Improving The Honda 160 Vintage RoadRacer


So now you've gotten your Honda 160 roadracer built, and it runs fairly well.   What now?   We here at Group W Racing won't let you down, we've got some ideas for you to play with.   This section will give some ideas for slight improvements to your 160 roadracer.   Several of the tips will be in how to recover some horsepower lost due to age and the realities of factory assembly.   None of these tips will be huge improvements, rather small incremental changes.   With the horsepower we have the biggest improvement you can make is in your riding.   Small mistakes make huge differences in lap times, and a good rider on a slow bike will still do just fine.   You should also make sure that you've gotten your bike running with a stock setup before you start experimenting.   Otherwise, you might end up in tuning hell, not sure which of the several modifications you've made is causing you problems.   I also suggest that when undertaking changes you do them one at a time so you know whether or not the modification is working.

None of us want to start a horsepower war or cause anyone to need to spend money to keep up - that would really be outside the spirit of our vision of 160 racing.   That's not the point of this article.   I'm willing to help any of you out with doing anything here if you need it.   You'd need to come over to my shop when I've got some free time, but you're more than welcome to come.   We don't want any of you left in the dust - let's keep our racing cheap and competitive!

I will eventually get some pictures up to match the tips, but for now it's just written tips.


I mentioned this in the original 160 building article, but it can stand mentioning again.   Proper gearing will go a long way toward making your 160 faster.   This is one place that can make a huge difference.   If you're geared too low you won't be able to keep up on the straights, and in fact the same goes for being geared too high.   If you're tapped out and the bike has quit pulling halfway down the straight then you're losing time and speed.   If your bike can't pull top gear down the straight then you're also losing time and speed, but that's not as likely as being under-geared.   It is good to remember you'll be drafting, and err a bit on the side of being over-geared.   Gearing also affects you in the corners, and changing overall gearing will likely change your shift points, and may even change what gear you take certain corners in.   Some thought will have to be given to whether the changes you make work for you all around the track.   My roadracer is now geared 18/36 from the Spokane race and it worked very well there.   I left it that way for the 6/23 Seattle race, and it worked great there too.   In fact, with that gearing and the draft from Tim Keane's Ducati 250 I was able to set a new 160 lap record of 2:04 point something-or-other.   The higher gearing allows me to shift down for some corners that I had to take in fourth gear before (like turns two and six/seven at Pacific Raceway), and I think I get a better drive out of the corners that way.

The gearing suggestions I'm making are for bikes running the smaller 80/90 rear tire.   If you're running the 90/90 rear tire you'll probably need to adjust the numbers by about 3% or one additional tooth on the rear sprocket.

Here's a gearing chart for figuring out final drive ratios - larger ratios are higher gearing.

Gearing Ratio

Sprocket Specialists makes gearing for the 160s and it's not too expensive.   If you're in the Seattle area give Sparky at Motorcycle Works of Renton a call and let him know you need gears for a 160 - he knows exactly what you need.   In the Portland area I'm sure Joe at Vicious Cycles can get what you need.   I keep 17/18/19 tooth front sprockets and 35/36/37/38 tooth rear sprockets in the spares box.   That's probably a bit excessive, but I'm still sorting out what gearing works best for me at which tracks so it's nice to have that many options.


The coils on these bikes work okay, but lately we've been having trouble with some of them.   They are 35 years old or more after all, and it's not suprising some of them are giving up the ghost. Tom Deem had two that had intermittent failures that were very hard to track down.   Michael Collins failure was easy to track down on the other hand, as visible smoke was pouring from the coil.   At a minimum, even if you're running a stock coil, you should check the spark plug cap connection at the end of the plug wire. They're threaded on the plug wire, and often the wire at that end is non-existant and the spark is merely following the carbon track of burnt copper where the wire used to be.   If that's the case you need to clip off 1/4" to 1/2" of wire to get back to where there's actual copper wire and thread the caps back on.   Make sure to measure the resistance of the cap to be sure it's within specs and not part of the problem.   At least half of us are now using aftermarket coils.   I have an Accel 3.5 ohm coil that I bought on ebay, and several of the Portland Mean Bees are running the same coil.   It mounts in the stock position with a small amount of metal removal with a file to make clearance for the coil posts, or in front of the engine mount with a supplied bracket.   It'll be fairly obvious what needs doing.   It seems to give a more consistant spark, but in my case the bike was running fine before and the coil is more of a mental talisman.   Make sure that whatever coil you get is designed for points ignition.

Also - check the timing regularly.   It does drift around some and if you've got a crusty points cam it will chew away at the points rubbing block, change the point gap and move the timing.   Bad spark timing can cause damage to the engine in addition to power loss, so it's a good idea to keep a fairly close eye on it.

Points Replacement

Speaking of timing - if you get tired of messing with the points there is an alternative.   Pertronix is a company making hall-effect sensor points replacements for cars.   They're very popular with the Volkswagen crowd.   They work by using magnets mounted in a plastic sleeve to trigger a hall-effect sensor which in turn triggers the coil.   This last race at Seattle (6/23) I experimented with (successfully) adapting the Pertronix "Ignitor" to the 160.   I'm using part nunber 1847a which is intended to fit the Bosch 009 distributor.   I machined an adaptor to mount the magnet sleeve (with two magnets removed) to the end of the cam and mounted the sensor on a spare points plate.   There is a write-up on this on the Moto Zastrow site, and I'll have one here (with pictures) soon.  

