Starting Problems Any Ideas?

[MartinL]

I've just built up a half decent 1981 R65 out of (nearly) two bikes that had languished in a mates garage for the last 15 years. I've got it going but I have a cold starting problem that's driving me mad. I won't go over all the things I've tried but here is a summary:

Problem:
Bike won't start from cold regardless of choke position / throttle opening. You can crank it till the battery is flat and it won't even try to fire. A squirt of petrol down the plug holes or a hand placed over the carb mouths gets it to fire and once warmed up it runs OK, pulls well for a '65 and is very smooth. It will restart from hot. Continuous cold cranking even with full choke does not result in flooding or even slightly wet plugs which leads me to suspect the carbs?

Basics:
I've had the top end apart and the motor is in rude mechanical health. Valve clearances good and 10.5 bar each side on the compression tester.
Filters, plugs etc. all new and alternative plugs have also been tried.
Ignition timing is spot on and the auto-advance functions as it should. I've tried different trigger units from my stash with no effect.
Carbs have been scrupulously cleaned several times, rebuilt with new gaskets and O-rings and have been tried with both new and old diaphragms.
Fuel tank is clean with rebuilt tap and fresh petrol.

Starter carbs:
In frustration today I popped on a pair of old carbs from another bike even though they were filthy and the slides a bit sticky the bike fired up pretty much straight away. I took (another!) close look at the "starter" carbs on my original set of carbs and although they seemed to be correct (spindles are stamped L & R to prevent mix ups) they are clean and don't leak air past the O-rings. I decided to swap the starter carb assemblies from left to right putting them apparently in the wrong place but when I refitted the carbs the bloody thing fired from cold on full choke virtually straight away. The rotary slides in the starter carbs have a tear drop shaped hole opposite a row of small graduated holes. When assembled "correctly" the tear drop hole meters the air and the small graduated holes meter the fuel drawn up from the float bowl but the bike won't start. When I put them on (apparently) arse about face the bike starts. Any ideas...????

 

[Deleted Member MS]

Heres a daft one. Are you sure the actual choke cables are coming on ok, not just at the lever, but actually at the carb body itself ?????

 

[MartinL]

Believe me mate after going through everything for the third or fourth time there are no daft suggestions but the cables are fine and the chokes are moving through their full range OK.

I am a pretty experienced mechanic but I'm the first to admit I could well be missing something obvious but I'm buggered if I can see what. I thought airheads were simple??

All suggestions daft or otherwise gratefully received......

 

[John Armstrong]

Any usefull info here: http://bmwmotorcycletech.info/techindex.htm

 

[Deleted Member MS]

Mmm, perhaps arse about face is correct after all. Maybe previous owner mixed em up ??????????????????
Frustrating isnt it !!

Have you tried looking at some exploded drawings etc !!!

 

[MartinL]

Thats what I'm leaning towards or at least I would be if the parts in the old carbs that "worked" did not look the same way round! I'm just about to surf t'interweb for some info as my old Haynes manual is typically lacking in the detail you want.

It would be handy if someone could explain exactly how the starter carb works. I think I know but that's not the same thing of course. The starter carb "body" is connected to atmosphere via a drilling in the carb mouth that aligns with the projection on the body. I'm wondering that if the internal parts are assembled incorrectly if the "choke" drilling downstream of the butterfly ends up open to atmosphere? This might explain why no end of cranking will create enough depression to draw fuel from the float bowl. Sticking my hand over the carb mouth causes a high vacuum which then sucks fuel up and lets the thing fire. I should also have mentioned that when it's warmed up and ticking over operating the choke causes it to cut out instantly.

 

[MartinL]

Any usefull info here: http://bmwmotorcycletech.info/techindex.htm

Good link John. It certainly looks like the discs in my starter carbs have been put in the wrong way around. Curiously it looks like the "old" carbs are wrong too so maybe they only worked because they are badly gummed up.

I'll have a go at sorting it out tomorrow night and report back.

Cheers...

 

[Rob Farmer]

Martin,

I had a similar problem with a 100/7. It turned out to be the cork gaskets had gone oval round the choke tube in the float bowl. This prevented the fuel being sucked through the choke. Took me ages to track it down.

 

[John Armstrong]

Martin,

I had a similar problem with a 100/7. It turned out to be the cork gaskets had gone oval round the choke tube in the float bowl. This prevented the fuel being sucked through the choke. Took me ages to track it down.

Could this cause an 88 R100GS only to start with a pump of the throttle when cold, but is happy when warm?

 

[Rob Farmer]

Possibly. For the price of a couple of gaskets it's got to be worth a go.

 

[John Armstrong]

Possibly. For the price of a couple of gaskets it's got to be worth a go.

Happened to get a pair as I suspected, correctly that the float level was set a bit low. Thanks

 

[MartinL]

Update

Well I checked out the build of the starter carbs this evening and (unfortunately?) they are assembled correctly as confirmed by t'interweb, the proper BMW manual I'd forgotten I had and checking the same on the old set of carbs I've got. The easiest way I can describe the alignment of the rotary slide is that the row of holes should align with the bottom passage in the carb body that feeds fuel up from the float bowl and the tear drop shaped orifice should align with the (upper) passage leading downstream of the butterfly.

