lawn mower gx390, needs more power

[chuff]

First off I apologize if I sound really stupid, i'm new to all this, and this may not be the proper place as it's not a minibike but looks like some of you have experience with the 390.

What I have is a stand up rider mower that had a 14hp k series kohler engine, this thing is pretty old but has always worked good and is handy as heck for some residential lawns we mow. It worked well until recently the butterfly fell apart in the carb and it reved to it's demise.

Anyway I have a gx390 on it now great engine, runs smooth but it would be really nice if it had just a few more horses for thick grass. I removed the spark arrestor in the muffler right away as I've never had any luck with em anyway; so here I sit on a $900 motor I wish had more.

The thing is it's operated by my employees so it has to remain dead reliable, start easily (has electric start) be quiet yet, remain at normal governed speed, run on regular gas, and last as long as possible.

I've read about advancing the timing with a special key, is it possible to expect any gains by doing that while everything else remains stock? Any other tips or tricks to squeeze a little extra out of it? I really appreciate any help or advise.

 

[MB165]

Since your running at 3600rpm, I dont recommend advancing the timinig, your not going to get more power doing it. Plus, its gonna run really hot especially in thick grass. Start by trying to get rid of the airbox and install a K&N type filter. If the deck is capped off for mulching try opening it up. On this application, theres not much you can do.
Pete

 

[texasfabguy]

bore the carb and rejet,advance the timing 4 deg. and mill the head. You can also get a low end cam. But really you need a bigger motor for longevity and more power.
Look at how the do it for the boats and water cannons.

 

[novaracer]

bore the carb and rejet,advance the timing 4 deg. and mill the head. You can also get a low end cam. But really you need a bigger motor for longevity and more power.
Look at how the do it for the boats and water cannons.

i would do the same but i would port the head. not relly making the port bigger bit i would do bowl and short side radious work. :thumbsup:

 

[The Restore Kid]

Your best bet is just tell them "It is what it is" And just take it easy on it.. When ya hit deep heavy grass.. Just pull back on the variable speed, hydrostat shifter, drop a gear or two.. To slow the forward speed of the mower and let it cut... :shrug: You don't have to be doing 12 miles per hour all the time...

Although time is money on a lawn crew.. So if it is absolutely imperitive to keep it rollig 7th gear wide open...

Buy a bigger engine, or another machine, and just keep it for a back up... For 900 dollars you could probably have the Kohler rebuilt.. They are expensive WITHOUT a blowup... But it can be done... There is always used on Craigslit stuff too.. You may be able to find a horizontal shaft, horizontally opposed 18 horse twin Briggs and Stratton for a few hundred bucks, and bolt that beast on there, and torque the blades right off the mower..

 

[chuff]

Thanks for the tips, the awkward place the engine has to mount made any v-twin impossible and this is about the largest single cylinder out there, could have gone igx440 but that thing looks way to complicated and expensive. The tank needed to be removed to fit in there which isn't a problem as it has a remote tank.

It's not any worse than the old Kohler was and it may not have been pumping out all 14hp anyway. I'm just wishing the 390 had a little more as i've put this much into already anyway.

I'm not afraid to throw a couple hundred bucks at it, I only need a few more horse maybe 4 to help it out. I've found a few bored out carbs for around $90, would that coupled with a high flow air filter help you guys think?

Being as we use it on only a few lawns buying a new one would be really nice but a little impractical for me especially with the current economic climate.

Thanks again though!!!

 

[The Restore Kid]

I think it is already fixed... :shrug: I would just use it as is.. You aren't going to notice any huge improvement anyways.. It's not like bolting on a Supergharger or stroking the crank an extra 2 inches or boring it out 3 inches wider some how.. :laugh:

If you really wanted to notice a difference in really dumping out continuous massive torque, you're talking building a $2,000 racing motor and defeating your purpose to begin with.. :doah:

It's a lawn mower.. It goes 4.5... If ya have to back off to 4.4 in the heavy grass for a minute.. Oh well... Thats cutting grass.. :laugh:

 

[chuff]

lol, I understand what you're saying... we're not trying to win races or anything and quite possibly it is what it is, I was just hoping there was something half way simple to get a little more out of er.

 

[tlizard2]

As noted above you can get more out of it. You just need to be conservative to keep it reliable. A slight boost in ignition timing, bump up the compression a little, clean up the ports in the head, a good intake and rig up a header with a large muffler so it stays quiet but can breath. You could maybe even boost the max rpm up a little, say from 3600 to 4000. Most of these engines make more power at rpms above the factory rated rpm. I'll bet you'll find that extra 4 hp easily. And change the oil regularly!

 

[chuff]

Thanks tlizard, i'm gonna mess with it a little, see what happens. I'm goona bump up the governor a little, so I got a little tach for it. I ordered a bored carb few different jets, high flow air cleaner and some header pipe bends with flange and universal muffler. I need to make my own header to get around frame and such. Is there a certain total length of exhaust I should shoot for, or is somewhere under 2ft and over 1ft good enough, ha?

I'm having a little trouble finding a 4 degree advance timing key, or is this something I need to file myself?

Also in anyone's best opinion would you just port the head or port and mill it a little? Anyone willing to do such for a small fee for me? Would better valve springs be recommended?

Thanks a ton

 

[jrzmac]

affordablegokarts: Advanced Timing Key