Valve setting 81 cb750k dohc

Rampton

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Hi 😊 to all I own a cb750 k Dohc 1981, I have had this bike for two years and it is really a great ride when it runs. The latest fix was regulator stator and rotor to sort the charging system. Now poor spark number 4 but great on number one also number 3 intermittent. When bike warm #1&2 cylinders plus headers hot but #3&4 cold to touch ordered a new /used set of pick up coils. For peace of mind did compression test highest reading 115 psi and lowest 105 psi so now got more problems old bikes are great but wtf.my question is I have read that know one adjust the valves and this burns the valves, so why don't they adjust the valves is it a major job to do on my own. Would I have to change the valves or just adjust/ change shim's never done these on a Dohc before. Anyone out there got some good advice 😃 thanks
 

fg1972

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Hi there, adjusting the valves is important and can be done yourself with basic tools. It's not hard to do but very fiddly, best bet is to get yourself a manual. There is lots of info online too. Basically you need to check the gaps between the valve shims and camshafts. If gap is within tolerance, leave it, if gap too big, put in a thicker shim, if gap too small, thinner shim. What you might find is a shim taken from 1 spot can be used in another spot.
 

Rampton

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Thanks for the advice I have the manual and have bought the tool so I intend to have a go. Just trying to get the connection block and pick up coils changed had to take the engine support bolt near the front sprocket and prise the engine down to give enough room to feed the old connection block out and new one in, got new plugs without resistors, also oil leak at the front tensioner, a small rubber and metal plug don't know what it's for other than stopping oil leaking from cams?? Anyway thanks for the reply didn't seem like I was going to get any help been researching on other sites a lot will let you know how I go on. Stay safe
 

Rampton

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Hi there, adjusting the valves is important and can be done yourself with basic tools. It's not hard to do but very fiddly, best bet is to get yourself a manual. There is lots of info online too. Basically you need to check the gaps between the valve shims and camshafts. If gap is within tolerance, leave it, if gap too big, put in a thicker shim, if gap too small, thinner shim. What you might find is a shim taken from 1 spot can be used in another spot.
Lived in QLD for a while now back in UK 🇬🇧🇭🇲
 

Rampton

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Finished installing new pickup coils new plugs and cam tensioner plug. Started bike full choke for a few seconds and then half choke until fully off. Tick over 1200- 1500 unable to turn any lower. Seems to be hunting between these two rooms. Headers on 1&2 hot headers on 3&4 warm any suggestions please all are welcome 🇬🇧🇭🇲
 

X7eater

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Another tip (I'm in QLD by the way), get yourself a flexible pick up magnet tool. I paid about $5 for mine, but something like this is good: https://www.supercheapauto.com.au/p/toolpro-toolpro-pick-up-tool-magnetic-flexible/12545.html

The reason I mention this is because some of the shims pop out easily towards the centre of the head, whilst a couple of others can be removed more easily from the inlet/outlet side of the cam, and thats where the little magnet tool comes in handy, i.e. get the cam lobes in the right place, rotate the buckets, gently prise loose the cam, and then the little magnet helps lift out the cam from either side.

Also, and this is important, the Honda workshop manual specifies a valve clearance of .003" (+.002" -.001"). That potentially puts you down at .002" which is too tight and will result in the premature burn out of valves. You might ask....."Honda must know best"....true, but it's now generally accepted within the Honda DOHC 750/900/1100 community that the ideal inlet and outlet valve clearances should be set at 0.005" or 0.127mm, and if you're struggling to obtain the correct shim size, then use a smaller shim (i.e. larger clearance).
 

Rampton

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Brilliant thanks have a magnet tool in my toolbox so will use that, good idea. Will set both ex & inlet @ 0.005" as you say. 🇬🇧🇭🇲
 

fg1972

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Ok, there is a bit going on in this thread, valves, coils, ignition so I'm trying to break it down into smaller chunks otherwise you could be frustratingly chasing your tail. So after sorting out the ignition, I'm assuming you're now getting good spark on #3 and #4? If that's all good, cooler headers on #3, #4 could be relating to fuel, ie; carbs. The stock carbs are known for the low speed or idle jets to clog up. If you raise the revs to say midrange, do the colder headers warm up? Also the fact that you can't get the idle lower and it hunts around tends to point to carbs. Valves should and need to be done anyway but may not rectify the issues you're chasing hence trying to address the issues one by one.
 

