Fuel Starvation Issue and the Cold?

Discussion in 'General' started by whereshaldo, Dec 4, 2018.

  1. whereshaldo

    whereshaldo Member

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    Riding to work yesterday and today the temperature was in the low 30s. Overall the bike runs fine, but when riding at WOT the bike seems like it loses fuel and dies. I'll have to pull over and then it will start right back up and I can ride away immediately. At 80% throttle it runs just fine, this only happens at WOT. I'm going to check the fuel screen in the petcock to make sure its not clogged, but any other ideas? Its not really warm enough for carb icing, but I'm wondering if the cold air is keeping the bike from getting warmed up properly.

    Hal
     
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  3. Deoodles

    Deoodles Active Member

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    I agree with your thought. There is no all season tune unless you live in Florida or someplace like that. The cold air is denser and it does make the bike run differently. If you fix it don’t forget to put it back the way it is for the summer. Just how to fix it is not something I can help with. Someone will be along soon
     
  4. racerx

    racerx Administrator
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    You are, effectively, getting "carburetor icing". It's probably not actually icing, but intake temps are dropping low enough to create the same fuel atomization/vaporization problems that exist at cold-startup. The engine "acts lean" because, even though the carburetor is delivering the same amount of fuel (the A:F ratio, by volume, may be virtually unaffected), the amount that is actually being vaporized and burned is reduced.

    Consider why more fuel is needed for cold-startup and to keep an engine running, until it's up-to-temp. With the venturi, intake tube, and intake port all cold, liquid gasoline tends to remain liquid. While the combustion chamber & intake valve are also cold, only a small percentage...the most volatile fraction of the fuel...is being vaporized and it's gasoline vapor that burns. The rest passes through the engine, unburned waste. Thus, the engine can run lean on a pig-rich mixture...until it warms up. As each part of the combustion chamber and intake tract warm up, more fuel is atomized, vaporized, and burned, resulting in a functionally/chemically richer mixture which is why you have to open the choke. Now for the big variable, ambient air temp.

    Due to latent heat of vaporization, the intake tract is, effectively, a refrigeration unit. Liquid gasoline, exposed to vacuum & vaporized, cools the air/fuel column and everything with which it comes into contact. The effects of this can be: seen, by intake valve coloration; measured by intake tube & carburetor surface temps. In typical riding weather, with ambient temps well above 60F, surface temps remain high enough and far enough upstream, to reach a sustainable equilibrium. Ideally, intake temp should be above ~120F+. However, it can drop well below that without causing major problems. Fuel droplets will condense out of suspension, but only to a point...and...there'll be enough surface heat, downstream, to re-atomize, then vaporize, the liquefied fuel. That is, until ambient air temp intensifies the refrigeration effect enough to overcome engine heat and cool the entire intake tract too much. An IR thermometer can quickly show you what's going on. It's not really necessary. When you're getting the lean stumbles, the intake will be icy-cold to the touch. With my own bikes, the intakes can get cool enough to sweat, like a glass of iced tea, in 85F ambient air...under just the right conditions. Stopping at a red light, after rolling along at (relatively) high speeds for a few miles, the intake will be cool to the touch. That changes quickly, once at idle...due to "heat soak". And that's what you're getting once the engine is shutdown for a minute or three. I've verified this, using a wideband O2 sensor & meter. A:F ratios can go from the high 12s into the low 15s and in as little as a mile, in temps below 50F. A brief shutdown restores the A:F ratio...briefly.

    Best you can do is to fit an O-ringed intake tube (manifold) to maximize heat transfer from the head. Being a hardcore fair weather rider, I've not sought such an intake and don't know if one is available. A machine shop could easily cut a groove, in your intake, to fit an O-ring. You might also remove the phenolic insulator/spacer, at the carburetor end, for the cold season; that may require substituting an aluminum. spacer, for fitment.
     
  5. cjpayne

    cjpayne Well-Known Member

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    Have you ever noticed that alot of trucks have an apron over their grill during the winter? Its the same issue your having, IMO. Once on the highway, the engine just simply runs too cool and cannot atomize the fuel properly.
    I live in NEA Oklahoma and have ridden in all kinds of weather all year round. I have ridden when its 15F or colder before and told "Your crazy!" and never had this issue happen......but I never held the bikes WOT for very long at all, just getting up to the speed I want.
    If you must ride at WOT alot, in extreme cold, I would try putting a thick layer of header wrap around the intake or some type of insulation. Maybe some kind of wind blocker fastened to the engine guard?
     
  6. racerx

    racerx Administrator
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    There are some big, fundamental, differences between liquid & air-cooled engines, not to mention diesel vs gasoline. With truck engines, direct airflow is being restricted to the radiator.

    Blocking/restricting airflow over the engine, with no radiator would make less of a difference but, it might make enough difference. Fabbing a heat stove, around the headpipe, then ducting it to the airfilter would likely be more effective...but...is it practical?
     
  7. OLD CT

    OLD CT Well-Known Member

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    The intake is getting ''too cold''. I would put the biggest main jet it will rev out clean with. For now when it stalls, get a temp reading on the intake manifold with a pocket infrared. Harbor freight sells them. The temp reading will give you a indication as to what's going on. That is why some other little Hondas have a copper heat tube from the cylinder to the carb. S65 I believe, is one. You could also run a thin piece of copper tube from a heat source, ''the cylinder'' and tightly wrap the intake with it.
    Neat little trick if needed as a last ditch effort, if a re-jet doesn't work.
     
  8. Gary

    Gary Active Member

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    S65's had the clutch case tapped and an insulated oil line run up to the carb. The case half where we normally drill out when fitting a larger oil pump was blocked off. The hot oil circulated thru the carb then returned thru another pipe to the cylinder and then continued on it's normal way to the head.
    Over the years I have run alot of bikes,many times at below 0,never had trouble with carb freezing. I suspect it's the different fuels we now have,might be worth your time to try different brands,octanes or alcohol content
     
  9. OLD CT

    OLD CT Well-Known Member

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    It's 31 degrees out now.. There's no way in hell I would ride anything out there now over 30 mph. Screw, coffee and Coast the eye opener soap! You must be wide awake when you get to work!!!
     
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  10. racerx

    racerx Administrator
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    No need to tap the clutch cover to add oil tap & return fittings. Source an oil tap plate...as used with auxiliary oil coolers...for a 6V head.
    You could then wrap the intake with a coil of copper tubing, using short lengths of hose to join it to the tap plate fittings. Cometh the summertime, you'd have the option of installing an oil cooler...if desired/needed.

    I'll drink to that!
     
  11. cjpayne

    cjpayne Well-Known Member

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    Yes, I know, but was just trying to give an idea of whats going on.

    I was actually envisioning a thin board or something thick, like plastic, ziptied to the engine guard rails, just in front of the head. Never done anything like that before though.
     
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  12. OLD CT

    OLD CT Well-Known Member

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    That is a neat trick Bob. That's easier than re jetting in the freezing cold too. There aren't many places that sell the 6 volt style tap anymore. All the oil cooler setup's are for the 12v head. I remember wrapping my intake with a old sock one year, to keep the heat in the intake. I think it worked too!
     
  13. kirrbby

    kirrbby Well-Known Member

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    Stinky!
    :)
     
  14. whereshaldo

    whereshaldo Member

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    Yeah it was 27 today and the bike just would not go with any oomph at all. I could chuff along at half throttle but at speed it just bogs down. I need to get the CR6HSA plug put in to see if that helps. I have some velcro backed neoprene, so I was going to try to make an insulator that I can just velcro on to the carb and intake to keep in the heat that does soak up from the head.



    Also the front brake is frozen on -- i'm pretty sure its the cable but I was just too irritated this morning to dig into it. I think the over-mitt that I have on filled up with rain last week and got water into the brake cable housing which has now frozen. hahaha, this is fun.

    Hal
     
  15. racerx

    racerx Administrator
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    Changing to a different heat range plug isn't going to solve this problem. Spark plug heat range is a widely misunderstood term. The "Cliff notes" version: forget about going to a colder plug (it won't help) and don't even consider a hotter plug as it will make things worse. You're getting a functionally lean mixture, due the fuel condensing out of suspension, from the incoming airstream and those big droplets don't have enough time to vaporize inside the combustion chamber. Until you can warm the intake tract to at least the minimum temp where the engine will run normally, you're going to have fuel atomization/vaporization issues.

    Have you tried adjusting jet needle height? I'd drop the the C-clip to the lowest groove and see what happens. If that makes a substantial difference, then it'd be worth upsizing the main jet...until warm weather returns. If it corrects the problem, then you'd have a simple, seasonal, adjustment. Be advised, if/when the intake should warm up, even after a brief shutdown (with its attendant heat soak) the engine will run rich.
     
  16. Roy Chambers

    Roy Chambers Member

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    Did you try the simplest thing first? Sometimes It depends on not what the actual Temperature is, but what the Relevant Humidity is that day !!! I had a Mazda Truck that would ICE the Carburetor up on a 60 degree day.

    They way i use to deal with it was like this, if it was raining when I left, and i just started it up and left, it would cause Big problems the whole 26 miles home. And if i kept driving it, it never went away all the way home. You could even see ICE down in the Carburetor, if you were real quick in oping the hood and taking the top of the air filter. Mazda know of the problem, cause it had a Heating Element built into the Carburetor insulator from the factory. And I tried all kinds of things to fix it. None of them worked!

    They way i got completely around this problem was to do this simple thing!
    On Rainy days OR on high Humidity days, I just let the Truck completely warm up!
    If I did this, it never ICED up and drove just like it was a summer day!

    SO, try to let the Engine warm up a lot longer before you drive. It wont involve any work at all.
    Give it a try, and see what happens.

    Thanks Roy.
     
  17. whereshaldo

    whereshaldo Member

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    Today I finally had some free time to monkey around with things. Granted its a balmy 42 right now, but the issues were happening at this temperature too.

    In round 1 I switched from a CR7 to a CR6 plug. RacerX I know you say it wont help at all but the manual says to do it, and so I do it. I also topped up the oil, which should make it run cooler, but it should also help keep a warm engine warm.

    I drained the puke tube, which had a ton of water in it. I also lubed up the front brake cable, which had frozen solid last week -- makes the front brake WAY more responsive!. I also pulled off the petcock valve and confirmed that the screen was clear. Lastly I removed the phenolic spacer between the carb and the manifold to see if the carb would run warmer that way.

    The CR6 plug did help with the initial starting and warm up but I was still having the starvation issues on longer WOT runs, so that didn't fix it. At these temperatures I don't think there is enough thermal mass in the manifold to really heat up the carb with the spacer removed. The airflow through the intake and around the outside of the engine will have a much larger impact (at least on 4-5 mile runs).

    In round 2, I pulled the carb, took it apart, blew everything out with carb cleaner and put it all back together. The only change I made other than cleaning it out was to increase the jet from a 94 to a 98 and i replaced the carb spacer. With that change the bike ran fantastic -- and I don't just mean that it ran like it used to, but it actually had more pull and more power at the higher RPMs than ever before and also there were zero fuel starvation issues both pulling up a slight grade at WOT and going down a slight grade a WOT at about as high an RPM as I like.

    So jetting fixed it. I probably had it jetted a bit too lean all along so I may be able to get away with a 96 for the summer months, but my concern is when I put in the 88cc jug and piston that I'll need to increase my jetting even more. Sadly the set of jets I bought topped out at 98, so I may need to enlist a friend to help drill out an 80 to 100 or 102. I guess the other option is to move up to a bigger carb, but then I believe i start running into issues with the the exhaust and intake being undersized and potentially not being able to use the stock airbox.

    hal
     
  18. Roy Chambers

    Roy Chambers Member

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    What carb it that! That's a big Jet! I don't even see them over 90!
     
  19. racerx

    racerx Administrator
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    Now, you see why I find the rich limit, when jetting a new carb/engine combo. Without doing this, you're guessing.

    From here, you'll just have to see how things work out when ambient air temps drop back below freezing...and then when they return to summertime levels. You might have to go back to a smaller jet, if it runs overrich. Might be able to get by with a jet needle height adjustment.

    Don't jump to foregone conclusions about jetting the 88cc kit. Crude as they are, carburetors are remarkably self-adjusting, since they rely on pressure differentials (vacuum) to work. Increasing displacement will also increase vacuum, which may also richen the mixture. This is known as "pullover effect"...to those of us who having been working with carburetors since dinosaurs were still being distilled into crude oil.

    Be careful...and prepared... when overdrilling jets this small. It's beyond too easy to either a.) make them too big or/and b.) not get the internal orifice perfectly uniform & smooth, thereby ruining the flow characteristic(s). The size numbers don't always correspond to a specific, numeric, cross-sectional diameter. So...if you're going to drill main jets (don't even attempt this with pilot jets, they're too minuscule) it's best to have some spares to sacrifice, just in case something goes a little pear-shaped.

    Of course, you could source a VM18, or VM20 Mikuni...and have a huge selection of jet sizes.
     

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