R26 won't start

[BSA Bob]

My R26 has become almost impossible to start. If I'm lucky the engine fires on the first kick but doesn't catch & can't be started after that.
I've checked the wiring connections. Fitted new points & condenser. Rigged up another coil which works. Tried another advance unit.
There's a spark, sometimes 2 in quick succession.
Timing & gaps are correct. Tried a few plugs.
The carburettor is in good overall condition. I've fitted new jet block & needle, 125 or 120 main jet.
Float needle doesn't let the carburettor flood.
Another carburettor in similar condition was tried.
Valve clearances are OK.
If the motor eventually starts it seems to run well.
Before I start going over it all again, does anyone have any suggestions as to what I should try?

[BSA Bob]

[Eppo]

Small question:
Did you check the valve timing for intake and outtake valve?

It could be that the chain has sprung one tooth.

[RTwenty7]

Good Morning Rob,

check your Ignition lock and it´s wirings or/and the Key, does it stay in place while kicking?

[patro]

well, there is fuel and a spark. How about air and compression? You could try to dismount the airfilter. What happens then?

Gruß, Ralf.

[Longo]

When starting a cold engine, you need to tap the carburettor until the seal is wet and then start the engine with the throttle closed.
If the engine is warm, do not tap the carburettor and start with half-throttle.

[4Taktix]

Hi Bob,
as you already checked and exchanged the usual suspects without any noticeable improvement,
there were two possible issues coming into my mind:
1. a loose valve-seat
  ( if sticking out during clearance check and adjustment, it woun't match any longer, when hammered back into place
    during the first few revs )
2. a sticking valve-stem (Valve doesn't seal correctly, maybe occasionally)

I'd check that with an Adapter for the plug-hole and pressurized air.
Or else, remove the valve-covers and check by hand during several start-attemps, if there is suddenly way to much clearance on
either rocker, or maybe absolutely zero tactile clearance.
Depending on the outcome I'd encircle the root-cause.

[BSA Bob]

Hi folks, thanks for the thoughts.
Unfortunately for me, 4Taktix, You're right that it's a cylinder head problem. Valve clearances stayed the same as I kicked it over a few times, & compression always felt the same. But when feeding air in there's a hiss from the top of the head around the valves. I can't isolate it & I should be able to take the head off tomorrow.

[felixcurly]

Just because I am curious: a compression test would leas to the same conclusion as the test with pressurized air I suppose?

Felix

[BSA Bob]

Hi felix
Feeding air into the chamber is an el cheapo leakdown test. All it does is give an indication that gases are escaping where they shouldn't. A proper leakdown test involves gauges to measure pressures more accurately.

[4Taktix]

At least you could probably determine, where to proceed with detailed investigations.
Use a Stethoskope or a big Screwdriver instead, tip touching exhaust or carb, your ear to the rear end
and listen. This way you could encircle, if the hissing leakage comes from intake or exhaust, or elsewhere.
Maybe you got a loose plug-thread-insert or broken pistonrings.
In case the latter is true, you can probably hear the hiss through the oil-fill-hole, dipstick removed of course.
Now you can call me smartass   

Regards,
Sascha

[BSA Bob]

There's been a lot of activity but the problem remains. I removed the head. put some petrol in the chamber & the inlet valve was leaking. A friend with the appropriate tools took the valve out. The valve guide is a special, made by the cylinder head shop previously as the outside diameter was larger than BMW makes. It's positioned by a circlip & it looked like the guide had moved, preventing the valve from closing. The valve face tested in good condition. So a bit was lopped off the length of the guide, the head reassembled & fitted. And it still won't start & there's still that hiss.
So the head comes off for further investigation.

[RTwenty7]

Tried another advance unit.

Is that assembled in correct position? It´s possible to put it 180 degreese incorrect position and then nothing starts

[4Taktix]

good point. If somebody e.g. replaced the little springs and assembled the two main parts twisted 180°,
you will have a spark at bottom dead center ! This leads to wild backfires every few kicks.
(but it woun't explain the hiss ... )
I'm curious: Have you got some pictures of the "DIY-Guides" and assembly ?
What about the tappets ? New or used and redone ?

