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Balancing the carbs

Roger Lee

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Pneumatic Carb Balance tips,

You may not perform this, but this gives you some idea on the how.


First your throttle lever in the cockpit should have a mechanical stop on it of some kind so when your throttle lever in the cockpit is all the way back at idle the carb idle stop screw hits it's stop at the same time. This is the most ideal setup. If you can pull the cockpit lever back any more then you stand a very good chance at bending the idle stop plates on the carbs.


You should have the plane wheel chocked along with the parking brake set.

If you have to turn your engine on and off because of its configuration then one person is ok, but 2 still make life easier. If you do your adjustments with it running 2 people are a must for safety. Making adjustments while it is running makes life easier, but of course carries more risk of injury so absolutely take all precautions. I always wear hearing, eye protection (goggles) and never have loose clothing flapping around.


With the Carbmate or Syncromate you can move the throttle lever on the carb while it is running to watch which way the light moves and just take note as to how much before shutting off the engine to make your adjustment. Most adjustments will be small.


The next big thing I have heard recently is syncing carbs down at 2000 - 2500 rpm for the high rpm adjustment. This is too low and too close to idle. Tell your mechanic that you want it done at higher rpms if he normally does it at the low rpms. I never sync a set of carbs less than 3500 and then check them up to 4000 rpm. If you set your carb sync up at 2500 rpm I guarantee that at 3500-4000 rpm they will not be synced. You fly closer to the 3500-4000 rpm and only taxi at 2000-2500 rpm. Your crossover tube can correct up to a certain point, but after that they will not be synced if you allowed them to be too far out from each other. I can usually feel carbs out of balance between 4200-4800 rpm in flight on many planes (not all). Learn your plane and its feel at different rpms. Put your hand on the frame when you fly and listen to it, get a feel for the way it vibrates and moves. Then when things aren't right you will know. As the rpms go higher than 4800 the frequency of the vibration becomes too fine and you may not feel it any more.


Knowing which carb to change can be a challenge for some.

At idle it is easy. If you are shooting for a final idle rpm of 1750-1800 then look at your tach. If you are idling at 2000 rpm then adjust the carb with the lowest number on the gauges because it is getting more fuel and controlling the rpm more. As you bring down this vacuum to the higher number the other will change slightly too. If you are only getting 1650 rpm and want to raise the rpm then adjust the carb with the highest number so it gets more fuel and the gauge number becomes lower.


The higher the vacuum number on one carb means it is getting less fuel than the carb with the lower number.


This is the only bad thing about Carbmate and Syncromate as you can't tell which carb to go after and need to wiggle the carb arms to see which one you want to adjust. The ideal electronic balancer would have a set of gauges in line so you could look at the gauge to see which carb to adjust then use the electronic balancer lights to make it perfect.


As far as adjusting the high rpm for sync and knowing which carb to adjust, I usually will take a look at the bowden cable screws and see how much adjustment each has left. I try to adjust the lower number carb getting more fuel to the higher number carb getting less fuel. Sometimes there is not enough adjustment left to pull the carb arm back any farther on the one getting more fuel. So you have to make a decision here. If you only need a little adjustment I may go ahead and adjust the higher vacuum carb to the lower vacuum carb if it is only a slight move. If it needs a fair amount then I will adjust the length of the cable on the one that is pulled back the farthest (getting more fuel, less vacuum) shortening the cable by 1/16th".


As far as hooking up your gauge or Carbmate lines there are a couple different ways. For me the easiest and quickest is to pull off the left side crossover tube on top of the manifold. I put one, smooth, end male fitting in the rubber tube and I have a female tube that slips over the fitting on top of the manifold. I do have a screw clamp to snug down on each to prevent any air leaks. No clamping lines, taking out screws, just no muss or fuss and the carbs are now separated. Carb balance on the normal plane takes me about 15-25 min. I have had a couple up around 45 minutes and 1-2 that took an hour+, but that was a long time ago. Learning when to quit messing with the carbs and make a cable adjustment right up front can save you 30-40 minutes of time.


The first few carb syncs take a little time and has a learning curve, but after a few they get easier. Carbs will be out of balance by every 100 hour inspection. Guaranteed. Cables relax, stretch, pulleys wear, parts in the carb wear, ect...


BALANCE those carbs and don't let anyone talk you out of it and tell you it isn't needed. That's as they say is "Hogwash". Think of your engine as a right engine (cyl 1&3) and a left engine (cyl. 2&4). If one side is getting a little more fuel than the other it may be trying to run at 5100 while the other side is trying to run at 5000 (just an example). The cross over can only do so much. Do it right and sync those carbs. It's only a $20K engine so why screw it up or shorten its life.


I hope this helps a few.


If you have any questions you can call me.



You should always check the carb bowls for leaks and fuel dribble. If you have fuel stains on the sides of the bowls or fuel stains in the drip trays get those gaskets replaced. They are about $12 each. It only takes a few minutes. If you have carbs getting up in age i.e. 5+ years or a lot of hours then think about getting them rebuilt. They do wear internally and "O" rings do split. If you have no one to rebuild them for you then you can send them to me. I charge $60 plus parts. I get them from all over the country and have a one day turn around.

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