'82 CB750K cylinder #1 plug sooting

[Beavis]

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.

 

[Davidson750]

Swap your plugs around or try a new plug and see if it still happens I've had bad plugs out of the box before sometimes we overthink things.

 

[Beavis]

Thanks for the tip! However, spark plugs have been totally renewed once in the process of troubleshooting, and Iforgot to mention that after the renewal I've swapped the plugs several times between cylinders, so that actually I think all four of them have been in cylinder #1, and they all collect soot there, but not in other cylinders. All other components of the ignition system have been swapped between cylinders also, so I think ignition as a whole is ruled out as the cause for the problems.

I also forgot to mention that I naturally did check the fuel level in cylinder #1 carb. Both when it was dismantled, but also by installing transparent hoses to fuel drainage holes, opening the valves and checking where the level stands. It all checked out ok, so fuel level should not be the cause.

 

[Davidson750]

Seems like you got it covered.. you could check valve clearances.. go through and look for any big discrepancies... .005 intake/exhaust

 

[Beavis]

I checked the valve clearances as one of the first things I did after trying out all the ignition part swapping. All clearances were within factory tolerances.

But I could check them again to see if anything has changed, especially clearances in cyl #1. It's been a while since the check and I have done some hundreds of miles.

When considering my options for continuing the troubleshooting, I've wondered what could be wrong inside the cylinders that causes this. For pulling the head, the most obvious reason (in my non-professional engin builder mind ) is that there is something wrong with the valves or valve seats, causing the valves not to close properly, but for some reason this not showing up in the compression test. A change in the valve tolerances could be a result of such a situation.

I'm just afraid that I do all the work to remove the engine and remonving the head, but I find nothing abnormal. Then I just reinstall everything and the same symptoms continue. To avoid this frustration is why I'm looking for any new ideas on where to look and what to look for.

 

[Davidson750]

I completely understand... I'm thinking more carburetor.. when you did Jets and the rebuild,.. did you use genuine kehein(jets,etc) parts.. aftermarket rebuild kits I've found have been pretty unreliable.

 

[Beavis]

I think the rebuild set I used was non-genuine, i.e. non-Keihin. I got it from Ebay if I remember correctly. But this was years before the engine rebuild, hence years before the soot problam started. I also swapped all these parts from #1 to #2 carburetor when troubleshooting, but the problem stayed in #1, so I guess the rebuild set parts should not have caused this.

I think I still have the original carburetor parts, so I could give it a try. But I'm not very optimistic about it helping.

Since swapping everything I could from one carb to the other did not move the problem to the other cylinder, the only way the carb is causing this problem would be if the carb body is somehow corrupted, because the body is pretty much the only part I could not swap. The fault should not be in the jets, needles, springs, float, valve etc. which were all swapped to cylinder #2 without any change in the problem.

 

[Davidson750]

If not carbs,rings..were the gapped?.. installation went smoothly?... I use a second pair of hands if available ,and I made blocks to keep the Pistons from going down looks like tuning forks..good luck

 

[dirtdigger]

factory clearances are too tight for valve clearances. need to be upped to .005" factory clearance will burn valves. compression needs to be at least 150psi to be healthy, 120 or less will cause running problems, 100 is basically a dead hole. 175 is ideal compression for a strong runner.

leakdown tests will determine if its rings or valve issue

piston to cylinder clearance would have to be extremely high to cause an issue, you would hear a piston rattle before you will notice a running problem. same with ring end gap, it has to be pretty extreme to cause an issue. Now rings that didnt seat or engine later overheating will cause compression loss and oil usage. you will get a wet plug if burning oil, dry black sooted plug is mechanical issue or spark issue. Over fueling normally cause wet plug if extreme but can cause black dry soot if only slightly rich. All sorts of running and fuel issues will occur if not running the factory air box.

 

[Beavis]

Thank you again for taking the time to comment and help, it is highly appreciated!

If not carbs,rings..were the gapped?.. installation went smoothly?... I use a second pair of hands if available ,and I made blocks to keep the Pistons from going down looks like tuning forks..good luck

Cylinder block installation went ok. This was the second time I did that for this engine and I remember the first time ca. 20 years ago was more of a pain, and I managed to break one of the new piston rings, so had to replace it. This time it went smoothly without problems. I think I also used some wood blocks as help as seen in your picture. The rings were renewed in the overhaul and I did check the gaps before installation, the gaps were within factory tolerances.

