Stromberg Synchonisation

[david.g.chapman]

Its' been a while since I posted - I've been too busy enjoying my +2

One thing has been bugging me about my Stromberg carbs though. I sorted the air/fuel mixture and temperature compensators out some years ago, and the engine has been running smoothly.

There have been occasions though where I have checked the synchronisation with an air flow meter, and it has been quite a long way out. When it was corrected (hopefully), the engine ran much the same as before. It is almost as if I am missing the optimum point where performance/pickup might get significantly better.

Any thoughts? Is the synchonisation point narrow and does it make much difference anyway?

Dave Chapman.

[69S4]

There was some talk here a while ago about which gadget to use for synchronising Strombergs and without delving into the archives I seem to remember one of the conclusions being that you can't use vacuum syncronisation (a la motorcycles), although I can't remember why and that cheap air flow equipment like Carbalancer doesn't actually do much balancing and you're probably better using the tubing in the ear method from the workshop manual. Or I could have got all of this completely wrong.

Being a cheapskate and actually having a Carbalancer already that's what I've used (or tried to use). I didn't find it that easy to get consistent results as the Stromberg engine does have an uneven kind of idle and what appeared to be in balance one minute seemed to be way out when I went back to check a minute or two later. That's probably why it's not the equipment of choice. Either way I've got results equally good (or bad!) just by using the equal hiss method or by slackening off the linkage and counting turns on the idle screws. None of this seems to have made the slightest difference to the drivability of the car. If there is a sweet spot I've yet to find it.

[bill308]

Hi David.

Probably the best instrument to use for synching Strombergs is an SK (small) or BK (Big) Synchrometer. These instruments measure air flow going into the carb and are not subject to orientation like a Unisynch. The tubing hiss method is a little too subjective for me but seems to work for some. I like a more objective measurement. Vaccuum measurements down stream of the throttle plates don't work well because they don't isolate the two carbs, as there is an air balance passage connecting the two carbs down stream of the throttle plates. The recommended Synchrometers do isolate the two carbs but sysnchroization is challanging because of the uneven pulses resulting from the firing order, 1-3-4-2 and then repeat. Notice that 3-4 fire in sucsession, followed by 2-1 in sucsession, then the whole process repeats.

Balance is most important at idle and just off idle, say up to 2000 rpm. Above that limit, balance is not so important. But, a lot of driving is done in the lower rpm range and good balance makes it's presence known when starting off from a stand still. It's really a driveability issue.

In my mind, there are 2 parts to synchronization. There is the time when the linkage sits on the throttle stops and there is the time when the linkage takes a set, just off the idle stops. Both are important. The former affects the quality of the idle and keeps the plugs clean and the second affects driveability.

The first thing I like to do is back off the throttle stops a turn or two then shorten the throttle cable a little so that all the linkage takes a set, hanging on the throttle cable, clear of the throttle stops. The engine should idle at say 1000 rpm. Adjust balance at this condition and you should be good throughout the critical 1000-2000 rpm range. When this is good, lengthen the throttle cable to obtain a low idle speed at say 600 rpm, with the linkage still clear of the throttle stops. Now adjust the throttle stops for a final idle speed of say 800 rpm and balance the carbs a second time just using the throttle stops. If balance is done using this procedure, you will have good idle balance and good off idle balance.

I hope this helps.

Bill

[twincamman]

WELL GUYS THE SECRET IS TO undo the fixture on the throttle shaft joining the carbs and treat each carb as a separate unit -- using your unysin --otherwise your twisting and turning knobs and screws and pissin in the wind---ed

[bill308]

twincamman,

The point is that the throttle linkage is not very rigid. It twists a little depending on how it it is supported. The loads are the return springs on the carbs.

When at idle, it is presumeably supported by two throttle stops and will take an appropriate set or slight twist or deformation in engineering lingo. When off the stops and supported only by the throttle cable, the linkage takes a slighly different twist than when on the throttle stops. If the throttle return springs are weak, then linkage deformation is less than if they are stronger.

The strategy is to obtain equal air flows under the more general case where the throttle shaft is supported only by the throttle cable, any time one is not at idle. If one can adjust the linkage for this condition than synchronization is assured thoroughout the critical rpm band.

The final step is to nip up the throttle stops to just maintain synchronization/geometry when on the throttle stops.

In order to obtain the desired geometry, one has to adjust the screw on the throttle stop lever of the front carb. This should be easier than the standard Dellorto or Weber setup where one doesn't have to deal with twist of the throttle shaft within a carb. In this case one would have to twist or replace the throttle shaft of a DCOE 18 where synchronization of some of the later carbs (40 DCO 131 and DHLA 40E) can be obtained by an air bypass adjustment to effect balance within a carb.

The best way of measuring balance (equal air flows) on a Stromberg setup (two carbs for 4-cylinders) is with a Synchrometer (an air flow meter). The best way with Webers/Dellortos is measurement of vacuum (manometer) as there is effectively one carb per cylinder.

Bill

[twincamman]

well Bill you just do that your way and good luck to you --- keep your powder wet and drink down stream of the herd - ed

[69S4]

Bill, I've come across your two stage sync method before in connection with balancing the carbs on older BMW twin m/cycles. They have a cable going to each carb and you balance initially at 2000 rpm with the engine "hanging on the cables" by adjusting the screw in stops where the cable outer is located. You then do it again at idle on the throttle stops. I don't own an older BMW but I'm told it does make a substantial difference to low down smoothness.

