New (hopefully) observation on weak HT 80cc motor - anyone seen this ?

Discussion in '2-Stroke Engines' started by chrisnbush, Sep 24, 2009.

  1. chrisnbush

    chrisnbush Member

    I have a new weak motor I have been diagnosing. Runs OK on the flat but DIES on hills, yada yada, you have heard it all before...

    I noticed something I hadn't seen before - when I run the motor without the air cleaner on, and hold my hand over where the air goes in (not blocking it) I feel air+gas mixture coming OUT of the carb.

    Maybe this is normal for a two stroke ? I have been wondering about these crop of 80CCs, I am using this motor with all equipment I had for my previous "65cc".

    Now I am thinking maybe I have too much back pressure from the exhaust. This is what I have done THIS time working on this motor ->

    1. Cleaned gas filter (the one on the petcock)
    2. Tried different carb needle settings (richer and leaner, actually left it one leaner as it seems to run a little better)
    3. Sealed intake tube, replaced tube gasket with silicone gasket cement
    4. Tried different plugs, tried moving gap back to about 0.04 - approx. the way they come (I gap them down to .16 or something as per the forum)
    5. Checked exhaust for blockage, appears to be clear.
    6. Changed cable settings on throttle at top of carb to pull more slide - thinking maybe I wasn't opening it up enough
    7. On DAX rec, tried 1/32 gas mix - this is a new motor and I have been breaking it in at 1/20
    8. Cleaning air filter, leaving it off

    The only thing that seemed to improve performance a little was to run the dang thing with the air cleaner off of it. I have noted oil in the bottom of the air cleaner sponge, which now I am pretty sure is due to a certain amount of backpressure out of the carb as I describe above.

    Tonight I am going to run it with the muffler baffle out. I am sure this will increase performance, so after I test this, I am going to remove air cleaner cover again, and see if I can feel this backpressure with my hand again. If not, I am going to consider an expansion carb, or drilling a hole somewhere in the muffler.

    I have been lead to believe that any 80cc motor is the same as a 65cc, and all the carb, pipes, etc. are completely swappable. Guess I am wondering if this is true, as spookytooth makes a carb that is "optimized" for 80cc. Anyone else seen anything like this back pressure, or is it a normal thing anyway ?

    Gotta love em...


  2. machiasmort

    machiasmort Active Member

    I've heard that the 80's run rich w/air filiter on, no-matter what needle setting!

    A tighter plug gap gives a hotter spark and better combustion.
  3. arceeguy

    arceeguy Active Member

    Port inducted two strokes will spit a little mix out the intake, totally normal.

    If you pull the air cleaner off of a weed trimmer, you'll see the same behavior.
  4. motorpsycho

    motorpsycho Active Member

    this is what causes a boost bottle to work. the excess oil/gas that comes out of the back of the carb, is forced into the boost bottler under pressure.
    this residual fuel/oil/air mixture is sucked back into the carb. under pressure and this fills the cylinder more and causes a "boost" effect.

    the same thing occurs on the exhaust will loose some raw fuel out of the exhaust. this is where an expansion chamber comes into play. it's the same idea as the boost bottle, except sound waves force the unburned fuel/oil/air mix back into the cylinder on the intake stroke...packing the cylinder fuller for more power. it's kind of like a supercharger effect. packing or forcing more fuel /air into the cylinder gives more power. on these motors you might see 1/2 h.p. gain (if that) with an expansion chamber.
    but alot of that depends on the way the pipe has been tuned. it could be tunde for all low end power, mid/upper range power or all top end power. it depends on the length of the pipe that comes out of the cylinder and leads to the expansion chamber part of the pipe.

    TREEWK Member

    Will A Intake Reed Valve Set Up Help?? Ron
  6. chrisnbush

    chrisnbush Member

    Well last night, I ran it without the baffle. Loud and tons of power. Then I took the air cleaner cover off again - still spitback from the carb (thanks arceeguY), but maybe not as much.

    I went back to the forum looking for ways to reduce backpressure without making too much noise - don't want to attract too much attention if you know what I mean. Someday I will get the tuned expansion from SBP, I think this is the total answer. I liked the "beer can" exhaust with the steel wool idea, but too much work, so I just cut the tube thing off the baffle with a hacksaw - the piece that extends back into the muffler.

    Now I am getting like 20% more power, it is almost back to what this thing SHOULD be with 25 miles on it (these pick up quite a bit at 200 or so miles in my experience). The muffler makes more of a pop sound, but really hardly anymore volume, just doesn't sound as smooth.

    You know, the one size fits all idea doesn't fit these range of 50-"80cc" motors. I don't think you can really use the same stock exhaust on the 80, the one I am using was from my previous 65. I know spookytooth makes a carb optimized for the 80, might be worth it - and an expansion pipe in any event.

    I don't know much about these motors, but finally figuring out that pressure at the intake and pressure at the exhaust is REALLY important on how these things run. Something that seems stupid, like a mildly clogged air cleaner (e.g. with spitback oil), or carbon buildup (not even close to blockage) on the baffle tube can have a large effect on performance. So I will be getting tuned exhaust and improved carb situaition at some point.

    Thanks for your help

  7. juliman

    juliman Member

    i do that on my dirtbike some times lol and your hand acts like a choke
  8. Fabian

    Fabian Well-Known Member

    It's normal for a piston ported two stroke engine to spit air/fuel mixture out the exhaust due to it's design.

    A reed valve setup is a much better system.
    When the crankcase pressure starts to go positive from a decending piston, the reed valves close, sealing more of the gas mixture in the crankcase; increasing the velocity when the transfer ports open, better atomising the mixture as it enters the cylinder.

    You get more power from greater gas quantity stored in the crankcase and more power through better air/fuel atomisation.

  9. chrisnbush

    chrisnbush Member

    Well the power has come on this engine now, actuallly I have about 600 miles on it now.

    I made another hole in the exhaust baffle, and bent the internal muffler routing "plates" to open it up a little. I am right on the edge of being too noisy, cops haven't pulled me over yet.

    Real happy on hills, I think I could climb anything. 4 strokes on straightaways. In detail, when I am climbing a moderate hill, I can back off the throttle till it 2 strokes again. If I floor it, it just 4 strokes. My take on it is that if my engine speed goes above a certain rpm it starts to 4 stroke. Since I am getting only 64 mpg (!) I don' think I have any intake issue, I think I can improve things by tuned exhauset.

    By the way, don't remember if I posted above, but I ground out the exhaust pipe intake to "match" the port on the cylinder which improved things quite a bit. I also ground out the intake side, but that didn't do much and I had to mess with a gasket to get it to match, ended up with the rubber gasket stuff (right stuff).

    I am looking at two options right now: 1. SBP tuned exhaust that gets a lot of thumbs up reviews. 2. Spooky tooth Grube expansion chamber + either CNS carb, or at least the one "designed" for 66/80 cc (Grube again). Looks like SBP has a lot of pieces to it, stuff to fall off. I put a lot of miles on these things and I am looking for durability in addition to performance.

    Again by the way, this is the engine I cryotempered (well SORT of crytempered, see my other thread). So 600 miles and counting, my goal is to beat 2500 - my record with this type of engine.

    My mileage count will slow down once I get my replacment Titan from DAX, that is my preferred commuting ride (rear mount chain drive). Compared to centermount, sucks on hills, but smooth on the straigtaways and certainly faster, at least until I get a better handle on this tuning situation.

    Hey, maybe I will try my homemade boost bottle again on the centermount in its current configuration (uh oh)...

  10. Ghost0

    Ghost0 Guest

    Your 4 stroking at WOT is usually a sign of a rich condition caused by too large of a jet. This is why when you back off the throttle slightly the pin on the slide partially blocks the flow of fuel and corrects your mixture.
  11. chrisnbush

    chrisnbush Member

    Huh, thanks. I have the clip as high up on the carb needle as it will go which I understand to be as lean as possible. I have noted that the gap was a little wet, not white and brown as desired I think. Again was thinking to go to 66/80 cc grubee carb, but wouldn't it be even richer ? I think ive seen threads on making the jet smaller somehow, is this easy ?

    Thanks again
  12. Ghost0

    Ghost0 Guest

    Adjusting the needle has the most effect at low throttle settings and does next to nothing at WOT. The jet size controls fuel flow at WOT. You can get different size jets from Dellorto, they are 5mm and the stock hole size is .7 mm and I would recommend a .65 mm. You can rejet yourself if you have a soldering iron and the correct set of bits.
  13. khillr48

    khillr48 New Member

    Carb fog

    The fuel fog in front of the carb is usually caused from the lack of scavenging of the exhaust. I've even seen it on 4-strokes. An expansion chamber will help both power and scavenging. But it's a fine line between torque and top end. The header pipe from the engine to the chamber will tune the power. A long header pipe makes torque and a short pipe makes top end. But the exhaust is where it's at.
    Good luck, let us know how the tuning goes.
  14. andynogo

    andynogo New Member

    I've got a 66/80 motor and that was 4 stroking constantly so I've had to make the jet smaller- down to slightly less than .5mm! That's with a proper air filter, expansion chamber exhaust, slight porting- mostly cleanup.

    Runs beautifully now and can fine tune it with the needle. It cruises at 50-55km/h and I've seen 61- with standard sproket on the back. I don't top speed it very often though as it is pulling some revs!

    Yours certainly sounds like it's running very rich. Get some jet drills off ebay or use fine wires if you have it. Solder up the jet and re-drill it smaller. You can also use mig welder tip cleaners to change the size of the jet orifice.
  15. AussieSteve

    AussieSteve Active Member

    Dellorto 5mm jets fit these carbs. I think you can buy direct from them. I'm not sure of the exact range, but I think that a #50, #65 and #76 are available. (0.5, 0.65 and 0.76 mm)
  16. andynogo

    andynogo New Member

    Buy jets? Went round four or five shops of different types and all I got was blank looks! Typical Perth response. Usually followed by "Yeah, well, I spose I could ask someone over East...but it'll take a couple of weeks..."

    Took me three minutes with a soldering iron and some solder...
  17. AussieSteve

    AussieSteve Active Member

    That's what the internet is for. For a range of sizes - again not sure of the range, try:-
    (You'll need to contact Tony, they're not listed on the site, but he says that he has them.)

    Also, if you want a larger jet, 0.76mm:-
    (These are listed on the spares page)

    As mentioned, I think that Dellorto sell them directly from their website, too.

    The problem with soldering and drilling is that you can't swap back to the old one or a different size without re-soldering and drilling. Also, not everyone has a range of drill bits in these sizes. They're harder to get than Dellorto jets. If you have a couple of ready-made jets, no dramas, especially if mods are ongoing and more re-jetting will be needed.
    (Jets cost about $5 each, no big deal.)

    ... Steve
  18. chrisnbush

    chrisnbush Member

    The Dellorto guy got back to me - the part number for the jet is 1486 (5mm), the orifice size range from .3 mm on up, by 0.01 so they have every thing. 3.28 pounds, which is about 5 bucks, but I would want to get a range of sizes I think. Wonder what shipping is.

    I am going to get some jet drills, but want to look at the jet first - take it out of the carb. My experience with soldering is electrical, and I don't think my iron is going to get that piece hot enough to melt the solder on it. I can drip solder into it, but that won't stick.

    Wondering if I should use pipe solder and if so if I need rosin core - I have some. Seems like not a problem to solder them, could just reheat and clean them out, right ?
  19. chrisnbush

    chrisnbush Member

    BUT you know as much as I like to mess around with these things, wondering if this "CNS" (whatever that means) carb that spookytooth isn't maybe a better answer in the big picture.

    You can "tune" your fuel + air mix with these, right ? Anyone have experience with a racing carburetor, which I assume as a group allow you to do this ?

    I have been using my centermount for basic transportation for 2 years now. I swap out motors when they go (got 2500 on one), maintain my rear wheel (spookytooth) but will swap it out when it goes. Tank and carbs are pretty stable, along with sprocket and my light setup. My frame is old schwinn cruiser, close to solid heavy steel - this is the base.

    If I get a tunable carb, then I can swap in pretty much any motor - 49, 66, 80 whatever and tune it appropriately. I have a couple of rebuilt motors in the wings I can drop in this way, and don't want to limit myself to any particular displacement. I am working to have them all setup with mounting studs, clamps, and carb input tube.

    I figure I could get the SBP tuned exhaust, and this would work pretty much for any motor, although I might have to have a supply of the header tubes as I understand the lenght of these has an effect on the tuning related to the RPM of the engine.

    I also want to get a supply of these slant heads with higher compression, both 49 and 80 cc so I can just swap this in for whatever I purchase. I like DAX as a engine supplier, he stands behind what he sells, but he doesn't have slant heads.

    Using these things for basic transportation I am trying to minimize my down time. If I come home broken, I want to be riding to work the next day.

    I guess I am kind of crazy about this ****. I have a lot of ideas - was thinking about rigging a tube extension for the exhaust so I can run it under my coat to keep me warm (it is cold in NH this time of year). And maybe some screws + bolts in the tires...

  20. mauicruzer

    mauicruzer New Member

    Thanks for all the info.

    Good to reading!!