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Topic-icon R75/7 3000 RPM Idle

  • Andrew Lane
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2 days 17 hours ago #6613 by Andrew Lane
R75/7 3000 RPM Idle was created by Andrew Lane
Hi All,

Running into an issue that I am having trouble diagnosing... I have just finished rebuilding the carbs and put them back on the bike. The issue I am having now is that the bike starts right away with no choke and will sit at a proper idle of ~1000 RPM for a couple seconds, then climbs very quickly to a little over 3000 RPM and sits there. The bike will occasionally backfire right after turning ignition off. Currently, the carbs are set with the idle speed screw fully off (butterflies closed) and the air mix screw out 1 full turn from being screwed all the way in. Also the rubber boots connecting the carb to the cylinder heads are brand new (put them on after the carb rebuild) and don't appear to have any air leaks. The right cylinder isn't pulling as much as the left based on putting my hand behind the exhaust, and the right cylinder is smoking and has quite a lot of valve clatter where the left does not. I will adjust the valves but is there anything else that could be causing such a high idle?
Thanks,
Andrew

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2 days 2 hours ago - 2 days 1 hour ago #6621 by Wobbly
Replied by Wobbly on topic R75/7 3000 RPM Idle
► A collection of random ideas....

• It is VERY common that high idle has to do with the ignition advance not returning to Full Retard at idle. This can be due to lack of lubrication of the advance mechanism and/or weak ignition advance return springs. When was the last time you serviced your auto advance unit ?

• If you disassembled both carbs at the same time, you could have placed the Enrichener Discs into the wrong carbs. There is most definitely a Left and Right to the carbs AND to the parts of the enriching circuit. Those discs are marked L and R for that purpose.

• Since you had to undo the choke AND throttle cables to work on the carbs, the correct re-installation of the cables is HIGHLY suspect. Do the chokes go to the FULL OFF position on both sides ? Is there 3mm slack in BOTH the throttle cables at idle ?


► Stop guessing at carb balance. Build THIS simple device.


► An explanation of what work was performed and what your experience level is would also help.

All the best.

Don't hide 'em, Ride 'em !!
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Last edit: 2 days 1 hour ago by Wobbly.

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2 days 22 minutes ago #6622 by Andrew Lane
Replied by Andrew Lane on topic R75/7 3000 RPM Idle
Thanks for the response Wobbly, you have been a great help to us. As more of a background... the bike came with two different carbs (you might recall I posted a question about this a few weeks ago). The right was the correct 64/32/14 while the left was a carb body from 1995 (which only had 2 cap mounting screw holes) with the 4 mounting screw hole cap that was supposed to be with the 64/32/13 carb that should have been there. We since bought the correct /13 carb, rebuilt it, and put it on the bike. But the original setup with the hybridized carb (although almost impossible to start) did not over-rev like it is doing now. Don't know if this means anything as the choke cables and choke did not seem to be on correctly. I guess this actually increased our confusion as out first startup of the bike after we had reinstalled the rebuilt carbs was great. Engine started right away with no choke and went straight to the correct idle (but as mentioned below, that changed quickly).

The work performed was a rebuild of both carbs(all new seals and gaskets and some replaced jets) with new cables, but they were done one at a time to avoid mixing up any parts. I have some general mechanical experience but have never done carbs before so I did the work carefully after watching many videos and following along with them. There is only one slight difference in the carbs, the left has a #50 low speed jet while the right has a #45 (this is the way they came on the bike when we bought it). I don't know how big an issue this is but I figured I should mention it. As to the cables, the coke lever does go to the fully closed position (lever on the carb rests on the stop) and goes through the whole range normally. The throttle cable currently has ~2mm free play on both sides but I can back that out a bit. Finally, I have not yet done work on the auto advance unit and the bike has been sitting since 1996 so its been a minimum 23 years since its been serviced. Is there a way to determine if the advance is the culprit and if so how do I remedy it?
Thanks again,
Andrew

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1 day 23 hours ago - 1 day 23 hours ago #6624 by Wobbly
Replied by Wobbly on topic R75/7 3000 RPM Idle
• The secret to getting any dual carb bike to run correctly is to make all the parts and adjustments identical. I do not think the mis-matched jets are causing your present issue, but I do believe it's not a "good thing". Your next parts order should include a matching #45 for the other side, and fairly soon.

• If you have the single cable at the twist grip, then you have a "2 into 1" device under the tank. That device needs some lube (like ATF) and the 2 lower cables need slack at the adjustment on the carb's top. IOW, ALL the throttle cables need slack at idle if you expect it to actually "idle". The 2 cable systems can be checked at the twistgrip alone, but the 3 cable systems require a lot more work.

• Americans are crazy about cable ties, and that's OK... until you start lashing things like throttle cables hard to the frame. Throttle cables need to be routed such that they make the most gentle bends possible, and are free to move around under tension. I spent an hour or so on my new bike experimenting with different routings to get the best possible. If you head into a sharp turn at speed and expect to shut the throttle and get the required engine braking, then these "small details" seem to matter. The last thing you need is a revving engine when it should be decelerating.

• No Airhead cranks without choke. And the later the model gets after about 1977, the more choke us usually needed due to EPA leaning out the mixture. So the fact that your bike cranks easily without choke is telling me that your mixture is WAY too rich. That's why I'm keying on choke adjustment. Next would be float needle and float height. You can follow that trail for yourself. Do both sides have the manometer ports blocked ? Did you replace ALL the o-rings on ALL the jets ?

Hope this helps.

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Last edit: 1 day 23 hours ago by Wobbly.

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1 day 18 hours ago #6627 by Andrew Lane
Replied by Andrew Lane on topic R75/7 3000 RPM Idle
Adjusted the valves and replaced the spark plugs today and started the bike up, the over-revving issue seemed to get a little better. The bike would not start without choke and idled at 2000 RPM. It would only get "stuck" at 2800 after blipping the throttle. This seems like it could be a weak springs in the mechanical advance issue so tomorrow I will get the front cover off and investigate. The right cylinder is seems to be running very rich as smoke is only coming out of the right exhaust (large amount when blipping the throttle). I have a 2 cable system not the "2 into 1" and there seems to be ample free play in the cables (I don't use ties). When rebuilding the carb, I replaced the 5 O-rings in each of the carb and have just ensured that the manometer ports are blocked. How would the float level cause a "too rich" problem in one cylinder and how might I check the fuel level in the float bowl. There is a lot I still have to learn here and really appreciate your advice.

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1 day 6 hours ago - 1 day 6 hours ago #6629 by Wobbly
Replied by Wobbly on topic R75/7 3000 RPM Idle
The physics of the carburetor is akin to drinking liquids through a soda straw. The closer the pool of liquid is to the vacuum source, then the easier it is for a set vacuum to draw liquid up through the straw. IOW, the closer the pool is to the vacuum, the greater the volume of liquid drawn up. So the height of the fuel inside each float bowl is critical when you have 2 carbs you need to run exactly alike.

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Last edit: 1 day 6 hours ago by Wobbly.

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