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Surging at Crusing Speed - Troubleshooting Help

14K views 48 replies 9 participants last post by  N1ckel  
#1 · (Edited)
After solving the off-idle stumble, I have a new problem. When riding along, at low throttle and low rpm, I'm getting a surging. it feels like the bike is moving back and forward below me. Its most notable at 1/8" throttle and about 3k rpm.

I've also noticed that pulling the enrichment out a bit helps with the problem but not entirely.

About the bike: It's a 2004 Bonneville. The SAI has been removed. The intake and exhaust are factory stock. The carburetor has #42 pilots and the pilot screws are 2.5 turns out. I live in Boulder, CO, which is about 5,300 feet above sea level. The plugs are DPR8EA-9's and they are gapped to 0.85mm.

I've checked for air leaks by passing propane around the vacuum ports and intake o-rings and didn't notice a change in idle. The vacuum caps and intake boots were replaced with OE parts last year as part of the troubleshooting process for the off-idle stumble. The carburetors have been synchronized using a Motion Pro manometer and the idle is set between 1200 and 1300 rpm (according to the factory tachometer).

A photo of the plugs is provided below:

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Where do you think I am on the lean-rich scale? The plugs look lean to me and that would fit with a lean surge.

And what do you advise I do next? Increase the pilot jet sizing further to #44? Run the carbs through another cleaning and inspection cycle? Do a more thorough check for air leaks?
 
#2 · (Edited)
Considering your altitude It seems to me the #42 pilots two and a half turns out would be way rich, but from the looks of the plugs and the surging you are lean. Check again for air leaks. Also are you running stock needles? Have the slides, springs or needles been changed? Also overly advanced timing, by modifying and moving the crank sensor too far advanced will cause a surge. If the pilot was changed to cure a stumble maybe you covered up a vacuum leak or clogged carb passages or air leak at the manifold to cyl head ''O'' ring seal.
 
#3 · (Edited)
Considering your altitude It seems to me the #42 pilots two and a half turns out would be way rich, but from the looks of the plugs and the surging you are lean.
I had the same thought. It doesn't seem right that I'd be running larger jets with the thinner air here.

Check again for air leaks. Also are you running stock needles? Have the slides, springs or needles been changed? Also overly advanced timing, by modifying and moving the crank sensor too far advanced will cause a surge. If the pilot was changed to cure a stumble maybe you covered up a vacuum leak or clogged carb passages or air leak at the manifold to cyl head ''O'' ring seal.
I'm not sure if the needles or springs are stock. I hadn't thought to look at them. I'll pull the carbs and see what's in there for needles.

The slides/diapragms were replaced last year because the slides had considerable wear. I replaced the crank sensor last year with a new one (genuine) while renewing the side cover as I had read they were prone to failure. It's in the stock location, set to the modified air gap of 0.8mm.

I'm going to go back and dig into air-leaks further and see what I can find. Thank you for the advice. I have a few avenues to explore.
 
#4 ·
I pulled the slides out. Both are in like condition with a bit of polishing in one spot. They look just about as good as they did when they were new last year:

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The needles are NAGb needles and are not shimmed. I don't see any damage or wear on the needles or in the emulsion tubes.

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The springs look like this. Their free length is 4.055". I couldn't find a spec on what that should be in my FSM. Do these look stock and is this free length in the ballpark? Both carbs are about the same length.

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I'm going to take it out and get it warm, then check for air leaks again. I'll let you know what I find there.
 
#5 ·
Sometimes just bad seating of the diaphragms can cause all sorts of problems, and they're easy to pinch - if one side doesn't seat properly it causes all sort of problems. Be worth just checking.


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#6 · (Edited)
Sometimes just bad seating of the diaphragms can cause all sorts of problems, and they're easy to pinch - if one side doesn't seat properly it causes all sort of problems. Be worth just checking.

Thanks, N1ckel. I pulled both diaphragms and they look good.
 
#7 ·
I should probably also add: The bike has 8k miles on it. I bought it with a tick over 7k in San Antonio TX and rode it to Denver. When I bought it, the guy said it'd been sitting a while. Early on, it had trouble with dieing at idle and off-idle bog. I fixed both of those with an initial carb cleaning and a new coil. The surging was present too and I compensated for the ride to Denver by using the enrichment.

I suspect the previous owner never could solve the surging problems and now here I am, chasing those gremlins too.

To be 100% certain, I pulled the carbs and am in the process of giving them another good and thorough cleaning in my ultrasonic. Both carbs look clean and I found no evidence of fuel varnish upon disassembly. Both carbs have #42 pilot jets and #110 main jets installed. The float heights on both are set to 17mm.