[BSA Bob]

Thanks for these latest thoughts, but I don't think they apply to my case. The alternate advance unit is functional & was properly keyed to the crankshaft extension. Rockers, tappets etc are in good condition.
I took the head off, put petrol in the chamber & the valves are sealing. I filled the chamber & after about an hour the depth had dropped a couple of mm.
As I said in my first post, if the bike does eventually start (after a long time & many, many kicks) it runs well. I should've mentioned that it tends to stall almost immediately but if I can get it past that it runs well.
I can't quite get out of my mind the thought that it's a fuel delivery problem. I've read the in these carburettors that were fitted to R26/7s it's possible for a piece holding the atomiser to move, altering the settings. I don't know what to look for about this, though.
Further thoughts are welcome because I haven't got a bloody clue. I reckon I'm missing something small but no idea what. I'm going to bed now, I'm sure I'll dream about the thing.

[BSA Bob]

I should have added here that, even after many kicks using the throttle, there doesn't seem to be a lot of fuel in the combustion chamber, the plug isn't wet. But the carb's been cleaned & cleaned &... Good fuel supply from the tank.

[4Taktix]

OK, it would help to know, which exact type carb you have.
But in general it sound like a potential problem with the float & Needle-valve.
Check the blind hole in the float-chamber lid for clogs and burrs.
The needle topend has to move freely in the blind hole, so the float can go up and down easy.
Another potential problem could be an aftermarket float. Sometimes these have a burred joint/seam
and tend to stick to the chamber walls.
Or else you could simply have the wrong float, there were some 11 grams and some 7 grams.
If we knew your exact type carb, we could say, which float weight is correct.
If your R26 really lacks fuel, I'd say the float is rather too light.
Or the float needle isn't correct or bent or manipulated ( manually added notches, Wire clip not latched proper )
I'd first try and dab longer/more.
Which idle-jet-size is installed ? you could try one size bigger. Maybe 40.

Good luck !
Sascha

[BSA Bob]

Hi Sascha.
The regular carb is a 1/26/93 & the other I've tried is a 1/26/55. Both have new jets, atomisers & needles. The slides are OK. They have their correct floats & the float needles seal well. The 1/26/93 has the accelerator pump removed. Until this problem it was running well.
I tried a loaned head today with exactly the same results. Almost impossible to start, when it finally did it ran OK for a minute & then cut out, wouldn't restart. For the test I re used the head gasket I'd put on only a couple of days ago. Initially it had little compression, I squirted a bit of oil into the cylinder which improved things. I suppose this sounds a bit like a rings issue, but compression has been OK until I re used the head gasket. I checked the compression with a warm motor & it was 100psi- nearly 7bar.
I had an embarrassing Eureka moment about the air hissing from the head. Air fed into the combustion chamber seeps past the rings on a cold motor & vents through the pushrod tubes, creating this sound. The loaned head did exactly the same.

[4Taktix]

I'm a bit puzzled.
So eventually the main culprit were the pistonrings, this is what I understand so far.
Or piston/barrel were worn out. Thus there's no change with anothr head.
There is no other way for air/gas from the combustion-chamber to the pushrod-tubes,
unless one or both valves weren't closed completely during your test and the valve-guides were also worn out.
But I doubt that this applies for both heads.
Also the breather has to be in a closed position to force gas through the pushrod-tubes.
I feel like we overlook something - so far the story isn't really plausible in total.
Most awkward is the fact, that she runs a minute or so, if she ever starts, then starves and woun't start again.
I think the hissing is a red herring ...
I'd focus on ignition and fuel supply.
Did you check, if she has sparks after running a minute and not willing to fire ?
Petcock free both positions ? Some debris in the tank ?  'old' fuel ? Condensed water ?
You could try fuel supply from an external tank ( coke-bottle or whatever ) to exclude theses things in one go.
No more ideas up to now.
(apart a forgotten rag or a small rodent in the exhaust ... )

[Einzylindär]

What about the ignition coil? How do you know the 2nd coil is ok?
If the coil gets warm after some time a short circuit appears due to bad isolation.
This could be after a minute or after more.
I remember my SR500...
I was a CDI ignition system, but with a faulty coil it ran cold 800m around the block, then misfiting occur and the enginge stops. After some time the coil cools down and you could repeat it...

So try another coil or check spark length in cold and warm condition. You can heat the coil with an air blower to 80°C.
Another idea is the spark plug connector. Is it with resistor or without?

