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Topic-icon '74 R75/6: Mixture screw and a few other questions

  • Winter
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2 weeks 5 days ago - 2 weeks 4 days ago #6410 by Winter
A few weeks ago we bought a 1974 R75/6 with just over 140,000mi on it. For most of its life it was owned by one man, who kept it running to his satisfaction for 42 years, though he tended not to change anything unless it was actively broken. The next owner did a 1200mi road trip tinkered a bit, but he was more a Japanese bike guy, which is how we ended up able to buy it.

We decided we'd go through and check over much of the systems, and replace parts that were worn out or in danger of being so. I also felt it wasn't running as smoothly as it should be, and the behavior of the enricher worried me, namely that putting on the enricher would almost immediately kill the engine. Along the way, we've run into some issues that needed addressing, such as a split open nut on the left intake rocker arm, comically worn out teeth on the throttle assembly, and the left carburetor (Bing 64/32/10) had the enricher disk installed backwards, which shocked the heck out of me, even after reading that it was something to watch for.

Here's where my question comes in: we've got dual plug heads, which from what I've read, means we have to use a vacuum tool for balancing rather than the shorting method, as we can't just disconnect one set of plugs without risking the ignition system (which was changed out to a Dyna coil system and electronic ignition, so I'm not sure if that guidance still applies). So after adjusting the valves, and replacing all the nuts under the valve cover with appropriate replacements, I set the mix screws to 0.75 turns out and the idle adjust screws to 1.5 turns in to give a baseline idle, then after letting the bike warm a bit, tried to get things set.

On the left carburetor, when I turn the mix screw all the way in, the engine dies, which is the behavior I expect. On the right side however, even turning the mix screw all the way down doesn't have much impact on the engine idle, which is rough as hell. Now I'm trying to figure out if that is pointing to a problem within the carburetor, or if I should be looking elsewhere, such as replacing plugs, or tinkering with timing. I'm an old aircooled VW guy, but I always ran single carburetors, so figuring out about balancing dual carbs is intimidating, even with all the good info out there.

Which brings up another question, I don't have points, but I still have an advance mechanism, and I think there's still a condenser, though I don't associate that with electronic ignitions as a rule. Is there anything I should know in particular about timing with that set up, most of what I find only relates to timing with the stock configuration?

And finally: when I pulled the front engine cover off, I disconnected the *positive* lead out of habit from cars, rather than the negative as is widely instructed. The bike starts up and all lights work, so I'm guessing I escaped killing the diode board, but for future reference, does the bike use a positive ground, or is there some other reason to disconnect the negative terminal?

Just for fun, here's a picture of the nut I removed from the left intake rocker mount. After seeing this, we bought all new replacements to be on the safe side

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2 weeks 4 days ago - 2 weeks 4 days ago #6412 by Wobbly
Some mixed thoughts....

• The Bing enricheners are a constant issue. Not only can they be reversed, but they can also be swapped Left into Right, causing a different kind of trouble. Good eyes. Glad you caught that one, most people don't.

• These days, the "shorting" and the "disconnecting the plug cap" methods are NEVER advised. Time has proven it's a great way to burn out an ignition coil... no matter what type, brand, or the number of plugs per head. Just DON'T do it. Go ahead and make a good manometer set up LIKE THIS . That''s all you need.

• You got a slight mix-up there. The Idle mixture screw setting is more like 1-1/2 turns out to start. The Idle speed screw is set by the tachometer (engine RPM) and not the number of turns.

• Jap and Brit bikes are ready to have their carbs tweaked after a 1-2 minute in-garage warm up. This procedure will absolutely not work on a BMW Airhead. They simply run too cool to effectively make any carb adjustments without first riding the bike for several miles. Most sources quote 8-10 miles, You can adjust all you want in the garage, but after 15 minutes on the road, all your work is going to be for naught, and more adjustments will be required upon return.

• Idle should be set around 950-1000 rpm, with the vacuum gauges showing even balance. Then, the throttle has to be increased at the twist grip to 1700-2000 RPM and the throttle cables have to be adjusted to balance out the vacuum gauges. Everyone forgets that last part.

• ALL carb adjustments are wasted time and energy unless all other tune-up settings (plugs, valves, air filter, ignition timing, plug wires, plug caps, float height, compression readings, et al) are adjusted/ checked/ measured first. Carb settings are completely dependent upon all other settings. If you intend to start the tune-up with the carbs, then be prepared to repeat the entire process at the end.

• Dual plug timing is a specialty unto itself. Sorry, I can't help you with those specifics. But timing is always best set using the 'F' mark at high RPM. In other words, use a strobe lamp to set the ignition at Full Advance where the engine runs while going down the road. 'On the road' is where you want the engine to do it's best. There's only one brand of motorcycle where the owners want the engine to run its best at idle, and that brand is not BMW. :P

• More research is required on your ignition. In 1976, electronic ignitions (EI) were in their infancy. You are much more likely to have one of the spark boosting "capacitive discharge" systems. There were some early EI systems that used the mechanical advance, which is not as good, since the mechanical portion is the weak point of the entire ignition anyway. Modern EI (since ~1990) systems have zero moving parts and the advance is done in software. No EI system that I'm aware of incorporates the use of the old ignition capacitor ("ignition condenser"); the capacitor goes away with the points. If you can't figure out what brand or type system your bike is running, then you may need to replace it simply to tune your bike correctly. Allow me to point out the obvious and say, if you have trouble 200 miles from home, how can you troubleshoot an unknown ?

