Suspect ignition problem!

450twincafe

450twincafe

Active Member
Could someone help? I have a CB450K7 that is misfiring at high RPM's and think it is an ignition problem. The jets are dead on, the timing is good. This is a bike that has points and I am not very good with accessing issues with such. All my bikes have electronic ignitions in the past.
I am sure that there has been discussion with this issue but I can't find the info.
Please help as this is the only thing that is keeping this bike from the twisty roads we crave so much.
 
I've got one of them and am very experienced with the points system. So, stock carbs, aftermarket silencers? What sort of tools do you have? You need at least the basic stuff plus a voltmeter with continuity tester. And a manual?
Let's nail down the symptoms first. High RPM misfire.....what RPM does this start at? Every gear? In the flats? Going uphill?
Is the battery charged up? Condition of the plugs? Have you been put-putting around town?
You say the timing is good, but question the points. They are one in the same. The points condition, gap and opening position(in relation to one-another and the cam) determine the timing.
450s are a pain in the ass as far as getting the timing spot on. Basically it's a compromise between proper gap and opening position.
Do you know about the Honda Twins site? http://www.hondatwins.net/forum/viewforum.php?f=29
In particular: http://www.hondatwins.net/forum/viewtopic.php?f=18&t=883&st=0&sk=t&sd=d&start=0
NE
 
Thanks for the reply.
Here is the info to evaluate.
Battery is good and charging
Problems start at 4500 continuous RMP's or hard throttle(more than 3/4)
Plugs spec good after hard throttle for 20 sec. and pulled.
Bike has only run in aggressive throttle, no idling more than one minute.
Bike starts with a quick touch of the starter.
Thanks CurbTigher for the assistance.

Here is a pic of the bike in question!
 

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I see it's got pods and pipes with no silencers. Have the main jets been upped a bit? Guess your at altitude, so upping the jets may result in a rich condition.
Will the bike not run beyond 4500rpm? Just bogs down? Or does it just quit working? Meaning, can it actually rev to 8000rpm with no load.
Is the misfire more of a stutter?
How 'bout if you let off the throttle just a wee bit when it's starting to miss. Does it clear up then?
Have you tried a plug with different heat rating? A hotter plug so the air/fuel is more willing to ignite.
Original coils/wires/caps?
Have the points been filed and cleaned lately? Could be that one of the points is carboned up somewhat, not allowing enough current to the coil, resulting in a weak spark. Similar to if the gap was to large, resulting in the points not closing long enough at higher rpm, not allowing the coil to charge up far enough for a solid spark.
I use one of these to clean up the points occasionally: http://www.foredom.com/foundations/store/shopdetail.asp?params=A-10342*0
Hold the point open with a flat screwdriver, dip the diamond hone in rubbing alcohol and give the contacts a good scrub. Keep an eye on the portion of the hone being used for how much carbon is being removed. When the points appear to be cleaned up, use a 1/4" wide strip of non-shiny business card, dipped again in rubbing alcohol to pull through the points, removing any crud from the filing. Watch that no tiny nib is left on either side of the contact surfaces. It'll grab the paper, leaving a bit of paper in there, resulting in reduced or no current passage.
NE
 
Thanks for the tips.
To answer the questions, the bike will rev at the way with no load and does more like stutter instead of misfire. Letting off throttle a bit does seem to stop the problem.
I will try hotter plugs, but think it is in the points. I opened the cover and ran the bike with no load. The result was in the high revs one of the points was arcing out of the side. Could be the issue?
I will clean and hone the points and see if it helps.
 
450twincafe said:
Thanks for the tips.
To answer the questions, the bike will rev at the way with no load and does more like stutter instead of misfire. Letting off throttle a bit does seem to stop the problem.
I will try hotter plugs, but think it is in the points. I opened the cover and ran the bike with no load. The result was in the high revs one of the points was arcing out of the side. Could be the issue?
I will clean and hone the points and see if it helps.
Click to expand...

Another test you can do:

Let the revs climb up at say 1/2 throttle in first and second. If it revs to redline at part throttle, odds are your points are fine. If it only bogs at high RPMs and high throttle, it could be your main jet is too large. You say you are sure of your jetting, but from looking at your intake / exhaust mods, the right size can only be determined by trial and error. There are so many factors at play... Altitude, temperature, humidity...

Look at the points work... Do they arch at all? If so your condenser may not be grounded properly, maybe just take it off and clean the ground. It could be dirty / rusty. Clean the points with contact cleaner.

Make sure the points gap is within spec. Ideally, you want to be at the smallest gap withing the specs. More dwell angle (ie points are closed longer) means the coil builds a better charge.
 
