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Old 20-03-2011, 05:22 PM
Billy Balthorpe Billy Balthorpe is offline
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Default Air cooled Honda DOHC woes

My CB 750 Nighthawk which i was going to use for a base for a Bobber is blowing smoke and has low compression on all cylinders, i was hoping against hope that it was the valves slightly propped but no, a squirt of some oil down the bore and the compression shot up to over 200 psi. Conclusion, bores are knackered. Its a shame coz the engine runs very sweet with no rattles, clunks or tapping. I've priced up the replacement pistons for a rebore from David silver and gave up when the total got above 500 for parts alone. So im thinking of putting a CBX750 motor in it. Has anyone had any experience of this or could anyone point me in the direction of a Honda forum which would give me some advice??

Anyway i'm off to put my head in the gas oven, i'm gutted, i MOT'd the thing a week ago as well!
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Old 20-03-2011, 06:35 PM
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do wiseco or similar not do a kit?
i went that route with my gsx1100 because the oversize genuine suzuki pistons were way too much
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Old 20-03-2011, 07:09 PM
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Anyway i'm off to put my head in the gas oven, i'm gutted, i MOT'd the thing a week ago as well!
Not much point in that these days. We use natural gas now which is only lethal after its burnt! You'de roast yer face off before you keeled over from the fumes!

Fungus the Gas (Safe) man
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Old 20-03-2011, 08:34 PM
Billy Balthorpe Billy Balthorpe is offline
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Not much point in that these days. We use natural gas now which is only lethal after its burnt! You'de roast yer face off before you keeled over from the fumes!

Fungus the Gas (Safe) man
I was waiting for some sad person to point that out.

But i had cunningly sorted a seal from around the open door of the oven over my head and around my neck, so, although i wouldnt die from Town Gas poisoning, i would however die from asphyxiation. Well, i would have done, but i realised that we now have a lovely Smeg dual fuel cooker which replaced the all gas jobby we had before, i actually realised this just after my hair set on fire.
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Old 20-03-2011, 08:41 PM
Billy Balthorpe Billy Balthorpe is offline
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do wiseco or similar not do a kit?
i went that route with my gsx1100 because the oversize genuine suzuki pistons were way too much
I doubt if they would do a kit for my Nighthawk, they would probably do one for the CBX750, but that has a different compression ratio. As they probably use the same heads they probably have messed about with the Gudgeon Pin to piston crown sizes. As i dont fancy finding out it wont work after ive paid for the kit i think ill give it a miss and try to fit a complete CBX750 engine and loom/CDI.
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Old 20-03-2011, 09:21 PM
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I think I can help..

A flip lid bin bag should fit nicely over your head, tie it round your neck with a scarf and you can do it in the comfort of your armchair.

hope that helps..

John.
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Old 20-03-2011, 09:56 PM
Billy Balthorpe Billy Balthorpe is offline
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Thanks John, Just the bike to sort out now then!
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Old 20-03-2011, 11:20 PM
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you mention new pistons..why? a rebore and oversize rings would be far cheaper...
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Old 20-03-2011, 11:25 PM
Billy Balthorpe Billy Balthorpe is offline
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I didnt think you could do that?

So you could overbore to the first size, which i think is .25 and still use the original pistons? Wouldnt you get piston slap?

Have you ever done it?
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Old 21-03-2011, 12:20 AM
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why would you get piston slap?..the piston doesnt come into contact with the bore,the rings do..having the bores re-ground to the next size and fitting the new rings is no different than running original..
i had a 250n rebored and ran oversize rings...and it ran as crap as it should..lol
na seriously look into it..
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Old 21-03-2011, 12:24 AM
Billy Balthorpe Billy Balthorpe is offline
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I always thought that with an increase in bore size (through wear usually) and the piston staying the same size, the piston woul have enough room to pivot on the pin and have the skirt come into contact with the bore so creating piston slap?

I stand to be corrected because i've never actually heard this piston slap, there has always been something much worse going on when they get to that stage.