I feel that cleaning the carbs again might be a bit of tail chasing but this way I'll be certain. More to come.
 
#8 ·
The other easy thing to try is shimming the needles. My 05 always needs a shim, despite other configs that seem ok without it. If you are lean just off idle the shim will richen it up a bit, at least you will know if it's a lean issue. A couple of small washers from Home Depot work just fine.


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#13 · (Edited)
Carbs are back on the bike and I've taken it for a series of test rides. The problem persists.

Observations:

  1. As the engine warms up, the idle speed increases and I have to turn the idle screw a couple of times to keep moving it back down
  2. When hot, the bike feels down on power right off idle, say, when accelerating from a standstill. I have to rev it more than I should when letting the clutch out it bogs down. This improves when moving to 2.0 turns out on the pilot screws.
  3. When not, after the bike has sat for some time, say 10 minutes, I have to open the throttle slightly to get it to start. Subsequent starts don't require this until it sits again.
  4. Turning the pilot screw in to 2 turns seems to make it worse at low RPM's (say cruising at 20mph in 2nd gear in a school zone; 2800rpm or so).
  5. Turning the pilot screw outward seems to improve the surge but the exhaust note at idle sounds mushy.

I'm going to confirm proper synchronization of the carbs. More to come but could use advice on next steps.
 
#14 ·
When you say the idle screw are you talking about the single idle knob on the side, or the 2 a/f screws under the carbs?


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#15 ·
The idle speed increasing as the bike warms up is normal so I wouldn't count that as a problem


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#16 ·
By idle screw, I meant the black knob on the left-hand side of the bike that adjusts the butterfly opening for idle. Not the pilot jet screws on each carb. And to be fair, I had not adjusted it after the carb cleaning so it did need adjustment.

Another note or observation: The exhaust does not smell of unburned fuel at idle.

That being said, I just finished synchronizing the carbs. To my dismay, they were out of sync. I had to turn the synchronization screw about 1/8 turn to get them back in. I should have checked this first. :surprise:

So when it cools down a bit, I'm going to take another ride and see what impact that had.
 
#17 ·
You should be able to set the idle knob and forget it. Set it when the engine is warm for around 1100rpm. When it's cold just use the enricher to keep it alive long enough not to stall. Mine gets too rich with the enricher pretty quick, but isn't quite warm enough to stay alive sometimes, so I just keep a bit of throttle for a few minutes - one of the trade-offs of having a load of mods. I used to mess with the idle knob a lot but now I've got it sorted I never have to touch it.


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#18 ·
And ignore the official 950 - 1000 idle rpm stuff in the manual, most carb Bonnie's are much happier idling at around 1100.


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#19 ·
If you say opening the idle knob improves things - you are effectively adding more air, so I still think you could be too rich in the pilot circuit. It gets worse as the bike gets hot, another symptom of too rich. If you don't get anywhere, just try putting the 40s back in. It could just be a too rich bog as you expose the transfer ports.


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#20 · (Edited)
If you say opening the idle knob improves things - you are effectively adding more air, so I still think you could be too rich in the pilot circuit. It gets worse as the bike gets hot, another symptom of too rich. If you don't get anywhere, just try putting the 40s back in. It could just be a too rich bog as you expose the transfer ports.
I'll take your advice and do just that. Once the bike cools down (its too hot to even look at it right now), I'll pull the carbs and install the original #40 pilot jets. I'll set the screws back to 2.5 turns out and then resynchronize the carbs (just to be absolutely certain). That should put me back to stock (minus the SAI deletion).

I checked fuel economy on the last tank and I'm getting about 57mpg. Does that sound right for a stock bike?

BTW, the synchronization and leaning up the pilot screw settings seems to have made a noticeable difference. The surge is still there but its much less pronounced. I think we're on the right track.
 
#21 ·
Wish I could help, but instead I can confirm that while carb fiddling on my very neglected '02 Bonnie some amazing mpg's can be expected. I wasn't scientific enough to isolate each small tweak to its results, but could go from 37 mpg to 55mpg in one week while still experiencing what I thought was 'poor running.'

Now back to your regularly scheduled program.................
 
#22 · (Edited)
Latest set of observations:

I reinstalled the #40 pilot jets and set the pilot screws to 2.5 turns out. When riding, the surging feels worse at low speeds. It's almost a bucking that feels like a small misfire. I don't quite know how to describe it other than the bike hopping underneath me slightly at low throttle. Happens in all gears at all speeds when engine RPM is below 4k and the throttle is 1/8" or so (light throttle cruising).

Oddly enough, too, I do now smell a hint of unburned fuel in the exhaust at idle.

