1982 CB750F...Better Devil

[pidjones]

I've pulled so many "sealed" relays, etc. apart and fixed. Even the ABS and pulse wiper controllers in our old S-10.

 

[Jimbonaut]

May have a...situation. Installed the exhaust (properly, with gaskets) - while this has most certainly taken care of the rattle coming from the headers, that absence of noise means I can now hear a bit of a clatter coming from...somewhere else. I've adjusted the cam chain tensioners without any noticeable difference, so now I'm thinking it's more significant. It kinda sounds like it's coming from the bottom end, and I'm wondering if it's the primary shaft.

Furthermore, the base gasket is leaking. So that can't be good. I don't know if torquing down the head bolts is in order - they're already pretty tight at over 30 ft lbs if I remember correctly. I'm kinda ok with the situation, not too bummed out. I mean, shit - it's hardly great news. But if I have to pull the damn thing out and do it over then that's what I gotta do. Shit.

 

[irk miller]

Is your head gasket leaking, too? Leaking gaskets like that will give you the sounds of rattling metal or gears, as will a leaky header gasket.

 

[Jimbonaut]

Head gasket looks ok Irk


Sent from my iPhone using DO THE TON

 

[Maritime]

Have you run it enough to know for sure? The head takes a bit of a run at RPM's to show a leak sometimes where the base will leak easier. I wonder if one of your locating dowels is a little proud?

 

[Jimbonaut]

It’s just idled for a while, that’s all. There’s also the clatter from the bottom end - wildly optimistic that a carb synch may take care of that?

I haven’t googled it yet, but a search for “how to remove CB750 engine without fucking up fresh paint” may not be far off dammit.


Sent from my iPhone using DO THE TON

 

[Maritime]

some clatter is normal, some is bad. I don't know the normal for a CB750 but my CM450 when valves we set right had a real clatter at idle and some other parts of the rev band and you could hear it way more depending on where the bike was running at. In the open driveway it was minor, in a garage or ally etc it was damn loud and sounded like it was going to implode but it never did. Case clatter on that was if the counter balancer was getting lose and it was an easy solve.

 

[Jimbonaut]

I have my man Gabriel coming over today - initially the plan was just to vacuum synch the carbs, but I fear my job ahead may be slightly more substantial. I've decided to be totally ok with whatever situation arises. I'm deciding that now. Ask me again in a few hours...

 

[doc_rot]

Im not familiar with these CBs, i think they use a primary chain. If so, the clatter from the bottom end you are hearing is probably that. Syncing the carbs will make it run smoother and by making the power pulses more even it will reduce primary chain clatter. This is a common issue on KZ650/750 that use a primary chain.

 

[Jimbonaut]

I replaced both the primary chain and the rubber dampers inside the primary shaft when I rebuilt the engine. Maybe the clatter will go away/subside/be normal after a carb synch and valve clearance check. Not sure about the base gasket leak though - 99.9% sure there was no crap in the locating dowel holes but that leaves 0.01% of absolute possibility.

 

[cbrianroll]

Edit....can't read lol.

 

[doc_rot]

the sound doesnt have to do anything with the status of the chain, uneven power pulses will cause the chain to rattle as there always must be some tolerance between the gears and the rollers. if you are way out of sync it can be pretty bad.

 

[Jimbonaut]

Thanks for the clarification doc, that's interesting - here's hoping a decent carb synch will make a difference.

High hopes - followed by the crushing flashback to oil seeping around the base gasket. The lord giveth and the lord taketh away and all that

 

[CarbsAndCylinders]

Re the oil leak, are you sure you torqued everything properly?

 

[pidjones]

My GL1800 has rattles at idle. Always has, they all do. The exhaust is so quiet you hear everything else, and when Honda went to chain driven cams the old silent GL1500 (belt-driven cams) owners thought something was wrong. The DOHC CBs have twice the chance for valve train noise as the SOHC models.

 

[Jimbonaut]

I know the cylinder studs in the F models can be torqued down more than the manual states, 33 ft lbs I think. Mine are pretty much torqued down to that, maybe a little less. That will be the first thing to try - check the torque and see if I can tighten them down some. As for the clatter - I'm cautiously hopeful that a carb synch and valve clearance check may help.

 

[Jimbonaut]

So this happened last night -

Base gasket leak and disconcerting rattle from the engine amounted to one thing - pull the bastard and have at it. My friend - Honda mechanic of 42 years - didn't hesitate when we fired it up and he heard the noise, and we got the engine pulled in 30 mins. Helluva guy to have pop round to the garage every now and then. So, in I go. Again.

 

[Maritime]

well better to sort it than have it implode later

 

[CarbsAndCylinders]

 

[SquidHunter]

I’m a little late. Looks like you already pulled it, but it will rattle. I can always tell when I’m about to start having carb troubles.... lol.

I probed around on mine for about 45 minutes the other day before riding. I never could pin point the rattle.


Sent from my iPhone using DO THE TON