Sportster poor running at low RPM

[Motormed]

Hi everyone, looking for some advice please. I have a 91 Sporstster XLH1200 that's running atrociously at low revs.
Seems to run fine when accelerating and at higher revs but when the revs drop it coughs and backfires, occasionally through the carb.
I've tried different sized pilot jets and needle height settings, changed plugs and checked the timing to no avail.
Any ideas out there? It's really testing my patience now!

 

[Toddmeister]

Firstly a disclaimer - I’m a bit shit at all things mechanical so this may be completely off the mark, however…..

This just came up on my subscriber feed and I wondered if it could be relevant to your issue:

Hope it helps
Toddy

 

[Motormed]

Firstly a disclaimer - I’m a bit shit at all things mechanical so this may be completely off the mark, however…..

This just came up on my subscriber feed and I wondered if it could be relevant to your issue:

Hope it helps
Toddy

Thanks for that suggestion, Toddy.
I'll definitely investigate the VOES again. I have checked the connections on it before but not to that degree.

This all started 18 months ago, I got fed up, put the bike away under a sheet and forgot about it!
I just wished then that I'd logged everything I did because it now feels Like I'm starting from scratch again.

 

[Lord Snooty]

An air leak somewhere around the carb maybe?

 

[theoneandonly]

start at the beginning .

run engine
spray carb cleaner around carb/inlet/head port .

what happens?

 

[twizzle]

CV carb?

 

[solidstate100]

start at the beginning .

run engine
spray carb cleaner around carb/inlet/head port .

what happens?

This ^

Also a bit more info - is it standard, or has it been fucked about with (drag exhausts, air clear mods etc etc).?

Also if it was running fine before why did you go down the re-jetting route??

 

[Motormed]

CV carb?

Yes, Keihin CV40. Same one as when you had the bike .

I've already replaced the inlet rubber, but I haven't tried the carb cleaner trick yet so I'll give that a go.

As you can see, the bike's a long way from being standard!
I think the coughing and spluttering started after I fitted the current air filter, but only happened occasionally
(Usually after setting off in the morning to go to work and maybe knocking the choke off a little too early).

I was guided towards changing the pilot jet by a few of the Yank forums, bit of a rabbit hole maybe?

Thanks for all your suggestions so far .

 

[Lord Snooty]

Probably best to back off the changes you have made to the carb as you have introduced more variables into the equation. Put it back to how it was and start your process of elimination from there.

 

[twizzle]

Yes, Keihin CV40. Same one as when you had the bike .

I've already replaced the inlet rubber, but I haven't tried the carb cleaner trick yet so I'll give that a go.

As you can see, the bike's a long way from being standard!
I think the coughing and spluttering started after I fitted the current air filter, but only happened occasionally
(Usually after setting off in the morning to go to work and maybe knocking the choke off a little too early).

I was guided towards changing the pilot jet by a few of the Yank forums, bit of a rabbit hole maybe?

Thanks for all your suggestions so far .

Ha! The penny dropped after I’d posted….

Could it be stale fuel gumming it up…

 

[Motormed]

Probably best to back off the changes you have made to the carb as you have introduced more variables into the equation. Put it back to how it was and start your process of elimination from there.

I agree, back to basics, one variable at a time, apply logic and keep calm!
Just come back from the garage after step 1 which is draining the tank ready for some fresh E5 after work tomorrow.

 

[twizzle]

Looks good, like the Seat and tank combo…

 

[Deleted account 240901001]

So it started running rough when you changed the air filter…then went into storage for 18months? First thing is to take the carb off and clean it properly. Reinstall the jets and settings as when you bought it. Put the air filter it came with back on.
Modern petrol is shocking when left, especially outside (moist air).

 

[Motormed]

Looks good, like the Seat and tank combo…

Thanks, Twizzle.
It's over 4 years since I bought it from you. It was delivered a week before we went into the first Covid lockdown!
Gave me time to tinker with it......a lot.

And Doris, I've spent a lot of time researching today, learn a lot.
I just read that when I fitted that air filter I should have fitted one of these to support it and the carb!
That should hold the whole lot together and form a better seal from the carb hopefully.
Another thing on the shopping list then.

 

[theoneandonly]

Thanks, Twizzle.
It's over 4 years since I bought it from you. It was delivered a week before we went into the first Covid lockdown!
Gave me time to tinker with it......a lot.

so it's Twizzles fault then !

 

[twizzle]

so it's Twizzles fault then !

Most things are…….

 

[Motormed]

so it's Twizzles fault then !

I'm not saying anything

 

[twizzle]

I'm not saying anything

Pop my air filter back on…………and all will be well

 

[Motormed]

Just a little update on the situation....
The carb support bracket arrived, before fitting it I thought I'd remove the carb just to check things over (again).
Two things I found - float height was a touch high, more concerning however was the jet needle.
The number on it is N72A and from my research it should be an N65C.
I've just ordered one, hopefully it'll arrive in time for me to sort everything out before the MOT which is booked for Friday.

 

[The Other PaulG]

It's always good when you find something that appears to be a likely cause. I hope that sorts it.