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

[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.

[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......

[algophobe]

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

the shop manual (digital form on my ipad) is what's i've been working off and the hard copy of the clymer manual (which leaves a lot of details out). what fun is it when i pay someone to do the work robbing me the opportunity of learning or falling flat on my face  but i'll leave the precision machining to the experts.  so far i think i'll try my hand at de-shrouding the combustion chamber and cleaning up the intake and exhaust ports before i send it off to have new guides and valves placed by cycleX.

[algophobe]

got some work on the head. took work to clean off all the carbon from the exhaust ports. my main goal with the exhaust ports is to smooth it out as best i can, especially remove the casting lines and not necessarily widen the ports. what i do notice is that 1/4 ports are noticeably wider than 2/3. but it does look as if 2/3 are a little shorter.  i sanded it with a die grinder using sanding mandrel with 80, then 100, 120 grits. then using the dremel with the abrasive buffing wheel to 180, 220, 320. For #$%*s and giggles i polished them with the course rouge. its no where near perfect but a learning opportunity and hopefully makes the exhaust flow a little easier.

i won't bore you with pics of every port, but here are some samples.

the valves took some beating, but they'll be sent out to be replaced at the end along with a new valve seats cut.

for the intake ports, i'll be grinding down the transition zones and deshrouding the combustion chamber but not go crazy like this.. going by the theory that the turbulence from the surface will help mix air and fuel better since its a carburated system. more to come.

[algophobe]

the combustion chambers were de-shrouded and sanded to 180 grit and on the in-take side, i opened up the passages a little from the carb boots to the port just above the intake valve. then the head and cylinders just got shipped to Cycle X for some work.

[Terry in Australia]

Looks good mate, and just smoothing the casting marks will improve the engine's performance, so worth your while.

[algophobe]

Finally the head and cylinders came back from CycleX! So peeerty. Blasted, cylinders bored and honed to 2+, new valves and guides and seats re-cut. New matching 2+ pistons from CycleX as well.. The shine from my home port job is gone from the sand blast.. but I'm ok with leaving it this way.

It's been a while since I've been back in the workshop. Excited for this weekend.