Topic: Couple of newbie questions

[Dime]

So I just wanted to be sure of something and ask another question besides.

From what I can tell it was the 77/78 F cb750's that had issues with the valve guides not the 78K models correct?

Also I would want to switch over to electronic ignition instead of points. What is the cheapest way to do this on a 78 K model?

Thank you

[denward17]

Not sure about the F model, but my '78 K model (US) had worn out exhaust valve guides when I took the head off at 22K miles.

I purchased from EBAY a knock off Dyna electronic ignition, it has been working well so far (about 2500 miles).

[rocket johnny]

hondaman sells a electric ignition that is easy to install and works great

[Dime]

Not sure about the F model, but my '78 K model (US) had worn out exhaust valve guides when I took the head off at 22K miles.

I purchased from EBAY a knock off Dyna electronic ignition, it has been working well so far (about 2500 miles).

Good to know. I will have to put some cash aside for guides if that is the case. It is a 1978k I am looking at.

[bryanj]

Cheap and electronic should not be in the same sentence unless you want problems.
If you want the head done right send it to M Reike(not sure of spelling) on the forum, superb work.

EDIT forum name is MRieck

[Dime]

Cheap and electronic should not be in the same sentence unless you want problems.
If you want the head done right send it to M Reike(not sure of spelling) on the forum, superb work.

EDIT forum name is MRieck

Affordable then. I can get the head done locally. My old boss does amazing work. I will pocket ported the head myself. The bike has 14k kms so it should be good for a minute yet before I pull it down.

Hondaman looks like a good option.

[rotortiller]

I have 60,000 miles on my K7 and the engine has not bee taken apart. Burns absolutely no oil. My brother's K7 has more miles and same thing. From what I can see so far with our two examples is the 77K guide defect is likely invented, maybe a carry over from the 750 supersport or does not apply.

I have had a Tytronics electronic ignition since 2017 (under 100 USD online) with no associated maintenance and it totally replaces the points and condensors. Lots of brands to choose from and my advice is to totally replace the points system with a magnetically fired system.

[Dime]

I have 60,000 miles on my K7 and the engine has not bee taken apart. Burns absolutely no oil. My brother's K7 has more miles and same thing. From what I can see so far with our two examples is the 77K guide defect is likely invented, maybe a carry over from the 750 supersport or does not apply.

I have had a Tytronics electronic ignition since 2017 (under 100 USD online) with no associated maintenance and it totally replaces the points and condensors. Lots of brands to choose from and my advice is to totally replace the points system with a magnetically fired system.

Just checked out that ignition and it looks good. What coils are you running with it?

[rotortiller]

stock coils

[Dime]

stock coils

Awesome this looks like the best option then. Thank you

[Tracksnblades1]

A 78k. “750” unmolested should be wearing a 392 cylinder head..

A very good one. Especially when fitted with the larger valves..😇

The 410 cylinder heads were installed on 77-78F 750s. Had a larger combustion chambers (more ccs).
Had higher domed pistons to get the same compression or a tad more.
Had different valve locks and larger 34mm intakes and 31mm exhaust..
Had different oil drain back holes and had some water issues with a few head studs IIRC…🤔

Your k should share the same double roller output shaft and spacing as the two later F’s..
You can free up some power with a 530 chain and toss the 630…

[Dime]

A 78k. “750” unmolested should be wearing a 392 cylinder head..

A very good one. Especially when fitted with the larger valves..😇

The 410 cylinder heads were installed on 77-78F 750s. Had a larger combustion chambers (more ccs).
Had higher domed pistons to get the same compression or a tad more.
Had different valve locks and larger 34mm intakes and 31mm exhaust..
Had different oil drain back holes and had some water issues with a few head studs IIRC…🤔

Your k should share the same double roller output shaft and spacing as the two later F’s..
You can free up some power with a 530 chain and toss the 630…

Excellent info thank you. All about the hot rod tips. My old cb750 dohc was built on them. I don't mind spending money but I like it to be smart money. Hot rodder for life. Better rodded than rotten.

[Tracksnblades1]

You can get a lot of printed info on Kenny’s site, Cycle Exchange.

There are many wrenches on here that’s been frequently knee deep in CBs that can save you money and time here also..

If you’re going to spend money, don’t forget the head. A professional port job and big valves can put a smile on your face. A bigger smile when used in conjunction with a big bore and camshaft upgrade..

