Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Help with SU Carbs

Last year I had the idle and running of the car (1960 +4 with TR3 engine) dialed in perfectly and she purred like a champ, and have adjusted the SUs many times on the car and feel confident futzing around with them.

Since last year I have replaced the interior, brakes, parts of the gearbox, and had the engine completely serviced and 83mm liners replaced with 87mm liners. After the engine work, the shop ran the engine on their stand with their shop carbs and it did well. I have since put the stock SU H6 carbs (SM needles) back on and am running into difficulties. When I first turned the engine over, fuel poured out of the front float bowl overflow pipe onto the wing. I cleaned out the float bowl valve and now it's doing well. I also disassembled and cleaned both carbs, even thought about polishing them. Installed new rubber/cork gaskets in both. Both carbs do have some degree of throttle shaft play and this will be something to address in the near future (they were doing fine when the car was last run). The front carb was leaking some petrol around the jet bearing, but after an overnight soak in oil to renew the gaskets it is no longer leaking.

When the car is running at idle and warmed up it is super-d-duper lean judging from the exhaust color, pulse, smell, piston-lifting-pin causing rough idle and RPM to fall, and the temperature creeps up quickly. While running, I sprayed all over the carbs and linkage with carb-cleaner but found no evidence of a vacuum leak (I expected to). When I place a carbsynch tool over either carb, the idle speed increases substantially. The question remains though - why is this sucker so lean?! Should I have to replace needles when bumping up the displacement? If so that's awesome and an easy fix.

Another think I noticed when cleaning out the carbs is that mud-daubers had built nests in the passageways that connect the area above the carb piston to the outside air (the two small holes on the face of the carb). Not sure if this could have been 'naturally' enrichening the mixture and now that I've removed the mud they are running lean.

Any suggestions as to where to go next from here?


As an interesting aside. Mud daubers really do like to plug up tubes and cause problems, I'm unsure if you guys in Europe or OZ ever have to deal with them:

I've never heard of mud daubers before, interesting little blighters. I don't think we get them in the UK.

I expect Andy G (MOG615) or SpannerJuggler will be along soon to advise properly, but logic tells me that with an increase in bore you will be sucking in a greater volume of air so therefore you are bound to need to increase the fuel to maintain a correct mix at any given setting.

I originally thought that, but at the same time if it's sucking in more air wouldn't the air just "pick up" more fuel from the jet as it goes by at an increased velocity?

Mud dauber nests are creepy, each "chamber" contains about a dozen paralyzed but alive spiders. The little baby dauber eats the spiders alive. Or if you break the nest off of your house/ceiling you get dozens of paralyzed spiders falling on your head. Another cool quote on them:

I'm so glad that there isn't a human version of that parasitic wasp...

As to the SU issue, there is useful information here:

Burlen

They own the SU Brand and make all the parts, including new carbs. A number of data sheets worth reading.

If the carbs were set up "just right" before the rebore it is possible that the new engine will need re-jetting. Anything that increases gas flow may make the mixture lean.

CV carbs use the interplay between main jet and needle profile to manage the mixture. Both needles and main jets will wear and should be replaced if there is any doubt.

To be honest SU carbs, like any CV carb, once set up shouldn't need any adjustment. If they need constant adjustment something is wrong, or they may just be worn out.

Increasing volume from 83mm pistons (1991cc) to 87mm (2188cc) will almost certainly need a different needle on the SUs.

I would start by trying to assess if you are indeed running weak (as I suspect) .
What do the plugs look like , are they blanched white, a sign of a weak mixture. Or you can use an analytical device, one of the popular ones was Gunson ColourPlug.

Once you have this I would start with the "normal" rich needle for these carbs which is the RH. Again run it up through the rev range to see where it is running well.

The Burlen website, that Peter has posted, has a lot of useful data, and most of the information is pretty clear and logical. If you follow this you won't go far wrong.

Plugs didn't look too weak, wish I had a colorplug and I know they're fairly inexpensive but haven't gotten around to purchasing one.

Also looking at www.sucarbs.com I might get this guy to go ahead and clean them up even more, and replace the throttle shafts.

Per WINSU, the RH and SM needles are pretty similar at least at idle/slow running speed.

I did some work with Burlen a number of years ago trying to fit Amal carbs to the "New Triumph Bonneville" (an interesting journey) and learnt a lot about old British Carburettors.

One piece of advice I thought was a sales pitch, but realised wasn't, is that once the body of the carb has worn it is better to buy new, not re-manufactured. A new 2014 SU will be made to tighter tollerances than the original new SU, made of better alloy and will simply work properly.

I know there is a certain attraction to makeing the old one work, and many people ride this wave offering all sorts of services, but the soft alloy used in the 60s and 70s wears, not just the piston bore and the throttle spindle bearing surface but the internal airways. They corrode and get bigger...and once they are no longer as they should be it is game over.

SUs were better made than Amals, btw...old 70s Amals are just horrible.

Might be worth a search of the TR Register forum. Quick search of 'carb needles' brought up lots of threads including this, which indicates that SM is the one recommended for all engine displacements

http://www.tr-register.co.uk/forums/index.php?/topic/47505-su-carbs/?hl=%2Bcarb+%2Bneedles#entry379185

(don't know why it hasn't come up as a link, it normally does when I copy and paste?)

If the revs are increasing when you hold the syncro-check against the carb body its a classic sign of a weak mixture. If you have increased the gas flow through the ports you will need to alter the mixture and this may or may not require a new needle. SU jets (old memory now so check these details) of that era came in 2 sizes, plus left and right handed for single and twin installation, its unlikely you will need to rejet unless they are worn but what you describe does not suggest jet wear. Step one is to try and reset the mixture, remember to follow the Plateau tuning method and set to the rich side then back one flat. Short version is to go rich until she stutters when you raise the piston 1/16" to 1/8" and then the revs increase then fall back, from that position weaken one flat only. If you lift the piston from there you should get a very slight 100-150 RPM increase and then it drops back. Much smoke and mirrors about Dashpot oil but before I taught the subject at BL Service Training I spent 4 weeks working at SU Butec in the workshops fitting and setting up the carbs and they ALWAYS used 20/50 engine oil, anything else is too thin unless your in an extreme cold climate then use ATF Fluid. Some will bitch about this and claim 3 in 1 is the way to go, make your choice but engine oil is what SU and BL used in production (the training school was attached to the Triumph plant at Canley and I know what they used on the line!). Syncro both carbs air flow rates and balance them so you don't have a 'lead' carb, I used to use tissue paper under the throttle stops (idle screws) as you pull the throttle cable the airflow from the cooling fan should blow both pieces of tissue away together if both linkages are lifting at the same time.....simple and a whole lot cheaper than a pair of air flow gauges ;-).

Old school i like it.....