Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Wenman on July 03, 2020, 04:00:29 PM
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Okay so here’s the problem..
Honda CB750 k2
Stock carbs
Pod filters (don’t kill me)
4-1 (again don’t kill me)
Standard idle jet
+4 on the main jet
Toming correct
Cam chain correct
New coils
New leads
New plugs
New points
New condensers
Bike runs okay, pulls well above 2500rpm suggesting the main jet is okay, really bad idle and running very rich so I put the stock idle jets back on.
The main issue I am getting is the revs hang around 2500/3000 rpm when at a stand still.
I’ve put them on a vacuum balancer which was impossible as I’m struggling to get the bike to run at idle.
Everything seems okay on the carbs, they were fully dismantled cleaned new jets, seals, float heights done etc.
The throttle return spring is new and if anything on the heavy side.. after holding the throttle open for 3 miles of 50mph my hand was hurting 😂
If I apply A downward pressure to the tops of the carb lifters and blip the throttle it returns quickly to idle every time .. Almost like Ive made an error when putting them back together?
It feels like they are not closing properly?
I know this could be a multitude of things but was wondering if anyone has experienced this and it was a stupid error made when putting a set of these carbs back together??
Thanks for your help
Alan
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Have you air leaks between carbs and head?
Try removing the pods and checking.
You could see if the slides are dropping correctly.
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I’ve sprayed the inlet rubbers with carb cleaner and didn’t notice any increase in rpms?
Also slides are all seating correctly I believe?
Thanks tho
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Do you have both throttle cables fitted?
If so, can you bring the idle down by "forcing " the twist grip closed? ie. Rotating it clock wise to make the secondary cable work.
If it closes off the high revs by doing the above, have you tensioned the normal throttle cable adjustment too much as it should have about 10 degrees of rotational slack at the twist grip. This would also make the throttle really heavy too if it's not set correctly.
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Do you have both throttle cables fitted?
If so, can you bring the idle down by "forcing " the twist grip closed? ie. Rotating it clock wise to make the secondary cable work.
If it closes off the high revs by doing the above, have you tensioned the normal throttle cable adjustment too much as it should have about 10 degrees of rotational slack at the twist grip. This would also make the throttle really heavy too if it's not set correctly.
Pushing the throttle grip forward does not reduce the revs sadly...
I def have play about 10 degrees at the throttle grip. I can see a small amount of slack in the ‘pull’ cable at the carb rack but unsure on the ‘push’ cable? I’ll have a look
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Push cable usually has no adjuster, so just fit and leave. But that one shouldn't give you a problem unless something is misplaced on brackets.
As it has an affect when you push the top of slides it may be that there's not a parity of the slide home position from assembly. Are you familiar with Bryan's bench synchronisation method? As that should remove an error like that if present.
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Push cable usually has no adjuster, so just fit and leave. But that one shouldn't give you a problem unless something is misplaced on brackets.
As it has an affect when you push the top of slides it may be that there's not a parity of the slide home position from assembly. Are you familiar with Bryan's bench synchronisation method? As that should remove an error like that if present.
No? Happy to whip the rack off and go through a proper sync.. where can I find that document please??
Thabsk
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http://www.sohc.co.uk/index.php/topic,17769.0.html
It should (it does) remove any discrepancy that inadvertently arises from reassembly of the carb rack to leave the slides in correct position relative to the single hand adjustable idle screw for setting overall tickover speed.
Afterwards, I'd not try anything with vacuum gauges until you've got the jetting more competently in range.
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This thread is worth a read too as it looks specifically at accommodations for running without air filter box etc.
http://www.sohc.co.uk/index.php/topic,22031.0.html
It'll be interesting to hear how you get on with it.
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This thread is worth a read too as it looks specifically at accommodations for running without air filter box etc.
http://www.sohc.co.uk/index.php/topic,22031.0.html
It'll be interesting to hear how you get on with it.
Thank you.. I think this is def the best starting point! I’ll be sure to get back and let you know how I get on!