Stock Carburetors

You can free up a small amount of horsepower by improving the flow through the stock carburetors.   If you look down the carburetor throat, you can see that the carburetor is bored in a sort of figure 8 shape, roughly 18mm wide by 22mm high, with a 20mm wide slide.   You can remove some of the metal from the left and right side, making it closer to round.   I use a combination of a dremel, a half-round file and a rat tail file.   It's good to have a junk set of carburetors to practice on first.   Be careful when you do this, you don't want to make it so wide that you uncover the slide. 19.5mm is probably the practical limit. Also be careful to not remove any material from the roof or the floor of the carburetor throat.   You can also smooth down some of the casting bumps in the intake side of the carb throat that disturb the airflow.   As mentioned in the building article, running velocity stacks will also help smooth the flow into the carburetor and allow the engine to breathe better.   These are small improvements and hard to quantify, and sometimes it seems it doesn't work.   We tried the larger carbs on one of the spare bikes and it didn't work well at all.   Brian Halbert has done this with his carburetors and while his bike runs fine, he is unsure of whether or not it made any difference at all.   Make sure if you do this that you have a working stock setup to fall back on if it doesn't work for you.   It may be just as easy to make problems as it is to make improvements here.

Mikuni Carburetors

Simon-Pierre Smith and Dan Hill have gotten frustrated with the stock worn-out 35 year old Kei'hin carburetors, and have gone for aftermarket Mikuni Carburetors. They've been spending considerable time and energy trying to get them dialed in, and Simon says he thinks he's got it figured out.   Once he does, he'll do a write-up for the web page here and you can see what his set-up ended up being.

Other Carburetors

SuperHawk carburetors are 22mm, and should work fine on the bikes if they're breathing well from the exhaust.   You'll need to make an adaptor to get the carbs to fit the stock manifolds, just like Dan and Simon-Pierre had to do for the Mikuni Carburetors.   I've got no idea where to start with jetting though, as I haven't experimented with these carbs.

Cam Degreeing

According to Craig Hanson, one of the big differences between "fast" and "slow" 160 engines is in the cam timing.   This is a place where you pick up lost power, not make new power.   It won't make a bike faster, but it will bring a slow bike up to the same speed as the others.   After 35 years, the cam chains have stretched, the cylinders may have been decked to make them flat, the head may have been decked to make it flat, all of which retards the cam timing.   On top of that, there's natural variations from the factory.   If you take the time to check the cam timing and set it, you'll be assured that you're not losing power to a retarded cam.   This isn't hard but it is tedious and requires some specialized equipment.

This will be the subject of a separate article soon, but for now here's some information to get you started.   A search on the web for "cam timing" will get you started on what it is and how to do it.   You'll need a degree wheel, a piston stop and a dial indicator and base.   You'll also need a hydraulic press as the gear is pressed onto the cam.   Once I get a good piston stop made for the 160 that doesn't hit the intake valve, it'll be available for loan.   Depending on what's going on in the shop I may also be able to help you here at the Bateman Racing Skunkwerkes.   You're shooting for an intake lobe center number of 101 to 104 degrees. Anything larger than 104 is bad.   Mine was at 106.5 degrees, and one degree can make a difference.   If you find yours out like that you'll have to remove the cam, mark the current position of the cam, press the gear off and press it back on in the new position. Remember that the cam is spinning at half of the engine speed, so a one degree change at the cam will make a two degree change on your degree wheel.   I know that for most of you this is gibberish, but I promise you it isn't hard and will make sense if I show it to you.   You will most likely be advancing the cam (which will make the lobe center number go down). To do this you will be moving the cam forward in relation to the gear.   Think of the engine being assembled with the cam chain in place, then taking ahold of the cam on either side of the gear and turning it forward (the same direction as normal engine rotation) while the gear stays still.   That's the direction you want to move it when you press the gear off and back on.   You then assemble everything and check it again, doing it over if need be to get it right. It took me several tries to get mine where I wanted it, as it takes such a small change at the cam to make a big change on the degree wheel.


Most of these bikes have what appears to be a pretty horrible setup when it comes to the front forks.   In spite of this it works great anyway.   The upper triple clamp doesn't actually clamp the forks, the fork caps just go through it and locate the tops of the forks.   This doesn't resist twisting very well at all.   Some of the later model 175 forks do have a clamp-type upper triple clamp which should work better.   For what it's worth, until this last race I've been racing on the stock 160 setup and it's been working great for me.   The stock suspension setup really is fine as long as you use good shocks in the rear.   These little bikes handle way better than you would think.   One of the Portland riders commented that he's invested over $1,500 in the forks of his 350 roadracer, and the stock 160 forks handle better than his expensive setup on the 350.   I experimented with later model 175 forks this weekend and they worked just fine, but I don't think they were any better than the original stock 160 setup.   It does mean that if your 160 forks are shot you can find replacement front ends in the boneyard cheap - look for vertical model 175 heaps that have good forks.

If you haven't gone to Hagon shocks yet and you're having difficulties with suspension, it's probably time to pony up for the good shocks.   Give Dave Quinn a call and order some Hagons.

I'll try to get some pictures up here to go with these tips, and if you've got any tips you'd like to see up here feel free to send them to me.   Good luck, and I'll see you on the grid!




Copyright © 2002 Michael Bateman
Photographs Copyright © 2002 Michael Bateman