Topic: 1972 CB 350 F Engine Rebuild

[Markk71]

Good day!

Long time lurker here but ill start by introducing myself. Name is Mark! Gotta say this forum has been worth its weight in gold for all 3 CB's i've owned. Currently im working on a CB 350 F, has about roughly 35 thousand miles.

Did a compression check on it and had pretty well 65 psi right across ( pretty sad ) but felt optimistic and thought hey, maybe from sitting for 2 decades the rings are just stuck. Got it running the other day and it ran well but sure enough its burning oil. Still feeling optimistic and threw some engine restore into the engine and it did work (shot to about 90 across) but still not good enough to get the rest of the season out of.

Anyway. I bought new pistons and rings. Obviously the heads will be off but just curious on when the timing chains get changed on these bikes? Or anything else that i should consider during that time.

Heres what the bike looked like a few days ago!

[JWilde]

Nice bike. Welcome to the forum and welcome to the 350F club! Here is my 1972 350 F. Hope all goes well with yours

[Markk71]

Yea it will be an over bore kit I purchased online. Primary chain to huh. Any write ups or good sources for a part like that?

But loving the 350f you got there JWILDE.

[JWilde]

Yea it will be an over bore kit I purchased online. Primary chain to huh. Any write ups or good sources for a part like that?

But loving the 350f you got there JWILDE.

Thanks Mark. Buchanans just shipped out replacement aluminum rims (Excel) to me, fully laced etc. Will post pics when they are on the bike.

[bochnak]

Just out of curiosity, what brand of compression gauge did you use?

[camelman]

I wouldn't necessarily expect a 350f to "need" a rebuild by 35K miles. Your compression numbers seem to have recovered from running the engine, and I doubt your compression tester is reading the actual compression value.

Most compression testers have a hose with a threaded end that threads into the spark plug hole. On the other end of the hose is the gauge and a pressure relief valve. The volume of the hose becomes part of the "compressed" volume of the cylinder when it is threaded in, which effectively increases "compressed" volume and decreases peak pressure. In other words, there's a good chance your engine is perfectly fine and doesn't need to be rebuilt because your compression tester is reading incorrectly. Can you post a link to the compression tester you are using?

Something like this will ALWAYS give low compression readings for a small engine, although it'll be somewhat more accurate for a larger engine like a 350 V8: https://www.summitracing.com/parts/aaf-all96520?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-allstar-performance&gclid=CJqwvau0r80CFZRlfgodIVQCfQ

Something like this is more appropriate for a small engine, although it still isn't 100% accurate: http://www.sears.com/stens-compression-tester/p-07184269000P?sid=IDx01192011x000001&gclid=CMTcofu0r80CFQJsfgodGLkJPA&gclsrc=aw.ds


One final point. Turning the engine over more times with the incorrect compression tester will NOT get you closer to the actual pressure reading. The pressure relief valve must be near the spark plug for multiple turns of the engine to have an impact.

[bochnak]

I wouldn't necessarily expect a 350f to "need" a rebuild by 35K miles. Your compression numbers seem to have recovered from running the engine, and I doubt your compression tester is reading the actual compression value.

Most compression testers have a hose with a threaded end that threads into the spark plug hole. On the other end of the hose is the gauge and a pressure relief valve. The volume of the hose becomes part of the "compressed" volume of the cylinder when it is threaded in, which effectively increases "compressed" volume and decreases peak pressure. In other words, there's a good chance your engine is perfectly fine and doesn't need to be rebuilt because your compression tester is reading incorrectly. Can you post a link to the compression tester you are using?

Something like this will ALWAYS give low compression readings for a small engine, although it'll be somewhat more accurate for a larger engine like a 350 V8: https://www.summitracing.com/parts/aaf-all96520?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-allstar-performance&gclid=CJqwvau0r80CFZRlfgodIVQCfQ

Something like this is more appropriate for a small engine, although it still isn't 100% accurate: http://www.sears.com/stens-compression-tester/p-07184269000P?sid=IDx01192011x000001&gclid=CMTcofu0r80CFQJsfgodGLkJPA&gclsrc=aw.ds


One final point. Turning the engine over more times with the incorrect compression tester will NOT get you closer to the actual pressure reading. The pressure relief valve must be near the spark plug for multiple turns of the engine to have an impact.

I have experienced this first hand with the HF unit. The design is junk.

Here is the one I use:
https://www.howtomotorcyclerepair.com/compression-tester/

It comes with all the adapters you'll ever need for various motorcycles and costs about $40.

[Markk71]

I'd really like to see how those excel wheels turn out. Used to run them on my motocross bikes and the quality was impecable..

Wow, I never really put those factors into consideration. I'm simply using a cheapo compression tester from the local canadian tire, it's a equus brand. I am suddenly feeling optimistic once again. So maybe once it's all assembled and ready to go, I should just ride it and get it nice and hot. Hope for the best kind of thing? The parts are on there way but no reason why I can't return them.

[bochnak]

I'd really like to see how those excel wheels turn out. Used to run them on my motocross bikes and the quality was impecable..

Wow, I never really put those factors into consideration. I'm simply using a cheapo compression tester from the local canadian tire, it's a equus brand. I am suddenly feeling optimistic once again. So maybe once it's all assembled and ready to go, I should just ride it and get it nice and hot. Hope for the best kind of thing? The parts are on there way but no reason why I can't return them.

If it truly has low compression, the next step is a leak down test, which will figure out where the air is going:

[MoMo]

If it truly had 65 psi I doubt it would have started.  Think you reading is misleading...Larry

[Markk71]

Yea a leak down test would certainly give me a few answers. Like I said, the engine runs incredibly smooth and really responsive. Just once you get into the meat and potato of things around the 5 thousand rpm mark is really poors out the smoke at idle it's incredible faint but still visible. I did toss some seafoam in the crankcase before putting some engine restore. Could that be a culprit?

I'll go out and pick up one of those leak down testers and get back to you guys with some results. Any other feedback would be great.

[camelman]

What color is the smoke? White is condensation, black is poorly combusted fuel, blue is oil burning. If it's blue, then typical culprits are valve guide seals, head gasket leak, or rings. I'd check the other items first before rings. If it's black, then likely old air filter, dirty carbs, incorrect jetting, or mismatched intake and exhaust. I've seen worn out and incorrectly tuned ignition cause poor combustion and black smoke too.

[Markk71]

Now that you mention it. Its more gray than anything.. It certainly smells incredibly rich of fuel ( which it should be ) I,ve rejetted it for pod filters but havent started them with the pods on. I started it today momentarily to see what kind of effort would be involved and sure enough first kick and it was running like a champion. But still tons of smoke coming out of the pipes.

But i did run into a weird situation a few days ago. To start the bike the kickstart would slip and take a few rotations to engage again. IT felt like it was still rotating something but hardly any resistance. Anyone have a similar problem?