Power Bicycle
The Ultimate guide to Motorized Bicycles
What is the best gas motor for a bicycle?
There is a massive variety of motors for bicycles which can confuse consumers who can seldom test every option available. Bicycle motors ranging in size from 32cc to 150cc are being advertised, and the choice of engines to motorize your bicycle is bordering on ludicrous.
To help clarify the confusion, I examine the range of gas bicycle engine kits on the market. I break down the advantages and disadvantages of each kit in order to clarify what to expect when you purchase. I provide my analysis from over 20 years of selling these engine kits, discussions with mechanics, and customer feedback about each option.
The best bicycle motor kit is the standard 80cc, 2-stroke, bicycle motor kit. 80cc motor kits are strong, reliable, have respectable power, and are among the lowest cost of any bicycle motor kit.
There are good reasons why these motors continue to dominate the gas motorized bicycle market.
Of course, one size does not fit all. There are alternatives that may be better for the motorized bicycle project that you have in mind. I analyze the benefits of each of the major motor kit options to give an understanding of where their advantages lie.
What Is a Motorized Bicycle?
Imagine your favorite bike, but with a motor that can power up hills making it effortless to get around. This was the foundation of the motorcycle. Motorized bicycles are a classic concept with roots in the early years of motorized transport.
A motorized bicycle is a bike fitted with a specially designed engine which fits into the frame. Bicycle motor kits provide sprockets, chains, gas tanks, cables, mounts and every part that you will need to put the motor on your bicycle.
Motors for bicycles are small, barrow engines with mounting brackets that fit onto a bicycle frame. The bicycle engine kit contains the bicycle motor and all the parts that are required to turn your bicycle into a small motorcycle.
These days, motorized bicycles are more focused than they used to be. There are many sports and pastimes that demand different motor options. Hunting bikes, commuters, road bikes, and mountain bikes all have different engine requirements creating the demand for a wider range of bicycle motor kit options.
Low-cost bicycle motors are not motorcycle engines
The first thing to recognize is that many people have preconceived ideas about bicycle motor kits. Some enthusiasts prefer 4-stroke engines over 2-stroke, while others feel that bigger is better, believing that there is “no replacement for displacement”. This can lead us to build motorized bikes that could be better. Misconceptions may result in motorized bicycles being more expensive, more difficult to build, or are lower powered, than they could have been.
To further complicate choice, marketing rhetoric does not always live up to reality. I’ve learned to test every motor, and performance part before I listen to a word of the marketing spin that accompanies the product.
To help you cut through the jargon that is spread across every product website, I’ve ranked the best gas bicycle engine kits on the market. I make these recommendations considering performance, affordability, and of course, customer feedback. I have placed a lot of emphasis on the price points of each kit to help you find your perfect match. After all, the big selling point of motorized bicycle kits is that they are low cost.
The first point of clarification is to cut through the marketing hype of engine capacity. The true cubic capacity of bicycle engines is often mis-represented because vendors can bump up price for larger engines.
The table below clarifies what the true capacity of each bicycle engine against the capacity stated on the sales tag:
REAL CUBIC CAPACITY
38cc
50cc
70cc
80cc
90cc
100cc
125cc
150cc
MARKETED CUBIC CAPACITY
36cc
49cc
54cc
66cc
72cc
80cc
92cc
138cc
Performance of bicycle motor kits vary
The easiest initial test when beginning our troubleshooting is to check for spark.
For most motorized bicycle engines, it is quick and simple to remove the spark plug to see whether there is a spark.
- Remove the spark plug from the cylinder head.
- Fit the plug cap back onto the spark plug terminal (the top of the spark plug).
- Leaving the spark loose, earth the ground electrode (screw threads) of the spark plug against the engine.
- Engage the clutch and turn the motor over. For a motorized bicycle engine you will have to push the bike up to around 5MPH. This should be easy as there will be little engine resistance as the spark plug is not in place.
If the ignition is good then you should see a few blue sparks jump the air gap in the spark plug. In addition, you should also hear the “click” as the spark jumps.
If the spark is good, jump to section 2. If there is no spark, and you have the common magneto / cdi ignition, then you have an electrical fault.
Further spark testing is required for single piece ignition motorized bicycle engines. As single piece ignitions are unusual in motorized bicycles, additional spark tests have been placed towards the end of the document.
Which bicycle Motor is best?
There is a huge choice of gas bicycle engines. We analyze which is best for your motorized bicycle project
What size motor is best for a bicycle?
