79cc Predator with Tav 2 CVT

jeffreykogan

Member
Mar 1, 2021
53
31
18
Jerry he said there is plenty of torque, so first try the sprocket change, maybe even a 38 tooth. Also that carb would not heart even if stock, the way it is the carb would be a big difference., not so much speed just response. exhaust would be the next thing from affordable go-karts.com would also be a good choice...................Curt
I think I have a pretty good exhaust, as I replaced the stock exhaust with a nice straight pipe. And in regards to the carburetor, you're saying that if I go for that reworked carb that I might squeeze out a bit of extra power without increasing the rpms, right? I don't need the engine to go faster, just for it to have a bit more oomph. For the time being, I will take your advice to wait and see what its like with a 40t rear sprocket gear.
 

curtisfox

Well-Known Member
Dec 29, 2008
5,391
2,052
113
minesota
Ya that would be the best, Ya a little more power, more response in between, and maybe up the RPM's a tad, you already done that with the exhaust..........Curt
 
  • Like
Reactions: jeffreykogan

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,336
205
63
59
Rockwood, TN
I think I have a pretty good exhaust, as I replaced the stock exhaust with a nice straight pipe. And in regards to the carburetor, you're saying that if I go for that reworked carb that I might squeeze out a bit of extra power without increasing the rpms, right? I don't need the engine to go faster, just for it to have a bit more oomph. For the time being, I will take your advice to wait and see what its like with a 40t rear sprocket gear.
Maybe I should start with the basic information first.

1) What is the total weight (rider/bike)?
2) What is the terrain like (hill grade/frequency)?
3) Load pulling?

Answering these basic questions can determine how wide of a reduction range you'll need for the power output of a certain engine.
 

curtisfox

Well-Known Member
Dec 29, 2008
5,391
2,052
113
minesota
Maybe I should start with the basic information first.

1) What is the total weight (rider/bike)?
2) What is the terrain like (hill grade/frequency)?
3) Load pulling?

Answering these basic questions can determine how wide of a reduction range you'll need for the power output of a certain engine.
LOL, he is not going racing. I built my first bike in 1958 with a 1/1/2hp Briggs, it did 30mph then I ported and relived the intakes and exhaust, take in mind these old engines turn around 1800rpms I gained 5mph, until the crank went flat. These new engines rev about 7,000, so shouldn't take much to get 35mph like he wants just by changing sprockets..............Curt
 

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,336
205
63
59
Rockwood, TN
LOL, he is not going racing. I built my first bike in 1958 with a 1/1/2hp Briggs, it did 30mph then I ported and relived the intakes and exhaust, take in mind these old engines turn around 1800rpms I gained 5mph, until the crank went flat. These new engines rev about 7,000, so shouldn't take much to get 35mph like he wants just by changing sprockets..............Curt
What I asked has nothing to do with racing but rather versatility. If no load is being pulled. The rider isn't a large person and there's no steep and or/frequent hills to contend with then simply changing the rear sprocket might be sufficient.

However if the variables I mentioned do exist then widening the reduction range, using a fixed geared IGH jack shaft would be a good way to do that. It'd be like having 3 different variable reduction ranges you could choose from.

1) Pulling a load/ Hill climbing

2) Around town use.

3) Open road use.
 
Last edited:

jeffreykogan

Member
Mar 1, 2021
53
31
18
Maybe I should start with the basic information first.

1) What is the total weight (rider/bike)?
2) What is the terrain like (hill grade/frequency)?
3) Load pulling?

Answering these basic questions can determine how wide of a reduction range you'll need for the power output of a certain engine.
I weigh about 150, I don't have super steep hill grades (but at most 20%-25%), I'm not planning on taking it off road, and I wasn't planning on pulling any load
 
  • Like
Reactions: Sidewinder Jerry

jeffreykogan

Member
Mar 1, 2021
53
31
18
I just have a few questions about this reworked carburetor (in case I decide to go for it). Would I need to buy an intake manifold for it? If so, what is the purpose of it? I have attached a picture of what I am talking about below.

Screenshot (5).png
 

jeffreykogan

Member
Mar 1, 2021
53
31
18
Before I make any investments into this carb and manifold, is affordablegokarts.com a reliable place to order from?
 

JerryAssburger

Active Member
Mar 11, 2017
152
76
28
Salem Oregon
Hey, jeffreykogan.... Here's my madness to the method:

My logic goes like this. I view the TAV2 as a variable jackshaft with a starting ratio of 2.68>1, and if Engine HP permits, an ending ratio of 1>1.

