How to install a top end kit on your Honda Express or Urban Express Noped
Installing a new top end is similar to painting a room in your house. Most of your time will be spent on the prep work for painting. In the same way, most of your time will be spent on prep work before actually installing the new top end. Below I have listed the major areas you should address before, during and after installing a new top end. Don’t be in a hurry to ride after installing your kit or you may risk skipping a step or two or hurrying through a step or two. Mistakes often result in starting over from the very beginning, lost time and wasted money. It’s a great idea to have another bike that you can ride while going through this upgrade process. You’ll be more likely to take your time and do it right on the first go round.
1. Make sure your Express is running well in its stock configuration. It should have a top speed near 28-30 mph; start easily; not excessively four stroke; have good throttle response; and accelerate smoothly. A common maintenance item that can lead to poor starting, hurt acceleration and reduce top speed is a dirty carburetor. Two areas to concentrate on are the pilot jet (aka idle jet) and the seat for the float needle. You can clean the pilot jet with a thin guitar string. You can clean the seat with a Q-tip in a drill and some metal polish. Cleaning the carburetor can often resolve several issues related to poor running and starting all at the same time.
2. If your plan is to upgrade several components at the same time, change your plan. A common mistake is to buy a new exhaust, carburetor, top end kit, reeds, etc. and install them all at once. If something goes wrong, where do you start to troubleshoot? You’ve changed just about everything. A better progression would be to install the new exhaust. Tune your stock carb to work with that exhaust. Once you have that running right, install a bigger carburetor. Tune the bigger carburetor to work with your current set up. And after all that, only then install the new top end. Tune your carburetor again to work with the new top end.
3. If you don’t know the history of the crank shaft bearing seals, change them or have them changed. Bad seals will allow air into the crank case, cause your engine to overheat and ruin your new top end. I have a Honda Express (at the time I’m writing this) with a bad left side seal. I know because after a few minutes of running, my Express starts spewing out white smoke from the exhaust. It’s burning oil in the transmission case because oil is seeping past the left seal into the engine. If the left seal is bad then chances are the right one is too. Skip this step at your peril. You can ruin your top end plus jetting will be a rear nightmare.
4. Remove all of the old base gasket from the cylinder mating surface. 200 grit sandpaper does a good job without being too abrasive to the aluminum surface. For easy removal next time, smear both sides of your new base gasket with 2 stroke oil prior to install.
5. Even though your kit may have come with the rings already installed on the piston, take them off and measure the ring gap. Here’s a handy guide for measuring and calculating ring gap.
5b. Take some 400 grit sandpaper in small pieces and sand the edges of the exhaust port and the other ports as you are able. You may need a chop stick or similar device to reach inside the cylinder easily. You can wrap the sandpaper around the end and reach in to sand the hard to get to port edges.
6. After reinstalling the rings on the piston, I install one of the circlips in the piston. After that, smear the piston and rings in 2 stroke oil as well as the small end bearing (get a new one of these too as they are not often included in kits) and wrist pin. Install the small end bearing (needle bearing), piston and wrist pin into the connecting rod. Install the final circlip. Compress the rings while keeping the ring ends on either side of the ring stops. While compressing the rings, insert the piston into the cylinder and slide it in (Arrow on piston dome pointing to the exhaust port or “EX” on the exhaust side or, in the case of the DR kit, window on piston towards intake). You didn’t forget your base gasket, did you?
7. On Hondas, I often install the cylinder head, head gasket, cylinder jug and base gasket as one unit over the piston. In thinking about it, it may be easier just to install the cylinder jug and base gasket over the piston and then bolt the exhaust to the jug. You can later add the cylinder head and head gasket along with the cylinder bolts.
8. If you are installing a bigger top end kit, purchase a cylinder head that is machined to match the new piston diameter. You can often reuse the stock cylinder head but at the expense of cooling capacity and to the detriment of performance.
9. A different cylinder head will often be bigger but thinner. In that case, you may have to use cylinder studs or bolster that are shorter than the stock ones and even add washers to the bolts/studs.
10. Obtain a torque wrench and torque the head bolts/nuts to the proper torque.
11. Perform a comprehensive break in in order to seat the rings.
12. For bigger kits like the DR kit, I prefer to run premix at a 32:1 ratio with a reputable 2 stroke oil (high viscosity and high flash point).
13. Reeds. I imagine performance reeds can improve throttle response. Stock reeds will work just fine as well.
14. Tune your carburetor to the new kit by increasing the main jet size and tuning the needle as necessary. A bigger carburetor will offer more cooling ability and will help you achieve the potential of the new kit. When tuning the carburetor, consult the moped community for advice on a proper main jet range. Purchase several main jets and start with the biggest one and work your way down.
15. A temperature gauge is a $39 insurance policy to keep your kit alive otherwise you are running blind. For most kits, temperatures approaching 400F are a potential disaster. You can save your kit by shutting down when temps get this hot. Walk it home and figure out the issue. Fighting high temperatures will be your main concern with bigger top end kits.