Correct way to port match a chinese 80cc engine

Discussion in 'Intake & Exhaust' started by micargicycles, Jan 6, 2016.

  1. micargicycles

    micargicycles New Member

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    I have finally busted out the bits for doing port work. Never done this before but looking to get at least 30mph out of a Micargi fat tire im building. I did my best to port match the intake and also make the exhaust a bit bigger. Not much just a bit. So does this look correct? Thanks for any advice. Oh and i will be adding one of these racing carbs to it.

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    Here is the pic of the bike im trying to get 30mph from.

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  2. bairdco

    bairdco a guy who makes cool bikes

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    Make sure there are no sharp edges in the cylinder. Chamfering the edges a bit is the way to go.
     
  3. sbest

    sbest Member

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    Like Bairdco says, chamfer the edges where the ports go into the cylinder so there are no sharp edges in the cylinder for the rings to catch on.
    My 66cc Grubee is still mostly stock with a few little clean up tuning tricks and is on a fairly heavy Mountain Bike with 2.3" tires. I weigh 210 lbs buck naked and dry. Currently my top speed with a 44t sprocket is 35-37mph with no special parts and no mods you couldn't do with sandpaper.

    I do one modification at a time so I know if that mod is an improvement or a detriment. I have a long history with modifying engines, but not particularly piston port engines so I am going slow to learn what works and what doesn't.

    One mod that didn't work for me was port matching the intake. I would guess that it is due to reverse pulses going up the intake tract. A large port mismatch (cylinder larger than manifold) helps to block these pulses. I got better speed and power with the mismatched intake.

    Other items I see from your pictures, square up the top of the ports into the cylinder a bit. Not completely square, but more than you have. Round cylinder windows are good for ring longevity and noise reduction but a squarish port gives a strong exhaust pulse and clears larger volumes of gasses. Here is a drawing of an ideal exhaust port. Ignore the two small ports on either, they were for another engine:
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    The slight curves on top and bottom are to spare the rings. The intake port doesn't touch the rings at the bottom and it can be absolutely square if you want.

    Larger windows into the cylinder is good, but larger cross-section of the port is not. The cross sectional area should be fairly consistent with the carb size and gently ramp up or down from the carb to achieve increasing or decreasing velocity. This is the big problem with reed-boxes, a huge volme/area increase that causes fuel to drop out of suspension. Here are some pictures of spacers and epoxy stuffing I used to solve this problem on reed valved engines:
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    Steve
     
    #3 sbest, Jan 7, 2016
    Last edited: Jan 12, 2016
  4. micargicycles

    micargicycles New Member

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    Im 240lbs and i seem to get a top speed of 23 to 25mph out of these with a 44T sprocket. I started a full time business building and selling motorized bikes from home and online. I keep having customers asking about port work but i don't know much about the internal workings of these small engines. Some have said or claim to have reached over 50mph with porting work but i cant seem to gain even 5mph more. Can this be done just reworking the intake and exhaust ports?
     
  5. sbest

    sbest Member

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    As well as my new Grubee GT5A, I have an older 66cc engine, its ports were worse than the new bike in most respects. More casting and plating slag in the ports, and conservative port timing. It had less top speed and hill climbing power than the newer motor. As well, its crankshaft balance was worse too, so that over 25mph it vibrated terribly.

    New, the Grubee GT5A was capable of 20-25mph (35-40kph) as it came out of the box and had no undue vibrations in this range. The speeds picked up as it it broke in through the first couple tanks of fuel to 25-30mph (40-50kph), I also started using less oil in the fuel mix too, which seemed to help, 20:1 to 40:1. I might recommend using 32:1 from some things I've seen now. I used varying power and rpm to break in the engine, using full throttle about 20% of the time in 10-30 second bursts in the first 30 miles.

    After break-in I started doing some hill climbs to the limit of what the engine could stand and heard a "popcorn popping sound" rattle coming from the engine. This was detonation and eventually it blew a headgasket and likely warped the head. The squish area of the head was poorly shaped, so I sanded the head flat, and reshaped the squish area to match the piston with sandpaper on the head of a piston, and set the gap at 1mm (0.038"). This was a huge benefit, stopped all detonation rattle and overheating of the engine.

