Hey y'all. I am running a 1936 flathead in my '34 Ford. The engine has an aluminum flywheel, a 3/4 race cam, Eddie Meyer finned heads and 2- 2bbl carbs on a Fenton manifold. I have run this combo for over 17 years, but, until today, I have never tried to check the 'balance' of the two carbs. So I got one of the Uni-Sys flow meters to see whats up. The rear carb can be adjusted so the gauge float is in the middle of the scale, but if I pull it off and place it on the front card, the float stays bottomed out- in other words, it doesn't move.. So, I use my hand to cover the rear carb - and the airflow sucks my hand down and shuts off the air- engine goes dead. I crank her back up. I do the same to the front carb and, while the engine does idle down some, it Stays running. There is maybe a Quarter of the suction that the rear carb has. Not good. Anyone have any idea as to what my next move should be?? Looking for thoughts before I go thru the front carb again. All I can think of is a vacuum leak, internal to that carb. Hoping someone has been thru this before and won't mind telling me about it. Thanks y'all,
Did you disconnect the linkage? do you have it so that both carbs are working - not 'progressive'? If 'Yes' after measuring the rear carb then increase idle speed until the rpm increases via the front carb and then you know both carbs are working, then start lowing the rpm going between the two carbs back and forth until you get the proper idle, set the unisyn on each carb and it should read about the same. With two carbs you don't need it when going back and forth between the carbs. With 2 carbs at idle you should be able to have rpm change at adjustment of either carb. It sounds like the rear carb is acting like a primary and it is what the engine is idling on. Don't set your hand on the carb as it'll get wet for reasons I can't quite get my head around.
I run dual Carter W-1 carbs on a 250 inline six, neither carb had enough flow to lift the float of the balance tool due to the low rpm idle. I took the tool apart and cleaned out the casting holes and made sure the seals were tight, this allowed for better adjustment of the tool. Pull the linkage off one or both carbs so idle speed can adjusted independently. Start with the front carb and adjust the idle up till the float comes off the bottom, move to the rear cab and lower the idle till the float drops down, keep going back and forth till the float level is equal on both carbs, don't worry about rpm. Once you have the float equal on both, back each idle screw off just a touch till the idle rpm is where you want it. When done, the float should still be equal distance from the bottom, you may have to adjust the tool a bit as you go. When set correctly, the slightest turn of either idle screw should raise or lower the rpm, if not, one carb is still over running the other. Mixture screws can effect the adjustment also, keep tweaking the idle and mixture till you are happy and the float stays in the same place, its a lot of back and forth. Hook up the linkage, it should go on without effecting the rpm. Adjust the linkage to remove any play so both carbs open at the same time, but not so tight it holds one open or binds.
If the above doesn't work, pull the front carb for a cleaning. I had a similar problem when my front carb idle port plugged up. Engine still ran and I could drive the truck, but it wasn't right. When both are working, the engine is much happier. I knew mine was off since I have an Air Fuel Ratio gauge in the cab, the gauge showed way lean at idle.
Oh Man- you guys have given me exactly what I came here for- thank you so much!!! Now, to go to work... Mike, I have thought as hard as I can...that's why I posted here.
I'd check the plugs to see how it is running. Then swap the 2 carbs around and compare the results. If it runs the same either way and performs fine I'd leave it alone.
Just think of it as one carburetor cut in half and spread apart. They need to be adjusted equally so all the cylinders receive the same amount of air and fuel mixture.
The carbs sit on a common plenum, they don't need to be sync'd the way carbs on individual runners need to be.
this comment is contrary to most of the information that I have read on here, but is exactly what my untrained mind says also, as everything is going get mixed under vacuum, - more or less………….
With multiple carburetors above a common plenum, there are going to be areas where there will be unequal fuel/air ratios unless the carburetors are synchronized. There's a lengthy but informative article from Hot Rod, Aug. 28, 2017, detailing the set-up of a 4 carb installation on a 291 DeSoto Hemi. Here are a couple of pics from the article. The carb/manifold assembly was reinstalled on the DeSoto Hemi and the fuel line connected to the fuel block/regulator. For good measure, a new inline fuel filter was also installed. With the engine running and the first carburetor adjusted for the highest idle, a Uni-Syn carburetor balancing tool can be used to ensure all the carbs' air/fuel mixtures are synced. On that first carb, note the position of the red float within the Uni-Syn's tube. Then, move the tool to the rest of the carbs and adjust them until the float matches the same position of the first carb.
