79cc Predator Build

[Tony01]

Yeah, I really have to baby it till it reaches 25 MPH. If I don't the whole bike shakes and vibrates. Feels like the clutch is locked because the engine really labors. The magic number is 25 mph then I can give it throttle. When it hit 43-45 mph I was on a long level stretch with a light wind to my back. I like the low RPM's at 30 MPH but I'll probably increase the number of teeth to 36 on the rear sprocket to improve the low speed performance. My Max Torque six shoe clutch came Saturday and then I found the 415 chain was to narrow to fit on the #41 clutch sprocket so now I am awaiting the #41 chain to arrive.

It's shaking because your setup is wrong. The magic number is 25 because your gear ratio is probably close to a 8:1 which puts you at about 100rpm per mph and 25mph is about the speed that would be your clutch locking up. Put the green spring in it and it will get better. A stock clutch works great because most karts and minibikes that use a Max torque have gear ratios set for about 30mph. To go faster on one gear your clutch needs to slip a lot more. Think of it as driving a manual car in 1st gear vs 3rd only. In 1st you lock it pretty much at 10mph, but starting in 3rd you need to slip it till about 25mph or else risk under revving the motor. Also when you slip it starting in 3rd you rev the motor to about 2800 to 3000, because your foot instinctively knows the best torque is right there. This is why you need a heavier spring. Setups that have worked for me have been black spring with 3 heavy shoes, black spring with 6 heavy, and green spring by itself. My local karting shop doesn't carry heavy shoes so I just get the one spring. Then when your clutch shoes and bell get really worn like .050" each, you can go back to stock spring for almost the same effect. Get a short piece of pvc about 1.25" ID to make the reassembly easier. The acceleration will still be slow in general terms, you might not beat a Prius or even a bus but you'll beat a bicycle off the line. You need horsepower to accelerate fast, and to go fast, and a 79cc doesn't have much of it so the setup needs to be tits.

Basically; gear the bike for the speed you want to go (and if motor can handle it) and set your clutch to lock up at around the torque maximum of the engine. For example, my bike is geared 7:1 to top out around 55mph at 5k with the clutch locking up at 30mph (green spring). Even the green spring is a bit too close to stock for me, but it's good enough and my motor has enough power to force the clutch through its chatter.

Also cancel your order for that #41 crap and get the #420 at your local mc shop.

 

[Kletus]

Thank you Tony01 for that comprehensive theses you wrote on the clutch modifications. I have decided to go that rout instead of increasing the gearing ratio and loosing top end speed. I can live with slower take offs. I ordered a green spring for my Max Torque clutch today. I noticed that the silver spring in the MT clutch is made with much larger wire then the wire in EBAY clutch I am now using and I wonder if a green spring in stalled in it might improve its performance in my application, might be worth a try.

 

[TheNecromancer13]

If you remove the governor you can get a lot more top speed while still running a better gear ratio.

 

[Kletus]

If I had a billet rod and a billet balanced flywheel and stronger valve springs I would consider removing the governor. Also these engines have dipper oiling for the rod bearing, no oil pressure, you could end up welding the bearing insert to the crankshaft throw. What we need is a needle bearing rod. I know people are doing this and getting away with it but its risky. It might be that the governor could be modified to let it build another thousand RPM's. Say 5000 RPM's for short bursts. If the engine flies apart with you sitting on the bike you could be seriously injured.

 

[TheNecromancer13]

If I had a billet rod and a billet balanced flywheel and stronger valve springs I would consider removing the governor. Also these engines have dipper oiling for the rod bearing, no oil pressure, you could end up welding the bearing insert to the crankshaft throw. What we need is a needle bearing rod. I know people are doing this and getting away with it but its risky. It might be that the governor could be modified to let it build another thousand RPM's. Say 5000 RPM's for short bursts. If the engine flies apart with you sitting on the bike you could be seriously injured.

I've been running mine for over 6,000 miles with the governor removed and the stock rod, stock flywheel, and stock springs, and it hasn't blown up. The valves start to float and limit the rpms before they get high enough to do any damage.

 

[Kletus]

Installed a stock Max Torque clutch with a silver garter spring and 46 gram light shoes. Improvement. Now I can give it throttle at 10 MPH with no shaking. I installed a green spring with not much improvement. I am awaiting a black spring to arrive. I am going to try that before I adjust the gearing. One thing at a time. The no name clutch that was previously installed that shook up to 25MPH had 60 gram shoes and a garter spring that had a wire diameter the same as the Max Torque green spring. The only differences I saw between these two clutches was shoe weight, a brass spindle bushing and a replaceable sprocket. The shoe weight being the most important as the 60 gram shoes would cause the clutch to engage at a much lower engine RPM. I'll let you know how that black spring works out. My gearing is 6.6:1. Clutch sprocket=10 tooth, jack shaft sprocket = 22 tooth, jack shaft output sprocket =10 tooth and the back wheel sprocket is 44 tooth. Many Thanks to Tony01 for pointing me in the right direction with respect to the clutch engagement and Necromancer13, help also.

 

[TheNecromancer13]

6.6:1 gear ratio is kinda steep even if you were using a 212cc.

 

[Kletus]

I have a 212cc predator on a I have a Schwinn Meridian trike and its geared 4.2:1. It does standing starts well. I have no idea how fast it will run because the front wheel starts to shake and vibrate and hop around at 30 mph. These both are King motorbike kits.

