1981 Honda C70 - Stumbles when given throttle, seems to misfire

1981Honda

Member
I need help with my 1981 Honda C70, It starts up and idles fine but when given large throttle imputs in netrul or when in gear it sort of stumbles and seems to misfire. I've replaced the point set since the original one was super burned up and pitted and it seemed to help some, but it still stumbles and sort of backfires through the intake. I tried advancing and retarding the spark timing by moving the points plate and every time I did it always got worse. So I have the timing set back to where it was and I don't think its an issue.

I have checked the bike pretty thoroughly for air leaks at the top end like carb gaskets, cylinder head, head gasket, base gasket, etc and there are no air leaks there. Carb is clean, valves are pretty close to spec, spark advancer is free, points are new and gapped correctly, and the air screw is set to 1.5 turns which is stock. What's interesting is I can get the bike to run somewhat fine if I slowly open the throttle gradually when accelerating, but when giving large or full throttle inputs it just sputters around. Seems like when I get to a certain lower rpm when downshifting it stumbles when trying to rev match. What could I be missing? I've ordered a new condenser and i'll see if that could be causing the issue, maybe it is shorting out? Any help would be greatly appreciated.
 
Your experience sounds similar to mine with my 1978 CT90 with a Chinese carb. I described the motor behaviour with quick throttle opening a little different (gulping and stumbling), but this might just be different words for a similar behaviour - I do not know). I thought that my problem was that the mixture was too rich at higher throttle, so I tried moving the needle clip toward the top end of the clip (came stock at the middle position). This resulted in my motor running smoother. I hope you might have the same positive experience. I have only a few months experience with my Honda (i.e, no expertise disclaimer), but what I tried is zero cost and may work for you too.

-Will
 

lukelaw1

Active Member
Are you using the stock air cleaner assembly? you need to find out if the stumble/misfire is caused by a lean or a rich condition. You can do this by opening or closing the choke when the bike is stumbling/misfiring. Also low compression will cause hard carb tuning and adjusting.
 

1981Honda

Member
Are you using the stock air cleaner assembly? you need to find out if the stumble/misfire is caused by a lean or a rich condition. You can do this by opening or closing the choke when the bike is stumbling/misfiring. Also low compression will cause hard carb tuning and adjusting.
Yes I am using the stock air filter, the bike is completely stock like it was in 1981. Puts out 150 PSI of compression so it's very healthy. It has around 4,530 miles on it so its not terribly high, I've put over 2,300 miles on it since I got it the beginning of June this year. It has run super great up until I had some cam chain noise which is fixed now and then this problem with it stumbling. I've been very diligent keeping up with the maintenance such as oil changes every 800-900 miles, valve clearance adjustments, chassis lubed up, etc.

I'm taking the carb apart again just to ensure it is spotless, I've got an Ultrasonic cleaner so I'm going to put it in there to make sure it's seriously cleaned out. So far all the jets look completely clear so I'm not thinking its the carb.

I also have a new condenser coming from eBay, maybe the one on there is shorting out?
 

1981Honda

Member
I have the carb sitting in the ultrasonic cleaner right now so I will check that out tomorrow. I have tried to turn the air mixture screw in and out about a half turn each and it has always made it worse. right now it is set at the stock setting, its like at 1 1/4 - 1 1/2 ish turns.

Thanks to everyone for the help so far! I'm hoping the carb was just a little dirty or something
 

OLD CT

Well-Known Member
Time to look at the condition of the plug. I would fully warm it up and then put a brand new plug in and get it to misfire and stumble for a minute, then pull it to check if it's black and sooty. I have a feeling you are running too rich.
 

1981Honda

Member
Time to look at the condition of the plug. I would fully warm it up and then put a brand new plug in and get it to misfire and stumble for a minute, then pull it to check if it's black and sooty. I have a feeling you are running too rich.
I'll try that out too, what is weird is once it gets past the stumbling it revs up and runs fine. Still gets up to around 40-43 mph.
 
I had that same stumbles at about 2/3 or 3/4 throttle. I was told that the mixture screw is only controlling the fuel mix at idle, so I turned to the needle position, and I found improvement. Good luck.
 

1981Honda

Member
I had that same stumbles at about 2/3 or 3/4 throttle. I was told that the mixture screw is only controlling the fuel mix at idle, so I turned to the needle position, and I found improvement. Good luck.
That is exactly the problem I am having, you give it 1/2-3/4 or full throttle and it stumbles and backfires into the intake and air filter (you can feel the air coming out of the intake when it pops). What is also weird is if you very gradually give it throttle it doesn't seem to do it almost at all. It is just large throttle imputs that cause it to do that. I may try moving the needle clip up or down 1 notch and see what happens, It is at the stock setting of 3 so I would Ideally like to leave it there because other than this weird stumbling the motor runs great and is very healthy. I just want to leave it as stock as possible for reliability. Once the bike is running at its top speed it runs just fine so this is just weird how its doing this now
 
My guess is that as the engine spins up with gradual throttle the suction from the piston movement pulls air into the carb which allows all the fuel to burn, but when you are running too rich and you quickly increase the fuel with the throttle it cannot burn it all with the air it has, and at full throttle the throat is wide open so the throttle slide is playing a smaller role. The needle slide going toward the top end (away from the point) means that more air gets past the slide for the same amount of fuel being allowed through the main jet. Trying to keep to past settings may be a mistake as perhaps through the years the needle and main jet have worn larger so too much fuel is getting through for the air to burn it, or new gas is not as well suited to these old motors, or many things I cannot imagine. With a change in the needle setting (if you leave everything else the same) is pretty easy to reverse if it does not help you as it did me. Look at me, writing like I know whats what - I only "learned" it on the internets (but my CT90 is running much better than it was).
 

