Chronic 4-stroking

[PeterG]

Engine: Grubee SkyHawk GT5A (66cc)

From brand-new this engine has been chronically 4-stroking despite all efforts to solve the problem.

Here's a time-line of what I've tried and the results and symptoms:
All stock, out of the box. NT carb/black plastic air filter. Fuel mix (25:1 to start then 33:1) - 4-stroking much of the time, occasionally breaks into two-stroking and shows good power. If you are careful with the throttle you can often ride the two-stroking right to red-line. Backing off on the throttle will cause 4-stroking again. Going up a slight incline (putting under load) will break it out of 4-stroking and into 2-stroking. Going down-hill with throttle off will cause it to 4-stroke for several hundred yards when you get back on the throttle. Seems like it's loading up with too much fuel. Spark plug colour looks ok (medium brown). Moving needle clip up/down has little effect. Carb is spitting fuel back into filter. Will not idle when warm (too rich) but starts and idles well when cold (no choke required).

Replaced spark plug with NGK B6HS
Replaced head gasket
No change

CNS carb/red plastic air filter. Factory main jet. Fuel mix 33:1. Long "Z" intake. No appreciable change. In addition to 4-stroking, was bogging (lean) at full throttle. Raised needle one notch, solved bogging problem. Did not solve 4-stroking problem. Carb is spitting fuel back into filter. Plug colour not bad (medium brown). Will not idle when warm (too rich) but starts and idles well when cold (no choke required).

CNS carb/SBP low profile filter/SBP main jets .72mm, .74mm, .76mm, .78mm
z-intake. Same symptoms.

CNS carb/SBP low profile filter/SBP main jet .74mm/BBR billet intake (short) & boost bottle. Same symptoms.

Disconnected kill switch.
Replaced CDI.
Replaced Magneto loop and magnet.

No change.

Original NT carb re-installed with BBR billet intake and boost bottle. Use CNS needle in NT carb with clip on one notch below middle. Ran quite well for about three days. Still 4-stroking at certain points in the throttle but I can ride around it. Gets progressively worse over the next few days though I haven't touched the carb.

Is now 4-stroking nearly 100% of the time.

Flush out exhaust with kerosene. Replace exhaust gasket and seal with form-a-gasket. Doesn't help.

Blow oil out of air filter. Doesn't help.

NT SPEED Carb, New SBP Air filter, BBR billet intake and boost bottle - Same symptoms; 4-stroking 100%.

If this were a horse I'd have been forced to shoot it long ago.

Time for another engine?

 

[crassius]

With the original NT, I'd try lowering float level and leaning needle to see what happens - if it changes, then consider soldering & drilling main jet.

 

[ZipTie]

I can only offer suggestions on what I would try or double check, besides what was already suggested. I know this all can be frustrating. Below is what I would do since a few lean conditions or weak spark symptoms and timing issues can also mimic the rich caused 4 stroking sound and loss of power. Your intermittent symptoms seem odd but the prob is most likely very simple.
( after you fix it) It sure sound like you do know what your doing...
DO ONE AT A TIME and not in the order below. ( do the free ones first) .
1 Spark plug gap, make it smaller. start at .025
2 Order a new stock CDI, timing could be off even though its producing spark.
3 Replace the carb/ with a cheap NT carb/ to see if prob goes away, great to have a spare anyhow as they are 7 bucks shipped
4 Remove carb make sure choke intake blocker arm is moving out of the way
when chock is off, these levers are junk. also make sure carb slide is operating smooth inside the carb.
5 Replace the spark wire and or at least reseat the wire connections into the CDI and the boot.
6 Make sure the spark plug cap snaps on solid
7 Fresh gas and maybe less oil ratio in mix/ may have some bad ethanol gas saturated with water. Happens more than you think because most vehicles are fuel injection and this gas still burns fine.
8 Add a O ring to the intake to carb/ carb air leak
9 last but not least Magneto swap or check settings
10 compression test
good luck man, let us know what happens.

The forum search engine has all the answers or specifics

 

[PeterG]

Thanks for the suggestions. All have been tried at one time or another.

Strange thing is that I'll think I'm making progress then it nose-dives into bad performance again. Vis-a-Vis the swap back to the stock NT carb (with the CNS needle). Performance was better than it had ever been...for a few days...then it gradually gets worse (higher percentage of time 4-stroking) until now it's basically unusable.

