'78 R100/7 Loosing Power High Speed

0m3nc0w · Jun 18, 2019

I bought this bike in stock condition 3 years ago and when I would hit about 65-70mph it would bog down like someone was pinching the fuel lines on and off really quick, or flipping a switch back and forth really fast disconnecting the spark plugs - sort of a sputtering behavior. I ruled out a vapor lock in the tank by opening the lid at speed. Strangely this behavior does NOT repeat itself in 4th gear hitting the same RPM, only in 5th gear at 65-70mph. It's definitely not the clutch going bad as proposed in another similar post.

So I thought it might be a carb issue and since I was using this as a donor bike for a custom restoration (see below) I figured rebuilding both carbs would correct the behavior. Unfortunately after total rebuilds of both 36mm Bing carbs with new diaphragms, O-rings, needles, floats, jets etc. and a thorough cleaning the problem persists.

Any ideas?

m_stock10506 · Jun 18, 2019

Diaphragms were going to be my first suggestion, but you say that you replaced them. With Bing replacements?

Have you checked valve clearances, and have you checked compression?

ecjjce · Jun 19, 2019

Maybe check or replace your inline fuel filters, and also check the petcock screens. A partial clog may be reducing the fuel flow just enough at certain speeds to cause the sputtering. I would also check that the floats are set to the correct height.

You might also check that your carb slides are working smoothly, and that the new diaphragms are installed with the notch in the right position.

Good luck!

ECJ

20774 · Jun 19, 2019

Welcome to the forum! I'll add check your air filter...had an R100RS that was restricting high speeds...it was a dirty air filter.

ccolwell · Jun 19, 2019

Just to be clear, the bike accelerates and cruises normally at all rpm ranges in the first 4 gears but not in 5th? That's very odd. I would try to repeat the symptoms going up hill in 3rd and 4th. If they don't repeat, it doesn't sound like a fuel/air issue. I would still replace air and fuel filters, and fuel lines, as others suggest, low hanging and cheap fruit. If not, it's speed related and all I can think of is wind moving something that either blocks the air intake or causes a bad plug wire to flutter and cut spark. Are the plug wires new?

stevenrankin · Jun 19, 2019

Tune up Mantra

The tune up Mantra for airheads as I understand it is as follows.

1: Compression, Check to see if it is proper 150psi or so. (If you have a leak down tester perform a leak down test.)
2: Valves, see if they are set to proper clearance. (Not sure the mileage on the bike so I will assume they are not worn out. IF they will not hold proper gap, it is a sign they are worn or receding.)
3: Timing and in your case points. Check the points for proper gap and wear. After the points are properly gapped check for proper timing. (A possible problem could be the points condenser has gone bad. I have seen this happen in two bikes of the same year.)
4: Carbs, You have rebuilt the carbs. As suggested, check the filters in the tank. Check for water in the gas. IF the bike has been sitting with gas in it, you could have rust, water, or both causing fuel flow problems.

Do your troubleshooting after you have done the steps above.

From my armchair here at home, I suggest replace the condenser. Sometimes, the points get replaced but the condenser doesn't and they quit or break down with time. St.

manicmechanic · Jun 19, 2019

Float level.

barryg · Jun 19, 2019

Check your gas tank cap vent. Could be stopped up.

20774 · Jun 19, 2019

barryg said:
Check your gas tank cap vent. Could be stopped up.

He said he opened the "lid" while at speed. Seems to rule out the gas cap vent.

But float height sounds like a good candidate!

69zeff65 · Jun 20, 2019

Open exhaust = none stock carb settings

Looks like you have some low restriction mufflers on there. Have you tried bigger main jets and or needle setting change?

khittner · Jun 20, 2019

MANICMECHANIC said:
Float level.

+1.

I had precisely the same behavior from my ‘91 R100 that I did my first carb rebuild a month or so ago. Steve Strickland at Motoworks (Chicago) re-did my rebuild, and adjusted the float levels. Voila! The bike now keeps up with interstate traffic without a problem. Fuel depth in the bowls should be 18-22mm when the floats are properly adjusted. Yours feels like it’s running out of gas because it probably is.

brook.reams · Jun 20, 2019

khittner said:
+1.

I had precisely the same behavior from my ‘91 R100 that I did my first carb rebuild a month or so ago. Steve Strickland at Motoworks (Chicago) re-did my rebuild, and adjusted the float levels. Voila! The bike now keeps up with interstate traffic without a problem. Fuel depth in the bowls should be 18-22mm when the floats are properly adjusted. Yours feels like it’s running out of gas because it probably is.

Khittner,

I don't think those numbers are correct. According to Bob Fleischer, aka Snowbum, the depth depends on if the carbs are 32 mm (64/32) or 40 mm carbs (94/40). Here is his quote from this article: https://bmwmotorcycletech.info/bingcv-2.htm.

"For the 32 mm carburetors, use 24 mm fuel height. For the 40 mm carburetors, use 28 mm fuel height. "

I believe this bike has the 40 mm carbs. In any event, Bob's recommendations are for deeper fuel levels than you state.

In this article, https://bmwmotorcycletech.info/earlybingr75cv.htm Bob says the "early" /5 carbs (I believe this is the 64/32/3 and /4 models) have a recommended fuel level that is lower, 19-20 mm.

"... you can measure the fuel depth in the float bowl decently enough. Optimum from the center bottom middle depression area of the bowl, to the fuel level, is 19 to 20 mm. This is for the stock one piece floats!"

I hope this helps.

Best.
Brook Reams.

guenner · Jun 21, 2019

I had the very same symptom on a car only when in the top gear. I didn't believe it when the mechanic said it's a spark plug wire. :scratch

Argued with him! But after he replaced the wire the problem was gone and never re-appeared.

He explained it to me that in the higher gear this spark plug failed - due to the bad wire - to always cause an ignition.

/Guenther

Anyname · Jun 22, 2019

I would check the gas tank breather as well.

0m3nc0w · Jul 18, 2019

Anyname said:
I would check the gas tank breather as well.

I opened the tank lid at cruising speed and it didn't change the behavior so it's definitely not a vacuum lock in the tank.

0m3nc0w · Jul 18, 2019

khittner said:
+1.

I had precisely the same behavior from my ‘91 R100 that I did my first carb rebuild a month or so ago. Steve Strickland at Motoworks (Chicago) re-did my rebuild, and adjusted the float levels. Voila! The bike now keeps up with interstate traffic without a problem. Fuel depth in the bowls should be 18-22mm when the floats are properly adjusted. Yours feels like it’s running out of gas because it probably is.

I'm running the 32mm Bings and checked the fuel level in the float bowls when I rebuilt the carbs and they are perfect.

0m3nc0w · Jul 18, 2019

m_stock10506 said:
Diaphragms were going to be my first suggestion, but you say that you replaced them. With Bing replacements?

Have you checked valve clearances, and have you checked compression?

Yep, carbs were completely rebuilt with bonafide Bing parts. Valves are good and compression is 141 (L) and 143 (R).

0m3nc0w · Jul 18, 2019

ecjjce said:
Maybe check or replace your inline fuel filters, and also check the petcock screens. A partial clog may be reducing the fuel flow just enough at certain speeds to cause the sputtering. I would also check that the floats are set to the correct height.

You might also check that your carb slides are working smoothly, and that the new diaphragms are installed with the notch in the right position.

Good luck!

ECJ

Thanks ECJ, I have brand new fuel filters and petcock screens and the float bowl levels are perfect. Carbs were just completely rebuilt and are in perfect working order.

0m3nc0w · Jul 18, 2019

20774 said:
Welcome to the forum! I'll add check your air filter...had an R100RS that was restricting high speeds...it was a dirty air filter.

Brand new air filter, thanks

20774 · Jul 18, 2019

You mention fuel filters. Have you confirmed the fuel flow rates through those filters? In 5th gear, that will be maximum flow required. You say it runs fine in 4th gear...stay in 4th out to a much higher RPM...should be a high fuel demand...see if the condition repeats in 4th.

'78 R100/7 Loosing Power High Speed

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