Hello all
This is my first post in the forum, but have been an occasional reader for some years now.
I have a 1982 CB750K DOHC European version (i.e. without acceleration pump in the carbs, but otherwise same mechanics as in the American version, as far as I know). The problem is cylinder 1 spark plug sooting. The background is that I've had the bike for 25 years now and I rebuilt the engine back in 2021. Included in the overhaul were cylinder honing, new piston rings, head planing, all new valves, all new main and connecting rod bearings, new cam chains and chain tensioner, new carb insulator rubbers etc. Naturally new gaskets, oil seals, filters, plugs, oil were used in reassembly. The head job, valve installation and adjustment and honing was done at a professional machine shop, engine assembly was done by myself. This was a fourth car or MC engine rebuild for me so I'm not a pro but not a full rookie either. I synced the carbs myself using a DIY tool.
I did not experience problems in the first two summers after the engine overhaul but in the summer of 2023 I noticed the cylinder 1 spark plug started fouling due to soot buildup. The first symptom was lack of immediate power when accelerating on top gear after several km's of low power cruising. Hence, cruising at 4500 rpm for some time and then just twisting the throttle, it took a while with engine "coughing" for a while, feeling like it was firing on three cylinders, and then after a couple of seconds, it "found" the fourth spark and full power to accelerate. After a WOT acceleration, the operation of the engine seemed to get back to normal, until cruising for some tens of km's again on lower power.
It seemed like the WOT acceleration would clean the plug so that it worked ok again, but cruising then accumulated soot on the plug, causing the symptoms to reappear. If I renew the sparks or thoroughly clean them the bike works ok for like 60-100 miles and then starts showing the symptoms again. WOT acceleration cleans the plugs for 20-30 km and then the symptoms start again. I have a slight feeling that over the past two summers the problem has slowly gotten worse so that the symptoms appear earlier after plug cleaning or removal, but am not 100 % sure.
I renewed the spark plugs with no change. The plug wires had just been renewed, so I just swapped them between cylinders, but the problem remained in cylinder 1. I swapped the coils with each other, but the problem persisted in cylinder 1. So I guess all ignition related had been tried out without result.
I checked the compression and it was good, actually cylinder 1 had the highest compression of all cylinders with a small margin. I checked the valve adjustments, nothing to rework there.
I resynced the carbs, no difference. I Opened the carbs, they had been rebuilt with overhaul kits some years earlier and had worked well all their life (apart from very difficult starting when the bike has been standing for several days). The AF mixture screws had never been touched by me. I opened the cyl 1 carb, cleaned it, checked everything, found nothing wrong with it. I checked that the carb insulators were undamaged and found nothing wrong with them. No help, still soot in cyl 1 plug. The choke plate's structure is such that if it was stuck, so would cylinder 2, and all choke and throttle plates work properly anyways.
I removed the carbs a second time. I also removed the vacuum springs of all four carbs and checked their spring tension against a letter scale, all four showed similar numbers. I swapped all the possible carb components (springs, needles, jets, nozzles, floats, float valves, diaphgrams, anything that could be removed from the carb core) between cyl 1 and 2 carbs, but no difference, the problem stays in cylinder 1. I tried turning the cylinder 1 AF screw in half a round, no difference. I then turned it back out to original position.
I removed the exhaust and the pipe in cylinder 1 is very sooty, also the exhaust port for cyl 1 is covered in soot. Not a suprise.
I've been wondering if something went wrong in the engine rebuild, and the first two summers the problem was so mild that I didn't notice it (I drive only 1000 km per summer) and now the problem has progressively gotten worse so I started noticing it. Other than that, I think nothing extraordinary has happened to the bike which could have caused this problem to suddenly appear.
When I took the engine parts to the machining shop in 2021, there were some marks in the cylinders at the location where the piston is at Top Dead Center. The experienced machinist told me that honing should be enough, and after honing there were still small marks left. He told me that he did not want to hone it any further so that the cylinder to piston clearanc would not become excessive. Upon reassembly I measured the clearance and it seemed ok but on the high end of the tolerances. I've been wondering if cylinder 1 did end up having excessive clearance and that could cause the problems. But could excessive clearance soot the plug, and wouldn't it cause any compression decline?
The obvious reason for a sooty plug would be rich mixture. This could be caused by a faulty carb, but I have found nothing wrong with it, and the problem does not follow the carb components to the next cylinder, but remains in cyl 1. The carb core can't be interchanged between cylinders, and sure the core of cyl 1 carb could be faulty but what would have broken it so that I haven't found the problem in it? If the problem is in the carb, at least in that case the problem should not be developing so that sooting slowly gets worse and worse, because I can't come up with any way the carb could be getting slowly more and more faulty in time. But I could be wrong here.
Could there be some issues with intake or exhaust valves that don't have an effect in the compression measurement, but cause plug sooting? I can't imagine how that would happen, but have found some guessing on the internet on that kind of issues causing plug sooting.
At the moment I have two possible ways to go forward: buy a new carb for cylinder 1 (costly) or remove the engine from the bike, remove the head and check what is inside the cylinder and how the valves are doing (a lot of work, and I don't even know what to look for). Either way, I would not like to do it for no result.
Any ideas, experiences and suggestions are welcome before I use my precious time and/or money to just find out the problem persists. I'm kind of getting crazy with this neverending mystery here.
Thanks for reading, if you really did have the energy to get to the end of this report.