If the throttles are not set equally, same position, as you drive, one will always be open more than the other one, does that sound right? Why would you not sync them together?
I couldn't get a link to the article, no matter what I tried. The example shown in the magazine article is of 4 carbs with non-progressive linkage. If you read the article, it describes in detail what is involved, from re-jetting the carbs to linkage adjustments.
Before I did anything major I would swap the carbs around. While you are doing that you may get lucky and find a busted gasket on the front carb. if you don't, fire it back up and repeat your check. You may find that the front carb gets a reading and the rear does not. What does that indicate? bad carb. If you get no change then you will need to dig deeper.
This is not accurate. There may be some difference in the amount of air/fuel flowing through the carbs, but a Uni-Syn will only help you equalize the air flow, not the air/fuel ratio of the carbs. The air/fuel ratio is controlled by the jet's/power valves, etc (and the air/fuel screws at idle). Once inside the plenum it is all mixed together anyway. Synchronizing the air drawn through the carbs at idle really isn't going to change much if any of that. How many thousands of cars run progressive linkage? On a multi-carb intake with progressive linkage how often are the throttles of all the carbs in the same position? (only at WOT). If that works OK, why wouldn't it work OK on non-progressive linkage carbs? As long as you can get a decent idle quality, and both carbs are fully opened at WOT you're good to go. Now things are different with individual runners, for that the carbs should be synchronized.
Thanks for the comments to my post. I have synchronised mine, and it seems to run well. I am just not convinced that it is as vital as everyone says, and the original posters first post seems to suggest that, assuming that he is running fixed linkage - (he doesn’t actually say so). Maybe he will come back and let us know how he got on, once he has sorted it all out. .
Most multi carb set ups with progressive linkage, use a more centered carburetor for the main and outers for the add-ons. Mine is an inline six, if they are not sync'ed, it runs bad, it's a long way from one end to the other. A V8 can get away with more
What's not accurate? If the amount of air flowing through the carbs is the same and the carbs are all jetted the same, there will be a more consistent air/fuel mixture in all parts of the manifold. If you read the article I referred to, the fellow setting up the system removed the stock jets and replaced them with smaller units so that the fuel/air mixture wasn't entering the manifold too rich. When the air/fuel mixture first goes into the manifold, the mixture isn't uniform. There are larger droplets of fuel that fall out of suspension and puddle on the floor of the manifold while the air continues on it's way to the combustion chamber. Not every part of the manifold has the same amount of air and fuel in it. The closer you can get the carburetors synchronized, the better your fuel/air mixture will be which will result in better performance.
Throw a wide band A/F ratio gauge on it, you will very quickly see how little it takes to move the needle from idea settings. I run one all the time, and can tell right away if the fuel system is not right. 1/8 of turn on just one mixture screw can change the ratio a 1/2 point !
That’s exactly what I thought. I thought that tool was only to be used on individual runner carb setups like Webers etc. it’s only to get the airflow matched between each is my understanding. As for plenum type setups you are limited to intake runner lengths etc for individual cylinder performance.