• You are correct. Disconnecting either battery terminal effectively disconnects the whole battery from the system. We generally recommend the Negative terminal simply because the Positive side may have multiple cables on it and the single connection is much easier to deal with. Make sure to disconnect at the Battery Terminal because the Negative cable connection at the gearbox is physically weak and cannot withstand multiple re-connections.

Hope this helps !

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Last edit: 2 weeks 4 days ago by Wobbly.

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2 weeks 4 days ago #6415 by Winter
Thanks for all the info! I'll admit, trying to get this bike in tune is intimidating the hell out of me, it seems like there are so many moving parts (so to speak) that have to be balanced. You mentioned backing out the idle mixture screw 1.5 turns to start, but where do you go from there? What I read basically said to adjust it until you find the highest revs it induces at idle, but the right side screw at the moment has virtually no effect on the idle speed.

I set the idle speed screws to 1.5 turns in just to get the bike so it will idle when cold, but I could back them out and use the enricher to try and hold the idle, since it seems to be behaving itself now, and yes, I did check that the enricher disks were marked for the correct side of the bike, in addition to fixing the orientation on the left side unit.

New plugs will happen today, and I did change the air filter, which seemed not to have been done in a long time. Checking the float height wasn't on my list though, so I'll add that to my list. I'll also see about a compression check, as while I was assured compression is good, and I've got records of a top-end rebuild not so many miles ago, it's worth confirming.

The news on that ignition is concerning, and I can see why potentially replacing it with a known-quantity modern unit might make sense (I'm open to suggestions). Here's a photo of what the current set-up looks like

I know it's advised to ride the bike prior to adjustments, but I wanted to touch base about whether I'm risking any damage to the bike by riding it when the two cylinders are clearly out of synch (bike rocks from side to side noticeably at idle for instance. Is there anything I should watch out for as a warning sign that I *shouldn't* ride it as a warm up?
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2 weeks 4 days ago - 2 weeks 4 days ago #6416 by Wobbly
More mixed thoughts....

• Highest idle speed is the ultimate goal when setting the Idle Mixture Screw, but you're not going to get that until the engine is thoroughly hot. The "sweet spot" should be within 1/8 turn in or out from the starting position of 1-1/2 turns. If you need to turn more than +/- 1/8 turn, then something else is wrong. Truth is... when I'm tuning my bike I simply set them to 1-1/2 turns and walk away.

...but the right side screw at the moment has virtually no effect on the idle speed.

• That's because your Right Side slow speed jet is clogged. Replace the small jets on both sides. And squirt carb cleaner through the Idle Screw Mixture passage. You probably also have a clogged enrighener jet, here...


Again... the Idle Speed Screw is set by engine RPM, and ONLY engine RPM. I have no idea where you heard or read about number of turns on idle speed, but you should wipe that little nugget from your cranium. I'm glad to volunteer time to help you, but because it takes so long to answer these type posts with the necessary detail, there's no time to repeat stuff. You're getting the benefit of several decades of professional m/c mechanic experience, accept the knowledge or let it pass.

• Plugs: NGK BP7ES. Along with new plugs... check out installing new plug wires and plug caps. The trio is under $20. Cheap insurance.

• If you intend to set the float heights, then Bing has new ethanol-proof floats, which I highly suggest, along with 2 new float needles.

• If you'll remove that center screw on the auto advance unit, it will pull straight off the shaft. Then you should be able to read a brand name on the tiny blue circuit board. That in turn will allow you to visit their web site, and pull down the PDF instruction manual. BTW... that mechanical advance is going to need to be lubricated and possibly receive 2 new springs.

• As long as the fuel is high test with a built-in cleaner, the ignition timing is pretty close, the valves are checked, the engine oil is the correct grade and level, and the tires are inflated... then no harm will come. In fact a few hundred miles it is probably the best thing you can do.

• I will also refer you to an article about getting Airheads going after storage. It used to be stored as a Stickie in the root maintenance directory, but the powers that be have decreed it 'non grata' and have pushed it off to some obscure corner. It only took me 2 months to write, so it's quite complete with ideas, photos and charts. Worth your while to look up. Lots of what you're asking is covered.


All the best.

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Last edit: 2 weeks 4 days ago by Wobbly.

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2 weeks 4 days ago #6419 by Winter
Thank you again for sharing your knowledge!

You're definitely right about the ignition being an older version of points-replacements, I just dug through the records that came with the bike, and it looks like the conversion to dual plug heads and coil packs, as well as (I think) the solid-state points replacement, was done in '87. It's those lower plugs that have been stressing me out in terms of replacements btw, since they are totally different than the top ones, but I found the info in the bike's files, looks like 12mm x 1/2" NKG D6HS plugs are what were originally used in the lower holes.

New plug wires and caps make a lot of sense too. Do folk tend to buy ones specific to their airheads, or just buy spark plug wire by the length, a bunch of caps, and assemble themselves?

Looks like I'll be digging back into the carbs, in more detail. Do you have any sources you recommend, or *don't* recommend for gasket kits and those replacement slow speed jets? I know that Bing carries most parts, though I didn't see a slow speed jet on their parts list.

I've put about 150mi on the bike since getting it (rode it home 90mi the day I picked it up), and that's really good info about tolerances for riding.

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2 weeks 3 days ago - 2 weeks 3 days ago #6423 by Wobbly
It's the older, stock OEM spark plug caps and wires you'll want to worry about. They seem to go out at about 40 years.

Read this... airheads.org/forum/wrenching/992-new-own...ead-back-on-the-road

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