Hi I've got a problem on my 1995 750 Zephyr, on a cold start it is misfiring on one cylinder, it's takes a little while to clear, and when warm it runs fine. I've had the carbs off approx 18 months ago when the valves where checked, and they were Ultrasonic cleaned. Can anyone suggest what I need to check.
Regards
J.W.
Misfire on cold start up.
Re: Misfire on cold start up.
Hi J.W. has the bike been in regular use or laid up for months? do you know which cylinder is misfiring? the very first thing I'd check would be the spark plug, try a new replacement don't f about with a possibly dodgy old one.
Re: Misfire on cold start up.
Hi it is the No 2 cylinder, it has only been used occasionally in the last couple of months, the spark plugs where replaced last year.
J.W.
J.W.
Re: Misfire on cold start up.
Interesting...
I have the exact same issue, number 2 not running initially. Bike is in winter storage with a trickle loader, did start it this weekend (I do this once a month while in winter storage) and it took a while before the down pipe of number 2 became warm...
I have the exact same issue, number 2 not running initially. Bike is in winter storage with a trickle loader, did start it this weekend (I do this once a month while in winter storage) and it took a while before the down pipe of number 2 became warm...
Re: Misfire on cold start up.
Blocked pilot jet, or mixture adjustment circuit.
Try adjusting the mixture first. The process of seating the mixture screw may clear the offending crap if the blockage is in that area. This is more optimism than anything else as the blockage is most likely to be in the pilot jet itself.
Try adjusting the mixture first. The process of seating the mixture screw may clear the offending crap if the blockage is in that area. This is more optimism than anything else as the blockage is most likely to be in the pilot jet itself.
Re: Misfire on cold start up.
Could be a blockage however in my case I've cleaned those carbs multiple times. I've always had issues with cilinders not all running from the start. Used to be number 3 and 4, persistently at cold start. After al the work I did last year I thought the issue was over only to discover the issue is now with #2. It's only for a few seconds though but noticeable.
Re: Misfire on cold start up.
Partially blocked pilot jets. The buildup inside the jet makes it smaller and results in a lean AFR. The offending cylinder won't run on this lean AFR till it warms up a bit.
The reason it persists after cleaning the carbies is because the jets haven't been adequately cleaned. Ultrasonic or chemical cleaners are quite poor at removing the rock hard concrete type buildup that can accumulate inside jets. Physically cleaning of the jet hole or replacement of the jet is the only guaranteed solution. Appropriate size genuine Keihin pilot and main jets are readily available at a quite modest cost. http://www.pjmotorsports.com/keihin-jet ... low%20jets
The problem occurs in infrequently used motors with fuel left in the bowls to evaporate away. That creates a concentration that sets rock hard on the walls of the jets as it dries completely. As this process repeats so the buildup. It is not a matter of if it will happen if not managed, just when.
The way to prevent, or at least significantly delay, the re-occurrence of the problem is to always drain the carbies before putting the engine away if you don't use the engine (lawn mower, outboard, motorcycle, whatever) quite frequently, like every couple days at least.
The easiest way to do that on a Zephyr would be to fit a manual fuel tap, and after turning the tap off run the engine till the bowls suck dry and the engine stops.
The reason it persists after cleaning the carbies is because the jets haven't been adequately cleaned. Ultrasonic or chemical cleaners are quite poor at removing the rock hard concrete type buildup that can accumulate inside jets. Physically cleaning of the jet hole or replacement of the jet is the only guaranteed solution. Appropriate size genuine Keihin pilot and main jets are readily available at a quite modest cost. http://www.pjmotorsports.com/keihin-jet ... low%20jets
The problem occurs in infrequently used motors with fuel left in the bowls to evaporate away. That creates a concentration that sets rock hard on the walls of the jets as it dries completely. As this process repeats so the buildup. It is not a matter of if it will happen if not managed, just when.
The way to prevent, or at least significantly delay, the re-occurrence of the problem is to always drain the carbies before putting the engine away if you don't use the engine (lawn mower, outboard, motorcycle, whatever) quite frequently, like every couple days at least.
The easiest way to do that on a Zephyr would be to fit a manual fuel tap, and after turning the tap off run the engine till the bowls suck dry and the engine stops.
Re: Misfire on cold start up.
Your explanation makes absolute sense now as I remember I did notice a darker color on some area's of the jets I couldn't get away completely. So next time I pull the carbs I will replace those pilot jets! Thanks for the feedback again 
Re: Misfire on cold start up.
On today's 30 C temperature ride (sorry guys just got to rub it it) it occurred to me how you can test the pilot jets.
Pull the spark plug caps from the spark plugs and sit them lightly back in place on top of the plugs. Start the bike up, and bring it up to operating temperature. With it idling, using a pair of long nose pointed pliers (preferably with thick insulated handles) to lift well clear each plug cap one at a time and observe the change in idle speed and 'beat'
Assuming compression pressures are all ok and roughly equal, the engine should respond reasonably similar to taking each of the cylinders one at a time out of action. If you find that's not the case and some cylinders are 'lazy' in comparison to the others, there is really only reason to explain this (assuming the mixture screws have all been set correctly).
And that reason is ................ you guessed it, partially blocked pilot jet/circuit.
Pull the spark plug caps from the spark plugs and sit them lightly back in place on top of the plugs. Start the bike up, and bring it up to operating temperature. With it idling, using a pair of long nose pointed pliers (preferably with thick insulated handles) to lift well clear each plug cap one at a time and observe the change in idle speed and 'beat'
Assuming compression pressures are all ok and roughly equal, the engine should respond reasonably similar to taking each of the cylinders one at a time out of action. If you find that's not the case and some cylinders are 'lazy' in comparison to the others, there is really only reason to explain this (assuming the mixture screws have all been set correctly).
And that reason is ................ you guessed it, partially blocked pilot jet/circuit.
Re: Misfire on cold start up.
Sounds convincing Freddy
Do have a remark though. Removing te spark plug means high voltage energy will be build up in the ignition coil, energy that needs to go some where, it simply tries to find the path of lowest resistance and in some occasion it will pull a spark within the high voltage coil itself damaging the internal insulation (shortening it)...
At least this happens frequently on 2 stroke mopeds from the 70s.
Do have a remark though. Removing te spark plug means high voltage energy will be build up in the ignition coil, energy that needs to go some where, it simply tries to find the path of lowest resistance and in some occasion it will pull a spark within the high voltage coil itself damaging the internal insulation (shortening it)...
At least this happens frequently on 2 stroke mopeds from the 70s.