Crazy Manometer readings- help!

[elansprint71]

I've been trying to balance my newly-rebuilt carbs but have come to the end of my tether!
They are Dellorto 40 DHLA, with fine-thread mixture screws and without idle by-pass circuit. I had them refurbed and rebuilt with new jets, etc. to the spec in the Lotus workshop manual (my thinking being that this would be a good starting point prior to rolling road tune up).
The engine has been running pretty much OK, without any major popping and banging, or hesitations; pulling really well in fact. I adjusted the idle mixture with the screws 3 turns out and fine-adjusted using Colortune, so that they were as good as can be expected.

Today I finally un-earthed my four-branch manometer and have spent all day trying to make sense of what is going on- see photo. Thinking there might be an air-leak at cyl 2 I removed the carb and confirmed that the o-rings were correctly in place and did the leak-test with Easy-Start, I checked that the carbs were not too lose; they move only slightly when I tug them up and down; Thackray washers fairly-well compressed.
The readings for cyl 4&3 (left pair in photo) are low on the scales but the indicators are moving up and down slightly with pulses from the engine; no matter what adjustment I make to the mixture screws they do not move far up or down. Cyl 2 adjustment makes no difference at all to the indicator position and cyl 1 moves when adjusted but I can't get it as low as 3&4.
I swapped over the idle jets on 1 & 2 to see if that made any difference- nothing!

Any suggestions chaps- I'm out of ideas.


EDit: cylinder numbers corrected!

[Howie146]

Looks like front and rear carb are out of sync use the balance screw on the linkage between the carbs to adjust the balance between the two carbs then final check/adjust idle and mixture. Agreed there is an imbalance in the front carb but see if this improves when they are in sync if that still exists and there are no air bypass screws to adjust it could be a slightly twisted shaft in that carb.

[twincamman]

Pete ?.try a romote thermostat focused on the exhaust runners should be around 480 degrees on each runner hot ?gives you a visual on the balance .ed

[miked]

Pete,
I use one of these devices but only on Webers. Assuming that the "take offs" are the same on Dellorto's and on the engine side of the butterfly?s means that you are showing less air flow and more vacuum on No 2. Arse about face from putting a flow device in the ram pipe! That's means that No 1 butterfly is more open (or drawing air) dropping the No 1 reading Vacuum reading. I would suggest that perhaps No 1 is pulling air or the butterfly is not seated as well in the throat. I have just sorted some 45's that were way out (from new). I always hold them up to the light with the venturi and choke out to set them best as possible with minimum light around them. You need to bottom that imbalance on the front carb and then you can use the centre carb to carb balance screw to synch' across the two carbs. The rear carb looks great choke to choke.

PS adjusting the butterfly position affects the vacuum. The nearer to closed they are the higher the readings with this type of take off. Mixture adjustment will go from to rich to too lean to not firing at all by playing with the screw. I am a layman and many more on here will know better but that's what I would think and have cured in the past.

Mike.

[twincamman]

you can use your manometor to learn to play bag pipes by metering your breathing ,,,,,just before you pass out too?..ed

[lotusfan]

Pete

The manometer measures the depression at each cylinder inlet. The first thing to do when setting up the carbs is to balance the airflows. Having done that the next thing to do is to set the mixture. The manometer is not used to set the mixture, that is s job for the Colourtune.

Looking at your photo the right hand carb, presumably cylinders 2 and 1, are sucking much harder than 3 and 4. You need to adjust the screw on the linkage between the 2 carbs to get the 2 carbs much closer together. Your tick over must be very rough. Ideally all 4 manometer readings need to be the same. It may be necessary to twist the throttle spindle to achieve a perfect balance.

[twincamman]

[/attachment]

geez DONT go bending throttle shafts ?..try a slightly stronger return spring or a new one or adding a second return spring from a vw bug or another brit car first ?it solved my balance problem ?..the return spring gets tired or the bearing/ bushings from the rebuild are yet to wear in with use ?ala Are You Being Served .ed

[twincamman]

Or the throttle shaft. Nuts may be too tight on that carb ..ed

[mark030358]

As above carbs are out of sync.... And throttle butterflies out of alignment. As Ed says stronger spring initially.

[billwill]

Remember that the front carb manifold has the vacuum takeoff; you should probably remove the vacuum pipe and blank that hole when doing manometer test.

[elansprint71]

Thanks guys, have made some progress but now have ipone6 to master too! I"ll update you Asap. Thanks again.