The piston rings I used were a part of a rebuild set I got from 4into1.com: https://4into1.com/engine-rebuild-kit-honda-cb750c-f-k-l-sc-1979-1982/

Not sure if the quality of the rings is good, they definitely were not genuine Honda rings.

I checked my notes on the latest valve clearance check, cyl #1 had 0,10 mm (0.004") and 0,16 (0.006") mm for exhaust and 0,12 (0.005") and 0,10 mm (0.004") for intake valves. The factory spec is 0,06-0,13 mm so one of the exhaust valves was a bit over the spec, but I can't imagine how such a small deviation could cause soot buildup.

Valves were all renewed upon the engine overhaul. I got them off Ebay from a German seller AC-Motorradteile (not sure if they're selling the exact same valves now, but I could find them based on the part code of my 2021 order):
https://www.ebay.com/itm/2908681775...PDCTEE&hash=item43b9182ab7:g:wKwAAOxyQj9RJ1SI
https://www.ebay.com/itm/3631307834...7RW39H&hash=item548c4806ef:g:8BQAAMXQAx9RJ1SP

It says "Honda original Ersatzteile!" (Genuine Honda spare parts) for the current product for sale but I think my valves are not genuine Honda parts.

My notes from compression testing (stone cold engine) show 10 bar (145 psi) for cylinders #1, #3 and #4 and 9 bar (130 psi) for cylinder #2. I haven't been more precise in writing them down, because I was more concentrated on differences between cylinders, and the absolute amounts seemed fine to me. The precise measurements could be 10,x and 9,x bars, so the psi numbers are quite approximate.

I haven't noticed any oil consumption in the engine, but I have only driven some hundreds of miles per summer (oil change every year) in the past three summers, because of this problem. The soot is definitely of the dry and black type, the plug in cyl #1 hasn't been wet or had oil on it. It is black powder soot on the plug as well as in the exhaust port. It builds up quite fast, so I think if it was caused by a rich mixture, it should be really rich. A friend who was riding behind me once told me that at WOT acceleration, he could see some black smoke coming out of the exhaust.

The motor has the factory air box with a standard, renewed, clean air filter. All other parts of the engine are standard, too, no modifications.

dry black sooted plug is mechanical issue or spark issue

Since I believe that ignition problems have been ruled out by component swapping between cylinders, I would like to learn what mechanical issues would be the most likely causes for sooting in one cylinder?

 

[Davidson750]

factory clearances are too tight for valve clearances. need to be upped to .005" factory clearance will burn valves. compression needs to be at least 150psi to be healthy, 120 or less will cause running problems, 100 is basically a dead hole. 175 is ideal compression for a strong runner.

leakdown tests will determine if its rings or valve issue

piston to cylinder clearance would have to be extremely high to cause an issue, you would hear a piston rattle before you will notice a running problem. same with ring end gap, it has to be pretty extreme to cause an issue. Now rings that didnt seat or engine later overheating will cause compression loss and oil usage. you will get a wet plug if burning oil, dry black sooted plug is mechanical issue or spark issue. Over fueling normally cause wet plug if extreme but can cause black dry soot if only slightly rich. All sorts of running and fuel issues will occur if not ru

 

[Davidson750]

We've been doing .005 intake and exhaust... factory specs are a little tight as I mentioned earlier...750s are known for running lean..it's the exhaust valve on that cylinder..... We are dealing with thousands of an inch so yes it can make a difference.and of course proper ignition timing.. if you need shims I have quite a few

 

[Beavis]

US models have accelerator pumps, right? European models don't have the pumps and I've understood it is because of different emission standards, which required bikes sold in the US to be adjusted leaner than the ones in Europe. Overall leaner adjustment caused problems in acceleration, so the pumps were added to US models to solve that.

Don't remember where I read this from but this is my impression. So the carbs are a bit different in North America and I guess the running lean -issue is also mostly for US models.

Not that this would have any connection to my problems, just a thought that popped my mind after the comment on running lean.

 

[Davidson750]

The kehein 43's and 42's (750/900)all have accelerator pumps. I live in Dallas Texas very hot very dry I have to fatten up everything on the carbs... The weather is getting nicer you might have to just start digging into it and see what you get but I would start at those valves easy to do don't have to pull the motor.good luck.