Lotus Elan

A question of balance

PostPost by: mark030358 » Sun Jul 24, 2005 10:52 am

Gents,
A quick question on balancing air flows on carbs.

When measuring and setting up the balance between the carbs, in my case by measuring the vacuum, what is an acceptable deviation between:-

1) Both barrels on the same carb.
2) Between both pairs of carbs.


I am measuring in mm Hg. But any units will do.

I have read an article on GG Lotus site that it must be within several mm Hg, but this seems really really small.

thanks in advance
Mark
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PostPost by: john.p.clegg » Sun Jul 24, 2005 1:11 pm

I know this isn't very technical but i would connect the guages and do the best you can,you may be able to get all four spot on?
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PostPost by: type26owner » Sun Jul 24, 2005 4:03 pm

Wouldn't it make more sense to measure and balance the volume of air going through the engine rather then it's suction? Measuring the volume is the direct method and is more accurate.
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PostPost by: twincamman » Mon Jul 25, 2005 2:39 am

welll Kieth at 10000 rpm and a 1/4 IN tube in the right ear the sound should be around f sharp on the minor scale-----but if your not a musician close the carbs and measure the butterfly as the sinc screw is moved until there is a slight pull on the feeler you have inserted previously and then both carbs match----HOWEVER --I use a unisyn and that works on both my tc Webber heads as all the plugs are brown after a 5000 rpm shut off and plug read :roll: ---ed
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PostPost by: bill308 » Mon Jul 25, 2005 3:16 am

Mark,

I think John is right on. Just do the best you can. If you are able to measure each barrel's vacuum, and it's best to do them all simultaneously (i.e. all manometers hooked up and positioned right next to each other and drawing mercury from a common source) all you have to do is match the column heights. This method will instantly show you how an adjustment to the airflow of one throat affects the others. Also, depending on how much damping you have in each circuit (usually a precision restriction - the smaller the hole the more damping) the column heights will pulsate up and down. With the columns next to each other, your eye will easily tell when they are flowing equally.

Keith,

I have found the simultaneous measurement of vacuum, using a bank of manometers,to be far superior to any other method I have tried. The SK Synchrometer is the best on-engine flow meter I have ever used and even when paired with a matched mate, I believe it to be a distant second in balancing usefulness. Although the SK will yield a reasonably accurate measurement of absolute flow, the actual value is of little use. It the relative flows, throat-to-throat that count. Also, single point flow measurement devices can only see one throat at a time. Select a throat to to be your master and balance the rest to match its flow and guess what, by the time you go back to your master, its flow has changed. Onec you have tried the 4-bank manometer method, you will never do it any other way. The down side, yes there is almost always a down side, you must have a vacuum tap for each throat placed somewhere down stream of the throttle plate. In general, smaller is better, more damping. As far as I know, the DCOE series do not have provision for vacuum tap. I don't know about DHLA's. So, you must drill and tap the intake manifold for a suitable hose barb. I drilled and tapped the underside of the intake manifold while my friend Tim drilled and tapped the top side of his manifold. Top side access is much easier to use but bottom side machining preserves a stock apperance. Finally, one can remove the hose barb and and plug the holes when done or simply run a small rubber hose (silicone hose is best) between adjacent hose barbs left on the intake manifold. Any flow through the tube is very small and has no affect during normal operation.

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PostPost by: type26owner » Mon Jul 25, 2005 1:42 pm

Mark,
Get it as close as you can by whatever means. The final measure is how little the engine shakes about on it's compliant mounts. If you've done a good job getting the same amount of combustion pressure in each cylinder the engine will hold the idling rpm steady and will not be visibly shaking. Setting the idle on carburetors that don't share a common plenum is a challenging task.

Hey Bill,
Rather than hijack this post let's start another one and compare experiences if you'd like to discuss this subject in some depth.
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PostPost by: mark030358 » Mon Jul 25, 2005 1:59 pm

Bill,Keith,
Thanks for your replies. I am using accurate vaccum gauges which have a "snubbers" fitted to avoid pulsation of the measurement. The DHLA series have (well mine does) vacuum tappings down stream of the butterflies. I will post some results and readings later as this may be of help to others.

Thanks again
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PostPost by: type26owner » Mon Jul 25, 2005 2:40 pm

Hey Mark,
There in an airflow balance adjustment that MUST be done prior to performing the idling one. Run the idle speed screw up slowly to 3000 rpms. Wherever the engine shakes about the most that's where the airflow balancing between the carburetors via the linkage must be set. The SK Synchrometer works at all rpms and is ideal for doing this measurement too. Does the vacuum manometer work well enough up there at that rpm? If you don't have any sensitivity with the manometer then ignore it and just adjust the linkage so the engine runs as smoothly as possible so it shakes about the least amount. To establish a baseline test on how bad the shaking can really get try blocking off all the airflow on one of the air trumpets. After getting the partial throttle flow balanced only then adjust the idling vacuum settings.
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PostPost by: twincamman » Mon Jul 25, 2005 4:11 pm

really guys --your making this far to complicated----use a balance instrument such as a unysin----set the idle at 1000 rpm after a spirited run and forget them -- :lol: carburetor-is French for leave it alone ---ed
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PostPost by: type26owner » Mon Jul 25, 2005 4:29 pm

