Sputtering top end after porting and milling head?

mhornfeck72205

mhornfeck72205

New Member
#1
#1
Hi all. I've run into a problem that I cant quite figure out. Sorry for the long read but I hope you can help.

I've recently ported my engine head and I have milled the head by .015" and no doubt the flow has increased, but I seem to be getting the results of an extremely rich jet, but its only the stock jet size... I have even sized down from like .038" because it was only running slightly rich then. The spark plug comes out dark black and the engine used to top me out at 48mph but now I struggle to top 30mph. The engine sputters and the clutch jerks and engages/disengages because the engine isn't keeping a stable rpm. I can only get it to about 20mph before it gets rough.

The engine should be maxxing at around 6400 based on my speed and gearing. Heres all of my engine info:

clone GX200
flat top piston
arc rod
stock head milled .015" and ported
UNI filter
staight pipe
6 degree advance stock flywheel
 
novaracer

novaracer

New Member
#2
#2
i would say its lean. you are pumping alot more air and with that teh ve has changed and needs rejetted to what i have no idea i am going to try to hook up my wide band to mine in the morning. thats the only true way of telling you how lean or rich you are. :weld:
 
minidragbike

minidragbike

Supporting Speed Nut!
#3
#3
If you only milled the head the carb should be slightly rich. But it sounds like your lean for some reason. That normally happens when someone ports the head letting in more air, but your still runing the same old fuel ratio. Which makes it lean.
 
Oldschool

Oldschool

New Member
#4
#4
I just tried a 160 E tube in my 200 ( along with a drilled .021 pilot jet ). I'm run rich.....

These Chinee carbs just suck thats my theory....
 
125ccCrazy

125ccCrazy

Well-Known Member
#5
#5
get a tilly or mikuni

Here's a thought also.... Did you put the timing key in the right way?? if it's backwards you retarded the timing, that will decrease your RPM's and power and cause a rich condition as it's firing after the valves start to close..
 
125ccCrazy

125ccCrazy

Well-Known Member
#8
#8
125ccCrazy said:
get a tilly or mikuni

Here's a thought also.... Did you put the timing key in the right way?? if it's backwards you retarded the timing, that will decrease your RPM's and power and cause a rich condition as it's firing after the valves start to close..
Click to expand...
Scratch that, it will fire after the exh valve starts to open

I dont think that pz19 is gonna be enough fuel, your gonna want a 22 with the mods you have... I plan on running pz19's on my 3hp briggs...
 
Oldschool

Oldschool

New Member
#9
#9
Message - Old Mini Bikes Photo Gallery

Well the smallest is the PZ 19 largest is a JYUL 29 with something marked PCKF in between.....

I'm out of my zone here with this stuff. But I was kind of leaning towards the PCKF carb because it looked about half way between a 5/8 and 7/8 ventury Tilley.

I don't have any mods yet short of drilled jet and E tube swap in my all stock engine....

At this point I have started to hijack a thread so I should back away and let mhornfeck72205 continue on and hopefuly you guys can help him solve his issue.

I'm going to start one of my own to discuss these carbs and discuss this further.
 
mhornfeck72205

mhornfeck72205

New Member
#10
#10
Ok so I richened it back up to .038" and it runs fine and its back to normal but the plug is still coming out black and it seems too rich. If the plug blackened when it was lean, what would be causing that? Should I look into a better plug? Right now I am only running the stock one.

A new carb is likely in the future. The stock one is quite annoying and restricting.
 
Oldschool

Oldschool

New Member
#11
#11
Ya tell me about it....

Black means your rich or/and possibly running a cold plug.

You need to get the engine hot and run it hard to check your plugs. If you let it idle this changes everything, you may have it running good on the main jet but a rich idle could colouring the plugs and you loose what ever info you may have gathered from the test.

mhornfeck72205 said:
Ok so I richened it back up to .038" and it runs fine and its back to normal but the plug is still coming out black and it seems too rich. If the plug blackened when it was lean, what would be causing that? Should I look into a better plug? Right now I am only running the stock one.

A new carb is likely in the future. The stock one is quite annoying and restricting.
Click to expand...
 
novaracer

novaracer

New Member
#12
#12
could be the plug heat range and/or knock. a engine can knock with a a a/f near stoch 14.7 to 1 some time we need extra fuel to cool combustion chamber temps enough to prevent knock at mid to high rpm under load.:scooter:
 
TT

TT

#14
#14
Nobody has addressed fuel octane? Are you running 91 octane on a modified motor? Is the carb. correct for the increase of fuel and air mixture? Is the carb. too small to handle the increase?

I ran a 22 Mikuni on a stock engine 6.5 gx-200 with a 12.5 pilot and a 130 main. Now, if you have a dark plug and spuddering in mid range it's the pilot jet. If you have no top end it's the main. You cannot modify one thing without changing the other. More air and more fuel equals new carb. specifically made for the application. Listen good to MDB and get a Mikuni book and read it...

Good luck,

TT :thumbsup:
 
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