HF 79cc question for new build

[sanelson]

Hi folks,

Just got a 79cc new for $55. I am going to put a Qmatic on it with a final ratio of 12.4:1. I was wondering if those of you who use these engines take off the govenor or leave it in. This is going to be mounted in-frame on an old mountain bike. Nothing fancy about this, I just want a reliable mode of transportation. Top end speed is not a priority but being able to climb steep hills without pedeling is. I currently have a 49cc HS geared at 20:1 which is giving me top end speed in the low 20's and enough umph to tackle most of the hills I ride up. I'm hoping the HF will give me the same top end speed but with a little more hill climbing ability. Your thoughts?

 

[MotorBicycleRacing]

Hi folks,

Just got a 79cc new for $55. I am going to put a Qmatic on it with a final ratio of 12.4:1. I was wondering if those of you who use these engines take off the govenor or leave it in. This is going to be mounted in-frame on an old mountain bike. Nothing fancy about this, I just want a reliable mode of transportation. Top end speed is not a priority but being able to climb steep hills without pedeling is. I currently have a 49cc HS geared at 20:1 which is giving me top end speed in the low 20's and enough umph to tackle most of the hills I ride up. I'm hoping the HF will give me the same top end speed but with a little more hill climbing ability. Your thoughts?

Take off the governor for sure.
Have you considered AGK's gearbox for the 79 cc HF motor?
Quite a bit cheaper than a Q Matic.

Just make sure that your rear sprocket is large enough, how many
teeth are you going to use?

AGK has a good article on how to remove the governor.
AGK - Home Page - Affordable Go Karts

 

[sanelson]

Thanks. I have an HD wheel with a 56 tooth sprocket.

 

[Elmo]

Disconnect the gov mechanism on the top of the engine. With a 12.4-1 you will have good hill climbing and a top speed of at least 35 mph. A straight pipe exhaust and a free flow air filter will help.

 

[LS614]

yes, remove the external governor, the internal does not need to be removed. I think you'll really like that engine

 

[sanelson]

Thanks for the info everyone. After taking the gas tank off and looking at the current linkage, if I bypass all that 'stuff' and can figure out a way to connect the throttle cable to the lever on top of the carb then I presume that would give me the max rpm's for the engine? Or do you still have to crack open the engine and take out the govenor gear as shown on the AGK web site.

Steve

 

[wayne z]

Hey Steve, you dont hafta crack the case.

On my Predator(same thing) I added a throttle spring from the back of the bellcrank link to the lug that was to support the tank over the pto.

This spring has to be strong enough to pull the throttle and cable closed.

I then made a small loop with saftywire, and saftywired the cable tip lug to the hole next to the throttle wire to the carb.

I then doublewrapped saftywired the cable housing to the frame over the valve cove arear.

Works great and easy to figure out.

I'll see If I can get a pic

 

[agk]

Thanks for the info everyone. After taking the gas tank off and looking at the current linkage, if I bypass all that 'stuff' and can figure out a way to connect the throttle cable to the lever on top of the carb then I presume that would give me the max rpm's for the engine? Or do you still have to crack open the engine and take out the govenor gear as shown on the AGK web site.

Steve

You have a couple options with the govenor. One thing our customers have done successfully is stiffen the govenor spring. This will cause the govenor to kick in at a higher rpm.

You can also bypass the govenor like what is being suggested here.

We go in the motor a pull the govenor out. With the 6.5hp engines, if you bypass the govenor and leave it in the motor, the govenor can come apart inside the motor causing damage. This doesn't always happen, but often enough so we do not leave them in the motor.

The 79cc engine is governed at 3600 rpm. Our 79cc engine turns 6000 rpm now. That little govenor assembly is not designed to spin that fast. When you hold one in your hands you will see why.

One of our 79cc bikes is geared 12.44:1. This bike does a great job climbing hills. It does a much better climbing than our 49cc bike that is geared 17:1. Actually there is no comparison because the 79cc bike has a whole lot more torque.

The 79cc bike I'm talking about went about 25mph at 3600 rpm (going off of memory here...I have some rpm and speed results written in my notes which are not here). We wanted to know how fast it would go if it had the govenor and we kept this gearing.

