Lotus Elan

Carb air flow balance

PostPost by: miked » Mon Oct 12, 2015 9:33 am

This saved me some time!
Butty box, tap hose adaptor, some MISAB's and grease and my old garage Vac'. Will have been done before. Would not lift my Carbtune slugs but did operate the snail device nicely. When on the engine the Carbtune finished the job.

Mike :D
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Last edited by miked on Mon Oct 12, 2015 11:57 am, edited 1 time in total.
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PostPost by: Chancer » Mon Oct 12, 2015 10:31 am

What were you trying to do with it, balance the idle by pass screws? assuming you have them that is.

Where is the time saving as you still had to re-balance it on the engine?

Or have i missed something as usual?

Whatever the aim I admire your style and pragmatic solution though!
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PostPost by: miked » Mon Oct 12, 2015 11:56 am

I had problems with butterfly fit and unequal air flow. It was on a Weber 45 - 152's and even with air bypass you can only tune so much out. The error was beyond the screw adjustments. This allowed me to play with the butterflies in the throats multiple times and even drill a small hole in one. It is easier to take the butty box off and make an adjustment, don't you think? I was then balanced and within the scope of the screws. To do this on the car would have meant on and off etc and would have been a real pain. The fit of the butterfly in the throat can be out even with new carbs. This Spanish one also had small casting holes in the throat.

When back on the car I put the Carbtune on (which is a doddle) to check/confirm my work and fine tune with the bypass. I will be doing this with any future carb. The accuracy of the progression holes in relation to the butterflies are also a subject on their own. Even the Weber book acknowledges this.

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PostPost by: Chancer » Mon Oct 12, 2015 12:14 pm

Smart thinking, the Spanish made Webers were a crock of sh1te, they are probably Chinese made now.

It sounds like you find your Carbtune more sensitive than the snail type, I have the former and have always been tempted by the latter because of the intake pulses jiggling the indicator slug on the Carbtune.

Is one better than the other or do both have their place? How do you use yours?
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PostPost by: miked » Tue Oct 13, 2015 7:15 pm

I think the snail is very accurate but I like the fact that you can see all 4 intakes at the same time with the Carbtune and also watch the total reaction when you operate the throttle (even with some jiggle). Also (if you have carbs with the adapters) you can leave the air box on and get connected within minutes. The snail operated with my set up but I could not lift the Carbtune slugs so it looks like the snail is very sensitive but appear nicely damped. I was lucky enough to be give this one and only used it for the first time on this job.
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PostPost by: Chancer » Wed Oct 14, 2015 7:38 am

OK I understand now, my carbtune I bought close to 40 years ago and it only has one manometer and a conical adaptor that doesnt fit very well on DCOE trumpets, you have a set of 4 with vacuum take off pipes, much better than the snail, but the snail would be much better than my carbtune.
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PostPost by: mbell » Wed Oct 14, 2015 7:31 pm

That's a great idea Mike. I am having a lot of trouble the the throttle plate position and balance on my carbs, basically unsuccessfully trying to balance them on the car by tweaking the shaft wasn't fun. Doing something like that would have made my life much easier than trying to do it on the car.

Basically come to the conclusion that the only way forward from here is new shafts but think I am going to just get new carbs rather than go through the hassle of changing the shafts bearings to just then have more issues. I want to get this car on the road!
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: miked » Thu Oct 15, 2015 5:40 pm

I had the problem with untouched carbs with the butterflies set poorly from the factory. After not getting any adjustment on no 3 cylinder with the mixture screw!
IMO some Spanish ones are not that good. I had little casting holes in the No 3 throat and it looks like somebody in the factory had fitted one butterfly out of shape and then air flow tested it and set the other badly to match. I mean really bad were you could drive a train through the gap on each. So much so that you can now see the old shaft line at an angle across the butterfly. Upon seating them properly I had a massive vacuum on one. Then there was the alignment of the progression holes. Drilled slightly differently on each throat. What a load of ?&****?

This is on a 2 litre Zetec. I am tempted by GSXR 1000 throttle bodies and an OMEX. I have it better than it was now but it is a compromise between balance and the progression hole position which I don't like.

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PostPost by: mbell » Thu Oct 15, 2015 8:13 pm

I ordered a new set of carbs yesterday so hopefully I'll get a good set. I will be giving them a good check over before fitting to make sure.