So, in the case of your bike, your starting ratio is (overall) 11.79>1. At that ratio, the TAV2 will max out at about 3500rpm/23mph, and it will then start advancing the ratio, (sort of like upshifting) until the engine can't handle the advancing without an RPM drop. IF you were to have Plenty-O-Power, it would advance all the way to a 1>1 ratio, driving your 44T sprocket, for an overall ratio of 4.4>1. Even at 3500 rpm, that's 61 MPH!

As you can see, you are potentially WAY over-geared for a 3hp engine. My experience with slapping a 3hp Briggs on the back of a mountain bike got me into the 33-35mph range on flat ground. (I'm beginning to think B/S under-rated their motors because the difference between a 80000 series 3hp Briggs and my Lifan 2.8hp feels like a LOT more than 1/5th of a HP!)

Soooooooo!.... (Yes, I'll get to The Point, I promise! )... If you were to slap a 54T sprocket on the rear wheel (pretty easy to find), your TAV2 set-up would give you a starting ratio of 14.47>1 (3500rpm/18mph).... it will then advance to a potential of 5.4>1 (3500rpm/50mph).

Even the 54T is really too small, but I know finding anything larger is kind of tough. I have a 60T on mine, and it looks like a surplus blade from a local sawmill!

If it was a perfect world, where we could easily obtain the "ideal ratio" for our set up (TAV2 w/ 3hp engine), it would put us at 3600 rpm at 30-35 mph.

IDEALLY... it would be our little 3hp engines/ TAV2 geared to a 85T sprocket! That would net you 3600rpm/32mph when the TAV2 advanced all the way to 1>1.

What I'm getting at is, through trial and error, we all kinda know that an honest 3hp can get us to 30 mph-ish. Even with a 85T sprocket and your TAV2, you're in no danger of over-revving at your potential top-speed of 30 mph-ish.


I need to explain why I use 3500rpm as the TAV2 "max" rpm.... With the stock spring on the driver left untouched, the TAV2 will let the engine rev up to about 3500 rpm, and then if the vehicle keeps picking up speed, the belt will advance, keeping the engine at very close to the same rpm the whole way. Unless you have a LOT of HP, it's hard to get it to max the ratio AND go higher than 3500 rpm. Yes, you could travel down a very long steep hill but that's almost what it would take.

One of things I would like to experiment with is the three hole positions on the TAV2 driven spring. Supposedly, you can set it up so that the "hold or shift rpm" will be higher than 3500 rpm. I have never heard any ACTUAL rpm #s gained by doing this, though. Also, tighter springs for the driven pulley are available to accomplish the same thing- letting the engine rev up a little higher before the TAV2 starts clamping down on it.

Many of the members here (too many to quote) have warned that the TAV2 is just a bit too much for anything under 5hp, and I would have to agree from a top-speed standpoint. I still think it's workable as far as having great take-off speed, but if you're seeking ultimate top speed, a centrifugal clutch driving at the proper final ratio is the way to go. I'm sticking with my TAV2, because with the little Lifan, it's good enough for what I'm doing.

Regarding your question on the carb, I think if you're after better performance, I would start at the exhaust.

There was a fascinating article that I found on another forum, about getting more hp from a flathead 5hp B/S. They dyno-ed it stock and then step-by step modified the motor, testing the results after each mod.

One of the biggest (and easiest gains) was axing the stock exhaust and going with a less-restrictive one. Gains were realized EVEN WITHOUT GOING ABOVE THE STOCK 3600 RPM! Let that sink in! So if I were to go after more performance from the Predator 79, I would start with governor removal, a header with less restrictive muffler and then a tunable carb for it. (If you get that NT carb, it'll eliminate the governor anyway!)

Geez, my typing fingers are tired. If I find that article, Ill try to post it as a PDF.
 
Last edited:
  • Like
Reactions: jeffreykogan

curtisfox

Well-Known Member
Dec 29, 2008
5,391
2,052
113
minesota
LOL and if you notice he already has a lot of torque, he just needs to speed it up, and that would be smaller sprocket.........Curt
 

jeffreykogan

Member
Mar 1, 2021
53
31
18
Hey, jeffreykogan.... Here's my madness to the method:

My logic goes like this. I view the TAV2 as a variable jackshaft with a starting ratio of 2.68>1, and if Engine HP permits, an ending ratio of 1>1.

So, in the case of your bike, your starting ratio is (overall) 11.79>1. At that ratio, the TAV2 will max out at about 3500rpm/23mph, and it will then start advancing the ratio, (sort of like upshifting) until the engine can't handle the advancing without an RPM drop. IF you were to have Plenty-O-Power, it would advance all the way to a 1>1 ratio, driving your 44T sprocket, for an overall ratio of 4.4>1. Even at 3500 rpm, that's 61 MPH!

