Stumbling coming off idle when cold

Discussion in 'General' started by whereshaldo, Jul 22, 2018.

  1. whereshaldo

    whereshaldo Member

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    I've got a really odd situation that cropped up recently that I can't diagnose. This started recently after I almost ran out of gas, and so I'm suspicious that its gas related, but I'd like someone else's opinion.

    Starting the bike cold, it will idle just fine but then as the throttle comes on it just chuffs and sputters. In neutral, i can turn the throttle and it just bogs and then will slowly chuff but without increasing rpm. After 10 seconds or so it will rev up to proper rpm but then when you let off the throttle it will repeat the symptoms. In gear it almost feels like the clutch is slipping except the engine doesn't rev. The bike just doesn't have the power to propel the bike forward. Once the bike does start moving however, it works just fine.

    Once the bike is warmed up, it runs just fine and the symptoms go away completely, but it will take 5 to 10 minutes for these symptoms to abate.

    In terms of the bike -- timing is spot on, i checked it today with a test light. Valves are set properly -- .002. I took the carb apart and cleaned it and all jets are clear. Plug is clean. The only other thing is new gas (and some sea foam that I added after the fact). Could it really be as simple as crappy gas?

    Hal
     
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  3. OLD CT

    OLD CT Well-Known Member

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    You are now running 20/50 oil! You HAVE to warm it up as in idle it only for 10 minutes! I run 20/50 and can actually 'slowly' hear the idle pick up after ONLY idling for 10 mins. You can't just jump on it with 20/50 and take off like it's warmed up to normal operating temp.
     
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  4. whereshaldo

    whereshaldo Member

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    ohhhhhhhh, ok. will give that a shot. sometimes the little things escape me.
    h
     
  5. racerx

    racerx Administrator
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    OLD CT is right. 20W50 is like molasses until it's warmed-up. With my daily rider, I can feel the engine being held back until oil temp reaches 80-85C...and won't take the bike out into traffic (where the engine will get a real workout) until it's at least 70C. That last ride, in the fall, when oil temp can barely top 80C, usually is with 3-4mph scrubbed-off the top end. I dunno that oil viscosity is the cause of your issue. I'm more inclined to think that it's carburetion...and it's protected you from yourself.

    My guess is that you've got the air:fuel ratio just about perfect...once everything is up-to-temp & normalized. Until the intake & head are hot, fuel atomization is poor; that's why (IMHO) the throttle response issue goes away as the engine heats up. Wanna do a little quick testing? Drop the jet needle C-clip 2 grooves and twist the pilot airbleed screw 1/2-3/4 turn clockwise; that'll richen everything...too much to be sustainable, I expect. However, if throttle response improves, you'll have an answer. I still agree with OC, running the bike above a fast idle, with cold oil, is asking for trouble. These machines were notoriously cold-blooded, back in the day when they were the newest models in dealer showrooms. Thorough warmup is just part of the deal...
     
  6. cjpayne

    cjpayne Well-Known Member

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    I think OC is right too. These old bikes don't have EFI.lol. We have gotten spoiled over the years and just don't think about it anymore.
     
  7. whereshaldo

    whereshaldo Member

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    That definitely did the trick. A bit of warming up has it running like a well oiled clock. The engine sounds much better with the heavier oil and smokes much less. For those of you running 20w50 what do you change to in winter? I may be one of the few on this list who rides year round, rain, ice and shine, so I may just have to experiment on my own but understanding the warming up issues helps understand what the bike needs a bit more.

    Hal
     
  8. cjpayne

    cjpayne Well-Known Member

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    I use Valvoline MC wet clutch 10w40 pretty much all year round. I live in NEA Oklahoma. For breaking in rebuilds and exceptionally hot weather, I use the 20w50. I can actually feel the difference between the two. The 20w50 stuff does take a little more time to warm up.
     
  9. racerx

    racerx Administrator
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    Sounds like a motor in need of a rebuild...

    I select oil based on peak operating temperature. The compromise, obviously, is the lengthy warmup needed for 20W50...which is the closest you can get to the OEM-spec straight 30W. I'd not go thinner than 10W40. The manufacturer spec'd the thinner oil for ambient temps fall below 60F. They knew what they were doing.

    For your type of usage, an oil thermometer would be a good investment. You're running the engine at close to peak output for a high percentage of its total mileage. It's close to amazing that these machines survive a on steady diet of this. Imagine what would happen if you pushed a car engine this hard, i.e. 80-100% of peak output(!).
     
  10. whereshaldo

    whereshaldo Member

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    I thought I'd give a general update about where I'm at with this project. I've made a couple of different changes and overall the bike is running as good as I'd ever hoped.

    First, I switched from conventional to full synthetic oil. I'm currently running Lucas 10w30 and its made a huge overall difference. Most notably the exhaust is cleaner and the amount of oil I'm burning has really dropped.

    Second, with the slightly hotter cam I installed I've had to play with the jetting of the carb a bit. It turns out someone had already upped the main jet from an 88 to a 90, so when I put in the new cam I only went up to a 92. Overall the bike ran fine but I was having issues with the bike sputtering off idle when it was cold, and also at high speed (30+) when I let off the throttle, the bike almost felt like it was dragging -- there just wasn't enough fuel to maintain the deceleration of the engine.

