How to build a carb balancer

[Tig]

I wanted to share a simple little tool I made for balancing the carbs on my three cylinder Honda. I hope you guys find this useful. It's a homemade 3 way manometer. It is extremely sensitive and measures relative vacuum between carbs. It will detect even the slightest carb adjustment.
During use you would want to hang it straighter than in my pic but I had to pose it for the camera. Click on a pic to enlarge.



I connected it to each of the three vacuum test ports and and started the motor up. Initially I could not let the engine run because the carbs were out of balance and quickly sucked fluid up the one line. The fluid is automatic transmission fluid, that way if it gets in the engine there is no harm. I started and stopped the engine several times while I dialed in enough balance in the linkage to allow me to idle with the manometer connected. Once I had it idling I was able to dial the carbs to a very fine balance following the procedure outlined in the manual. The goal is to get the fluid levels balanced in all three tubes and you are done.

Material list for a 3 carbed motor:

20' of 1/4" clear PVC flexible tubing
3 way 1/4" barbed hose connector
yardstick
3 Tie wraps
3 barbed fittings for the carb end
Automatic transmission fluid
Fittings to connect the tubes to the manifolds

How to build:
Refer to the picture for details of arrangement.
Cut the tubing into equal parts and connect one end of each tube to the three way barbed 1/4" hose fitting.
Tie wrap the three tubes to the yard stick so you will be able to compare fluid levels side by side.
Pour a small amount of ATF into one tube until all three tubes are filled 3/4 of the way up the yardstick.
The only tricky part is the connectors for the manifold. If I was more patient I would have threaded 5mm connectors from nylon or brass, but I found rubber plugs were a quick fit. I drilled passages in these plugs with the smallest drill I had so that they would act as restrictors to moderate the pulses from the manifold. This seemed to work well because there were no problems with pulsing levels.
Labeling the tubes would make it easier to keep straight which tube is which carb. Tape or my favorite, numbered wooden clothes pegs, will work for this.

Overall I think it worked very well. Very easy to set up, use and put away. After balancing, the motor starts much better and the idle seems smoother.
I ran out of daylight otherwise I would have liked to check the balance off idle at a moderate RPM just to see if the carb linkage works as it should.

Although I built this, in no way did I invent it. I got the idea by googling "build a manometer" after I discovered that a regular vacuum gauge is ill suited for this task. I found Hugh Kenny's article on building a manometer on a BMW site. Hugh credits Marty Ignazito (a powered-parachute enthusiast) as his inspiration. Thank you Hugh and Marty for sharing.

 

[4JawChuck]

Re: How to build a carb balancer

You can use vacuum gauges but you need to damp the port, I used aquarium needle valves on the four gauge set I built...works very well. If your interested I can post a pic.

 

[Tig]

Re: How to build a carb balancer

Hi Chuck,
Originally I was looking to see how others dampened the line so I could use my gauge. I was thinking restrictors and/or expansion chambers but then I came across this design. I favor it for a few reasons such as:
1) It will never fall out of calibration like multiple gauges will. Just watch out for air bubbles.
2) I cost me about $3 to build.
3) The accuracy exceeds what is required.
4) It is simpler than sharing/switching my one gauge between three carbs. This also makes it more accurate than sharing since it allows me to see all vacuum signals all the same moment in time. I found that there was a small amount of drift in the vacuum signals back and forth if you stand there and watch it work.

I fix my stuff as a hobby and complicated systems for a living. My personal bias is always towards the simple solution.

 

[kenmyfam]

Re: How to build a carb balancer

I use vaccuum gauges also. Very good, simple and cheap method you made.

 

[4JawChuck]

Re: How to build a carb balancer

I originally made a manometer set similar to yours but out of 1/4" ID tubing years ago, it worked well but I got tired of constantly sucking all the fluid out when I would blip the throttle on the motorcycles and snowmobiles I was tuning at the time so I built the vacuum gauge set.

 

[Tig]

Re: How to build a carb balancer

Once I had things roughly balanced the fluid stayed put. I did blip the throttle in between tweaks just to make sure that all carbs were sitting as the linkage would set them during normal use. Being a closed system the vacuum level is relative and things shouldn't change as RPM increases.
I quickly checked off idle balance but I did not have enough daylight to fully explore it. It wasn't off to the point that fluid was sucked out of the tubes but it was noticeably different. Which makes me wonder why that is. Is it that my linkage does not operate all carbs evenly once it gets off idle? Or is it that any slight imbalance at idle gets magnified once the RPMs go up? Or is it just a sign of the flow characteristics of the engine changing unevenly. In a perfect world the sync would hold through the RPM range. I did come across one thread where the poster mentioned that he set his balance at off idle RPM(1800 I think?) I wonder if that is a better idea?
What RPM do you gets set balance at?
How even does it stay as RPMs increase?

 

[JustJason]

Re: How to build a carb balancer

The 1 problem with your setup on some engines the carbs are not set equally, And they specifically want a difference between the top and bottom carbs. You really need to have a manual and understand what the manufactoer wants.

 

[CATransplant]

Re: How to build a carb balancer

Reminds me of working on twin SU carbs on old MGs. There was a really cool manometer tool you could buy to balance those carbs. Being poor and a college student, I listened to the sound of the hiss of the carb at idle to do it. Worked OK.

