Fellas,
Been chasing down this issue for a while now. Here's the deal:
Once warm, after a 20 minute ride, idle hiccups a few times before dropping. I can hear the engine "skip a beat' and feel it on the bike---at each hiccup. The tach needs moves in accordance with this. The idle eventually drops from a warm 1k to 700 or so. I can blip the throttle and the process repeats itself.
See here.
https://www.youtube.com/watch?v=uQyTrvhAHiQ
What I've done/checked:
Started with timing, set and checked in spec.
Disassembled cleaned carbs/jets/passages with compressed air and carb cleaner.
Replaced all o rings (sans butterfly rod). There were a few that needed replaced.
Installed new float needles.
Float needle seat is clean and shows no wear.
New throttle piston (slide) with new needles set on 3rd clip.
No tears or noticeable defects in the diaphragms.
Checked float weight (12.9 and 13.1 grams respectively) and fuel level in bowl. No irregularities.
Sprayed carb cleaner and opened unlit map gas around all carb areas and potential leak areas. No change in idle speed.
Used shorting rods to sync balance carbs. Idle/air/fuel mix screws are set, as prescribed, to allow for engine to run on on side, with fuel mixture screw set for highest idle (then backed off just a touch, so its not too rich).
Valves are adjusted at 20/15, and a loose 15 on the intake.
Plugs are new, not too white or black.
No gas foot or fuel leak at float bowl.
Ignition system is a Alpha from Rick at ME, just a few years old. Original bean can went a few years ago.
I can get a great idle/balance then after another 20 minute ride. I get the hiccup and unsteady idle and loose the smooth sync.
I might be missing or have overlooked something in the carbs?
My thought is that if this was a carb issue, I would also be a problem when cold and when trying to sync/balance.
Otherwise, I am uncertain where else to trouble shoot mechanically or electrically.
Spark plug leads (wires) are original to the bike (1981) and I plan to try the set from my 90S to see if that makes a difference.
Alpha ING module is riveted together with a heat sink pad.
Otherwise bike runs great thru the rev range and performs as it should.
It can be slightly cranky to start from cold but nothing out of the ordinary, by my estimation.
Open to any suggestions/questions, etc.
Scott,
Good call on timing light during idle drop. I'd not thought of that.
Carb spigots, will check those again. I did install new rubber boots there, just recently, as well. They were tight then.
I did replace the butterfly o-rings a few years back on the first rebuild but my experience is when the BF is not functioning idle speed can't be adjusted, which is not the case. Still...maybe of concern.
Coil is new, Dyna brand from Rick.
Noted on ING unit and re pasting. I will call Rick about this, as well.
100 will keep things posted here. Thanks for chiming in with your thoughts.
Very interesting problem. Anxious to hear what you find. I’m betting the timing light will reveal something. I had trouble with an Alpha Classic ignition (‘78) and the timing light clearly displayed the problem. It would idle ok, but when you tried to raise the rpm the timing marks would dance about. It almost seems like you get a few miss-fires, then lose your 6 deg advance at idle? I am totally not familiar with the bean can ignition, or it’s replacement.
Some follow-on ideas....
- Not clear from your wording, but you shouldn't even want to attempt to sync the carbs until you already have at least 10 miles on the engine. Everything needs to be completely, fully warmed, including the carbs and engine oil.
- How old are your plug caps and wires ?
- When was your last new air filter ?
- What are idle mixture screws set at ?
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Short follow up here.
Carb adjustments are made after about a 20 minute ride, yes.
Good call on the plug caps and wires.
I installed a new set and that immediately got rid of the hard hiccup/skip I was experiencing when the bike got up to operational temps.
I was able to get the carbs synced/dialed in much better but still have a slightly rich condition on the L side AND the idle still does fall a bit when stationary but it's much better.
Not calling this fixed, yet but much closer. Hopeful a bit more careful adjustment on the carb settings will take care of this.
We'll see....
Appreciate everyone's comments. It appears the plug leads were a contributing factor here.
@bremoit I take it your assessment of the "slightly rich condition" comes from plug color. Some further comments...
- Pull both Slides and using a caliper, check that the exposed extension of the needle is the same on both. Since the needle clip cannot be directly viewed, ala Mikuni, Bing needles are often unknowingly inserted to different depths. And too, both needles should be clean and shiny over their entire length.
