backfire/missing when cold

[seestheday]

Hi Guys,
I have been having some battery/starting issues lately with the weather here in Canada getting colder so I put my battery on a tender last night and was able to start my bike up this morning.

As usual when it's cold it didn't start right up and I had to hold the starter down for a while before it really started firing well.

Anyways, I let it warm up the usual amount and pulled away.  Everything was fine until I hit around 4.5K rpm's and it started to miss/backfire.  I pulled over with it still running to see if there was anything I could see visually and tried revving the engine when it was out of gear to try to replicate the problem.  With the bike in neutral there was no backfiring/missing so I tried to start moving again and the backfiring/misses started again.  I ran it gently for a few km's and then after warming up some more the problem seemed to go away.

Is this normal?  Should I be worried and need to take immediate action to fix the issue (e.g. clean carbs etc)?

I was planning on working on the bike throughout the winter and wanted to use the time that I have left to ride.

[seestheday]

Oh, on a side note, the only other thing that changed was that I gave it a good wash last night (no pressure washing or harsh chemicals, just soapy water and rags).

I was inspired by all of the awesome looking bikes in the builds section

[seestheday]

Based on this thread: http://honda-cb750-s.456789.n3.nabble.com/dyna-ignition-and-coils-td459927.html#a459930

It sounds like I need to pick up some iridium plugs .

I also should probably not leave the gas always in the "on" position and turn it off when I'm not riding it.

[shinyribs]

Do you recall if the backfire sounded like it was thru the carbs or thru the pipes.Im not 100% sure,but it sounds like timing perhaps.If it is timing it is probably the same thing that causes it to start slow.Usually when you have to keep holding the starter button,waiting for it to "catch up"so to speak,your timing is low or retarded.Retarded timing may also backfire thru the pipes since the engine is trying to fire with the exhaust valves open.Not being able to replicate it with load suggests to me it could be the timing advance is stuck.If it were me i would check on the timing advance.Of course...if you follow my advice that may cause you problems all by itself.Good luck

[seestheday]

Makes sense.  Looks like I need to get myself a timing light.

[shinyribs]

You probably should ask around a little more before you buy anything.That was just an idea of mine.You might want to get a few different references first

[sgtslag]

The Iridium plugs will help it start faster, easier (they work flawlessly if the rest of the ignition system is working properly), but they don't explain the backfiring.  They're just an inexpensive upgrade to the weak, OEM ignition coils.

[seestheday]

I've had a feeling that the timing was off from the way its been riding. Doing some research tells me that advancing the timing can be a good idea on engines with a lot of wear, and mine has 87,000 km's on it.
And I was hearing the backfire from the exhaust. I am also planing on doing some carb cleaning this winter.

On Wed, Oct 5, 2011 at 4:37 PM, shinyribs [via Honda CB750'S] <[hidden email]> wrote:

You probably should ask around a little more before you buy anything.That was just an idea of mine.You might want to get a few different references first

Montvale,Virginia
DUE TO RECENT ECONOMIC CONDITIONS, AND THE RISING COST OF ELECTRICITY, GAS AND OIL, THE LIGHT AT THE END OF THE TUNNEL HAS BEEN TURNED OFF.
Remember:Computers are fun...but you can't download a finished bike.   Get to work!
Common sense is not all that common
 Piute said it best:  "wheres my 10mm?"

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Simon Austin

Kremental.com

[TOOLS1]

These things are cold blooded. I would bet that the water from washing the bike was still on some of the ignition parts. The coil/plug wires perhaps. Ride it tomorrow, and see how it does.
TOOLS

[seestheday]

Happened again for the 3rd day in a row :-(. Back/misfires when cold after about 4.5K rpms, but runs fine after warming up significantly.

I'm going to pull the plugs tonight to see how they look and then head out to pick up some new plugs if there aren't more serious issues. Probably won't fix the back/misfire issue but should hopefully let it start much more easily.

I also noticed that the cables for the spark plugs are really cheap and I should upgrade those to a decent set. Not sure why the prior owner would put on such a cheap set.

How easy is it to pull the gas tank off? I am guessing I need to do that before I can replace the wires and I want to be sure I leave enough time to put it back on because I need to ride every day (work).

Simon

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "TOOLS1 [via Honda CB750'S]" <[hidden email]>

Date: Wed, 5 Oct 2011 18:23:10 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: backfire/missing when cold

These things are cold blooded. I would bet that the water from washing the bike was still on some of the ignition parts. The coil/plug wires perhaps. Ride it tomorrow, and see how it does.
TOOLS

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[Lucky 1]

Everyone here is all over the map guessing.

Did you use the choke to get it started?
How long did you leave the choke on?
You may have to leave the choke on for a couple blocks in cold weather.
Also you can use partial choke.

You said it ran fine after it was warmed up.
What year is  this CB750?

You have 6 posts on this issue, and 27 replies. Not one person asked about the choke.
Unbelievable.

Why don't you just disassemble the entire motorcycle and replace every part you can think of,then maybe it will run right.

BTW don't wash a motorcycle if you do not have too.
Just clean with a soft cloth and spray cleaner by hand.

I saw two girls ride up to a car wash one day and asked to have their motorcycle washed.
What do you think happened?  Thats right ....They called their daddy to come pick them up.

[seestheday]

Its a 1981 ch750K.

Ya, I have to use choke to get it started, otherwise it'll stall.
I usually start it up and have to pull the choke out manually as far as it will go. Then after it warms up a bit I give it a half turn so it will stay out. Then I put on my boots and helmet and by the time I'm done that its warm enough to ride.