I've just run a series of tests with the new NT-SPEED carb at various needle settings and am convinced the 4-stroking is not carb related (at least not any more). I dropped the needle and got lean-related bogging (but still 4-stroking and poor performance). Raised the needle and bogging went away but 4-stroking remained.

The only thing I haven't swapped out is the exhaust. I have tried a less restrictive end cap with no decisive results.

This leads me to a question I have a question about the "black cat" exhaust (stock). How restrictive should they be? I've tried blowing through mine and it's surprisingly restrictive (I can definitely feel back-pressure).

 

[ZipTie]

PeterG, Sounds like a haunted Engine...yikes... but seriously its odd that it will run decent and then get bad again.
That's why I though bad gas, old gas that's clogging the jet, gumming up etc. If your 100 percent on the carb and fuel system then it has to be ignition related like a loose connection thus the cause of running good then bad. Are your connections soldered? Is your carb pretty level?
I have also heard of people getting a number of bad CDis in a row.
Do you have the internal CDI? They can get hot and burn out on the internal types. Its why lots of guys drill cooling holes in the cover.
Muffler may be defective or clogged giving to much back pressure,
Also try a different name brand of synthetic oil and lighten up oil that ratio a tad. But after reading your post about the carb sputtering gas sound like that float needs to be lowered as crassius stated.
Its possible you have more than one problem going on also.
If you give us a very brief run down on the complete long list of what you have done. it would help when some other real creative guys read this and can offer some suggestions I haven't thought of. These China Girl Engines are so easy compared to other engines that a solution should be something very obvious. Posting pictures of your bike would also help maybe.

 

[PeterG]

I know. It's driving me nuts. Should not be this hard.

I spent most of my tuning time on the CNS carb:
-multiple jets (but all larger than stock, smallest alternate was .72mm, largest was .78)
- raised and lowered float
- raised and lowered needle
- disabled idle circuit
I did experience lean-bog with factory supplied CNS jet which was MUCH smaller than SBP supplied .72 mm (though SBP says factory supplied jet is .70 mm)

Other parts replaced:
CDI
Mag loop
Perm magnet
Spark plug
Plug cap
Intake manifold
Exhaust gasket
Head gasket

I'm now using NT speed carb with same symptoms.

Seems like there's a lot of knocking along with the 4-stroking
Using premium Canadian petrol (no corn-fed ethanol) and castrol super two-stroke oil at 33:1
I'm near sea level.

 

[Tyler6357]

I know. It's driving me nuts. Should not be this hard.

I spent most of my tuning time on the CNS carb:
-multiple jets (but all larger than stock, smallest alternate was .72mm, largest was .78)
- raised and lowered float
- raised and lowered needle
- disabled idle circuit
I did experience lean-bog with factory supplied CNS jet which was MUCH smaller than SBP supplied .72 mm (though SBP says factory supplied jet is .70 mm)

Other parts replaced:
CDI
Mag loop
Perm magnet
Spark plug
Plug cap
Intake manifold
Exhaust gasket
Head gasket

I'm now using NT speed carb with same symptoms.

Seems like there's a lot of knocking along with the 4-stroking
Using premium Canadian petrol (no corn-fed ethanol) and castrol super two-stroke oil at 33:1
I'm near sea level.

You need to jet down. 4 stroking means you are running too rich. Smaller jets will solve this. I had to go with a .60 on my RT carb to get it to stop 4 stroking (I'm at sea level). Get some smaller jets, start high and move down until you dial it in.

http://www.ebay.com/itm/281779435448?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

http://www.ebay.com/itm/MOTORIZED-B...ash=item419b5ce8a7:g:~kUAAOSwPcVV16dE&vxp=mtr

 

[ZipTie]

Yes, Tylers probably dead on when he had the same problem. Especially after replacing all that and swapping out all ignition parts as well.
It has to be the float or the jet needs to go way down or both. Yes true 4 stroking is caused by way too rich mixture, like leaving the choke on, but like I said some other electrical probs can at times mimic it also. I feel bad for ya man, hang in there you will get it running smooth.

 

[exokinetic]

Like others have said, all the carbs for these engines are jetted rich from the "factory".

Adjusting the clip will help, and on some engines it is all that is needed.

