If your Honda Express won’t start, it could be due to poor compression, timing, fuel delivery, and/or spark. You may have to evaluate all four in order to determine what is making your Honda Express hard to start or not start at all.
Compression – The easiest thing to determine is your bike’s compression – or it should be. Some automotive compression gauges will give a very low compression reading because they are designed for bigger engines. You should get a compression gauge for small engines. While there is no exact reading or Mendoza Line below which your Express won’t start and above which your Express will start, your compression should be at least above 80 psi to allow the bike to start.
Of course, you may have multiple issues contributing to the inability to start your bike. So your compression could be 90 psi but you also have a fuel delivery issue. The two combined (relatively low compression and lack of fuel) may be working together to cause your starting issue.
If you don’t have a compression gauge, you can test compression by holding your thumb over the empty spark plug hole while you kick over the bike. If you have a wind up starter, this is even easier to test. If your thumb gets pushed away easily when you turn over the bike, you should have enough compression to start the bike.
If your compression is poor, you will need to remove the cylinder head and check the condition of the cylinder, piston and rings. A visual inspection of the cylinder with the piston moved back towards bottom dead center should tell you quite a bit. Significant scoring in the cylinder walls usually suggests that the rings are also worn and compression is suffering as a result. A cylinder honing, new piston and rings may be in order.
Before removing the cylinder head, you can visually inspect the areas underneath the head and base gaskets. If you see gas/oil mixture leaking out from under either gasket, you have a leak that is causing compression loss. New gaskets may correct the problem or you may have uneven mating surfaces between the head and cylinder or cylinder and case. In that case, you can sand the mating surfaces flat (at least between the head and cylinder) by using sandpaper taped to a piece of glass and sanding in figure eight pattern. Be sure to thoroughly clean all metal dust from your cylinder and head before re-installing.
If your crank case bearing seals are original to the bike, they may have hardened up over time and are no longer doing their job of keeping additional air from entering the engine. If they are bad enough, they will make starting difficult. With the proper tools, you can do a leak down test to determine how the seals are performing. If they are original to the bike, it’s very unlikely that they are still good, and you will need to replace them.
Fuel Delivery – You may need to go over every component of your fuel system to check for adequate fuel delivery. Your gas cap has a vent hole in it. If this vent hole is blocked, fuel will not flow as it should through your fuel system. You can test this by trying to start your bike with the gas cap tightened on the tank and then with the gas cap removed from the tank. If your bike starts and stays running with the cap off but not with the cap on then it’s likely your vent hole is blocked on the cap. If your bike won’t start to test this then you may have to locate the vent hole and clear it of obstructions. You can usually do so with a small wire.
Gas tank – Is your gas tank free of rust, varnish, and other foreign material that would inhibit fuel flow? If not, you will need to clean your fuel tank. Here is a splendid article on various methods for removing rust from your fuel tank.
Petcock – You will also want to check for adequate fuel flow from the petcock. You can do this by removing the fuel line from the petcock and then turning your petcock valve to the “on” and “reserve” positions to check for fuel flow. Have a container ready to catch the gas that hopefully comes out. I’ve had petcocks that were clogged with rust preventing fuel flow. You can either clean your petcock or install a new one.
Don’t make the rookie mistake of over-tightening your petcock when you re-install it. Over-tightening will crush or deform the o-ring inside the petcock and result in leaks. Some petcocks (especially aftermarket ones) will draw fuel from a higher level than your original petcock. So it may appear that you have some gas in the tank but no gas is flowing from the petcock. You either need to add more gas to the tank or adjust the level where the petcock draws fuel.
Fuel line and fuel filter – Don’t make the other rookie mistake of not running a fuel filter just because the petcock has a little screen built into it. That screen does a poor job of filtering. You should install a paper fuel filter between the petcock and carburetor. Don’t make yet another rookie mistake of thinking that a fuel filter will filter out all the rust from your rusty gas tank either.
In relation to starting issues, your old fuel filter may be clogged or partially clogged. Test fuel flow after the filter by removing your fuel line from the end of the filter and turning your petcock on. Add a new fuel filter and test again to make sure fuel flow has improved.
You might as well replace your old fuel line as well. I recommend transparent fuel line so you can easily see whether fuel is flowing from the tank to the carburetor.
Carburetor – Neither the main jet nor the idle jet controls the amount of fuel in the float bowl. Fuel in the float bowl is determined by the float, float needle and seat. I’ve worked on carburetors so badly clogged that I’ve seen the brass fuel inlet (where you connect your fuel line to the carb) and the passage leading to the float seat clogged with foreign material. Run a small wire through the fuel inlet and then through the float seat to determine if any issues exist. Take a q-tip and attach it to your drill and with some polishing compound, clean the float seat after clearing the passageway to the seat with a small wire. Follow with a clean q-tip to remove any polishing compound left behind.
Idle or pilot jet – The biggest issue with Honda Express carburetors (especially original ones) is likely to be a blocked idle jet. The idle jet is just a long brass tube right next to the main jet. If you remove the chamber cap and slide from the carburetor, you can hold it up to the light and you should be able to see light coming through a tiny hole inside this long brass tube. The hole is only about 0.30mm in diameter so you will need a very thin wire to clean it. I like to use the high string from a mandolin to do so. The thinnest guitar string usually won’t work because it is still too thick. You can remove the idle jet but I don’t recommend it. Clean it with a wire while it is installed in the carb.
If you remove the main jet, you can do the same thing and you should see a slightly larger hole with light coming through. If not remove any debris in the passageway or in the main jet. You can go a step further and remove the emulsion tube above the main jet. I just take a golf tee and push down on the emulation tube through the slide chamber. Take a wire and clean all the small holes in the tube.
You can clean the choke passage with your wire as well. You’ll see a small hole leading up to the choke chamber if you look carefully. Finish by removing any rust or sediment in the bottom of the float bowl. An ultrasonic cleaner with appropriate cleaning solution can do a good job cleaning your carburetor but you will have to address the areas above by hand.
Spark – Test your spark by turning the key to “on” or “run”, turning your kill switch to “run”, attaching a spark plug to your plug wire and holding the tip against the cylinder, and then turning over the engine. If you get spark, it should be a fat blue spark that is consistent throughout the time that the engine is turning over. If you get a yellow spark and/or a spark that is inconsistent, you need to improve your spark. Take care to avoid getting shocked by the plug by keeping your hand and fingers on the insulated boot.
I usually begin by cleaning and re-gapping the points (if you have a 1977-1980 Express). Here’s a good video illustrating the process of cleaning and setting points on a Honda Express. You may have to clean the points several times. I should repeat that. You may have to clean the points several times. The same applies to the carburetor. You may have to clean the carburetor several times.
You may have to systematically go through the entire system related to producing spark to discover your culprit. That system would start with the spark plug > connection of spark plug to boot > connection of boot to ignition coil wire > ignition coil > wiring of ignition coil > proper wiring of battery to 4 pronged wiring plug (if you have owned a lot of Express bikes, you will eventually come across one where the previous owner used a copper wire to rig the four pronged plug) > wiring of stator to wiring harness > as well as the stator.
You may even have to check your kill switch and your ignition switch and the wiring associated with both if it comes down to it.
These tips should pretty much cover almost everything that would affect starting on your Honda Express. I will add additional information and links to this article over time.