A question of balance

[type26owner]

piercemanifolds.com has a new tool offered called a Universal Synchro. Be wary of the gimmicky type tools that are not all that useful upon closer scrutiny.

A better solution to monitoring all the cylinders all the same time would be an array of mass airflow sensors (MAS). (Hopefully those are the ones I remember as being absolute sensors requiring no calibration.) To cobble together a system ought to cost a small fortune though. The STE Synchrometer is the analog equivalent and one heck of a lot cheaper. Just keep resetting the idle rpms back to the position so the throttle plate when opened the tiniest amount will start to extract emulsion from the first progressive hole. BTW, if you know that rpm then there is a direct correlation between it and the absolute scale on the STE meter. Got my DCOEs setup so that number is 4.5 Kg/hr. Therefore I can reset the idling rpm by the airflow alone.

[twincamman]

kieth ---I can lend you my unisyn ---ed

[berni29]

Hi Guys

I am about to set up the carbs on my car and am getting a synchrometer for the purpose, but have a question. I read in the manual that you twist the spindle to even up the butterflies on each carb (if required). This is more easily done with the carbs off the car. How about I empty ALL of the fuel out of the carb, clean them, and then connect my powerful vacum cleaner hose (via an adapter) to each inlet in turn and use the synchrometer to adjust the butterflies using the twisting method. Is there any benefit to doing it this way (as a starting point), or is it just a bad idea?

All the best

Berni

[bill308]

Keith,

Yes, I guess I am making the assumption that the throughput (pumping ability?) of each cylinder is equal. If you have a non sealing intake valve or the valve lash is way off on one cylinder, you are unlikely to get good balance with the manometer vacuum method or any other method. To a large extent, the vacuum method automatically compensates for throttle shaft and O-ring leakage so the measurements tend to be forgiving and repeatable. The engine does not know if the air coming down the inlet tract slipped by the butterfly valve or past a leaky throttle shaft bearing. It only sees the available air in the intake manifold. In contrast, a Synchrometer does not see air leakage past its sealing surface to the carb, past the throttle shaft bearings, or past the O-rings. A Synchrometer only sees airflow passing through itself.


M100,

Yeah, about a 12-ft column of manometer fluid with a specific gravity of 2.95, where specific gravity of water is 1. Now I've switched to mercury (Hg), with a specific gravity of about 13, so 2.95*12/13=2.72 ft=32.7 inches. At idle I see about 24 inches of HG. If the engine is reved and the throttle suddenly shut, vacuum gets quite high for a short period of time.


Bernie,

You are essentially constructing a poor man's flow bench. If you are careful and pay attention to details (sealing of the Synchrometer to the carb face and the adaptor to the manifold side of the carb, you should be able to measure relative flow with reasonable accuracy, certainaly good enough for a first order approximation. I've tweaked my throttle shafts on the car, but it's a crude way to adjust barrel to barrel balance. The adjustment also doesn't seem to hold for a long period of time. The later carbs, 40 DCOE 131's I believe, have a separate balance circuit to solve this problem.

Bill

[mark030358]

Hi there,
Well set evrerything up as best possible and got the following (at idle):-

No1 = 13.5"Hg average
No2 = 11.0" Hg average
No3 = 11.0"Hg average
No4 = 13.5" Hg average


Interesting that Nos 1&4 and 2&3 are identical, I even moved the gauges about to check for measurement error. I thought this may be to do with the way I refitted the nuts on the spindles. but I discounred this as they are righthand threads and tightening in this direction would do the opposite on nos 3&4 (open 4 and close 3 with excessive force) However at higher RPM's the balance is almost perfect. Can anyone explain the difference at idle and more importantly is ths acceptable. There are no leaks etc etc

Bill,
Very ointeresting, 24"Hg is approaching full vacuum, does that mean your butterflies are much more closed than mine or do you have a more efficient air pump of an engine than I. I'm still running the engine in (440 miles as yet) but I had a compression if 200psi on each cylinder +/- almost zero. Would be interested in your thoughts.
cheers
MArk

[twincamman]

welll it seems to me this a lot of trouble to set the idle circuits as the high speed jets are set and all you have to do is adjust one screw to set the synchronization of both carbs --and that can be determined with a hi speed key off and a cllutch dump and a plug read --ed

[bill308]

Mark,

Your vacuum numbers seem low to me. How are you measuring them?

