CB750 Barn find project - First Timer

[Underdog1]

Check compression! Burned or leaky valves could be an issue.... build up of carbon on the stems can cause the valves to stay open once warmed up  (i had this issue back in the 80’s)... a re-lap of the valve seats transformed the motor. A compression test will help, when cold and also at running temp... keep going, you will get there.

Ok sounds easy enough, lapped all the valves and cleaned hem up during the rebuild but I’ll recheck clearances and do a compression test this week.. just need a compression gauge.

Are the cheap ones that come in a red plastic case (all over amazon and eBay) for about £12 good enough?

[Nurse Julie]

10-15 minutes riding sounds quite a long time for something fuel or electrical related to show itself. Could it be the top end of the engine getting very hot for some reason and partially seizing due to metal expansion? Cools down, contracts and off you go again. My theory is based on nothing more than Women's logic here 😁😁

[Underdog1]

10-15 minutes riding sounds quite a long time for something fuel or electrical related to show itself. Could it be the top end of the engine getting very hot for some reason and partially seizing due to metal expansion? Cools down, contracts and off you go again. My theory is based on nothing more than Women's logic here 😁😁

I had something that felt similar on a hornet I had, would conk out after 15 mins and not restart until it cooled down, was a burnt connector from the stator and a duff rectifier causing the battery to not charge.

I get 14.5v over the battery with bike on or off which seems fine. Would be good to rest when the battery is depleted a bit I suppose to make sure the voltage jumps up when it starts

[Bryanj]

The 12 quid ones are crap mate

[Underdog1]

The 12 quid ones are crap mate

Maybe I’ll stretch to the £18 ones then

Not had time to work on the bike today but quickly whipped the carb bowls off and fuel flows fine, embarrassingly I thought  the off and reserve settings on the pet cock were the other way round so half the time I was probably trying to start the bike in the off position!

Now it’s even possible that I ran out of fuel on the on position as I put a Jerry cans worth of fuel in  then continued to try and start the bike in the off position and I did fanny about with the peacock a fair bit during breakdowns, perhaps I filled the carb bowls up when I though it was switched off and then drove a mile or so with it actually switched off thinking I was on reserve until the bowls emptied again..

Anyway I can’t see any sparks at all at the points one of the spark plugs seemed loose and this is the colour they’re at

[Rob62]

Lol... hopefully you have solved the mystery... 

[Nurse Julie]

I know you have a few issues to attend too but it must be a good feeling that she's up and running now. We'll done.

[Underdog1]

I know you have a few issues to attend too but it must be a good feeling that she's up and running now. We'll done.

Yeah it’s been a great learning experience, never messed about with engines before but now feel as if I’ve at least got a basic grasp!

When I do the next one it will be a lot easer 

[SteveD CB500K0]

I did fanny about with the peacock a fair bit during breakdowns

I should resurrect the old “Quote of the Month” thread



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[K2-K6]

Plug doesn't look crazy,  bit dark but the main electrode is burning clean-ish so not too far off and would probably show better if it ran for longer.

They really will do over 3 miles on one fill of the float bowls as I routinely turned mine off on the way home to leave it parked as good habit when riding them.

Hope you've found the reason and nailed that one down.

[Underdog1]

I did fanny about with the peacock a fair bit during breakdowns

I should resurrect the old “Quote of the Month” thread



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Hah peacock! Bloody iPhone autocorrect always makes me post weird stuff 

[Underdog1]

Plug doesn't look crazy,  bit dark but the main electrode is burning clean-ish so not too far off and would probably show better if it ran for longer.

They really will do over 3 miles on one fill of the float bowls as I routinely turned mine off on the way home to leave it parked as good habit when riding them.

Hope you've found the reason and nailed that one down.

Will be nice if it’s that simple, going to to some laps of the block so if I break down I won’t be too far and also bring a backpack full of tools to check plugs, fuel, coils, battery etc.

As I mentioned earlier the carbs have only been bench synced, needle clips are in the middle position so could move  them down a notch or two to raise the needle for a leaner mix. I’ve ordered a carbtune pro to properly sync them too.

I’m happy I’m in the ballpark with the jets as it was basically a guess

[Underdog1]

Seemed ok on a 15 minute ride, came back in as the front brake was feeling pretty absent. I’ve got braided lines and a rebuilt caliper (reused the piston as it seemed ok) and new master cylinder, it’s been ok up until now but just felt like it wasn’t stopping the bike to the point where I met a car in the lanes and had to use the rear brake heavily to stop. Haven’t yet adjusted it as I can’t get the front wheel in the air without taking the headers off which means new crush gaskets, will get around to it soon though but I think it’s maybe more than just out of adjustment. It’s been bled properly and the lever is feeling hard.

Something else I’ve noticed is one of the oil lines has a small drip where the end fixing is crimped on. I take it this can’t be overhauled?



With regards to the carbs overflowing, I fitted the overflow hoses and since then not had anything out of them, can’t see why that would make a difference though.

UPDATE: just seen the price of those oil hoses 

[Underdog1]

So thankfully it seems it’s the washer on the oil tank drain bolt leaking rather than the oil hose, I’ll replace the washer at some point and see if it makes a difference.

The seats currently with a local upholsterer getting sorted, have received a Morgan carbtune pro so I’ll hopefully have the carbs dialled in soon (unable to work on it this weekend though).

The carbs are still dumping fuel seemingly randomly. I’ll put the bike away and switch the fuel off, tinker around with a few things and still no fuel dumped.. but when I come back into the garage the next day there will be fuel ok the floor. Sometimes it doesn’t happen for a few days and all of a sudden it justs dumps the fuel whilst sat doing nothing...

They floats seem ok, if they were at incorrect heights or whatever surely it would just overflow straight away?

[Honda Al]

So thankfully it seems it’s the washer on the oil tank drain bolt leaking rather than the oil hose, I’ll replace the washer at some point and see if it makes a difference.

The seats currently with a local upholsterer getting sorted, have received a Morgan carbtune pro so I’ll hopefully have the carbs dialled in soon (unable to work on it this weekend though).

The carbs are still dumping fuel seemingly randomly. I’ll put the bike away and switch the fuel off, tinker around with a few things and still no fuel dumped.. but when I come back into the garage the next day there will be fuel ok the floor. Sometimes it doesn’t happen for a few days and all of a sudden it justs dumps the fuel whilst sat doing nothing...

They floats seem ok, if they were at incorrect heights or whatever surely it would just overflow straight away?

I’ve had this recently on my 500. At first I thought I left the fuel tap on, but realised that the fibre washers in the petcock housing had failed and were leaking fuel through the tap down into the carbs. They would then overflow onto the garage floor.

Quick test is to lift the fuel tank, and place it on a flat surface with a tray under the petcock. If the petcock is switched off but fuel starts gathering in the tray, there’s the source of the problem.

Hope this helps.


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