Thread: Low Budget Performance Upgrades

I have a 1973 Honda CT70. A few years ago, I over revved it and bent both valves. While I had it apart replacing the valves, I decided I would add a few upgrades to the head, including a bearing race cam and heavy duty valve springs. I also went with a thin copper head gasket (.36 mm thick) thinking that would help on compression ratio.

Now, after adding the new cam, I think the carburetor needs to be rejetted. I have adjusted the main jet all the way in, and it still is running way too lean. Since I put the larger cam in there, I was wondering if I would see much of a performance difference if I went with a 20mm carb, larger manifold, and after market air filter. Dratv has a kit for $80, that I am considering if this will make a noticable difference. Please give me your opinions.

I have found the race cam will actually hurt overall performance, you may find different, IF you get the right setup

With a larger cam you will likely target higher rpm horsepower and sacrifice some low and even mid range torque, depending on how big the differences are in the cam itself

Without going into unnecessary details you can play by simple rules:

- Larger cam needs more air flow in, more fuel in, better exhaust flow out
- Higher compression is better
- Ignition advance probably needed
- Needs to stay higher in rpm band to achieve useful power


Tuning a cam to engine marriage is a form of art, and never is achieved first try, you will need to tune your setup, likely with trial and error

This is exactly where misconception on what horsepower is, and what it means takes place

Often times bikes with a huge peak horsepower number are slower overall than bikes with a wide rpm power band and mid range torque

The kicker is, without adding displacement it is very difficult to achieve wide power bands and low end torque, hence the reason so many people are moving to larger displacement engines in these bikes

As the old saying goes "there is no replacement for displacement"

In short, if you don't change the setup around your race cam, you may actually see a decrease in performance compared to stock setup

So, what you are saying is that with this large of a cam and the stock carburetor, I am in a sense starving the engine of fuel and air? In order to improve fuel and air flow in, going with the larger carb and manifold would help. On the other hand, I don't have many options for improving exhaust flow out except changing out the exhaust system. Also, what are the sizes of the intake and exhaust port on this bike? Theoretically, it wouldnt do any good to add a manifold larger than the intake port would it?

As far as increasing compression, the thinner head gasket should help in that some. I am not sure how thick the stock head gasket is, but the one I put on is .36mm thick. It is almost paper thin. Other than that, I don't have any other options for increasing compression without adding more displacement.

Eventually, I would like to add a big bore kit to this bike, but right now I don't have the money to spend on one of those. Since I put the larger cam in, I actually have noticed a difference in the high rpm range. It seems to have a little more horse power. On the other hand, I did notice a big difference on low end torque too. The bike is very sluggish taking off, but once it gets wound up, it is pretty quick for a 70cc. Are there any other things you would reccommend me doing to increase the power of the bike with out dishing out a load of money?

You might be able to jet the carb to a point where it can supply enough fuel. Keep in mind lean conditions do not just run poorly, they often lead to overheating and damage if it gets lean enough

You should step up your jets and probably play with the timing a little bit

If the cam is a race cam (meaning much different than stock), it really should be used with a race head, big bore kit, and larger intake, carb and exhaust

There is no single piece of magic to bump up engine performance, the entire engine is a system and should be treated as such

Immediate thoughts are to change the sprockets to get you into the power band quickly, but then you lose cruising speed

On my stock 70cc, I decided to put the stock cam back in, as even with a well tuned setup I really couldn't drive it on street, with decent cruising speed and still have the low end torque I needed

The only option was to keep it wide open all the time, and for street riding that isn't practical

On a race bike where the throttle is open all the time, a race cam works well, if that is what you are doing

I don't ride this bike on the street, so cruising speed is not something I am concerned about. I have a track in the back yard that I ride on, and when I am riding it, I normally keep the rpm's up. This being said, I will probably keep the race cam in, since I am not concerned about low end torque for this type of riding. However, I will try rejeting the carb and playing with the timing. Do you know what size main jet comes with a stock carb? Right now, I have it screwed all the way in. What size would you reccommend me starting with? Also, will the aftermarket air filters that are sold on Dratv fit on a stock setup, without the 45 degree turn out manifold? Would I want the straight air filter or the turned out air filter for this setup?

Maybe eventually I can rebuild this engine and put a big bore kit along with a stroker kit to bump up the displacement. I can get both kits along with the carb and manifold for around $325 from dratv.

Also, whats your opinions on spark plugs? I have always ran the stock NKG plug. I saw on dratv he has a "Twin Fire" racing plug. It claims to produce a small amount of extra power. Would this help going along with the large cam? More spark will mean better combustion, and a larger cam allows more fuel and air in the engine to be combusted.

Be carefull with that bearing cam,i have seen them tare up the top end on a ct70.they tend to walk in he head causing the rockers and cam to prematurely wear,eventually wiping out.also a great choice for you,and ''pricewise'' is a pz19mm carb with the left hand choke and dual inlet on ebay for around 30 bucks delivered.you may be able to just bolt it on with no jet change required.and a loud freeflow exhaust,search ebay ''pit bike exhaust'' is in you future too!

I have had no luck with that bearing cam in a larger eng and IMO its to large for a 70cc eng. You might try a SL or XL hot Honda cam, It will run a lot better all around, also PC's right with that 19mm carb. Good riding!

After looking at the specs of the race cam I have now and comparing them to the hot cam, here is what I found:

Race Cam
Lift: 6mm
Overlap: 40 degrees
Intake duration: 255 degrees
Exhaust duration: 265 degrees.

Honda Hot Cam
Lift: 5.1 mm
Overlap: 10 degrees
Intake duration: 215 degrees
Exhaust duration: 225 degrees


Am I looking at the correct Honda hot cam or are there more than one? Also, where is the best place to get this cam? Dratv has the hot cam for $45. Are there other places to get it cheaper? How much of a performance increase would this be from the stock cam?


Also, about the 19mm carb. What size carb comes stock with a k2 ct70? I was thinking it was either an 18 or 19 mm. If I went with a 19mm carb would I need to replace the intake manifold as well?

I think the Honda hot cam is just an SL70 cam. you could get a good used one of those on ebay for considerably less than $45.

The stock carb is 16 mm if memory serves.

I could not find any SL70 cams anywhere. The only place I have seen the hot cam was on dratv. I searched ebay and other websites.

Also, would removing the spark arrestor from the muffler help improve flow, which would also improve power? If so, how do you remove the spar arrestor from these mufflers?