Karting1.co.uk

[knighty]

HI folks, does anyone know where I can get a simple adjustable rev-limiter for my GX390 engine, I'm running a briggs & stratton raptor coil, engine is specced for 25-30hp but its only running 32lb single valve springs and I want to protect buzzing the engine bejond about 7,000rpm........I had a skim on ebay the otherday and found something for a 12,000rpm monkey bike, will try to find the link tonight.......surely someone has already done this for a Honda GX or a briggs????.........just noticed dan also sells a nice set of 45lb dual valve springs, but will these fit the GX390???

http://www.gxtuningstoreuk.com/inc/sdetail/126/158

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[karterdan]

Just look at your rev counter down the straights, simple.

Dan,
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GX Tuning Store UK - The Home To High HP Honda's.
www.GXTuningStoreUK.com

[knighty]

its not that simple and thats not a way to limit revs in my experience. I did this at tilbury on saturday and nearly had a big crash as I should have been looking for my braking point. its just not safe for me or the engine!

[karterdan]

What Lbs Springs do you have in there at the moment?

7000Rpm is not a lot of Revs at all, and we have turned over this with the 18lb Valve springs, but ideally you want 26lb Valve Springs in there and you're set for 8200rpm.

Once you get use to it, you can just tell what RPM it is by ear, or just put a smaller rear sprocket on.

Remember that's all GX200.

Thanks,
Dan
_________________
GX Tuning Store UK - The Home To High HP Honda's.
www.GXTuningStoreUK.com

[thunderkart]

Hi Steve,

Unlike most of the gx390 guys in this country, i never rev my engine above 6600! But then i also dont run any american tuning parts!

At the british championship meeting last year there were guys reving their engines up to 9000, with all the fancy bits in from america . . . but i actually set the fastest lap time for a gx390, and in the races i was in was not overtaken by another gx390!

Get the gearing right, and there is no need to rev them so high, and this will also help with your reliability, after all it is only designed to do about 3700revs with 13hp standard!

I dont have any sort of limiter on my engine, i have a rev counter with a max recall after each run, and i gear it so that i never go over 6600, and like i say, i rarely get passed by any other 390's reving higher!

Its not all about how high you can rev like many in the gx karting world believe, its about putting the power down on the track!

Stu.

[knighty]

Thanks Stu & Dan, thanks for the replies.......currently I run a standard size exhaust valve and a 38mm intake valve, the cam is a 360" lift, 260 degree duration AGK item, which is quite a pokey cam associated with high revs on US oval karting tracks (not the ideal cam really!), the valve springs are single 32Lb items, upon recently removing the cylinder head the valves have "just" been touching the piston due to either valve float or valve bounce, but the previous owner was revving well over 7000rpm, I have since properly clearanced the piston to clear the valves via "eyebrowing" the piston with a 30mm dremel and nicely finishing it with some wet & dry, so no nasty grinding stress-raisers in the piston.

currently the kart is geared for bayford meadows a 1100m circuit, but I run on the 665m Tilbury circuit, so I'm not using the full potential of the engine on the main straight, currently I run a 21/72 sprocket which works out at 67mph at 7000rpm.......this is just not working at Tilbury.......my gearing calculation matrix indicates an 80 tooth sprocket will reduce that to 60mph at 7000rpm, which I think is far more appropriate for tilbury, but I wanted to investigate a rev limiter to be extra safe, the conclusion seems to be that they just arent available, anyway my gearing calcs and what you say both imply all should be ok (fingers crossed) so lets give the 80 tooth sprocket a go and see what happens

[thunderkart]

dont forget steve, its not just the sprockets you run that will give you the gearing effect, the rollout circumference of your tyres (determined by the tyre and the rim size your running) will give you a gearing effect too! we play around with this quite alot and it does make a big difference.
and im not sure on the layout of tilbury, but you could also use the depth of the tyre to aid stability . . .
Or even introduce a bit of stagger to the rear of the kart to allow better drive in one direction if its a majorly one direction track! again, i have played with stagger on my kart, and i found about 4 tenths in it on a majorly one direction track . . .

As i said, i never rev my engine above 6600, and i dont struggel for pace (im also coming in at 200kg all up!), but with your cam you need the revs to make it work.
I dont run any fancy valve springs (none of this double valve spring stuff some people say you have to run!), i have a longer exhaust torque tube to aid lower end power and then with the correct gearing you should have the pace your after!

[knighty]

thanks Stu, I forgot to mention, my new moto-schuette S-bend exhaust is now installed and fully operational, I added about another 6 inches of length for correct silencer placement if I'm honest, but this will also help the power come in at a lower rpm, it looks the dogs nuts I must say, the kart generates lots of interest in the predominantly Rotax pit lane............on the last run the engine was properly coming "on-cam" at about 5000rpm, which was never happening with the old exhaust, the trouble was the tilbury straight is so short and the gearing is so wrong that it was only momentarily "on-cam" for half a second or so, hence my desire to sort the gearing pronto, as the kart is bogging down on the slow corners in the rest of the circuit, its a real compromise at the moment........stagger & roll-out.........got you, will give it a try

[TheTRYALL]

i have a Honda gx390 and a clone 420 (gx390 clone) with the Honda internals such as the cam and what not, all that has been done to it is dual valve springs gx390 head on the 420 ( to increase compression) and a arc con rod with a suco clutch plus a standard crankshaft enhanced for lubrication just like this one

http://www.ebay.co.uk/itm/Honda-GX390-Thunder-Kart-Modified-Crankshaft-Enhanced-Oil-Lubrication-Kart-/180888456470?pt=UK_Cars_Parts_Vehicles_Other_Vehicle_Parts_Accessories_ET&hash=item2a1dcacd16#ht_500wt_1303

and i have been running 13-46 with my gx390 and this was the result

http://www.youtube.com/watch?v=cIYk20uXT_g&feature=plcp

it was okay in the corners and fine down the straight but i got a whole new kart basically and my gearing was 13-52 which hit its revs way to fast and i had a standard copy rod at the time and it tor apart and kind of melted at the same time which meant nearly losing the crank hence why i got a arc rod by the way AGK do a rev calculator which could help you and what clutch are you running.

i would like to know how this turns out for you to help me lol and if i find a decent gearing i will pm you. one more link this guys gx390 is fult tuned with a 13-52 and this is his result

http://www.youtube.com/watch?v=VXLx1jFWHIk&feature=channel&list=UL

it probably worked for him better than me because how much tuning was done to that engine
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