/5 tuning advice, please!

[life101]

I'm stumped and don't know where to look next. Can I beg a little sage advice from you all, please?

1971 R75/5 runs like a champ, all the way up to 75 mph. above 75, it begins to fumble and stutter and really doesn't want to be pushed any faster. I know an R75/5 is capable and comfortable up to and around 90 - so where do I look next?

I have good fuel flow, and the spark plugs are correct and look great. Idle up to 75 she never misses a beat, and will maintain 75 no problems.

Where do I begin?

[StephenB]

Your best bet is to start with fuel supply and mechanical advance: float bowl level, clogged petcocks, even sticky advance.

1) I'd try float bowl level first: here check whether it actually floats, opens/closes the valves correctly (consult your manual on how to check) and when you remove the bowl carefully after running the bike it should be about 3/4 filled with gas. If you are an experienced rider use this method : use the kill switch to shut down your engine when your bike starts sputtering at 75mph, close the petcock immediately and then check content of floatbowl on the road once stopped. If it is empty or fairly empty you've found your problem.
2) Check petcocks: remove line they should provide a steady stream, not dripping. But you said you had good fuel flow (whatever that means for you).
3) Use a timing light to ascertain that the the timing mark is on Z when at or above 3000rpm, that deals with sticky advance.

If all that is ok ... it is getting trickier. So you better try 1-3 before we go any further.

My money is on the float bowl level. Experienced that with my G/S: I could accelerate through first, second and third before it would start to cut out in fourth. Ran a couple of seconds on low rpm in fourth, and then could crank the throttle again ... for a couple of seconds until it cut out again. Cause: flot bowl level to low. Adjusted the tongue: problem solved.

Just my 2c.

Stephen

[DanOGrande]

Check the little springs on the ignition advance mechanism. They wear out.
Also try this site for more ideas: http://w6rec.com/duane/bmw/index.htm

[RocketJ]

Dollars to doughnuts, Stephan is right on. R90 did the same thing.

[Tim McKittrick]

Originally Posted by DanOGrande

Check the little springs on the ignition advance mechanism. They wear out.

Bad springs will cause the advancer to stay in the advanced position, and will lead to a mysterious wandering high idle- but should still allow full advance at higher engine speeds, so its probably not that. I recently came into an R100RT that (as reported by it's former owner) fell on it's face at 80mph. He sent me the carbs before I took delivery of the bike, and I rebuilt them. I found the float to be a little high- not enough to be a big issue- and they were remarkably free of dirt and goo. I replaced the diaphrams while I was at it, even though there was no visable holes, cracks, or tears to the old ones. With the reconditioned carbs on, the bike instantly fired up and was able to hit redline in top.
I think the problem was a diaphram issue- most likely a leak around the rim of the diaphram. I would go ahead and put a full carb kit into your bike if you have the time and inclination- that way you will eliminate float height, clogged jets, and bad diaphrams in one go and be able to move directly to other engine systems if the problem isn't solved.

[jtwind]

I think Tim may well be onto something, diaphrams were the first thing that I though of. JT

[life101]

Thanks for the replies, guys:

1. I've just rebuilt the carbs, including diaphrams.

2. I will check fuel level in the bowl as suggested (shut it down at speed), but I think fuel flow really is OK.

3. Isn't the advance at full setting at about 3,000 rpm? This problem doesn't occur until well above 5,000...

The description was right on, though - it "falls on it's face" at about 80 mph.

Anything else I should check?

PS - I always check Duane's site first - couldn't find anything about this issue.

Thanks!

[jtwind]

If you are sure the diaphrams are ok, I'd be suspicious of the coil or plug wires next. JT

[Tim McKittrick]

I feel silly even mentioning this- but have you confirmed your gas cap is venting? If you are producing even a slight vacuum in the tank at your highest fuel consumption rate (going fast in top) it will reduce fuel flow to the carbs. This is easy to rule out- just twist the cap open enough to make an air gap when the motor is misbehaving and see if the problem resolves. I suspect that were your cap the issue, you would be able to make brief runs up past your magic velocity and the problem would be much worse with a full tank than a nearly empty one.
I would also point out the obvious- if your diaphrams are not seated properly they will leak around their perimeter (I believe this is what was happening to my RT) causing the slides to drop under some conditions. If I get my physics of CV carbs right, full throttle and high load will have the least differential of pressure above and below the diaphram, so it is under these conditions that a tiny leak will show up. I'd take a careful look at them again- pull off the cables and remove the tops with the carbs in place.
Your problem is almost certainly a fueling issue as the bike operates as it should elsewhere. If it will pull strongly to redline through the lower gears it is unlikely to be an ignition problem or a mechanical one. About the only way I can see the plug wires failing at speed is if they are so loose that they are flapping about at 70+ and loosing their connection at the sparkplugs.
Let us know what you find

[Wirespokes]

At high RPMs systems are more stressed than at lower RPMs. So it still could be ignition - points perhaps? If it's electronic then maybe the plug wires are beginning to break down.

But what I'd check after the diaphrams and gas tank vacuum, would be air cleaner and fuel lines. Does it have the in-line fuel filters? Are the fuel lines the 7mm or 1/4"? I think that the smaller lines and fuel filters only make a difference on the 90s and 100s, but it might on the R75 as well. Did you do the fuel flow test to see how much comes out in the specified amount of time?

As far as setting float level, it's worked well for me to just see where the fuel turns on and off while lifting and lowering the float. The seam on the side of the float should be parallel to the flange where the bowl mounts when it turns off.

[Flash412]

When your float level is too low, the symptom is that the bike WILL get "up to speed" if you rip right through the gears, then it will slow down to whatever speed it can maintain with the flow provided.

If it ain't float levels, replace the spark plug caps with NGK 5k-ohm ones.

[dlearl476]

I wish I could offer some specific advice, but in my haste to get my brother's R75 back on the road after 10 years I pretty much did ALL THE ABOVE to cure the exact same problem.
I will offer a couple of "FWIW's" though:
My favorite mechanic said "change the advance springs before you do ANYTHING else"* In my haste to do that, I grounded out the diode board with the ignition cover, so I replaced that as well as all the aformentioned mods with a new-fangled diode board from Motarrad Elektrik.
Given that everything else was in working order, I think the two things that did the MOST to correct the problem, was a good fine tune carb adjust and the new spark plug wires as Flash suggested.

Now the bike runs excellently. From an 800 rpm idle to smooth as a sewing machine at redlline.


*according to him, unless the springs are right, you'll never get a correct timing setting or carb adjust.

[life101]

Yes - it really runs perfectly at all but WFO, so probably a carb problem. Points are new, and I've checked the float levels - all OK.

I'm running a large Heinrich tank w/ toolbox, so I'll see if the old gas cap is restricted - hadn't thought of that. Petcocks and fuel line are new.

Diaphgrams are new, but I'll see if they're not seated properly.

Also I'll check the spark plug wires - I think I have some spares available.

Thanks!

[monzablue]

You're getting good advice here, and I would not be at all surprised if you find you problem as described in this thread. BUT, I thought I'd throw out another resource. . . over 1200 members on a /5 specific board, and many are ready and willing to offer good technical advice. If you're still stumped, it's a great resource.

http://groups.yahoo.com/group/5united/