Topic: My first complete build 72 CB750 - completed for now...

[algophobe]

I haven't read your thread lately, but did you confirm the valve clearances before disassembly? Is it possible that one of the offending cylinder valves was not seating? Is it possibly bent?

I am huge fan of a leak down test prior to disassembly (for future reference).

from what i've read on performing leak down tests, it requires more equipment? but my understanding is that if the compression does not improve with adding oil into the combustion chamber then the next step would be leak down. which in this case did improve, therefore i'm opted not doing it. in retrospect, i wished i had done it anyways.

[algophobe]

Well looks like the engines going to come out of the frame once again. I decided to run some basic diagnostic test on the bike before really digging into the carburetors to get it fine tune. I decided to check the timing which appears decent. When starting the bike from cold, she seems to have a little more trouble than before. Once she warmed up I checked the temperature of the headers with a infrared heat gun and cylinder number 4 always seems to be about a hundred degrees below the other three. So I ran a compression test. From left to right, the values were 135, 130, 130, 90. I dropped a little oil into the top of cylinder 4 re-check the compression it was shot up to 125. I do recall some issues with the 4th piston ring during initial installation, where it got caught between the cylinder and case. It may have compromised it and the symptoms appeared after some riding. Well at least this will give me the opportunity to install a frame kit with the engine out.

here's a little recap

but i guess at this point i'll have to take apart the #4 valves and see.

[PeWe]

Leaking valves reduce compression and visible with a cheap compression tester, especially when 1 or 2 cylinders have >10-20 PSI lower than the other. Carbonised seats can look like that.

When head is off, check all valves and seats are OK. No wiggling valve in a guide, usually ex side. No bent valves that will not fully close.

[algophobe]

Leaking valves reduce compression and visible with a cheap compression tester, especially when 1 or 2 cylinders have >10-20 PSI lower than the other. Carbonised seats can look like that.

When head is off, check all valves and seats are OK. No wiggling valve in a guide, usually ex side. No bent valves that will not fully close.

two questions: 1. would the compression improve with adding oil to the combustion chamber in the case of leaking valves? my understanding is that it wouldn't be the case. 2. when you refer to carbonized seats.. are you referring to carbon build up on the valve seats?

[scottly]

two questions: 1. would the compression improve with adding oil to the combustion chamber in the case of leaking valves? my understanding is that it wouldn't be the case.

If compression goes up after adding oil, it indicates a problem with rings. If it doesn't go up, it indicates valves.
BTW, there was a member a while back with low compression on one cylinder. After much head-scratching, it turned out the carburetor slide for that cylinder wasn't connected to the throttle linkage, and was never opening, so very little air made it into the cylinder to be compressed.

[calj737]

BTW, there was a member a while back with low compression on one cylinder. After much head-scratching, it turned out the carburetor slide for that cylinder wasn't connected to the throttle linkage, and was never opening, so very little air made it into the cylinder to be compressed.

I would think a leak down test might have revealed this? Or am I wrong?

[scottly]

BTW, there was a member a while back with low compression on one cylinder. After much head-scratching, it turned out the carburetor slide for that cylinder wasn't connected to the throttle linkage, and was never opening, so very little air made it into the cylinder to be compressed.

I would think a leak down test might have revealed this? Or am I wrong?

No, a leakdown test would not have shown the carburetor slide was not opening.

[algophobe]

two questions: 1. would the compression improve with adding oil to the combustion chamber in the case of leaking valves? my understanding is that it wouldn't be the case.

If compression goes up after adding oil, it indicates a problem with rings. If it doesn't go up, it indicates valves.
BTW, there was a member a while back with low compression on one cylinder. After much head-scratching, it turned out the carburetor slide for that cylinder wasn't connected to the throttle linkage, and was never opening, so very little air made it into the cylinder to be compressed.

i'm the amateur here but don't think a leak down test would detect this since you're pressurizing the chamber via the spark plug channel. the carb slider not working prevents air from entering the intake valve to pressurize the chamber during compression testing???

