Topic: #1 cylinder not firing all the time 74' CB550

[valt]

Alright so I have been chasing down this stupid miss in cylinder one for months now. I have replaced the condensers, plugs, coils and wires (coils were replaced with CB900 coils). But yet I am still getting a intermitent miss in cylinder 1 and I'm honestly at a loss as to what could be causing this. I gapped the points, had the carbs synced, and adjusted the timing but it's still happening. Is there something else that I'm not thinking of that could be causing this? The plug always looks sooty, but the other plugs look just fine and with the timing light on that cylinder I can see its not firing as constantly as the other cylinders.

Any ideas?

[TwoTired]

If the plug is sooty enough, it won't fire. Either swap it for one thats is cleaner from another cylinder, or get a new one.

If a clean plug soots up an stop firing, you going to have do some carb work/cleaning.

Cheers,

[valt]

If the plug is sooty enough, it won't fire. Either swap it for one thats is cleaner from another cylinder, or get a new one.

If a clean plug soots up an stop firing, you going to have do some carb work/cleaning.

Cheers,

I have replaced the plugs since this started and the new plug got nasty too. I would think that its a carb issue as well, but I can't think of a reason that a dirty carb would make the cylinder run rich. Also regardless of whats happening with the carb I should be getting a light constantly from the plug wire and timing light correct?

[Brian G]

O.K. then.., if it's not a plug, coil or wire issue and #4 is firing O.K. (1 & 4 fire together?) , it's got to be carburetion (unless your valve clearances are totally awry....).

The plug always looks sooty

So she's running rich.... idle air screw? Needle position? Slide cutaway?

Might I ask how you know for sure that it's #1 which has the intermittent miss? My BSA has a once-in-a-while miss, But I have no idea which particular cylinder it is...

I'd certainly be looking at the carburetor(s). I'm new to SOHC 4's (up until now, Brit triples have been my area of preference) so  I may well be corrected, but there are only so many reasons that a cylinder (either British or Japanese) does not fire. Back to basics....

Hth,
Brian

[valt]

O.K. then.., if it's not a plug, coil or wire issue and #4 is firing O.K. (1 & 4 fire together?) , it's got to be carburetion (unless your valve clearances are totally awry....).

The plug always looks sooty

So she's running rich.... idle air screw? Needle position? Slide cutaway?

Might I ask how you know for sure that it's #1 which has the intermittent miss? My BSA has a once-in-a-while miss, But I have no idea which particular cylinder it is...

I'd certainly be looking at the carburetor(s). I'm new to SOHC 4's (up until now, Brit triples have been my area of preference) so  I may well be corrected, but there are only so many reasons that a cylinder (either British or Japanese) does not fire. Back to basics....

Hth,
Brian

Well for one I rebuilt the carbs and all of them were the same. Second of all cylinder number 1 is the only one that when I put the timing light on it, it doesn't light the light everytime. The timing light I use looks like the one below, all of the other cylinders seem to fire all the time except the one. I think my issue is probably ignition related, but I'm not sure how the one cylinder wouldn't be firing. I know the coils and wires are fine, and I have replaced the condensers so I don't really know what else ignition wise would cause this. As for the one carb being richer then the others I don't think thats possible since they are all exactly the same.

[Brian G]

Hi Valt -

Yup! It's a puzzle.... 

What I can't get my head around is that if #4 is firing O.K., #1 should be as well unless one of the components which is applicable only to #1 (i.e. plug or wire) has failed - or is in the process of failing. But you've taken care of that possibility.

Does she miss throughout the rpm range?

One of the things I have done with my R3 when I've experienced combustion problems is to swap the carb on the affected cylinder with another. If the problem moves with the carb, you know for sure that its carburetion-related.

Regards,

[dustyc]

Try taking the spark plug boot off and cutting the wire back 1/4 inch.  You might have a bad connection.

[TwoTired]

I have replaced the plugs since this started and the new plug got nasty too. I would think that its a carb issue as well, but I can't think of a reason that a dirty carb would make the cylinder run rich.

Ok, here are some.
The slow air jet is clogged/restricted.
The main air jet is clogged/restricted.
The main emulsion tube air bleeds are clogged/restricted.
The main jet oring is leaking
The main jet has fallen out.
The float is sticking or adjusted wrong leaving too high a level in the one carb, possible dumping raw extra fuel in the cylinder.

Also regardless of whats happening with the carb I should be getting a light constantly from the plug wire and timing light correct?