Hey Ed,
I threw three Unisyns away after I used the SK unit for the first time. Anything flow instrument that restricts the flow while it's doing it's thing is not very precise or useful IMO. It's nice to have the engine purr perfectly at idle. It's a fine adjustment that the better you can set it, the better it will idle.
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PostPost by: twincamman » Mon Jul 25, 2005 4:59 pm

Kieth --- you can adjust the instrument so it wont stall the motor-----I ve used them for years with no problems -- :lol: along with a 5000 rpm shut off and plug read ---attention to plug condition and gap and points [if used]-- and of course correct timing I feel has more effect on idle and performance when the carbs are correct ---ed
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PostPost by: bill308 » Mon Jul 25, 2005 11:43 pm

Well I must say this discussion is getting rather interesting and lively.

The first car I owned was a TR-3A with twin SU's. I balanced these carbs using the hose/hiss method, which worked ok for 2-carbs with a balance pipe but was really subjective in nature and sensitive to how one positioned the source end of the hose.

Later, I graduated to a Unisynch, which worked better in that it delivered a smoother idle yet was sensitive to orientation. If the glass tube was perfectly positioned vertically and the engine idled reliably, the readings were somewhat repeatable and surely better than the hose/hiss method.

Later, I got my S2 SE Elan with twin Webers. Instead of the near steady state flow of the SU?s, there was the unsteady state or slug of airflow characteristic of Webers, where one throat supplies fuel/air for each cylinder. Suddenly, the hose/hiss and Unisynch methods just didn?t work as well. While still a young pup of 30 ish and studying for my mechanical engineering degree, I decided to try measuring/equalizing manifold vacuum among the throats to see if this would work better than the previous methods. Well, I tapped the bottom of the intake manifold runners and inserted grease fittings, minus the check ball, as my vacuum taps. Although they didn?t work as well as special purpose hose barbs, they did the trick. I used Merium manometer fluid (specific gravity of about 2.95) and had to use about a 12-ft tall column of fluid as I recall, and be very careful not to ingest the fluid after reving the engine and closing the throttle (a high vacuum condition).

Decades passed and I found myself the owner of a 78 Ferrari 308 (8-cylinder). As my TR3A?s were long gone, I purchased a pair a matched pair of BK (bigger brother of the SK) Synchrometers to tune the 4-DCNF, twin choke, down draft carbs. These instruments promised absolute precision. My plan was to idle the engine down as much as possible, select the highest flowing throat and position the ?control? BK to continuously monitor flow on one on the high flowing cylinder and adjust the remaining 7-throats to agree with it. As each throat was dialed in, the rpm would change. Flow to the throat to be adjusted was matched to the flow of the control throat. The process seemed logical to me in that it accounted for changes in rpm due to adjustments. What one couldn?t do well with this method was look at the dynamic behavior of all the cylinders when the throttle was just off the idle stop. To do this, one needs to be able to look at the flow/vacuum of all cylinders simultaneously. Here is where the 2 x 4-bank manometer truly came into its own.

With a bank of mercury (no friction) manometers, one could blip or slowly increase the throttle, and see simultaneously the effects on all cylinders. If the levels are equal, the flows are equal. While the BK/SK Synchrometers work pretty well at idle, it?s also the off idle operation, up to about 2000 rpm, that is important for drivability and low-end torque. Indeed, one should set the off idle synchronization, prior to setting the idle synchronization, to achieve the best driveability.

Finally, synchronization is mostly important during ?every day? driving. If one is racing, I think you only need to be close as you are at open throttle much of the time and a smooth idle and progression are just not important. From an every day, live ability point of view; synchronization is very important and attainable.
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PostPost by: type26owner » Tue Jul 26, 2005 12:07 am

Hey Bill,
But aren't you making assumption that the throughput of each cylinder is equal though. It all goes wrong if they are not because they can appear to be flowing at the same airflow rate by just varying the leakrate past the throttle plates. I'm pretty sure the vacuum levels in the induction throats is an indirect measurement which is prone to large errors potentially. A similiar argument can be stated for the unequal combustion pressures obtained if the throughput differs greatly too. Best practice would be to have pressure tranducers in each cylinder. No one offers those yet. :cry: I'll continue this later.
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PostPost by: twincamman » Tue Jul 26, 2005 1:03 am

welll guys it sounds like your trying to make 40 year old carbs work like fuel injection [which may solve your problems]----I still live in the stone age and prefer unisyns and plug reads and one tc has 180 000miles and the other is starting out newly rebuilt so ill be dead before i can get that kind of mileage on it --- :wink: ed
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PostPost by: M100 » Tue Jul 26, 2005 12:42 pm

"and had to use about a 12-ft tall column of fluid"

The mind boggles!

I've used the SK Synchrometers on my Dellortos - happy with them.
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