125 cc and 150 cc 4-stroke bicycle engines are made for motorcycles. 125 cc and 150 cc engines are pit bike engines. Although they can be fitted, these engines are very difficult to mount onto bicycles. If we can squeeze the motors inside the frame, they often require special brackets in order to fix the engine in place.
By comparison, bicycle engines have mounting brackets designed to bolt onto a bicycle frame. It is a straightforward task to bolt a bicycle engine onto a bike.
The most practical bicycle motors are between 50cc and 100cc engines with the best, and most popular engine found in 80cc bicycle engine kits.
Which motor is best for a bicycle?
2-Stroke vs 4-Stroke motorized bicycle
Choosing the correct engine could make or break your motorized bicycle project. As a large factor in your choice will be down to your preference, lets clarify some fundamentals.
2-Stroke Bicycle engine kits
2-stroke bicycle engines kits have advantages in speed, reliability, price, and fuel consumption. In addition, there are an endless amount of low cost tuning parts available for 2-stroke motorized bicycles. It is simple to increase power output if you like performance.
A standard 80cc bicycle engine kit can reach 30 MPH, and a few simple, after market performance parts can easily bump the speed up to 35+ MPH.
2 stroke bicycle engine kits can reach speeds of over 40 MPH with basic tuning. The average DIY enthusiast can achieve these speeds without breaking the budget.
Speeds exceeding 40 MPH takes a little more effort and costs ramp up quickly.
Motorized bicycles are small motorcycles
Apart from the size of the engine, there is little difference between a motorized bicycle and a motorcycle.
Less Maintenance
One of the best parts about 2-stroke engines is that they are easy to maintain. You don’t have to replace the whole engine if something goes wrong. Just about every component is available for a few dollars on Amazon if you ever need a part.
More reliable
2-stroke motorized bicycle engines are very simple and have far fewer parts than 4-strokes making them more reliable.
Disadvantages
Although 2 stroke bicycle engines have many advantages, they have a high-pitched engine tone. They also require 2-stroke oil to be mixed with the gas which can cause smokey exhaust fumes.
4-Stroke bicycle engine kits
Automatic clutch
The biggest advantage to most 4-stroke bicycle engine kits is that they are automatic clutch motors with pull start. The automatic clutch allows the rider to pull from a standing start rather than having to pedal assist the engine away from a stop. Motorized bicycles with automatic clutch motors are easier to ride as first time riders don’t have to become familiar with a manual clutch.
Quiet
4-stroke bicycle engine kits are also quieter than 2 stoke motors and have a deeper exhaust tone. Quieter engines are an advantage for hunting bikes and can be less offensive if you ride in the suburbs.
Pull start
Pull start motors are essential for many conversions (For example, if you are building a slider trike)
Disadvantages
A Huffy dragster motorized bicycle
How can you make a great bicycle better? Put a bicycle motor kit on it!
1. Pull starts are very unreliable
It is rare that an automatic clutch is set correctly. They usually need to be adjusted to operate correctly.
most automatic clutch mechanisms are unreliable.
2. 4-stroke bicycle motor kits are more complex to fit.
4-stroke engines are wider and bulkier than their 2-stroke counterparts. They require a larger bicycle frame to accommodate 4-stroke engines. In addition, the pedal shaft has to be widened to allow the pedals to clear the motor.
3. 4-stroke bicycle engines require more maintenance as the engines are more complex
4. They require regular oil changes.
5. Spares are not as easy to find
6. There are few tuning options for 4 stroke bicycle motors.
7. There are far fewer engine parts available for 4 stroke bicycle engines. If problems occur, a minor fault can require a replacement engine.
8. Cylinder heads on 4-stroke engines are far more complex housing cam shafts, valves and timing chains. The complexity means that the cylinder head is relatively difficult to remove.
9. There are far fewer 4 stroke bicycle motors and they are more complex to work on. Performance components for 4-stroke bicycle motors are, therefore, not widely available. Some vendors offer big bore kits but these provide little improvement in speed.
Ranking the best bicycle motors
The best gas bicycle motor
We rank the different bicycle motors detailing their advantages and disadvantages.
Now that we have covered the fundamentals, the following section delves into the major bicycle motor kits available. I have ranked the kits from best to worst, highlighting the advantages and drawbacks of each motor type.
The winner of the best gas bicycle motor is the 80cc engine kit. 80CC bicycle engines are perfect for anyone who wants to build their own motorized bicycle. 80cc engine kits are easy to install, provide the best bang for buck, and they have heaps of power for a bicycle!
Don’t think that if you put one of these low-cost motors on your bicycle that it will be unreliable. 80cc bicycle engines are more reliable, more powerful, lighter, and more fuel efficient than their 4-stroke counterparts. You will see many thousands of miles on these engines as long as you use good 2 stroke oil mixed in the correct ratio.