Ultimately, unless it is ONLY 4-stroking in mid-throttle positions, something other than the clip position will require adjustment.

If the clip position that produces the best/smoothest/snappiest mid-throttle performance results in 4 stroking at WOT, a leaner main jet must be installed to provide proper fueling at mid-throttle and at WOT.

If the best clip position for mid-throttle results in 4 stroking at just off-idle throttle positions and/or too rich idle, then the slide needs to be modified.

Here is an image of a stock NT carb slide, and one I used a file to modify for more "slide cut-away".

-Modified on Right

-Modified on Right


"Slide Cut-Away" refers to how much material is removed from the inlet side of the Carb Slide in ALL Slide Valve Carburetors. The slide cut-away controls mixture from idle to 1/8 throttle, and continues to have an effect, in combination with the needle clip position up to 1/4 throttle.

A slide with MORE cut-away (modified slide in picture) results in a leaner mixture.

The stock Slide cut-away for an NT Carb (pictured) is 1mm. I have multiple slides I have modified, 1.5mm, 2.0mm and 2.5mm.

I use these to tune the idle, and off idle mixture after I find the perfect clip position and main jet combo.


.....this also might have nothing to do with your issue >.<

Check the nut that secures the magneto magnet, it has some play side to side on the woodruff key when loose, and all the way one direction may give perfect timing and no 4 stroking, while the other way is off timing-wise enough to cause 4-stoking no matter what you do with the Carb.

It loosening and moving might explain it working really good after re-assembly, but then exhibiting the same condition over time, intermittently at first, then just all the time, as it becomes more and more loose... Or not more loose, but just as it works its way from the perfect timing position, and rotates slowly over to the poor 4-stroke inducing position.


Also, a restrictive exhaust *can* cause a rich condition, and thus 4-stroking, but it wouldn't be intermittent. The stock exhaust generally provides relatively good performance, especially in the lower RPM's, it gives up in the higher RPM's, but doesn't *generally* lead to 4-stroking, just a loss of power.

To troubleshoot the exhaust system for restriction (stock kit exhaust) remove the cap on the end of the muffler (secured usually by a single nut) and run the bike. It will be loud, but you should be able to see if the symptoms instantly get better, or generally remain the same.


Good luck man! We feel your pain.

 

[PeterG]

All good information.

Just to clarify, the condition isn't actually intermittent. I'd describe it more as re-occurring and persistent. That is, it'll seem like I'm making progress and things are getting better (through carb tuning) but then it'll degrade over a few days until it's crap again.

It's bewildering, I know.

Right now it's 4-stroking 100% of the time. I'm using the NT-SPEED carb.
I've dropped the needle until I get lean-bog at mid-throttle and above but the 4-stroking persists; i.e. there is no point in the throttle where it isn't 4-stroking or lean-bogging (or what seems like both). Surely, if the main jet was too large there'd be some point where it didn't 4-stroke if you dropped the needle enough (or is this just misguided thinking?).

I've made a less restrictive end cap for the exhaust and at first thought this helped but it doesn't seem to anymore.

Here's the real kicker. I spent a lot of time trying to get the CNS carb tuned. I had it where I was getting good mid and top end but it was still running too rich at idle (wouldn't idle when warm). I could take the idle air mix screw right out of it and it wouldn't fix it. I reasoned the idle circuit was mal-functioning. I cleaned up the flash at the outlet to the idle circuit and other flash in the venturi. Put it back together and it was running stupid-rich and flooding. Lowered float, tried everthing. No fix. Figured I ruined the CNS so went back to the original NT. Ran ok. I was running an adjustable end-cap ( that I made myself) on the exhaust. Two discs with four 3/8" outlet holes in them. The two discs could be rotated to either have the full 3/8" outlets open or progressively less opening down to nothing. After I ran the original NT again I realized the discs had rotated themselves and totally closed the exhaust outlets (still ran ok with the original NT). Put the factory end-cap back on the exhaust and it ran even better (with the original NT, didn't have the stomach to put the CNS back on at this point).

Did some tweaking of the original NT (changed to the CNS needle, moved needle up/down, checked float level which was spot on at 21mm). Was running pretty good but not as much zing as with the CNS at it's best. Ordered the NT speed carb to see if I could get some of the zing back.