Alan
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I know i wrote it but it works and if all the mechanical service items are right i find the vac gauges are normaly within tolerance. Idealy if you have done all this and it runs ride it round till next service due, 500 if engine rebuilt 1500 if not, then do vacs AFTER doing rest of service items.
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I feel it's the best way to set them correctly Bryan, and probably better than vacuum gauges as it effectively calibrates each separate carburettor with exactly the same reference tool as opposed to introducing any gauge to gauge tolerances into the carbs.
It also very neatly has the advantage of being able to synchronise the carbs without any outside influence from the other components, which allows synchronisation to be taken out of the equation in problem diagnosis, and proceed with assurance that's not part of any running issue.
A very accurate way of achieving it.
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Push cable usually has no adjuster, so just fit and leave. But that one shouldn't give you a problem unless something is misplaced on brackets.
As it has an affect when you push the top of slides it may be that there's not a parity of the slide home position from assembly. Are you familiar with Bryan's bench synchronisation method? As that should remove an error like that if present.
Thank you so much!! And thanks to you Bryan as well..
So last night I whipped the carbs off and bench synced them using a red tipped welding electrode 1.6mm (1/16) adjusted all the floats again and went through the jets with a guitar string.
Popped them back on and it’s purring like a kitten.. idle drops back down responsively and it’s sounds great!!
I have the air/fuel mix screw out 1 turn and it seems bang on!
Not had chance to get her on the open road Yet but it’s a far cry from what I was dealing with!!
Once again this forum comes to the rescue! What would I do without you guys!
If you lived closer I’d get you a pint!!
I’ll keep you posted after the run out but needless to say I am very excited!!!
I’ll be clapping for you guys on Thursday night!!!
Cheers
Alan
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Okay so went out for a ride today.. it seems a lot better but still getting the occasional hang up??
What is going on.. I’m at breaking point now.. feel like I’ve tried everything and not sure what direction to go.
The only thing I can think of is an air leak in the inlet rubbers but when I had the carbs off I inspected them all and they all looked good??
Don’t really want to start throwing money at it when I don’t know what the exact problem is?
Any more help would be appreciated if you guys/girls have any ideas??
Thanks
Alan
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Unfortunately as you well know, pods are ALWAYS a compromise when it comes to carb set up. Honda spent a lot of time on R&D of the carbs/filter set up to suit the engine and standard exhaust. You are probably about as far as it can be sorted in it's current form, incorrect filters and incorrect exhaust both play such a huge part in the overall running characteristics of the engine. There is a school of thought around using part of the standard airbox to carb rubbers to act like a bell mouth inside the pod filter. It does something to help sort the airflow through the carbs, but I have no practical experience of this, but a couple of others on here have. Finding their posts may be the next move, rather than keep throwing money at it with no real benefit.
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If it's only at idle that you're struggling to hit the sweet spot, it might be that you need to look at a set of different size pilot jets. As I'm finding with a different bike and non-standard carbs/filters, the wrong size jets for any given application will cause some odd things to happen.
In my case, I was told the jetting was spot on, but it clearly wasn't - after a short time running at idle, the bike would die. I was adjusting the air screws and it was making no significant difference. And pulling the plugs showed they were black with carbon. So what that told me was the slow/pilot/idle jets were too big. I've gone down one size and the bike doesn't die at idle, but the plugs get blacker than I'd like (but nowhere near as bad as they were) so I'll try going down one more.
As the others have said, pod filters/different exhaust are likely to change the airflow characteristics to the point where they move away from the book settings and you won't be able to iron out the idle/fueling without changing something. Fortunately, that something is likely to be pretty small in the grand scheme of things, but it might take some messing around to get to where you want to be.
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Thanks for those responses I have some 38’s that I can try so that will be my next move. I feel like it’s not a jetting problem tho!
