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Carby Again!
https://www.ausmini.com/forums/viewtopic.php?f=2&t=55016		 Page 1 of 1
Author:  scotta_74 [ Fri Jul 10, 2009 6:43 pm ]
Post subject:  Carby Again!
HI all,

Once again I seem to be having issues with my carby. I have an 1 1/2 inch SU which most of the time idles and responds perfectly. Intermittently when I pull up at a set of lights the engine just stalls, other times it returns to an idle speed of about 800rpm and purrs like gerg's cat!

I cant find any air leaks and am scratching my head for any other ideas as to what is causing my problem. Any ideas????

Cheers,

Scott
Author:  gerg [ Fri Jul 10, 2009 7:24 pm ]
Post subject: 
Hey!, leave my cat out of it... :lol:
Author:  gafmo [ Fri Jul 10, 2009 7:32 pm ]
Post subject: 
three thing I would check straight off. Fuel Filter, Airfilter and Oil in the Dashpot
Author:  TheMiniMan [ Fri Jul 10, 2009 7:43 pm ]
Post subject: 
Scotta,,, do this for me & see what happens

--->

richen the mnixture just a tad,,,, reach under the carb & find the nut that will adjust the jet up or down (the mixture nut) ,,, then just wind it down a couple of flats/hex of the nut or even half of a turn (3x hex/flats) even one full turn (6x hex/flats) won`t hurt things but it`s just a test

then drive it & see if it keeps doing the problem thingy :-)
Author:  1310/71 [ Fri Jul 10, 2009 7:50 pm ]
Post subject: 
gerg wrote:
Hey!, leave my cat out of it... :lol:

I was expecting some Mrs Slocombe and her pussy jokes at least.
Author:  gafmo [ Fri Jul 10, 2009 8:45 pm ]
Post subject: 
1310/71 wrote:
gerg wrote:
Hey!, leave my cat out of it... :lol:

I was expecting some Mrs Slocombe and her pussy jokes at least.


Your wish is my comand, She wil be missed... :cry: yes she passsed away during the week
http://www.youtube.com/watch?v=19AQgpsIMVY

By the way as per Matt...to richen is Clockwise
Author:  scotta_74 [ Sat Jul 11, 2009 7:45 pm ]
Post subject: 
TheMiniMan wrote:
Scotta,,, do this for me & see what happens

--->

richen the mnixture just a tad,,,, reach under the carb & find the nut that will adjust the jet up or down (the mixture nut) ,,, then just wind it down a couple of flats/hex of the nut or even half of a turn (3x hex/flats) even one full turn (6x hex/flats) won`t hurt things but it`s just a test

then drive it & see if it keeps doing the problem thingy :-)


I went down (Clockwise) 6 flats and she ran like a pig with a massive flat spot. I went back up 3 flats and it runs like a scalded cat! So whatever was the problem now seems to be ok. Me so confused :?
Author:  TheMiniMan [ Sat Jul 11, 2009 8:02 pm ]
Post subject: 
ha,,, 3 flats down & she`s sweet

must have been a tad lean originally then & 6 flats down is obviously too rich

:-)
Author:  drmini in aust [ Sat Jul 11, 2009 8:06 pm ]
Post subject: 
TheMiniMan wrote:
ha,,, 3 flats down & she`s sweet

must have been a tad lean originally then & 6 flats down is obviously too rich

:-)

Get thee a real carby, they have proper calibrated jets you can change to get it right. Then, it stays that way. 8)
Author:  TheMiniMan [ Sat Jul 11, 2009 8:22 pm ]
Post subject: 
ha,,, true constant velocity & lovely progression was very hard to beat Kev,,, it took electronic fuel injection to better it (slightly) :-)

But Kev,,, I`m not arguing,,, i like webers/dellortos too :-) horses for courses :-)
Author:  aaron [ Sat Jul 11, 2009 11:35 pm ]
Post subject: 
The Su has it's uses, as does the weber and the weber clones.
SU's I believe are a better daily which also makes them good for motorkhana.

Weber's sound great, but meh, not for me.