Didn't mess with the stock NT anymore at this point. Just wanted to ride for a while. Of course I did some tinkering with the bike but can't think of anything I did that would have caused the problem to recur, but it did. Performance got progressively worse over the next few days until it's at the state it is now. 100% 4-stroking.

New NT-SPEED arrived. No change to symptoms.

 

[exokinetic]

When you installed the new magneto magnet, did you notice it having side to side play when installed on the shaft with the woodruff key?

The magnets position on the shaft sets the initial timing advance. With the magneto magnet nut loose, rotating the magnet clockwise will advance timing, rotating it counter clockwise will retard timing.

Timing that is too retarded will cause the symptoms you are experiencing.

I recommend loosening the nut that secures the magneto magnet, and try to rotate it clockwise as much as you can and re-tighten the nut while holding the magnet in this clockwise turned position.

If this doesn't help at all, then go ahead and loosen the magneto nut, and try to re-tighten it with the magnet turned counter-clockwise as far as it will go against the woodruff key. If it IS a timing issue, you should feel the condition at least change a little bit with these adjustments.

If it get better going one direction (advanced or retarded) but isn't completely fixed, you may need to file on the woodruff key to allow the magnet to rotate even further in one direction or the other, in order to find a few more degrees of advance or retard, whatever is necessary.

 

[crassius]

did you ever try cleaning motor really well along the case seam, then running it a bit , then look for the slightest dampness along the seam?

I've had case bolts slightly loose than caused lack of power just a bit with just the least dampness along the seam - hitting all bolts with the impact driver cleared it up

 

[PeterG]

Good info, thanks. Didn't notice any play but will check that next.

ANYBODY: Has anybody blown through the flange end of their exhaust with their mouth? How much resistance do you feel? With mine, it's like blowing through a soda straw (lots of resistance) even with the end cap off.

 

[exokinetic]

Just to go through some brainstorming:

It doesn't seem like an air leak anywhere as that would cause a lean condition. And you would generally feel it as lean surging (with no 4-stroking during the surging).


It doesn't *seem* like it is the fuel as it has persisted through multiple tanks and oil mixes.


It doesn't *seem* like it is the Carb, as that has been switched out and the same condition persisted. Although rich settings in the Carb (Main Jet, Slide Cut-Away) can be making things worse, I do not think they are the *root* cause.


SO.

It's not too much air.

It might be too much fuel, but we attempt to reduce fueling (leaner Carb settings) and it still persists way more than it should for the amount of fuel removed.


So let's assume it's not too much fuel.


So, if a decent air/fuel mixture is maxing it into the cylinder, why is it 4-stroking?


You changed all the ignition components with no change in the symptoms.

So, I am going to assume it is not *faulty* components.

So, why is it not able to fire the good fuel/air mixture on so many strokes (felt as 4-stroking)?


At this point we have narrowed it down to either:

A. The timing is too retarded.

- With retarded timing, the spark is firing the mixture too late, the piston has already compressed the air/fuel mix on the compression stroke, but the spark didn't come until after the piston started falling again, and cylinder pressure had fallen too far for the mixture to be fired.


B. Spark Plug Gap too big.

- I doubt it, but if the gap was too wide, the spark would be unable to jump the gap if the mixture was too rich. If the gap was too wide it may result in 4-strokeing even though you have the perfect air/fuel mix. Anything 0.032" or smaller should be able to fire even a too rich mixture on stock ignition components.


C. Compression too Low.

- If for whatever reason compression was too low ( there can be a lot of culprits) cylinder pressure would not be getting high enough to fire even a perfect air/fuel mix. It may fire if the mixture is set too lean, but as soon as the mixture approaches rich enough to be correct, the cylinder pressure just isn't high enough to ignite that mixture and it 4-strokes, or just doesn't fire at all.


It is easier to fire a mixture that is lean than it is one that is rich. The richer the mixture, the more ignition voltage is required for the spark to jump the plug gap.

The higher the compression ratio, the richer the fuel mixture required. The lower the compression ratio, the leaner the mixture reuired. Compression too low, and the required mixture will not be enough fuel to provide any meaningful torque or HP.


If timing adjustments don't help at all I highly recommend buying/renting/borrowing a compression tester, and figure out what kind of PSI the cylinder will achieve.

Anything below around 50 PSI and we have found the problem. If it's above 80 PSI it's something else.

