HF 79cc question for new build
- Thread starter sanelson
- Start date
wayne z
Active Member
Yea, seems like the 99cc runs a lil lean too. Especialy after I changed to a custom air filter and non-stock exhaust.
Seemed kinda lean stock too. Must be that C.A.R.B. required jets and settings.
Even when warmed up good, they still like to be choked to start easily. But I am getting 60 miles per 2 litre tank, and that's running it hard.
Seemed kinda lean stock too. Must be that C.A.R.B. required jets and settings.
Even when warmed up good, they still like to be choked to start easily. But I am getting 60 miles per 2 litre tank, and that's running it hard.
wayne z
Active Member
Hmmm, If the 79cc se anywhere close to the performance of the 99cc, it should pull hills way better than a HT.
I'm running mine at 8.75 -1 and getting 40 mph at 4500, with fricton drive.
Are yout tires at optimum pressure? makes a BIG difference. What type intake/ exhaust are you running? is the chain or wheel bearings too tight? All of these added together could cause bad performance.
cannonball2
Well-Known Member
For decent climbing of "real" hills you need to be in the 13-14:1 area with 2.5hp.
daRog
New Member
Altitude is a major factor. I live about 4100 ft above sea level and engine performance difference between here and down in the valley is quite noticeable.
Try inputting some numbers in a calculator and see what you get:
Relative Horsepower Calculator
Try inputting some numbers in a calculator and see what you get:
Relative Horsepower Calculator
I have a 79cc engine in a bike that is geared 11.10:1. The only modifications to the engine are govenor removal and an exhaust that I made. This bike will climb a very big hill near my house. The hill is a 8% grade and about a 1/4 mile long. I usually get a running start at the hill. The other day I started from a dead stop at the bottom of the hill. RPM climbed to 4100 and the bike climbed the hill with no problem.
Usually our 79cc bikes use a 12.44:1 gear ratio. With this ratio the engine will climb to about 4600 rpm and will carry the bike even faster up the hill.
Our typical 49cc bike is geared 17.11:1. These bikes simply can't make it up the hill. The 8% grade is too steep and the 49cc engine doesn't have the torque to tackle the hill with the gear ratio we are using.
Based on the gear ratios we are using on our 79cc bikes, you should be able to handle the hills with your 11.55:1 gear ratio. Have you pulled the cover off the Q-matic to inspect the belt tension and for signs of slippage? Seems like torque is getting lost somewhere. Is modifying the spring tension on the govenor the only modification? Have you checked the plug to see if it's running lean?
cory151
Member
Perhaps you got a "runt" (hence the $55 price tag display, or returned model) I once had two 5.0 Mustangs of the same model, same Nitto tires, same year, same T-5 trans, and same mileage both coupes and each weighed within 40lbs of each other. One would consistantly run 3/10ths faster at test and tune in the 1/4 miles. Even with a different driver, no matter what.
Now these motors probably have less qaulity control than Ford so I wonder if theres not some issue with it. Variations can take place in manufacturing where inches add up to feet.
Quenton Guenther
Dealer
Here are a few facts that may help.
The motors vary greatly, some are real strong and some are not. I have found the lifters set from .035" to .018" on different motors. 11.55 X 1 should easily pull most hills.
The primary pulley on the Q-Matic can be reduced to 2-1/4", however the clutch will enguage much later.
I am currently testing a factory prototype Q-Matic on the HF 79 CC motor, and have easily exceeded 40 MPH will ratios near 10 X 1 [I am a light weight rider]using an adjustable primary pulley. I ended up setting the adjustable at 2.5" for best overall performance [11.55 x 1]
The HF motors run much stronger with a less restrictive intake and exhaust. Many 79 CC owners installed the "tuned' flex pipe with major improvements in power. Careful, as the motor is border line "Lean" to begin with. I increased the main jet by 3 sizes to get it to work correctly with the alternate intake & exhaust system.
Have fun,
The motors vary greatly, some are real strong and some are not. I have found the lifters set from .035" to .018" on different motors. 11.55 X 1 should easily pull most hills.
The primary pulley on the Q-Matic can be reduced to 2-1/4", however the clutch will enguage much later.
I am currently testing a factory prototype Q-Matic on the HF 79 CC motor, and have easily exceeded 40 MPH will ratios near 10 X 1 [I am a light weight rider]using an adjustable primary pulley. I ended up setting the adjustable at 2.5" for best overall performance [11.55 x 1]
The HF motors run much stronger with a less restrictive intake and exhaust. Many 79 CC owners installed the "tuned' flex pipe with major improvements in power. Careful, as the motor is border line "Lean" to begin with. I increased the main jet by 3 sizes to get it to work correctly with the alternate intake & exhaust system.
