PD running rough at low rpm

[Shurv]

I've borrowed a 100PD from a mate to do my Scotland trip next week as my Moorespeed/ Dakar rep isn't finished yet.It runs roughly under 3k rpm but fine otherwise.I've blown through the jets and cleaned out the carbs and all seems well. I've adjusted the rattly bits and balanced the carbs and checked the airfilter which is new.The plugs have been changed but not made a difference, and the old plugs were black and sooty. Any ideas as to what is acusing the rough running ?. I was wondering if the choke was staying on a bit but both plugs were as black as each other, so if that is it then they are both out.

 

[Ogmios]

... I was wondering if the choke was staying on a bit but both plugs were as black as each other, so if that is it then they are both out.

does sound like a choke problem. stating the obvious i know, but - the lever/cables not sticking on is it?
i had similar problems and it was one choke gasket wasn't sealing properly.
i think i sussed this by spraying WD40 around the carbs until the revs changed (the WD found the leak) and that was how i found it.
fixed with a bit of cornflakes box.

dunno if this helps, but worth a go. Good Luck

 

[Rob Farmer]

Drop the needles one notch.

You didn't mention if it had a y piece or original collector.

 

[Shurv]

Rob, it has a collector box.It points to the chokes or just running too rich so when it needs more fuel it's getting it, so ok, but when it dosn't need it, it's getting it anyway so runs rough. The plugs aren't wet, just black and soooty.

 

[Devon]

Drop the needles one notch.

You didn't mention if it had a y piece or original collector.

Does a Y piece affect the carbs much? I put a Y piece on my Basic and apart from the engine running a bit lumpier it doesn't seem to make much difference.

 

[MickDB1]

Shitty air filter Marcus ?

 

[Shurv]

Hi Mick, you ok? Your old bike's nearly finished, you won't recognise it!!!
The air filter is fine. The bike has been sat unused for 12 months , so poss a bit gummed up. It has been run to the NW200 and back last week where the low speed running issue cropped up.

 

[Rob Farmer]

Does a Y piece affect the carbs much? I put a Y piece on my Basic and apart from the engine running a bit lumpier it doesn't seem to make much difference.

That lumpier running is usually because the engines running too rich. I had the same thing with my 100PD with a Y piece fitted. Put it on the dyno and it was obvious that the Y piece (A keihin) was cocking up the jetting. They are more of a T piece than a Y so the gases interrupt the flow of the exhaust gases instead of helping them. Dropping the needle actually increased the midrange power because it bought the jetting in line.

If you get a chance take your bike to a local dyno (around £30) worth every penny and will pay for it's self in improved fuel economy, it's also quite an eye opener. You'll be amazed how badly the 40mm bings actually work. The 32 carb bikes work a treat with the standard airbox but with the 40's the airbox just doesn't flow enough air.

 

[Shurv]

Tempted to try it with the filter removed and see what that does.It's definately chucking in too much fuel.

 

[Rob Farmer]

I've had a couple of pattern air filters swell up so much they only left the tops of the filter passing air. I always use genuine filters since then. No problem since.

 

[Devon]

That lumpier running is usually because the engines running too rich. I had the same thing with my 100PD with a Y piece fitted. Put it on the dyno and it was obvious that the Y piece (A keihin) was cocking up the jetting. They are more of a T piece than a Y so the gases interrupt the flow of the exhaust gases instead of helping them. Dropping the needle actually increased the midrange power because it bought the jetting in line.

If you get a chance take your bike to a local dyno (around £30) worth every penny and will pay for it's self in improved fuel economy, it's also quite an eye opener. You'll be amazed how badly the 40mm bings actually work. The 32 carb bikes work a treat with the standard airbox but with the 40's the airbox just doesn't flow enough air.

Thank's for that Rob, I will check it out. I tried to buy a new balancer box but they are no longer in production so I fitted the BMW Y piece which I presumed was a straight replacement. I had the original balancer box cleaned up but have not refitted it as it seems a shame to get it dirty now it looks so good

 

[Beemerboff]

If it has been standing a year the carbs carb could be gummed up somewhere - stick a bottle of Fuel injection cleaner in a full tank , should be all clear by the next fill up.

And try without the filter - as Rob says there were a few around that couldnt cope with a bit damp air and just stopped flowing properly - I went to foam to avoid them BMW boxer Supplies have them.

Timing may be a tad advanced too--.