Indicated Uneven Weber Airflow

Rick · 09-18-07, 03:02 AM

What can cause an uneven air volume reading while adjusting the idle on 48 IDA's? Yesterday the air volume between throats on a single carb was uneven. Throttle plates are phased the same. Compression is the same throughout the engine, valves adjusted. Gaskets were checked and are not compromised. The mixture screw for the cylinder in question has no impact on runability. I realize this is a question of High Pressure (above the throttle plate)/Low Pressure (below the throttle plate) why are the two runners different if they are opened at the same moment & rate.
If a plug fouls will that impact the indicated flow?
What am I overlooking? I'm thinking too hard on this.

Mal · 09-18-07, 03:53 AM

Could there be an air leak btn throttle plate and inlet valve on the low flow cylinder. This would explain the reduced flow and ineffectiveness of the idle mixture screw.

Rick · 09-18-07, 04:15 AM

That is exactly what I was looking for yesterday. We used a smoke generator to pressurize the crankcase in an attempt to see if the manifold gasket was leaking into the lifter area. There was no indicated leakage whatsoever. Looking into the runner the gasket is not displaced. Nor did it leak in the above described testing procedure.

Peter D · 09-18-07, 05:47 AM

Rick, not sure if this is relevant, but it may help. I have an original-style DC&O 8xTB EFI setup & each pair of TB's have a common shaft which moves the butterflies. Synchronising the 2 butterflies on the one shaft was an issue - there is one idle stop for each pair of ganged butterflies, & they did not provide equal openings at idle.

My engine guru sorted it out by simply adjusting one butterfly with a screw-driver & a small hammer - tap, tap, tap, until they were matched !
Then it was a matter of balancing each of the 4 pairs with eachother, using the screw adjustments on the shaft joints / linkages.

I had originally thought that I had it sorted by setting the gap at the edge of the butterflies with a piece of 8-amp fuse wire (pretty small, I thought). When I started the engine, it idled at about 2000rpm ! So we are looking at some really, really small gaps & tiny variations between TB's amount to quite large percentage differences at idle.

Maybe you have some slight differences between the butterfly openings on each carb pair which could be addressed via the "tap, tap" method ??

Kind Regards,

Peter D.

Dean Lampe · 09-18-07, 09:25 AM

Rick, can you check the synch at 2500 rpm's? Does that cylinder come back on line? It almost has to be a bent throttle shaft. I know you checked it but it is hard to see with the naked eye. If the carbs can be synchronized just off idle, you will probably be OK. I have been there before with my kirkham. As soon as i touched the accelerator, everything evened out. You know, I am fighting weber demons too. We will get it all worked out. And when it is done, no one will catch us!

Rick · 09-22-07, 10:58 AM

One of the great aspects of this site is the ability to share and occasionally benefit from the situations and difficulties that we all encounter from time to time. I try to learn from them and apply the remedy if the situation or symptom happens to me. Hopefully others some of you feel the same way. Sometimes it is the small things or somthing that has been mis-diagnosed or even over looked that ends up being the cause. Well... my recent problem turned out to be the throat that was actually flowing the higher volume of air. It turned out that the throttle plate in THAT throat was improperly fit and was allowing excess air to bypass it when closed, While the original one in question was actually sealling correctly. (brain cramp) I readjusted the other plate and all seems to be ok now, and I will continue on my journey to find the final jetting package.
Somtimes it's the simple things........................

Thanks to all of you that replied.
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Rick