Also noted, after a 40 mile ride (not spirited but speeds of 75mph) that the exhaust glowed red where it connects to the cylinder head.

The surging or bucking does not occur when the clutch is pulled in likely ruling out chain/clutch/drivetrain issues.

I've disconnected the TPS and will report back to see if it changes anything. The thought is that, since it modifies the ignition timing, if it were improperly adjusted, the CDI might change the timing resulting in a lean burn at that specific throttle position. EDIT: Disconnecting the TPS did not improve/resolve the issue. I think we can rule that out too.
 
#24 ·
Well that definitely sounds too lean then in the pilot circuit, which shouldn't be the case given your setup. If it was a fuel issue you'd expect it to be worse at larger throttle openings.


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#25 · (Edited)
Here's a summary of the symptoms, actions I've taken, and what my next steps are:

Observations about the problem:
  1. A surging or bucking sensation is noted from the bike at low-throttle openings and low engine speeds. It is notable between 1800 and 3200 rpm at 1/16 to 1/8 throttle while cruising. It is not present at throttle openings above 1/8.
  2. This surging sensation is not present when the clutch is pulled and appears again when the clutch is let out
  3. The exhaust pipes glow red for the first 3" from the cylinder head. This is only observable at night when it is dark but is present while idling and while moving along at 65 mph.
  4. The bike idles smoothly.
  5. The bike starts well on full choke when cold and starts without choke when hot. When hot, it takes about four cranks for it to fire.
  6. When accelerating from a stop, the bike does not bog as the clutch is let out and it accelerates cleanly to 6k rpm.
  7. When revved, the idle speed drops to the set level and does not fall below that. It drops smoothly and quickly and does not hang.
  8. Symptoms do not change when the TPS is disconnected

What I've done to help remedy the problem:
  • Confirmed that no air leaks are present at the carburetor intake boots, intake o-rings or at the takeoff caps. This was done using the propane method. No change in idle observed at any point.
  • Removed and cleaned the carburetors in my ultrasonic. Verified all passages are clean using copper wire. Inspected diaphragms, slides, enrichment plungers and air cut-off and all were in good condition.
  • Removed #42 pilot jets. Installed fresh #40 pilot jets and set pilot screws to 2.5 turns from seat.
  • Installed new spark plugs gapped to 0.85mm

This problem has been present from the moment I bought this motorcycle. The previous owner bought it new and blamed it on carburetor gumming due to sitting without being ridden much. I have confirmed that the problem is present with #40 pilot jets at 2.0, 2.5 and 3.0 pilot screw settings. I've also confirmed that it is present with #42 pilot jets at 1.5, 2.5 and 3.5 pilot screw settings. I cannot confirm whether the changes in jetting made the problem better or worse. I can say, though, that general ridability is much better with the #40 pilot jets at 2.5 turns.


Where should I look next? Are we certain this is a carburetor issue?

My next steps are:
  1. Replace the chain. I haven't performed significant chain maintenance on this bike so it's due for chain maintenance anyway. But I thought maybe a chain with some stiff links might reveal itself as a surge.
  2. Replace the CDI. I was having coil issues last year that really felt like carburetor issues. Replacing the coil helped fix that but I wonder if I have a CDI that is failing at certain ignition frequencies (say 1.8k to 3.2k ignition triggers per minute). Given the reliability of the Gills units, I'll replace it as a matter of course and see if it has a positive affect.

BTW, this is what the plugs look like after an 80-mile ride with the #40 pilots at 2.5 turns.

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#26 ·
Those plugs too look too clean, suggesting lean, and the headers shouldn't glow red, that also sounds lean. Do you know if the valves have been checked? I'm reaching now, but thinking through some basics that could make it overly lean if it's not carbs. It could also be worth checking the fuel flow as others have suggested.


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#27 ·
Also I never had much luck with propane or WD40. Liberally spraying starter fluid on the rubber was the only way I identified my air leak.


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#31 ·
Still working on this:

While riding at night, the left side is running hotter than the right as I can see more orange/red on the exhaust. I rechecked for air leaks, this time by adding some pressure through the synchronization port and spraying the outside with soap. I had bubbles at the right side intake boot, engine side which seemed fairly significant.

The boots that are presently on the bike were replaced last year when I went through the carbs and other issues trying to resolve the ignition problems. Its odd that they would fail this quickly. I did order new boots, new clamps, new intake o-rings and new vacuum caps from Triumph, though. When those get here, I plan to pull the induction system, check everything out again, reseal it and see where that gets me. I also read a few posts about casting marks on the intake manifolds causing odd indentions in the boots which leaked. I recall seeing similar casting marks on my intakes and so I plan to file those out smooth and check other sealing surfaces for defects.