[scottly]

Correct, this why compression tests are done with the throttle fully open. A leak-down test is done with both valves closed, so the carburetor is not involved.

[scottly]

Decided to go with the no weld frame kit. However in my excitement I did not take into account the ignition key switch mounting point to the left and front of the frame and started cutting. Guess I'll have to figure something out down the line.

Can you mount the key switch behind the frame clamp; slide the mount over the removable part of the frame tube, fit the removable section in place with the kit clamp in place, then slide the key switch mount up against the back of the clamp.

[algophobe]

Decided to go with the no weld frame kit. However in my excitement I did not take into account the ignition key switch mounting point to the left and front of the frame and started cutting. Guess I'll have to figure something out down the line.

Can you mount the key switch behind the frame clamp; slide the mount over the removable part of the frame tube, fit the removable section in place with the kit clamp in place, then slide the key switch mount up against the back of the clamp.

thanks, will try that, hopefully it doesn't come too close to the valve cover.

[algophobe]

i had a old rusty C clamp sitting around and decided to make a valve compression tool. drill a hole thru the top of the clamp to fit a drill attachment for a socket cut to retain the string.

[algophobe]

the exhaust valves were pretty dirty, I wouldn't expect this much for 250 miles only on the bike. pictured is only the 4th exhaust valve which was the dirtiest. alot more carbon build up than expected.

the 4 exhaust valves slides out, all straight, from what i can tell has minimal play. the 4 intake valves won't come out as you can see because the edge of the end of the valve has rolled over causing it to mushroom and not pass thru the valve guides. the intake valves have little play... probably would have to the cut or ground down to come out.

i looked at the spark plugs and they look like either fouled by oil or running too rich. not knowing the source of compression loss on #4, which is likely from accelerated carbon build up from oil leak into the combustion chamber... potentially from the valve guides. 

during the initial build i had the valves redone (new guides, seat cut, and lapped) by a local machine shop, not knowing what kind of quality work they do. the valves guides i had bought from ebay, stating they were Kibblewhites.

the questions, should i just start from scratch and have cycleX or some other reputative source redo the valves altogether? possibly even getting it ported to maximize the usage of the CR31 carbs? so easy to fall into another rabbit hole of spending too much when its not really needed.

[algophobe]

had a chance to check out the play on the valves/guides. by eyeball, seems kinda loose considering the manual recommendation for replacing the valves and guides is 0.003". i attempted at being more precise with the Harbor Freight digital gauge... after all it is "harbor freight" but it consistently reads about 0.012 to 0.02" of play. what do you guys think? am i paranoid or its worth going down the rabbit hole of redoing the valves and guides?

[scottly]

the 4 intake valves won't come out as you can see because the edge of the end of the valve has rolled over causing it to mushroom and not pass thru the valve guides.

There is something wrong there!?? That edge is where the keepers sit, and it is Not normal wear! How many miles on this motor?

[algophobe]

the 4 intake valves won't come out as you can see because the edge of the end of the valve has rolled over causing it to mushroom and not pass thru the valve guides.

There is something wrong there!?? That edge is where the keepers sit, and it is Not normal wear! How many miles on this motor?

yeah, there's quite a mushrooming effect. only 250 miles. so much that i can't remove the valves from the guides on the intake side. all the exhaust side comes out.

valves and guides were brand new as of 250 miles ago. would you trust a harbor freight gauge and believe that play is signficant to cause oil leak into the combustion chamber?

[scottly]

Are the keepers correct for those valves? Please post a pic of the inside of the keepers. Is the cam "high lift"? Any evidence of the valve retainers hitting the top of the guides?

[PeWe]

New guides and valves should not have a play possible to feel with valve lift 10-15mm.
Ex guides usually worn in used old heads. I replaced some at 65.000km. A few again soon after (8000km) due to severe overheating.