This is my first post in the forum, but have been an occasional reader for some years now.
I have a 1982 CB750K DOHC European version (i.e. without acceleration pump in the carbs, but otherwise same mechanics as in the American version, as far as I know). The problem is cylinder 1 spark plug sooting. The background is that I've had the bike for 25 years now and I rebuilt the engine back in 2021. Included in the overhaul were cylinder honing, new piston rings, head planing, all new valves, all new main and connecting rod bearings, new cam chains and chain tensioner, new carb insulator rubbers etc. Naturally new gaskets, oil seals, filters, plugs, oil were used in reassembly. The head job, valve installation and adjustment and honing was done at a professional machine shop, engine assembly was done by myself. This was a fourth car or MC engine rebuild for me so I'm not a pro but not a full rookie either. I synced the carbs myself using a DIY tool.
I did not experience problems in the first two summers after the engine overhaul but in the summer of 2023 I noticed the cylinder 1 spark plug started fouling due to soot buildup. The first symptom was lack of immediate power when accelerating on top gear after several km's of low power cruising. Hence, cruising at 4500 rpm for some time and then just twisting the throttle, it took a while with engine "coughing" for a while, feeling like it was firing on three cylinders, and then after a couple of seconds, it "found" the fourth spark and full power to accelerate. After a WOT acceleration, the operation of the engine seemed to get back to normal, until cruising for some tens of km's again on lower power.
It seemed like the WOT acceleration would clean the plug so that it worked ok again, but cruising then accumulated soot on the plug, causing the symptoms to reappear. If I renew the sparks or thoroughly clean them the bike works ok for like 60-100 miles and then starts showing the symptoms again. WOT acceleration cleans the plugs for 20-30 km and then the symptoms start again. I have a slight feeling that over the past two summers the problem has slowly gotten worse so that the symptoms appear earlier after plug cleaning or removal, but am not 100 % sure.
I renewed the spark plugs with no change. The plug wires had just been renewed, so I just swapped them between cylinders, but the problem remained in cylinder 1. I swapped the coils with each other, but the problem persisted in cylinder 1. So I guess all ignition related had been tried out without result.
I checked the compression and it was good, actually cylinder 1 had the highest compression of all cylinders with a small margin. I checked the valve adjustments, nothing to rework there.
I resynced the carbs, no difference. I Opened the carbs, they had been rebuilt with overhaul kits some years earlier and had worked well all their life (apart from very difficult starting when the bike has been standing for several days). The AF mixture screws had never been touched by me. I opened the cyl 1 carb, cleaned it, checked everything, found nothing wrong with it. I checked that the carb insulators were undamaged and found nothing wrong with them. No help, still soot in cyl 1 plug. The choke plate's structure is such that if it was stuck, so would cylinder 2, and all choke and throttle plates work properly anyways.
I removed the carbs a second time. I also removed the vacuum springs of all four carbs and checked their spring tension against a letter scale, all four showed similar numbers. I swapped all the possible carb components (springs, needles, jets, nozzles, floats, float valves, diaphgrams, anything that could be removed from the carb core) between cyl 1 and 2 carbs, but no difference, the problem stays in cylinder 1. I tried turning the cylinder 1 AF screw in half a round, no difference. I then turned it back out to original position.
I removed the exhaust and the pipe in cylinder 1 is very sooty, also the exhaust port for cyl 1 is covered in soot. Not a suprise.
I've been wondering if something went wrong in the engine rebuild, and the first two summers the problem was so mild that I didn't notice it (I drive only 1000 km per summer) and now the problem has progressively gotten worse so I started noticing it. Other than that, I think nothing extraordinary has happened to the bike which could have caused this problem to suddenly appear.
When I took the engine parts to the machining shop in 2021, there were some marks in the cylinders at the location where the piston is at Top Dead Center. The experienced machinist told me that honing should be enough, and after honing there were still small marks left. He told me that he did not want to hone it any further so that the cylinder to piston clearanc would not become excessive. Upon reassembly I measured the clearance and it seemed ok but on the high end of the tolerances. I've been wondering if cylinder 1 did end up having excessive clearance and that could cause the problems. But could excessive clearance soot the plug, and wouldn't it cause any compression decline?
The obvious reason for a sooty plug would be rich mixture. This could be caused by a faulty carb, but I have found nothing wrong with it, and the problem does not follow the carb components to the next cylinder, but remains in cyl 1. The carb core can't be interchanged between cylinders, and sure the core of cyl 1 carb could be faulty but what would have broken it so that I haven't found the problem in it? If the problem is in the carb, at least in that case the problem should not be developing so that sooting slowly gets worse and worse, because I can't come up with any way the carb could be getting slowly more and more faulty in time. But I could be wrong here.
Could there be some issues with intake or exhaust valves that don't have an effect in the compression measurement, but cause plug sooting? I can't imagine how that would happen, but have found some guessing on the internet on that kind of issues causing plug sooting.
At the moment I have two possible ways to go forward: buy a new carb for cylinder 1 (costly) or remove the engine from the bike, remove the head and check what is inside the cylinder and how the valves are doing (a lot of work, and I don't even know what to look for). Either way, I would not like to do it for no result.
Any ideas, experiences and suggestions are welcome before I use my precious time and/or money to just find out the problem persists. I'm kind of getting crazy with this neverending mystery here.
Thanks for reading, if you really did have the energy to get to the end of this report.