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Discussion Starter · #1 ·
First project.
I recently bought a 1980 CB750F to turn into a cafe/brat style. Got it from non running to a steadyish/rough idle. I can hear some chattering in the engine but not sure where from, sounds like the middle. All of the cam chain tensioner bolts have broken off from the previous owner. But everything else seems to be alright. Is separating the engine to put new gaskets in a bad idea?
I guess I'm just looking for some advice from some more experienced folks. I'm in no rush, I'd like to get this bike tuned and built correctly.
 

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Sounds like the engine is a little sick.

The clattering could be many things.
Lets start with the basics.

Have you checked the valve clearances with a set of feeler gauges?
If not get those set right first of all. Easy to do and could be the source of your rattling.

Buy or borrow a compression tester. Theres no point in anything if you dont have compression. A low compression reading on any cylinder means either piston rings not sealing or valves not sealing.
Either of these require the head to come off so you may as well put a new timing chain and tensioner in there whilst you are at it.
These are a consumable parts and could also be the source of your rattling. In fact i would almost bet money on this being your noise.

check your timing and gap on your points ignition.

Do all these tests first as that will give a baseline.
Then come back to us with the results. Once we have this info we can help further.

All the above can be carried out by a novice (the compression test and adjustments to points and valve clearances) with a couple of relatively cheap specialist tools, a socket set and a haynes/clymer manual.
YouTube is your friend also if you are struggling to picture what the manual is saying.

A knowledgeable friend to help will make this more fun, sociable and easier to learn also.
Get involved in your local clubs.
 

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First project.
I recently bought a 1980 CB750F to turn into a cafe/brat style. Got it from non running to a steadyish/rough idle. I can hear some chattering in the engine but not sure where from, sounds like the middle. All of the cam chain tensioner bolts have broken off from the previous owner. But everything else seems to be alright. Is separating the engine to put new gaskets in a bad idea?
I guess I'm just looking for some advice from some more experienced folks. I'm in no rush, I'd like to get this bike tuned and built correctly.
The bike has a chain primary drive. How bad is the original primary drive chain stretched?
With the engine off and transmission in any gear, how far does the bike roll forward and backwards before the internal drive chain comes tight?

If you're going to split the engine casings you might better replace every coil spring, chain and chain tensioner in it.
 

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Discussion Starter · #4 ·
Sounds like the engine is a little sick.

The clattering could be many things.
Lets start with the basics.

Have you checked the valve clearances with a set of feeler gauges?
If not get those set right first of all. Easy to do and could be the source of your rattling.

Buy or borrow a compression tester. Theres no point in anything if you dont have compression. A low compression reading on any cylinder means either piston rings not sealing or valves not sealing.
Either of these require the head to come off so you may as well put a new timing chain and tensioner in there whilst you are at it.
These are a consumable parts and could also be the source of your rattling. In fact i would almost bet money on this being your noise.

check your timing and gap on your points ignition.

Do all these tests first as that will give a baseline.
Then come back to us with the results. Once we have this info we can help further.

All the above can be carried out by a novice (the compression test and adjustments to points and valve clearances) with a couple of relatively cheap specialist tools, a socket set and a haynes/clymer manual.
YouTube is your friend also if you are struggling to picture what the manual is saying.

A knowledgeable friend to help will make this more fun, sociable and easier to learn also.
Get involved in your local clubs.
Thanks for the detailed response! I did have valve clearances on my to do list, what is the proper clearances? I've got mixed answers when doing my research. Some say .008mm, .005mm and I've even seen some say .002. I also purchased a haynes manual, which is awesome to look through.

Tonight I will loan a compression test kit and timing light from my auto parts store. Where is the best place to get valve shims?
The bike has a chain primary drive. How bad is the original primary drive chain stretched?
With the engine off and transmission in any gear, how far does the bike roll forward and backwards before the internal drive chain comes tight?

If you're going to split the engine casings you might better replace every coil spring, chain and chain tensioner in it.
From what I recall, when in gear, the movement is negligible, if any. The exact measurement? No idea.

The plan is to replace everything that needs to be without question.
 

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With the chain it doesn't actually stretch (unless you are talking very high horsepower) it is usually more due to wear at the bearing surface of the chain pins causing a bit of slop. That slop multiplied by the number of pins can add up. Modern sealed chains are less prone than the old ones but it still happens. You are more easily able to see wear on the sprockets. Both "should" be changed out at the same time although it isn't always done. They will have worn together at the same rate and changing one out to new will most likely accelerate wear on both as they try to match up again.

