Got A Little 4 Strokin' Goin' On

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MM7

New Member
Jan 2, 2010
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I want pick everyone's brain if I may.

I have a china girl build that consists of:

1) one used late70's early 80's Huffy 21 inch frame (from garage sale)
2) custom built 26 inch wheels with 13 guage spokes on Alexis rims.
a)front wheel has Sturmey-Archer Drum brake
b)rear wheel is 7-speed SRAM cassette -no brake on rear.
3) Steel lined cylinder 48.7 cc China Girl from e-bay
4) Sick Bike's jackshaft kit.
5) One of Dax's muffler/exhaust. (will upgrade to expansion chamber in a month or so)
6) 6 months educating myself in the basics of 2 strokes. A Graham Bell's & Jenning's Handbooks.
7) Carb is the slim-line type from a pocketbike with the jets filled and bored out w/ #72 drill-bit.(bike seemed to run best @ #72)
8) Fuel is my own mix 70% premium gasoline + 30% Toluene using Mobil 1 synthetic blend 2 stroke oil @ 25-30:1 {China-Girl seems to like this blend best.}

I am still getting some 4 stroking going on in the last 1/4 of throttle travel & I am a little worried about reducing the main jet any further. I may have to go with a small Mikuni carb so I can adjust the needle size and such eventually.

Can anyone see what I am overlooking? I have a sneaky feeling the answer is right in front of me. I will be glad to have any comments/ideas/suggestions 'cause it just don't seem right to only run smooth for parts of the throttle travel. Oh well.

Thanks Everyone!!
Matt:-||
 
Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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My first question is 'how many gallons of gas have you run through the engine?' It should keep running better & better the more it breaks in... (Even up to 5 gallons.)

My second question is 'have you tried running it with just gas & oil?' (without the toluene.)
 
bairdco

bairdco

a guy who makes cool bikes
Aug 18, 2009
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i've solved the four stroking at WOT problem on a few bikes just by changing to a good quality petcock. (after trying everything else.)


my theory is that the stock kit "dribbler" can't fill the float bowl fast enough at WOT and starves the engine for fuel.

my theory is unproven, but my results are, at least to me.

besides, whaddaya got to lose? the stock petcocks are junk anyways.
 
Goat Herder

Goat Herder

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Apr 28, 2008
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bairdco said:
i've solved the four stroking at WOT problem on a few bikes just by changing to a good quality petcock. (after trying everything else.)


my theory is that the stock kit "dribbler" can't fill the float bowl fast enough at WOT and starves the engine for fuel.

my theory is unproven, but my results are, at least to me.

besides, whaddaya got to lose? the stock petcocks are junk anyways.
Click to expand...
4 stroking is when the motor is running too rich. :p
 
Goat Herder

Goat Herder

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Apr 28, 2008
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bairdco said:
you just like to contradict me, don'tcha?

read this, smarty pants...;)

http://www.mzrc-cornwall.org.uk/mod/resource/view.php?id=15
Click to expand...
Its fun sumtimes:)

From the going consensus then the term 4 stroke is a silly useless term and should be replaced with do a plug chop. This will be the only way anyone will be able to know what they got. http://motorbicycling.com/f3/plug-picture-lean-rich-24382.html

Most folks always say it comes predominantly from running too rich. How ever any thing that can make the motor cut out can now be solemnly called 4 stroking now? So the term has lost much of its diagnostic meaning..

4 stroking - Google Search

When I had fuel starvation I just eventually reached a cut out point where the motor died. Let off my throttle for a brief second and it all cam back until I got to the same R.P.M.

Then there are those suddle changes in the exhaust pipes. But that would be Scavenging.
 
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scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
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Man it's the craziest thing......everytime I start my bike engine up, it starts 4-stroking right away. I can seem to make it stop.....1/4 throttle, 1/2 throttle, WOT......all it does is 4-stroke.

Do ya think it has anything to do with being a 4-stroke? My 2-strokes rarely 4-stroked but my 4-strokes do it all the time, is that normal?









laffrotfldance1

sorry, i've gotta sick sense of humor sometimes....
 
scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
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Just lean it out baird, it'll seem twice as fast......

My 2-strokes 4-stroked when running "fat"......when running "lean" they run mean. When starving for fuel, they crapped out. Ya run too lean however.......yup, they fry! Don't run too lean.
 
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Goat Herder

Goat Herder

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Apr 28, 2008
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scotto- said:
Do not lean off the throttle while leaning over the bars.....unless one likes to plant faces:D
Click to expand...
:D agreed..

Norman said:
I went and checked on my metric drill sizes for the 70cc main jet mine was drilled at 0.70mm and was a little to rich I soldered it shut and drilled it for 0.65mm which is working good this was done on a Dax 70 cc my other china girls are 0.065 some that I've looked at are slopply drilled and could use a solder shut and redrill my elevation here is3200 ft above sea level and the temp is in the 90s so air density is like running at a higher elevation so a guy will need maybe need a slightly smaller jet during the summer.
Best to check with the plug nice light brown is in the ballpark.
Ok for you guys who want it in american measurements
0.70mm= 0.026"
0.65mm=0.025"
Hope this will help
Click to expand...
The OP and Norm may be at similar altitudes? Both being from Texas.
http://motorbicycling.com/f13/ht-main-jet-size-1353.html
More comparisons here for the OP:)
china jet size - Google Search
 
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wayne z

wayne z

Active Member
Dec 5, 2010
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Hey Matt, Don't change carbs to have more needle adjustment. The needle controls mid range rpm .

The main jet controls mostly 3/4 to full throttle.

My 66cc ran like yours before I soldered up and re-drilled.

I bought a set of torch tip cleaning bits from NaPA.

Soldered up thr jet and back drilled it some first with a much larger bit , then drilled it with the smallest drill in the kit .018"


That was too lean. Drilled it with the next size, .023 and it worked perfect after fine adjusting with the needle clip. Just barely get a little 4-stroking now at WOT.

Not sure but I think .023 is close to #66 or 68 jet size?

Don't be afraid to solder it up and try smaller. If it's too lean, it'll be immediatly noticable. Easy to drill it larger again.
 
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M

MM7

New Member
Jan 2, 2010
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Texas
Goat Herder, Scotto, et.al. Glad to see we got a few giggles out of this..laff

However to respond to Wayne, If I read what you're saying correctly, it sounds like I need to re-solder and drill smaller. (which in my twist drill set is to use a #72 or #73 or # 74.) larger numbers-> smaller holes.
What you said about jets controlling the last 1/3 to 1/4 of throttle travel fits; I think? Now you got me wondering just exactly what the bike is doing and I am going to have to go for another longer ride to conduct a little more research. Damn, I ride that thing almost everyday and I thought I could describe it for another person so they could understand. That's what I get for getting old!

Matt
 
scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
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Ridin' inSane Diego, CA.
Matt, might it be that running tolulene in your gasoline is causing it 4-stroke? And I'm really curious as to why you run that much.....to keep the combustion chamber and exhaust port cleaner?? It sounds to me that you still need to jet down.....it's running too fat. Let us know how it goes.
 
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