Standard 80cc engine kits are a great platform to start your motorized bicycle experience. Most often, they are the best option for motorized bicycle projects. These motors have heaps of torque, fuel consumption of just under 40 MPG, and a cruising speed is around 25 MPH.
The 80cc engine kit has been the most popular option for motorize bicycles for over 20 years. Fabrication has matured over the years and brackets, barrel porting, and they have improved bearings to better the product.
In addition, there are lots of low-cost performance parts available to tune the engine if you feel you need more grunt.
There are a number of upgrade options for motorized bicycle engines. The most popular upgrade is the Rongtong carb.
The Rongtong is more complex than the NT carb and has a number of advantages. From our troubleshooting point of view, the Rongtong carb has fewer adjustable parts and a different float setup. The Rongtong carb is, therefore,
- less prone to flooding the engine,
- it can tolerate a greater fitting incline, and
- it is unlikely to require tuning adjustments.
If you have verified that the carburetor is working correctly then the following tests are the next stage in troubleshooting starting faults on your motorized bicycle:
Every vendor promotes high performance bicycle engine kits with a myriad of performance components. Some vendors offer engines with oversized barrel/piston, high compression cylinder heads, Boost Bottles, special barrel porting, reed valves, and many other performance options.
Most vendors just want to bolt on a few eye-catching parts and spin the benefits. Very few “racing engines” perform any better than the base 80cc model.
Looking for a performance boost? The standard 80cc bicycle motor kit with aftermarket carb, power pipe, and 36 tooth rear sprocket is the best option.
Change the standard NT carburetor for a Rongtong carburetor and swap the muffler for a good performing expansion chamber. These modifications provide the additional torque to drive a smaller rear sprocket. Swapping the standard 44 tooth rear sprocket for a 36 tooth sprocket will increase speed to around 35 MPH.
These simple options provide the best platform to increase power with little effort, and without breaking the bank. Other components add little value unless you are knowledgeable on tuning 2 strokes.
High compression cylinder heads, Boost Bottles and reed valves add no speed or acceleration, without additional engine adjustments. “Staged” motors are most often just standard 80cc engine kits with a few bolt on components that simply add cost.
Reed valve engines
I will mention here that factory built reed valve engines perform very well. It is, however, hard to know whether the reed valve engine has been correctly designed. Most reed valve engines are simply a standard 80cc motor with an after market reed valve bolted on. Reed valves do not add power without changing porting.
Factory built reed valve motors have different barrel porting to improve fuel intake. Unfortunately, bolt on reed valves are common. For improved performance you need to source a rare, factory built reed valve engine.
If you’re looking for a 2-stroke engine with tons of torque and power, the 100cc bicycle engine kit is the way to go. As the saying goes, “There is no replacement for displacement” and a 100cc bicycle engine kit is the largest 2 stroke bicycle motor available.
The top speed of the engine is very similar to the 50cc and 80cc motorized bicycles but with more torque. In standard form, 100cc engines leave the lower capacity engine kits behind on hills and acceleration. The enormous advantage of the 100cc bicycle engine is its potential. If you want to tune your engine, then the 100cc engine kit is the best starting point.
The factor that limits the speed of all bicycle motors is the balance of the crankshaft. Bicycle motors rev to around 4,000 RPM in standard form. (We have found that the lighter 50cc engine cranks typically rev a little higher typically maxing out at around 4,500 RPM *). A few engine kits (Zeda built motors for example) have a better balanced crank which can, occasionally, rev as high as 6000 RPM when tuned.
The advantage of the 100cc engine kit is that the torque allows the use of a smaller rear sprocket. 100cc engines can use higher gearing to provide higher top speeds than other bicycle motors.
The downside of 100cc engines is their cost. 100cc engine kits are around twice the price of an 80cc engine kit. An 80cc engine with expansion chamber, sports carb and smaller rear sprocket will outperform the 100cc engine for less money.
Standard 100cc motorized bicycles will cruise at 35 MPH when fitted with a smaller rear sprocket.
This is the first 4 stroke bicycle engine on the list. The key feature of 4 stoke bicycle engine kit is that they almost always have automatic clutches. (You can, of course buy 2-stroke engines with automatic clutches, however, they are very hard to find.)
Advantages
Automatic clutch motors are an enormous advantage for some riders. The feature makes the motorized bike easier to operate and provides it the ability to pull away from a standing start.
4 stroke motors always feature pull start mechanisms which can be essential for some projects (slider trikes as mentioned before).
