Topic: To Change or Not to Change the Gaskets?

[YoungBlood]

Opinions wanted:
1975 Honda Cb750F, 20k miles, leaking head gasket and valve cover. #4 cylinder has ~90 psi, #1 has ~100 psi. Engine backfires on deacceleration: unclear as to why.
I am a broke college kid, who would like to replace all the motor's gaskets this winter. However, I am trying to avoid undue hardship in "rebuilding" the motor after inspecting cylinder, piston, head specs, etc., and finding I need to get replacement valve stems, seats, seals, machined head, etc. I would like to hear everyone's opinion on whether it is viable to only change the gaskets when tearing into this 20k mile engine.
If you all think it's unlikely I won't find something wrong, then what exactly should I keep my eye out for? And how economic, financially, can I keep my costs - price wise.
Thanks!

[RAFster122s]

Most auto compression testers are not good a measuring motorcycle compression.
Backfire like you mentioned usually related to air leaks... Look for them using brake cleaner or carb cleaner or starting fluid around intake boots between carbs and intake manifold or intake manifold and head. Sounds like deceleration is sucking in air somewhere besides through the carb throat and filter...

Oh brake cleaner is likely going to be tough on paint on engine cases may wish to use carb cleaner... spraying on boots while motor is running...changes in engine speed tell you it is getting suckedin near where you were spraying...

[bryanj]

At 20k i would expect to need to lap in the valve seats and relace stem seals(should be in the kit), possibly replace rings but not necessarily, at neary 50 years old the camchain tensioner rollers will be as hard as rock and need replacing with genuine Honda, intake rubbers the same.
You really can not do these engines on a small budget

[RAFster122s]

Typically the cam chain is worn out by 30k miles and tensioner and sliders need replaced by then if not before. Old motors like these might need new primary gear cush rubbers too. They get hard with age and heat cycles. Plan on replacing the front fork springs as they never were really good steel.
If you have the cylinders bored, make sure they are align bored by the machine shop. The aluminum alloy in the cylinder cooling fins allowed them to drift out of alignment as the engine wore in. The misalignment was a one time deal and it robs about 5% power according to HondaMan.

[YoungBlood]

Thanks RAFster and bryanj, so as a running list of what is likely going to need attention in the engine (*** = definitely change, ** = likely, * = somewhat likely)
- Camchain tensioner **
- Camchain slider **
- Cam chain roller ***
- Primary gear cush rubbers *
- Lapping valve seats ***
- Gasket kit including stem seals *** https://www.denniskirk.com/vesrah/complete-gasket-set-vg157.p260693.prd/260693.sku

For lapping the valves, can I do this myself as this video shows?

Now talking costs, is it plausible I could get away with doing the entire list above under $500?

[ekpent]

Once the head is off its easy to check if the valves are leaking by filling the chambers and looking for fluid passing through. May not need any attention. 20K is pretty low miles as far as rings go unless it was mercilessly beat to death.. If your on a major budget concentrate on the head gasket and rubber pucks.

[Alan F.]

Would it be advisable to simply re-torque the existing head gasket and check again?

[YoungBlood]

Hey Alan, the head gasket is in pretty bad shape. I can attach some pictures this weekend.
Revision: The head is carbon fowled all the way around the engine, some of the gasket appears blown out in cylinder 1 and 4
But given ekpent's comments, I'm feeling confident I can get a $100 Vesrah gasket set, and hopefully not incur too many other expenses (asides from Rafster and bryanj's suggestions).

[RAFster122s]

Cam chain, oil seals (not in head gasket sets). If your valves are properly set, check your compression numbers. If low, then wet compression test tells you ring health if head isn't leaking at valves ...
With 20k miles you may need to overbore one step larger while you have it torn down. It would be foolhardy in my opinion not to.
Expect to spend $800-1000+ for full rebuild.

[PeWe]

Tighten the headgasket needs to pull engine out of the frame with lots of work plus valve cover, cam with holders off.
If bike should have a frame kit,  much easier to test tighten the head nuts to see if they are loose.

A complete head job doing valves. + guides OK etc should be OK longer, at least 65.000 km, +40k miles.
Bronze guides wear quicker than stock iron.

Honda designed the CB750 wrong, frame kit should have been stock from the very beginning.
Cutting frame on a pure stocker might lower the value.
But it is a must on a bike that is frequently used.

I replaced headgasket due to a leak last summer.(wrong type of MLS gasket)
A weekend job, quicker with stock and no cam timing.

I have just done another bike after adding seals between sleeves and cylinder. Cleaning pistons and head due to too rich jetted carbs included

No need to pull engine.
3 hours to take cylinder off.
3 days to assemble it to be ready starting engine and synch carbs.

