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post #1 of 45 (permalink) Old 09-02-2019 Thread Starter
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engine problems

hey guys,


Currently I have a 49cc cag with a big bore kit, ported carby and ive done the petcock mod and what I did was after I ported the carby I installed I think it was a .67 jet and the bike was running good, still had a small flat spot when taking off. I went and purchased some cf reeds and now after I installed them the bike is still bogging down when you give it throttle and cuts out when you go 1/2 throttle.


I accidentally mixed up my jets so im not sure what is the .64 or .67 as I still have some .70 .75 jets in the packs. I checked my plug and it appears its a black so its running a bit rich unless my bike doesn't like the cf reeds I put on it ? or the carby I ported may be the best ?


Any tips on what to do ?
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post #2 of 45 (permalink) Old 09-02-2019
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Re: engine problems

Welcome to the site.....

Sounds to me you have a lot of stuff to re-go over...……..Also if you can post pics of how you modded the carb and tell us the process in which you did it because when doing so theres a right way and a wrong way...…….Its hard to do it the right way and easy to muck up...…

I taperport my carbs by hand with a very sharp pocketknife and some sandpaper.....I don't go ramming a 9/16" drillbit down the center and hope for the best like others do...…..




You should also go over the reed installation and put the tin reeds back in for starters...…...

The cf or fiberglass reeds should never be cranked down on when being set with the setscrews.....I know if the screws are just installed snuggly the reeds will have a tendency to loosen up if they aren't tightened down but the CF and the fiberglass reeds also get warped when they are...

CF or fiberglass reeds must be installed with the set screws set snug and with the aid of Loctite...I use slightly longer screws and install tiny nuts on the other side then I modify the slot on the intake manifold to fit the nuts if they don't fit......…..

Seeings you ported the carb you should need more fuel and thats a given...If you cant tell the jet sizes hold them up to the light or use a welding tip cleaner tool as a gauge..........

Check your carb float setting and the setting on the c-clip on the needle...It seems to me like the float could be set too rich..

When you installed the topend kit which way did you install the copper sealing ring?..It matters how its installed as it wont hold compression too long when installed wrong..

Did you use a base gasket under the cylinder and did you split the cases to install a full circle crank?...FCC cranks turn the engine into a shakerbox and offers zero extra performance….

Whats the specs of the kit....How many extra ports and what type headcap?...Take pics of it and post it...




What was your squish calculation after the build?...Its pretty important to know your squish setting also your compression ratio so you can ballpark the fuel type and the jetsize to start with...You dont get performance if it isn't built with-in performance specs....Although the topend kits seem like a plug n play deal;- they are and they aren't at the same time which means you just build a potluck engine and hope for the best if you do......

Did you do a leakdown test on the mill after assembly to see if the crankseals are good and theres no leaks anywhere else..?Did you change out the crankseals?

Did you install a new coil when you installed the new topend kit?...If not a new coil should be installed too.......Its a rule of thumb with small 2 stroke engines to replace the coil when doing a tune-up or changing out the topend….

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Last edited by CAM2; 09-02-2019 at 10:02 AM.
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post #3 of 45 (permalink) Old 09-02-2019 Thread Starter
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Re: engine problems

Hey mate thanks for your reply

I bought the cag with the headkit already on it so im not sure what the squish calculation is but it did run good for a while until i started bogging down then i left the bike to sit there and finally got around to it last weekend to repair it.

Ive attached a photo of my bike with the head kit. The way i did my carby if this is wright or wrong was i drilled it with a 14mm drill bit on both sides but i didnt go right through and i cant see any gaps between where the slides sits.

When i installed my cf reeds i didnt go crazy tightening them, only a little so they dont fall off inside the motor but ill put my stock tin ones back on and start from there.

