Jetting the carbies

[merseybluenose]

Need help, am looking at a Concorde Sealine 99 model with an oil injected 30hp Yammy 2 stroke (06 model, 35hrs, elec trim and tilt).

Add didn't mention motor size and I'm ideally after a 40hp, spoke to bloke and he reckons you can re-jet the carbie on this model to make it a 40hp although he couldn't tell me anymore about it i.e. someone who had done this before, cost etc.... has anyone had experience with this???

[Stuart]

Uping the jet size doesnt always mean power. In fact it could very well flood the donk. 10hp increase is a 2 stroke is a huge jump, I could be wrong but I doubt you can go up and add 10hp.

[dreemon]

I had a 40hp mariner made by yamaha that I did some mods on, bigger jets, dremiled the rough castings on the intake, fit boyesen reeds and added intake breathers to the cowl, ( the cowl is calibrated for air flow to each engine hp)

in the end it was a hell of alot of work/time/money just to go 5k's faster at wot, ran great ! ! ! but an outboard is very well designed and makes the best power stock from the factory, along with it being more reliable by just leaving it alone.

it was on a 4.1 brooker tinnie and got 64 kph @ max fuel drinking rpms ,

[TheSaint]

In the Yamaha's (most motors too) the 30hp & 40hp are different block & you can't turn 30hp into 40hp.

On the another hand if you have 40hp you can turn it into 50hp as their share the same block. On the yamaha I beleive you would need to get the carbies rejetted, reed valves changed, ports changed, exhaust changed & need a new prop & this would cost about $1800 & would work out cheaper to buy a 50hp.

If you have Tohatsu (Mercury Lighting XR) 40hp you can tune it up to 50hp by rejetting the carbies, prop change & dyno tune will get you up to 50hp & willl cost about $1000 to do so & I know of speeds at WOT from this model doing 65kph in tiller steer & reduced to 63kph when turned into a side console on a Stacer 4m bass elite.

[merseybluenose]

Cheers boys - sounding like too much work for not enough improvement!

[centrefire]

Yeah, can't be done. The 40 and 50hp Yamahas are 700cc and the 30hp is only 500cc.

[BM]

To increase hp you need to increase the fuel charge. Petrol AND air.

So increasing jet size without increasing airflow means an engine running rich which will inevitably run poorly.

If within the same engine block group you want to increase hp you will normally need to replace the carby/ies with larger units and often the exhaust tuner requires replacing also.

In some models of engines (usually low hp) they have a restrictive plate on the carby. These models will run a jet to suit the restricted airflow. Changing jets and removing the restriction will usually up the hp on those engines. They are simple because the engine is really a higher hp but has been detuned. Ie: 15hp taken down to a 9.9hp.

Always keep in mind that there is a reason for the cost differential between hp's. If it was $10 to get from a 40 to a 50 then as a new motor purchase they couldn't justify a large price diff.

[BM]

And as has been mentioned, outboards are typically very highly tuned for maximum reliable performance from the factory. Fiddling doesn't usually achieve much more hp but does affect reliability.

[merseybluenose]

Thanks all

[Gilli]

I looked into doing the same thing mate. I priced the 40hp tohatsu then compared it to the 50hp tohy. wasnt worth re-jetting, just got the 50. too many dramas plus not covered by new motor warranty and what not. Although i got my 50 on a 4 meter tinny and havent got the balls to go in it by myself... far too quick.