I have a 71 mk3 Cooper 998 w/ dual su carbs. It is a little sluggish on the bottom end and hesitates until you get up to rpms and then the car smooths out and runs good. Its sometimes spits and sputters a little and then if you give it more gas the other carb kicks in and it runs out good. Once you get up to speed it runs out fine. Is there something simple i check and do as far as tuning the carbs. To me it seems like one of the carb is a little lean. Im going to clean and check the points and everything to make sure all of that is up to par.

 

Check the float adjustments, check for proper needles, double check your ignition timing.

 

These carbs are not sequential, one should not "kick in" after the other.

There is a very specific and simple procedure to tune these carbs, but you need to make sure all your other ducks are in line first....the carbs should be the last thing you do.

If you buy any Bentley or Haynes or SU manual, the procedure will be in there, and you'll benefit from having it on hand in the future to refer to.

Basically, if everything else is good (points, timing, valve adjustment, compression etc) you loosen the connection between the two carbs. Adjust the idle speed equally using a unisyn, then adjust the mix, then reset the idle speed if it's climbed, reconnect the two carbs and you're done. Be sure the tops have light weight oil in them and that the pots are not sticking in the tops.

 

I've check the oil in the tops of the carbs in filled w/ marvel mystery oil. I haven't done anything else to them though. The car idles perfect, and runs good. If you are from a dead stop and are easy on it and keep the rpm's low the car seems to studder, and wants to hesitate like it is very cold, but it continues to do it after is has warmed up. You can mash the peddle down a little more and give it more gas and it takes off and runs like it should. Is there such a thing as a low speed needle on these carbs. It seems like the low speed part needs to be richened.

 

Did you see the article I spoke about in the last few days?

http://classicmotorsports.net/articles/simple-carbs/

Your basic tune procedure is in there. As well as a good explnation of what's in there and what ain't. The SU is a very simple beast and many things you "look for" in a typical carb, simply ain't there, and the ones that r typically don't operate as they do on other carbs! If you want a good SU reference, I recommend ""SU Carburetters Tuning Tips & Techniques" Brooklands Books LTD. I got it from Mini Mania. It will give you complete rebuilt guides for everything SU (if rebuildable)

You may want to start with a simple top end cleaning. 3 screws hold down the 'pot cover' and removing those will allow u to remove the cover, and the piston and the spring and the needle. When I did that the first time I found a bit a varnish buildup that kept things from moving freely (in particular the spring was sticking to the sides and causing the piston to hang up). A little brake cleaner degunked everything and I filled the pot damper with 3 in 1 oil and everything has been honky dory since (I am a single HIS carb tho....no worry about balancing)!

Other than that ...
1. never mess with carbs until you r 110% sure the ignition is absolutely totally good.
2. do you have stock air cleaners? Cone filters need 'other than stock' needles because of the increased airflow. (do U have a clue what needles are in there????)

Good Luck

 

Everything looks to be stock. It still has the stock aircleaner. Me and my dad fooled around with them some more and never got the darn thing to run any better. The carbs look like they havent had a thing done to them since 1971. There was nothing left to the gaskets in the cover for the float bowl. They were all in the bottom of the bowl ( which can't be good). I bit the bullet and took them off the car and sent them to chester at sucarbs.com. He has a special going on right now and he completely restores the carbs, puts new bushings in, complete new kit, and checks for vacuum leaks etc. For the price he does and guarantees them, I couldnt afford to buy a kit and be pissing in the wind again. Ill keep ya posted when i get them back from him. It should be a couple weeks.

 

There has been a lot of information written on the various issues with SU carbs, A good deal of that info can be found here:
http://www.minimania.com/web/SCatago...ticle_Type.cfm

 

Just as long as you're aware that if something else is causing the problem - leaking vacuum line, stuck distributor advance, points closed up etc, you won't have "fixed" it by overhauling the carbs, although from your description they still needed it..........good luck and let us know what happens, OK?

 

Yeh i figured it was a good place to start since the bad shape that they looked like. When i get them back im putting in new points, condenser, & double checking the timing etc.... I couldn't believe all the gaskets were gone in them. Can't wait till they get back. Anxious to drive it some more. https://www.northamericanmotoring.co.../impatient.gif

 

UPDATE

seen the latest MiniWorld? Yet another article on this topic. Wanna tune a pair of SU's?

Get on a Dyno and have a handful of needles available. Altho you can clean up an SU you won't change the running 'mix' until you change the needle....and picking the right needle is an art, not a science. Change the jet - you mess with the idle....the running mix is controlled by the needle.

Not intended as a told you so...intended as a don't waste your money...

btw...what did the plugs look like? Oil coated and black or a nice "biscut" tan color? Fouled? This tells you a bunch about how the fuel is burning.....

 

The plugs weren't black ,or very wet. They looked fine not very white (but similar to the biscuit tan look). I checked the points with a dwell meter, and they were set way to wide, so i closed them up to the 60 that the manual stated. No change ,but i set them to the manual specs. I've got to borrow a timing light from a friend, but the carbs won't be back for a couple weeks. In the mean time i'm going to clean the engine bay, repaint the engine, and clean it all up so its ready to put the polished carbs back on.

 

Do you what camshaft is installed as from what you are saying its lumpy at low revs and then fine at a higher rev range this could be a special tunning cam fitted IE 731 profile mid rev range or maybe 649 high rev range...........

 

As far as i know car is completely original. Car has a hesitation at low revs, like it isn't taking fuel. Car doesnt have a lumpy idle or anything, so im assuming cam is the stock grind. All the gaskets fell apart on the carb on inspection ,which is one of the reasons i sent them off. Chester was supposed to have them back in the mail on friday. Still waiting for ups to bring them. I also have a new distributor cap, points , and condenser im going to install, and set the timing when i put the rebuilt carbs back on.

 

Here is a tip ..............
remove rocker cover and remove spark plugs put car in 4th gear with H/brake off, and pull car forward untill the rockers on No: piston (ie valves 1-2) are rocking this means that this is the firring stroke............
now remove dizzy cap and then put a dot of white paint opp the rotar arm wich should be pointing between 1-2 o'clock noe replace dizzy cap and see if No: lead is opp dot with lead rotation of 1-3-4-2 in anticlockwise order this will give you a rough place for timming to begin now replace rocker cover with new gasket and replace plugs and put on H/brake and take it out of gear...............good luck...........and hope this helps............