.com
| Building a 125 Classic Class legal MX engine By Terry Miller |
Now that basic motor is finished it is time to deal with finishing touches.
Cam installation can be set to "straight-up" and this works well most of time. Powroll makes an index-able timing
sprocket that is a great addition. Advancing by 2-4 degree's tends to help mid range and bottom. Retarding is
usually not recommended as acceleration is diminished and MX use needs as much acceleration as possible.
PS use Loc-tite on bolts on cam gear as they do come loose.
Many people tend to run a stock carb or too large a carb. A basic XR200 carb is about right size. 26mm
maximum(Unless full-race) and by using a Honda carb the jetting is pretty close right out of box. There are
specific "Clone" carbs that work great. I have found one brand that is near perfect and only requires minor
mods. Other clone carbs just seem to work poorly or only in certain area's. I can sell you one pre-jetted(with a
couple extra jets) for less than you will spend screwing around trying to get yours to work. I am working on new
manifold/Mikuni combo and if I get all slide/needle jet/pilot/main jet set properly I can offer those later this year
also.
Airbox really needs opened up. Quite restrictive and filter is too small. Do not just put a foam filter on end of
carb as this just doesn't work. You need some additional runner length from carb to filter to get these to carb
right. A friend is making a fiberglass airbox but not finalized yet.
A proper exhaust really wakes these up. It took a while on dyno and track to work out best pipe. Length and
diameter is critical along with muffler design. I will try to get some pipes made for people to buy as when we got
everything "right" it was worth a lot of power. Right now I will only make the down style with swept up exhaust but
may make a Up-pipe later. I personally prefer down pipe as it keeps heat away from rider AND motor.
Ignition is a "Bear" as AHRMA has specific rule on Honda only. It specifies points type ignition. This leaves you
choice of stock or total loss type ignition. Only failures we have had at races is due to ignition. Part design and
part human error. I dislike this rule as Honda 150 is ONLY bike in all of AHRMA to require this. Rule was
originally in place to keep people from using XR185-200 parts(cheater bikes) as very similar in appearance. If
running stroker with 11-1 or higher I suggest you set timing to first of total advance marks at rpm. Don't base
timing off of initial as every advance unit is different. Always set timing at total when running 4-5000 rpm's and
rev it up higher and check that it doesn't go any higher than factory marks.
Set valves to camshaft specs, add oil and fire it up. Don't let it idle but bring up to 2-4000 and run up and
down. Listen for any noises. Usually you will hear valve tapping for 3-8 seconds before oil gets to top-end(you
did use good assembly lube??). Leave intake tappet cover off and verify that oil is flowing. Run for a minute or
so and shut-down. Let cool for 30 minutes or so and fire up and run for 2-4 minutes blipping throttle but don't
wind too high. Now drain oil and remove screen and check and clean. Let cool till completely cool. Re-set valves
and re-torque head.
Now you can refire and check timing again and take a short easy ride. Get motor warm but not hot. shut down
and let cool to touch. Fire it up and ride again. Now drain oil after 30 minutes or more of running. Re-check
valves etc. Ready for first moderate hard ride. If any carb problems this is time to fix them. On Powroll's site
there is a pretty good article on how to set 4-stroke jetting. It works...
Cam installation can be set to "straight-up" and this works well most of time. Powroll makes an index-able timing
sprocket that is a great addition. Advancing by 2-4 degree's tends to help mid range and bottom. Retarding is
usually not recommended as acceleration is diminished and MX use needs as much acceleration as possible.
PS use Loc-tite on bolts on cam gear as they do come loose.
Many people tend to run a stock carb or too large a carb. A basic XR200 carb is about right size. 26mm
maximum(Unless full-race) and by using a Honda carb the jetting is pretty close right out of box. There are
specific "Clone" carbs that work great. I have found one brand that is near perfect and only requires minor
mods. Other clone carbs just seem to work poorly or only in certain area's. I can sell you one pre-jetted(with a
couple extra jets) for less than you will spend screwing around trying to get yours to work. I am working on new
manifold/Mikuni combo and if I get all slide/needle jet/pilot/main jet set properly I can offer those later this year
also.
Airbox really needs opened up. Quite restrictive and filter is too small. Do not just put a foam filter on end of
carb as this just doesn't work. You need some additional runner length from carb to filter to get these to carb
right. A friend is making a fiberglass airbox but not finalized yet.
A proper exhaust really wakes these up. It took a while on dyno and track to work out best pipe. Length and
diameter is critical along with muffler design. I will try to get some pipes made for people to buy as when we got
everything "right" it was worth a lot of power. Right now I will only make the down style with swept up exhaust but
may make a Up-pipe later. I personally prefer down pipe as it keeps heat away from rider AND motor.