- While the carbs are being sync-ed, we not only balance the readings at idle, but also at higher speeds. Use the throttle friction device to hold the engine at ~1800 RPM. Throttle cables stretch over time, and to re-match the position of the butterflies at running speeds we use the cable adjusters... found where the throttle cables enter the carb.
- Plug readings should only be made with a "plug chop". After riding at a fixed road speed for several miles, enact the Kill Switch and simultaneously pull in the clutch. Coast to the verge of the roadway where you can remove the plugs and check their color. Make sure your plug threads are lubricated to make this job easier.
- Unmatched fuel heights in the float bowl can cause plug color mis-match. With the bike on the centerstand, check this by turning the petcocks OFF and pulling the float bowls. Then you can use a caliper to measure the liquid height from the lip of the bowl.
- Check that both fuel enricheners (chokes) turn completely OFF with positive tension (actually compression) in both cables.
- Replace both the Needles and the Needle Jets if they are over 10 years old. The pulsation of the intake air hammers the needle against the interior of the needle jet, thus opening it up. You cannot see or measure this without lots of precision instruments.
- You may have slight restriction of jets or fuel passageways due to prior storage. You can try a cap-full of StarTron fuel conditioner/cleaner in each tank of fuel. This will slowly clean the interior of the carb.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Noted on the plug chop, that's a new one to me.
All other considerations you've mentioned, I've been rather thorough in covering including new slides/needles, float needles, weighing floats, checking fuel bowl level, etc...in the past 2 months.
The butterfly on one side could be slow to return---which maybe a result of a cable issue. I do have 2mm of freeplay (per owners manual) and the cables are relatively new (3-4 years). Still, will double/double check things there.
Really appreciate the tips and suggestions.
@wobbly Noted on the plug chop, that's a new one to me.
If you think about it, you've got 3 stages of jetting (slow, needle & main), plus idle mixture. If you're looking at the plug color, then you've got to be sure which "stage" is leaving that color. You wouldn't allow the bike to idle for 2 minutes, pull the plugs, and then change the main jet due to the color. I think you can understand the engine was nowhere near the main jets in that extreme example. So the only way to read plugs is by riding in one of the 3 jet ranges and then suddenly cutting the engine to read that specific range.
This doesn't mean it's not OK to look at your plugs when you have them out... for instance... to help with a valve adjustment. Sure you can get general information about the bike's general condition, but don't try to glean anything specific. And definitely don't take any more drastic action than to tell yourself that you need to follow up with more detailed investigation.
And while we're on the subject... You've probably heard of people that pull their plugs and then swap their main jets. Then they scratch their heads wondering why they don't see a change. Well, the main jet only comes into play at about 7/8 to full open. I don't know anyone that rides at 7/8 to full open for 5 minutes. You'd almost need to live in Nevada AND be crazy. I bet 85% of the riders on the road today have never even been on their main jet !! But, dealerships sure sell a lot of main jets !! ?
All the best.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly Yes, ... I read (somewhere) that the only real way to judge your main jet size would to get on a flat straight road and run the bike up to top speed in top gear, holding the throttles wide open. Once at steady maximum speed, close the throttles slightly. If the bike actually gains speed, you should step up your main jet size.
Ya, like I’m gonna go out and try that! My stock #160 main jets are fine.
Yes, and the other one is reading plugs after idling. No one in the universe cares about plug color at idle. No, what everyone wants at idle is a smooth, even beat... that doesn't allow the engine to stumble and stall... just as the light turns green !
?
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Curious if you found the cause of your hiccup and idle speed drop. Did the timing light reveal anything? It’s an interesting issue you were having.
Good call on the plug caps and wires.
I installed a new set and that immediately got rid of the hard hiccup/skip I was experiencing when the bike got up to operational temps.
Another happy customer ! ?
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Follow up:
After installing new spark plug leads and swapping the blue Dyna Coil back to the original BMW coil AND re balancing the carbs, I seem to have resolved most of the issues.
There were just the slightest adjustments needed on the air/fuel mixture to get things dialed in properly (to a touch richer setting--as there were some slight backfires going down hill using engine braking) even after balancing with the shorting sticks.
Note--- the original bean can/ICU were swapped to the Motorrad Elektric along with the Dyna coil a few years ago.
For whatever reason the combination of new leads and the old coil resolved the hard blip/drop I was getting at idle. I'll need some more seat time to fully say this is resolved but after a few 20 mile rides, all seems well.
Advertisement