Like any carb, you have to tune them, once you got it right you don't need to touch it. Simple really. :wink:
Author:  TheMiniMan [ Sun Jul 12, 2009 11:27 am ]
Post subject: 
well Aaron,,, you`re "sort of right" ,,, the SUs do tend to occasionally (over a long period of time) get their pistons & bells quite dirty & can do with (or rather "Need") the occasional cleaning,,, that`s one little thing to keep in your head

& Also---> the "later" SUs were fitted with a "Self centering" needle (Bias needle or sprung needle--whatever you like to call it) ,,,, now that was good in one sense, because the average Joe/Jane Doe was not "in tune" (pardon the pun) with the need to actually physically "centre" a new jet (when ever one was fitted) & """IF""" that wasn`t done, then there is the good "Possibility" of the new jet not being perfectly inline (or central) to the needle,,,, so there was an accasional jamming of needle in jet,,, then you`d have a stuck piston & car won`t go.... some smarty pants people would see the problem ,but may not have totally understood the cause, but could cure the symptom by sliightly en-richening the mixture,,, in doing so it would pull the jet slightly downward & of course the needle wouldn`t jam anymore,,, problem fixed yeah???,,, not really tho,,, because the "REAL" problem is the jet isn`t central & by winding the jet a little lower has cleared the jet & needle jamming but at the same time it has richened up the mixture,,, maybe too much, maybe not,,, depending on the correct tune needed.

so,,, that was one big reason why SU decided to make self centering needles,,, the other reason was that after a few years of driving, the needles & jets would wear conciderably & therfor they would obviously make more money by selling more needles & jets & at the same time they wouldn`t get bad mouthed for their pistons jamming from the accasional dunce head who dosen`t know that they have to "Centre" the new jet when fitting into the early fixed jet carbs

sorry for the long post again :-) (my trade mark i spose now) :-) But that`s the only "real" issues with SUs,,,, well aprt from the ones that are 5 million years old & have cronic wear in the shaft & butterfly because of another agricultural idea of having the return spring mounted such that it pulls the dam thing one way dramatic,,, the later SU`s fixed that with their "Spiral" wound return spring,,, far far better.

OH!!! that is only "If" you have any idea on needle choices for any particular engine spec combo :-) If you don`t know that, then you probably don`t know what jets to fit into a weber for any given engine spec combo either :-) so,,, tuning wise, they (SU`s & webers) are much the same.... you will still need to set it up to suit whatever engine your`re fitting it onto

ok,, class dissmissed :-)

don`t forget to do your homework, brush your teeth , have a piddle & wash your hands before you go to bed,,, OH !!! & give your mum a kiss & tell her that you love her.
Author:  aaron [ Sun Jul 12, 2009 7:34 pm ]
Post subject: 
Yes matt you do write long posts.

What your are talking about re build up is a std maintenance issue, and one that really wouldn't be an issue for day to day as in my mind the carb will most likely outlast the motor.

Re Needle choice, as you say most people don't have a clue as to what needle to use, i for one am one and will happily tell you so. BUT, when you build a motor or do any mods most people have enough common sense to take it to someone to retune the thing, thats what i do :wink:
Author:  ddk848 [ Mon Jul 13, 2009 4:24 pm ]
Post subject: 
Scott, the symptoms you are describing are identical to what I had been enduring for years with my 850 - in my infinite wisdom I had thought it would be a good idea to fit extractors and a ram-flo filter to the 850 and retain the standard 1.25" SU. After taking it to several places to get tuned, I could never get it to run smoothly, there would be flat spots, it would stall when I pulled up to the lights and I couldn't run the car without the choke on.

After having my carby rebuilt, reinstalling the standard exhaust and air filter and a complete tune, the car is running better than I can ever remember. With hindsight (the wonderful thing that it is...) I realize that the problems I experienced were caused by either:

- Wrong combination of extractors/filter/needle - I had taken it to several 'specialists' but could never get it right - maybe the guys who tuned it weren't as good as I thought they'd be, or they were just doing the best they could with the crap setup I've provided! With the improved airflow, I think the little 1.25 just couldn't keep up (hence needing to use the choke!)

- Dashpot oil - If your dashpot oil is low or not thick (viscous) enough, when you open the throttle, the piston (which is attached to the needle) rises - if the oil is too thick or too thin, the piston will rise too quickly/slowly and you will either end up with a very rich or very lean mixture, causing a flat spot. When you pull up to the lights and take your foot off the throttle, the piston will fall too quickly or too slowly which will again lean out or enrich the mixture - at low/idle rpm this will stall the engine. Some people use engine oil, I've even heard of people finding ATF in there, but you can buy penrite SU Carby dashpot oil which is the correct viscosity (it should be fairly thick!) - it comes in a little 200ml squeezy bottle and you can buy it off ebay or at decent car parts stores.

It sounds like you've had a good look at your carby and checked all the usual tuning suspects - changing the dashpot oil is a pretty simple and inexpensive job as opposed to opening up the carby again - fingers crossed it will at least reduce the severity of your flat spots and the stalling issue!
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