As you can see, you are potentially WAY over-geared for a 3hp engine. My experience with slapping a 3hp Briggs on the back of a mountain bike got me into the 33-35mph range on flat ground. (I'm beginning to think B/S under-rated their motors because the difference between a 80000 series 3hp Briggs and my Lifan 2.8hp feels like a LOT more than 1/5th of a HP!)

Soooooooo!.... (Yes, I'll get to The Point, I promise! )... If you were to slap a 54T sprocket on the rear wheel (pretty easy to find), your TAV2 set-up would give you a starting ratio of 14.47>1 (3500rpm/18mph).... it will then advance to a potential of 5.4>1 (3500rpm/50mph).

Even the 54T is really too small, but I know finding anything larger is kind of tough. I have a 60T on mine, and it looks like a surplus blade from a local sawmill!

If it was a perfect world, where we could easily obtain the "ideal ratio" for our set up (TAV2 w/ 3hp engine), it would put us at 3600 rpm at 30-35 mph.

IDEALLY... it would be our little 3hp engines/ TAV2 geared to a 85T sprocket! That would net you 3600rpm/32mph when the TAV2 advanced all the way to 1>1.

What I'm getting at is, through trial and error, we all kinda know that an honest 3hp can get us to 30 mph-ish. Even with a 85T sprocket and your TAV2, you're in no danger of over-revving at your potential top-speed of 30 mph-ish.


I need to explain why I use 3500rpm as the TAV2 "max" rpm.... With the stock spring on the driver left untouched, the TAV2 will let the engine rev up to about 3500 rpm, and then if the vehicle keeps picking up speed, the belt will advance, keeping the engine at very close to the same rpm the whole way. Unless you have a LOT of HP, it's hard to get it to max the ratio AND go higher than 3500 rpm. Yes, you could travel down a very long steep hill but that's almost what it would take.

One of things I would like to experiment with is the three hole positions on the TAV2 driven spring. Supposedly, you can set it up so that the "hold or shift rpm" will be higher than 3500 rpm. I have never heard any ACTUAL rpm #s gained by doing this, though. Also, tighter springs for the driven pulley are available to accomplish the same thing- letting the engine rev up a little higher before the TAV2 starts clamping down on it.

Many of the members here (too many to quote) have warned that the TAV2 is just a bit too much for anything under 5hp, and I would have to agree from a top-speed standpoint. I still think it's workable as far as having great take-off speed, but if you're seeking ultimate top speed, a centrifugal clutch driving at the proper final ratio is the way to go. I'm sticking with my TAV2, because with the little Lifan, it's good enough for what I'm doing.

Regarding your question on the carb, I think if you're after better performance, I would start at the exhaust.

There was a fascinating article that I found on another forum, about getting more hp from a flathead 5hp B/S. They dyno-ed it stock and then step-by step modified the motor, testing the results after each mod.

One of the biggest (and easiest gains) was axing the stock exhaust and going with a less-restrictive one. Gains were realized EVEN WITHOUT GOING ABOVE THE STOCK 3600 RPM! Let that sink in! So if I were to go after more performance from the Predator 79, I would start with governor removal, a header with less restrictive muffler and then a tunable carb for it. (If you get that NT carb, it'll eliminate the governor anyway!)

Geez, my typing fingers are tired. If I find that article, Ill try to post it as a PDF.
First off, thank you very much. This was very helpful and made me realize that may have purchased the wrong sprocket. I do like the torque from the tav 2 quite a bit, and I feel like I would miss that torque with the centrifugal clutch. I just have a quick question about the rear sprocket; does your bike with the 60t rear sprocket struggle to go up hills? Like what speed does yours go up a 15% hill? I think I have geared mine a bit too high, and I should probably go to a 60t to get the full range of the tav 2. Again, thank you for this information.
 

JerryAssburger

Active Member
Mar 11, 2017
152
76
28
Salem Oregon
First off, thank you very much. This was very helpful and made me realize that may have purchased the wrong sprocket. I do like the torque from the tav 2 quite a bit, and I feel like I would miss that torque with the centrifugal clutch. I just have a quick question about the rear sprocket; does your bike with the 60t rear sprocket struggle to go up hills? Like what speed does yours go up a 15% hill? I think I have geared mine a bit too high, and I should probably go to a 60t to get the full range of the tav 2. Again, thank you for this information.
You're welcome! I don't have any real hills to try mine out on (gotta put on brakes, first!) It has great take-off and I'm pretty sure would do okay on hills. Our (Us 3hp-ish 4-stroke guys) limitations are dictated by the engine's lack of hp, but it looks like you're on the right track to an excellent ride. I haven't seen any exclusive "79cc Predator HP Build Up" threads, but I'm sure they're out there. My Lifan flathead is a bit of a dead-end, beyond minor exhaust work. I already did the RT Carb on it, but it'll need some sorting out. I see no reason why the 79cc Predator can't be massaged for more power. Not having owned one, I really can't say I know enough about them.

