1982 CB750F...Better Devil

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
The Jimbonaut said:
Insulators? What be they - another name for the cam guides?
What most people call the carb boots are actually called insulators. They're numbered, so you don't have to worry about mixing them up, and they're a different size for the head side and the carb side.
 

Jimbonaut

Well-Known Member
DTT SUPPORTER
Ah, those things. I’ve been calling them jug boots. Yeah, I need to get those things off, but it’s so damn cold in my garage they’re rock solid. Need to - ahem - borrow the wife’s hair dryer and soften them up a bit.

Fuck it’s cold here. “How cold is it?” So cold I saw a cop tazering himself to keep warm.


Sent from my iPhone using DO THE TON
 

doc_rot

Oh the usual... I bowl, I drive around...
DTT SUPPORTER
DTT BOTM WINNER
I would replace the base gasket otherwise you probably will be tearing it down later to replace it anyway. Doesn't matter about the timing marks, you will have to re-time the cams upon assembly. I would also consider measuring the cylinder bores to see if they're out of spec and if that checked out I personally would do a flex hone and maybe new rings. Just takes one cheapy engine rebuild to make you do it twice, but to each their own
 

pidjones

Well-Known Member
If you post about the boots on an F site, you will be corrected. They say it is because their search engine won't find it unless you use insulator. I've used a heat gun to soften the boots both for installation and removal of the carbs. And that is in a heated garage. It helps to keep from cracking the old rubber. I also smear a light bit of silicone vacuum grease on them to install, but that may not be necessary. At least the last time I didn't have to used a ratchet strap to assist installing them. Maybe third time is the charm, 'cause they finally are all four working and not leaking. Hint: hook up throttle cables BEFORE putting the carbs on the insulators. I prop the at ~45 degree angle part-way behind the head to hook them up, the lower them in position and commence heating the insulators.
 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
There ya go Jim. Looks like you've got a couple burnt up looking exhaust valves so wouldn't hurt to make sure they are all still sealing up (alcohol in the ports test) and you'll definitely want to check your valve clearances and probably loosen those particular exhaust valves up a bit.
 

Jimbonaut

Well-Known Member
DTT SUPPORTER
Thanks chaps so that's what a burnt valve looks like. Knew pulling the head was going to school me. Pretty sure I did the alcohol test with the KLR but will need a revise on the process - essentially I'm looking to see how tightly the valves seal by pouring alcohol into the ports, correct? Both intake and exhaust?

Doc, good advice. I wasn't really planning on doing a complete top-end rebuild, largely because it seems (to amateur me at least) largely unnecessary. The compression is was excellent (hopefully will be again once I put everything back together again) and so I didn't plan on going any deeper than necessary. That said, I was looking at the jugs yesterday thinking, well, I've got this far...

Pulled all the side covers, oil pan and whatever else I could off the crankcase - not long before it's ready to paint. Thanks for the info on the timing - it's starting to make more sense now. I can't get my head around the importance of the timing mark before pulling the cams, but I totally get the importance of getting that mark in the right place before putting them back.

That happened yesterday, but not before the first decent skate of the year. When you live in the deep freeze, you take the perks where you can get 'em.

 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
Man, its been years since I've skated. We used to have a blast playing pond hockey when we were younger.

To check the valves, pour alcohol (not beer ;D) into the intakes, give it a good 20 minutes or an hour or so and check for any leaks. Take note, empty and dry them out and repeat on the other side.
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
Just because they're light, or appear a little lean, doesn't mean the valves are burnt. A burnt valve will literally have part of it's edge melted away. As the pressurized and fuel rich, super heated air is looking for oxygen, it takes the path of least resistance. Once it finds its weak point, it always pushes through that spot, melting it away. If you had good compression, then it is unlikely that the valves are burnt, as they would hold little or no compression.

Also, nothing wrong with just going with a hone and putting it back together. Depends on how deep down this rabbit hole you want to go.
 

Jimbonaut

Well-Known Member
DTT SUPPORTER
Excellent Nick, I'll do that. Any excuse to pick up some alcohol.

irk miller said:
Just because they're light, or appear a little lean, doesn't mean the valves are burnt. A burnt valve will literally have part of it's edge melted away. As the pressurized and fuel rich, super heated air is looking for oxygen, it takes the path of least resistance. Once it finds its weak point, it always pushes through that spot, melting it away. If you had good compression, then it is unlikely that the valves are burnt, as they would hold little or no compression.