You know you're over the hill if you still have an SU wrench on your key chain. I haven't adjusted a pair of SUs in over 20 years, but I keep that wrench on my key chain...just in case.

 

[coolguy147]

Re: How to build a carb balancer

what's a carb balancer?

 

[Tig]

Re: How to build a carb balancer

Cat,
I've been away from multiple carbs for a while but back in the day my eyes and ears were also the instruments of choice. I've never tuned a British sports car but I have had the opportunity to admire some of those multiple carb setups. They do look like they would be interesting to tune.
CG,
What I called a carb balancer is probably more correctly called a carb synchronizer. As Jason mentions above the procedure may vary from motor to motor but for mine the Honda shop manual calls for measuring the intake manifold vacuum at test ports while idling. Carb linkage is then adjusted so that the the vacuum is equal on all carbs, within a certain tolerance. Once synchronized/balanced the engine should run smoother.
I'm probably over thinking things when I wonder about monitoring off idle balance but I'm sure I'm not the first to notice this and I'd like to benefit from any experience out there.

 

[CATransplant]

Re: How to build a carb balancer

SU carburetors are an amazing piece of engineering. Made more like a motorcycle carburetor, they work by pulling a tapered needle out of the main jet as the throttle is opened, by pulling a piston up to open the venturi. There's an oil damping system, too to keep the piston from fluttering.

It's really about the simplest carburetor around and works very well when it works well. The wrench is used to adjust the main jet up and down to balance the fuel flow. In most cases, there's also a balancing tube between the carburetors, which also keeps them in synch.

There aren't any carb kits for these carburetors. There aren't hardly any parts in them. Needle and seat in the float chamber, and a couple of gaskets.

As I said, they worked great when they worked great. When they didn't, they were a PITA. I love SUs.

N.B. SU stands for Skinner's Union, after the inventor.

 

[4JawChuck]

Re: How to build a carb balancer

Once I had things roughly balanced the fluid stayed put. I did blip the throttle in between tweaks just to make sure that all carbs were sitting as the linkage would set them during normal use. Being a closed system the vacuum level is relative and things shouldn't change as RPM increases.
I quickly checked off idle balance but I did not have enough daylight to fully explore it. It wasn't off to the point that fluid was sucked out of the tubes but it was noticeably different. Which makes me wonder why that is. Is it that my linkage does not operate all carbs evenly once it gets off idle? Or is it that any slight imbalance at idle gets magnified once the RPMs go up? Or is it just a sign of the flow characteristics of the engine changing unevenly. In a perfect world the sync would hold through the RPM range. I did come across one thread where the poster mentioned that he set his balance at off idle RPM(1800 I think?) I wonder if that is a better idea?
What RPM do you gets set balance at?
How even does it stay as RPMs increase?

I think a lot of confusion exists on how to properly adjust carbs on a multiple carb system, for example my 2003 Merc says to adjust the idle mixture on the top carb different than the other carbs but it starts and runs way better with them equal.

I have had the best performance by making sure all the carbs close equally, this would be the sync part. Lots of ways to do this, on a snowmobile you just stick your finger in the carb throats and feel that they all open at the same time, on an outboard I just make sure they are fully closed and have some one hold the plates in that position while I lock the throttle shaft adjustment.

Next adjust idle mixture for the highest reading on each cylinder, finally adjust the idle speed to the lowest it can go before stalling and adjust the idle mixture again. Adjust ilde speed lower and do the idle mixture adjustment again using the vacuum gauge as the indicator.

Last I do quick throttle snaps on the carbs to make sure it doesn't stumble, some engines like a little more fuel so it may take an extra 1/4 turn out before it runs up properly and doesn't stumble...do this adjustment equally on each carb.

Last, at very low idle I would check the idle vacuum using the gauges to ensure they were equal on each carb, vacuum levels at higher rpms may not be comparable since there are differences in exhaust flow...really all your concerned about is idle mixture and carb balance at idle speed.

 

[Tig]

Re: How to build a carb balancer

Thanks for your thoughts Chuck. I used to dial Mikunis in real low on my bikes and sleds. I like my two strokes to idle on the edge of lumpy. As for the Honda, I do find the 950 RPM idle spec pretty high. Too fast for trolling pickerel and if I approach a dock in gear it feels like a juggernaut. I'm still feeling out this boat so I'm going to leave things at spec for now. I'm just setting up for a prop change.
Cat, I don't think I've ever seen an SU. The TR6 I was thinking of had Strombergs. That dampening reservoir is an interesting idea.

 

[4JawChuck]

Re: How to build a carb balancer

My Merc idles at 700rpm and with the 22" prop feels like its charging the dock at times also, I just got used to popping in and out of gear as required and got good at the reverse left turn technique to pull me into the dock nice and square.

As an interesting comparison I learned to tune carbs with the old "hose to the ear" technique and wondered how close the gauge methods would be to the old timer way. The hose worked just as well with no difference on multiple engines I tried it on, pretty amazing that you can judge idle mixture and sync with a hose and a set of Mark I ears just as closely as a fancy gauge.

Guess thats why I don't use my gauge set anymore, the old ears are always handy and need no complicated hookups.:redface: