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Old 09-28-2013, 03:55 PM   #1
BalefireX OP
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Question R75/5 only running on one cylinder

Somewhat perplexed here...
Today I was riding along on the highway at about 70, cruising happily, when suddenly I lost power and could barely keep the bike at 50. I pulled off and determined that I was only running on one cylinder. I limped the bike to a nearby (not BMW) shop where we tried a bunch of things, including:
- removed and checked the plugs, wires, coils, etc. cleaning all contacts
- removed the float bowls and checked the carbs, blew air out the needles
- reset fuel mix and idle settings, resynced carbs
none of this worked.

A couple times we got the bike going on both cylinders but it would always die after riding it down the road for a mile or two.

Strange things we noticed:
-The bike would run well after removing and replacing the carb bowls, but only for a few minutes.
-It seemed like air bubbles were coming back up the fuel lines (looking at the clear fuel filters)
-Going over a bump would sometimes cause the non-firing cylinder to kick in for a second; bumpy roads caused surging as power came on and off.
-When I stopped along the side of the road, the carb bowl on the side that was not firing was nearly empty. Gas also didn't seem to flow nearly as freely on that side.

Do any of you gurus have an idea what might be the issue? Based on that last piece of info I'm thinking it's an issue internal to the right (problem) side carb, and that the cylinder is being starved of fuel, but I know enough to know that I don't know enough to take apart a carb, working or not!

Thanks!
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Old 09-28-2013, 04:15 PM   #2
disston
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Take the gas out of the tank. Take the petcocks off and clean the screens. Hose out the gas tank. Allow to dry. Remove clear plastic filters and replace fuel line if cracked much. You need 3 feet of new fuel line to do one bike.

The OEM tank liner constantly flakes off and plugs the petcock screens and the clear filter. As the liner is worn away more and more of the tank interior is exposed to moisture in the gas and can rust which causes more debris in the tank. Your gas tank may need a proper relining. I like the tank relining kit from POR-15.

I run no clear filters and have had the petcocks clogged enough to cause problems. The filters don't really do much for you. The screens will catch the bigger pieces. Cleaning the gas tank once every couple of years or even more often does work. A good tank liner is the best preventive medicine.

It is rather obvious the shop was not familiar with Airheads.

BTW, it is also possible the debris is plugging the inlet port on the carb or the hoses themselves?
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disston screwed with this post 09-28-2013 at 04:21 PM
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Old 09-28-2013, 04:25 PM   #3
photorider
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Did this happen after a fuel stop perhaps? Do you have a tank bag? Sounds like your gas tank may not be venting properly.
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Old 09-28-2013, 04:32 PM   #4
villageidiot
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check your valve clearances.......

had a bike on a trip (purchased just for said trip for a friend from denmark to ride) and the valves closed up on one cylinder.

we set em, and it was fine, made it home the 700 miles.
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Old 09-28-2013, 05:11 PM   #5
Plaka
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Quote:
Originally Posted by BalefireX View Post
Somewhat perplexed here...
Today I was riding along on the highway at about 70, cruising happily, when suddenly I lost power and could barely keep the bike at 50. I pulled off and determined that I was only running on one cylinder. I limped the bike to a nearby (not BMW) shop where we tried a bunch of things, including:
- removed and checked the plugs, wires, coils, etc. cleaning all contacts
- removed the float bowls and checked the carbs, blew air out the needles
- reset fuel mix and idle settings, resynced carbs
none of this worked.

A couple times we got the bike going on both cylinders but it would always die after riding it down the road for a mile or two.

Strange things we noticed:
-The bike would run well after removing and replacing the carb bowls, but only for a few minutes.
-It seemed like air bubbles were coming back up the fuel lines (looking at the clear fuel filters)
-Going over a bump would sometimes cause the non-firing cylinder to kick in for a second; bumpy roads caused surging as power came on and off.
-When I stopped along the side of the road, the carb bowl on the side that was not firing was nearly empty. Gas also didn't seem to flow nearly as freely on that side.

Do any of you gurus have an idea what might be the issue? Based on that last piece of info I'm thinking it's an issue internal to the right (problem) side carb, and that the cylinder is being starved of fuel, but I know enough to know that I don't know enough to take apart a carb, working or not!

Thanks!
Gas isn't flowing freely from where? The bottom of the carb with the bowl off?

Your carbs are connected by a crossover fuel line. If there is fuel starvation up at the tank, it affects both sides, even with only one petcock on. So I would suspect an issue at the one carb. if you don't have a crossover, why not?


If your float needle is sticking closed, it will restrict the flow into the carb bowl. On some carbs the float needle is attached to float with a fine wire clip. not always and it's tricky to install so someone may have left it off. So the weight of the float pulls the need down (open) by the clip. With the bowl off and petcock on and the float fully descended you should have fuel flow. Do have a clean can under it. Don't smoke. If you raise the float with your finger you will see it pivots on a pin. There is a tab there in with all the pivoting stuff and the float needle is directly above that tab. look for a connecting fine wire clip. The needle is sort of a triangular chubby thing and may have a ball bearing pressed into the bottom. if there is no wire, or it's broken, and you don't see the bottom end of the float needle. Rap on the carb body with your knuckles and see if it descends.

Pull the bowl off the working carb, do the same thing and see if there is some difference in that area.




