Improving throttle response

[Toycrusher]

Looking for "snappier" throttle response on a mini-monster truck that's really just used for mud, and (as usual) don't want to spend a lot of money

It had a 5.5 briggs OHV, 30-series Tav2, 12t on the Tav, 36T jackshaft in, 12t jackshaft out, 54t axle. Roughly calculated 2.68:1 converter x 3:1 jackshaft x 4.5:1 axle = 36:1 low, and 12:1 high.

It's spinning 22" or 23" tires so the ratio seemed adequate but I had some trouble on inclines if I tried to start from a dead stop at the bottom without momentum. It could spin the tires pretty good in the mud but the lag between hitting the gas and the wheels breaking into a spin made timing turns a bit of a guessing game.

My goal is for the wheels to spin on demand for better control of muddy "drifts" around corners. Since speed is not safe in this thing, and acceleration is what I'm looking for, I targeted gearing first. I swapped the 35t jackshaft input for a 60t sprocket, bringing my gearing to 60:1 low, and 20:1 high.

I was secretly hoping that nearly doubling the already low gearing might let me do a wheelie of some sort and have that burnout inducing acceleration, but what I found is that it really doesn't accelerate much better than before and as expected, speed topped out at about 15 mph.

On an incline, it does walk up it now without drama (or belt smoke), but I'm disappointed that it doesn't accelerate any harder than before.

Is it because...

Governor is intact. Is it the delay in the governor opening the throttle?

Do I need a carb with a fuel accelerator pump?

Can I make changes to the 30-series converter for more violent launches? Is the converter not happy with the lower gearing?

I can't imagine the 5.5 hp is just too small for the task when it's geared this low!

 

[B.M.800]

Is there any slop in the throttle cable? Might be worth looking into?

 

[Poboy kartman]

DADGUMMIT! !!! I really hate to say this....because I'm a big fan of B&S ....but...for $100.... right off the bat...you get an extra horse....remove the governor...and you'll get a lil more....crap....get a Predator....( dang that hurt)....

 

[Toycrusher]

If I changed engines I would go with electric start Lifan cause it's a bear to get in and out of this thing, especially if it stalls.

 

[Toycrusher]

Is there any slop in the throttle cable? Might be worth looking into?

No slop in the throttle. It performs well, accelerates smoothly, I just want more...

 

[2SlickNick]

Is that Briggs an Intek? Because they can fit some of the Briggs animals internals... A slide carb always gives better response than these industrial carbs.

 

[Toycrusher]

Is that Briggs an Intek? Because they can fit some of the Briggs animals internals... A slide carb always gives better response than these industrial carbs.

It is an Intek. I have a little 110 cc in a chinese quad that I don't use much (well, I don't FIT on it...). I had just put a new slide carb on it, maybe I'll "borrow" it.

 

[Poboy kartman]

OH WAIT!!!!! You just need more jackshafts! !!! Sorry....I know it's getting old....be he asked for it!!!!!

 

[Toycrusher]

OH WAIT!!!!! You just need more jackshafts! !!!

I KNEW there had to be a simple answer!

 

[minibikegi]

add 3 jack shafts and you will be at original starting point, what ya need is an oak board for engine flex torque free rev no load value but down hillclimbs? cant use pump station gas, gotta get fuel from airport. prolly need a black necker-chief to be around the fumes. write us how far ya gotten?

---------- Post added at 01:37 AM ---------- Previous post was at 01:18 AM ----------

Sorry, but reminds me of growing up in a trailer park in my youth, My dad kept telling the neighbor kid we had a pet horse that lived under the trailer,,,That kid would always ask how that horse was doing? theres always them ppl that Don't get-it?

 

[chancer]

add 3 jack shafts and you will be at original starting point, what ya need is an oak board for engine flex torque free rev no load value but down hillclimbs? cant use pump station gas, gotta get fuel from airport. prolly need a black necker-chief to be around the fumes. write us how far ya gotten
Don't get-it?

What are you talking about?

 

[itsid]

Since you ruled the cable out..
Main thing left is inertia to be the cause for bad throttle response.

Most important for inertia (that you have control over) is the fllywheel.. the lighter it is, the quicker it'd be at the needed rpms (2000 for the TC to engage IIRC)
downside: you'll pay with engine torque, with a ratio like that nothing to worry about though

Setting the TC driven to hole three will increase the amount of time the TC will run at it's lowest ratio.
The TC itself is slightly laggy due to the belt slip..
all you can do is make perfectly sure you keep the TC perfectly clean..
especially mud on the sheaves is bad, it sticks and is wet and cause more and longer belt slip.
Do not run without cover; only have the smallest reasonable holes for the chain cut in etc.

yes, the governor itself adds to the lag as well..
but if everything visible is clean and well lubricated it shouldn't be too much of an issue at all.

But you can of course still remove the governor... but remove it, not bypas it

'sid

 

[Toycrusher]

Since you ruled the cable out..
Main thing left is inertia to be the cause for bad throttle response.

Most important for inertia (that you have control over) is the fllywheel.. the lighter it is, the quicker it'd be at the needed rpms (2000 for the TC to engage IIRC)
downside: you'll pay with engine torque, with a ratio like that nothing to worry about though

Setting the TC driven to hole three will increase the amount of time the TC will run at it's lowest ratio.
The TC itself is slightly laggy due to the belt slip..
all you can do is make perfectly sure you keep the TC perfectly clean..
especially mud on the sheaves is bad, it sticks and is wet and cause more and longer belt slip.
Do not run without cover; only have the smallest reasonable holes for the chain cut in etc.

yes, the governor itself adds to the lag as well..
but if everything visible is clean and well lubricated it shouldn't be too much of an issue at all.

But you can of course still remove the governor... but remove it, not bypas it

'sid

I don't have a cover for it and I do have plenty of mud flying around (only thing it's really used for). That being said, sounds like a good cleaning is in order. I'll plan to check the TC driven setting.

A lighter flywheel? Not happening on this engine, but governor removal has been on the to-do list, just not looking forward to having to break the chain again the get it all apart...

The slide carb on my little ATV looks like the bolt spacing is nearly spot on. Since throttle linkage needs to be redone after ditching the governor anyway, I think I'll try the carb swap at the same time.

Thanks!

 

[bob58o]

I've read taping the holes on recoil / fan shroud has same effect as sawing off flywheel fins. Since these engines are designed to be stationary, a moving kart should help cool it down.

 

[itsid]

first things first..

get a TC cover and use it!
I almost bet that alone will make a huge difference

'sid

 

[chancer]

first things first..

get a TC cover and use it!
I almost bet that alone will make a huge difference

Shopping today, I found this $1.00
http://www.bmikarts.com/Clearance-Center_c_174.html

 

[Poboy kartman]

add 3 jack shafts and you will be at original starting point, what ya need is an oak board for engine flex torque free rev no load value but down hillclimbs? cant use pump station gas, gotta get fuel from airport. prolly need a black necker-chief to be around the fumes. write us how far ya gotten?

---------- Post added at 01:37 AM ---------- Previous post was at 01:18 AM ----------

Sorry, but reminds me of growing up in a trailer park in my youth, My dad kept telling the neighbor kid we had a pet horse that lived under the trailer,,,That kid would always ask how that horse was doing? theres always them ppl that Don't get-it?

It's always refreshing to have knowledgeable folks join the forum...who KNOW! Oh well....the biggest problem always has and will be...relating knowledge to those unable to relate!