Have fun,
Thanks guys,
This bike does exceptionally well on the flats - gets up to 30mph quickly and maintains that speed pretty easily it seems. The bike is completly stock, carb and exhaust. Checked the tension on the belt, makes a nice high pitched note when strummed. I see no slippage there. Maybe it's just the alltitude 5500 feet. I would guess the hills I am riding up are closer to 10-15 degrees, maybe a bit more. The engine pulls but the speed drops drastically to 5-8mph. I would be OK with substituting the 56t to a 62-64t sprocket if that would keep the hill climbing speed in the teens. The only mod was tightening the govenor spring. The engine rev's in the mid 4000's now.
This bike does exceptionally well on the flats - gets up to 30mph quickly and maintains that speed pretty easily it seems. The bike is completly stock, carb and exhaust. Checked the tension on the belt, makes a nice high pitched note when strummed. I see no slippage there. Maybe it's just the alltitude 5500 feet. I would guess the hills I am riding up are closer to 10-15 degrees, maybe a bit more. The engine pulls but the speed drops drastically to 5-8mph. I would be OK with substituting the 56t to a 62-64t sprocket if that would keep the hill climbing speed in the teens. The only mod was tightening the govenor spring. The engine rev's in the mid 4000's now.
Take a plug reading. 5500 feet is really up there. The engine isn't going to run it's best if it's not jetted properly for that altitude. A quick plug reading will give you an idea of where your at on the jetting.
Quenton Guenther
Dealer
Did you check plug after WOT run? The HF motor tends to run rich at low throttle settings and lean above 3/4 open [throttle position, not RPM]. When I installed a flex pipe exhaust [tuned], and a less restrictive air breather [from 1999 Whizzer], I had to increase the main jet size by 3 sizes. Even though the motor went extreemly lean at WOT, it still tested rich at lower settings. Adjusting the air/fuel settings have some effect, but I suspect the pilot jet is too large. The carburetor leaves room for improvement, and hopefully I can try a few different types on future tests. BTW the main jets from the new edition Whizzer carburetors fit the HF carburetor.
You might want to check the valve clearance, as I checked 3 new motors and all were set differently.
On 2 motors [one 2 years ago, one recently] the ignition coil needed some TLC, and in both cases it was damaged during assembly.
Have fun,
You might want to check the valve clearance, as I checked 3 new motors and all were set differently.
On 2 motors [one 2 years ago, one recently] the ignition coil needed some TLC, and in both cases it was damaged during assembly.
Have fun,
MarkSumpter
New Member
The bike looks good but if you are having a rich condition I have found that the air box on the 79cc HF engines to be one of the worst offenders as they tend to greatly restrict air flow. Give one of the universal filters that AGK sells HERE I have heard from others that they work well and allow a greater amount of air to get to the carb. Getting that mix to lean out would be the priority before changing the gearing as the hill climbing problem might just fade away if you are running a good air to fuel ratio.
Last edited:
Quenton Guenther
Dealer
You can also gain a lot of power via the exhaust system.
About 3 years ago we tried different exhaust & muffler systems on the HF motors, the difference is major. During testing we found the "lawnmower" style muffler to be almost as bad as the stock muffler.
At one of the shows [DE] there were 2 almost identical bikes with the HF motor [both with the Q-Matic drive], and one was much faster than the other. Both had the flex pipe exhaust, however one had the "lawnmower" style muffler attached at the end, after removing the "lawnmower" type muffler, both bikes ran almost the same.
Have fun,
About 3 years ago we tried different exhaust & muffler systems on the HF motors, the difference is major. During testing we found the "lawnmower" style muffler to be almost as bad as the stock muffler.
At one of the shows [DE] there were 2 almost identical bikes with the HF motor [both with the Q-Matic drive], and one was much faster than the other. Both had the flex pipe exhaust, however one had the "lawnmower" style muffler attached at the end, after removing the "lawnmower" type muffler, both bikes ran almost the same.
Have fun,
MarkSumpter
New Member
Right you are Quentin and after I posted my reply I wondered why I hadn't posted about the exhaust too but was too lazy to come back and re-edit the post again...
I also set the valve lash on both the intake and exhaust to a tight .002" clearance and internally remove the governor as well as the oil sensor (I have had too many sensors fail or become intermittent in the past being a pain) and plug the holes.
Sometimes I wonder how much power the EPA/C.A.R.B. certification regulations and modifications reduce power as well as their effectiveness in reducing emissions.