When rejetting do i start from the largest jet and work my way down or from the smallest and work my way up ? ill change my carby to a non ported one and see if that makes a difference.
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post #4 of 45 (permalink) Old 09-02-2019
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Re: engine problems

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Originally Posted by johnni View Post
Hey mate thanks for your reply

I bought the cag with the headkit already on it so im not sure what the squish calculation is but it did run good for a while until i started bogging down then i left the bike to sit there and finally got around to it last weekend to repair it.

Ive attached a photo of my bike with the head kit. The way i did my carby if this is wright or wrong was i drilled it with a 14mm drill bit on both sides but i didnt go right through and i cant see any gaps between where the slides sits.

When i installed my cf reeds i didnt go crazy tightening them, only a little so they dont fall off inside the motor but ill put my stock tin ones back on and start from there.

When rejetting do i start from the largest jet and work my way down or from the smallest and work my way up ? ill change my carby to a non ported one and see if that makes a difference.
Usually when they sit the carby starts to leak out and fill the crankcase jug up which also fills the j-bend of the exhaust pipe up when trying to start it up causing the engine to run very rich until flogged out properly by goosing the throttle wide open for a minute or two or by manual means but seeings you stated you put the largest jet in to start with that also flogs the engine and the pipe............

On these small engines you usually only need to go up 1-2 jetsizes and adjust the c-clip on the needle...…On some builds the kits give such a good velocity the same stock jet works as the increased velocity pulls more fuel through the same jet even with a ported carb...…..

Going to the largest jet to begin with will never work with these small engines unless you know your build specs and know the engine will handle that much fuel like a 5 port cylinder with 19mm phbg carb tuned on methanol....

Start with a 70 mainjet and also go back to the tin reeds to start with and remove the exhaust pipe and sparkplug and de-fuel the j-bend and pull the rope to evacuate the jug of any excess fuel and put a brand new sparkplug in so you start off fresh....

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Last edited by CAM2; 09-02-2019 at 10:25 AM.
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post #5 of 45 (permalink) Old 09-02-2019 Thread Starter
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Re: engine problems

hopefully this week ill be modding the stock exhaust by adding in a extra belly and removing a bit of the straight pipe after the J so hopefully that can help. Does it sound like i ported my carby correctly or possibly i stuffed it up ?

Maybe ill do the exhaust mod first and return the stock reeds and carby and work my way up by doing one thing at a time.
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post #6 of 45 (permalink) Old 09-02-2019
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Re: engine problems

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hopefully this week ill be modding the stock exhaust by adding in a extra belly and removing a bit of the straight pipe after the J so hopefully that can help. Does it sound like i ported my carby correctly or possibly i stuffed it up ?

Maybe ill do the exhaust mod first and return the stock reeds and carby and work my way up by doing one thing at a time.
Pretty nice looking paintscheme you have on that bike......Its also one of the models with the better built frames too...…..

Judging by the pic of the bike I can see a pretty wet stain on the upper part of the pipe which indicates its possibly filled with fuel and oil mix....

I cant say if you did the carb right or not unless you tell us the process of how you did it and pics of how it looked when reassembled as when the carby is drilled out wrong creates airleaks on the sides of the barrel rendering the carb useless...

modded stock cag pipe vs a stock 39cc Polini 910 Carena watercooled pb exhaust pipe


In any event I prefer pumper carbs over the floater carbs hands down on these small engines....No re-jetting required....just twist a few screws and its always in tune....




Its not hard to figure your squish.....alls you need is thin 5/32" rosin core electrical solder and a digital caliper

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Last edited by CAM2; 09-02-2019 at 10:55 AM.
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post #7 of 45 (permalink) Old 09-02-2019 Thread Starter
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Do you have a link to the pumped carb as I'm considering purchasing one for my cag maybe
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post #8 of 45 (permalink) Old 09-03-2019
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look on ebay you need the one that comes with the plastic manifold

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post #9 of 45 (permalink) Old 09-03-2019 Thread Starter
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Is this the carby I'll need boys ?

https://www.ebay.com.au/itm/Racing-C...8AAOSwHRpZeYIB
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Re: engine problems

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Yep that's one of them......The aluminum kit costs 3 times as much...

