SOHC.co.uk Forums > CB750
Hanging idle please help
the-chauffeur:
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.
Wenman:
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
Johnwebley:
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
Sent from my SM-A750FN using Tapatalk
the-chauffeur:
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.
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.
Wenman:
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!
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version