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Old 03-21-2012, 04:09 PM   #1
Tankad OP
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Can’t find the issue… Need help R75/6

I am currently running a well maintained 1974 R75 /6. Over the past several years I’ve changed out or replaced most if not all of the common troubled areas dealing with this motocycle, but I am still am having a power (horsepower) issue. It periodically feels sluggish and while riding at highway speeds I lose power and either I need to downshift to maintain speed or sometimes adjusting the choke helps maintain speed (but never increases it). Furth more, I have never been able to pull more than 5,600 RPMs on the engine since I bought it.

Here is a list of a few things I have done already:

New plugs, always 93 octane fuel, new battery, switched to an electric ignition, changed the diode board, cylinder compression seems fairly good (not sure what the standard should be), oil changed regularly, valves adjusted at every oil change, cleaned the carbs and checked the internals (everything seems fine), replaced the ignition coils.

Is there something else I am missing / should check, or is issue sound more like an engine internals (valves, pistons,…) problem?

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Old 03-21-2012, 05:33 PM   #2
disston
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Some more information might help. How many miles on this machine? What settings are you using on the valves? Have you rebuilt carburetors? I think not, you don't say. But since you seem to have done a conscientious job with other things then I bet you have changed the air filter, yes?

A lack of rpm can be a lack of total timing. You should have an older style timing light to check and see that the ignition is doing what it should. That is at idle on the proper mark for idle. This is the same mark used for static timing, in general. More important is actually the total timing at a higher rpm. Usually before 3,000 rpm the ignition should be finished with advancing and there should be a single image of the light on the flywheel for total advance.

I'm not familiar with the marks on a '74 machine. Marks vary somewhat through the years. Tell us what marks your flywheel has and somebody will tell us what marks you should use. My bike is one year later. I have three lines for static, I use the middle line. And I have a dot for total. There is also a mark for TDC, marked "OT", on my flywheel. Yours might be the same or very similar.
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Old 03-21-2012, 10:38 PM   #3
pommie john
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Fuel filters.

I can't remember what kind they have on the /5 but check they are not blocked and there's probably a filter of some kind in the carb too.
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Old 03-21-2012, 11:33 PM   #4
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I'll go with the filters suggestion, plus the tank might not be venting properly. Do a flow test to see how much fuel runs out of the taps in a minute. Also, when you encounter the problem, try lifting the gas cap to see if that relieves a vacuum in there.
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Old 03-22-2012, 12:43 AM   #5
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Originally Posted by Wirespokes View Post
I'll go with the filters suggestion, plus the tank might not be venting properly. Do a flow test to see how much fuel runs out of the taps in a minute. Also, when you encounter the problem, try lifting the gas cap to see if that relieves a vacuum in there.

Hmmm...

Thats pretty much what I'd look at but I'd stress the fuel flow check.

My /6 had road speed issues that disappeared once I rebuilt the fuel petcocks. OTOH, the old /6 tank lids usually leak enough air that they won't fuel starve an engine but the later model recessed caps sure do. Boy they do!

1) Make sure the tanks clean
2) New air filter.
3) Do a fuel flow check or simply rebuild the petcocks. (You always ride with both petcocks open. Right? Right??)
4) Try riding with the tank lid cracked open.
5) After that, get an experienced Airhead to look the bike over with you and double check the timing. (Quite frankly, I doubt that thats the problem)
6) Ounce you find the problem, report back. OK? It's gotta be simple!
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Old 03-22-2012, 03:32 AM   #6
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Check the float height on the carbs, If it is too low you will get the same symptoms that you complain of.

To check, turn off fuel tap, remove carb bowl, hold float bowl up, turn fuel back on and lower bowl slowly, noting where the fuel just starts to flow. It should be when the bowl is aligned with the bottom of the carb. To adjust bend the tang that bears against the bottom of the fuel valve. It's a fiddly job.

Charles
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Old 03-22-2012, 07:36 AM   #7
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And double-check the carb diaphragms. They can seem fine when they are not.
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Old 03-22-2012, 08:29 AM   #8
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Thanks guys for all the information and suggestions to fix my issue! I really appreciate it. I will try an answer everyone’s questions to help this process along…

Here is a little more information on the bike… it has about 100K on it and i've owned it since 2003. I am running the standard Bing carbs from that year. I have checked the rubber diaphragms and they seem to be in perfect order. Should I replace them anyways? Or should I do a full carb rebuild? I have changed out the air filter and checked the timing last riding season. Everything seemed fine on that end. Should I go to cone air filters and take the stock air-box out of the picture to see if that helps? I know that the stock air-box can be restrictive.

I have had in the past an issue with the clutch side carb sticking and overfilling. To fix this, I just shut off both petcocks while riding to let the fuel level drop down… a couple of little taps on the side of the carb with my foot, and it is back to normal. Is this generally a sign of a bigger problem?

I have never rebuilt the petcocks. So I will definitely give them an additional look. They currently don’t seem too weak and give a good “thunk” when moving from one position to the next. I have never run an inline fuel filter… Should I? Is there another fuel filter I should be aware of and check?

I will do a flow test and crack the gas cap open when riding this weekend (it is suppose to rain here tonight and tomorrow).
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Old 03-22-2012, 08:47 AM   #9
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If you have crap in your bowls when you drop them, then an inline filter is definitely in order, but a tank cleaning is the best medicine. The fact that you have a fiddly carb tells me that they need some love. A rebuild is probably in order and ChasBMW gave good advice on setting the float height. Wrong float height negates everything else done. EUBMW has decent gasket kits but you need to hit BMW ( or Bing)for throttle plate screws, float needles, jet needles and needle jets to do it right. If all else is stock, the stock airbox is fine and truly not restrictive. Do double check that all jet numbers are correct for your model. I had a bike that had leaner main jets that were installed by the PO to combat a rich running situation that he did not know was caused by very worn needle jets.
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Old 03-22-2012, 08:55 AM   #10
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Well OK. If your are sure it's not ignition.

The petcock drill with the toe tapping sounds like some debris in the system. So a little piece gets in the float needle and you clear it but how much debris is this system holding all together?

I think you need to clean the entire system, new fuel lines, new in line filters, rebuild the petcocks and wash the inside of the gas tank.

It also sounds like a carb rebuild might be a good idea. Tiny pin holes in the diaphragms are hard to see.

If there is a lot of rust in that gas tank you may want to reline it sooner rather than later.

Here is a link to the type of in line filter seen on most Airheads. There are many sources. You might be going to your dealer to get some stuff and/or another source, this type of filter is available at most of these places. Using the auction just for the illustrative purpose:

http://compare.ebay.com/like/230729201162

There is a screen filter in the tank that's part of the petcock. It's not much of a filter, just a brass screen. There's a new style you'll use when you rebuild the petcocks. It combines the screen with the washer that seals the petcock to the tank.
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Old 03-22-2012, 09:04 AM   #11
Tankad OP
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Quote:
Check the float height on the carbs, If it is too low you will get the same symptoms that you complain of.

To check, turn off fuel tap, remove carb bowl, hold float bowl up, turn fuel back on and lower bowl slowly, noting where the fuel just starts to flow. It should be when the bowl is aligned with the bottom of the carb. To adjust bend the tang that bears against the bottom of the fuel valve. It's a fiddly job.

Charles
Thanks for the info... i will check this this weekend as well.

Quote:
The fact that you have a fiddly carb tells me that they need some love.
i think you are right. looks like i'm about to tackle a carb and fuel delivery rebuild. i'll let you know how it goes.
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Old 03-22-2012, 09:21 AM   #12
OZbeemer
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In Milwaukee contact Matt at Motorwest. He is one of the best local airhead guys around. Nice to support a local shop if you can. I have dealt with him a bunch.

Wade
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Old 03-22-2012, 09:25 AM   #13
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Quote:
Originally Posted by disston View Post
There is a screen filter in the tank that's part of the petcock. It's not much of a filter, just a brass screen. There's a new style you'll use when you rebuild the petcocks. It combines the screen with the washer that seals the petcock to the tank.
You don't need an in-line filter, the petcocks have a fine-mesh brass screen.

You need to remove the petcocks and check that screen, if it is clogged that could be your issue.

Rebuilt 'em while you have them off
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Old 03-22-2012, 09:32 AM   #14
Tankad OP
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In Milwaukee contact Matt at Motorwest. He is one of the best local airhead guys around. Nice to support a local shop if you can. I have dealt with him a bunch.

Wade
Yep… Matt at Motorwest is a great guy and awesome shop! I was just there on Saturday. He is currently swamped. If I can’t figure things out… he is always the first person I go too. He has been servicing the bike since I got it. That is why I know it is a well sorted moto.
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Old 03-22-2012, 06:58 PM   #15
Wirespokes
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Quote:
Originally Posted by SOLO LOBO View Post
You don't need an in-line filter, the petcocks have a fine-mesh brass screen.

You need to remove the petcocks and check that screen, if it is clogged that could be your issue.

Rebuilt 'em while you have them off
That's exactly what I'd suggest as well! I don't use those inline filters as the stock ones work just fine! If too much of the red tank liner is flaking away, take care of it. If the tank and stock screens are up to snuff, you won't need any added filters.
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