Tuning Twin 40 Weber DCOEs

[Susan S.]

I am in the process of trying to tune/balance twin DCOEs on my SuperSprint. I have a balancing tool and I have set the airflow of each carb to match each other and then set the idle to about 1000 rpm, having set the mixture screws to two turns open. However at this point the backfires are really quite bad and adjusting the mixture screws one at at time as the manual suggests does nothing alter the RPM. Is there any tricks to this or am I just being a Doofus?

 

 

 

 

[Big Brother]

"womble"

I have a similar problem adjusting the mixture screws on my supersprint.

It can be very difficult to pick out the slight rise or fall on the engine note.

 

I am about to try using a colortune.

These cost around £20 and are recommended by several blatchatters.

See here

Also, if you have a 4-into-1 competition type exhaust, these suffer from a built in leak - where the 4 pipes go into the collector.

Applying sealer here will stop most pops and bangs on the overrun.

 

Hope this helps

 

Steve

 

 

Se7en-Up!

Less is more!

 

[Graham Sewell]

Steve,

 

Sorry that I could not get the colortune to the meeting last night - I had email'd the reason to you. You are still welcome to pop round to borrow it.

 

Womble,

 

A trick that I use is to adjust the idle speed (upwards) slightly until someting in the engine bay just starts to resonate. You can then use this as a very accurate measure of max rpm while adjusting the idle mixture screws as the resonance will get louder while the engine speed increases.

 

However, I am thinking of buying the Gunson GasTester CO analyser to see how close to 3% CO this method gives or whether it is way over rich - although it might be quicker/cheaper to pop to my local MOT centre and bung them a few pounds....

 

Low tech luddite - xflow and proud!

[pete east]

Womble - Use of balancer will go a long way towards getting a useful set up on your carbs - If this is followed by a colourtune this will be about as good as you can get short of a session on a rolling road. A technique I have used when a my colourtune was not available,was to start with an initial set on the idle mixture screws of 2.5 turns out from fully shut. This will be a bit on the rich side but gives generally very good performance (But relatively high fuel consumption) After driving the car a few miles I turn down the mixture screws in small increments going for a drive after each adjustment untill I get a setting which gives good performance with no spitting back (usually caused by weak mixture) and reasonable (for a Caterham) fuel consumption with a good colour to the exhaust (not soot black or light grey but somewhere in between) Not very scientific but it does seem to work and my car goes like the clappers!.

[Nigel Riches]

Pete your a stalwart to go the long twiddle and try method, good on yer, but the comment about exhaust colour, black to light grey, won't apply if unleaded fuel is used, black is what you get, in or out of tune, something to do with the muck that's sold as petroleum spirit not being the same as real leaded petrol I can remember from years ago, still that's progress, not, best regards Nigel.

 

1982. 5 speed, clamshells. B.R.G / Ali. The True Colours.

[sjwb]

Gents,

Please be aware that the (sic) mixture screws only control idle and consequently should have no effect on fuel consumption.

 

Steve B

[stevefoster]

Try half a turn more on all screws to start with 2.5 to 2.75 turns is ball park.

From there I richen by half a turn. Wait. The rise and fall in revs can take ~10 secs to happen and can be slight. You kind of feel the thing running a little sweeter. If you richen by half and it is lumpier or the revs fall lean out by a quarter at a time untill you are back to where you started and then try a half a turn leaner etc. Do not go on to the next until you are happy unless it is so lumpy that you need to get it running well enough to start with.

 

After sorting all 4 you will most probably need to reduce the idle speed a few hundred rpm.

Then re-balance the carbs again.

 

I always found the Webers / the XF need a rich mixture so that the colour tune just stays orance all the time at tickover.

 

 

 

My racing pics, 7 DIY, race prep. Updated often here

Photo's of the year here

Hants (North) and Berkshire Area club site here

 

[microlightman]

 

A couple of related questions, without wanting to hi-jack the thread totally from the good carb cook book...

 

1) How much is it going to cost me to get carbs sorted on a rolling road (and where would I go)?

 

2) The chap I bought my car from had previously tuned the carbs. He also checked the float levels using info given by Caterham. Why would this need checking?

In fact the carbs are 45's and Caterham sent float levels for 40's. Are they the same?

 

 

 

 

[Bob_r]

sjwb is correct to say that the mixture screws only affect the Idle, have a look at this page -

 

http://www.geocities.com/MotorCity/9526/carbs.html

 

hope it helps

 

Bob

[puremalt]

I got a nice balancer from Burton which worked easily (even I got it sorted realtively easily). Cost £40 ish though which seemed steep.

 

Mine Farts and Belches