1981Honda

Member
My guess is that as the engine spins up with gradual throttle the suction from the piston movement pulls air into the carb which allows all the fuel to burn, but when you are running too rich and you quickly increase the fuel with the throttle it cannot burn it all with the air it has, and at full throttle the throat is wide open so the throttle slide is playing a smaller role. The needle slide going toward the top end (away from the point) means that more air gets past the slide for the same amount of fuel being allowed through the main jet. Trying to keep to past settings may be a mistake as perhaps through the years the needle and main jet have worn larger so too much fuel is getting through for the air to burn it, or new gas is not as well suited to these old motors, or many things I cannot imagine. With a change in the needle setting (if you leave everything else the same) is pretty easy to reverse if it does not help you as it did me. Look at me, writing like I know whats what - I only "learned" it on the internets (but my CT90 is running much better than it was).
Thanks for your help! I have the carb back on the bike and I started it up today and it seems to do better. There must have been a small blockage somewhere. When giving it full throttle at idle like you said it still stumbles a bit, but yeah I guess I'm pretty much just dumping gas into the engine when doing that so it can't burn it all (at least quickly). I'll try the needle clip setting tomorrow, It's currently pouring rain where I live so I can't really do much of a test run right now... oh well.

Also lukelaw1 I tried closing the choke almost all the way and about halfway, and it made it worse. So looks like if anything I'm running rich.

I'd really like to get the bike warmed up good before doing any real big adjustments, right now I've only been able to fire it up in the garage. Who knows, maybe once it is warmed up and everything when I give it full throttle it may very well be able to burn all the fuel, I'll have to see.

Thanks a lot to those who have helped me so far!!
 

Tucuda

New Member
Thanks for your help! I have the carb back on the bike and I started it up today and it seems to do better. There must have been a small blockage somewhere. When giving it full throttle at idle like you said it still stumbles a bit, but yeah I guess I'm pretty much just dumping gas into the engine when doing that so it can't burn it all (at least quickly). I'll try the needle clip setting tomorrow, It's currently pouring rain where I live so I can't really do much of a test run right now... oh well.

Also lukelaw1 I tried closing the choke almost all the way and about halfway, and it made it worse. So looks like if anything I'm running rich.

I'd really like to get the bike warmed up good before doing any real big adjustments, right now I've only been able to fire it up in the garage. Who knows, maybe once it is warmed up and everything when I give it full throttle it may very well be able to burn all the fuel, I'll have to see.

Thanks a lot to those who have helped me so far!
Hello, Could you solve the problem? I have the same issue and can't find how to fix it.

Thanks you
 

1981Honda

Member
Hello, Could you solve the problem? I have the same issue and can't find how to fix it.

Thanks you
For me it was a combination of things. I had a Partially clogged jet in the carburetor and I also replaced some really old points and that seemed to help a lot. One thing I realized is that when you open the throttle all the way, when it’s just idling, it’s just dumping a bunch of fuel into the engine and the engine isn’t spinning fast enough to burn it efficiently. So it’s gonna run a little bit rich, and maybe even cause the engine to die out. I’m not sure where you live at, but I am in Michigan, and the weather is getting cold here so naturally bikes are running richer so it’s a little bit of an issue with these older bikes. It definitely helps once the bike is warmed up, doesn’t seem to do it as much for me. Just let it warm up good and if it’s still stumbling a little bit just slowly is ease into the throttle when starting from a stop. Hopefully this helps!!
 

kirrbby

Well-Known Member
You can usually help that by adjusting the air/fuel screw. Tune to the best idle initially...by the book. But if you have than rich misfire when you crack the throttle...turn the screw towards the lean side. You might need to re-adjust the idle speed screw as you go.
See if you can tune it out that way.

If it's still bad...go one size smaller on the idle jet, and re-tune.

your goal is less fuel at low throttle...pilot jet, and AF screw controls that.
 
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kirrbby

Well-Known Member
I have to take that back^^^ reverse it.
Typical, is a lean stumble when you crack the throttle. It goes too lean...not, too rich. So you need to ADD fuel to the pilot circuit...reduce air.
So, tune the AF screw towards rich...and/or go BIGGER on the pilot jet.

If none of that works...you need a smaller carburetor, and/or intake.
 

Lcron

New Member
My c70 Passport backfires when I try to start it. Does anyone know which way to turn the points plate to eliminate this problem? Set the timing?
 
There are lots of guides to setting the timing on these motorcycles. I remember doing it in the early 1980s visually (without a light), and think this was not adequate. All you need is an cheap LED flashlight that you can open and interrupt the wires so you can attach it to each side of the points gap and then rotate the engine (open the rotor cover to access the bolt to rotate; check the manual or stamped on the rotor for the direction to turn) and then set the points so the light goes off as the engine passes the fire mark ("F") on the rotor on the compression stroke.

I am not sure how this will affect backfiring (never had that problem), but setting the timing is standard maintenance on these motorcycles, so good to eliminate this as an issue. There are others here who know much more than me, so I hope you will get better advice on the question of backfiring.

Good luck!
 
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