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Poor idle, weak spark.

nigxl

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I have a rough running and generally lack of idle on my bike (a 1979 CB750K UK bike) when I saw another similar post.
I believe my wiring may have been swapped on my coils as my +ve connections are the yellow and blue cables, could this be my issue??
The bike will start however it is not happy to idle below 2K revs and generally sounds like it is running on 2 cylinders as it is very knocky.
I have done the electrical tests as per manual and everything seems to be Ok but if I disconnect a wire from coil 1 the bike runs the same.
I have swapped coils over and the fault is still the same on coil 1 (which is now coil 2 if that makes sense) so I'm assuming the fault lies further back.
When testing the pulser assy I get a spark across plugs 1 and 2 but it does seem to be quite weak (the plugs are new).
I'm not sure if one of my spark units is faulty as it seems to have leaked most of its potting out also, although I have changed these over banks to test and I still have the issue on cylinders 1 and 4 on pulling the coil cable she runs the same.
 
IMG_20170721_142523[1].jpg Pic of the spark units.
 
The spark units leak the tar on early ones all the time but can still be good for years as all that has to happen there was cover all parts in it, if still covered then good. They are NOT overheating like so many say when they see that. I had them leak the compound even when sitting but in a Texas summer. Honda realized the issue later and changed the compound. It being tar though coats the parts pretty good and all that's needed, just clean up the excess.

Switch the coil wires, the blue and yellow go on minus side.

Spark is not nearly everything to making a good idle, the carbs are notorious on these for clogging up the idle circuits to make people clean them 5 times before getting them right and every time they swear they are clean.

The engines never get the valves set either due to needing shims and 'too much work' once an owner knows that, and it leads to crap idle as well when engine compression goes south. I used to buy and sell them based on that fact and made some pretty big money at times. Meaning you need a compression test right off the bat or likely wasting a lot of time and money.
 
Hi thanks for the reply.
I have switched the wires to no avail unfortunately.
I did attempt to do a compression check but my tester has what I thought was a standard plug thread that I have used on many outboard and car engines in the past, guess the bike engines use a smaller thread. Guess I'll have to get an adaptor made up.
I'll admit to not checking the valve clearances, should have done when I replaced the leaking cam cover gasket :banghead:
I didn't realise how critical it was until joining this forum.
I did remove the pulser cover and started the bike to check the ignition timing yesterday which seemed ok but the rattling was amplified when I did this which leads me to think that the starter clutch is shot, the large cog definitely seems lose when prodded with a screwdriver.
Think I have a bit more work to do around this area before progressing further, carbs have been rebuilt I should add with new jets and needles so I was expecting great things.
 
Here in '79 the idle jets are pressed in place and have to be pulled to change them, that may not be so with Euro models. 'New jets and needles' may not address idle issues at all either. The air cuts MUST be in good shape and the mixing chamber under the butterfly must be clear as well.

The 'large cog' WILL seem loose but not the issue. It's the smaller diameter part in front and right behind advance unit with the 3 bolts on the face, they come loose all the time.
 
Here in '79 the idle jets are pressed in place and have to be pulled to change them, that may not be so with Euro models. 'New jets and needles' may not address idle issues at all either. The air cuts MUST be in good shape and the mixing chamber under the butterfly must be clear as well.

The 'large cog' WILL seem loose but not the issue. It's the smaller diameter part in front and right behind advance unit with the 3 bolts on the face, they come loose all the time.

I did have a few issues with the carbs.
On no 4 cylinder the air slider was stuck and not returning under spring pressure.
2 of the pilot air screws had snapped their ends off in the carbs and all 4 rubber bungs over the idle jet (hope I'm right there) were hard and split so I had new aluminium bungs made in a machine shop with o rings fitted instead. Found that idea on a CX500 forum.
The carbs had been cleaned but not very well and also the o rings were leaking on the fuel rail.
I did find press in idle jets in the carb kit but didn't fit them for fear of damaging the carb bodies when attempting to remove the old ones.
I did a bench sync on the carbs and blew through every orifice I could find with carb cleaner and everything seemed clear.
Excuse my ignorance but what are you referring to when you mention 'air cuts' ?
Thanks for the guidance so far.
Also I have been having a conversation with another member on here and here are a few more points I discussed with him.

