1970 cb175 compression/coil prob

Ok, I'm very new to all this engine stuff! I've had the bike for about three months, was running pretty good aside from it idling at about 3k. But then it just started cutting off after it got hot, sometimes after just a min and most of the time when it would run, it would back fire and only get up to about 25 or 30 mph!

So I took it to the shop and he got it running again. He changed the plugs, set the timing so on and so forth. He told me that my compression is 60 on one side and 80 on the other. He said it has to idle at 3k because of the compression ans he showed me how to move the slides. He basically told me that since the bike is so old, its just a show piece and would never be reliable enough to be a daily driver, unless I put a lot of money into it.

So now it wants to run.....It hates to idle.....and still after about 15 min of driving, it cuts off and needs to cool off before it will start again!

A friend told me it might be the coil, but like I said, I have no idea, I just want to ride my very sweet vintage bike!

Anyone know how I might go about getting her back to "reliable"?

Thanks a lot...If you have any more questions, just let me know!

Those compression numbers are very low and indicative of an engine that needs a healthy serve of TLC. The mechanic is half way right. If you had to pay him to do the work at regular shop rates it would be a bank breaker.

But fortunately there's a lot that you can do yourself if you have the inclination. If you want to learn, start with a shop manual and a parts list. I find the parts list to be the best exploded views of parts. I can't always tell what every part looks like but I can get a really good idea of what goes where and in what sequence.

Next you have to decide what you can afford to do and what you would like to do and work up a To Do list.

Start with the chassis. If the tires are cracked or worn out and the brakes don't work and the forks are leaking, it might not be cost effective even if you do it yourself. Post some pics so we can see what you are dealing with and i'm sure people will chime in with ideas. They are very simple bikes and lots of people here learned how to wrench on them.

Parts are reasonably cheap and mainly available and there are lots of helpful people hanging around here.

That's some good info - big +1. You can either buy a run down bike for cheap and pay to fix it up, or buy a new bike that is reliable (but still needs maintained). IMO, I would rather become very intimate with every nut and bolt on a two wheeled machine so I KNOW when a problem is coming up.

You're running for a while, dying, cooling down, and running again problem sounds like a coil on it's last spark. Also, an old coil will lose its spark at higher rpms, so that would explain that too.

And fortunately Coils and condensers are relatively cheap as long as you choose aftermarket parts and not genuine Honda. PartnMore have condensers for 6 bucks and various places sell double lead coils from 3 to 5 ohms for 50 to 80 bucks a pop and less on ebay.

Thanks for a start guys! I would really like to know how to work on my own bikes! I do most the work on my cars, and am not afraid of a wrench, but I just dont know much about small engines. My other prob is that I really would like a bigger bike, cb350 or so. But at the same time I have tons of ideas for this bike and it would really like to cafe it out and use it a sometimes rider/show bike! So ill prob end up keeping it! I'm only into the bike about 8 hundred bucks, but I dont have a title, so it will be another $200 to get a bonded one.

Here are some pics!

That's pretty clean and more or less complete. Too nice to trash, but the compression numbers are not good. These engines are just like lawnmowers and weed whackers - small and easy to work on. OK so they are a little more complex than that but not by much.

If you really want a 350 or 400, might as well sell this one on now and put the cash toward that bigger bike.

If you want to hang onto this one, get a manual first and read it twice while the title is coming through. Without a title it has no value and no point in sinking cash into it until you know for sure it's yours to keep.

The only shop I know down your way that I trust is AF1Racing.com. Micah and I worked on Mototech RS250 Cup bikes a few years ago. It's his shop and he's a good guy and he knows his shit. At the very least he should be able to point you at the best places to get things machined if he can't do it.

Cool, thanks so much for all the help! Ill try to get my hands on a manual and get my title. I think I should use this bike as a learning experience and get elbow deep into some grease! Hopefully I don't have to spend much more than about $500 to get her into a good working order. I'm sure yall will let me know if I'm getting myself in to deep!

You can download a copy of the factory manual or buy a hard copy (remember them?) I prefer paper copies because I can read them and scribble notes and highlight sections. And it's hard taking a PC into the bathroom, bedroom etc.

If you do it yourself, take your time and double check things as you go. They are easy to work on. An engine stand is a good investment. I usually make them out of a length of 1/2" or 3/4" square steel tube or 1" Aluminum from the hardware store and I get someone to weld them up. You could probably make something out of wood scraps just to stop the engine from moving around while you work on it.

Drain the oil first and be prepared for more oil to ooze out when you pull cases off. I use lots of newspaper on the bench and when I remember I put the engine plus stand into a large baking tray to catch oil. This stand was made out of aluminum angle and 1" square. You can use an engine bolt to hold it in. I get T rods made because they are quick and easy to use and no threads to damage.



Buy your mom/girlfriend/wife/boyfriend a new baking tray and liberate the old one. That way you get what you need and they give you extra brownie points.

Ok, downloaded the manual.....Looks pretty thick! Ill get it all printed out so I can highlight stuff and such.

I have a few questions!
Well first of all I guess I'm just gonna change out the coil and such to try to keep it from dying.....
But my question is how do I actually fix the compression, just rip the engine apart and put it back together. Or just replacing parts inside or what?

I guess I don't really know enough about compression to really understand. I just know its how many pounds of pressure is in the cylinders. I don't even know what my compression is supposed to be, but I guess this manual should help!

Thanks again for all the help!

sorry to but in, but where did you get the manual?

A 4 stroke cycle in simple terms is

Intake

Compression

Power

Exhaust

That happens over two complete engine revolutions. As the piston rises it compresses the gas above it and that's the pressure we measure. if the valves don't seal because they are too tight or burned or the seats are pitted, gas escapes and the compression is low. In the case of that motor, around 140 is fine and anything less than say 120 is getting weak. Not even sure if the manual lists that number.

If piston rings are broken or stuck in a piston, pressure is lost.

The only way to determine what needs to be machined or replaced is to take the top end off and have a look. When you get in there you clean things up and measure them and compare those dimensions to those in the book and then it becomes clearer.

There is a tool that you probably don't have and that's a leak down tester. They are really useful tools for those of us that build lots of engines. What they do is to pressurize the cylinder so that leakage can be measured. Pretty nifty, but the real issue here is not how much is escaping, but where is it escaping and we don't need to measure that, we need to listen to it.

What you need is a compression gauge hose that screws into your spark plug hole (after you take out a plug obviously) and you put the bike in gear or find another way to stop the engine from turning and then connect that hose to a compressed air supply and listen. If you here gas hissing out of the exhaust pipe, the exhaust valve is leaking. If it hisses out of the carb, it's the intake valve that needs attention. And if it hisses past the piston and out of the oil filler cap or engine breather, the piston or barrel need attention.

In your case it's probably all 3 and and they used to say in England - off with its head.

It's not a random process of pulling things off and sticking them back in or of throwing parts at it. It's strip, clean inspect, measure and decide. Simple really.

There's lots of people on this board with experience building those motors to help you along the way.

Well, when you put it as "OFF WITH ITS HEAD" It doesn't sound so bad!
I'm actually kinda excited about the whole ordeal!


Dean, I got the manual at tradebit.com! I paid $7.....Messege me your email and Ill send you the PDF!

Just take your time and check things before you strip them,. Take tons of pictures. When you start pulling things off, bag and tag them all. Have fun