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MG MGB Technical - Su Carb Parts Wear Factor?

Being a person blessed, or cursed, with a ongoing need to question generally accepted norms, I pose this question - what significant wear is considered as excessive with regard to fairly static SU carburettor parts?

It is fairly obvious when an older throttle spindle is worn significantly in just two sections, because of it's limited pivoting action, and I accept linkages wear etc, then we also have the suction chamber piston and shaft, but what of the actual fuel needle, and it's jet, and the butterfly? Where would the wear be occasioned to a butterfly, and more so to a needle and jet?

I would like to have your views on this,

Maurie P
R M Prior

Throttle spindle and bushes is the main point of wear, but even then only after many tens of thousands of miles.

Fixed needles and jets shouldn't wear at all if they are set up correctly, the only problem I've heard of with HS jets is the seal failing where the jet pipe screws into the float chamber.

swinging needles and jets will in theory but over an even longer period than spindles and bushes. My V8 HIFs were 'old' when they came to me, I've done over 100k, and not found any need to have them rebuilt or any parts replaced from 'wear' except one float and both valves.

Butterflies shouldn't wear, they don't touch the throat except perhaps right by the spindle and there may be something preventing sideways movement there.

HIF enrichment valve is possibly the biggest source of problems when the O-rings wear, but again they are 'original' to mine. Again in theory HS jets and guides may wear with them going up and down for enrichment, but I've never heard of it.

SU carbs, particularly HSs, are incredibly trouble-free when set-up correctly and not fiddled about with afterwards. There is the adage "90% of problems with SU carbs are caused by Lucas ignition", but when that is set up correctly it is trouble-free as well!
paulh4

I agree with Paul if the carbs, linkages and cables are set up properly and not constantly fiddled with they should last a very long time. A very occasional clean and lube I think helps especially if the car isn't kept in a pampered garage or used where the air carries a lot of grit or other stuff.

I think as often prevention is better than cure, if the carbs have previous been neglected or set up wrong and have started to wear badly then if not repaired/refurbed the wear and constant fiddling will increase.

I had some SU carbs that didn't look nice but had a little work done on them but after I wasn't happy with them so bought a brand new set and wished I'd done so well before. I only had trouble with them after one of my usual not completely successful interactions with professional specialist garages.

I don't know if it's the same for the B but with the Spridget even SU Burlen don't sell the correct full set of linkages (not that they want to know or care, unlike others I don't find them a good company to deal with and I'm not alone in this). Normally you shouldn't need new linkages anyway - but that does depend on what previous owners have done.
Nigel Atkins

While what Paul says about items that shouldn't wear is correct, the key word is "shouldn't". In practice, over their 50 years or more of life most carbs have had attention from people who don't know how to set them up correctly and who have dived into them because they have heard twin carbs are trouble. (When in fact they don't change settings on their own and as Paul notes they don't wear much; it's the people who fiddle with them to "tune" a fault elsewhere who are the problem).

OK, enough of my favourite rant and back to what could wear (noting again that most shouldn't if set up correctly) and when to replace. It should be remembered that HIF jets and needles inherently wear due to being in constant contact.

Jets: . Mixture adjustment can compensate by and large for jet wear - up to the point where the jet reaches level-ish with the top of the bridge. After that it will rapidly reach the stage where the piston rests on it at idle and become impossible to make much sense of. Then it is time to renew it if needle wear isn't the cause of getting out of range. We have a gauge we use to measure them, and have found this gets close to happening with a bit over a thou of wear.

Metering needles: the most wear occurs around the idle position, which effectively changes the profile of the needle. The end result of too much wear is that when the mixture is correct at idle it becomes a tad lean everywhere else due to the jet having been moved up to compensate. This difference is largest once clear of the worst wear on the needle, typically around a constant 30-35 mph in top gear on a level road. If the engine starts hunting in that load condition when the mixture is right at idle then it is time to renew the metering needles.

Throttle discs: they can wear around the sides, making it impossible to slow the idle. But check first that it is not the more common fault of incorrectly aligned discs preventing the throttle from closing fully.

Needle valves: wear in the original brass tipped valve is easy to spot. poor sealing results leading to excessive fuel level creep (makes mixture get rich fairly rapidly while idling) or overflows.

Throttle spindles: principally affect idle quality. when bad they also result in idle speed often being different when the throttle is blipped. Air leaking through badly worn spindles can also contribute to run-on.


Paul Walbran

Paul, just re-read your post and agree 200% with that last statement. Carbs go out of tune only because people think they have and so adjust them in an effort to fix a problem that is actually somewhere else, and adjustment doomed to fail because that's not where the problem is. And so they tell the next person that yes, it's true, twin carbs are devils, I've spent the whole day trying to get them right and they still aren't.

My mother and sister both had MGBs on which I overhauled their (worn) carbs and was the sole pair of hands for 20+ years after. Left the carbs till the end of each tune as should be the case, checked them, and every time no adjustment was needed.
Paul Walbran


Thank you Paul, Nigel and Paul, for the considered opinions. The reason I started this thread was due to the expectation by SU Burlens, to suggest/insist? that we should be replacing all these parts, even if they are not worn as we have currently discussed.

Throttle spindles and bushes most definitely yes, if obviously worn, but jets? needles? butterfly discs? I might accept the latter, especially recently, as I noted on a rebuild that I am doing, that the disc had definite wear adjacent to both of the bush sides of the throat, which is logical, given the constant scraping against the carb body.