Rampton

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Thanks for the reply, when I hold it at about 3000rpm the headers get warmer but not hot like #1&2 I think your right in being a problem with fuel and not spark. Like you say I could end up chasing issues. The fact is when I got the bike it wouldn't run far and thought I had fuel or spark problem, took carbs off had them sonic cleaned I changed stock jets for stock jets plus new float valves. Set mixtures at 1 3/4 turns out. Went for a ride and cut out after half an hour pushed bike home 🤣 found battery flat so went to charging system, rotor was problem when hot shorting out so by then had changed regulator stator and rotor, charging great 14.7v then found this problem with pick up coils so changed them plus spark plugs. And that is where I am. I did compression test twice 2 time with screw in fittings and engine warm, readings around 130s average don't think that doing valve shims will alter the problem but like you said on a 40 year old bike they want doing. This bike has taught me so much in two years, but can't take it to a shop to get her sorted not many old school mechanics around anymore. So hence asking you guys and gals for advice which is appreciated so much. So still got a fuel problem slow jet side but doing valve clearance tomorrow. Then order shims required from David silvers in the UK from£2-£4:50 each depending on size. Then back to carb fault#3&4 need help on that run out of ideas, I even bought the old "E" 🎸 string to assist cleaning passageways. 02:20 in UK now so going to get some Kip. Thanks again keep helping!!!! Stay safe🇬🇧🇭🇲
 

fg1972

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Thanks for the reply, when I hold it at about 3000rpm the headers get warmer but not hot like #1&2 I think your right in being a problem with fuel and not spark. Like you say I could end up chasing issues. The fact is when I got the bike it wouldn't run far and thought I had fuel or spark problem, took carbs off had them sonic cleaned I changed stock jets for stock jets plus new float valves. Set mixtures at 1 3/4 turns out. Went for a ride and cut out after half an hour pushed bike home 🤣 found battery flat so went to charging system, rotor was problem when hot shorting out so by then had changed regulator stator and rotor, charging great 14.7v then found this problem with pick up coils so changed them plus spark plugs. And that is where I am. I did compression test twice 2 time with screw in fittings and engine warm, readings around 130s average don't think that doing valve shims will alter the problem but like you said on a 40 year old bike they want doing. This bike has taught me so much in two years, but can't take it to a shop to get her sorted not many old school mechanics around anymore. So hence asking you guys and gals for advice which is appreciated so much. So still got a fuel problem slow jet side but doing valve clearance tomorrow. Then order shims required from David silvers in the UK from£2-£4:50 each depending on size. Then back to carb fault#3&4 need help on that run out of ideas, I even bought the old "E" 🎸 string to assist cleaning passageways. 02:20 in UK now so going to get some Kip. Thanks again keep helping!!!! Stay safe🇬🇧🇭🇲
Cleaning the carbs ultrasonically alone is probably not enough. Fully dismantling is the way to go. The primary and main jets shouldn’t be too much of an issue. The problem is idle jet which if you’re lucky, you’ll have the model where they are removable and easier to clean otherwise the guitar string method is your only option. I can’t stress how important it is that these idle circuits are clean. I dismantled mine 3 times to get it right. Good luck, with it all.
 

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Morning fg ready for the world now haha,Getting the carb bank off is a bad enough job but did before when I had it cleaned. Had a nightmare refitting them with new rubber boots it took me hours, is there an easy way??? I did slide the air box back. I changed mixture screws and two jets marked #100 second main jet and the other marked #68 primary main jet. Plus the needles and rubber tipped float valves. Set the floats with a micrometer to 15.5 mm. Carbs are vb52s and under the rubber plug on each carb is a pressed in jet which im thinking that these are the slow jets???? Didn't know how to remove them. I did put the guitar string in but couldn't see where it came out or even if it did. Thanks for the great advice keep it comming I will get her running if it kills me. I have a CBR 1997 fireblade that needs some time on the road and a 1996 triumph adventurer Hinckley 900 but the best is the cb750. Thanks again me old mate (Yorkshire saying) look forward to hearing from you soon.🇬🇧🇭🇲
 

fg1972

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Putting carbs back on can be a pain as you know. A trick that makes it a lot easier is to heat the rubbers with a heat gun. The jet under the rubber plug is the troublesome low speed or idle jet. They are either pressed in or have a slot in the top which can be unscrewed. Putting the guitar string in there and also in a tiny hole just in front of the throttle flap with lots of carb cleaner. Not sure if have this carb manual that is really useful. http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf
 

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Thanks again great advice will have a go! Will have to be with guitar string cos jet pressed in thanks for the carb diagram and great idea about heating up rubber boots to make them more playable will definitely try that spent hours trying to get the new boots back on were they didn't want to go cheers pal🇬🇧🇭🇲
 

Rampton

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Ok here we go just checked valve clearances on the cb750 Dohc 1981 Honda, had no problems (due to good advice) the results are as follows,
#1 ex .007" & .008"
#2ex .006" & .004"
#3ex .004" & .004"
#4ex .006" & .006"

#1in .003" & .004"
#2in .005" & .005"
#3in .004" & .003"
#4in .007" & .004"

As I understand it shims will need changing for different size on any below .005" and any above .006" is this correct???? Stay safe 🇬🇧🇭🇲
 

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Rampton

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Hi there wow!!! No wonder no one does the shims bit wobbly at first but with a great tip from x7eater (using the Flexi magnet) and a couple of picks I got into a swing got easier as I went on, simple job really but frustrating at times haha. Thanks again for the help this is much appreciated 😊😊. The shim sizes are as follows:-

#1ex 265 & 265.
#2ex 270 & 275.
#3ex 265 & 265.
#4ex 280 & 280.