You can measure a set number of links and determine if the chain still falls within allowable spec. I can't recall where the data for that is at the moment.
 

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I believe it might. The procedure should be in there. It would be in the Suzuki shop manual but that info would only apply to the OEM chain. If you have an aftermarket chain you would follow that manufacturers spec.

Carlsalter.com has downloadable OEM service manuals for older bikes. I quickly looked and saw a 1979 CB750 but didn't check very careful. It is safe to download from there, I have done so many times.

 

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Of course it will be in a Haynes manual.
 

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With the valve clea
Thanks for the detailed response! I did have valve clearances on my to do list, what is the proper clearances? I've got mixed answers when doing my research. Some say .008mm, .005mm and I've even seen some say .002. I also purchased a haynes manual, which is awesome to look through.

Tonight I will loan a compression test kit and timing light from my auto parts store. Where is the best place to get valve shims?

From what I recall, when in gear, the movement is negligible, if any. The exact measurement? No idea.

The plan is to replace everything that needs to be without question.
With the valve clearances all of these will be detailed in the Haynes manual. Both in terms of proceedure and tolerances.

In a haynes manual all of the tolerances and specs will be listed at the beginning of the chapter.
There we usually be different specs for inlet and exhaust as the exhaust valves run hotter and therefore tend to have a larger clearance for expansion.

With the primary chain lay it out flat and the bend it sideways as much as it will go to try and form an arch.
Take a pic and post. The more banana shaped you can make it the more worn it is.
 

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I had a CB350 four cylinder that had such a bad primary drive it lurched when you went from acceleration to engine braking and it sounded like a bag of hammers right from when I bought it. I just figured the previous owner liked to do wheelies with it. When I sold it she lurched when you went from acceleration to engine braking and still sounded like a bag of hammers because I didn't want to split the crankcase on it and put the money into parts so I bought a new bike. I was impulsive like that sometimes when I was young and that bike is likely scrap by now. ymmv.
 

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Just to be clear: When TR is talking about a “primary chain”, he is referring to the chain linking the crankshaft to the transmission input shaft.... not the drive chain connecting the output shaft to the rear wheel.

This won’t be able to be checked unless the OP gets to the point that it is deemed necessary to split the crankcase.
 
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Discussion Starter · #14 ·
With the valve clea

With the valve clearances all of these will be detailed in the Haynes manual. Both in terms of proceedure and tolerances.

In a haynes manual all of the tolerances and specs will be listed at the beginning of the chapter.
There we usually be different specs for inlet and exhaust as the exhaust valves run hotter and therefore tend to have a larger clearance for expansion.

With the primary chain lay it out flat and the bend it sideways as much as it will go to try and form an arch.
Take a pic and post. The more banana shaped you can make it the more worn it is.
I'm have to separate the case to look at that drive train, right?

Well pointed out.

OP Do the tests and adjustments i detailed and get back to us.
Hoping we can help you further once you have done these as a baseline
Yep thats the plan this weekend hopefully.

I won't mind if I have to take the case apart, a good learning experience.
 

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I'm have to separate the case to look at that drive train, right?


Yep thats the plan this weekend hopefully.

I won't mind if I have to take the case apart, a good learning experience.
The bottom ends of these motors were pretty bullet proof.
I would bet money that there is nothing wrong down there.
Its the top end where pretty much any air cooled hinda suffers.
So at most i think you will need to remove head and barrels, so no splitting cases.
 

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Discussion Starter · #19 ·
The bottom ends of these motors were pretty bullet proof.
I would bet money that there is nothing wrong down there.
Its the top end where pretty much any air cooled hinda suffers.
So at most i think you will need to remove head and barrels, so no splitting cases.
And as of my findings last night, the cam tensioner adjustment bolt seems to be missing/broken and the previous owner must have put a rubber stopper in as a replacement... it looked like they tried to dill it out and retap but failed. Looks like a new cylinder head? Or could there be another fix?
 

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Oof there is a lot of excess silicone sealant oozing out of that motor.
That coupled with the snapped bolts doesn't make me feel all warm and fuzzy inside.

The head should be perfectly salvageable.

Personally i would pull the head and send it off to someone who knows what they are doing to unfuck the snapped tensioner and give it a refresh.
Then a fresh tensioner and chain.

Still though i would want to get those all important compression readings first!
 
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