71 R75/5 dies when warmed up

[stevenrankin]

Warmed up

One thing I have learned over the years is warming up an airhead to adjust carbs means more than a ten minute ride. ANY adjustments I make on my two bikes are done after at least a 50 mile ride. St.

 

[cvhd94]

Whelp, new needle jet and atomizer day was today, and no improvement. I also installed new BP6ES plugs, and have non resistor wires on the way. But not sure where to go from here. I have rechecked valve clearance, timing, points gap, new coils points and condenser and I am just not making any real headway. I will say it no longer smokes, and plugs look much cleaner, but that’s it. It also still starts right away without choke.

The more it warms up it seems the engine has more trouble transitioning from open throttle to idle, if I can blip the throttle just right and make it land on idle then it will keep running but if I let it down normally or try to open the throttle from idle it just feels like mush unless I blip the throttle and get the revs up. And that’s also where it dies, in the “mush” zone. Once it’s dead it still won’t restart until it cools off.

I feel like I have run out of parts to throw at it, and now am really stuck.

 

[20774]

Is the advance working properly?

 

[cvhd94]

Is the advance working properly?

As far as I can tell, when I got the bike the advance had two different types of springs on it, and I replaced them with new appropriate springs and cleaned and greased it. The advance unit does feel a bit loose/ sloppy, but when I check the timing with my timing light the S is quite consistent in the window, and the advance mark comes in at around 3k rpm and stays solid.

 

[stevenrankin]

Too Far

I peeked at your profile and sadly you are a bit too far away for me to ride over to help in person. Or to bring the bike to me or my friend's shop.

I know I keep saying check your work. I don't do it to mean you are not competent to work on your bike. I mean everyone makes errors at one time.

For you bike, if you still have the part numbers for the carb needles and any parts you have installed double check them with a good microfiche to be sure you have the right set up for the particular carbs you have. Go back over things with a fine tooth comb and if possible pull the old parts out and take a look at them.

I ordered new needles one time and got the wrong ones, the new ones fit and the bike ran but not quite right. It wasn't until I checked the bag the new needles came in that I discovered the error as part numbers were off.

Man, I know how frustrating it is to be where you are at. I really would like to hit the nail on the head for you. Good luck, you are not far from getting it running right. St.

 

[ccolwell]

I had a carburetion problem with my R90S (I do not remember the symptoms). It turned out that I had not tightened the hose clamps on the sleeve between the carburetor and the head enough. As such, there was an air leak there. Perhaps try spraying carburetor cleaner at the junction with it running to see if anything changes.

 

[kinterridge]

I have read through this post and agree that it really sounds like carburetion. I did not see where you have yet balanced the carbs after the valve check. If you have balanced the carbs what procedure was used? Loose throttle cables, slightly, adjust idle screw to desired rpm, use either shorting or twin max or similar to balance, then adjust cables to achieve same rpm at around 2000 rpm with shorting or other method? Of course bike needs to be at operating temp, with fans on cylinders. I hope you get it sorted soon and can ride.

 

[20774]

I have read through this post and agree that it really sounds like carburetion. I did not see where you have yet balanced the carbs after the valve check. If you have balanced the carbs what procedure was used? Loose throttle cables, slightly, adjust idle screw to desired rpm, use either shorting or twin max or similar to balance, then adjust cables to achieve same rpm at around 2000 rpm with shorting or other method? Of course bike needs to be at operating temp, with fans on cylinders. I hope you get it sorted soon and can ride.

It seems he hasn't been able to keep the bike running long enough for that:

https://forums.bmwmoa.org/showthrea...en-warmed-up&p=1307201&viewfull=1#post1307201

 

[pauls1150]

re Post 22, paragraph 2 - Valves too tight? Either by wrong spec, worn feeler gauges, technique, or my continuing rant that the factory specs on air- and oil-heads is a bit too tight.
Or poor valve-to-seat mating/sealing?

 

[guenner]

Following the discussion reading "warmup", starting fine cold with enricher in off position and finding 'something' wrong with the enricher positioning, I would check for the correct assembly of the enricher...again.

 

[cvhd94]

First I just want to say thank you for everyone for taking the time to read my post and provide suggestions, I sincerely appreciate everyone’s contribution, it’s kept my hope alive!

I have made some progress today, I think?

I thought the idle circuit may still be clogged so I cleaned them out again, and also tightened my clamps on the carb to cylinder head seals.

I fired it up with a bit of choke, and it ran well when cold as usual. It seemed to run better when warm and didn’t die on its own. When going from blipping the throttle to letting it idle again the idle speed came to a crawl and then slowly built up again. But it didn’t die.

I then turned it off to see if it would start again. Nope. Engine spins over and get some galloping from one cylinder with half throttle as usual but does not start.

