Carb tuning and synchronisation on a Yamaha F40B

[Pascal]

Hi all,

I have a 2001 Yamaha F40B de 2001 with carbs (3 cylinders, 3 carbs) on a Zodiac Pro II 470.

The engine was running flawlessly at the start of the season. However, after heading out from a beach break, it developed some issues with acceleration.

It still starts effortlessly and runs smoothly at low RPMs (< 2000 RPM), but when I try to throttle up to get the boat planing, it struggles, and the speed increases very slowly until it finally planes. Once on plane, it runs well but with a slight, noticeable ticking sound. If I try to accelerate harder to plane quicker, the engine floods.

I’m planning to check compression, valve clearance, and carb sync at the end of the season.

Can anyone recommend a decent, reasonably priced vacuum gauge for syncing the carbs?
I came across this one: https://www.amazon.co.uk/dp/B07MT4PTWQ, but I’m unsure of its quality.

The engine has only 110 hours on it, but it was winterized for 8 years by a previous owner. The owner before me had similar issues after bringing it out of storage, which were resolved by cleaning the carbs. If I end up having to disassemble the carbs, I’ll put them through an ultrasonic cleaner and do a full rebuild for peace of mind. I’ll need to sync them afterward, hence my question about a reliable vacuum gauge.

Cheers
Pascal

[FoxZulu]

Carbtune Pro. £80-90. Excellent product used by a lot of top tuners. I have one myself (4 cyl.) and very easy to use.

[Pascal]

I'm a bit puzzled by what I read in the workshop manual: they say there should be a 5mmHg difference between carburettors. All synchronisation tutorials I've seen tune all carbs to the same vacuum reading.

Any idea?

[Peter_C]

The quality of the gauges isn't so important as you can easily connect them all to one carb and verify they are roughly the same before beginning tuning. I bought a crappy Chinese set that works just fine as I do not work on boats for a living (Cars are far more profitable in the USA @$175 an hour).

I skimmed that article you posted and my take away is get them all close and you are good to go. They should be as close as possible to each other but don't expect perfection.

DO NOT continue to the run the engine until it is running properly. Internal engine damage can occur from a lean condition. Excessive heat from being lean can destroy the engine. Fix it now.