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CB350/400 / Gummed up Carbs and modern fuel
« on: September 01, 2010, 10:25:44 PM »
Best grab a mug of coffee - this is a bit of a long post.
Two years ago, after my 400/4 had been laid up for a couple or more years, I have had to jump through all of the usual hoops to get the carbs cleaned up and working again.
On opening them up I was faced with a solid coating of brown varnish and the floats stuck solid on their pivots.
I completely stripped them and soaked everything in methylated spirit for a couple of days. Most of the varnish turned to a brown powder which came off easily with a tooth brush and soapy water. I found that the usual carb cleaners evaporated too quickly to be of any use. After an initial clean up I could remove the main jet , emulsion tube and pilot jet. I was still left with the stuck floats. After mangling one float trying to work it loose by spraying carb cleaner over the pivot and wiggling it, I eventually had some success with using a gas soldering iron with the miniature hot air attachment to locally heat up the pivot enough to soften the varnish and was then able to push out the pin.
The whole lot was soaked again in jam jars of methylated spirit and the jars were dropped into an ultrasonic bath in the hope of cleaning the hard to reach places. After rinsing off and blowing through with an air line the small passages were treated to a blast of carb cleaner, and catching what came put on some paper to make sure it was clean.
Main jet, float valve and pilot jet were replaced with genuine Honda items together with all of the O rings. Float heights set. After statically balancing the carbs everything seemed to be in order apart from one incontinent carb. As I have since found out this was the float that was mangled during disassembly. Basically the pivot and tang was out of alignment causing the tang to press on the needle at an odd angle and also the float was able to jam in the fully open position, hence the need to tap the bowl with a screwdriver handle when first switching on the fuel.
Fast Forward 2 years.
I finally got around to pulling off the carbs and sorting out the float problem by swapping all of them for a spare set. Reset the float heights again. Just for good measure I put an airline on the fuel feed pipe at a very low pressure and was able to verify that the needles were sealing properly and at the right height. Once again, I checked all of the small passages were clear.
After all of this I am still left with a richness problem at low to medium throttle settings. After a few days in 30-40 mph limits the plugs are black with dry soot. After wringing it's neck down the dual carriageway the plugs are a dark brown. Sometimes when sitting at traffic lights the pong from the exhaust makes your eyes water.
Also adjusting the pilot air screw does not change the engine speed. I haven't balanced the carbs properly, there seems no point yet if the idle mixture is off.
Other symptoms - runs better from idle to 3000 rpm when half warmed up, stumbles a little 2000-3500 when gently on/off the throttle when up to temperature.
Other snippets that I have found courtesy of google.
The black plastic float material can absorb fuel as the plastic ages and so give an artificially higher than normal fuel level
Modern fuels are more dense and rely on complex injection systems to correctly atomise the fuel
Modern fuels atomise poorly in carbs- droplets too big giving the symptom of running rich wheras the mixture is actually lean (?? )
Continued flooding of carbs can cause fuel in the oil which then gets recycled back to the air box causing over rich mixture
Some people have drilled extra cross feed holes in the emulsion tubes to improve matters
Also see this
http://www.ksrc-au.com/Bike_Optimax.pdf
So, does any one believe any of the 5 snippets above or had experience of them?
By the way I am using the bog standard Shell unleaded and have used unleaded since it was first on the forecourts, prior to that I always ran on 3 star leaded.
Airbox , filter jet sizes - in fact everything is as Mr Honda intended.
Other sites recommend checking the needles again and oval wear to the top of the emulsion tubes, but at low throttle settings there shouldn't be any thing coming from them any way - or should there be?
I'm now just starting to add a fuel stabiliser so that I don't have to put up with gum and varnish again, Never had any problems like this with the good old leaded stuff.
Perhaps we could have a poll on here - how black are your plugs after pottering around town
One thing I do know that causes problems is the tool tray cover migrating towards the air inlet and nudging up to the cut away in the inlet rubber.
My next move is to try a hotter plug for local riding, this is what Shell in Australia advised their customers to do
I'm clean out of ideas now - any suggestions?