I did find a couple of things wrong though: The dip tubes that draw petrol up to the choke chamber have a small hole in the side part way up that was blocked on both carbs but not on the "old" set. The float heights were also too low so I tweaked them too.

On re-assembly the bike initially refused to start on any choke setting or throttle opening. Leaving it for a minute I tried again with no choke and it fired up. Half choke picked up the revs but full choke made it way over rich and "choked " it to a stop. (Ambient air temp about 14deg C). Anyhow I set tickover, balanced and synchronised the carbs and went for a run around the mountain. It ran fine other than the idle speed ending up too high.

The bike was running well at speed but developed a quite noticeable rumble from the front of the motor when pulling hard. Stopping at a junction I noticed lots of smoke in the headlight beam. Back in the garage it was evident that there was a major oil leak from the timing cover joint spraying onto the exhaust. The timing cover proved to be loose explaining the rumble from the front end! I could kick myself, these must be the only fasteners I have not checked for security when going over the bike. They were certainly nipped up as there were no leaks or untoward noises initially but obviously they had been slacked at some time during the lay up. Unfortunately the gasket must have blown as tightening the cover has not cured the leak. Crankcase pressure is sending out a good spray from the bottom corner so it will have to come off.

I not convinced I've resolved the starting issue either. If it doesn't start from stone cold tomorrow then I've failed. The only thing I can think to do then is to clean up the old flat top carbs I have and give them ago. Although they are covered in crud they look better internally than the newer units on the bike.

Watch this space...

 

[ianboydsnr]

Did it start from cold then and how about some pics

 

[John Armstrong]

Could this cause an 88 R100GS only to start with a pump of the throttle when cold, but is happy when warm?

Had more of a play before trying to strip the bike into 1001 bits, and as long as I don't try and fully open the choke / cold start lever, it fires into life without a problem, and idles happily away Could it be over choking ie far too much fuel, which is why a touch of throttle gets it going?

 

[Kenny]

Could be an oversized needle/jet set-up ( I'm no carb expert mind )...if I was you...I'd enjoy the bike for now, regroup and attack it in the springtime

 

[John Armstrong]

Could be an oversized needle/jet set-up ( I'm no carb expert mind )...if I was you...I'd enjoy the bike for now, regroup and attack it in the springtime

But it'll easily start when warm without any choke and seems to do about 45mpg.

That's not holding back on motorways etc Probably very wise words of if it's working don't mess with it.

 

[MartinL]

Easy Fix!!

Thanks for the suggestions guys but it turns out the problem was ignition not carbs at all. I eventually decided the problem had to be common to both pots and went for the coil as the obvious even though it was visibly sparking with the plugs out. Motorworks wanted 45 quid for a used coil so I bought a pair off a K100 on Fleabay for a fiver. A bit off hacksawery to the brackets had one of the pair fitted and bugger me if it did not start first touch of the button. It was that eager it didn't even spin over, just hit compression, fired and ran.

If there's a lesson to be learned here it's to have a bit of faith in your own work and once something is checked and maybe double checked move on because doing the same things and expecting a different result gets you nowhere

 

[Deleted Member MS]

I bet you feel ssssssssooooooooooooooooooo much better now its ok eh ?

 

[John Armstrong]

Could be an oversized needle/jet set-up ( I'm no carb expert mind )...if I was you...I'd enjoy the bike for now, regroup and attack it in the springtime

But it'll easily start when warm without any choke and seems to do about 45mpg.

That's not holding back on motorways etc Probably very wise words of if it's working don't mess with it.

Wise words, who listens to them

I wanted to get the starting 100% or if not there then at least 99%. Have had the carbs off a couple of times now. Used carb cleaner to blast through the jets etc and around the "choke". Thought I'd made sure the choke orrifices in the rotating bit and choke were clear by using the carb cleaner. Put the chokes back on, with new gaskets on the choke and float bowl. Exactly the same. Only starts on full choke with some assistance with the throttle.

Looked into getting some carbs to try and eliminate them, but the guru said check the choke orrifices. So, off with both carbs and noticed I couldn't get some fishing line all the way through the smallest hole (1/2 choke?). So decided to completely dismantle the choke. Once in pieces noted that there was a plate on the rear of the bit you can see which is why the fishing line wouldn't go through So how does the air get past it? Messed about and tried to prise the two bits apart and they're sprung loaded in the closed postion. Put it all back together, and it stays sprung. Unlike before, when it was solid.

Checked the right-hand side, sprung loaded when I took it off. Built the bike back into one piece, and now it starts on full choke with no throttle Admitedly it's not an immediate fire up, but at least it does start now without any throttle.

The bit that is sprung is item #47 in the Bing manual page 25. Or item 11 on the diagram below. Obvious when you look at it and know what happens.

Thanks Rob for your help and kind offer