'Thanks for the useful info. The starter does not seem to be disengaging as when the starter button is released the starter still sounds as if it is spinning as the bike is spluttering to run (maybe on 2 cylinders).
Also when the bike stalls the starter also seems to be heard disengaging.
Think I will need to sort this out first as I have already bought new LHS crankcase gasket as it appears to be leaking, that and the amount of red Hermatite around the sealing face makes me think this area has had issues in the past :rolleyes:
The screw heads are also a bit chewed around the plate that holds the pulser coils so I might just as well disassemble everything and give it a good sorting out :thumbsup:.'
 
Air cuts are the deceleration valves on the left sides of all 4 carbs as they mount on bike and you sitting on it. They mainly stop backfiring in the pipe at decel due to too lean at that time. They work by having two idle air jets in top of carb under slide rim, one gets cut off at decel only to richen the idle pullthrough mixture then. At all other times both the air jets are open for leaner mixture for emissions. Too lean at high vac decel makes the mixture not burn fast enough, it then continues burning back inside the exhaust pipe. If one of those diaphragms leaks then you have an idle vacuum leak and the idle will be flawed.

As well, broken idle mix screws scream that someone has bottomed them out hard on the seats and then that damages the seat to mean you no longer can turn the screw a certain number of turns to get what you want. With carb bank off bike so you can see the outlet holes you lightly seat all 4 needles and then look in carb bores to see if any one needle tip is sticking way up out of the hole unlike the others, damage indicator. No tips seen is good. You are looking for the furthest forward hole in front of the butterfly in the BOTTOM of the bore, the curb idle discharge hole. The hole you see higher on the right side wall (while looking in carb fronts) is the air cut vacuum supply port.

The idle FUEL jet (pilot, under the ex-rubber plug) has a very small hole in the top that people use an 'E' guitar string to get to to open up, it is the correct .013" size needed. The problem is there are holes punched sideways to add air to emulsify the fuel to make it mix better and you cannot get to those when the jet is pressed in.
 
Thanks for all the helpful info, looks like I'll be pulling the carbs again then, I only shot a load of carb cleaner through the idle fuel jet, didn't actually probe it.
I think I read somewhere (probably on here) that you can bypass the air cuts by tweaking open the pilot mixture screws is that correct?
Appreciate all the guidance :thumbsup:
 
If the bank was never broke apart into 4 separate carbs the air cut diaphragms are ruined if you dunked carbs into cleaner.

The idle screws do get tweaked but that is not all of it.

http://www.motorcycleproject.com/text/keihin_idle_circuits_white_paper.swf

Page 15 starts the aircut section, 22 shows what you do, elsewhere in there discusses the pressed pilots too.
 
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If the bank was never broke apart into 4 separate carbs the air cut diaphragms are ruined if you dunked carbs into cleaner.

The idle screws do get tweaked but that is not all of it.

http://www.motorcycleproject.com/text/keihin_idle_circuits_white_paper.swf

Page 15 starts the aircut section, 22 shows what you do, elsewhere in there discusses the pressed pilots too.

Thanks I'll look into that.
The carbs were not immersed in cleaner but they did have a lot of spraying into most orifices, I didn't remove the air cut covers however.
I think another stripdown is on the cards though :thumbsup:
 
Just a quick update on this, I needed to sort out the starter clutch rattle so I removed the advance assembly and found that the pin holding it was damaged and had also chewed on the assembly. The starter bolts have now been cleaned and loctited and the e metal is curing on the assembly ready for refitting tomorrow. I'm guessing that I was getting a false TDC reading due to this, more tomorrow.
 
I have used liquid metal, used it for many other applications with good success so far.
 
I have used epoxies of various types for many years and under loads of up to 750 psi but it has to be done with some solid thinking to it. I said what I did based on seeing what most people do with epoxy while I was in parts sales, most people misapply the product in ways that absolutely will not work. Of course if you ask they do not like being told that. The honest ones would then come back and admit it though.

The assembly there will not be held by the pin, it is only a locator for position and not intended to hold the entire thing in place. The TORQUE there is the true part holder. Honda had issues with them coming loose in the field and they started loctiting the center bolt after tightening the crap out of them.
 
So carbs are pulled and air cut no1 inspected. IMG_20170801_144740[1].jpg
The plunger was seized in the carb body and as can be seen a lot of corrosion but the rubber seemed fine.
IMG_20170801_144746[1].jpgIMG_20170801_150001[1].jpg
 
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