At least it was until a few days ago when it started misfiring at around 4.5k rpm's until I rode it for 5-6 blocks, after which it would start riding fine.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "donald branscom [via Honda CB750'S]" <[hidden email]>

Date: Fri, 7 Oct 2011 13:05:58 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: backfire/missing when cold

Everyone here is all over the map guessing.

Did you use the choke to get it started?
How long did you leave the choke on?
You may have to leave the choke on for a couple blocks in cold weather.
Also you can use partial choke.

You said it ran fine after it was warmed up.
What year is  this CB750?

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[Peterwh]

You have probably worked it out by now but getting the gas tank off these bikes is the easiest thing.  Lift the seat.  Pull a rubber boot back off the rear of the tank, pull the tank back to get it off the rubber forward mounts and that's it.  If I'm not removing the tank completely, at this point I usually place a piece of timber under the gas tank tunnel and rest it on the handle bars.  That secures the tank fairly safely and gives me enough room to do whatever I want to do.  If you want to remove the tank completely, you just need to disconnect the gas pipe from the petcock.

[seestheday]

Thanks, I haven't got to this yet tonight so that really helps.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "Peterwh [via Honda CB750'S]" <[hidden email]>

Date: Fri, 7 Oct 2011 14:40:42 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: backfire/missing when cold

You have probably worked it out by now but getting the gas tank off these bikes is the easiest thing.  Lift the seat.  Pull a rubber boot back off the rear of the tank, pull the tank back to get it off the rubber forward mounts and that's it.  If I'm not removing the tank completely, at this point I usually place a piece of timber under the gas tank tunnel and rest it on the handle bars.  That secures the tank fairly safely and gives me enough room to do whatever I want to do.  If you want to remove the tank completely, you just need to disconnect the gas pipe from the petcock.

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[seestheday]

Oh, and by wash I really just meant wipe it down with a cloth.  Yes I had a bucket with soapy water, but I was squeezing the cloth quite a bit and drying with another cloth as I went.

[seestheday]

Ok, update from tonight.

I pulled the leftmost and rightmost plugs and noticed that they both had a little carbon fouling (not much, just a little).  The left was worse than the right and is pictured below.

I also checked the air filter and noticed that it was pretty oily, so it's time for a new one according to my Haynes repair manual.  It's just a standard air filter (not K&N).  Do you guys recommend going with a K&N?

I plan on picking one up tomorrow along with some iridium spark plugs and decent spark plug wires to help the starting problem.

[TOOLS1]

The K&N filters are very good. Just a little pricey. I myself just run the stock filters, but I do not ride on gravel, or dirt.
TOOLS

[shinyribs]

I see what the problem is!Those fingernails are waaay to clean to be on a bike mechanicjust pickin' on ya

[free2ride]

seestheday wrote

Hi Guys,
I have been having some battery/starting issues lately with the weather here in Canada getting colder

well there's your problem right there -- Canada -- it just gets too cold too quickly here!

As usual when it's cold it didn't start right up and I had to hold the starter down for a while before it really started firing well.

this is totally normal.

I ran it gently for a few km's and then after warming up some more the problem seemed to go away.

normal until the bike is warm

Everything was fine until I hit around 4.5K rpm's and it started to miss/backfire . . . I tried to start moving again and the backfiring/misses started again.  

a theme coming here . . . normal for us Canucks

I have discovered [ymmv] . . . that once we hit the middle of September it takes a lot more time for my bike to warm up. full choke to start, easing off gently until it is fully off [been told that driving with the choke on will foul the plugs] and RPMs at ~1500 . . . then I'm usually good to go.

should say I'm riding an 80 CBK, same mileage [unless there's a "1" at the front that the odo didn't catch

[seestheday]

Ha,
No I won't be riding through the winter. I'm in waterloo ontario, so I will be getting plenty of snow. I plan on working on the bike through the winter when I can't ride :-).

Thanks for the tips.

I switched in iridium spark plugs and at first it started great, but quickly started to run rough and stalled. I don't have a new air filter yet, so I think the mix is running too rich, and won't be fixed until I get my new air filter. None in stock locally for a reasonable price so I'm ordering one tonight along with some other parts I've had my eye on.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "free2ride [via Honda CB750'S]" <[hidden email]>

Date: Sat, 8 Oct 2011 10:39:07 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: backfire/missing when cold

seestheday wrote:

Hi Guys,
I have been having some battery/starting issues lately with the weather here in Canada getting colder

well there's your problem right there -- Canada -- it just gets too cold too quickly here!

As usual when it's cold it didn't start right up and I had to hold the starter down for a while before it really started firing well.

this is totally normal.

I ran it gently for a few km's and then after warming up some more the problem seemed to go away.

normal until the bike is warm

Everything was fine until I hit around 4.5K rpm's and it started to miss/backfire . . . I tried to start moving again and the backfiring/misses started again.  

a theme coming here . . . normal for us Canucks

I was planning on working on the bike throughout the winter and wanted to use the time that I have left to ride.

OK, you aren't near to me . . . I certainly won't be riding through the winter!

I have discovered [ymmv] . . . that once we hit the middle of September it takes a lot more time for my bike to warm up. full choke to start, easing off gently until it is fully off [been told that driving with the choke on will foul the plugs] and RPMs at ~1500 . . . then I'm usually good to go.

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