Hole in Cylinder Block... 1979 CB750K

[jwaltmire]

Hi all,

Brand new poster here on the forums, although I've been doing a fair bit of reading since I purchased my 79 CB750K about four months ago. Y'all do great work.

I received some pretty devastating news from my local shop this week after taking the bike in to check on a pretty hefty oil leak. Turns out there's a 1/2" hole in the lower front of my cylinder block, right where the front cam chain is housed. See picture below for reference:


Looks like the previous owner (who told me nothing about this) had patched the hole with some kind of epoxy, but it began to crack, thus the oil leak. The picture above shows the hole after I cleared the cracked epoxy away.

I have two questions at this point: 1) is it worth trying to patch a hole this big with a JB Weld high temp putty, or something similar? And 2) how difficult might it be to locate a "new" cylinder block and install it? I'm not afraid to get my hands dirty and spend some time on the project, but I've also never taken apart an engine, and cant afford to put more than $500 into the effort.

Any thoughts or guidance would be greatly appreciated.

Cheers,
J

[TOOLS1]

I would be more concerned with what caused the hole to start with, and has it been corrected. As for finding a cylinder, that is easy as I just happen to have one. However you could have a patch welded to yours, or even JB weld a piece of beer can over it. Should last a little while.
TOOLS

[jwaltmire]

Thanks, Tools! Certainly curious as to what may have caused the hole in the first place as well. Possible that it was corroded somehow from the inside? I'd guess that'd give me some bigger problems to worry about. Bike had been running perfectly before the oil leak, though.

I'll try the aluminum patch and see how that fares. How much might I be looking at for your cylinder block?

[TOOLS1]

I doubt it was corrosion. Looks like something either came out, or went in. I would take $50.00 plus shipping for the cylinder.
TOOLS

[shinyribs]

There's no mechanical stress or oil pressure at that area. I'd JB Weld it shut and never worry about it again.

The good ( original) JB Weld is pretty runny. A scrap of metal screen would help it bridge that gap without dripping in.

[jwaltmire]

Thanks, shiny. Its good to know I don't have to worry about a lot of pressure in that area destroying the patch.

I used a combination of high temp putty from JB weld to bridge the gap, followed by some extra-high heat sealant from JB weld to close up any gaps and hold everything together. Test rode today after letting the weld cure overnight, and things seem to be holding up okay. No more oil leak.

The worrying thing now is that the engine seems to be misfiring/backfiring. I noticed this when I rode home from the shop, but it hasn't been happening when I dropped it off. Was hoping it was just a low oil issue due to the leak, but it's still happening after adding new oil. It's a major loss of power in all gears, and some loud popping from the exhaust pipe when revving in 2nd or 3rd gear.

According to the mechanic at the shop-- he just adjusted the o-ring on the oil filter before he noticed the hole in my engine. No other work was done. Why might I suddenly be having this misfiring issue? Possible that something got into the engine through the hole and seized up one or more cylinders? Or would running on low oil have fouled up the ignition system somehow?

Thanks again,
J

[jwaltmire]

Update: changed the spark plugs, still backfiring.

[TOOLS1]

Did the shop do anything to the carbs, or ignition?
TOOLS

[jwaltmire]

Not that I was told. The extent of the work that they did was to clean off the underside of the engine to check for leaking gaskets, and check to ensure that the oil filter was seated correctly. The stark change in performance definitely has me wondering what else may have happened, though.

Any other easy checks that can be done for this problem? Air filter? Ignition wiring?

On Oct 2, 2016, at 1:17 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

Did the shop do anything to the carbs, or ignition?
TOOLS

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[TOOLS1]

How old is the gas? Has the bike sat for awhile with gas in the carbs without being run?
TOOLS

[jwaltmire]

Brand new tank, and has been ridden consistently as my commuter for the past three months. Only began seeing this problem after picking it up from the shop.

On Oct 2, 2016, at 4:17 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

How old is the gas? Has the bike sat for awhile with gas in the carbs without being run?
TOOLS

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[jwaltmire]

Hey all,

Update on the bike, for anyone who's curious:

1) The hole in the cylinder block was likely originally caused by a loose or ill-fitted cam chain which chewed right through the steel. I don't believe that this is still happening-- or else I'd hear it, right? In any case-- the hole is patched with JB weld and seems to be holding up.

2) The misfiring issue has now been solved for. I replaced the spark plugs and ran SeaFoam through the carbs to try and clean them up-- then added 4oz to the two gallons of gas in my tank to continue cleaning as I ride. The bike is now idling and accelerating like a dream, and I'm getting none of the loud popping I'd been experiencing before. I figure that running on low oil for a little while probably gummed up the works a bit-- and SeaFoam helped clear that up (thanks to those on this site that have recommended it's use).

That's all for now. Bike's back up and running just in time for the rainy season here in Oregon!

[Jim13]

How long did the jb weld hold? I have a cb750 k and same thing has happened.

[jwaltmire]

JB Weld did not hold, unfortunately-- although it may have if I'd taken the engine apart and applied the epoxy from the inside and outside after a thorough cleaning.

I purchased a new cylinder block on eBay and took the opportunity to bore the cylinders out and install new pistons, cam chains, cams, and gaskets. It's been worth the investment and the bike is running like a champ.

I'd say that, unless you can correct the problem with the alignment/tension of the cam chain-- you will keep running into the same issue when the chain eats through the epoxy. With the block out of the engine, you could likely have it welded up and not need to purchase a new one on eBay, although either option could be more expensive than the other depending. 

Best of luck!

On Sun, May 20, 2018 at 1:37 PM, Jim13 [via Vintage and Classic Honda's] <[hidden email]> wrote:

How long did the jb weld hold? I have a cb750 k and same thing has happened.

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