A little initial help for a noob

[jdkno]

I getting ready to venture into building a kart. I have all of the equipment needed to fabricate the frame, I can weld, and I have easy access to a spray booth and have painted cars in the past so I think I have my bases covered on getting it built.

I am building this cart around a Honda GX390 motor, which will probably stay stock until after the cart is built and running then I can add to the motor over time if I feel it needs it.

My plans right now are to run a live axle and TAV on it. Most of the riding will take place in loose mud and grass, might go on the street every now and then but not alot.

With the TAV running on a GX390 and say some 14" tires what do you guys think I could get out of it top speed?

How many RPM's can the GX390 turn?

Lastly what is a jackshaft used for and would I need one??

 

[mike75925]

honda has all those specs available on their page.
http://www.honda-engines.com/engines/gx390.htm
jackshaft can be used if your sprocket is on the other side of the motor (from the clutch unit), or it can be used to get a different ratio (more torque/acceleration). normally for the first thing listed. your tav pulleys will dictate how fast it will let you go (besides engine power, final drive ratio, tire size, weight of vehicle with driver etc). tension on the driven, tension on the separtaors/weights, diameter of pulley(s). the larger tav units can give you up to a .78:1 ratio. which means overdrive.

 

[r_chez_08]

stock rpm is about 3600 for these motors

 

[newrider3]

You should use a Comet 40 series if you are running a GX390. With a TAV2 you're going to burn up belts, and have crappy power transfer with that much HP. The jackshaft is like a secondary axle, mounted behind/under the engine and connected to the rear axle (final drive sprocket) with chain. That is where you would mount the driven clutch of the 40 Series TC.

 

[jdkno]

I had gotten the specs off of Honda's website but I figured Honda is going to tell you what they are designed to run, not what they acctually can do.

Theoreticly I don't need a jackshaft right? Couldn't I mount the drive part of the TAV to the engines output shaft and then the driven part directly to the live axle??


If I want to keep the Honda at say 3800rpm max how would I limit that?

 

[theo]

You have to have a jack shaft, to get the gearing. If you want the honda to spin a 3800 leave the governor on.

 

[t0x1k]

Theoreticly I don't need a jackshaft right? Couldn't I mount the drive part of the TAV to the engines output shaft and then the driven part directly to the live axle??

I you want to accelerate at a snails pace and reach over 200mph.

 

[jdkno]

You have to have a jack shaft, to get the gearing. If you want the honda to spin a 3800 leave the governor on.

There in lies the point of my original question. If Honda says its good up to 3600, but people take the governor off then what are they spinning them up to without grenading them?

If I took the governor off how would one keep the motor from spinning to 5 or 6k rpm?

I you want to accelerate at a snails pace and reach over 200mph.

Jackshafts it is, while 200mph would be cool I don't think I would like the time it would take to get there haha.

Sorry if these are really elementary questions. I have tried searching for the info but it seems that everything I come across is mostly people who already know talking about it not really giving descriptions of it all.

So the carts powertrain would consist of the motor which drives the jackshaft, which drives the TAV, that then turns the live axle. Is that correct?

 

[theo]

CVT to the jack shaft then to the axle. Like this
http://www.mts.net/~hpokrant/Piranha_Buggy/Jack_Shaft.htm

 

[modelengineer]

If you did direct drive, I don't think it would even move. Certainly wouldn't reach 200mph, or even 80mph.

 

[jdkno]

Thanks guys your input is really appreciated.

theo that link was great really helped me visualize and realize what the jackshaft is doing.

 

[NJOHC]

There in lies the point of my original question. If Honda says its good up to 3600, but people take the governor off then what are they spinning them up to without grenading them?

If I took the governor off how would one keep the motor from spinning to 5 or 6k rpm?

Go out to autozone or some place and pick up a little box of springs that resemble your current governor spring. Try them one at a time along with your current spring and see how high it lets it rev. A stiffer spring just delays when the gov. kicks in. I did the same thing with my Subaru and only let it rev up to 4500. You basicaly have the same problem I did. I didnt trust myself enough to back off when I needed to. And Id rather just floor it knowing that it's spinning as high as it safely can without me having to regulate that myself.

 

[Kenny_McCormic]

With heavy valve springs, a billet rod and billet flywheeel, you can(safely) rev as high has the carb will flow.