Before you begin, make sure the linkage ARMS that attach to the carb throttle rods are all at the same angle and that the angle left/right of center is about the same when closed and when fully open (on each side of 12 oclock). You do this before you start the sync procedure. Also, the modern repop Elco ball-bearing style arms may not clamp the throttle shafts on the carbs well enough to not slip. I've had a LOT of problems with slippage on the throttle shafts - making the whole setup/sync procedure useless. Or, you have it setup correctly, then the arms slip later on - screwing everything up. What I do now is setup the arms angles to match and drill the arms and throttle shafts for a small 1/16" roll-pin. Once installed, this insures that the arms do not slip on the shafts. In many cases a sync issue is related to how the linkage is setup and how the idle stops on the carbs are used. When running multiple carbs that are running an inline or synchronous linkage, I only use the idle RPM screw on ONE of the carbs to set idle airflow/speed (usually the one tied to the accel pedal). On the others I either remove the screw or back it way out. This is to ensure that the linkage for the one is actually controlling the other carb(s) and they are in sync. I then use a Uni-Syn to ensure that all the other carbs are flowing the same/similar amount of air as the one with the hard RPM stop screw. I typically turn the RPM screw up enough to get the red-ball at least 1/2" off the bottom. You then adjust the linkage to raise/lower the red-ball on all the other carbs that don't have the hard RPM stop screw. This also ensures that any slop/play in the linkage is accounted for during the sync procedure - as the linkage is "under load" during the sync process. The linkage should have left/right hand threads in the connecting rod (if using the Elco style ball-bearing ends) - so you rotate the rod to change the amount of air on the carbs that are not on the hard stop. I can't see how it hurts to have them in sync - regardless of whether a plenum exists and/or the configuration of the plenum (some have 2 plenums)
Syncing carbs only really becomes an issue on a vehicle with separate plenums or in come cases no plenum at all just runners to the ports. It can be a way of determining overall health. But you really need a baseline to start with to determine if anything needs to be done. Poor flow on one carb compared to another can be indicative of a problem with the carb, or a bad gasket, or in extreme cases a bad lobe on the cam or improper valve lash. Then you have to take into account that engine in question. Some V8s flow better on one end of the intake than the other. Some intakes have quirks too. For example, some 2x4 intakes have a very small equalizer passage between to bigger plenums. In theory if your mill has a better flow to one end then the other you will have a problem making the carbs flow equally. When I am syncing carbs I prefer to have all the carbs hooked up at once. Its a quirk of mine. I want to see what changes when I make changes to the carb in question.
@porknbeaner , thank you for your comprehensive answer. - and one that I can understand ! As I said, I am still learning (and enjoying doing so).
A plenum will help mix up the fuel, and is more forgiving than individual runners if one carb is different to the other. It is not a silver bullet though. There is still "unmixed" gas in parts of the plenum. Give your plenum the best chance it can get, and feed it the same mixture from both carbs. How do you get both carbs giving it the same mixture? Put the same jets, valves and idle tubes in both carbs. I figure you already have that. Then make sure that the jets, valves and idle tubes are given the same amount of "suck" to vacuum up the same amount of fuel. The amount of suck that a carb gets is the difference in vacuum from the air horn to the manifold. If that pressure difference is the same, the two carbs flow the same mixture. That's what the Unisyn measures... pressure difference, not flow. Use the Unisyn to make the pressure difference the same by adjusting the throttle plates, giving more or less pressure drop across them. Cheers, Harv.
On a side note, we've been talking about air-flow (really vacuum) into the carbs - which is about as close as you can get to attempting to get them to flow the same amount of fuel (assuming all the same jets, bleeds, power-valves, dump tubes, etc). But - it is CLOSE - as no two carbs flow the same amount of fuel as one would think. There are many reasons - including manifolds, differences between jets, etc.. This is also true in both mechanical and electronic fuel injection systems. Racers have been having their jets or injectors flowed for years - trying to equalize fuel delivery to every cylinder - so creating a "matched set" of jets/nozzles or injectors is part of the game. Fuel Flow Measurements: When one is setting up an engine on an engine dyno, we have other options (depending on the dyno setup). When I recently built a pretty high horsepower street SBC (stroked 383, 520 HP), we had precision fuel turbines on our fuel lines to measure the amount of fuel the primaries and secondaries were running as we made dyno pulls (modern square bore QuickFuel Holley). The dyno data would give us the precise amount of fuel we were using on both circuits. This enabled us to "tweak" the fuel circuits to ensure that we were flowing the same amount of actual fuel on both ends of the carb. Obviously, one can't do this at home, but I thought I'd post some additional "tuning details" for how one can address the issue when you have a lot of computer equipment (data collection) on the dyno. We ended up making small jet changes to "square up" the fuel delivery/usage as part of the final tuning procedure. Of course, we also made 20 dyno pulls! LOL
Found the link to the article I referenced. It's a good read with pictures https://www.bing.com/ck/a?!&&p=cfc2...XBsZS1zdHJvbWJlcmctOTctY2FyYnVyZXRvcnMv&ntb=1
Wow, the information coming in is worth it's weight in gold- thank you a bunch. Currently, I am rebuilding a carb to replace the one that seemed to not be working. I do not have 'progressive' linkage. Throttle works both carbs the same. I'll be back after the fresh carb is on and I have something to repor.t
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