Honda 50 hp carburetor synchronization

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opie
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Honda 50 hp carburetor synchronization

Post by opie »

In the following closed topic, Luke makes a good point about Honda 50hp engines being temperamental. I agree. I also have posted here about the especially frequent need to clean the carburetor jets.
https://macgregorsailors.com/forum ... ental!

In 21 years of ownership of my 1999 26X I have probably cleaned the carbs 10 times, replacing gaskets regularly but the jets only a few times.
I always had success immediately, but this season there is a feeling that things are not quite right.
There are at least a dozen things to check when the old Honda ain’t right and I did all those. I want to adjust the carburetor synchronization for the first time. And I am too cheap to buy a three gage vacuum set. So, in the next week or so I will try two methods that I will mention here. I will post results later.

Method 1. I will remove the carb intake shroud and start the engine and after warmup I will place a 1/4” piece of tubing at my ear and listen to the intake whoosh sound at each of the three carb intakes and then adjust number 2 and 3 to match number 1. I learned this procedure in my classic British car club in tuning SU carbs.

Method 2. Having in my shop a single accurate vacuum gage and lots of various 1/4” fittings and tubing, I have come up with a plan that uses the single gage connected to all three carb test ports and utilize the easy crimping of the soft tubing to look at a single carb at a time. (Pic at bottom)

Comments welcome. Results in a week or so depending on my schedule.

Ps- yes, the male “NF” die threads I used do not perfectly match the female metric threads in the test ports, but close enough, a tiny bit loose, so I plan on a bit of teflon tape on the threads.

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Re: Honda 50 hp carburetor synchronization

Post by Be Free »

My experience over the years syncing the carbs on a Honda outboard, Honda motorcycle, and BMW motorcycle would cause me to question your plans. I've found every multiple carb engine I've ever worked on to be touchy and difficult to synchronize even when using the correct tools. Just saying.
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Re: Honda 50 hp carburetor synchronization

Post by kmclemore »

I’ve successfully used one of these for literally decades, on boats and cars. Works perfectly to synchronize the airflow of multiple carburetors. Then I use ColourTune plugs to sort out the mixture balance. Job done.


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Re: Honda 50 hp carburetor synchronization

Post by Be Free »

Kevin, this is the type of synchronizing tool I know how to use.

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How does the one you use work?
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Re: Honda 50 hp carburetor synchronization

Post by kmclemore »

Be Free wrote: Fri May 30, 2025 6:19 pm How does the one you use work?
Place the flat flange against the opening to the carburetor. Adjust the sight glass to vertical. Start the engine, and adjust the flow regulating disc inside the unit such that the bead in the sight glass is mid-way in the tube. Then, without changing anything, move the unit to the next carb and check the sight glass bead… if it’s higher than the other carb, this carb has higher airflow - adjust that carb to bring the bead to the same position as the last carb. Then go back and check the first carb to ensure nothing has changed - if so, adjust that carb to suit. Continue until all carbs are showing the same lift of the bead.
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Re: Honda 50 hp carburetor synchronization

Post by Be Free »

Got it. It's measuring air flow rather than manifold vacuum (which are obviously directly correlated).

On my BMW which has two cylinders and two independent carburetors I can see where that would make the balancing much easier.

On an engine where the multiple carbs share a common intake manifold (like the Honda outboards) having an input for multiple manometers (or gauges) lets you see how the adjustment of one carb affects all of the other carbs in real-time. It is quite literally a balancing act. It made a major difference on my old Honda outboard when the everything was in sync but I can't say I miss doing it on the new fuel injected version. :wink:
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Re: Honda 50 hp carburetor synchronization

Post by opie »

Tried my methods tofday, but failed. Details when I figure out what's wrong.

In the meantime, I found this Honda manual on carburetors.

https ://www. virtualware. nl / wp-content / uploads / Honda-carburation-manual.pdf
https://www.virtualware.nl/wp-content/ ... ual.pdf

I have not read it yet. Does anyone know if you have to pinch them closed or disconnect the vacuum equalizer connection tubes that connect all three carbs together before synch?
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Re: Honda 50 hp carburetor synchronization

Post by Be Free »

All of the carbs are connected to the gauges at the same time. The bottom carb is the reference; the other two are adjusted to match it.

You will notice that as you change one carb the others will often change as well. It will be a back and forth adjustment until everything is reading the same or at least very close to the same.

It is also very important that you bring the idle speed back to the same (800 IIRC) setting before making a new adjustment. Bump the linkage to rev up the engine and let it settle back between adjustments as well.
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Re: Honda 50 hp carburetor synchronization

Post by adudinsk »

When I had a 4 cylinder, I bought a set of gauges (4 in the set) and it took only minutes.
Commonly used for Motorcycles...
About $65 Canadian pesos... for that price, if you sync even once a year.. its WELL WORTH IT!

This year I repowered though.. sick of carb tweaking/cleaning/adjusting...

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Re: Honda 50 hp carburetor synchronization

Post by opie »

Thanks dude,
Thank you for your advice that I will follow. I found what I think is the same synch tool for $29 American pesos. I will order it.
My failures mentioned above were:

- the sound-by-ear-thru-tubing method failed due to the noise of the engine drowned out the “hiss” sound. (For some reason, that sound method works on my old sports car.)

- My home-made sync tool using one gage was cumbersome…….

More results after I follow everyones’ great advice.
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Re: Honda 50 hp carburetor synchronization

Post by adudinsk »

One nice thing about having a easy to use set of gauges is that you can check sync often and quickly, this can indicate if carbs are gumming up... wearing.. etc..

If possible.. sync while under load .. cruising... at your most common RPM. So if you motor often at 2500rpm.. hook it all up.. take it out.. get it warmed up.. hit 2500.. sync then..

This will dial in best fuel/performance.
If you do it at idle or in neutral at RPM.. not bad at all.. but... if your a perfectionist.. do it under load at RPM.


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Re: Honda 50 hp carburetor synchronization

Post by opie »

AD,
Thanks. Will do.
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Re: Honda 50 hp carburetor synchronization

Post by opie »

I learned alot in this topic. When I used my new synch tool this morning, the three carbs were perfectly synched….. however, the vacuum was 7” across the board instead of 16, 20 or more inches.
Horrible. So, my last compression test was last spring. Not great readings but ok. Then the inactivity was severe. I used the Mac maybe twice, then put it in the storage lot without any winterizing. So no fogging of the cylinders. The local Honda mechanic thinks water got into the bottom of the cylinders and rusted out the rings in all cylinders. My compression readings this morning were around 14 psi. I even put the compression gage on my car to make sure it wasn’t broken, but it was true.
So, I will research the repowering process by reading the archives. The mechanic says a 26 year old Honda is not a candidate for reboring as something else may soon break…..
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Re: Honda 50 hp carburetor synchronization

Post by adudinsk »

Sorry to hear...
Are you just going to replace rings, or are the cylinder walls scored? (ie:repower)


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Re: Honda 50 hp carburetor synchronization

Post by Be Free »

That is really bad news. I feel so sorry for you. :(

I'm with your mechanic. Unless you do the work yourself A BF50A is not worth what it would cost to do a top-end rebuild.

On the plus side, anything you replace it with will probably be fuel injected so you won't have carbs to sync. On the down side, you have a new sync tool and nothing that needs synchronizing.
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