PD not idling when cold

Mark_S

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Hi All,
My PD won't idle when cold. It starts OK, but needs a few mins on the throttle @2k before it will idle. If it is allowed to idle when cold with no throttle the revs drop and eventually a carb will be popped off.
Do they 'all do that sir'?

Timing, valves, carb balance all OK.

Can anyone help, waiting on 2k throttle before riding off, and anticipating a popped carb is getting tedious.

All useful help gratefully revived.

Cheers
Mark
 
Carbs popping off definitely doesn't sound right. Probably worth checking valves and timing and compression, you say valves OK BUT how do you know that?
 
Sounds like a carb clean is needed, or maybe an air leak between carb & head that heals once hot.

TBH I am shooting in the dark. Why not talk to Mikeyboy of this parish?
 
Has this problem just started happening ? was it running ok previously when cold ?. How long have you owned the bike ?

You make no mention of using the choke ? You do know that it has a choke ? You do know airheads need a lot of choke when cold, and take a while to warm up ? Does using the choke make a difference ?

I do love a technical question that's woefully short of information and takes many many replies before we can even get to a starting point.
 
Mark_S said:
Hi All,
My PD won't idle when cold. It starts OK, but needs a few mins on the throttle @2k before it will idle. If it is allowed to idle when cold with no throttle the revs drop and eventually a carb will be popped off.
Do they 'all do that sir'?

Timing, valves, carb balance all OK.

Can anyone help, waiting on 2k throttle before riding off, and anticipating a popped carb is getting tedious.

All useful help gratefully revived.

Cheers
Mark
Click to expand...


This sounds like normal airheadness to me. I start mine with choke when cold, and if I'm in a hurry, move choke to half, then ride up the road a few hundred yards, then, on to the main road, choke off then continue on my ride. Bike won't fully warm up for a mile or so.
 
Thanks for the helpful replies guys, and apologies for the lack of minute detail. I was originally asking if anyone had carbs 'popping off'.
I guess that this doesn't happen much, it's just mine then?

OK I'll try to provide more detail, in the winter I need full choke to start the bike, once it is started I select 1/2 choke but still have to have the throttle open so the engine is running at approx 2 thousand RPM.
If I close the throttle the revs decrease (so, guess the choke is not working well) if the throttle is not opened again the revs keep decreasing and, you guessed it, there is a pop (I think somehow, the revs drop so low that there is ignition between the carb - happens to both - and inlet to the engine, no idea how that happens) and off comes a carb.
When warm / hot the engine idles OK.

During warmer weather, I can start the engine with no choke (the carbs prolly don't work anyway), but there is still no idle until warm.

The ignition timing is set correctly
The gap for the inlet & exhaust valves is set correctly,
And the compression readings were L130 and R120 - so I guess the valves were OK. (Someone from this parish said 10 difference is ~ok)

The bike has always behaved like this - I didn't think how long I owned would be a factor!
The speedo shows 75k miles, I have been told the engine has had a complete and professional rebuild 10 or so ago so, mileage may not be a huge factor.

I am thinking that the carbs can do with a clean. Any other help is greatfilly received. Cheers
Mark
 
Mark, sorry to sound daft, but -popping*- do you mean that the carb gets blown off the inlet pipe physically? Becomes detached? If that's the case I've never heard of that. If it's not the case then forgive the following...

THe carb inlet joint seem not to be done up tight enough. Excess petrol could be pooling in the inlet tract and then it blows when that part heats up enough and an explosive atmosphere is reached. You say all your valves and timing are correct but all you say about your carbs is that they need a clean. None of us can do that for you - get them off and clean them! Make sure all your jets are clean (and the correct size) and replace worn ones. Check your diaphragms. Float levels. Set your chokes. Check your cables. Most of it is covered in the Haynes manual.

*'Popping' can also (usually) mean the sound you get on the overrun when your mixture is a little rich.
 
Hi Gelandestrasse (cool name BTW), thanks for the reply. yes the / a carb will get blown off the inlet pipe when 'trying' idle when cold. I'm surprised that not many of the collective have heard of this. The bike has always done it, when i bought it i was told that 'there will be no tickover when cold', being new to airhead ownership, i accepted this.

Do air heads tickover when cold, probably with some choke?

i have had the carbs apart and all appears to be clean / seals are good etc. Thinking a bit harder about the problem, i can understand that there may be a leak between the carbs and engine that seals up when at operating temp. I will focus on the seal between the carbs and engine.

cheers
Mark
 
Hi Mark, my name was tough to live with through school; I never forgave my parents. But now it's cool so that's good!

