Synchronizing throttle valves

[BSNMOE]

On page 22 of CMA "SYNCHRONIZING THROTTLE VALVES" it states to measure air flow at 6000RPM after "CHECKING AND ADJUSTING CORRELATION".
The EXAMPLE-measuring air flow illustrates all 6 cylinder with a range of air flow using, a synchrometer between 10.0 to 13.5 at 6000. That seems like a wide range of unequal air flow after correlation with protractors.
It goes on and states to average and then adjust air flow by adjusting up or down the air screws to get equal flow. All at 6000RPM.
Now that this is done nothing is said about idle synchronizing. It seems that if the flow is way off( between 10 and 13.5)at 6000 then you adjust equal from there the idle will then be way off.
Am I missing something or does the idle air flow not matter that much.

[snbush67]

I think (theoretically rather than categorically), that the reason you are going to see such different numbers at 6000 rather than 1000, is because the air flow is at an exponentially higher flow rate. Any subtleties in the difference at 1000, are magnified at 6000.

By making the adjustments at 6000, you are fine tuning the air flow. With this theory, you can expect to make smaller adjustments to the air screws at 6000.

To prove this, the air flow should all be equal at 1000 once you have adjusted at 6000.

Please let me know how this turns out.

I checked mine at 1000 and 3000, no adjustments were needed at 3000. I didn't want to maintain 6000 with no load.

The adjustment in air will not likely effect your idle, but may slightly effect your mix.

[Porboynz]

I can not see how adjusting the idle air flow screws with the throttle plates open far enough to get the engine up to 3000-600rpm is going to work? Surely the resulting air flow past the throttle plates will greatly exceed the small airflow through the idle air bleed slots? With my old MFI 2.4 I set all the throttle plates just off the fully closed position with the backstops (so the plate does not actually jam shut against the throttle body) then I equalise each stack using the air bleed screw with the flow meter. Works a treat. Maybe its different on more modern fuel injection systems, seems strange though.

[BSNMOE]

Sorry everyone but I meant to say that the CMA manual states to adjust air flow at 3000 RPM not 6000.

Actually, with my car, I can get perfect air flow at idle and all stacks will pull around 5 KGH on the meter. This is with air screws all 1 1/2 turns out. At 1500 RPM there still all even but at 3000 they are between 10 and 11 KGH which is closer than the example given in the manual. If I average the air flow equal at 3000 my stacks pull slightly unequal air flow at idle.

[Bill Verburg]

Originally Posted by BSNMOE

Sorry everyone but I meant to say that the CMA manual states to adjust air flow at 3000 RPM not 6000.

Actually, with my car, I can get perfect air flow at idle and all stacks will pull around 5 KGH on the meter. This is with air screws all 1 1/2 turns out. At 1500 RPM there still all even but at 3000 they are between 10 and 11 KGH which is closer than the example given in the manual. If I average the air flow equal at 3000 my stacks pull slightly unequal air flow at idle.

I'd live w/ unequal flow at idle, the geometry of the intake is not perfect, the end stacks will always flow differently from the middle one because of the bends and unequal lengths.

[1QuickS]

When at idle the throttle plates are nearly closed and air for idling is provided from the leakage past the throttle valve and the bore of the carb. Additionally, there is air supplied through the machining tolerances of the throttle plates and the bores, wear of the throttle plates with the bores (edge wear from axial movement of the throttle shaft over time), through the radial clearance between the throttle shaft and the bearings in the throttle housing which over time increase more in the outer bores than in the middle bore. Also, there is misalignment of the throttle plates in the bores from initial installation and due to twist in the long throttle shaft and relative twist between the long shaft and the short shaft. All these are magnified and variable air flow contributors when the throttle plates are closed. Achieving smooth idle required balancing the individual air flows with the air adjusting screws at idle or perhaps slightly above idle. The, idle mixtures need adjustment to account for the various air flow settings at each cylinder.

Adjusting air flow at 3000 RPM is meant to be sure all internal clearances in the throttle linkages have been eliminated by application of the throttle. (At larger throttle openings than when at idle, the minor variances in air flow from cylinder to cylinder are lost in comparison to the larger flow area.) To do this properly, the throttle cross bar should NOT be activated by hand, better to have an assistant hold the throttle open from the driver's seat or if perfomed by yourself then the horizontal link from the bell crank on the intake manifold to the cross bar should be actuated. Once the engine speed is at 3000 RPM the side-to-side balance may be adjusted.