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post #11 of 45 (permalink) Old 09-03-2019 Thread Starter
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Re: engine problems

Fantastic thank you for that.

ill return the stock tin reeds back on the bike and see if that makes a difference as im doing the exhaust mod this week. Ill try get around to measure the squish as well sometime this week but since I didn't install the top end kit and not sure if a new coil was installed, should I change the coil to another one ?
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Re: engine problems

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Originally Posted by johnni View Post
Fantastic thank you for that.

ill return the stock tin reeds back on the bike and see if that makes a difference as im doing the exhaust mod this week. Ill try get around to measure the squish as well sometime this week but since I didn't install the top end kit and not sure if a new coil was installed, should I change the coil to another one ?
I always do as not everyone does and you should too.......Its cheap insurance...........

Id check out the coil and flywheel....Seeings that bike looks like its been stored in a humid cubbyhole or a very dirty shed for 3-9 years theres gonna be rust on both and rust on a flywheel and coil thongs can cause bad running problems...........

A stock 1pc cylinder squish can vary between .065"sae and .058"sae......I custom grind my cylinder bases to get the squish between .040" and .035" for my 1pc cylinders which gets the compression around 185-195psi..

If you don't get a squishband of atleast .045"sae you aren't making much performance.....

Heres a vid of a 43cc 1pc cylinder I modded with a tight squish,aggressive porting and a 3rd boost port

https://flic.kr/p/24bMkE7


trophy cag modded exhaust 2 by Dizzy Gillespie, on Flickr






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Last edited by CAM2; 09-03-2019 at 09:18 PM.
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post #13 of 45 (permalink) Old 09-03-2019 Thread Starter
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Re: engine problems

if I end up buying a new flywheel and coil, are those lightened flywheels on ebay any good ? and will another stock coil do the job ?
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its best to mod the stock one
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Re: engine problems

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if I end up buying a new flywheel and coil, are those lightened flywheels on ebay any good ? and will another stock coil do the job ?
The lightened flywheels are too lightened for the performance you seek....

The stock coil will handle the job if it's up to par...not all Chinese crap works as intended..

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the china performance parts were never intended to be stacked if you have just the head kit or just the lightened flywheel you will be fine and notice some performance improvements but if you make the mistake of many YouTubers and try to open your wallet and spend your money on all the performance parts and Bolt the crap together you will be building a impossible to tune shaking pile of crap it may feel like it's making more power but it's just shaking all of it away...

you can make a lot more powerful engine spending your money on a Harbor Freight drill press and a Dremel...

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post #17 of 45 (permalink) Old 09-05-2019 Thread Starter
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This is what my Ignition coil and flywheel look like now as I pulled apart my motor, what do you think ?

My.poor bike I left outside under a cover for too long and finally got around to fixing it now.
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Last edited by johnni; 09-05-2019 at 07:26 AM.
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sand paper or steel wool + some quick creep penetrating oil and that will clean write up...

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Re: engine problems

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This is what my Ignition coil and flywheel look like now as I pulled apart my motor, what do you think ?

My.poor bike I left outside under a cover for too long and finally got around to fixing it now.
I never use steel wool as it gets very stuck to the magnets and is a pita to remove...

Looks like you better get some crank seals too as the flywheels side looks like there's a crack or a hole in it at the 6 o'clock position also the rust on the crank snout isn't good either and should be addressed when the seals are changed out.....

The seal size is 25mm x 15mm
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Last edited by CAM2; 09-05-2019 at 05:07 PM.
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post #20 of 45 (permalink) Old 09-05-2019 Thread Starter
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Do I do the same to clean up the contact points on the flywheel with some penetration oil and some sand paper ?

I didn't know I had a busted crank case seal, I'm guess I need to pull apart the whole engine or can I just take off one side to replace the seals?

Last edited by johnni; 09-05-2019 at 05:26 PM.
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