Weak 80 cc motor improvements + dellorto main jets

Discussion in 'Performance Mods' started by chrisnbush, Dec 15, 2009.

  1. chrisnbush

    chrisnbush Member

    I posted some of this on a related thread (search dellorto) but as it was getting long and a little off topic, thought I would put a summary here. I have seen a lot of posts about weak 80 cc engines. I have (had) one, that I got from DAX, the largest of the 3 sizes he sells. I won't get into the argument about what the displacement actually is, I believe it to be the largest available as DAX sells three sizes (starting with 49) and I don't think Duane would sell them as 3 different ones if they weren't.

    To recap: I have improved this "80" cc motor which was weak from Dax, by first grinding open the exhaust pipe entrance (on the pipe) to more closely match the engine port, and also the input side (which didn't make much of a difference).

    I improved it further by also drilling a couple of holes in the baffle, and bendingthe internal routing plate in the exhaust to make more of a straight shot for the exhaust. Louder, but improved power. Low end torque now takes me up any hills. BUT still not running as high as an RPM as other engines have for me.

    SO, as my plug is usually wet, even with the carb needle now as low as it can go (this is the "standard" NT carburetor I guess built for the 49cc engine if this is to be believed), I ordered a set of carb jet drills off of ebay, soldered my main jet, and attempted to drill it out to like 55 or something. Failure, as the drill chuck in this cheap set was ****, and I am not very patient and broke some drills.

    I purchased the 5mm Dellorto jets, got a 55, 60, and 65 - understanding that the stock jet is somewhere around 70 - 79. They came last night ! I am telling you the stock jet looks real close to 65 so now I am thinking that it might be 70 instead of 79 as I have heard through others.

    In ANY event, I put the 60 jet in, and there is quite a bit of difference. Still 2 strokes at higher RPM, but at least it goes to higher RPM - I can tell I will get a lot more speed out of it. And low end is even better than before, I declare this bike a winner now.

    So, I would recommend "matching" the exhaust pipe to the engine port, drilling some holes in the muffler baffle / or purchasing an expansion pipe, and REDUCING the main jet size for weak 80 cc engines (if the PLUG is wet after running at higher RPM for a couple of minutes). My somewhat lower cost approach.

    Just in case this helps


  2. give me vtec

    give me vtec Active Member

    I had a weak 80cc motor too but mine was a grubee skyhawk from spookytooth.

    Long story short...

    I got bored yesterday and took the head off a zoombicycles angle fire that I had laying around. Slapped it on the skyhawk, fired it up, and BAM instant torque... and I mean plenty of torque.

    I put a nos kit from boost bottles on it too and DOUBLE BAM!!!!. The motor made soo much torque the clutch started to slip. I am going to get a speedo for my felt now cause I think I am easily in the 50+ mph range.

    I will post some pics of the different heads and the bike if you are interested...


    for anyone interested in the nos... beware, a little goes a LONG way. I installed it on the intake tube and it didn't work well... so I installed it into the back of the air filter. The air filter worked much better. When engaging the nos trigger pull very very slow... too much will lean out your flow and either backfire, detonate, or maybe just stall your engine. Remember a little goes a LONG way.
    Last edited: Dec 15, 2009
  3. tskrem

    tskrem Member

    post some pics if u would give me vtec.
    i have an 80cc. i assume its week because it wont idle. but it does go fast ish if i really pedal. also what spedo do u have because the 1 i have doesnt work when i start the motor.
  4. AussieSteve

    AussieSteve Active Member

    Last edited: Dec 17, 2009
  5. give me vtec

    give me vtec Active Member

    here are some pics of the different heads I was talking about...

    Here is a top view side-by-side camparison... The high comp head is the one on the right with the spark plug slanted at a 45 degree angle. That is the one you want.

    Bottom view... high comp head on the right.

    Bottom view of the low comp head...

    Bottom view of the high comp head...

    I bolted that sucker on and it was easily the cheapest and most effective mod I have done yet. If you can find one, put this sucker on with just 1 head gasket and an expansion chamber from sick bikes.... I promise it will be all the power you will need.
  6. tskrem

    tskrem Member

    are they easy to take on and off? will i need to take my motor off my bike?
  7. give me vtec

    give me vtec Active Member

    nope... motor stays on the bike. Easy to swap out... its only the 4 bolts on the top of the head. Just make sure you only have 1 gasket between the barrel and the head.
  8. tskrem

    tskrem Member

  9. chrisnbush

    chrisnbush Member

    Some final observations on my swap to 0.60 mm carb jet on my 80cc DAX.

    The engine is "pulling" at higher RPM's. Before it would dog out when going fast up a slight hill - having no torque. Now it has plenty at higher RPMs.

    BUT if I floor it on a hill when going slow (a bigger hill), it dogs out. I can still do about the same hill as before if I keep my RPMS up coming into the hill, then NOT floor it, let it pull at midrange as it does best. If I keep the throttle there, it will slow quit a bit, but still about the same hill climbing ability (a little less) as with the 0.70 jet, which was stock in mine.

    My mpg has gone from 64 (which was just wrong) with the 0.70 jet, to 89 mpg with the 0.60 jet, and performance is way better over all. If I were to do anything further (which I won't as being a perfectionist with these things can kill you), I would try to nudge it up to a 0.65 jet to see what happens (as suggested by someone way above, guessing more low end but less high end torque. Since I now have 0.55, 0.60 and 0.65 jets I may try it anyway some time.

    To recap: I got a 80CC motor only from DAX, and installed it on my bike which had the kit for the DAX 65 cc installed on it (carb, muffler, tank, everything else). THE POWER AND SPEED MADE THE BIKE UNUSABLE WITHOUT THE FOLLOWING MODS:

    1. Put carb needle on its lowest setting (clip at highest ring), so that needle is lowest into needle jet. NO IMPROVMENT IN HIGH END.

    1. Drilled 2 additional 1/4" or so holes in muffler baffle (the part that comes out), and cut off the tube from the baffle - that inserts into the main exhaust. Also, bent the internal plates which route the exhaust to make it more of a straight shot. BIG IMPROVEMENT IN LOW END TORQUE BUT MORE NOISE - NOT TOO BAD THOUGH. A GOOD BIT BETTER, BUT STILL DOGS OUT AT HIGH RPM

    2. Ground out exhaust pipe, the end that attaches to the motor, to more closely match the diameter of the port on the side of the engine. NOTICEABLE IMPROVEMENT IN LOW END TORQUE STILL DOGS OUT AT HIGH RPM. AT THIS POINT STILL AROUND 64 MPG.


    The big take home I got from this is that I don't think the "true" 80 cc, i.e. the "3rd size up" was ever optimized for input ad output - they just threw it in with the same 49/65 cc stuff, at least as obtained from DAX. My 80cc as it came stock installed on my bike was a lot less engine than the original 65cc ! Unless the vendor is truely selling something different on these, like a "high flow" exhaust, if you buy these kits you will need to modify them, at least to realize the true difference from the "65cc".

    Got 900 miles on it now, this was the engine I cryotempered 3 days at -109F. Shooting to get over 2500 - my personal best. Although my MPG is much better, I don't know if even at the 20:1 ratio I am using, if I am getting sufficient lubrication.


    Last edited: Dec 25, 2009
  10. chrisnbush

    chrisnbush Member

    And YES I want one of those slant heads for 80cc (with the 8mm stud holes). Zoom sells them, but they havent been in stock for like 2 months. Do you know of a different source ?
  11. give me vtec

    give me vtec Active Member

    nope... I didnt even know about them until I chance swapped it out cause I was bored one day. It was off a zoom engine though, so I would assume that that might be the only place to source them from.
  12. sportscarpat

    sportscarpat New Member

    Head Swap

    Hey Vtec,
    I just tried the same slant head swap onto a Grubee. I used the BGF 80cc slant head off of ebay and bolted it to my 80cc Grubee. Noticed a couple things right off. The slant head has larger stud holes in it, the O.D. of the head fins are just a little smaller than the cylinder diameter of the Grubee, and finally the piston would just contact the head using the stock replacement Grubee head gasket. Looking at my new PK80J I just bought for another build I can see that engine has two head gaskets. I understand the current PK80J have the pop-up piston so the two gaskets are necessary. When I had my Grubee head off I could see that it also has a slight dome to the piston as well. Vtec, was your Skyhawk engine piston flat or a pop-up design? I may try two head gaskets to run the slant head or may elect to just deck the Grubee head. Currently this Grubee has an expansion chamber and CNS carb and runs really nice.
  13. give me vtec

    give me vtec Active Member

    my piston was flat... I did notice the holes were a little bigger on the slant head. You just have to feel around until you find the original groove in the gasket.

    I rand mine up and down the street first with no spark to see if the plug would hit... took the head off and there was no contact. I am currently running one of the thin gaskets from sick bikes, and the 8.8 head studs too (I highly recommend them).
    Last edited: Jan 8, 2010
  14. sportscarpat

    sportscarpat New Member

    Flat top, huh? That's why it worked on your set-up and not mine. I have not pulled my jug so do not know what it would take to upgrade the cylinder/head studs from 6mm to 8mm. Well, I know what it would take which is redrilling and the taping for the larger size, I just don't know if there is enough material around the existing holes to enlarge. I mentioned to a machinest friend about what I ran into. He is really into two strokes. He has a tool he made from the threaded end of a spark plug welded to a piece of round bar. The tool threads into the spark plug hole on a non-slant head. The round bar gets chucked up on a lathe so you can easilly spin the head and do a skim cut on the gasket surface to raise the compression. I hear 1mm or .040" cut is pretty standard. I will try a little less at first and see what kind of results I get.
  15. give me vtec

    give me vtec Active Member

    sorry... not 8mm studs. Grade 8.8 studs... sick bike parts sells them. Make sure you get the correct thread count. If you are using a grubee I think they use the 1.25 thread pitch, and the zoombikes engine used the 1.0 thread pitch.

    Stronger steel studs with good bolts/lock washers will allow you to really tighten the head down good for a better seal. The stock studs will just sheer off.

    And I am pretty sure my piston was flat... it might have been slightly domed but not much if any. The spark plug definitely clears... I took the head off and checked the piston and the plug for any signs of contact.
  16. John Ryan

    John Ryan New Member

    Just getting back to you. Yesterday, I mounted a chainsaw carburetor. I took it on a 30 mile run. Runs GREAT!! I tried the stock NT the so called improved CNS. They are garbage! (as you know). Chainsaw carb should come stock with these kits. My bike looks sweet too. 1936 Shelby frame, Monark forks, copper fuel tanks. I'm usein a iPhone to do this, I have no PC. I can only Email pics. I'm dyin to show my machine off.