My '73 R90/6 on Bings only starts on choke when it is cold, and ticks over fine on choke until it starts to warm up (30 secs) when I ease it off full choke onto a mid-way point. By this time I am normally riding so it goes fully off. But if not, after another 30 secs I back it off more and after a couple of mins it will tick over without choke, albeit 700-800 rpm and like a Swiss clock. After a run and al is hot, the tickover is a little higher but I can forgive her that.

Your PD (and my Paralever GS) should do exactly the same. My GS is not set up, but ticks over when warm and is reasonably behaved on choke (but no way perfect). If she spat off a carb I'd be highly amused and then a bit worried! I've yet to work on the carbs, but I am expecting to strip, clean and check the chokes at some point and also balance her on the vacuum gauges. Also, I'll check the cables. That always does the trick with the airheads I've owned (but I'm not saying I'm an expert - there are plenty available on this site. If it doesn't go like the Haynes manual says, it's normally because someone has messed with it at some point or completely failed to maintain it. They are a basically sound design. Aeroplanes use this engine/carb combo ffs!
 
Assuming the valves clearances are to spec, it seems like she is running lean on one cylinder when cold, suggesting the choke isn't enriching the mixture one side, or you have an air leak or a blocked jet ...........or all 3.

Given the age of the inlet connectors/grommets (#3 in the carb diagram) these could be where the problem lies ..... https://www.realoem.com/bmw/enUS/partgrp?id=0474-EUR-03-1989-47E1-BMW-R_100_GS_PD&mg=13
A spit-back should not be blowing the carb off for sure.

Otherwise, I would suggest the carbs are both stripped down & given a nice ultrasonic bath & the tank cleaned out & filters checked. Modern E5 fuels don't help, so best use a stabiliser if the bike stands unused for long periods.
 
Kenny said:
Seriously, get the carbs rebuilt by Steptoe or Mikeyboy etc. Also fit new carb mounting rubbers, they do go hard.
Click to expand...

And don’t overtighten the inlet clamps thinking it’ll help stop airleaks, it does the opposite..... I’ve also seen many examples of rebuilt carbs with the choke body put in 180 degrees out :D
 
Thanks for the information all,
The RHS inlet clamp was finger tight, i tightened it up, checked the LHS = OK. Started OK but ran like a bag of spanners (probably due to the different orientation of carbs?), and on a few occasions the revs dropped right off but there was no 'Popped carbs'.
:thumb
I fettled the throttle cable sheaths took it for a run when i got back I put the emergency manometer on (tube + Automatic Transmission Fluid) on and had to give the RHS a slight tweak.
seems to have worked, even the idle was good.

i await the next cold start

cheers
Mark
 
Mark,

Is that the PD you had off me?

That compression ratio is a bit worrying. Bottom end was built by moorespeed and the barrels were all replated by moorespeed at the same time, new piston rings were fitted. All this a short while before you bought it. Carbs had been ultrasonically cleaned and all gaskets were genuine BMW. Inlet rubbers were all new and were genuine BMW. Damned if i can remember what was done to the heads. If they haven't got morespeed Colisbro valves and guides then the heads were refurbished by scriminger. That 10PSI difference is huge so i suspect you may have a valve problem. The spitting through the carb is unusual too and is suggesting there is a valve issue.


Looking through the rebuild post (cant believe it was 2016!) no mention of the heads so they were rebuilt by scriminger at some time in the past. Cant remember when though. Is there anything in the paperwork?

https://www.ukgser.com/forums/showthread.php/427882-Time-for-a-100PD-rebuild
 
hi Rob, thats the fella,
have been busy hence the delay, havent bad time took thorough the paperwork so bavent confirmed what happened. bit concerned about the valves so i did another test, 5psi diff now.
not sure the problem lies there, but happy to be corrected.
i have ordered gaskets and o rings for the chokes, see if i can get them working.

sparkplugs are a bit black so prolly running a bit rich, did you rejet the carbs during the rebuild?

cheers
Mark
 
Mark_S said:
hi Rob, thats the fella,
have been busy hence the delay, havent bad time took thorough the paperwork so bavent confirmed what happened. bit concerned about the valves so i did another test, 5psi diff now.
not sure the problem lies there, but happy to be corrected.
i have ordered gaskets and o rings for the chokes, see if i can get them working.

sparkplugs are a bit black so prolly running a bit rich, did you rejet the carbs during the rebuild?

cheers
Mark
Click to expand...

mark,

I put everything back to standard as best I could including the jetting. I had a problem years ago with aftermarket air filters swell in up and reducing airflow. Bit of a long shot but you never know. I also had the o rings that sit under the carb slides blow up that made things rich just off idle. New o rings sorted that. I think the boring material reacted with the new fuels.
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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