[Dime]

You can get a lot of printed info on Kenny’s site, Cycle Exchange.

There are many wrenches on here that’s been frequently knee deep in CBs that can save you money and time here also..

If you’re going to spend money, don’t forget the head. A professional port job and big valves can put a smile on your face. A bigger smile when used in conjunction with a big bore and camshaft upgrade..

I plan on keeping it stock as long as possible engine wise. If the head needs to be addressed I can pocket port the head myself as I have done with several of my bikes. Not going to be as good as a full port job but will increase throttle response and run a lot smoother.

Bigger valves would be nice but if the head has good guides and the stock valves are in good condition I would probably do valve job and pocket port and put the money towards a bolt in cam.

The priority I have in my mind right now for this bike is electronic ignition and cleaning/rebuilding the carbs. Which would include bench setting them.followed by a sync.

I have gone the performance route with everyone of my last 4 bikes, two cb750 dohc, a sportster and my current electra glide. I plan on only digging into the engine when necessary on the 78 cb750.

My belief is when I go in there make it better. How much so is always a matter of money available and what I can do.with as little of it as possible.

Being able to do all my own work except for guides and valve job keeps the costs down thankfully.

Being a professional engine builder in a former life has payed off in the area of my personal performance hobby many times. LOL

Thanks again for the info. Will take anything you good folk wish to.impart on me.

[BenelliSEI]

FYI….. my ‘78 cb750K had over 40,000 km’s on it the first time I pulled the top end to address a few oil leaks. I did change the guides, but it was not “urgent”. No need to rush.

[RAFster122s]

Car techniques on CB sohc4 heads often do not work as you might imagine. So, before you dive in and mess with your head you should use some Google foo and read the articles and see the images if still available where people have shared successful mods to their head on the sohc4.  Just a word of caution to spare you the potential for an expensive mistake.

Simple things like port matching the intake manifold to the head and eliminating any protruding screw heads are safe, but things like deshroud8ng valves and changing the intake shape and texture sometimes can backfire.

[HondaMan]

Cheap and electronic should not be in the same sentence unless you want problems.
If you want the head done right send it to M Reike(not sure of spelling) on the forum, superb work.

EDIT forum name is MRieck

Affordable then. I can get the head done locally. My old boss does amazing work. I will pocket ported the head myself. The bike has 14k kms so it should be good for a minute yet before I pull it down.

Skip the pocket porting IMHO: the 750 breathes far too fast for pocket-porting to be any help, and if it goes too far it can make the engine stumble at about 2000-2200 RPM. It can be 'undone' afterward, but it's expensive enough to do that getting another head is then easier and cheaper.

The exhaust guides on the K7/8 will often tell you if they are significantly worn: there will be oil weeping out at the exhaust pipe headers, right at the head.

[Dime]

FYI….. my ‘78 cb750K had over 40,000 km’s on it the first time I pulled the top end to address a few oil leaks. I did change the guides, but it was not “urgent”. No need to rush.

Good to know. Thank you.

[Dime]

Car techniques on CB sohc4 heads often do not work as you might imagine. So, before you dive in and mess with your head you should use some Google foo and read the articles and see the images if still available where people have shared successful mods to their head on the sohc4.  Just a word of caution to spare you the potential for an expensive mistake.

Simple things like port matching the intake manifold to the head and eliminating any protruding screw heads are safe, but things like deshroud8ng valves and changing the intake shape and texture sometimes can backfire.

Thank you for this information. I am.still in research mode so any and all info is appreciated

[Dime]

Cheap and electronic should not be in the same sentence unless you want problems.
If you want the head done right send it to M Reike(not sure of spelling) on the forum, superb work.

EDIT forum name is MRieck

Affordable then. I can get the head done locally. My old boss does amazing work. I will pocket ported the head myself. The bike has 14k kms so it should be good for a minute yet before I pull it down.

Skip the pocket porting IMHO: the 750 breathes far too fast for pocket-porting to be any help, and if it goes too far it can make the engine stumble at about 2000-2200 RPM. It can be 'undone' afterward, but it's expensive enough to do that getting another head is then easier and cheaper.

The exhaust guides on the K7/8 will often tell you if they are significantly worn: there will be oil weeping out at the exhaust pipe headers, right at the head.

Thank you for this. It will probably be left stock if that is the case and just address guides as need be.

[Dime]

I have been reading a few posts here and looks like an F cam from a 77/78 would be a nice bolt in upgrade in the k model.