[tobluforu]

I’m going by memory and I think what is meant by CMA in regards this type of synching starts at the butterflies. If you have new ones then they should be setup perfectly, if not, then you might have stacks like mine that are 40 years old and need readjusting. You first disconnect all linkage, and of course this is after car is warmed up. With idle adjustment screws turned out 1.5 turns (some years call for more turns), you then synch the stacks using the little adjustment bolts/screws on the stacks/butterflies trying to achieve 850, give or take. This is a pain in the ass due to the location of some and you should not have to turn them much at all. You then connect all rods at the stacks making sure that the idle/synch has not changed, if so adjust rod length. After this is done you synch one last time at 3000 rpm and you then use the idle adjustment screws to dial it in at this rpm, and I think this is what Bill is referring to? In a perfect world with perfect stacks, butterflies etc, then this would turn out peachy, but you have to give and take on these old MFI cars.
If your stacks have been on the car for a long period of time, do not forget to tighten each nut that connects the stacks to the butterflies and then to the head as I could not believe how loose mine had come. This fixed my popping at idle.

[BSNMOE]

In the past I have removed TBs held them up to a light method along with the shop vac method. When installed with new gaskets I got a pertty even air flow and only 2 were a little low so I turned those 2 TB bolts out very little,leaving air screws at 1 1/2 turns out. I got a even flow at 900 RPM idle. It's only when I go past 1500RPM that they go uneven, but again, I'm off 1 KGH on # 3 and #4 at 3000RPM. #6 is slightly off( call it 1/2 KGH) All others pull even at 11 KGH. That's not as far off as the CMA Manual's example which is 10 to 13.5 KGH.

[BSNMOE]

I went out to my garage while leaving the computer on and it must have sent my last post without me finishing it. A friend came by and he held the gas pedal at a steady 3000 RPM prior to the car becoming too hot. I wanted to see if heat expanded the stacks, linkage etc. which would give me an indication of why I get the uneven reading. It was the same as when hot; therefore it must be something else. Maybe as small as the cup ends of the linkage are worn. (I have played around with adj linkage to try to get equal readings)
What i was getting at before was in the example given in the Manual the readings are all over the place(3 to 3.5 KGH off). Much more off than I have. This example provided makes me think that the ony concern is 3000RPM not idle because if they are so far off at 3000 then adjusted for even flow then the idle will be way off.
Sunbush67 seems to have equal flow idle to 3000 does anyone else?

[TR]

Hi
I couldn't balance mine either.
Finally i pulled them off and inspected them. After 40 years the shafts were a little worn and the air bypass was a little fouled.

the cost of repair is expensive and the specialist in australia said he didn't trust any locals to recondition them. Euometrix and Henry S do a great job but i didn't have that sort of money spare at the time.

I found a another 40 year old set which were like new. I put a vacuum cleaner hose on to them and just checked the flow values with a meter. Closed the butterfy valves till no air could pass, set the butterfy valves so the were all even then adjusted the air screw. The one thing I also did was replace all the linkages. A guy in the Uk sells them 80 bucks a set. I bought two sets so I could change all the sockets on every linkage.
I also honed the stacks, they had lumps and bums from the original castings. I carfully wrapped course emery paper around the head of a cylinder hone and spent a couple of hours carefully going in and out of the stacks eventuelly they were smooth. Finished off with finer emery then the engine hone. Now I know it is not the profesional way but they are smooth and look great. I constantly measued each end so i wasn't making them too big.

When I put it all back on the car it ran absolutely smooth, I measured all the flow values and then did some very minor sdjustments which mad no difference anyway. The car absolutely poweres now.

I have since put a Dank stainless steel muffler on with the 75mm tip, get a little bit of popping --think thats the unburn fuel doing a bit of after burn. The CMA mfi manual said that happens sometimes and cannot be resolved, I have a new air fuel meter and will try to get a better result.

There was a really nice set of throttle bodies for sale a few weeks ago on pelican. 450dollars---no affilliation, they might still be for sale. I spent 750 but it was well worth it.


good luck with it.

[snbush67]

Originally Posted by BSNMOE

Sunbush67 seems to have equal flow idle to 3000 does anyone else?

They weren't dead on but close, I did not make adjustments at 3000 just left them where they were at idle. I have PMO 46 Carbs with less than 2000 miles maybe the fact that they are new on a freshly built top end has something to do with it.

Do you think that the subtleties of I/E valve adjustments and a combination of the air adjustment could impact and create a significant difference?

[BSNMOE]

Terrance, My TBs were rebuilt 20,000 miles ago, so I believe it may be worn sockets on linkage.
Sunbush67, that's a goood point about valve adjustment, but I just did that and was carefull to re-check before installing covers. Also, I only have 20,000 mi on engine and compression is equal.
when I pull plugs they all have equal amounts of soot and the car idles ok(smooth) at 900/950. Car pulls smooth to 7000RPM.
Bill Verburg's point is probably the best approach, check at 3000.
I did see in Lee Rice's method that any change from idle to 2000/2500 RPM is linkage. i'm pretty close and I tried to adj that out, but, a I do have a little play in the sockets.

does anyone know where I can get the sockets or end caps?

[tobluforu]

Euometrix sells the linkage, and there is a guy on UK ebay that sells these as well.