On a whim I decided to take the intake tubes off that run from the carb to the air box to see if maybe the diaphragms were getting hung up. I tried to start it and I’ll be darned it started. I didn’t run well at all, but with a quick adjustment to the carbs it was purring. I let it run, turned it off and it fired right back up. Throttle response was great, and if felt just like a normal healthy bike.

The only thing I can assume now is that the intake air duct or somewhere between it and the air filter is blocked? It has a brand new filter and when I looked inside the housing when replacing it was clean.

Anyone have any other ideas?

I plan to take the tank off, and open the air box again to have a better look around.

 

[cvhd94]

Hmm. Well took off the tank and checked the air filter and air box and path to the intake duct and it’s all clear as day. So not sure where that leaves me now.

 

[stevenrankin]

Leaves you with

Well, from what I gather it leaves you with a running bike, LOL. Don't knock it. I spent two years trying to determine why my RT would cut out while riding. I NEVER found a smoking gun as to the cause but boy by the time I got done, every electrical connection was spotless, every ground grounded.

Sorry I could not have helped you more but I am happy you can ride at last, enjoy, St.

 

[cvhd94]

Well, from what I gather it leaves you with a running bike, LOL. Don't knock it. I spent two years trying to determine why my RT would cut out while riding. I NEVER found a smoking gun as to the cause but boy by the time I got done, every electrical connection was spotless, every ground grounded.

Sorry I could not have helped you more but I am happy you can ride at last, enjoy, St.

Haha, well it runs with the air tubes disconnected - but I'm worried when I put everything back together tonight I'll be in the same boat as before. Not sure if this is just a red herring, or what's going on.

I don't get how a slightly less restricted airflow would be the difference between starting when warm vs not. And running so much better.

I even took the starter cover off to verify there is no obstructions that I couldn't see before, and it's all clear and clean.

 

[pglaves]

F



On a whim I decided to take the intake tubes off that run from the carb to the air box to see if maybe the diaphragms were getting hung up. I tried to start it and I’ll be darned it started. I didn’t run well at all, but with a quick adjustment to the carbs it was purring. I let it run, turned it off and it fired right back up. Throttle response was great, and if felt just like a normal healthy bike.

>>>>>>>>>>>>

Hmm. Well took off the tank and checked the air filter and air box and path to the intake duct and it’s all clear as day. So not sure where that leaves me now.

I don't have a /5 to look at here so /5 owners, help us out. What is the path for the air to flow into the filter area? Is there any chance this can be blocked?

Then for the OP - take out the filter but reconnect the tubes and see what happens.

 

[kinterridge]

On my /5 there's really not much difference than on my R100S. Unless there's something blocking the rubber air scoop. He did take the starter cover off and said all was clear. I've opened 2 up that were packed full of mouse nests from one and of the starter cavitiy all the way to the air filter, they ran, albeit poorly but they ran. Some air inlets have a screen in place.

 

[guenner]

Running the motor without the intake tubes tells me - again - the carburetor is adding too much fuel to the mixture.

That's would have been the logic with my 1989 R100GS. May not apply to a R75/5.

 

[cvhd94]

Running the motor without the intake tubes tells me - again - the carburetor is adding too much fuel to the mixture.

That's would have been the logic with my 1989 R100GS. May not apply to a R75/5.

Yeah I agree. I think that’s the only reasonable option. I thoroughly inspected the path between the intake scoop to the carbs and there’s definitely not an obstruction or restriction.

I assembled everything again today (and set carbs back to base settings) and fired it up, and tried to take it for a spin around the block, but it barely made it 500 feet before dying and not restarting. Pulled the tubes and boom back in business.

I bought leaner needle and main jets, but it doesn’t seem right that it would need leaner jets to run. Especially since the needle is already at its leanest (lowest) setting.

I’m curious if there’s something off with the enrichment circuit. Like it’s continuously adding fuel? When I tore the carbs apart the first time the enrichment discs were in the opposite carbs. It’s my understanding that doing so causes the enrichment circuit to basically be blocked, but I may be wrong. Makes me curious if the swap was maybe done on purpose originally?

In either case I know I installed the discs back in their proper carb, with new o rings and new gaskets oriented correctly, dimples pointing to intake. I checked 3 separate times also know they are fully on their closed stop on the carbs when the choke lever is off.

Is there anything I could be missing that could still be causing the carbs to run rich?

 

[20774]

I believe you said you changed around the plugs and the wires so as to not have too much resistance. Have you used and ohm meter to check the total resistance from one spark plug cap all the way over to the other spark plug cap? Should be in the neighborhood of approximately 20,000 ohms. Could you possibly be having coil failure?

 

[barbeemer16]

You say its a new air filter,It sounds like filter is clogged.Its not a K@N filter with to much filter oil om it .I had a Yamaha RD350 I had to much filter oil on it and it would not run,until I wash oil out and put less oil on it.