Author Topic: CB750 Barn find project - First Timer  (Read 53255 times)

Offline Underdog1

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Re: CB750 Barn find project - First Timer
« Reply #555 on: September 01, 2018, 10:20:31 PM »
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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Thanks for the advice but as this has happened with a remote fuel tank too I don’t think it’s petcock related

Offline Rob62

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Re: CB750 Barn find project - First Timer
« Reply #556 on: September 02, 2018, 09:58:22 AM »
I had the leaking oil line problem..... apparently the oil lines rarely fail so i messed about with various washers but to no avail. I ended up having the oil line re-hosed at mendahose (not for the purist but very cheap and looks great imo).... solved the problem and a week later the 2nd hose also started dripping so i had that re-hosed exactly the same. Now both ok and look great, cost £35 each. You can get original looking pipes from dss for £300 ! Or they can be re-tubed back to standard but I think you would be happy with pipes done at mendahose or some other similar industrial hydraulic repair shop...http://www.mendahose.co.uk

Offline Underdog1

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Re: CB750 Barn find project - First Timer
« Reply #557 on: September 02, 2018, 05:21:15 PM »
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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Thanks for the advice but as this has happened with a remote fuel tank too I don’t think it’s petcock related
Well the bike has been standing a week and it still looks like fresh fuel is coming out the overflows despite petcock being switched off also one or two of the overflow tubes themselves are falling apart at the ends.. is this ethanol related?




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Re: CB750 Barn find project - First Timer
« Reply #558 on: September 02, 2018, 05:29:39 PM »
Take the float bowl off and secure it level,  fill with fuel and see if it drops then.

You can get the internal brass "standpipe" tube cracked and it let's the fuel leak out even without breaching the float/fuel level.

Check each in turn to find any errors.

Offline Underdog1

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Re: CB750 Barn find project - First Timer
« Reply #559 on: September 02, 2018, 06:41:53 PM »
Take the float bowl off and secure it level,  fill with fuel and see if it drops then.

You can get the internal brass "standpipe" tube cracked and it let's the fuel leak out even without breaching the float/fuel level.

Check each in turn to find any errors.

Just to clarify you mean fill up the carb bowl when they are off the bike?

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Re: CB750 Barn find project - First Timer
« Reply #560 on: September 02, 2018, 07:45:02 PM »
Yes.

They sometimes get a crack in the tube that allows the fuel out through the overflow pipe without the level of fuel being above the top of that brass tube.

It would do as you indicate by leaking out even if the fuel tap is turned off.

Offline Bryanj

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Re: CB750 Barn find project - First Timer
« Reply #561 on: September 03, 2018, 12:46:00 AM »
If it is leaking with the tap turned off either the tap sealing disc is duff or you did not fit the proper washers under the heads of the retaing bolts

Offline yozzer74

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Re: CB750 Barn find project - First Timer
« Reply #562 on: September 03, 2018, 04:23:58 PM »
I had this problem mine was the over flow tube had a crack in it couldn't see it but it leaked .I fitted a piece of rubber tube over it think it was 4mm had no trouble after.

Offline Underdog1

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Re: CB750 Barn find project - First Timer
« Reply #563 on: September 03, 2018, 06:46:28 PM »
If it is leaking with the tap turned off either the tap sealing disc is duff or you did not fit the proper washers under the heads of the retaing bolts
I just usescooper washers - didn’t look like anything special when I replaced them

Offline Bryanj

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Re: CB750 Barn find project - First Timer
« Reply #564 on: September 04, 2018, 07:32:13 AM »
There is your problem as they should be thick fibre washers that you can not get genericaly. I have used 6mm "Dowty" style washers with some success. If a total seal does not happen fuel leaks down past the bolt threads and bypasses the tap flooding the float bowls

Offline Underdog1

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Re: CB750 Barn find project - First Timer
« Reply #565 on: September 04, 2018, 05:05:16 PM »
There is your problem as they should be thick fibre washers that you can not get genericaly. I have used 6mm "Dowty" style washers with some success. If a total seal does not happen fuel leaks down past the bolt threads and bypasses the tap flooding the float bowls
Aha nice that would explain part of it, but the bowls overflowing is a separate issue I think

UPDATE: just under £14 to have 2 delivered from DS..
« Last Edit: September 04, 2018, 05:17:31 PM by Underdog1 »

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Re: CB750 Barn find project - First Timer
« Reply #566 on: September 04, 2018, 05:20:23 PM »
As above,  even with fuel tap leaking (if confirmed)  the float bowls should be limited by the float closing the supply with needle valve.

I know they can be iffy,  but if all clean they generally work ok.

I'd still work to confirm those internal "weir" tubes are not leaking.  If you have just one (put each one into a jamjar to verify)  then swap float bowl with another to see if problem switches with it.

Offline Bryanj

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Re: CB750 Barn find project - First Timer
« Reply #567 on: September 05, 2018, 02:13:37 AM »
With hravity feed from tank and engine off float needles have to be perfect to stop flow. With screws sealed you should not get more leakink and floats will seal enough for normal running as long as you remember to turn off when stopping engine. I think this was one of the reasons vac taps became more common

Offline Underdog1

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Re: CB750 Barn find project - First Timer
« Reply #568 on: September 05, 2018, 07:49:25 PM »
With hravity feed from tank and engine off float needles have to be perfect to stop flow. With screws sealed you should not get more leakink and floats will seal enough for normal running as long as you remember to turn off when stopping engine. I think this was one of the reasons vac taps became more common

Yeah will investigate further this weekend, have ordered a few of those dowty washers you mentioned, will hopefully have some luck. The seats at the upholsterers so can’t ride anyway

Offline Bryanj

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Re: CB750 Barn find project - First Timer
« Reply #569 on: September 05, 2018, 11:06:11 PM »
If using dowty washers you might get better results using allen bolts instead of screws as the underside of the head is better

 

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