    I just increased to a 94 and all of the above symptoms have gone away. The bike warms up much more quickly and idles much better cold. On deceleration it just feels much smoother and the bike does not feel like its being dragged down. Additionally, my ability to hold speed on grades and even accelerate up grades is much much better. I've even been able to lean out the mixture screw a bit and see a change in the idle speed.

    The only issue i'm now having is that the bike will occasionally die coming off idle. It an odd symptom, its not the coughing/sputtering i used to have but instead it is a hollow sounding pop that happens as soon as I turn the throttle just a couple degrees of rotation.

    Overall, i'm happy with the upgrade, i can now cruise at 35 with throttle to spare and i can hold 30 mph on a long gentle grade.

    Hal
     
  11. racerx

    racerx Administrator
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    A lean mixture, on deceleration, may be annoying but it won't hurt anything. With electronic fuel injection the ECM is mapped to deliver this, it cleans the combustion chamber. I could recognize "tuned" car engines just by the sound they make on closed-throttle deceleration. With a carburetor, this kind of precise fuel control isn't possible, it's luck-of-the-draw. And, most commonly, popping on deceleration means that the exhaust port gasket was leaking.

    As for the off-idle stumbling, that's a transient lean condition. A number of parameters can affect the functional air:fuel ratio. When you go from cruising speed, to idle, then sit at a light, temperatures throughout the induction system change, as does fuel flow through the carburetor and fuel atomization. I get the same thing with my bike...different engine & carb, same principle. The fuel flow gets "lazy" due to low airflow velocity at idle rpm. The only options are a smaller carb (not gonna fly), adjusting the pilot airbleed to make the idle mixture richer, increase idle speed. I've gotten used to just goosing the throttle a little bit when stuck at a long traffic signal. That's sufficient to keep the fuel circuits flowing and the fuel atomized & in suspension...rather than condensing inside the intake & intake port.

    FWIW, a hot intake can make a big difference. If possible, the intake should have the phenolic spacer/insulator placed at the carburetor end...only a gasket between the head & intake.
     
  12. whereshaldo

    whereshaldo Member

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    I've got the spacer in the right place and yes I'm just keeping the throttle slightly revved to keep it running. I've also filed a cross-hatch into the top of the idle set screw so that I can easily turn the set screw with my thumb to adjust for varying conditions as the bike warms up.

    So it sounds like I'm in good shape then. What I find most interesting is that the more I tinker and tweak the more I come to appreciate just how dialed in Honda had this bike. With the hotter cam and slightly taller gearing, I have a bike that is a good cruiser, but I definitely have given up torque off the line and especially torque starting off on a steep incline. Short of offering a different bike for different purposes, the stock setup is pretty close to maximized for the most possible scenarios of use.

    h
     
  13. racerx

    racerx Administrator
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    You've got that right! Can't even deviate too far from the stock gearing, without adding displacement, unless you have some very unusual riding conditions...like a swap meet, flat terrain everywhere...or better yet downhill with a tailwind everywhere. And that goes for road, offroad or dualsport riding.

    The next step up the performance ladder is a stroker crank. That's where torque, horsepower and gearing all change dramatically.
     
  14. whereshaldo

    whereshaldo Member

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    I keep struggling with this issue and I keep thinking about what you wrote here RacerX. I've experienced what you're talking about but my problem is not the same issue. This is not a gradual fuel laziness issue, its a pop and you're dead issue. What about ... vapor lock? The engine/carb is small enough that this could be a very intermittent issue simply because of the small thermal mass of the carb.

    I could try insulating the fuel lines or changing them out -- I've currently got 3/16" ID (8mm OD) blue polyurethane fuel hose and I've always been suspicious that its a little undersized based on how difficult it was to install the lines on the barbs. The fuel line is also much smaller OD than stock Honda fuel line. 3/16" translates to about 4.7mm and I'm guessing the original was 5mm ID and much larger OD. The 4.7mm equates to 90% of the original volume if the Honda lines were indeed 5mm ID. That extra .3mm ID would allow for greater fuel flow and the extra thickness of the hose would give more insulation.

    Hal

     
  15. racerx

    racerx Administrator
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    Actual fuel flow volume is a lot less than any fuel hose you could possibly install. Take a look at the diameter of the inlet seat, where the float needle contacts. That's more than adequate to surpass what the engine can inhale. Seriously, I can be rolling along at 55mph, have the carb bowl run dry, the engine cut out, and still have more than enough time to switch the petcock to "reserve" and pick right back up where things left off, without dropping below 40-45mph. That's with a much larger (VM22) carb, feeding 110cc.

    If your carb is getting hot enough to boil the fuel, the heat is coming from the head and is flowing by conduction. Even without a heat insulator between the carb & intake, while riding, latent heat of vaporization will chill both the intake & carb. Touch the carb body, if it's hot enough to do what you suspect, the high temp will let you know...instantly.

    IMHO, upsizing the fuel lines would be like installing a firehose nozzle...on a garden hose.
     

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