Other idea try some breake cleaner or other burnable fuel. If the motor starts better, than it is a carburetoe problem...

best regards and good luck
Stefan, the one cylindrist

[BSA Bob]

Thanks for the latest thoughts.
First the piston rings. This is strange. When I fitted the test head I re used the head gasket I'd put on only a few days before. When there was little compression I thought the gasket wasn't sealing, but a bit of oil seemed to get the compression back up. But there was reasonable compression before.
I haven't checked the plug immediately after the engine had briefly run. It's not a fuel supply issue, I've used both the proper tank & a slave tank to get it running briefly.
I'll be getting a new coil soon & will try that. My new friend AI says that even a nice blue looking spark can break down if the coil's faulty so we'll see what happens then.
I should mention the coils I'm using. The one on the bike was NOS fitted when the motor was rebuilt a couple of years ago. The second is an automotive type that years ago was fitted as for an R27. That's what I'll be getting now.

[4Taktix]

What about the spark-plug-connector ? This is a common case for troubles.
Either loose or oxidized wire-connection or also common is an internal resistor.
Mostly stamped or embossed on the connector.
Also there were sparkplugs with an internal resistor. BOSCH-Plugs e.g. do have an 'R' stamped/engraved.
No problem for healthy ignition components, the engine will run despite this.
But if these do have a weak spot somewhere (corrosion, almost broken wires, loose connections, things like that) it can mean run or not.
Very small, almost invisible from outside, but crucial for a good spark:
The tiny little isolator washers for the blade spring of the points.
This spring must not have ground/earth-contact by no means ! -> measure
In the past I had aftermarket points - despite delivered in a yellow and red "Bosch" box, they were made in Korea and were crap.
Points were offset and hit at an angle - could be dressed somhow with pliers.
But worst was the pinched isolator bushing - the breaker lever sporadically stuck in open points position ....

[BSA Bob]

Thanks again Sascha.
The plug cap is new & of good quality, no resistance. A couple of new plugs of the right grade have been tried. Today I put in a new top hat insulator at the points spring mounting & checked the other insulators. I put the proper cylinder head on today & went over the electrics. I was going to buy a new coil but on checking the high tension lead it gives a 4mm spark to earth which should indicate an OK coil. I'll try starting again tomorrow using fresh fuel but what I've been using should be OK. The coil's done about 12 000km.

I like your story about the pinched bushing. My new points are a very snug fit on their spindle when the small lock washer's fitted but fortunately they don't bind.

[Einzylindär]

4mm spark length is poor in my mind...
I remember something 7..10mm as good.

What about power supply of the coil?
I remember a bad connection on the connection plate of the alternator on Ruedigers R26 @NordmannR26 on our Edersee meeting in summer 2024.

To check this put a cable from battery plus directly to the coil and try to start or check spark length first with this setup.
Please check also the ground connesction from engine/gearbox to battery minus. In doubt use a cable from an upper engine bolt to battery minus.

best regards
Stefan, the one cylindrist

[Eppo]

4mm Spark between cable end and ground at atmosphere is not a good value.
At atmosphere it should be like Stefan mentioned around 10mm according to what type of coil you use. Internal coils have a shorter spark than a good external coil.

On the ignition test stand (Prüfrex) good external coils give up to 12mm when cold and still 8-10mm when warm.

My experience is, that most of the cheap new coils are not worth the money, cause these are not nearly as good as the original ones.
(during a test, I brought a new 6V labeled external coil which only get a spark length like a 12V coil when powered with 6V, so maybe the 6V labeled coil might be better used at 12V)

But there are some shops on the market that sell good ones.

Kind regards

[Taz]

Hello Bob.

From own and painful experience...have you considered the insulation washers at the contact breaker spring? These little bastards have cost me half the 2026 season and innumerable nervous breakdowns...

Fingers crossed, Ute

[BSA Bob]

I'm fairly confident I've got the bike fixed though I'm not really sure how.
Firstly I checked the coils properly. Better than I thought, the internal coil fitted to the bike was tested with a spacer to keep the lead from the bike & threw an 8mm spark. The spare external coil was tested by a professional & also threw an 8mm spark.
But it didn't improve things & nor did improving the insulating arrangement at the points. I finally noticed that the bike started better with a slave tank & the float bowl cover off. I cleaned the cover (again), shortened & rerouted the fuel line to the carburettor & it's starting again. All I can think of is that there was an air lock in the fuel line even though that arrangement has run OK til now & has been dismantled & put back together several times during all this.