Any ideas where i should start looking?
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Old 21-03-2011, 12:49 AM
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tbh id give draken a holler..
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Old 21-03-2011, 02:17 AM
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here ya go - get them to mark it as birthday present

http://cgi.ebay.com/ebaymotors/Honda...Q5fAccessories

also found this

http://cgi.ebay.co.uk/CB750-DOHC-Com...item33662c1987

set of exhaust caskets of ebay around 6.00

So with postage should be back on the road for 300.00 or so !
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Old 21-03-2011, 08:22 AM
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you cant rebore and use standard pistons so forget that idea.
there is a proper tolerance for piston clearance.
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Old 21-03-2011, 10:18 AM
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Quote:
Originally Posted by hacky View Post
why would you get piston slap?..the piston doesnt come into contact with the bore,the rings do..having the bores re-ground to the next size and fitting the new rings is no different than running original..
i had a 250n rebored and ran oversize rings...and it ran as crap as it should..lol
na seriously look into it..
The rings "seal" the piston in the bore but the piston skirt guides the piston in the cylinder. OK the piston doesn't come into contact with the bore normally cos there's a layer of oil between them.
But the correct clearance is essential to keep this oil film in place. This is why modern engines which run tight clearances require very thin oil to suit.

Standard pistons in over size bores NO.

You may have had the bores honed to glaze bust them and rings are available to suit, but that aint a rebore as honing hardly takes any metal away.
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Old 21-03-2011, 03:12 PM
Billy Balthorpe Billy Balthorpe is offline
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Ok, how about this.

I strip the head and barrels off and measure the bore (if it isnt visually screwed) if the bore is within the next oversize bore i should be able to hone the bore and fit new piston rings? Also i should be able to measure the ring gap and the width of the rings to see if its the rings that are worn. I also think that the rings may be "gummed" dont ask me why i just do, a new set of rings and a hone may bring the compression up and stop the thing smoking like and Kipper factory.

What do you think?
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Old 21-03-2011, 03:15 PM
Billy Balthorpe Billy Balthorpe is offline
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Originally Posted by Creature View Post
here ya go - get them to mark it as birthday present

http://cgi.ebay.com/ebaymotors/Honda...Q5fAccessories

also found this

http://cgi.ebay.co.uk/CB750-DOHC-Com...item33662c1987

set of exhaust caskets of ebay around 6.00

So with postage should be back on the road for 300.00 or so !
Yeah, ive seen the first one, i dont want to go and buy it if im going to get problems with the new pistons having a different pin to crown distance as mine has a lower compression ratio than a standard DOHC. Also it may have different pin sizes than mine, its a lot of money for a risk dont you think?

The second one looks good tho.
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Old 21-03-2011, 05:08 PM
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Originally Posted by Billy Balthorpe View Post
Ok, how about this.

I strip the head and barrels off and measure the bore (if it isnt visually screwed) if the bore is within the next oversize bore i should be able to hone the bore and fit new piston rings? Also i should be able to measure the ring gap and the width of the rings to see if its the rings that are worn. I also think that the rings may be "gummed" dont ask me why i just do, a new set of rings and a hone may bring the compression up and stop the thing smoking like and Kipper factory.

What do you think?
I have a GS1000 which was smoking badly and the compression was down. Oversized piston rings and a glaze busting sorted it out, no worries. Took a day. Later put a stage 3 dynojet kit in it... No problems since all this was done, approx 12 years ago, ish, but been in the back of the shed for 4 years now in favour of a 1200 Bandit and a Harley..........
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Old 21-03-2011, 06:34 PM
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Quote:
Yeah, ive seen the first one, i dont want to go and buy it if im going to get problems with the new pistons having a different pin to crown distance as mine has a lower compression ratio than a standard DOHC. Also it may have different pin sizes than mine, its a lot of money for a risk dont you think?
Nope they are the same engine - check your manual you will find apart from piston dome differences they are identical. will not cause yopu any problems. if you want to confirm this ring Wiseco 01322 669469 - they are in Kings Lyn and make and fit these kits all the time
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Old 21-03-2011, 06:44 PM
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Originally Posted by Billy Balthorpe View Post
Ok, how about this.

I strip the head and barrels off and measure the bore (if it isnt visually screwed) if the bore is within the next oversize bore i should be able to hone the bore and fit new piston rings? Also i should be able to measure the ring gap and the width of the rings to see if its the rings that are worn. I also think that the rings may be "gummed" dont ask me why i just do, a new set of rings and a hone may bring the compression up and stop the thing smoking like and Kipper factory.