Regards
Dave
Two years ago, after my 400/4 had been laid up for a couple or more years, I have had to jump through all of the usual hoops to get the carbs cleaned up and working again.
On opening them up I was faced with a solid coating of brown varnish and the floats stuck solid on their pivots.
I completely stripped them and soaked everything in methylated spirit for a couple of days. Most of the varnish turned to a brown powder which came off easily with a tooth brush and soapy water. I found that the usual carb cleaners evaporated too quickly to be of any use. After an initial clean up I could remove the main jet , emulsion tube and pilot jet. I was still left with the stuck floats. After mangling one float trying to work it loose by spraying carb cleaner over the pivot and wiggling it, I eventually had some success with using a gas soldering iron with the miniature hot air attachment to locally heat up the pivot enough to soften the varnish and was then able to push out the pin.
The whole lot was soaked again in jam jars of methylated spirit and the jars were dropped into an ultrasonic bath in the hope of cleaning the hard to reach places. After rinsing off and blowing through with an air line the small passages were treated to a blast of carb cleaner, and catching what came put on some paper to make sure it was clean.
Main jet, float valve and pilot jet were replaced with genuine Honda items together with all of the O rings. Float heights set. After statically balancing the carbs everything seemed to be in order apart from one incontinent carb. As I have since found out this was the float that was mangled during disassembly. Basically the pivot and tang was out of alignment causing the tang to press on the needle at an odd angle and also the float was able to jam in the fully open position, hence the need to tap the bowl with a screwdriver handle when first switching on the fuel.
Fast Forward 2 years.
I finally got around to pulling off the carbs and sorting out the float problem by swapping all of them for a spare set. Reset the float heights again. Just for good measure I put an airline on the fuel feed pipe at a very low pressure and was able to verify that the needles were sealing properly and at the right height. Once again, I checked all of the small passages were clear.
After all of this I am still left with a richness problem at low to medium throttle settings. After a few days in 30-40 mph limits the plugs are black with dry soot. After wringing it's neck down the dual carriageway the plugs are a dark brown. Sometimes when sitting at traffic lights the pong from the exhaust makes your eyes water.
Also adjusting the pilot air screw does not change the engine speed. I haven't balanced the carbs properly, there seems no point yet if the idle mixture is off.
Other symptoms - runs better from idle to 3000 rpm when half warmed up, stumbles a little 2000-3500 when gently on/off the throttle when up to temperature.
Other snippets that I have found courtesy of google.
The black plastic float material can absorb fuel as the plastic ages and so give an artificially higher than normal fuel level
Modern fuels are more dense and rely on complex injection systems to correctly atomise the fuel
Modern fuels atomise poorly in carbs- droplets too big giving the symptom of running rich wheras the mixture is actually lean (?? )
Continued flooding of carbs can cause fuel in the oil which then gets recycled back to the air box causing over rich mixture
Some people have drilled extra cross feed holes in the emulsion tubes to improve matters
Also see this
http://www.ksrc-au.com/Bike_Optimax.pdf
So, does any one believe any of the 5 snippets above or had experience of them?
By the way I am using the bog standard Shell unleaded and have used unleaded since it was first on the forecourts, prior to that I always ran on 3 star leaded.
Airbox , filter jet sizes - in fact everything is as Mr Honda intended.
Other sites recommend checking the needles again and oval wear to the top of the emulsion tubes, but at low throttle settings there shouldn't be any thing coming from them any way - or should there be?
I'm now just starting to add a fuel stabiliser so that I don't have to put up with gum and varnish again, Never had any problems like this with the good old leaded stuff.
Perhaps we could have a poll on here - how black are your plugs after pottering around town
One thing I do know that causes problems is the tool tray cover migrating towards the air inlet and nudging up to the cut away in the inlet rubber.
My next move is to try a hotter plug for local riding, this is what Shell in Australia advised their customers to do
I'm clean out of ideas now - any suggestions?
Regards
Dave