 

[Tony01]

Installed a stock Max Torque clutch with a silver garter spring and 46 gram light shoes. Improvement. Now I can give it throttle at 10 MPH with no shaking. I installed a green spring with not much improvement. I am awaiting a black spring to arrive. I am going to try that before I adjust the gearing. One thing at a time. The no name clutch that was previously installed that shook up to 25MPH had 60 gram shoes and a garter spring that had a wire diameter the same as the Max Torque green spring. The only differences I saw between these two clutches was shoe weight, a brass spindle bushing and a replaceable sprocket. The shoe weight being the most important as the 60 gram shoes would cause the clutch to engage at a much lower engine RPM. I'll let you know how that black spring works out. My gearing is 6.6:1. Clutch sprocket=10 tooth, jack shaft sprocket = 22 tooth, jack shaft output sprocket =10 tooth and the back wheel sprocket is 44 tooth. Many Thanks to Tony01 for pointing me in the right direction with respect to the clutch engagement and Necromancer13, help also.

6.6 is far too tall for a 79cc. You can remove the governor long as you gear it 8:1 on 26" wheel and keep stock springs. Not enough power to blow it up on level ground.

 

[Kletus]

Installed the BLACK Max Torque spring today and went for a ride. I like it. Now I can give it full throttle from a dead stop with no shaking or vibration. The amount of slipping depends on how much throttle I give it. Full throttle it slips to about 30MPH and starts to lock.

 

[Kletus]

Today I alternated 46 gram shoes with 60 gram shoes along with a black spring in the Max Torque clutch. I get shaking and vibration at standing starts and up to about 20 mph with full throttle but if II give it 1/4 throttle at a standing start and ease on the throttle it is smooth and vibration free. The clutch seems to completely lock somewhere around 25 mph. Acceleration is better with this setup than with the black spring and 6 light shoes due to the light shoe setup slipping badly all the way up to 30 mph. I have a 44 tooth back wheel sprocket and I'm thinking a 48-50 tooth rear sprocket may solve or improve some of this full throttle vibration issue. I may also take Necromancer's advise and remove the governor as I don't think the gearing or me getting chicken at any speed past 40 mph would allow a bad over rev to occur.

 

[JerryAssburger]

For what it's worth, my first project was a 26" mountain bike, 3hp B/S (stock) mounted on the rear at a 9:1 overall ratio and it did fine with my 170 lbs. It consistently hit 38-40mph on the level. (GPS verified). The Harbor Freight 212cc (6.5hp) engines were good up to 4500 rpm no worries at all, so I don't think the Original Poster's projections are far off, using the 79cc motor. Cobbling together a less-restrictive exhaust system and going a bit richer on the jet really wakes up these quiet, dependable motors.

 

[Tony01]

Removing the governor was the first thing I did after opening the box for the engine. Pain in the butt and you have to make a special hook tool to get the little clip off the gov gear shaft to get the gear off. 79cc won't push you faster than at most 44mph, so gear it for 5500rpm at 44mph and set the spring where you want it at.

Yes it's true, any flywheel will blow at some rpm, but the stock ones are good to at least 5700. You only need to worry about billet parts when you upgrade valve springs. In fact, I have upgrade my 212 to 18# springs and still run stock parts, but I don't push it like I used to.

 

[engineman]

In reguards to the 79cc predator engine.I was thinking of building a bike with that engine.I did a fair amount of research on it.Most importantly emailed a shope that gets heavily into modified small engines.So they took a 79cc predator and slowly modified it.Right away removed the governor and built a camshaft for this engine.Then increased the CR.To 10.5-1 added valve springs.Then took the bike out for a test ride.Said it ran obviously better.But still wasn't what they were after.To make a long story short.They reworked the cam lobes for a taller lift.I don't recall the cam specs now.But enough to say it was huge in comparission to the cam that came out.Then took the CR up to 12-1 added stronger valve springs to handle the lift and rpms.A straight pipe for exhaust and a reworked carb.The carb wasn't the same as what the engine came with obviously. As for the flywheel.Im almost positive it was the stock flywheel.As for the rpms way over anything any stock engine will run.Talking a honest 7000 ok.And it hauled for the first time ever.When I asked him how long it held up?I was shocked as he told me its still running fine after more hours in hard run times than any of them had figured!They finnaly left him take it home and he gave it to his son to ride.That was well over a years worth of run time.And letting customers ride! He told me that engine is a real winner!As who could care to disagree! However, make no mistake in understanding.There a go-kart speed shop! They know what there doing and yes they sell the same cam to the public. Along with afew other go fast goodys.But be prepared to shell out some cash for this one of its only kind cam.Which cost about 130.00Yes, they ran the bone stock rod and oil dipper!The only real major changes are all in this reply.So no the flywheel wont come apart do to any higher rpms this engines to achive.When the only reason at all any flywheel would.At any rpm level.Is do in whole to having a defect period. As for the billet rod.There lighter somewhat stronger and have slightly better oiling port as to the stock unit period.He told me there was no need to swap out this rod.They simply milled the stock head! Which happens to be the cheapest method and only way for any engine like this one.That you can not get any parts for but the same stock parts.So your aware you don't ever want to just install any larger cams without increasing the CR.And the bigger the cam lift and or duration.The more the CR will need increased. Do in whole to valve overlap.And Id highly recommend using a quality aircooled synthetic engine oil. Which is made when I had bought some along time ago now for a 150cc engine Id modified.By porting and polishing it and brought up the CR shlightly.Built a custom supertrap exhaust for as well added a mikuni flat slide carb to it. Valve springs and better ignition.That engine was a overhead camed. On the 79cc predator I would use a 18mm mikumi VM series round slide.Which this shop may have used as well.I don't recall but know my carbs and the mikuni is a serious good performance carb.That cant be beat!I asked him about balancing.He said don't mess with it at all.All that will do is change where the engine will vibrate worse .Not to mention the cost of having that done.As for the shops name wow.That has slipped my memory.One of them is AGK or AKGJust look under go-kart performance and predatore and you will find it like I did.