It is kind of guilt if one does not replace everything even when we might feel differently, but I guess Burlens feel that the average amateur mechanic won't understand the differences anyway, so they can sell more unnecessary rebuild kits etc.

I will take an image of a worn disc and post it up here this afternoon.

Maurie
R M Prior

This thread intrigues me a bit Maurie, mostly because I would have thought you should know the answers when you are advertising for sale-"fully reconditioned" SU carburettors
William Revit

Manufacturers and retailers are in business to sell you stuff regardless of whether you need it or not and regardless of whether it is right for your car or not.

I take a very dim view of SU Burlen. A pal bought a rebuild kit for a fuel pump and various parts didn't fit correctly. It turned out they had changed the base casting slightly, and as a result of that changed the components to match, but didn't change the part numbers. I've also heard that whereas there used to be a low pressure and a high pressure return spring for the diaphragm they now only have one, under both part numbers, and it is the high pressure one which can result in flooding in MGBs.
paulh4

I've put before about SU Burlen in my experience, and others, not being a good company to deal with and at one time at least (I don't know about now) the owners were concentrating on pedal car parts as they make more money with them.

SU loyalist won't hear of such stuff and are prepared to pay over the odds to get the brand name wrap on stuff, same with some other mg specialist companies, loyalty is good but not blind loyalty.

Not all manufacturers and retailers are in business to sell you stuff regardless, companies are run and staffed by people and different people have different ideas of what is acceptable or moral.
Nigel Atkins

Willy,

Two things - My "fully reconditioned SU carburettors" are exactly what I claim them to be. They are totally rebuilt, and are as described, and are supplied with, all new everything, except the carb bodies, pistons and suction chamber and float bowl. I fit new throttle bushes,spindles, butterfly discs, needles, complete jet assemblies and needles, float bowl needles and seats, gaskets, all necessary seals, brass piston securing screws, all new bolts, grommets, washers,and overflow pipes etc. Everything is new, whether it is needed or not, simply because, I can then sleep at night knowing that there can be no question as to the truth in my claim, of these carburettors, being "fully rebuilt"

The question raised in my thread - "Su Carb Parts Wear Factor?" is an another matter entirely - I was curious to learn whether others had a similar opinion, that the wear factors are indeed so bad that the parts referred to, had to be replaced due to excessive wear? The comments are interesting as not everyone agrees with SU Burlen and others with skin in the game, who try to coerce we mere mortals into simply buying more unecessary product.

Maurie
R M Prior

Just my ten pence on Burlen!

I have been using them for about 40 years off and on. Always found them helpful and benefited from trade discount, as I am trade.

That was until a few months ago when I needed to order quite a few kits. No trade discount any more and no reason given. Dealt with someone called Jamie who was worse than useless. Bad attitude and zero knowledge!

Placed the same order with Southern Carburettors who applied trade discount with no issues, and supplied all the parts required including the ones that Burlen were out of stock of. They also do Weber and Dellorto parts.

So I would recomend Southern and will never deal with Burlen again!
Colin Parkinson

Cheers Maurie-
Colin, thanks for the heads up on Southern Carburettors, I've been looking for a pair of Stromberg jet adjusters which they seem to have and some Dellorto emulsion tubes for a while now with no luck---I'll give them a try-Thankyou
William Revit

Willie, what emulsion tubes do you need. I may have some

Colin
Colin Parkinson

Maurie,
have you noticed the new parts aren't always the quality of the old parts. I think it was PaulW that suggested you have to go direct to SU to be sure of parts but I have and when comparing with the old parts they don't look as good to me (not an engineer or mechanic) and they look the same as the parts from other suppliers.

Decades back I took my MG to a rolling road tuner and it had an odd fault so I bought some linkage parts as they didn't look great to me and it'd save time and effort on the rollers. The tuner, a great old style chap that owned the garage but stuck to the rolling road tuning side, was horrified told me not to take the parts out of the package as he'd sort any issues. Took him about 15 minutes to sort when he had his eureka moment. Turned out to be a bent hinge pin on the organ pedal, he apologised and said he should have thought of that sooner as he'd had it before on a IIRC Leyland (DAF) van. Proper old school and honest guy, a couple of times he didn't even want to charge for his time and had to be talked into taking beer money even. Very unlike and opposite to the majority I've encountered in the motor trade.
Nigel Atkins

Colin,
I'm amazed you got so far for so long with SU.

A mate works for a company that was spending a few thousand a year on SU parts but SU didn't want to give them a trade account so they got trade discount by ordering through the chap whose name I always forget but is well known and been doing SU carbs for many years.

I rang SU and tried to tell them their Mini Cooper linkage kit doesn't fit the midget and Sprite as they advertise. I was put through to someone in "sales" but he was far from the stereotype, very unmotivated, unimaginative and unhelpful, just didn't want to know even though I was hinting and suggesting courses of action that could be taken having done such stuff in previous lives. They still advertise the kit as such on their very confused website, that won't find some of their own part numbers. It would have been bad enough if that was a one off but it wasn't.

Nigel Atkins

Hunt's Fifth Law of classic car ownership:

"Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals."
paulh4

Colin- Thanks for the offer, very kind of you.
Today I've been offered a set for a try which is great
6 x 7222.2 --I'll give them a go and if all is well which I'm hoping they will be, then talk him into selling them to me.
Thanks again
willy
William Revit

This thread was discussed between 27/09/2022 and 03/10/2022

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