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    All this was with the fuel mixture decidedly rich. With the head improved, I leaned out the carb by lowering the float level and dropping the needle and got a much more lively engine and speeds in the 50-59kph (30-35mph) range. Port matching the exhaust pipe helped broaden the torque, but port matching the intake made things worse so I went back to a stock intake Z-offset manifold. This is likely because the mismatch blocks reversion pulses. At this point I am running 54-59kph quite reliably, rarely and barely breaking 60kph.

    I pulled my cylinder to try reshaping the transfers, only to discover the stock filter had let dirt past and scored my cylinder. This is when I increased my oil to 32:1 and gained some speed and power. I tried a better foam in the filter but it reduced speed and power. I tried with no filter box and was able to break 60kph (37mph) even with the scored cylinder.

    My life history, sorry for the book, hope it helps. Speeds are Garmin 76 GPS supplied on a level stretch of private road and can vary as much as 4kph from day to day. Tire pressure, lube, brake drag, temperature, humidity and wind can affect the speeds so I try to do any testing all in the same day, within an hour of each other if I can. It feels like 60kph is the limit for rpm of the engine, probably limited by carb, port timing, crank balance, exhaust pipe and the 44t sprocket. The bike is a heavy framed mountain bike with 26"x2.3" tires.

    Hope there is something in here that helps.

    Steve

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    #5 sbest, Jan 11, 2016
    Last edited: Jan 11, 2016
  6. Kioshk

    Kioshk Active Member

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    Cool build.

    Would you please take a picture from the rear detailing the drive-chain alignment? I'm interested in building one of these fat-tire bikes, and am curious about what type of clearance you can get.

    Suggestion: get rid of the hardware you used for the rear-mount...ESPECIALLY the rubber. Make sure that connection is flush, and solid. Are you having pedal-clearance issue? It'll also reduce your chain-length. I see you have two master-links in there too. Tried that already? On that note...another minor suggestion: make sure the master-link's clip's fork is pointing AWAY from the direction of chain-travel. This reduces this chance of the clip getting snagged and popping-off.
     
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  7. 2door

    2door Moderator
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    I have to agree with Kioshk's assesment of the rear engine mount. Get rid of the rubber.
    If you're concerned about scratching the paint you can wrap a couple of layers of vinyl electrical tape around the frame but the rubber needs to go.

    Also keep a close eye on that chain tensioner bracket. They are notorious for loosening and rotating into the spokes. That will ruin your wheel and your day.

    I also concur with Steve when he says, "Do one engine mod at a time". You'll have a basis of comparison with each step and it won't be hard to pinpoint a problem if one comes up.
    On a more positive note your bike is looking good. Best of luck with your project.

    Tom
     
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  8. Motakitty

    Motakitty Member

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    I'm curious how did the engine react to the port matched intake? How did it run?I think I'm having similar results.
     
  9. bairdco

    bairdco a guy who makes cool bikes

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    Micargi, you'll never get 50mph with just port matching, or larger ports.

    You should get an increase of about +/- 5mph, or if you got a "good" engine, a little more.

    Gearing is one of the simplest ways to get a speed increase, provided your engine runs smooth, and the weight is kept down (the bike and the rider.)

    A stock, but tuned, port matched motor can run a 40t with no problems, usually, and even a 36t, again, depending on weight. The lower the gear, the more you'll sacrifice in hills and take off speeds, but your top end will increase in the straights.

    50mph is attainable with the right combinations of heads, carbs, expansion chambers, gearing, weight, tuning (and money, patience, trial and error, frustration, grenaded motors, snapped chains, cracked frames, blown clutches, more frustration, and more money.)
     
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  10. buba

    buba Member

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    well put Bairdco-- DTM running fast --- often 45-55 mph Los Angeles every
    Sunday/funday rides

    All Welcome at the Clubhouse-
    Sunday afternoons block and a half west of Arlington on 48th Street

    a whole lot of wrenching going on - gaskets pistons cylinder bearings magnetos cdis etc.
    wouldn't have it any other way--LOTS OF FUN
     
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  11. archtop67

    archtop67 New Member

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    Steve can you update your pics? im guessing you were on the photobucket scam they pulled on everyone.. seems 80% of pics on the net are down unless posters pay 400 bucks to them...
     
  12. Eric Ross

    Eric Ross New Member

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    ya could you up date the pic please
     
  13. Lukorice

    Lukorice New Member

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    Are these images still available? im unable to view them
     

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