#1in 265 & 270.
#2in 255 & 265.
#3in 265 & 265.
#4in 280 & 280.
Got work out which can be swopped and what size new required. Unless there is some genius out there 😂. Thanks again for all the help will keep plugging away to get this old girl back on the road. Stay safe. 🇬🇧🇭🇲
 
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Rampton

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Another tip (I'm in QLD by the way), get yourself a flexible pick up magnet tool. I paid about $5 for mine, but something like this is good: https://www.supercheapauto.com.au/p/toolpro-toolpro-pick-up-tool-magnetic-flexible/12545.html

The reason I mention this is because some of the shims pop out easily towards the centre of the head, whilst a couple of others can be removed more easily from the inlet/outlet side of the cam, and thats where the little magnet tool comes in handy, i.e. get the cam lobes in the right place, rotate the buckets, gently prise loose the cam, and then the little magnet helps lift out the cam from either side.

Also, and this is important, the Honda workshop manual specifies a valve clearance of .003" (+.002" -.001"). That potentially puts you down at .002" which is too tight and will result in the premature burn out of valves. You might ask....."Honda must know best"....true, but it's now generally accepted within the Honda DOHC 750/900/1100 community that the ideal inlet and outlet valve clearances should be set at 0.005" or 0.127mm, and if you're struggling to obtain the correct shim size, then use a smaller shim (i.e. larger clearance).
Hi x7eater thanks for the comments I have posted shim sizes and gaps, would like to know what you think regarding your comment on oversized shims and air on the side of larger clearance. My clearances range from .003 to .008 and as said the aim is .005. is this worth proceeding with shim change????? Opinions would be welcome thankyou.
 

Rampton

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Another tip (I'm in QLD by the way), get yourself a flexible pick up magnet tool. I paid about $5 for mine, but something like this is good: https://www.supercheapauto.com.au/p/toolpro-toolpro-pick-up-tool-magnetic-flexible/12545.html

The reason I mention this is because some of the shims pop out easily towards the centre of the head, whilst a couple of others can be removed more easily from the inlet/outlet side of the cam, and thats where the little magnet tool comes in handy, i.e. get the cam lobes in the right place, rotate the buckets, gently prise loose the cam, and then the little magnet helps lift out the cam from either side.

Also, and this is important, the Honda workshop manual specifies a valve clearance of .003" (+.002" -.001"). That potentially puts you down at .002" which is too tight and will result in the premature burn out of valves. You might ask....."Honda must know best"....true, but it's now generally accepted within the Honda DOHC 750/900/1100 community that the ideal inlet and outlet valve clearances should be set at 0.005" or 0.127mm, and if you're struggling to obtain the correct shim size, then use a smaller shim (i.e. larger cleara
Another tip (I'm in QLD by the way), get yourself a flexible pick up magnet tool. I paid about $5 for mine, but something like this is good: https://www.supercheapauto.com.au/p/toolpro-toolpro-pick-up-tool-magnetic-flexible/12545.html

The reason I mention this is because some of the shims pop out easily towards the centre of the head, whilst a couple of others can be removed more easily from the inlet/outlet side of the cam, and thats where the little magnet tool comes in handy, i.e. get the cam lobes in the right place, rotate the buckets, gently prise loose the cam, and then the little magnet helps lift out the cam from either side.

Also, and this is important, the Honda workshop manual specifies a valve clearance of .003" (+.002" -.001"). That potentially puts you down at .002" which is too tight and will result in the premature burn out of valves. You might ask....."Honda must know best"....true, but it's now generally accepted within the Honda DOHC 750/900/1100 community that the ideal inlet and outlet valve clearances should be set at 0.005" or 0.127mm, and if you're struggling to obtain the correct shim size, then use a smaller shim (i.e. larger clearance).
Hi sorry about confusion , I credited fg1972 instead of you regarding brilliant idea of using a small Flexi magnets, mixed up the reply, now as I have posted in a bit stuck as wether to try and find shims or not. I have posted the shim sizes and clearances I have, your comments are always
appreciated. Ps I lived in Emerald was a coal miner, true Yorkshire man. Stay safe 🇬🇧🇭🇲
 

Rampton

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Hi x7eater thanks for the comments I have posted shim sizes and gaps, would like to know what you think regarding your comment on oversized shims and air on the side of larger clearance. My clearances range from .003 to .008 and as said the aim is .005. is this worth proceeding with shim change????? Opinions would be welcome thankyou.
Looked at David Silvers for shims they only sell them in increments of 5 so I would not be able to achieve the desired .005" thinking I would be burnt as scolded. Does anyone have any advice on this matter thankyou. Stay safe 🇬🇧🇭🇲
 
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