Yamaha 40veo 6h4

[GAVMAC321]

Can anyone tell me if i can change the carbs on this engine ? Maybe to 50hp carbs ,?

[spartacus]

Originally Posted by GAVMAC321

Can anyone tell me if i can change the carbs on this engine ? Maybe to 50hp carbs ,?

Been covered a few times before, 40V shares same block as 50H (693cc). Don’t make same mistake as me thinking you can source 50G. You need to source carbs and exhaust manifold. Going by used prices, then it’s not going to be cheap. Carbs don’t usually come up that often. New they are £250 each, and you need three of them. Actually exhaust manifold not too bad, even new from Yamaha. To replace exhaust manifold will involve removing engine block plus new gaskets, etc. Can pretty much guarantee sheared bolts too. If I was buying used carbs, then you’d need to strip, ultrasonically clean, etc.

50G is completely different, detuned and shares same block as 70hp (850cc) and uses different gearbox.

Might be easier just buying the 50hp? Couple on EBay, including a 1999 autolube, electric start with autolube but no power trim for £1500.

[GAVMAC321]

My problem is that the middle cylinder isn't fitting correctly. There's good compression on all 3 cylinders. There's a spark to all the cylinders.

Then engine starts on the button but runs lumpy. Unplugging the coil packs one at a time, the middle one makes no difference to the running of the engine.

New plugs. Swapped coils around, no change. Removed carbs. Cleaned them ( they weren't dirty at all ).

I thought if i could get another used carb i would try it.

The middle carb had the emergency fuel start lever and the enrichment solenoid.

[beamishken]

A common issue with those is the trigger coils under the flywheel iir correct they run 1 coil for 2 cylinders & 1 coil for the other one some run a separate trigger for each cylinder the triggers are all the same, maybe slightly different lengths of wire but all do the same thing
Cant get at them without pulling the flywheel unfortunately. From the sound of your issue id definitely investigate the trigger coils

[GAVMAC321]

Any pointers on how i can test the trigger coils ?

[Roflhat]

The pickup coils under the flywheel can cause this, had it on my Yamaha 70ces. Sometimes they’re fine when cold but fail when they heat up. Can test the resistance in them then heat up gently with a hairdryer and check again. Not sure on 50hp but on the 70hp it’s meant to be around 130 ohms

[beamishken]

I cant remember the readings but the coils are very similar if not identical on a 70 so roflhat's readings probably close. In any case there will be 2 or possibly 3 so if there is a big difference in the readings you know there is a problem. Swapping them around works & if the fault follows the trigger you know its faulty, downside is you have to pull the flywheel to get at them.