From what I read both the quality of carbs and spare parts are hit and miss at the minute and didn't really fancy the having to fit the shafts, bearing and plates. Something bound to go wrong or be another issue. Rather spend a few $ and get the car driving well enough to put on the road.

All these issue with the carbs does make EFI look more and more attractive!
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: bill308 » Thu Oct 15, 2015 10:26 pm

I'm a little confused by the snail and Carbtune descriptions.

Is the SK Synchrometer shown is photo 2 the snail?

I thought the Carbtune was a set of four precision, transparent tubes, with precision slugs inside that float when exposed to intake vacuum. Is this correct? I believe earlier Cartunes were mercury manometers as is the Motion Pro I currently use. Mercury is toxic so needs to be used with care but does not require calibration and does not stick. Yes it pulsates in multicarb setups like the Elan, by it is a hands free device and is exquisitely sensitive. The pulse range can be easily compared when all four columns are placed next to each other. As the throttle is opened and rpms rise, the pulsing diminishes. Pulsing can also be damped by putting a restriction in the vacuum line to each column. This reduces the vacuum signal.

I had a miss drilled progression hole on one of my 40DCOE-18's. This caused the affected cylinder to lean misfire when the throttle failed expose the first progression when needed.The throttle plate fit in the bore was otherwise fine, as were all the other holes. A small 45-degree chamfer on the downstream edge, adjacent to the miss drilled progression hole and halfway through the throttle plate thickness, now enabled this hole to flow at the same time as it's cousins in the other three throats and all was well. Vacuum was not affected.

The throttle plate needs to be a smoochy fit in the bore and be the same across all throats for proper balance and operation. Unfortunately, the throttle shafts are soft and sometimes distort in twisting over time or due to abuse. This can sometimes be fixed by twisting it back into shape, but this is a time consuming and iterative process. Alternatively, a new throttle shaft can be fitted but again, this is not a fast or easy job.

The 45DCOE152g's (4-progression holes) I fitted to my new engine are Spanish made and appear to be good quality. These are made by Weber and I believe are the best available. There are Chinese made copy cats available at lower cost but I have no experience with them.

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PostPost by: miked » Fri Oct 16, 2015 12:11 pm

I'm a little confused by the snail and Carbtune descriptions.

Is the SK Synchrometer shown is photo 2 the snail? Yes it is. I could not remember the name.

I thought the Carbtune was a set of four precision, transparent tubes, with precision slugs inside that float when exposed to intake vacuum. Is this correct? Yes the one from Belfast. I believe earlier Cartunes were mercury manometers as is the Motion Pro I currently use. Mercury is toxic so needs to be used with care but does not require calibration and does not stick. Yes it pulsates in multicarb setups like the Elan, by it is a hands free device and is exquisitely sensitive. The pulse range can be easily compared when all four columns are placed next to each other. As the throttle is opened and rpms rise, the pulsing diminishes. Pulsing can also be damped by putting a restriction in the vacuum line to each column. This device has those. This reduces the vacuum signal.

I had a miss drilled progression hole on one of my 40DCOE-18's. This caused the affected cylinder to lean misfire when the throttle failed expose the first progression when needed.The throttle plate fit in the bore was otherwise fine, as were all the other holes. A small 45-degree chamfer on the downstream edge, adjacent to the miss drilled progression hole and halfway through the throttle plate thickness, now enabled this hole to flow at the same time as it's cousins in the other three throats and all was well. Vacuum was not affected.
Interesting, I have read about the chamfer of the plate in the Weber manual but not done that yet. For the time being I have it running pretty good but would like to look more into this.


The throttle plate needs to be a smoochy fit in the bore and be the same across all throats for proper balance and operation. Unfortunately, the throttle shafts are soft and sometimes distort in twisting over time or due to abuse. This can sometimes be fixed by twisting it back into shape, but this is a time consuming and iterative process. Yes I did a bend tweak and the box helps as you get to see what you have done quite quickly. Alternatively, a new throttle shaft can be fitted but again, this is not a fast or easy job.

The 45DCOE152g's (4-progression holes) I fitted to my new engine are Spanish made and appear to be good quality. These are made by Weber and I believe are the best available. There are Chinese made copy cats available at lower cost but I have no experience with them. These are I believe proper Weber and about 18 years old but have done very little work.

Bill
Mike

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