Ignition is a "Bear" as AHRMA has specific rule on Honda only. It specifies points type ignition. This leaves you
choice of stock or total loss type ignition. Only failures we have had at races is due to ignition. Part design and
part human error. I dislike this rule as Honda 150 is ONLY bike in all of AHRMA to require this. Rule was
originally in place to keep people from using XR185-200 parts(cheater bikes) as very similar in appearance. If
running stroker with 11-1 or higher I suggest you set timing to first of total advance marks at rpm. Don't base
timing off of initial as every advance unit is different. Always set timing at total when running 4-5000 rpm's and
rev it up higher and check that it doesn't go any higher than factory marks.
Set valves to camshaft specs, add oil and fire it up. Don't let it idle but bring up to 2-4000 and run up and
down. Listen for any noises. Usually you will hear valve tapping for 3-8 seconds before oil gets to top-end(you
did use good assembly lube??). Leave intake tappet cover off and verify that oil is flowing. Run for a minute or
so and shut-down. Let cool for 30 minutes or so and fire up and run for 2-4 minutes blipping throttle but don't
wind too high. Now drain oil and remove screen and check and clean. Let cool till completely cool. Re-set valves
and re-torque head.
Now you can refire and check timing again and take a short easy ride. Get motor warm but not hot. shut down
and let cool to touch. Fire it up and ride again. Now drain oil after 30 minutes or more of running. Re-check
valves etc. Ready for first moderate hard ride. If any carb problems this is time to fix them. On Powroll's site
there is a pretty good article on how to set 4-stroke jetting. It works...
Page 4
Finishing touches
. Once you get all tuning done I suggest you use same brand of fuel from now on. Also since these only use 1
quart of oil use the best and change it after every race date. Full synthetics seem to ruin clutch plates on little
Honda's so I suggest a semi-synthetic. Powroll has one of the best oils available and Honda has new oil but
very expensive(10.00 a quart). straight 40 or 20-50 is suggested.
Fuel is of importance on these. Don't use 120 octane thinking it will make your bike faster. The opposite is
true. You only use the least amount of octane your motor needs. A couple points extra can be used as safety
factor. 110 octane in a 10-1 motor will make less power than good 93 octane. Our little 100 runs 110 octane
and it is north of 13. 114 octane should only be used if over 14-1 and if you are running more than 14 you
better have everything right!!!!
Remember these articles are basics and there is many little things that can be done to increase performance
and reliability. Truthfully a good 11-12 to 1 150 stroker with moderate cam and porting will run pretty close to a
full out racer. The rider becomes more important once you get to that point. You will also find that gearing/tire
choices and throttle control will make up for that little bit extra you need. Don't run a 12-13 tooth countershaft,
use a 14-16 and use a larger rear. Use a quality chain. Make sure brakes are not dragging. Make sure wheels
rotate with least amount of drag. If you do all the little things then the whole package works like it is supposed
to.
Hope this helps anybody interested in these. I will offer advice as to combo of parts and can build entire motor
or just do individual parts. Fellow racers have had me build motors I wish I had!!!! If you see me at races tell
me your thoughts or e-mail me. Always looking for a little more info. Terry Miller
quart of oil use the best and change it after every race date. Full synthetics seem to ruin clutch plates on little
Honda's so I suggest a semi-synthetic. Powroll has one of the best oils available and Honda has new oil but
very expensive(10.00 a quart). straight 40 or 20-50 is suggested.
Fuel is of importance on these. Don't use 120 octane thinking it will make your bike faster. The opposite is
true. You only use the least amount of octane your motor needs. A couple points extra can be used as safety
factor. 110 octane in a 10-1 motor will make less power than good 93 octane. Our little 100 runs 110 octane
and it is north of 13. 114 octane should only be used if over 14-1 and if you are running more than 14 you
better have everything right!!!!
Remember these articles are basics and there is many little things that can be done to increase performance
and reliability. Truthfully a good 11-12 to 1 150 stroker with moderate cam and porting will run pretty close to a
full out racer. The rider becomes more important once you get to that point. You will also find that gearing/tire
choices and throttle control will make up for that little bit extra you need. Don't run a 12-13 tooth countershaft,
use a 14-16 and use a larger rear. Use a quality chain. Make sure brakes are not dragging. Make sure wheels
rotate with least amount of drag. If you do all the little things then the whole package works like it is supposed
to.
Hope this helps anybody interested in these. I will offer advice as to combo of parts and can build entire motor
or just do individual parts. Fellow racers have had me build motors I wish I had!!!! If you see me at races tell
me your thoughts or e-mail me. Always looking for a little more info. Terry Miller