I can send you digital photos showing the 308 vacuum across 8-cylinders at about 25 (not 24 as previously stated) inches of HG at an idle speed of about 1000 rpm. The problem with the carb 308 is getting it to idle low enough with the emissions equipment. The 4 x 40 DCNF 45-48 carbs are as closed as they will go and still stay balanced (i.e. all air flows/pressure drops are equalized to the highest flowing carb to maintain balance). My Elan's 40 DCOE 18's were set to idle at about 800 rpm. No emissions equipment on it though.
By the way, the Lotus twin cam is very similar to 1/2 a Ferrari 308 engine where 1588 cc's is about 1/2 of the 308's 2900 cc's and both are 2-valve hemi heads. HP of the Elan, as one would expect, is about 1/2 that of the 200+ of the 308. Weight of an Elan is also about 1/2 of a 308.

My friend Tim recently balanced his S3, now fitted with 2 x 40 DCNF 131's? with one of two of my 4-bank manometer systems. I'll have to ask him what his baseline manifold vacuum was and I'll report it here.

Bill

Bill

[bill308]

Mark.

Upon closer inspection of your vacuum numbers, it appears as if your throttle shafts are slightly twisted. The interior throats flow more that the outside throats. I believe the throttle torsion springs are on the outside throats of the throttle shafts. This constant torsion load will tend to twist the brass throttle shafts of 40 DCOE'a such that the throats closer to the throttle cable will be more open than those farther away from the throttle cable. If you could rectify this situation by twisting the shafts back, to attain equal readings, you will likely realize higher vacuum readings and a more reliable idle speed.

Bill

[bill308]

The following link is a write-up I did, with pictures, showing the use of manometers to tune carbs on a Ferrari 308:

http://www.ferrari-talk.com/discus/mess ... 1/971.html

Scroll down towards the bottom. After synchronization, all carbs show between 24-26 inches of mercury and the idle and off idle quality is very smooth.

Bill

[mark030358]

Bill,
I'm using glycerine filled vacuum gauges to measure the signals. All the gauges are within 1" HG of each other when attached to a commen vacuum line.

I'm not sure if I could attain 23" ish vacuum even by slightly twisting the spindles, more likley result would be that they average out. The only way to get more vacuum, I think and I'm far far from an expert, would to to such more air thru the carbs by means of engine induction and keeping the butterflies closed. As I said earlier the compression test I did seemed to be fine. The engine has been bored +30 so in effect I should suck more air?? What do you think?

All this effort by the way is to solve a drive problem I have. The car does not want to go slow, ie less than 30mph with out "trying" to judder. I expect that at this speed I am on idle jets only. Another thing is that all my mixture screws are only 7/8's to 1 1/8 turns open. This small opening, from what I have read and been told, is very small. The jets are standard sprint jets ie 50's and have been in since the car was a car. Any thoughts?

Cheers Mark

[type26owner]

Twisting the throttle plate spindles to adjust the airflow balance is a bad thing to do I my opinion. That gets the phasing of the throttle plates out of whack to the first progressive hole. You'll never get it to come off idle cleanly. The proper way to do the balancing is to drill a tiny hole through the plate of the throat that needs more flow.

If you've got massive airleaks then accurately measuring the airflow via the vacuum level is a bit of useless information IMHO. Most of the air MUST flow past the first of the progressive holes or it won't function properly. Fix the airleaks so the synchrometer can work properly.

[bill308]

Mark,

OK, when all gages are hooked up to a common signal they are within 1-inch HG. That appears pretty good but at what vacuum are you seeing these differences? Assuming its in the 20-25 inch HG range, I would make a note of which gage read lowest, designate it as the master, and note how much each of the other 3-differ from the master. Record these values and use them to correct your measurement readings during balancing.

The absolute vacuum values are unimportant. You just want you equalize vacuums between throats during tuning, at an acceptable rpm (800-900?). The fact that you have bored the engine 0.030 inches makes it a tiny bit bigger. All else being equal, the engine will breathe a little more air. For a fixed throttle setting, this will result in a slight increase in vacuum reading, but it will likely be too small to reliably measure with your equipment.

Your judder may have causes other than carburetor balance. At a steady state 30 mph, you will likely be on the progression circuit for both 3rd and 4th gears. I?m not sure exactly when the transition to the mains occurs, but a reasonable guess would be about 3000 rpm. Float levels are important here. Raise the levels and the progression phase will be extended and the mains will come in earlier. Lower the levels and the opposite will occur.