Also, nothing wrong with just going with a hone and putting it back together. Depends on how deep down this rabbit hole you want to go.
How deep indeed. Call it good and leave the jugs as they are or go full-on Heart Of Darkness on the thing. Either way, I think you've helped me name the bike - Colonel Kurtz, take a bow.
 

pidjones

Well-Known Member
BTW, that's isoprophy alcohol. The 91% is best, as the 70% "rubbing alcohol" is 30% water so it doesn;t evaporate so fast and lets the alcohol have time to kill germs. You can get 91% at most drug stores and Walmart these days.
 

Nybz

Member
I think you are on the right track to clean up inside the engine while you got it open.
Compression is good yes, but the peace of mind checking everything out before you put the engine back in the frame is priceless. Plus new skills is great!

Lapping some leaking valves is pretty easy with the right compression tool. Just don’t lap too hard as there is a hardened thin layer on the valve face and lapping will get rid of it and the valves life will be shorten dramatically.

Honing (can rent the tool if need) and reringing is pretty straight forward. Maybe not needed and these rings are pretty solid stock.

valve stems....these ones are way better then stock. if you are not on the Cb1100f web forum, check it out.
http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=75877
He also has upgrade cam holder bolts, which are higher grade so they don’t break as easy. FYI only 9ft/lb torque on those puppies. Manual says 12 ft/lb or something, you will snap bolts with that.

Cam chains and cam tensioners for sale....not cheap, but better then stock and will last longer then you/your bike.
http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=86245

You can get a full seal kit at a pretty good price. It would require splitting the cases, but who wants a oil leak on a fresh engine? Plus gives you a chance to check your rod and main bearings.

There are four o rings that go on the back (carb side) on the middle studs (at least on my 1979 model) that need to be replaced. Also larger o rings that go around the cylinder in between the base and cylinder.
You can still get the dowel pins, they are sometime just easier to just replace then to try and get them out and damage.

You will want a full engine gasket set and not reuse the base gasket. I use a spray on copper coat for base and head gasket that has worked great on cheap gaskets.

If your insulators are not just frozen and are rock hard from age/heat, they will have a hard time sealing against air leaks. I havent checked them out but there a super cheap aftermarket set on eBay. I opted to get genuine, as they are still available.
The carb-airbox insulators are always cracked and leaking, do your self a favour and get a new set of yours are old. If they aren’t cracked now they will be once you start playing with them more.

If you want some better performance, think about replacing your 750 cams with 900 cams. Direct swap.

Looks good! Will be checking on your progress.
 

pidjones

Well-Known Member
And to pull the dowel pins without damage (except possibly to the dowel pins); find a drill bit that you can just fit the shank into them, put it inside the dowel while you grip them with pliers. This keeps from collapsing the pin and gouging the inside of the case/jugs/head.
 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
pidjones said:
And to pull the dowel pins without damage (except possibly to the dowel pins); find a drill bit that you can just fit the shank into them, put it inside the dowel while you grip them with pliers. This keeps from collapsing the pin and gouging the inside of the case/jugs/head.
I like this tip. I've always just left those pesky dowel pins in place because I've only ever buggered them up trying to get them out. Good to know.
 

Jimbonaut

Well-Known Member
DTT SUPPORTER
Thanks as always gents for the excellent help and advice. Isopropyl alcohol - check (think I still have some from when I worked on the KLR). Lots of great info there Nybz, and many thanks for the CB1100F forum suggestion - I'm signed up now. Starting to weigh up pulling the cylinder block now, and glad that I cut my teeth with this kind of work on the KLR and its thumper engine. And pidjones I'm going to try that dowel suggestion, thanks a lot - it's jammed in the cylinder block tighter than a jar of peanut butter at a squirrel convention. If it does mangle, are they hard to replace?

Edit - apparently not, found the dowels for a few bucks on bikebandit.com -

https://www.bikebandit.com/oem-parts/1982-honda-750-super-sport-cb750f/o/m151786#sch18760

Good to know
 

Jimbonaut

Well-Known Member
DTT SUPPORTER
Maritime said:
Just add those should you need them to the pile O parts I'm building up for you LOL ;D
Ha! "We're gonna need a bigger boat greyhound"

They're on the list - $26 US shipped to Canada, $6 shipped within the US. Shiretown, make some room 8)
 

Jimbonaut

Well-Known Member
DTT SUPPORTER
Only 3ft? Damn, I thought these bars were wider ;)

Don't suppose those powdercoat samples had arrived had they? I need to add a petcock and top end gasket kit to the shopping list too - anytime I can return the favour and send a poutine your way mate say the word
 

DTT Bike Of The Month Gallery

DTT Light or Dark

www.cognitomoto.com
https://www.townmoto.com
www.speedmotoco.com
www.jadusmotorcycleparts.com
www.lostapostle.ca/
www.sparckmoto.com
Top Bottom