I'll report back after going through the induction system and let you all know how it goes. I did also buy a new set of 42 and 45 pilots as a last-resort and will install those if I get to the point of having the induction system sealed and still chasing this persistent problem. More to come.
 
#32 ·
Still working on this:



While riding at night, the left side is running hotter than the right as I can see more orange/red on the exhaust. I rechecked for air leaks, this time by adding some pressure through the synchronization port and spraying the outside with soap. I had bubbles at the right side intake boot, engine side which seemed fairly significant.



The boots that are presently on the bike were replaced last year when I went through the carbs and other issues trying to resolve the ignition problems. Its odd that they would fail this quickly. I did order new boots, new clamps, new intake o-rings and new vacuum caps from Triumph, though. When those get here, I plan to pull the induction system, check everything out again, reseal it and see where that gets me. I also read a few posts about casting marks on the intake manifolds causing odd indentions in the boots which leaked. I recall seeing similar casting marks on my intakes and so I plan to file those out smooth and check other sealing surfaces for defects.



I'll report back after going through the induction system and let you all know how it goes. I did also buy a new set of 42 and 45 pilots as a last-resort and will install those if I get to the point of having the induction system sealed and still chasing this persistent problem. More to come.


I had exactly this issue - casting marks on the manifold which I had to file. Using the starter fluid was the only way I found the leak. In the end I used some black hi-temp sealant to help seal the boots, been fine ever since.


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#33 ·
Maybe I missed something but it doesn't seem like you are adjusting your pilot air screws after the engine is well warmed up, say 10 miles or so, for best/smoothest idle. This is an adjustment, and should be set where the engine runs best. The factory setting is a starting point, not a law. You may adjust as many turns in or out as necessary. If you need to adjust more than 4-5 turns out, go up one pilot jet size. If you need to adjust to less than 1 turn out, go down a size. At that point if transition on throttle opening isn't smooth, you may want to shim the needle.
 
#34 · (Edited)
You haven't missed anything, duc. I moved the pilot jets in a few 1/4 turn increments (outward, richen) to see if they improved the problem and when they didn't, gave up on additional adjustment. Once I get her back together with the newer induction rubber, I'll spend some time with the pilot screws and report back on the results.
 
#35 ·
HI radare,

Sorry that I've nothing helpful to suggest but I do hope you find the answer to this annoying problem. I am following this thread because I have exactly the same problem with my 2006 790cc Bonnie and have done everything I've read here but not solved it yet. It's really annoying that my bike has done this since I bought it in 2008 and several times now I've thought I had cured it only for the surging to come back again after a few weeks. It's frustrating the h3ck out of me. Thank you for taking the time to give progress reports.
Jon
 
#36 ·
One more thing to try for the 790 is put Thrux needles in (NBZT). My 05 is so much happier with them in, despite other posts saying they get too rich. I have then with 1 shim as well. I've tested my setup with an AF gauge and it's pretty solid through the range.


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#38 ·
Thanks, N1ckel. I have the 42 and 45 pilot jets and I'll track down a set of Thruxton needles to go with them and have them on hand. I'm waiting on induction parts from Triumph When they get here, I'll go through the induction system and ensure I have no leaks and then start jetting and fueling modifications to correct the problem.

I think while I have the tank removed and the intakes off, I'll take the time to verify valve clearances so that I can cross those off my list too.
 
#39 ·
A quick summary of where I'm at:

Found an air leak on the righthand intake at the carb boot. Could not find a similar leak on the lefthand side but suspect one is there as this cylinder runs hotter than the right.

Ordered replacement boots and clamps and will install those along with filing the intakes smooth and then confirm that the intake is air tight.

I will then reset the carbs back to the stock 2.5 turns out, #40 pilots and will take a test ride.

After that, if the surging and lean conditions are still present, I'll begin adjusting the pilot screws. If that fails, I'll modify the fueling with jets/needles.

More to come.
 
#40 ·
Sounds like you are executing a very good organized trouble shooting plan !
Just curious, where on the boot was the leak you found ?
Mine are still the original boots and I have been thinking they are probably due replacement. They still are soft but the ethanol fuel may be killing them inside out. The Triumph rubber seems to be far less quality over the Japanese bikes I have owned over all the years.

Thanks and good luck.
 
#41 · (Edited)
They are leaking on the inward side (toward the center of the bike) where they clamp to the intake manifold. They were new last year and the clamps were tightened fully but they still leaked. Hopefully filing the intake manifolds smooth and fitting new boots and clamps will resolve the leak.