Jincheng dax 50 to 70cc woes

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Jc70dax
Posts: 6
Joined: Fri May 25, 2018 9:12 am
Rides:: Jincheng dax 50, gsxr 750 slingshot

Jincheng dax 50 to 70cc woes

Post by Jc70dax » Fri May 25, 2018 1:57 pm

Hi all, i have recently bought a dax copy and in my quest for a bit more umph i have also bought a complete 70cc top end kit with head off that popular auction site (brand new) to fit a crf50, however after i fitted it, it ran like a complete dog and was backfiring through the inlet manifold, i tried bigger jets, moving the needle, double and triple checked the valve clearances/cam timing but still no joy..... it would run with the choke on but i still feel theres more to it than fueling. Heres a couple of reasons why i think its sumat else.
1. The original 50cc piston is longer in the body with a pronounced dome on top of the piston that protrudes out of the barrel slightly with the head off.
2. The 70cc piston is shorter in length and doesnt have as much of a pronouced dome on top and also doesnt appear to travel all the way to the top of the barrel, (maybe about 4mm short from memory)

Is my 50cc engine a copy of a 6v honda in regards to piston/barrel assembly?
If so would i require a piston from a 6v c70 to remedy the issue or am i barking up the wrong tree and overlooking something alot simpler?
Looking forward to any replies as im stumped
Regards
Tom

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steveST100
Posts: 1551
Joined: Tue Oct 25, 2011 3:04 pm
Rides:: 1983 C50LAC 1976 ST100 1980 ST70
Location: Clitheroe Lancs

Re: Jincheng dax 50 to 70cc woes

Post by steveST100 » Fri May 25, 2018 3:03 pm

Hi Tom.
If you can post up some photos of the pistons and heads.
Old school builds on the JC50 engines was to fit 85cc(51mm)pistons and barrels most running the 50 head but some also running 70 heads.At that time you could get 70 pistons that ran with the 50 head for both 6v and 12v types.
This is one for the 6v SS50 to run with the 50's head but a 70 bore other piston is a stock 6v C70/ST70.
Image

Your JC50 engine should have had a piston like this(my old M50 engine) which is more like a 6v than the later 12v Honda/pitbike pistons.
Image
And the head on the 50 will have a bigger dome than later 12v types.
Image

With you saying the piston you have now is not coming to the top of the barrel is sounds like you need on with the right crown hight.
This is a 6v(back) and 12v(front) 70 on the same pin to show the difference.
Image

Hope that answers a few of your problems.

steve g

Jc70dax
Posts: 6
Joined: Fri May 25, 2018 9:12 am
Rides:: Jincheng dax 50, gsxr 750 slingshot

Re: Jincheng dax 50 to 70cc woes

Post by Jc70dax » Fri May 25, 2018 8:23 pm

It does indeed Steve, made things much clearer, i cant post any comparison pics as i have put the engine back together as a 50 for the time being but i could post a pic of the 70 piston in the barrel at roughly where tdc was when fitted to my engine (if that made sense lol)
If it is the case that i need a 6v 70 piston, where exactly could i get one from that wont cost an arm and a leg that will fit my barrel?
How do i post pictures which arnt to big? Appologies in advance if this is covered elsewhere on the forum.
Cheers
Tom

Jc70dax
Posts: 6
Joined: Fri May 25, 2018 9:12 am
Rides:: Jincheng dax 50, gsxr 750 slingshot

Re: Jincheng dax 50 to 70cc woes

Post by Jc70dax » Fri May 25, 2018 9:38 pm

Thinking about it now though, i wouldnt be able to use a 6v piston would i? As the 70cc head i have is a 12v style so surely it would hit? Shame i couldnt get a 12v style piston but with the right crown height. Or a 6v 50 big bore piston with the reduced shoulder.....
Bloody complicated things :lol:
Couldnt i have 2mm or watever the distance is, skimmed off the top of my 70cc 12v barrel to give me the correct squish or is that a no no also???
Sorry to bombard you with questions but im stumped lol
Just tried to post pics of my 70cc head/barrel/piston combo but it says file to large and i dont know how to resize images on my phone :oops:

Jc70dax
Posts: 6
Joined: Fri May 25, 2018 9:12 am
Rides:: Jincheng dax 50, gsxr 750 slingshot

Re: Jincheng dax 50 to 70cc woes

Post by Jc70dax » Thu May 31, 2018 10:51 pm

Sorted it now bud, bought a reproduction 6v piston for about 25 quid then set about the crown with a double cut bastard file and a sanding wheel in the dremel followed by a good polish, also had a play in the combustion chambre with the dremel to remove the sharp edge next to the bevel (minimal amouts of material took off both piston and head as piston barely kissed the combustiin chamber in stand form).
Runs quite well if im honest, definate inprovment over the 50cc in any case,.
Cheers
Tom

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