bore or displacement) you fall into a common fallacy.  The displacement of the engine has virtually nothing to do with what the needle profile should be.  To burn properly, the air/fuel ratio should be the same, regardless of engine displacement, and regardless of how much air that engine is breathing.  And one of the neat things about SUs is that they actually measure the amount of air flow and provide the same amount of fuel for that amount of air flow, regardless of what the engine displacement is.  It does get a little more complicated than that; the cam profile does affect how much backflow there is through the carburetors, and the air tends to pick up fuel each time it passes over the bridge, in, and out, and back in again.  But, to a first order of approximation (and perhaps even to the second order) engine displacement is irrelevant to needle profile.

4. "Idle profile" is something I would never have thought of.  Not quite sure what that might be.  Here's the way I look at it:  SUs are generally adjusted to give the proper mixture at idle.  Assuming there's sufficient adjustment available in the jet, and assuming that everyone does the job properly and achieves the desired running characteristics at idle, then at idle it really doesn't matter an iota what the needle profile is; everyone arrives at the same mixture on every engine and with whatever needle is in there.

Tom

Edited May 29, 2021 by Tom Bryant