I have seen Weber mixture jets range from about 1.5-turns open to 5-turns open to obtain a reasonable idle mixture. I believe the rule of thumb is if one needs to open the mixture adjustment screws more than 3-turns, one should install a richer idle jet. This of course assumes your float levels are correct. By the way, is yours a Sprint engine?


Keith,

The likely reason why airflow balance is off within a single carb is because the throttle shafts have twisted over time. This is likely a creep event where the throttle shaft is under a moderate torque, due to the return spring acting on the end of the throttle shaft, opposite the actuation (cable) end. One is merely trying to restore the original geometry when twisting the spindle back into its original shape. If one can do this, registration of the throttle plate and progression holes will be restored to that intended by the factory. By the way, the best way to judge the relationship of the progression holes to the throttle plate is to remove the brass cover plug exposing the 3-progression holes. One can look through the progress holes and see the relative position of the throttle plate. It?s interesting that on my 40 DCOE 18?s, one of the progression hole was not properly drilled. To compensate for this defect, I filed a small, localized, chamfer on the edge of the throttle plate (about a 45-degree angle extending through about ? the thickness of the plate. From the standpoint of the emulsified air/fuel point of view, in the progression circuit, the throttle plate edge was uncovered at exactly the same time as the corresponding holes on other 3-throats.

Intuitively, I really dislike drilling holes in the throttle plates to obtain balance, even though it is reversible in that it the holes can be soldered shut. Few carburetor installations have massive air leaks in my experience and those that do are almost always discovered during an installation. More likely, leakage will occur through the throttle shaft bearings or the anti-vibration, 0-ring mounts. Of course at low rpms, any leakage is an issue. Assuming leakage is moderate and there are no massive air leaks (like a visual gap due to failure of a pair of Thacker washers), most of the air will flow past the throttle valves and the measurement of manifold vacuum, as an indirect measure of airflow, will be valid. Idle and off idle smoothness will confirm this.

Let me be perfectly clear, the measurement and equalization of the individual vacuums of the inlet tracts, on Weber/Dellorto equipped twinks, is the most sensitive, forgiving, and fool proof method of setting carburetor balance, at the most important times, idle and part throttle, steady state operation. If you are racing and running wide open throttle, balance is not important. If you are driving the car, as every day mode of transportation, you will benefit from this balancing method, IMHO.

Bill

[type26owner]

Bill,
We're on the same page but prefer different methods to achieve similiar results. The reason one of the first progressive holes appeared to be not round is because I think the women that assembled it originally altered the downstream side by gouging out a chip with a metal scrapper. This is how they got the phasing perfect so when the throttles were opened ever so slightly from the idle position fuel flowed out of the pair of first progresssive holes at exactly the same time. One hole on each of my DCOE18s had been altered in this way. The only way to properly align the throttle spindle based on this fact is to remove it and twist it so both plates are coplanar again.

The original throttle spindle shafts were brass. The new replacement ones are steel and are way too stiff to twist anymore.

Drilling the holes in the plates is a royal pain because of the effort needed (a whole weekend). I've almost completed making replacement o-ring carriers that are thicker by another 1/8" so I can add air bleed bypass screws just like the latest carbies have.

[mark030358]

Bill,
Yes its a sprint engine. Maybe I forgot, but at wider throttle openings all readings are virtually identical. Thanks for your assistance with this thread. Going to get the car on a rolling road next week for a complete diagnostic.

cheers
Mark

[bill308]

Keith,

As long as you are making thicker O-ring carriers to support bleed airscrews, you might also consider adding vacuum taps and give my balance method a try. I think you will be glad you did. Your modified (thicker) O-ring carriers would be a great way to address the two biggest shortcomings in the 40 DCOE 18's, balance measurement and adjustment capability as a bolt-on, without having to permanently modify your intake runners. I would happy to buy a set of these animals.

The defect I saw on my carburetor progression hole pattern was a single hole that was clearly drilled in the wrong place. I saw no evidence of hand tuning to correct this defect on my carburetor. The modification I made to throttle plate however, fully compensated for this defect.


Mark,

Are you saying that off idle your balance is right on? How many idle stop screws are you using?

It seems to me that if off idle is perfectly balanced and you are not quite balanced with the throttles closed and you have multiple idle stop screws, the thing to do is use only a single throttle stop screw on the lever to which the throttle cable is attached. The carb-to-carb balance would be set with the intercarb fork adjustment, but would rely upon the single idle stop to set idle speed. This should preserve balance throughout the idle/low idle range where balance is important.

Bill