Carb balancing, you really should try it!

[pete east]

Put this up before, quite a while ago, but for anyone with Webers / Dellortos, carb balance is one of the first things I check when the Seven is feeling out of sorts.

The problem is that a reduction in performance can creep in gradually, so that at first you dont notice it too much. Mine gradually started hesitating when opening the throttle, tending towards overheating, not giving very good mpg (well, even for a crossflow) and the iding speed had fallen, so that it didnt idle all that well, the end result being that i wasnt enjoying driving it so much.

I got the flow meter out, and found that the carbs were quite a long way out of balance. Just a three quarter turn, on the balance screw, had the carbs all singing from the same hymn sheet.

Went out to see how it felt, and the results were (just like every time I use the flow meter) staggeringly good, clean pick up on opening the throttle with no stumbling, smooth clean acceleration, with no spitting back through the carbs, bags more power, better mpg, good engine temps, even on a hot day, and steady as set idling speed.

If there is one tool I can really recommend for those with carbs, it is the old flow meter.

I might try wire locking, the balance screw, to try to hold the setting more reliably, than the opposing spring set up seems to be able to.

[alextangent]

Locktite it.

 

Alex McDonald

Loud, louder, loudest...

[thesheep]

So is it just that balance screw that goes out, and not the idle speed screw, or the other 4 individual screws that control the mixture for each cylinder? I guess if it's always that 1 screw then it's not too bad to fix. But once you have to consider the balance screw and the individual screws it gets a bit more complex to do?

 

--------------------------------------------------------

Caterham7Junkie.com

[pete east]

Thesheep - It is really just the balance screw which goes out. you then have two cylinders which are dragging down (or trying to push up) the other two. The idle screw can only be properly adjusted, once all the cylinders are drawing the same amount.

 

With Webers there is really not much else which can be adjusted, other than changing the jets.

 

 

[IKB]

The K series owners don't know what they are missing 😬 😬 😬

[oilyhands]

Any K owners with throttle bodies will have a good idea.. I've yet to see a properly balanced set of roller barrels.

 

Oily

 

Edited by - oilyhands on 1 Sep 2009 09:28:43

[IKB]

Oily...you told me that Webers were engine jewellery and fuel injection was the answer. You never said that it was just as full of mystique as twin carbs ie roller barrels etc!!!!!!

[oilyhands]

Plenum based engines are remarkably adjustment free, however, any setup that has multiple barrels will need careful initial setting up to ensure the airflow is balanced.

 

Once properly setup and mapped everything will stay spot-on ad-infinitum, wear in the linkage notwithstanding.

 

To be fair TB setups are 10 times more reliable and consistent than carbs which are a box of compromises at best.

 

Oily

 

Edited by - oilyhands on 1 Sep 2009 10:04:43

[pete east]

I guess what I was trying to say, was for those on carbs, it really is quite staggering the difference a tiny adjustment on the balance screw can have.

The flow meter is one of the best bits of kit, i have for the seven.

[millsn]

Worth saying that loctite 243 is the oil resist version of 241/242 so may well be worth seeking out to help out XFs needing their balance screw fixing. For really heavy duty use 2701 (latest version of 270) but risk not getting it off again. Parts should nonetheless really be degreased beforehand.

 

[k80rum]

I've just used Oily's approach to balancing TBs and have to say it was well worth it. It's annoyingly iterative but incredibly rewarding, and an hours work turned a brand new engine and TBs into something that idles like a Swiss clock

 

Darren E

 

K80 RUM Website and Emerald maps library

 

Home of the long-term, supercharged Duratec project

[thesheep]

I just went to have my Weber twin 40s tuned, and was watching the guy work (Peter Landers at Sigma Engineering). He was playing with 4 screws, one for cylinder I guess, and turning them while he listened through a piece of hose pipe to the noise down the intake trumpets. He also turned the balance screw a bit. So I got the impression there are 5 main screws to play with... He adjusted the idle screw - which was a separate screw again - last of all.

 

--------------------------------------------------------

Caterham7Junkie.com

[millsn]

You've got to get the mixture right first which is the four screws assuming all is fundamentally ok, then you adjust the balance on the middle screw which sets the two bodies relative to each other as two pairs. On carbs the mixture screws seems pretty immobile (a good thing) but the balance screw cna be pretty useless at holding position I imagine it's because the two bodies can flex on theor O rings relative to each other and do thus releasing the screw. Typically I was always tightening mine. My Jenveys on the other hand just don't go out of balance.

[pete east]

Millsn and all, thanks for your comments,

I made the posting because I was struck, by the significant improvement that the use of the flowmeter, and just a tiny adjusment of the balancing screw can achieve, (when all other elements of the engines running condition, (timing, idle jets, no air leaks etc) were in good order.

As posted before the flow meter is one of the best bits of kit obtained for use on the 7.

Experienced people can do the same job listening to the intakes with a piece of hose,

but to make life easy, it really is worth getting hold of a flow/synchrometer.

[bstark]

Pete - as a xflow owner who has plenty of experience of the gradual creep down in performance, and how lovely the car feels after a service (which generally includes the phrase "balanced carbs") I'll give you a for making me think this sounds straightforward and quick to do at home.

 

Bob Stark

Supersprinter

[pete east]

Bob - There is a good description of how to balance carbs on our L7 club, but I have forgotten where! But essentially you set the car up with a 900 - 1000 rpm idle, and adjust the idle mixture screws first. Then use a flow meter to balance the carbs, Then go back to the idle mixture screws,,and tweak them again (You use engine speed to judge the effect of each tweak) then do the balance again, and if you feel like it a final tweak on the idle mixture, last action is to re adjust the idle speed screw as idle speed may have changed a bit.