My ported head with bronze guides 5mm stem (ampco 45) has some guides in need of replacement after only 25.000km. Too rich jetting and partly too lean helped to wear them quicker.

Mushroomed valves. Is it cheap aftermarket?
CB750 K stock, KPMI and CycleX have not worn that quick for me.

F2 stock 34mm in valves mushroomed really quick in K head, within 8000km. I have used them in a ported K head.  Last set got lash caps.
Guide height and high lift cams something I need to check.

Not good if retainer hit the guide seal. I read a thread from dec 2019 that 0.375" lift is a risk with stock guides and why Webcam's profiles have 0.360 lift.

Latest head on my K6 have stock guides and 125-75 cam.
F2 in valves, lift just under 0.400" at 0 lash.
My head to be swapped this winter so one thing to make sure that 0.400" lift is OK with 392 head, new stock guides, KPMI 33.5/28.5 valves, APE retainers and correct pressurized KPMI springs. Cut valves a little deeper and/or sink guides might help.
Mount cam to head on bench next....

[algophobe]

this is a stock cam, i believe the keeper and retainer is original. i bought the valves a while back from Vintage CB750, not sure who the maker are except for the lettering on the face of the valve "made in japan". didn't realize you have to have specific keepers for specific valves, most suppliers don't signate such. valve retainer and guide top and guide rubber seal seems to be in good condition. i tested the play at approximately 10-15mm, and they are noticeable.

are valve (or valves stems) and valve guides interchangeable between brands?

[algophobe]

so i spoke to Ken over at CycleX and asking around locally regarding work on the head. What do you all think about using the CX1 drop-in cams? Any experiences? I'm thinking of the CX1-R-975 to 1000 range. Supposedly its better bang for the buck than compared to a port job.

[RAFster122s]

Depends on who is doing your porting work I would wager... Maybe they optimized that cam offering for the typical stock head, but I have my doubts. But, I have been wrong about other things. Cam is likely to be less expensive than a good port job. 
Granted that Ken knows what he is talking about.
But, in my book a good port job is going to make more sense, especially if you are increasing displacement and opening up the exhaust with a free flowing exhaust system. Even with a stock cam a better port job is going to be worth the investment. I would do a MReich stage III port job on a stock motor with stock exhaust. Waste of money as you rob the motor of it being able to utilize the potential of such head work. It is a sustem that has to work together from intake to exhaust.

[SOHC4 Cafe Racer Fan]

Depends on who is doing your porting work I would wager... Maybe they optimized that cam offering for the typical stock head, but I have my doubts. But, I have been wrong about other things. Cam is likely to be less expensive than a good port job. 
Granted that Ken knows what he is talking about.
But, in my book a good port job is going to make more sense, especially if you are increasing displacement and opening up the exhaust with a free flowing exhaust system. Even with a stock cam a better port job is going to be worth the investment. I would do a MReich stage III port job on a stock motor with stock exhaust. Waste of money as you rob the motor of it being able to utilize the potential of such head work. It is a sustem that has to work together from intake to exhaust.

By the same rationale, go with a Stage IV if you are increasing displacement, and installing a performance cam and exhaust.

[algophobe]

Well.. I've been milling some ideas in my head while awaiting the arrival of Hondaman's book. Took a while but the lightly used copy arrived and started reading.. let's see what trouble I can get myself into down the wonderful rabbit hole!

[PeWe]

Perfect book for living room. Read with clean fingers from page 1 and forward!
When done you'll get the feel for it.

I hate good technical books to be used in the garage with oily fingers covering important data.

I have also a reprint of CB750 shop manual.
https://www.ebay.co.uk/itm/Honda-Factory-Workshop-Shop-Manual-Book-CB750-750-4-SOHC-Reproduction-HWM003-/251652308622

[newday777]

Well.. I've been milling some ideas in my head while awaiting the arrival of Hondaman's book. Took a while but the lightly used copy arrived and started reading.. let's see what trouble I can get myself into down the wonderful rabbit hole!

Watch out for rabbit holes......