What do you think?
Sounds reasonable to me. Dave at Z Power has this done to his restorations. I've seen some right smoky motors run like new again after the honing etc.
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Old 21-03-2011, 10:18 PM
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Originally Posted by Billy Balthorpe View Post
Ok, how about this.

I strip the head and barrels off and measure the bore (if it isnt visually screwed) if the bore is within the next oversize bore i should be able to hone the bore and fit new piston rings? Also i should be able to measure the ring gap and the width of the rings to see if its the rings that are worn. I also think that the rings may be "gummed" dont ask me why i just do, a new set of rings and a hone may bring the compression up and stop the thing smoking like and Kipper factory.

What do you think?
yep thats the best thing to do first, i wouldnt be too worried about ring gap, i read about testing done on this and they were up over 30 thou ring gap before any noticeable difference. piston clearance is much more important.
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Old 21-03-2011, 11:09 PM
Billy Balthorpe Billy Balthorpe is offline
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Originally Posted by griff2000 View Post
I have a GS1000 which was smoking badly and the compression was down. Oversized piston rings and a glaze busting sorted it out, no worries. Took a day. Later put a stage 3 dynojet kit in it... No problems since all this was done, approx 12 years ago, ish, but been in the back of the shed for 4 years now in favour of a 1200 Bandit and a Harley..........
What exactly do you mean by "oversized piston rings"? Do you mean rings that would normally be fitted to a new piston (+0.25mm) if you bored the cylinder out to the next size up (normally +0.25mm)?
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Old 22-03-2011, 08:24 AM
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billy, please forget the idea of boring out to next size and leaving standard piston with oversize rings. its asking for trouble, they dont use bigger pistons to fit the bore just for the fun of it. think about it.
and i fail to see how a honing job could make the bore big enough to use the size rings anyway? not without filing the ends to creat a gap anyway?
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Old 22-03-2011, 08:47 AM
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check the valves first, you may just have burned valves. putting oil down the bore will raise the comp cos it fills the gap round the pistons a bit, but it will also seal leaky valves too. checking your valve clearances will cost you nothing, so do that first, dont rush to spend money unless you really want to. if the budget is a factor, then look out for a used set of standard barrels on ebay.

isnt it the same engine as the cbx 750?
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Old 22-03-2011, 01:45 PM
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billy, please forget the idea of boring out to next size and leaving standard piston with oversize rings. its asking for trouble, they dont use bigger pistons to fit the bore just for the fun of it. think about it.
and i fail to see how a honing job could make the bore big enough to use the size rings anyway? not without filing the ends to creat a gap anyway?
I'd already given that up as a bad job mate. I'm going to strip it and check and measure the bores and rings. I'll check the ring gap and if its ok i'll hone the bores and re fit, if its a bit suspect i'll measure the width of the rings to make sure they have worn and buy some new ones, check the gap again and hone the bores and re fit.
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Old 22-03-2011, 01:49 PM
Billy Balthorpe Billy Balthorpe is offline
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check the valves first, you may just have burned valves. putting oil down the bore will raise the comp cos it fills the gap round the pistons a bit, but it will also seal leaky valves too. checking your valve clearances will cost you nothing, so do that first, dont rush to spend money unless you really want to. if the budget is a factor, then look out for a used set of standard barrels on ebay.

isnt it the same engine as the cbx 750?
I see what you are saying, but add the heavilly smoking exhaust into the equation and its looking like the bores. It could be inlet valve stem seals leaking as when the engine is running at higher RPM the vacuum created may be enough to pull oil down the guides. But i've had propped valves before and you mainly get trouble with starting, this starts like a good un. I'll check them when i take it apart anyway and probably renew the seals.
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Old 22-03-2011, 09:17 PM
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billy, i had a gsx1100 that smoked like fuck and it was the valve guide seals that were perished. the guides could be worn or seals fucked if your motor has them.
does it smoke all the time?
what was the compression before adding oil, how low?
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Old 23-03-2011, 12:59 AM
Billy Balthorpe Billy Balthorpe is offline
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55 to 80 psi, which went up to over 200 when I put oil down th bores.
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Old 23-03-2011, 11:16 AM
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I reckon' it's time to bite the bullet, and get the head off.

Then you'll know for definite.
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