But getting a synchrometer / flow meter is a really useful bit of kit.

Pete.

 

 

[oilyhands]

It may be this one from my website..

 

Setting the Idle and slow running

 

Rough running and idle is normally down to the idle mixture and balance settings being incorrect, below is a technique to establish a clean idle and progression. Before adjusting the carbs in this manner you must make sure that the following conditions are met.

 

i) The engine is at normal operating temperature

ii) That the throttle return spring/mechanism is working OK

iii) That the engine has sufficient advance at the idle speed (between 12 and 16 degrees)

iv) That an accurate rev counter is connected.

v) That there are no air leaks or electrical faults.

 

A reasonable idle speed for a modified engine on Webers is between 900 and 1100 RPM.

 

If you are adjusting the idle for a set of carbs already fitted then progress to the second stage, if the carbs are being fitted for the first time, screw all of the idle mixture adjustment screws fully home and then out 2.5 turns. If you are using DCO/SP carbs then start at one turn out. Start the engine and let it reach normal operating temperature. This may mean adjusting the idle speed as the engine warms up. Spitting back through the back of the carburettor normally indicates that the mixture is too weak, or the timing is hopelessly retarded. If this happens when the engine is warm and you know that the timing is OK, then the mixture will need trimming richer on that cylinder. Set the idle as near as you can to 900RPM.

 

Using an airflow meter or carb synchroniser adjust the balance mechanism between the carbs to balance the airflow between them, if the rearmost carb is drawing less air than the front, turn the balance screw in a clockwise direction to correct this. If it is drawing more air, then turn the balance screw anti-clockwise. If the Idle speed varies at this point, adjust it back to 900 RPM, to decrease idle speed screw in an anti-clockwise direction, to increase, screw in a clockwise direction.

 

When you are sure that the carbs are drawing the same volume of air, visit each idle mixture screw, turn the screw counter clockwise (richening) in small increments (quarter of a turn), allowing a good 5 - 10 seconds for the engine to settle after each adjustment. Note whether engine speed increases or decreases, if it increases continue turning in that direction and checking for engine speed, then the moment that engine speed starts to fall, back off a quarter of a turn. If the engine speed goes well over 1000RPM, then trim it down using the idle speed screw, and re-adjust the idle mixture screw. If engine speed decreases then turn the mixture screw clockwise (weakening) in small increments, again if engine speed continues to rise, continue in that direction, then the moment it starts to fall, back off a quarter a turn. The mixture is correct when a quarter of a turn in either direction causes the engine speed to fall. If that barrel is spitting back then the mixture is too weak, so start turning in an anti-clockwise direction to richen. During this procedure, the idle speed may become unacceptably high, so re-adjust it and repeat the procedure for each carb barrel.

 

After all the mixture screws have been set, the idle should be fairly even with no discernible 'rocking' of the engine, if the engine is pulsing, spitting or hunting then the mixture screws will need further adjustment. If the engine is rocking or shaking then the balance is out, so revisit with the airflow meter/ carb synchroniser. No amount of adjustment will give a good idle if the throttle spindles are bent or leaking air or the linkages are loose on the spindles!

 

That’s all there is to it.

 

Oily

[kenny.]

Oily, I followed that very advice of yours a week or so back when reftting my refurbed carbs and it worked a treat, thanks .

 

One thing though that is rarely mentioned and something I only realised when building them back up was the individual throttle body adjusters (Weber 45 DCOE 152's).

I'd just assumed that you could only balance in pairs but these give you the facility for individual balancing.

 

As it happens mine were "near enough" so I left well alone.

 

Would you have gone for ultimate balance via these air bleeds? and how much grief do they cause to fettle once you've committed yourself and started fiddling?....I didn't fancy doing something I'd never done or read anything about 😔

 

Many thanks

 

Kenny

HPC 218 Evo

[pete east]

Oilyhands

 

Thanks for your posting, that is the one I wanted to point out to Bob

 

Pete.

[thesheep]

And if it is *just* the idle speed that needs adjusting, on 40s there is a dedicated screw to do that - no need to mess with anything else. And that bit is really easy.

 

--------------------------------------------------------

Caterham7Junkie.com

[bstark]

Pete, Oily - thanks for the info

 

Bob Stark

Supersprinter

[JACK FLASH]

I didn' t find these individual throttle adjusters (air bleed screws) in my Weber 40 DCOE 4 (Twin Cam).

 

I suppose my carbs haven' t got them and that is a pity since there is a small unbalance between the throttles of 1 carb.

 

BTW, after experimenting and studying Weber carbs I found that the best way of balancing them is using a flow meter and checking the result afterwards by listening through a tube.

 

Jack Flash

 

[cookni]

Just wanted to thank Pete & his brother Des for hooking up with me to balance my carbs on Sat.

 

Throttle now feels a lot lighter and more responsive especially at lower rpm

 

Thanks guys.

 

I Continue to be bowled over by the helpfulness of the clubs members.

 

Best regards

 

Cookster

[Stationary M25 Traveller]

That's good news Nick. Cheers Des and Pete

 

 

[pete east]

Hi Nick, I hope that we were able to improve your cars running using the flow meter,

Stiil think it surprising the difference small tweak on one screwcan have on the way the engine runs, but I guess, if you have two cylinders trying to do more (or less) than the two next to them, there are going to be arguments in the engine.

Glad to be of help, and always pleased to see a new Sevener, Perhaps see you at the next Essex meet at the compass at Littley Green.

Ian Thanks for your help too, circumstances this year have stopped me getting over to the Plough, but I would really like to get to the next meeting, and see you Kent guys again.

Keep well.

Best Regards

 

Pete.