The issue is I can be going 20-30 mph and when I change gear with the clutch the revs maintain whatever I’m revving say around 5-6000rpm, I also feel like I’m braking against a reving engine.. like having one foot on the brake and the other foot on the accelerator (car reference before someone asks why I’m using pedals lol)
Occasionally when coming to a stop the rpm also remains high when clutch is disengaged??
Alan
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Two things. Make sure the slides always slam shut.if necessary shut them by closing the throttle hard using shut cable.
Also check again for air leaks
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Sorry - my earlier post was predicated on the fact that you'd mentioned idle being a problem. Based on your last post, it sounds as though you've got issues with your throttle cables. If the revs don't drop, chances are the slides aren't closing after being held open so there's either something odd about the cables themselves, or the routing's not right.
What sort of handlebars are on the bike - are they standard or aftermarket? And if they're standard, are they UK or US spec? The UK spec were the flatter type; the US have a higher rise on them. If they're the US spec and you're using UK spec cables, it's possible one (or both) of the cables is fractionally too short. And that being the case the throttle slides will be prevented from shutting freely/completely under certain conditions. Silvers sell both types - look down this page (https://www.davidsilverspares.co.uk/CB750K2-FOUR-1972/).
If it's not that, have a look at how you've routed the cables. Any sort of sharp bends in the way they're routed can cause the inners to get hung up on the outers. Another test - if the revs rise/fall when the handlebars are turned lock-to-lock, something about the cables is not right.
I'd also add that pictures (of both the front end and the cable routing/carbs from above) will help more knowledgeable folk here help you narrow down what needs attention.
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Okay thanks for all the replies!
The bike has aftermarket clip ons with venhill Cables. I have a domino aftermarket throttle too.
The venhill cables were cut to length by myself. I have slight play at both the carb end and about 5 degrees at the throttle grip.
Routing is somewhat questionable however there are no sharp bends.
On opening and closing the throttle when looking into the open carb bodies the sliders slam shut as the return spring pulls them down.
If the sliders weren’t closing properly surely I would feel that in throttle grip as if one holds open they all must be open due to all being on the same lifter?
Thanks again I’ll try and get some pics!
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Adding some additional checks to the list;-
Check that the ignition advance retard mechanism is working and drops back to baseline as revs drop ( probably worth removing and checking for operation if not sure of condition) as advance timing will certainly promote higher idle speed.
Same as you Alan, in that it doesn't appear like jetting as that is usually consistent. Either right or wrong. But uncontrolled revs are nearly always a scource of air that you aren't able to control. With that in mind, the intake head to carb rubbers are one of the prime suspects here, even if they look ok. Plus the clamping bands have to have good integrity to both fit in their groove and clamp to eliminate 100% of any potential leaks. If they are either very hard or you've any doubt about them, then a spray around them with carb cleaner (sparingly with tank removed and outside as it's a flammable risk) will ordinarily show you.
Specifically an air leak must be between the carb slide and the combustion chamber to achieve uncontrolled throttle effects, just to add a clear problem statement.
Another "potential" scource, if all of the above posts don't effect a cure. If you can get it to "hang" while stationary, place the engine breather pipe into a clear bottle (small plastic water bottle) with some engine oil to submerge the pipe, and closely monitor the level while revving and closing the throttle, it shouldn't suck the oil level to a lower position.
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https://youtu.be/wTdmuhMUE1Y
Okay so I think I’ve found the culprit.. you were correct in that the sliders aren’t seating properly.
If I blip the throttle the Return spring and Weight pulls them closed.
If however I slowly open the throttle to say 2500 revs and let go they struggle to fully seat and remain open about 2-3mm. They do slowly go down which can be seen in the last 3 throttle pulls in the video.
What could remedy this? I believe the cables are adequately routed with no sharp bends. Could it be a fault internally inside the carbs bent needle or faulty slider.
Any recommendations would be helpful thanks guys!
Alan
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Could it be slides gumming up? Mine did when I left it for a long time. I sprayed GT85 or similar via the air box which seemed to help and use Sta-Bil in the fuel.
