Yamaha 90A gets bogged down when driven onto a trailer

[CharlieH]

Hello, I have twin two stroke 90As both fed from the same fuel tank but via separate water filters. Both run absolutely fine at sea. I tried just one at a time and no issues and flat out at 5500rpm fine, so it would suggest plenty of fuel is reaching the engines. The bulbs are hard and all filters appear clear. However, when I come back into harbour and drive my Rib onto my trailer (engines down) the starboard engine has started to get bogged down and dies. It starts straight away but the same thing then happens. The only thing I can think of is as the boat goes onto the trailer it tilts upwards at the bow as the slip is at an angle which may cause fuel starvation but I can think why - or this could just be a red herring. I've been driving onto the trailer this for years with no problems. Any suggestions please?

[99yam40]

do these 90A's have carbs or are they injected motors?
if carbs, drain the bowls to see if you find anything in them that should not be
same goes for the VST if injected
the motor tilting along with the boat may move water around enough to get to jets

[CharlieH]

They are the old carb models - will do as you suggest thanks.

[CharlieH]

drained all three and collected the fuel in a glass jar which was completely clear, no water or other debris?

[zenoahphobic]

It will be to do with the carburettor as the load driving a boat onto the trailer tends to be more than normal takeoff where the boat moves freely increasing speed reducing the load.
What that suggests is that carburettor is running lean; the throttle is opened but extra fuel is limited by the restriction in the engine’s ability to rev up as it would by normal driving.
So carburettor clean is suggested with a check of float level setting (tilted engine will lower the level slightly).

I’m interested how you were able to individually get each engine to rev to 5500rpm. At what max revs do they do in tandem?

[CharlieH]

I guess then the only solution is to remove the carbs and send them to be cleaned - I'm not sure I can do this myself, but there is a company not too far away who say they can do this. Thanks for the advice. I have 17" props and with eight people on board I can cruise at 20Knts at 3600rpm both engines and top speed around 35knts at around 5800rpm (although I don't do this too often). On my own I can get around 17knts on a single engine flat out at around 5500rpm on a calm sea. The boat was designed to cope with one engine, just, but run comfortably with two at medium revs, so low fuel consumption and prolong the life of the engines. I dive in a fairly route area in Cornwall, England with embarrassing consequences to call out the lifeboat, so I need two engines that can give a reasonable speed on their own and to be totally safe.

[CharlieH]

Carbs all cleaned, new valves fitted and all tuned up, went out today and they purr like kittens, thank you for your advice

[brian3006]

good deal. Thanks for the follow-up. Could have been anything from debris to float level / or sticking. Glad you got it running right

[99yam40]

great to hear they run well again.
can you tell us what valves were fitted

[CharlieH]

I didn't do them myself, I had them cleaned by a marine technician who said they also replaced the needle valves. He gave me the old parts which included a bolt with a long point and a small rod with a cone - maybe you guys call these something else? Also various washers, a gasket, float chambers and drain plugs.