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Tuning weber carbs

PostPosted: Wed Jul 20, 2005 6:04 pm
by chrishewett
Now I have my +2 on the road I have found that it doesn't want to rev over 3,500 revs. It gets there in the end but it is very lumpy and slow about it.
I tried adjusting the carbs as per the manual. When I was adjusting the mixture screws I found that the screw for number 2 cylinder had no effect at all on engine revs. I could even take it out with no change.
When I checked the condition of the plugs they all looked the same.
Can anyone tell me where I ought to be looking now?

Re: Tuning weber carbs

PostPosted: Wed Jul 20, 2005 6:20 pm
by types26/36
Think you have to start from basics, point settings (if you have them) is the advance working/timing o.k./is the throttle cable opening fully/syncronize and balance carbs/is the air flow through carbs the same/compressions o.k./h.t.cables and cap etc.

Re: Tuning weber carbs

PostPosted: Wed Jul 20, 2005 7:41 pm
by chrishewett
I have fitted a new dizzy with aldon electronic ignition, new dizzy cap and leads, flamethrower coil, facet fuel pump with regulator( set at 2psi). I took the carbs apart ( partially, I didnt touch the accelerator pump) and cleaned them and fitted new float valves. It is possible I left muck in the carbs.
I have balanced the carbs as best I can using a rubber pipe. I am a bit worried about the compressions but I think I may not have run the last test properly, havnt retested yet. The engine was rebuilt less than 1000 miles ago ( but that was three years ago).
I have now reached the end of my capabilities on this one.

Re: Tuning weber carbs

PostPosted: Thu Jul 21, 2005 1:45 am
by bill308

The inability to adjust the mixture of a particular carb throat can have various causes, blocked idle jets, buggered mixture needle valve/seat, insufficient air flow for that throat, and probably others.

I've found that insufficient air flow is often the culprit and it's not obvious to recognize. You might try opening the throttle valve on the offending carburator a little bit relative to the good running one and see if this makes it possible to adjust the mixture for the one bad throat. I diagnosed this problem on my 308, which is fitted with 4-twin choke Weber DCNF's, so balance problems are even more of an issue. One throat was unresponsive to the mixture adjustment, from mixture screw closed to 8-turns open. The carbs were recently rebuilt and the offending idle jet was double checked with all appearing as it should. When I increased air flow for this one barrel, mixture setting was enabled and all was good in the world.

Others may disagree but use of a pipe/tube to balance the carbs is very crude in my opinion. A Unisynch is nearly as bad as it's orientation is important and it can markedly affect the idle air flow. A common air flow meter called an SK Syncrometer is a pretty good instrument as it has little effect on air flow and is reasonably repeatable. The down side is you have to move it from throat to throat and make linkage adjustments to equalize the air flows. If you have 2 Synchrometers that read the same, you can fit one to a "control" throat and move the second Synchrometer to the other 3-throats, always looking to equalize air flow readings. Get all the throats to flow the same and they will be much easier to dial in the mixture.

By far the best method is to tap into the intake manifold runners with a brass or stainless steel hose barbs and use the resulting vacuum signals as a measure of cylinder to cylinder balance. These vacuum signals are best read by a bank of 4-mercury manometers, like the motor cycle guys use. If you don't like mercury (it's environmentally unfriendly - toxic) there is a British firm that makes a pretty good replacment device called a Carb Tune II, that floats matched metal rods in a 4-tapered transparent bores. Height of the rods (they are next to each other) is a measure of the vacuum signal just like the height of a mercury column(s). The point is, with a bank of manometers, the task of balancing 4-throats is much easier and more precise. You would be astonished at the difference in idle quality with a really good balance and the difference it makes upon clutch engagement at small throttle openings. My twink would tick over reliably at 600 rpm if I wanted it to. The engine also becomes much more forgiving especially in around town driving. Again others may disagree, but I believe that around town, the bulk of one's driving is on the idle and progression circuits (I would guess the mains come in at about 2700 rpm), so it pays to get them sorted out.


Re: Tuning weber carbs

PostPosted: Thu Jul 21, 2005 9:04 pm
by chrishewett
I followed your suggestion and altered the airflow and it worked, I was then able to adjust the mixture control on that barrel. After a bit more tuning I took it out and same problem, not revving. I then decided to recheck the ignition timing and behold the distributor was loose! After resetting it - no problem, running ok!
Now I will try and find someone with the right equipment to fine tune for me.
Anyone know of a twincam expert in Lincolnshire?