In any event, unscrew the bottom nut on the petcocks to release the nipple the fuel line is attached to. There is a fine tubular screen in the top of the nipple, in addition to the ones in the tank. Check for crud. it is possible that you have two problems. A slightly sticky float needle and reduced fuel pressure from upstream clogs. With the pressure down, the flaky carb starts acting up. A full tank of gas should make it better, a low one worse and possibly running on reserve (bottom of tank and most clogged area of the in-tank screens) impossible.


Bing had a DVD on rebuilding the carbs. it's gotten good reviews around here. Combine that with the manual ($11) and you'll know anything your could want about those carbs. They are very simple and a good one to start on and lean some carb stuff. Next thing you'll be able to look at a row of 6 linked DCOEs on a Ferrari V12 and not blink.



yeah, OK, IDAs or somethng...

Plaka screwed with this post 09-28-2013 at 05:44 PM
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Old 09-28-2013, 05:21 PM   #6
disston
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Quote:
Originally Posted by villageidiot View Post
check your valve clearances.......

had a bike on a trip (purchased just for said trip for a friend from denmark to ride) and the valves closed up on one cylinder.

we set em, and it was fine, made it home the 700 miles.
In addition to the fuel delivery problems from debris in the tank, fuel lines and filters this one about valve clearances is the most likely. The valves can be tight enough that the bike starts but then dies when it heats up. Is the problem heat related at all? Like runs normal first thing in the morning but dies after 10 mins?
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Old 09-28-2013, 05:35 PM   #7
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Quote:
Originally Posted by disston View Post
In addition to the fuel delivery problems from debris in the tank, fuel lines and filters this one about valve clearances is the most likely. The valves can be tight enough that the bike starts but then dies when it heats up. Is the problem heat related at all? Like runs normal first thing in the morning but dies after 10 mins?
yeh seen the debris in the tank
bike sits crap floats away
bike starts no problem
runs for a while
debris move down to the outlet
stops bike
wait awhile
repeat
took me a while to work out
cheers
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Old 09-28-2013, 05:48 PM   #8
Plaka
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Quote:
Originally Posted by disston View Post
In addition to the fuel delivery problems from debris in the tank, fuel lines and filters this one about valve clearances is the most likely. The valves can be tight enough that the bike starts but then dies when it heats up. Is the problem heat related at all? Like runs normal first thing in the morning but dies after 10 mins?
never seen a bump change the valve clearances so much that a non running cylinder starts running. nor will a valve clearance problem be intermittent. in the manner the OP describes.


If we are willing to ignore his observations, which are careful and wide reaching, then I vote for an ignition problem...yeah, that's it...drys carb bowls right up...oh...we're ignoring that. sorry.
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Old 09-28-2013, 07:20 PM   #9
disston
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There's also the hole in a diaphragm problem that nobody has mentioned. Will cause one cylinder to not work but not common to be intermittent.

So I guess we are back to the gas tank debris being the most likely source of troubles?
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Old 09-28-2013, 07:34 PM   #10
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Except for the empty bowl observation, I'd remove the main jet on the affected side. It sounds blocked. It's a 5 minute job.

Has the front cover been removed and the wiring etc checked for problems?
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Old 09-28-2013, 08:43 PM   #11
Plaka
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Originally Posted by boxerboy81 View Post
Except for the empty bowl observation, I'd remove the main jet on the affected side. It sounds blocked. It's a 5 minute job.

Has the front cover been removed and the wiring etc checked for problems?
The bowl doesn't fill through the main jet. it empties through it. So if it's empty....

Theoretically all bowl vents AND the jets could be blocked. That would stop fuel flow. Lottery has better odds...

BTW, an easy way to diagnose a main jet problem is to just turn on the choke when the engine seems starved. You also cut throttle a moment to increase vacuum and suck some gas out the choke circuit, then rev it.

Plaka screwed with this post 09-28-2013 at 08:50 PM
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Old 09-28-2013, 08:44 PM   #12
Plaka
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Originally Posted by disston View Post
There's also the hole in a diaphragm problem that nobody has mentioned. Will cause one cylinder to not work but not common to be intermittent.

So I guess we are back to the gas tank debris being the most likely source of troubles?
We??? I thought it somewhat unlikely and an accessory if so. If that was a non-inclusive "we" however, or perhaps a royal "We", then...
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Old 09-28-2013, 08:48 PM   #13
Plaka
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BTW, do the carb bowls have two rods sticking up in them with two black floats strung on the rods? in this case the floats would not be attached to the carb body or brass pivoting frame at all and would come away with the bowl when you remove it from the carb.
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Old 09-28-2013, 08:55 PM   #14
Plaka
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Did this happen after a fuel stop perhaps? Do you have a tank bag? Sounds like your gas tank may not be venting properly.
On a typical /5 the cork in the tank cap (a spring loaded cork faced disc with a vent) is old and dried out. Venting problems are rare. if the cap has just been redone it's a possible. The bubbles seen in the filter do indicate air being frawn up towards the tank suggesting low pressure at the top. Still, a fuel feed issue should affect both sides unless one carb has another weakness.

I'd like to know how it ran well enough in the shop to sync. the carbs---unless that was a static effort, at which point they aren't particularly synced.
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Old 09-29-2013, 07:19 AM   #15
boxerboy81
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The bowl doesn't fill through the main jet. it empties through it. So if it's empty....

...
I knew that..why I wrote "except for the empty bowl observation". If it's empty, there's nuthin' to go thru the mj.
Still, until I got to that bit, I was thinking blocked mj.
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