 

[exokinetic]

ANYBODY: Has anybody blown through the flange end of their exhaust with their mouth? How much resistance do you feel? With mine, it's like blowing through a soda straw (lots of resistance) even with the end cap off.

Even with the end cap off? That doesn't sound right.

I've got a bunch of stock exhausts here... I'll go blow through em and report back.

You could test it with the exhaust off. It will be loud as ****, and the powe r delivery will be... Interesting. But you will be able to see if the 4-stroking goes away.

 

[PeterG]

Thanks exokinetic for the great efforts you are making to help solve this problem.

I agree with your conclusions and theories.

Today I was checking the woodruff key (did it fall out when I last changed the magnet? Turns out I didn't; was exactly where it should be) and took the following photos. I also had to take the head off because I had a piece of paper towel jammed in the exhaust port (cause I was checking for obstructions in the exhaust), forgot about when I spun the motor for TDC and sheared off a chunk of paper towel into the cylinder (!).

On the cylinder shot notice the flaw on the cylinder wall.
Shots of the magneto show it at TDC (for timing comparisons).

 

[exokinetic]

**EDIT**

Okay,


What those pictures tell me:

Your magneto is not installed backwards, that is roughly the correct orientation at top dead center.

I no longer think it is a timing issue.

That blemish on the cylinder is nothing to worry about. I routinely get cylinders with worse blemishes than that and they run great. It also does not look like the kind of vertical smear that is indicative of a piston about to sieze. In my opinion you can ignore it.

It looks to me like there is about 5-10mm (hard to tell) of carbon burnt onto the top of the cylinder wall that is not being swept off by the piston.

It looks to me like there is a solid 5mm's at least between the top of the cylinder and the outside edge of the piston at top dead center. Meaning that at top dead center, the piston is still significantly recessed in the cylinder.

This looks to me like the case of a long stroke cylinder on a short stroke crankshaft, or a long rod cylinder on a short rod crankshaft. Either way, the piston is not making it high enough in the cylinder at top dead center to provide the correct compression it is supposed to have from the "factory".

I can almost guarantee after looking at those pictures that if you did a compression test you would see a rather low number.

I bet it takes quite a few revolutions to get it started when trying to get it started.

If you don't have convienent access to a compression tester...

(although I highly recommend you purchase one fore yourself, they are invaluable when it comes to engine diagnosis. Fuel + Air + Spark + Compression = Running Engine. A fault in any of those and the engine will NOT run as intended. Make sure you get one from a reputable brand e.g. Snap-On, Matco, Bosch. Off brand compression gauges are known to have poor calibration)

...assemble the head on the cylinder with no head gasket. You may get *slight* weeping at the head/cylinder interface, but you will be able to see if the condition gets better.

Although it looks to me like the piston is quite low in the cylinder at top dead center, removing the gasket may not increase the compression enough to make an improvement.


You really need to get a compression gauge on that thing and find out what is happening in the cylinder.

When we know that, we can move forward and correct the issue.


Post back when you have a compression reading, or results from attempting to remove the head gasket, and I will have more stuff for you to do.

 

[Chaz]

Sounds like you may have a faulty muffler. There should be very little resistance if you are blowing through from the mounting flange. I remember doing this and with the end cap on I could feel no real back pressure.

Try loosening the flange to get a little gap and run it around the block a few times to see if that helps. An old Crassius trick.

 

[PeterG]

exokinetic: Gotta disagree with you there. Although it make not appear so on the pictures, the piston is right at the top of the bore at TDC. That bit of carbon on the cyl wall would be the distance from top of piston to ring.

EDIT: Sorry, you are right. It is about 5mm down from top of bore but I think it's a bit of a stretch to think I might have the wrong cylinder on there (but what do I know).

Compression has always felt outstanding; starts up right away.

Did you do the blow test on any of your stock exhaust pipes? vis-a-vis Chaz's comment I'm beginning to suspect the exhaust system.

 

[Tyler6357]

Did you try a smaller jet? The stock .70 that comes with most carbs is usually too rich, try a .65 or a .60. I still think you can save yourself all this effort by just screwing in a different size jet. Also, often times 4 stroking will occur when the engine is cold and will go away once its warmed up so start with a .68 and ride it until it's hot and then jet down until you find the right one.