 

[LS614]

I know this isn't a completely related question agk, but it still might help, what are the dimensions of your HF 79cc mount? Mounting is an issue I know I am facing and I'm sure others have faced. Peace
-LS

 

[scotto-]

This thread may be of some help as we put that same engine in an old Trek Mtn. Bike: http://motorbicycling.com/f37/new-build-hf-79cc-trek-24247.html

It was a fun build and the results are right what you're looking for, it climbs better than an HS 49cc and it pulls harder too.

 

[WayneC]

Does anybody know the valve lash setting on the HF79? And when removing the low oil sensor from the case, is there any rewiring/bypassing necessary? Oh, and what, if any air filter replacement are others using?

Thanks WC

 

[agk]

I know this isn't a completely related question agk, but it still might help, what are the dimensions of your HF 79cc mount? Mounting is an issue I know I am facing and I'm sure others have faced. Peace
-LS

The motor plate we have for the 79cc motor is 9" long. The height and width is the same as the standard 4-stroke motor plate.

 

[LS614]

1) Valve lash setting, NO CLUE

2) oil sensor removal: when you are done it should look like this

All you want to be left with is the black wire from your ignition coil (hot) and the white wire (ground) everything else can be cut off and removed.

3) Air filter you can get high flow material from AGK, buy a tractor airfilter from napa and cut your own, or keep the stock element but drill holes in the face plate for better flow.

Hope this helps

 

[LS614]

Oh, thanks agk, I just pm'ed you, sorry
it happened the same time this posted thanks for the info

 

[agk]

Does anybody know the valve lash setting on the HF79? And when removing the low oil sensor from the case, is there any rewiring/bypassing necessary? Oh, and what, if any air filter replacement are others using?

Thanks WC

The stock valve lash will vary as these engines are assembled in China and they are not exactly built with consistent tolerances. One of our 79cc engines had the lash set at .014 on the exhaust and .020 on the intake. We adjusted the lash to .006 on both valves. This gives us more valve lift. Going too tight with the lash could damage the plastic cam or bend pushrods. So far .006 has worked great and we will go a little tighter for the next race to see how far we can push it.

LS614 answered the low oil sensor question. For those that are not going inside the engine to remove the govenor you can just disconnect the low oil sensor wire coming out of the front of the block. If you are going inside the block you might as well remove the low oil sensor completely. As LS614 said you will only be left with the wire coming from the coil.

You can replace the stock air filter with a quality high flow air filter. We use UNI air filter element to replace the stock 79cc air filter. Drilling holes in the filter box cover made a big difference in air flow as well. Be prepared to drill out the main jet in the carb too. The increase in air flow will require more fuel. Use numbered drill bits. Find the drill bit that fits in the main jet. A #70 drill bit fit in our stock main jet. We suggest going to the next larger drill bit (#69) and drilling the main jet. Take the bike for a test run and check the spark plug. Repeat the drilling and testing process until you get a good plug reading. Don't skip drill bit sizes because you don't want to drill the jet too big.

 

[wayne z]

Check E-bay under"pocket bike parts" For some really nice and inexpensive custom air filters

 

[LS614]

good advice agk

 

[sanelson]

Holy cow Scotto , thanks for sharing that thread! I'm getting more stoked as the days go by waiting for the qmatic and motor mount. I hope my endeavor turns out half as nice as yours. I just can't make up my mind yet on whether to hook-up the throttle using the govenor or bypassing it. If the govenor is in use I can just ride if wot and not worry much but on the other hand it sure would be nice to have that extra boost to get out of ( or into) trouble. On my HS I have a tach just to make sure i'm within the limits - i guess i can do the same with this engine. choices......
Steve

 

[WayneC]

AGK (and others that are contributing),
Thanks SO much for this info. There is stuff here that the manual (such as it is) doesn't cover and will be invaluable to others.

I had my engine apart today to get the gov. and low oil sensor taken care of. It just seemed more prudent to remove them rather than take a chance on a "happening" inside the engine. While in there I could see that porting wasn't a priority with the "builders". I'm not looking to make a racing bike but for those that are, Ya might want to spend some time on the ports. About all I plan are the above mentioned filter upgrade, re-jet, and stronger valve springs.

Again, great info here--should be a way to make it a sticky.
Thanks again
WC

 

[sanelson]

So how do you stop the engine from turning when taking off the pawl nut? I must be missing something. Once that is figured out what do you need to do to take the flywheel off to get to that govenor pin?
Steve

 

[LS614]

AGK - Governor Removal on 79cc Harbor Freight Engine - Affordable Go Karts