Ok, her is what I know now. As suggested I ran the bike in first and second gear and let it you the rpm run up to 8000 and all was good when at less than half throttle. As suggested I also changed the plugs from B8ES to BR9ES to get a hotter spark, and same results. Honed and gapped the points and again no change.
Could the pods and open headers lead to a richer set up, when the plugs test perfect? Can I change needle settings on this type of carb.
Again the carbs are stock CB450K7.
Maybe I am wrong but every time I have adjusted jets for pods and headers I have gone to to a richer set-up to compensate for the increased flow. In this instance I went from a over rich setting(stock) to a leaner situation when adjusted for changes. The jets should be too lean? They are not to lean when inspecting the plugs after appropriate test run.
Can you over richen when adding better air flow?
 
450twincafe said:
Ok, her is what I know now. As suggested I ran the bike in first and second gear and let it you the rpm run up to 8000 and all was good when at less than half throttle. As suggested I also changed the plugs from B8ES to BR9ES to get a hotter spark, and same results. Honed and gapped the points and again no change.
Could the pods and open headers lead to a richer set up, when the plugs test perfect? Can I change needle settings on this type of carb.
Again the carbs are stock CB450K7.
Maybe I am wrong but every time I have adjusted jets for pods and headers I have gone to to a richer set-up to compensate for the increased flow. In this instance I went from a over rich setting(stock) to a leaner situation when adjusted for changes. The jets should be too lean? They are not to lean when inspecting the plugs after appropriate test run.
Can you over richen when adding better air flow?
Click to expand...

No I am pretty sure you are too rich. When you lean out, the engine will still run smoothly but with less than perfect power. The plugs will look melted and the engine will run hot.

You should be able to change the needle setting, but the needle affects the mixture mostly in the 1/4 - 2/3 throttle range. The stock needle setting is usually the best, although it's hard to say how your modifications will affect intake / exhaust pressure as the revs rise. WOT performance is mostly determined by the jet size, which is why you are only experiencing problems at or near full throttle.

If I was you I would pull off the main jets and double check which number they are, it may have been changed by the previous owner. Buy a couple of sets bigger and smaller than what you have.

Then test the right jet size by going out on the bike and running it up at WOT and high revs for a few minutes. Select the jet size that allows the engine to make the most top end power. Too big is definitely bad for power and will bog the engine right down. Only when you have determined which jet works best should you mess with the needle position. Then adjust the mixture at idle with the screw and sync the carbs.
 
Stock is supposed to be 145 mains. But I think they ran rich stock. Pods and open headers would make it lean out so 150 isn't out of the ballpark but may be high. I also don't think you can adjust the needles on these carbs. Might be wrong there.

I have the same issue with mine. When I would wind it out it would start to miss when I got up near redline. Haven't figured it out yet. I currently have the motor out to fix a valve and redo the rings.

Have you checked the float level?

Bogging at WOT does sound like a rich condition.
 
Turns out with a friend that is a Harley tech, the problem was actually a lean stutter and not a rich setting. I could not find jets local so my buddy bored the jets to 160 and with the fuel screws out two turns we found a setting that was close. I think that I will bore them to 165 and turn back the screws to one out, like the recommended stock setting.
I think the plugs were giving a false reading due to the warming of the motor prior to test. Maybe the pilot is a little to big or the needle needs to move.
Did I state the bike is running at 6700' to 8500', and the test is at the lower and richer altitude.

Wow did anyone ever consider that a motor could actually seem like an over rich setting and turn out to be to lean?
 
The shocks are the showa 328 that are 12.5 eye to clevis. Maybe the photos are off due to the slanted driveway. Here in the Alps nothing is level. ;D
This should blow your mind! I have to cross two passes at 11000' and 12000' to get to Denver. I would never try that on a vintage bike without a set of jets and many tools. In other words NEVER!!!
 
For a reply do you have to post a photo?
Has anyone herd of a lean out studder?
Here is a photo to entice!!!!!!!!!!!
Not quite sure what the pthot is but the help seems to come after a photo. :D ;D ::) :-*
 

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I didn't think there was a lean out studder. My bike leaned out before I changed the jets... The symptom was that there was less power at full throttle than at say 3/4 throttle. I then changed it to a jet that was too big, and it started missing at high rpm / full throttle like yours does. I changed down jet sizes 2- times and the same thing would happen, at higher rpms as the sizes became closer to the right one. The right jet size pull full throttle to red line cleanly.

Another part of your issue could in fact be the ignition. If you have weak spark at higher rpms, or if your advancer isn't working, it could have problems igniting the mixture. If your mixture is too rich or too lean, it will be harder to ignite, in this case a weak ignition with a lean mixture will cause it to miss.
 
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