70cc Engine Tuning Advise Needed

May 15, 2016
14
0
0
Auckland, New Zealand
Hi,

I have brought about all the upgrades you can have for my 70cc engine (jaguar cdi, banana muffler, performance carby, iridium spark etc). So I am now looking to do some work on the engine. I was thinking of starting off by just cleaning up all the ports & polishing everything. But otherwise I am pretty stuck.

I have read everything on www.dragonfly75.com and heaps on here so I'm not too much of a novice but for this job I am!

What would I need to do to change the port timings to maximise power across the whole rev range? If someone could I would love to be told precisely how many mm to change my ports.

Any help would be very much appreciated.

Cheers.
 

Agreen

Member
Feb 10, 2013
790
10
18
Southeastern GA
Unfortunately there is no straight answer since all manufacturers of these engines do things differently. Browse through Gordon Jennings' book "2 stroke tuners handbook". It'll give you some info on port timing, degree openings, and how to properly shape the ports so you don't break a ring.

Port work first. Then do one external engine "bolt on" at a time. See what works, and what makes it worse. And good luck with the jaguar. I've heard it is hit or miss for performance. Again, because of the difference in manufacturers.
 

2door

Moderator
Staff member
Sep 15, 2008
16,325
114
63
Littleton, Colorado
First thing you should do is forget everything you read from that person you mentioned.
Now start from scratch and read some of the information available here in the 'High Performance' section. Oh, yeah, and throw that junk ignition you paid too much for in the trash where it belongs. Your stock CDI will perform better and it didn't cost you anything.

Tom
 
Aug 26, 2015
472
5
18
Overgaard AZ
+1 to Tom.

The guy that makes the jag (dragonfly) is kind of a hack. There's some good info on his site, but it's mixed with junk. Follow 2Door's advice, haunt the forums, lurk, read, ask questions. For all things motorbike, start here, stay here. I've seen for myself, regardless what the problem is, the folks here can diagnose and advise on repair.

Just remember, Garbage In, Garbage Out. If you give us good info, we can help, give bad info, get bad advise.
 
May 15, 2016
14
0
0
Auckland, New Zealand
Yo, Cheers for the answers guys. Tbh I'm pretty shocked to hear what you guys think of jaguar. & I'll be pretty gutted if the cdi is a dud. His site seems so well researched.

I just hope I haven't destroyed my engine with taking his advise.

Surely if his bike performs as he says it does, then he must be somekind of knowledgeable.

Oh well - you live & learn. I've been meaning to get that book. It just seemed to talk way out of my knowledge level = baffling.

Wish there were more guys with these bikes in New Zealand!
 
May 15, 2016
14
0
0
Auckland, New Zealand
Also the more I think about engine tuning the more confused I get.

I am stuck on this problem:

Why would anyone ever port an engine for low rev power (like motorcross)? Wouldn't it be better to port for higher revs then just have a larger rear sprocket which would amount to low end torque.
Then if you wanted to race you could switch to a smaller sprocket?

I live in a hilly a **** area & am always pedal assisting up hills.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,586
382
83
Los Angeles, CA.
My best advice about porting is to NOT mess with the heights of the ports right beside the piston!!
(That changes the port timing & you can lose power where you needed it).

You can get a lot of power from making the intake & exhaust ports exactly match the exhaust pipe & intake manifold... & then if you want to go a step further you can make the transfer ports (inside the engine from the engine case to the cylinder) match each other so the air flows smoothly over the seams.
Just be very careful when re-installing the cylinder back over the piston because the ends of the piston rings have little locating pins in the ring grooves of the piston to keep the ends of the rings away from the ports.
 
Last edited:
May 15, 2016
14
0
0
Auckland, New Zealand
Yo, Cheers man. When I was doing some simple casting cleanups on my engine this weekend I had noticed that there was a bunch of metal covering some of the gasket on my intake - I made a note to clean it out at some point.

I'll do some research on port matching.

Thanks for the reply. :>