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Could it be slides gumming up? Mine did when I left it for a long time. I sprayed GT85 or similar via the air box which seemed to help and use Sta-Bil in the fuel.
Worth a try.. I just sprayed them with gt85 and it didn’t revolve interestingly it does NOT do it when the engines off!!!
So whatever is causing it is a result of what’s happening when the engine is running!!
Please help 😂
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Can you hear the slides metallic click as they hit bottom?
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Just another obscure random thought! Is the choke lever catching on the breather pipe. (my 500 did yours my not apply)
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At least you've now got a postive identification on the cause.
The little rods from outside to inside of the slides can get a bit "gummy" so worth a spray of lubricant there to see if it improves and helps you diagnose.
They should be clean and run dry though ultimately, along with the rubber boots to keep them clean and avoid impairment. You see many without the boot but they are important and effective.
Another is to clean the polished slides with something like turpentine substitute to degrease, and just dry them with a clean cloth, the carb venturi too along with the slide bore ( you really need slides out to properly inspect and clean this way thoroughly).
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I’m pretty confident the internals are clean and gum free, I stripped them down prior to rebuild and there was nothing untoward.. plus I’m a sucker for carb clean so they would have had a fair share of it.
Is there anything that may cause this effect when the engines running like vacuum?
Only it dosent repeat the fault when the engines off.. they seat well and quick. Only does it when the engines running?
Thanks for all your replies.. together we will sort it!!
Alan
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Ok, in that direction of "oddness" what have you ended up at when setting the idle mixture screws?
Projection to try and see if if makes a difference, turn the mixture screws IN a quarter turn from where they are, this should slow the tickover down ( makes it richer) forcing you to wind up the main tickover rpm control for the carb rack to bring it back up to normal level.
Let us know if that changes it.
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Is it possible for the return spring to bind in its shroud (if it has one )?
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Is it possible for the return spring to bind in its shroud (if it has one )?
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Great idea and on reading this I got very excited to think it might be something simple as that however as previously mentioned it dosent do the fault when engines off and if it was the shroud I’d expect it to do so both engine running and engine off??
Because it fails to do so when off I don’t believe it’s anything mechanical?
Could be wrong tho, have been many times before!
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Okay so this morning I removed the cables from the carbs ran it and still playing up, then I also removed the pod filters to see if they causing the issue.
Still defective
It seems no matter what I do when I open the throttle slowly raising the lifters only about 5mm they will NOT seat back quickly.
I have done another video which shows the slides themselves very slowly returning to where they should be.
https://youtu.be/yKIJlmy-rHc
Please have a look and see what u think?
Is there anyway of raising the sliders so they don’t go down as far as they do when they slow down by screwing the idle screw in raising the idle but then somehow reduce the idle on the air/fuel screws.
I’m running out of ideas now 😩
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Ok, in that direction of "oddness" what have you ended up at when setting the idle mixture screws?
Projection to try and see if if makes a difference, turn the mixture screws IN a quarter turn from where they are, this should slow the tickover down ( makes it richer) forcing you to wind up the main tickover rpm control for the carb rack to bring it back up to normal level.
Let us know if that changes it.
The routine I posted a couple back is trying to move it in the direction you are after, give that a try and see if you get an improvement to assess if it's on the right track.
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The only thing I can think of is an air leak in the inlet rubbers but when I had the carbs off I inspected them all and they all looked good??
Don’t really want to start throwing money at it when I don’t know what the exact problem is?
Any more help would be appreciated if you guys/girls have any ideas??
Thanks
Alan
this is what i said when i was having the initial problems with my pd42a carb setting (not quite sorted yet). even spraying carb cleaner/ether on them didn't raise the revs (but took the paint off the brace) but k2-k6 suggested i get my wallet out and i replaced the lot (i didn't realise the 'stacks' in the cleaner are tapered so act a velocity stacks) anyway at least it now runs and when, like you, i sort out the sync and pilot setting it should be all good.
