What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

balancing carbs

olderthandirt

Well Known Member
If carbs are balanced by adjusting throttle position why can't a person use a Vernier caliper to measure throttle position instead of using vacuum gauges and standing between propeller and wing with running engine...may try it anyway..thoughts...
 
If carbs are balanced by adjusting throttle position why can't a person use a Vernier caliper to measure throttle position instead of using vacuum gauges and standing between propeller and wing with running engine...may try it anyway..thoughts...

That makes the assumption that if the position of the throttle arms are exactly the same, that each carb. will flow exactly the same amount of air and fuel.
That assumption would be false.

This type of procedure has been commonplace for decades with engines that have multiple carburetors (cars, motorcycles, etc.). If there was a better/easier way, someone would have figured it out by now.

BTW, most gauge sets / tools used for sinking the carbs have long enough hoses to bring the tool into the cockpit (you can pass the hoses through one of the fresh air vents or just leave the canopy in the propped open / vent position.
 
Related to old stuff

I don't have a RV12 or a 4 cylinder rotax.
But I did work on motorcycles as a youngster.
the Honda 4's....aka 750-4's started the need for balancing carbs as they had one carb per cylinder and they would not run smoothly without balancing them. Balancing was hooking up a vacuum gauge (think manifold pressure) to each carb and adjusting the throttle linkage to achieve equal vacuum pressure. The gauge takes out or adjusts for small variances in the linkage.

You might also think of the procedure as a poor man's fuel injection balancing system.

If you use to tune up cars with points?? Remember the Dwell meter verses the feeler gauge? Same concept.
 
Last edited:
There is no need to stand behind the spinning prop when performing a carb balance. Here is the way I balance my carbs:

Not wanting to subject myself or any of my friends to standing near a rapidly turning propeller while looking at two carb balancing gauges and wanting to do both the engine operating monitoring and looking at the carb balancing gauges at the same time myself, I devised the following plan:

I positioned the carb balancing gauges inside of the cockpit by adding extensions to the two rubber hoses -- the extensions' were cut so that there is equal length between each gauge and the respective connection elbow on the intake manifold (not that having the lengths the same makes any difference at all). I ran the two extended rubber hoses through the open air vent on the copilot side and positioned the gauges on the copilot seat where they fall "readily to hand" as the British say.

Next, I tied a length of 50# monofilament fishing line to the top of each throttle arm and ran each line aft under the canopy fairing and into the cockpit where I terminated each line with a little "finger pull" loop.

Now I can start up the engine in the safety of the closed (and warm!) cockpit and gently pull on each loop to see what carb needs to be tweaked to balance the gauges. Then I shut down the engine, make the adjustment on the carb linkage, restart the engine and double-check my settings. Repeat, as necessary to achieve vacuum perfection.
 
If carbs are balanced by adjusting throttle position why can't a person use a Vernier caliper to measure throttle position instead of using vacuum gauges and standing between propeller and wing with running engine...may try it anyway..thoughts...

I had a Rotax 912 in a Kitfox and it made a HUGE difference in power and smoothness once I bit the bullet and bought a CarbMate (http://www.lockwood.aero/p-3301-carb-mate-diagnostic-tool.aspx) and finally got them vacuum synchronized correctly. I checked the sync at each conditional and it did change slightly with use.

Woody.
 
Last edited:
Try this - much less expensive. eBay $32.85 including shipping...
2wp5opx.png
 
I can't tell from the picture but the low-cost Honda vacuum guage set appears to lack the two hose needle valves that are necessary to dampen the guage pulses. These valves could be readily added, but that means a bit of additional cost. Otherwise, that looks like a reasonable price for two vacuum guages.
 
Last edited:
I think that eBay gauge set does have needle valves but they are plastic. It looks like a poor quality tool. Chances are one would buy this and find it unsatisfactory and have to buy the quality tool. I would rather spend $90-$100 for the proper gauges with the correct fittings, than waste $35 on that one. Skip the frustrations, you will enjoy working with a nice set of gauges.
 
I saw this idea in the bike forums and it seems to be pretty sensitive: It uses two loops of vinyl tube. The loops are about 6-8" across. One loop of tube merely holds one BB that always moves to the bottom/neutral position by gravity. The other loop is connected to two carbs and contains one or two BBs that are moved off neutral by pressure differential.

If there is a chance you could suck the BBs in the carb, insert something like a tie-wrap in the tube to stop the BB.

I can't post a pic so you will have to imagine it. :-(
 
I tried some of the homebrew setups to balance the carbs and, for whatever reason, was singularly unsuccessful. I ended up with a CarbMate and it works great. It's easy enough that I ended up checking the balance more than I need to just because it's so easy :).

Really does work well. I hook it up and set it in the cockpit. It tells me which side is high and it only took me a couple of tries to figure out how many lights on the CarbMate change when I move the hex adjustment nut one flat. Fire up the engine, let it warm to operating temp, check the number of lights the CarbMate shows out of balance (if any), shut the engine down and adjust the appropriate number of flats. Fire it back up and recheck.
 
Another reasonably priced alternative is to purchase a used manifold pressure gauge from a twin engine aircraft. The can be had for around $50 on EBay. These gauges are usually internally dampened, so you don't need to use external needle valves to keep the needles from bouncing. Add some fittings and tubing and you are good to go. Works great!
Alex
 
Another reasonably priced alternative is to purchase a used manifold pressure gauge from a twin engine aircraft. The can be had for around $50 on EBay. These gauges are usually internally dampened, so you don't need to use external needle valves to keep the needles from bouncing. Add some fittings and tubing and you are good to go. Works great!
Alex

Love ingenuity and thinking out of the box...
 
Back
Top