I can see how a similar approach would work with the Elan Strombergs but I'm not quite sure how you would actually carry out any adjustment at the first stage where the engine is hanging on the cable. The linkage doesn't really lend itself to any fine adjustment and if you screw in the throttle stop screws to act as a support you'll take the pressure off the linkage and thereby alter the "set".

Some Honda motorcycles face similar problems and their linkages have spring loaded screw adjusters between the carbs. You can adjust those with a screwdriver.

[david.g.chapman]

Interesting replies, and helpful as always.

I use a Gunson's device that consists of a shuttered funnel connected by a tube to a graduated flow tube with a small plastic "float" in it. The idea is to mount the tube vertically, place the funnel over the bare carb inlet, then adjust the funnel shutter until you get an arbitrary reading. You then carefully transfer the funnel to the other carb, then back again, etc.

I have found that the balance can change when the throttle shaft clamps are tightened up, and also when the cable starts the pull the shaft around. I have always tried to do this on my own, but I will get someone else to open the throttles a bit inside the car while I do my thing with the funnel. I would not be tempted to turn the shaft at one end to do this - too much flex - I think it has to be in the middle via the cable.

I will not get a chance to do this for a while - recovering from a bowel polyp operation.

Dave Chapman.

[Yum-yum]

I use an STE Synchrometer to balance the carbs, it works well. I had to make a right angle intake out of PVC pipe to get the synchrometer to clear the bodywork. The airbox needs to be removed, obviously, and the intake seats in the mouth of the carb. It has a by-pass hole for a different flow range, but I don't use that feature.
I set the balance and then the idle. My Stromberged motor settles into a very steady idle, but I attribute it to a good steady spark from electronic ignition.

[redskatejbf]

Hi.....Somewhere I have a note on balancing the air flows on the Strombergs that suggests that the ports that bleed air to the temperature compensators should be blanked off. I assume this is, with the engine hot, to stop the air flow meter giving a false reading because of air being bled off bypassing the meter. I don`t know if this is correct as I would have thought both carbs. would be at the same temperature and would have the same `leak` as each other, and with the balance pipe between the carbs. the inbalance would be very small. Has anyone heard of this, or used this method, or see any benefit in blanking off the ports?

Regards John.

[bill308]

Thanks for the kind words Stuart. It appears BMW used the same method I recommend but with a different linkage, 2-cables to one hand grip. The idea is the same.

I looked in the parts manual but I don't see a view of the throttle cable attachment to the carb throttle shaft. Some cables have an adjustment on the outer sheath that allows one to lengthen the sheath relative to the cable, thus effectively shortening the cable. Absent this adjustment fitting, one needs to be creative. Maybe you can wedge the accelerator pedal to give an effective idle rpm of 1000 rpm or shim the throttle cable sheath to effectively lengthen it or the cable end itself to effectively shorten the cable with something? My 308 has a threaded fitting on the end of the throttle cable sheath that allows me to lengthen or shorten its effective length. When I lengthen the sheath (outer covering) the throttle cable is effectively shortened and all 4-carbs can be effectively hung and adjusted for balance using multiple manometers reading manifold vacuum. Another possibility is to make up some sheet metal shims say 1 inch by 1 inch with a slot cut from one edge to past the middle. The slot would straddle the cable at the bulkhead and space the sheath away from the bulkhead. Use several of the spacers to effectively lengthen the sheath as necessary. Or maybe use a small pair if vice grips or a clothes pin to grab the throttle cable at the bulk head and space the sheath to make it longer? Just throwing out some ideas here. The important thing is to shorten the cable relative to the sheath if there is no other adjustment mechanism to effectively hang the throttle shaft off the throttle stops.

Please note this method is not covered in any text I've seen. It does try to address the reality of the real world.

David,

I have no experience with your Gunson device but from your description it seems similar to a Unisynch in operation. My main concern is that instruments of this type rely upon orientation for accuracy and they affect the air flow they are measuring. If you are careful, you can make it work better than the subjective hiss and placement of a simple tube to the ear.

A side note. Guys I'm really trying to help out here and give some insight on how things really work. After 30 years I find myself in semi retirement as a mechanical engineer specializing in heat transfer and fluids. After being laid off in January of this year, a contract engineering firm for my former employer, called me back to perform a special test for them. The question is whether a small screen in the fuel control electronics box that controls the engine on a Boeing 767 can be safely be removed to increase on wing longevity. The problem is that clearances are so tight between the compressor blades and housing of the engine that there is wear at the interface and aluminum in the form of aluminum hydroxide is liberated and is picked up by a pressure sensor tube that measures burner pressure. The aluminum hydroxide migrates up the pressure tube and over time clogs this little screen in the fuel control. When at altitude and very cold, moisture in the exhaust forms ice and can block the passage completely. Under these conditions, when the pilot wants to decend and throttles back, the fuel control cannot detect the reduction in burner pressure, due to the blockage of the screen, and the engine could shut down. So, the test is to determine if removal of the screen will result in longer on wing time before malfunction. The plan is to perform and A and B test. The A test is with the screen and the B test is without the screen. To do this test we simulate a lot of pressure cycles, equivalent to flight cycles, and we introduce aluminum hydroxide to the control's burner pressure port when the control is at different temperatures. The cumulative effect of whether it is better with or without the screen is the objective of this test. The contract employer came to me based upon recommendations that I was the best person to do this job as I had the necessary background.

I wrote the above note to try and establish some qualification for dealing with the question of how best to handle balance in carb tuning. For best results one should approach the task in a reasoned and methodical way. Not all will agree with my advice. That's OK. But please be assured that I write with the best of intentions and some relevant background.

Bill