The third benefit of a 4 stroke bicycle engine kit is that they are relatively quiet and most people prefer the engine tone.
Finally, you do not have to mix oil with the gas although 4 stroke engines require regular sump oil changes.
Negatives
2-stroke bicycle engine kits outperform 4-stroke engines in most cases. The very features that distinguish 4 stroke bicycle motors are also their downfall, as automatic clutches on low-cost engines are rarely adjusted properly. The engine often has to over revved to engage the clutch, which then grabs, and jolts the rider forward. Alternatively, the clutch engages at low revs and the motor struggle to pull away from the sidewalk.
Adding to the problem is the pull start which is rarely set properly. Often, the start cable will extend 3/4 of its travel before it catches the pull start mechanism. This can make the motors difficult to start.
- The pull start mechanism is also unreliable and I often encounter pull starts that break after only a few days of use.
- 4 stroke bicycle motors are underpowered and more expensive than their 2 stroke counterparts.
- 4 stroke bicycle engines are bulkier, and wider, making installation more difficult. They require pedal spacers to allow the bicycle pedals to turn, which are rarely included in kits.
- 4 stroke engines themselves are less reliable than other bicycle motors because of the increased number of moving parts.
If you purchase a 50cc 4 stroke motor, you can expect a cruising speed of around 20MPH.
Friction drive engines are compatible with just about every bicycle frame. Friction drive motors are perfect for anyone who has a frame that isn’t suited to a regular bicycle engine kit.
Friction drive motors drive direct to the bicycle tire. These bicycle motors can therefore power a bike with any size wheels.
These kits are the easiest to fit and can be attractive simply looking for ease of installation.
Negatives
The drawback to friction drive kits is that they are small capacity, high revving motors, with low torque. Friction drive motors can help push the bike to around 15 MPH. Friction drive motors lose traction in wet weather, or when trying to power the bike up hills where riders are looking for most help.
Reliability
They develop friction drive bicycle engines from high revving, brush cutter / chain saw motors. They are less reliable than purpose-built 50cc, 80cc, or 100cc bicycle motors.
How to motorize your bike
We have improved and updated our instructions that are provided with most bicycle motor kits.
Bicycle engine kit instructions
There are a number of upgrade options for motorized bicycle engines. The most popular upgrade is the Rongtong carb.
The Rongtong is more complex than the NT carb and has a number of advantages. From our troubleshooting point of view, the Rongtong carb has fewer adjustable parts and a different float setup. The Rongtong carb is, therefore,
- less prone to flooding the engine,
- it can tolerate a greater fitting incline, and
- it is unlikely to require tuning adjustments.
If you have verified that the carburetor is working correctly then the following tests are the next stage in troubleshooting starting faults on your motorized bicycle:
3.1 Rule of thumb compression test
3.2 Piston rings
Look for broken piston rings. There are two piston rings on motorized bicycle engines but starting problems occur even if only one ring is broken.
Remove the muffler from the barrel and move the engine until the piston rings are visible. Shine a strong light into the exhaust port and look for broken rings. The exhaust port is the largest port and the hottest part of a motorized bicycle engine. This is the most common point that rings break.
If you cannot see any fault, remove the cylinder head. Lower the piston in the barrel and check for scoring on the barrel wall. If the barrel is scored this indicates that the rings are broken. You will need to replace both barrel and piston rings.
The last stage is to remove the barrel completely to visually inspect the rings.
3.3 Oil seals
Finally, inspect the oil seals on both sides of the crankshaft of the bicycle motor. The most common cause for failed oil seals is engine overheating. If the engine is run hot, it melts the silicon seals. If one seal has failed, it is highly likely that the other is in bad shape.
Remove both front engine covers from your motorized bicycle and look for signs of oil. If the oil seals are faulty you should see some oil behind the magneto on the left casing or the primary drive cog on the right side of the engine.
If there is no visible oil, it is unlikely that the oil seals are faulty. If you can see oil on either side, then you should replace both oil seals.
Conclusion
If you are building a motorized bicycle, you will find that there can be a large variance in performance of one brand bicycle motor over the same product from another manufacturer.
If you want to motorize a bicycle, the best motor for a bicycle in most situations is the 80cc 2-stroke engine kit. There are, however, good reasons to purchase other bicycle motor kits:
- 100cc 2-stroke engines have most power,
- 50cc 2-stroke engines have the best fuel consumption,
- 4 stroke engines are quieter and have automatic clutches
- friction drive motors are easiest to install.
Whatever your choice of bicycle engine kit, motorized bicycles are great fun, cheap, useful, and environmentally friendly.