Retighten head the day after just for sure included.
That is a must with fiber head gasket

[Stev-o]

If the bike is running fine other than popping on decel just change the gaskets. These motors can be like opening a can of worms, I spent $600 on the last one I rebuilt.

Study BrandEn's thread for a leak free motor...

http://forums.sohc4.net/index.php/topic,107040.0.html

BTW - popping on decel can also be caused by a lean condition from the carbs.  Do you have an airbox and stock exhaust?  What do the plugs look like? [lean would show to be a very light colored tan or white in extreme conditions]

[seanbarney41]

If you are really on a budget, ride it until it stops.,  You will then be fixing all the same stuff these dudes are telling you to fix now.  I have re-torqued many old head gaskets with great success.  If your pucks are leaking, well, those gotta get fixed or it's gonna be messy.  I rode for years with leaky pucks and at some point you realize that there really is no reason to wear any other color pants than black.

[ekpent]

 Make sure you cheap tach seal isn't leaking also as they can get messy.

[PeWe]

The top cover on valve cover can cause oil leak that spread by the wind and make it messy.
- You can check if the rubber pucks leak. Remove spark plug and feel inside the cavity with finger. The ceiling there is the rubber puck/coin..

About low compression values.
- Did you test with throttle wide open which is needed?
- Check valve lash. Not completely closing valve will leak.

[YoungBlood]

I appreciate the comments everyone. I'm going to just going to go for it. What is a puck? I have a feeling this is a nick name for some type of gasket.
Ek- the tach has been replaced, I never sealed it, but it's never shown a noticeable leak.
Pewe- I'm just going to rip into it, I can't stand these questions I ask anymore. I'd rather just figure it out after tearing into it. Oh, and yeah, each time I've done a compression check I opened the throttle fully.
Rafster- no air leaks, I've checked more than once. I'm pretty sure it's related to lost compression (valves/gaskets)
Steve-o - bike is definitely rich, black sooted plugs -- all 4. I've been too lazy with the cold weather to bugger with the carb floats on my city side street. I have the stock air box, but I have this silly "shorty-pipe" exhaust pipe. I would love an oem exhaust, but that would be a 1,000 dollars I cannot find.
As to the valve lash, I've done plenty of adjustments, I'm certain the back fire is either related to a valve leak (internal), or my rich gas mixture. I'm tired of running this bike into lower maximum speeds. When I first got the bike it could hit 115, 7k miles later, 100 is pushing it. I know these gaskets need to be changed, absolutely certain. It's either I tear into this bike and become partially attached to her, or I'm going to give her up.

However... what tools do I need to check specs on all the odds and ends?
I have HondaMan's book, but it's not clear to me what is needed tool-wise for internal engine spec checks.



 

[Alan F.]

You can do it!

[YoungBlood]

I know I'm off topic Alan, but where can I find rear fork bushing parts that are reliable?
PS Thank you for all your positive enthusiasm in regards to my youngster attempts of fixings.

[bryanj]

Feeler gauges will do for ring gaps, measuring pistons and bores is better farmed out to an engineering shop.
Pucks are rubber discs under cam towers.
If you mean swing arm bushes get bronze ones from Nurse Julie a member on here and if you need the collar (probably) its Honda only and expensive

[Alan F.]

I know I'm off topic Alan, but where can I find rear fork bushing parts that are reliable?
PS Thank you for all your positive enthusiasm in regards to my youngster attempts of fixings.

It's your thread man, you set the topic.

I'd PM Hondaman about swingarm services.

And you're welcome.

[Stev-o]

Since it sounds like your ready to tear down the engine, I suggest starting a thread in the "Project Shop" area and we can follow along there and answer questions as needed. Good Luck

[MauiK3]

Please get Hondaman's book, it will save you time, money, frustration, heartache and cures the common cold.

[ekpent]

Unless you have a really good quality compression gauge don't really worry about the overall number as much as how all four numbers compare to each other. Harbor Freight reviews are fun to read about the guys who tore their engines down thinking they had low compression when they didn't 
  Torgue wrench is in your future to buy or borrow.

[Alan F.]

Here's the link for the Hondaman Book, it's not exactly inexpensive but it will be well worth having.

This link offers discounts now and then for the major holidays:

https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

(Also there are 'used' copies out there for sale for less money)

[Stev-o]

Please get Hondaman's book...

Here's the link for the Hondaman Book...

In reply #14 above, he states he has Hondaman's book.

[Alan F.]

Good, thanks Stev-o.

I'm not sorry for crossposting the link though, someone will find it in a search and buy one.

So start that project thread, state the issue, state your plan, and we can offer helpful info when necessary. I'm not going to be too useful but I'll follow along.

And don't worry, just keep moving on it. Lots of these bikes have been rebuilt far poorly by guys with less info than we have, and then ridden far more miles than we will. You'll be fine.