Velosity stacks horns mercury

[Davidpkl]

I am thinking on adding a set of velosity stacks on my mercury 60hp 3cyl. And on my carb cover it says not to run motor over 3,000 rpm with cover removed or engine damage will accrue. Why is this and what can I do?

[omcforever]

Don't know about Mercs but on my 140 crossflow Johnson, I had to jet up 3 sizes after removing front cover and installing stacks. Leaned it out badly which is very bad and can kill an outboard very quick.

[Slimm]

Removing the air box requires increase jetting so you don't melt a piston. I learned the hard way, when you remove that box and/or make any modification that allows more air in then you need to get more fuel in there also and don't waste time beating around the bush.




Slimm

[Davidpkl]

That is what I was thinking but had no idea it would lean it out that much on a mercury. I have taken off air boxes on lots of yamaha and suzuki a and never had a issue with lean conditions. Say I was just going to remove the air box how many sizes should I go up in jets.

[powerabout]

That is what I was thinking but had no idea it would lean it out that much on a mercury. I have taken off air boxes on lots of yamaha and suzuki a and never had a issue with lean conditions. Say I was just going to remove the air box how many sizes should I go up in jets.

How did you measure "never had a problem"
EGT?

[Davidpkl]

How did you measure "never had a problem"
EGT?

If you are asking me how I measured it on yamaha and suzuki motors i removed air box on. I did not mesure I hav just put lots of houres on them and never had one blow up or any issue with them.

[omcforever]

May take awhile for lean damage to occur. 1 tank of very bad octane gas or loading up engine could do it quicker. I used the "full throttle pulls" to evaluate my mixture. Plugs showed up very whiteish/light tan after air cover removed. After jetting, looked nornal tan/brownish....lot of variables like motor wear and condition/motor type/compression pressures/prop/load/fuel can dictate how long a lean motor lasts. Usually when you change something on an outboard, you have to compensate with something else(more airflow=more fuel). Lean in an outboard is bad. Some people do it and get away with it. Eventually making mods without knowledge or what needs to be adjusted/compensated for it will bite you and that bite is usually expensive. I know, I have melted down my fair share of outboards the past 20 years trying tricks and mods,,,Good luck..

[Slimm]

That is what I was thinking but had no idea it would lean it out that much on a mercury. I have taken off air boxes on lots of yamaha and suzuki a and never had a issue with lean conditions. Say I was just going to remove the air box how many sizes should I go up in jets.

Keep in mind that I didn't just remove the air box and this suddenly happened. I removed the air box, cut the vent slots on the removeable upper rear cover of the cowling so they were functional, then I cut out the air channels in the engine cover in hopes of removing any restriction of the air coming in to the inside of the cover. After that, I got the idea of cutting the front of the cover out and adding the front cover that comes on the HiPo engines. This allowed a LOT of fresh air right in there to the carburetors.

I did run it several times after doing these things but I did not run it hard, just idled out the no wake and throttled up for 30 seconds to my fishing hole then fished all day. Never ran it hard at all until the day the piston melted. Even then I wasn't real hard on it but I did stretch it out a bit more than hopping to a fishing hole.

Here are some pictures of all the things I changed that culminated in the melted piston. I'm sorry if I confused you, I have a lot going on upstairs right now and sometimes not the most clear with my explanations/comments.







Slimm

[FMP]

Look on the bright side, at least you know now the work you did to improve air flow to the engine was not in vain.

[Slimm]

Look on the bright side, at least you know now the work you did to improve air flow to the engine was not in vain.

Oh there's no doubt that what I did works, and it works wonders too by the looks of it.

Slimm

[ChrisCarsonMarine]

I have a somewhat different outlook on the airbox issue.Like the reed debate,as to will cut cages make an improvement in performance...the answer is...sometimes.Yes if the reeds are the restrictive agent in the intake\exhaust tract,no if they aren't.
Back on subject.Some motors have restrictive airboxes,some not so much...and remember what flows fine at stock rpm's may become restrictive at higher rpm's,just like reeds.
Now,more importantly...realize that by design some motors experiance much more reflux through the reeds than others...case in point,2.5 merc ...small amount of reflux,looper omc...lots of reflux.
Air is pushed into the carb or intake by ambient air pressure,trying to satisfy the partial vacumn in the crankcase caused by the piston moving away from the crankcase...increasing the area in the crankcase.This air is mixed with fuel and is then compressed in the crankcase as the piston moves toward it.The reeds are supposed to keep it in the crankcase...but some escapes...reflux...much more in some motor designs than others.Lighter,stiffer reeds reduce the reflux(escaping air),but do not trap it entirely.
Now to the point.When air escapes through the reeds it carries some fuel with it...normally trapped in the airbox and rebreathed by the next cylinder drawing it's air...and getting a little fuel with it.Because of this perpetual addition of fuel in the air in the airbox the carbs or injection is adjusted accordingly...so the cylinders receive the correct mixture.
Remove the airbox and the cloud of fuel\air mix is lost to the next cylinders intake gulp...resulting in a leaner mixture getting into the cylinders,how much leaner depends on motor design and reed performance.Chris

[eli]

don't have any problem turning 7500+ with stock air box 245 mercs so dont see a need to remove them

[Dave Strong]

My 245hp had small velocity stacks molded into the air box.

Dave

[Mark75H]

If air is escaping reeds, they are not working. Back in the 70's we converted a number of 2 strokes from piston port to reed intake. The jetting change was almost the maximum the carbs could do because the reeds only let the air pass thru the carb once. Piston port motors let the air back out, but reeds do not.

[powerabout]

Air and more importantly fuel definitely comes back out of reed valve outboards they all do it.
Just run one on a dyno with the airbox removed