No. Soot is a conductor that shorts the plug.  Normally, the voltage rises in the coil secondary (when the primary is triggered and the field collapses) and it builds till their is sufficient voltage to jump the spark gaps.  The voltage doesn't build when the plug is shorted with soot.  The timing light may not trigger threshold may not reliably fire with low voltage on the ignition wire.

Do you still have the resistors in the plug caps?  What value?

Cheers,

[TwoTired]

As for the one carb being richer then the others I don't think thats possible since they are all exactly the same.

You spark plugs say otherwise.

Soot is incomplete combustion from more fuel than oxygen when burnt.  Being burnt means that ignition had started.  Which says you had spark until the plug got too sooty to fire.

If you don't have spark, the plugs simply get wet, not sooty.

Cheers,

[KingCustomCycles.com]

+1 on all of that good troubleshooting advice.  I would like to add that we are blessed with 4 individual header pipes and each can be measured for temperature to determine combustion.  It is a cold cylinder check in airplane mechanic's language.  An IR thermometer from NAPA or online can go a long way to finding out which cylinder is not pulling its weight.  So will a crayon, heat stick, or spit on your finger.  Remember though, combustion is cooler on BOTH sides of peak EGT, (exhaust gas temperature) rich is too much fuel and cooler, and lean is too much air and is also cooler.  Any engine equipped with a manual mixture control and an egt gage will show this phenomenon easily. 

I can see many of the newer owners with problems want us to say "replace #1 idle jet with part# 1234 and it will be fixed.  It will never be that easy.  It is the theoretical knowledge and experience with the systems and their function, as they relate to other systems, that will save the day on a tough problem. "If you think education is expensive, try ignorance."   

[valt]

+1 on all of that good troubleshooting advice.  I would like to add that we are blessed with 4 individual header pipes and each can be measured for temperature to determine combustion.  It is a cold cylinder check in airplane mechanic's language.  An IR thermometer from NAPA or online can go a long way to finding out which cylinder is not pulling its weight.  So will a crayon, heat stick, or spit on your finger.  Remember though, combustion is cooler on BOTH sides of peak EGT, (exhaust gas temperature) rich is too much fuel and cooler, and lean is too much air and is also cooler.  Any engine equipped with a manual mixture control and an egt gage will show this phenomenon easily. 

I can see many of the newer owners with problems want us to say "replace #1 idle jet with part# 1234 and it will be fixed.  It will never be that easy.  It is the theoretical knowledge and experience with the systems and their function, as they relate to other systems, that will save the day on a tough problem. "If you think education is expensive, try ignorance."   

Oh I don't expect anyone to tell me part numbers and I have gotten plenty of good help already. I was not aware that a sooty plug would cause the timing light not to light. I guess I'm going to have to take apart that carb again and give it a look, which I must say I am not looking forward too. Thanks for all your help!

[kaysystems]

Do you still have the resistors in the plug caps?  What value?

I know how to get resistors ut of straight caps. How to on the bent ones (CB750) ..... I have a similar problem.

David

[speedy gonzalais]

does it smoke at all, loss of oil? maybe a sticky valve or valve guides

[TwoTired]

Do you still have the resistors in the plug caps?  What value?

I know how to get resistors ut of straight caps. How to on the bent ones (CB750) ..... I have a similar problem.

David

Are we talking about the stock plug caps? 
You don't need to take the resistors out of the cap.  The caps unscrew from the ignition lead and you can then measure their resistance from input to output terminal.  Depending on the bike, you can see nominal vales of either 10KΩ, or 5KΩ.  Plug caps are available with Zero Ω, too, but those aren't recommended unless you are using resistor spark plugs.

If your plug caps aren't within 10% of rated value, replace the entire plug cap.

Cheers,

[kaysystems]

Do you still have the resistors in the plug caps?  What value?

I know how to get resistors ut of straight caps. How to on the bent ones (CB750) ..... I have a similar problem.

David

Are we talking about the stock plug caps? 
You don't need to take the resistors out of the cap.  The caps unscrew from the ignition lead and you can then measure their resistance from input to output terminal.  Depending on the bike, you can see nominal vales of either 10KΩ, or 5KΩ.  Plug caps are available with Zero Ω, too, but those aren't recommended unless you are using resistor spark plugs.

If your plug caps aren't within 10% of rated value, replace the entire plug cap.

Cheers,

Sometimes I have trouble finding no-resistor plugs. I found out the hard way that resistor caps & resistor plugs can make my starting difficult, especially in cold weather.
So I'd like to have a set of no-resistor caps....just in case I can't get the right plugs. Where can I get no-resistor caps?

thanks

David

[martino1972]

i didnt see anywhere that you have checked the valve clearance....