I really hope I'm not violating any forum rules when I post this link- it's an article detailing a Predator 99cc engine with a check on the performance afterwards. I think the 79 and 99 are fairly close in design.
https://affordablegokarts.com/pages/99cc-hf-predator-modifications#step114
 
  • Like
Reactions: jeffreykogan

jeffreykogan

Member
Mar 1, 2021
53
31
18
You're welcome! I don't have any real hills to try mine out on (gotta put on brakes, first!) It has great take-off and I'm pretty sure would do okay on hills. Our (Us 3hp-ish 4-stroke guys) limitations are dictated by the engine's lack of hp, but it looks like you're on the right track to an excellent ride. I haven't seen any exclusive "79cc Predator HP Build Up" threads, but I'm sure they're out there. My Lifan flathead is a bit of a dead-end, beyond minor exhaust work. I already did the RT Carb on it, but it'll need some sorting out. I see no reason why the 79cc Predator can't be massaged for more power. Not having owned one, I really can't say I know enough about them.

I really hope I'm not violating any forum rules when I post this link- it's an article detailing a Predator 99cc engine with a check on the performance afterwards. I think the 79 and 99 are fairly close in design.
https://affordablegokarts.com/pages/99cc-hf-predator-modifications#step114
Yeah I'm gonna try and get my engine milled and ported from affordable go karts. My goal is to make maybe 4.5-5 horsepower. We'll see how it goes.
 

jeffreykogan

Member
Mar 1, 2021
53
31
18
Update: I have put on a 54t rear sprocket (thank you JerryAssBurger for recommending a larger sprocket size) and I now have some darn good take off power. I hit a top speed of 30, but the torque is incredible. The Tav2 is also very reliable, and I have not had any issues with it at all. My next upgrade will be a larger carb with an intake manifold. Thank you to everyone who gave me advice, as it was very helpful.
 

JerryAssburger

Active Member
Mar 11, 2017
152
76
28
Salem Oregon
AWESOME JOB, Jeffrey! One bit of tinkering you can try is switching the Driven Spring to the tighter hole on your TAV2. If you Google that, you'll find all sorts of step-by-step info. The issue is, STOCK, a TAV2 is designed to hold the engine it is bolted to at 3500-3600 rpm... since we know these little motors will make more HP if they were allowed to rev around 4000 rpm, it would probably improve things on the top-end by putting the Driven Spring in the tighter position. You will not suffer ANY loss of torque on the bottom end by doing this. It'll just let the motor rev up a little higher before advancing the ratio.

I'm going to take "The Walk Of Shame" and admit that the flathead Lifan just won't swing the TAV2 very well. There's little potential to be gained from it by it's design, even with the better carb and exhaust. The 79cc Predator seems to be a much better candidate, as you're on the road to proving. I took my Lifan off and am transitioning (SLOOOOWLY!) to a Predator 212, because that's what's sitting in my garage right now.

Just to give you an idea of the Lifan 152's limits... before I pulled it off, I gave it One Last Chance to redeem itself and the best I could pull on level ground was 30mph. I slapped on a Huasheng Knock-Off 53cc engine and geared it for a potential of 33 mph at it's stock hp peak, and without ANY tuning or tomfoolery, I hit 32mph. Same Red Cranbrook, same Fat Old Guy.
Me-Thinks the Lifan isn't even 2.8 hp, but I have no dyno to prove anything. ()My Butt-Dyno speaks volumes, though.)

IF I had spent $$ and got the 79 Predator, I would stick with it as it seems to be a manageable size and still puts out good power. At the time I was standing in Harbor Freight, the 212 was on sale for cheaper.

Now to find that shoehorn....
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
1,602
3,165
113
70
Rubicon, Wisconsin
Jeffrey, I run a 79cc pred., and just did The AGK port, polish, and head deck fifty thou.
(5) Bike builders that have machine tools/shops | Motorized Bicycle Engine Kit Forum (motorbicycling.com)
Starting at #191 (4) 79cc Sportsman Flyer build | Motorized Bicycle Engine Kit Forum (motorbicycling.com)
Don't waste time and money on a reworked carb.
Just get a 19mm Mikuni clone on Amazon mostly $20. Needle set middle notch AGK sells them too for much $.
Follow my two links you will see and learn alot. I did.
Lastly if you have the coin, a billet flywheel #6671 from ARC Racing is useful too. 5-7hp from a 79cc (^)
Jerry is telling you straight.
Tom
 
Last edited: