Tons of compression

Discussion in 'Performance Mods' started by Help, Sep 4, 2016.

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  1. Help

    Help Guest

    After adding a third tranfer port, arrow reed valve with keihin carburetor, reed piston mod, decked for a .70mm squish band, sickbike expansion chamber, a 6cc fred head and a jaguar cdi my compression is around 180 (so my compression tester reads). How do i lower my compression without compromising my squish band. I have crazy torque and with my sickbike shifter kit my top end is nice too. I don't want to lose any power. Any suggestions?

  2. Steve Best

    Steve Best Well-Known Member

    Open the combustion chamber bowl up, add another base gasket, or add another head gasket. Any or all. This is the ideal order to the solution but try another base gasket first. Cheap and quick.

    Up to 2mm squish will still keep much of its effect. Many stock engines run 2.5mm squish.
  3. butre

    butre Well-Known Member

    180 is nothing crazy, I've personally run 200+ with the only ill effect being the cost of higher octane fuel. you've already got the jag cdi, I would suggest using a colder spark plug with a smaller gap and more oil than you think you need to keep the chrome plating happy.

    you could also cut out the transfer walls to cut down crankcase compression a bit, with the side effect of giving you more top end power but sacrificing low end, that'll drop your dynamic compression ratio a surprising amount. a .1 decrease in crankcase compression ratio will equate to about a 15 PSI drop in compression assuming all my assumptions about which china girl you're running are correct
    Last edited: Sep 6, 2016
  4. jaguar

    jaguar Well-Known Member

    squish clearance isn't the same on all sizes of engines because the final number that matters is squish velocity. Larger engines have more area being squished and so need larger squish clearances to maintain the same correct squish velocity.
    You can read more on the subject and download my free squish velocity calculator at
  5. Frankenstein

    Frankenstein Well-Known Member

    Hey more about cutting the transfer port walls out, I was asked to work on a 47cc motor recently, it was for a pit bike of some sort, I was tearing it down and saw it had no transfer port walls. Thought it was an interesting look. Couldn't figure why they did that other than simplicity.

    If I were to cut the transfer walls out, but backfill the ports with jb weld to bring the compression back up would that change things or better or worse? Using a Reed valve too, also note the other motor comes with a Reed as well, and connects to the crank case, doesn't use the cylinder or piston to time or control intake, also doesn't expose raw fuel and air to a preheat like our cylinders can tend to do. Only useful feature that seems to have on our motor is the heat being transferred from the intake to the carb is an automatic carb defroster for those winter months.
  6. jaguar

    jaguar Well-Known Member

    you'd want to not reduce the width of the transfer channel. don't use JBWeld. no difference will be felt by the small decrease in crankcase compression.
    without that wall there is a zone of "interference" near the side edges of the piston which limits that width by air pressure/turbulence.
    I don't think it's a good modification.
  7. butre

    butre Well-Known Member

    there's no reason to cut the transfer walls out if you're just going to narrow up the ports again. transfer walls are good, but if you need more transfer volume in a small area (or simpler production) then removing them isn't the end of the world.
  8. Frankenstein

    Frankenstein Well-Known Member

    Interesting. Thank you.

    Still searching my head on cast iron cylinder vs a pro coated nikasil coating.

    Only I also find threads mentioning finely balanced cranks, perfect mods and types of oil brining about motors that last them a year and a half without too much issue.

    Course my luck tends to sway the opposite? Do everything to the T without error and everything goes to hell. Just slap some wd-40 on that bitch and throw a bit of dirt on it and what do ya know, it works like new. Wtf.

    Seriously explain.
  9. Frankenstein

    Frankenstein Well-Known Member

    I had main bearings fail in under 48 hours twice in a row, 3 different times. Yes this is magic but wait there's more!

    Hey replace every seal and gasket with brand new bona-fide true-blue gaskets and seals and it just doesn't do sh*t. Use a cereal box or old notebook for a gasket, and put a torn up burnt seal under that mag and what do ya know, it runs faster than that guy that sang that song about that guy who's name was Lola. Unless he was into that kinda thing then bad example, you get the point but wait there's even more!

    Disect that motor and install any part on any other motor and guess what it runs?!? Yet no single part new installed to the old motor makes it run.?

    Peanut brittle.
  10. Steve Best

    Steve Best Well-Known Member

    "Well, I'm not dumb but I can't understand
    Why she walk like a woman and talk like a man
    Oh my Lola, lo lo lo lo Lola, lo lo lo lo Lola"

    Whew, Frank, kinda scary thought for a Sunday morning.
    Just the kinda thought that keeps a single guy drinking responsibly!
    Never know what ya might drag home if you don't keep your wits about you...

    Cast Iron vs Nikasil:
    Lived with them both, in 4-strokes and 2 strokes. My Blasters are both actually, one with a KTM engine.
    Anyone living with cast iron is counting the over-bores. Counting down until none are left. Always looking for the next size piston.
    Last oversize is the end of life...
    Cheap pistons, oil and air cleaner bring it all the sooner, as do bearing and ring failures, quickly wiping out overbore re-life chances.
    Original Nikasil is thin, like our chrome flashing on our China Girls. It is what you get with any bought Nikasil cylinder.
    Rebuilder Nikasil is layers of plating, tough as nails. Will stand surprising abuse, like blown piston skirts, but not bearing bits.
    If you had a China Girl cylinder that was a wonder of porting power, a good Nikasil plating would keep it forever.
    Good oil and air cleaner, you could throw a $9 piston in it and $30 of rings every 5 years or 20,000 miles.
    No kidding.

    Everything to a "T" and it still fails?
    My paying job is to install complicated production machinery and get it to run as fast as possible, as long as possible.
    All things have a life cycle, all failures have a reason. Most are predictable if you know the factors involved.
    When things fail unexpectedly, there is a learning opportunity. Why?
    If you are not honest with yourself about the reasons, you will never learn a thing, and will repeat the mistakes.
    "Cheap Chinese metal" is a cop-out. Bolts strip or break because you overloaded them. Chrome comes off for a reason.
    Everything to a "T" implies impeccable quality control as opposed to "assemble and hope".

    Quality Control:
    I did a 2 year college course on this topic, could talk at length about any minute detail, but in the end it gets down to a balance of quality vs cost. How much do you want to pay for? So when you build your own engine are you doing the quality control? The inspection? The blueprinting? The quality testing?
    Did you:
    Dial the crank in to check for run-out or misalignment?
    Check for fit on the bearings and install them without putting the load across the rolling elements?
    Check for proper bearing side loads and clearance after assembly?
    Leak test the whole assembly with pressure to test sealing?
    Do a plug chop to assure proper mixture?

    Usually not, because most of us want that low $$$ value point but forget it comes with an increasing failure cost.

    My thoughts for a Sunday morning...
    zippinaround likes this.
  11. Frankenstein

    Frankenstein Well-Known Member

    Dial the crank in to check for run-out or misalignment?
    Crank was true and straight as a high end arrow.

    Check for fit on the bearings and install them without putting the load across the rolling elements?
    Yes, fit really well actually, with the satisfying snap, I went as far as using poor man's liquid nitrogen and a bit of non-direct heat to slip them (crank on bearing) together.

    Check for proper bearing side loads and clearance after assembly?
    Uh... No? But it was one of those by feel things, like after torquing the case together with a torque wrench at 25lbs (what I read somewhere) the crank had no play, was centered, was smooth and turned over without any resistance. I did drill holes in it prior, so it was better balanced.

    Leak test the whole assembly with pressure to test sealing?
    No. However the seals and gaskets were brand new soaked with 2 stroke oil (where applicable) so they made a good seal, this is the method I use always, no problem. Gave up on rtv, messy and doesn't protect the core of the gasket, if anything it's a crutch. Heavy dose of oil keeps the water clear of the gasket, which in an engine hot enough to boil water you might want to keep steam from forming in a gasket and destroying parts of it. Water in a gasket also freezes in the winter months. Oil suffers neither bad quality. Sorry got off subject. New head gasket, installed properly as well.

    Do a plug chop to assure proper mixture?
    Yes, easy as pie when you have a spark plug wrench and a dozen clean sparkplugs to work with.

    The engine had been running previously, no problems, only formed a small crank seal leak. Replaced it, saw my cylinder was getting a little dull, replaced and ported for a full cylinder fill from the transfers (offset the angle out slightly on one side, this results in less turbulence on the down stroke and creates more of a wall that pushes the old stuff out rather than a wet fart sorta push) , towards the back and up a bit you know blablabla. Corrected fill for third transfer (boost) port turbulence. Reed valve cleaned, checked reeds for imperfections, double checked assembly for problems. Motor ran before no problems, so not like I had a hole in the side of my crankcase.

    Did not change really anything... Oh wait I did! I pulled out my old bearings, they were getting rough, I think the heat on one side was making the mag seal leak. I also replaced my crankcase gasket with real ones, it was originally a checker board box, basically identical to a cereal box. It came off the engine without hesitation except at the clutch area, but it came off in one piece regardless. Still downstairs on a glass sheet I use for gasket prep, Preserved in oil.

    So tell me why after deliberate hard-assed assembly and quality control techniques I end up with a motor that either eats bearings or just runs like sh*t. Meanwhile just prior the engine was a genuine half assed project but still ran better with a leak and bit up main bearings!

    So the motor went to the junk box, hey somebody pulls the cylinder and Reed assembly it makes their engine run beautifully... Put that new piston and the crank into an old shell of another motor and it runs well enough to be worth a hundred bucks. Used the seals on a friend's bike and no problem. Leaves a crankcase with no found imperfections, and a clutch assembly with new pads in it. Mind boggling..

    I did not change my muffler set up, I shouldn't of have to, I'm using an identical motor, with the same set up as the other, with my original muffler, and it runs fine.

    I went as far as picking through the grease in the clutch spring area for signs of a leak into that, nothing. As of this time I do not have a tool for pressurizing the motor for a leak check, I do have access to a person who does. For checking that, what pressure do you think I should check to, and as far as the intake manifold goes, just tape it up? Would it be strange to put soapy water on the motor for additional leak checking during the test? Thing's covered in a brown film anyway, think a bit of dawn would be beneficial for it anyhow.

    Seriously why me...
  12. Steve Best

    Steve Best Well-Known Member

    Ha-ha! Well Frank, you are... ...special... :)
    I'm joking of course. I have black holes in my life too.

    A rubber cork from a beer and wine making store will seal the intake, a plate and gasket the exhaust. Drill the plate for a tubeless tire Schrader valve. You can put gauge line on a nipple in the cork or drill the plate for it. Use a bicycle pump to pump up to 5-7psi (no more!) and listen for leaks. See if the gauge holds. If it doesn't, use the soapy water to find the leak.

    Carefully examine the bad bearing, even cutting it in half to see the races. Where is the wear (as it were!)? This will tell you if it was installation, side load, misalignment or lube that killed the bearing and you could solve it.
  13. jaguar

    jaguar Well-Known Member

    don't forget that an unbalanced crank puts huge stress on the crank bearings, increasing greatly with more RPM.
    that's why no one should raise their exhaust port without first balancing the crank.
    rings are either made for use on iron sleeves or plated cylinders.
    If the rings these engines come with are truly made for use with plating then it's not a good idea to sleeve the cylinder with iron.
  14. gary55

    gary55 Well-Known Member

    Just checking to see if my thinking is right. I read where Jag had gone over how to balance a crank by drilling 2 - 3/8" holes in crank weights to the depth of the weights width. I am guessing this is for older model engines. The engines I am getting now have two holes approx. 3/8" in each weight already in them. Does additional material still need to be removed, or have manufacturers addressed this issue? I would like to add that the GT80 engines that are being advertised as balanced cranks have identical cranks with identical holes as all the other engines I have recently ordered.
  15. jaguar

    jaguar Well-Known Member

  16. Frankenstein

    Frankenstein Well-Known Member

    I have a zae50 rod, or at least I think I do. Has the same holes you tried to photograph in it. I noticed that my motor had a lot of additional vibration after I switched pistons, the original had the window cut out for the reed intake, I cut the entire piston skirt down and off, on my new one the piston has much more material left, I even left a small bit behind along the bottom, see pic.

    Obviously I should do my new new piston the same as the old, and balance the crank a bit more. I'll do a bit more math work and play with jags calculator more.

    Steve, I was thinking the same thing for a set up, only more complicated, I was going to gut (or make an effort at) a spark plug. I probably wouldn't have given up for a few days then decided I'd do it your way if I didn't know about it already.

    I'm thinking this port job over carefully right now, I have a reed valve but the only opening it has is about the size of a dime, that's kinda restricted but I guess not much more than the carb intake manifold, guess the intake side on the cylinder only needs enough room for a valve end. Anything else is surplus.

    I took a photo, think I found the main issue I was having, lean conditions. I bought this ported, but it appears the person who ported it went OD on it. The helicoil installed (by a third party) had its threads tapped right up to the edge of the intake. 2016-09-18 17.15.10.jpg
    The red arrow shows the part of the missing intake wall, the green box is a hairline crack formed in the wall, might be hard to see but it's there, also those helicoil threads are like at the bottom of the hole, way too deep, don't do it like this people.

    Sideways picture, bottom is old cylinder, look at that port job, some people might say it's beautiful. Nice and big, no restriction on flow in the least. Top pic is new cylinder which is a work in progress. Look at all the space between the intake and the stud thread. It won't change much. The larger port is probably twice the size as the intake manifold by surface area. Removing that much material won't magically make the manifold or the reeds any less restrictive. It will lower crank case compression if that was the goal. Will make sealing the intake harder too.

    The bottom cylinder was machined down on the intake face a bit, maybe 3mmish. That could explain the vertical over dosage of porting, because the reeds would rest a bit too low and get stuck shut. The left to right porting makes no sense.

    Also they did a deep port job for the exhaust, the hole is much larger than any exhaust manifold I've seen. This just creates a larger wall for escaping gasses to fight out, and more turbulence in that area. The exhaust port inside the cylinder was a wopping 35mm wide! The outside was only 25 wide. That means the exhaust port was cone shaped, don't know if that's bad or what, still seems a bit over done. Exhaust was ported very high too, guess this was designed for speed, but I wanted low rpm power, not high rpm speeds.

    The transfer ports are eh... Kinda large in my opinion, and they don't look like they would do exactly what I'd want as far as aiming the new fuel. I would have tried to compensate for the large exhaust by pushing the aim back further, a little less high, and offset the transfer angles to help them compete with the weird angle of the exhaust, it would fix the pressure difference on the sharp angle, this is what it basically looks like port wise, left side is intake.

    Illustration of fuel into the cylinder, green, orange is boost.

    This is sorta showing how the good fuel escapes. The low pressure (blue) sucks more than the high (red), the pressure difference is due to that sharp edge, long story short the transfer port does nothing to compensate for the lower pressure, so it looses gas by letting it out too near the exhaust.

    This image would show an attempt to work the new fuel away from the exhaust port, this is commonly done with jb weld, I don't like jb weld like that but it's sometimes my only choice. Otherwise I'd just go with a cylinder with too much material and work it out like that. The pink is something like jb weld. It would fix a major loss of fuel.

    I will probably leave the intake and exhaust as they are for the most part. The ports are more evenly set, I could remove more material on the hard edged side to help the gas escape a bit more evenly.

    Attached Files:

  17. Frankenstein

    Frankenstein Well-Known Member

    I guess the moral of the long ass story is : if you want it right, do it yourself
  18. jaguar

    jaguar Well-Known Member

    who was the beserko that ported it?
    the widest the exhaust port should be is 70% of the cylinder bore.
    yours is 74%. that wears the rings down at the exhaust side quicker.
  19. Frankenstein

    Frankenstein Well-Known Member

    Yeah tell that to the cylinder which is probably missing as much chrome on the exhaust side as the rings were. Look at these.

    20160918_194400.jpg 20160918_194325.jpg

    Oh and jag, I have a zaf80 crank, any specs on that?

    Attached Files:

  20. Frankenstein

    Frankenstein Well-Known Member

    More modern marvels.. This picture shows 2 pistons, both have a MNT raised lettering inside the skirt, both from the same seller, both are the same height hole, commonly called an A piston.

    The left image has a raised A, the second, B

    You see what I deal with?!?,..!