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Right so carbs off.. slides out all cleaned reassembled, slides move and fall freely off the bike, there is zero friction on the sliders, return spring added and they slam shut.
They still very slowly drop when I apply a very small amount of throttle.. this is def being caused by the engine running, maybe it’s characteristics??
Please if anyone has a CB750 can they try running it apply a small amount of throttle and watch how fast the sliders seat?
Maybe I’m just going mad!
Thanks you
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i don't have that type of carb so cannot test however this video clip (first couple of revs or so) seem to indicate that the carb should snap shut
https://www.bing.com/videos/search?q=honda+cb750+k2+carb+setting&&view=detail&mid=F17208F1C8D28AAFB071F17208F1C8D28AAFB071&&FORM=VRDGAR&ru=%2Fvideos%2Fsearch%3Fq%3Dhonda%2Bcb750%2Bk2%2Bcarb%2Bsetting%26FORM%3DHDRSC3
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Mine operate the same as that and they do slam shut when opened up anymore than 10%.
It’s only occurring when I apply a very small amount of throttle.
I posted a video of it earlier in the feed and you can see where the hang up is
If I open the throttle anymore than 10% the weight and force of the spring slams them shut.
If I apply a very tiny amount of throttle that closing momentum created by spring and the weight etc isn’t present so they are closing but very slowly?
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I’m clutching at straws here but could there be a little suction pulling the needles up in their holes? If that’s not nonsense, maybe allowing more air via the pilots (1/4 turn anti each) might help.
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so your saying with the engine off they snap shut but with it running thye shut slowly if slightly activated but ok at a larger movement of the throttle. presumably with cable on and off and if you press them down either running or not they close 'correctly'. only thing i can think of if everything is running freely it could be the vacuum pressure in the venturi acting against the return spring - is the spring the correct poundage for this carb?
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I’m clutching at straws here but could there be a little suction pulling the needles up in their holes? If that’s not nonsense, maybe allowing more air via the pilots (1/4 turn anti each) might help.
I understand your logic here, however if the lifters were closing fully and the revs hand that could be a cause.
The revs hanging is being caused by the lifters and subsequent sliders are not falling quick enough
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I’m wondering if the return spring is incorrect?
Running out of ideas now
Does anyone have a spare throttle return spring I could purchase? DSS have them as discontinued??
Thanks
Alan
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part no 16123300004 ebay 401385919944 but you could try using another spring or reducing the existing one
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I’m wondering if the return spring is incorrect?
Running out of ideas now
Does anyone have a spare throttle return spring I could purchase? DSS have them as discontinued??
Thanks
Alan
Sliver do the spring Link >>> https://www.davidsilverspares.co.uk/CB750K2-FOUR-1972/part_221951/
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I’m wondering if the return spring is incorrect?
Running out of ideas now
Does anyone have a spare throttle return spring I could purchase? DSS have them as discontinued??
Thanks
Alan
Sliver do the spring Link >>> https://www.davidsilverspares.co.uk/CB750K2-FOUR-1972/part_221951/
Thanks trigg, currently showing out of stock tho 😩
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"Is there anyway of raising the sliders so they don’t go down as far as they do when they slow down by screwing the idle screw in raising the idle but then somehow reduce the idle on the air/fuel screws.
I’m running out of ideas now 😩"
Ok, in that direction of "oddness" what have you ended up at when setting the idle mixture screws?
Projection to try and see if if makes a difference, turn the mixture screws IN a quarter turn from where they are, this should slow the tickover down ( makes it richer) forcing you to wind up the main tickover rpm control for the carb rack to bring it back up to normal level.
Let us know if that changes it.
The routine I posted a couple back is trying to move it in the direction you are after, give that a try and see if you get an improvement to assess if it's on the right track.
This is what the above setting is trying to do. By turning the screws inward it should make it richer and slow the rpm, to the point where you have to raise the slides with the master tickover control screw setting, and so bring the throttle slides up a little at idle.