What I Learned Porting an Earthquake

[Chris-PA]

Given how long the Dissecting an Earthquake thread has become, I decided to make a separate one to document what I did and what I learned in the 4 attempts I made to get this thing to run like I thought it should.

The Earthquake CS3816 is a Chinese clone of the Zenoah G3800, but with some changes to get it to pass emissions. This is interesting because they followed a different, more conventional path than Zenoah did in creating the strato GZ4000 - basically they modified the transfers, porting and stuffed a cat muffler on it, just like other manufacturers have done. Here are some shots of the stock engine (they are from other members):



For reference, this is a G3800 cylinder I saw on eBay (probably also a clone, but I expect it is representative of the stock transfers):

So you can see that they really reduced the transfer volume, as well as the intake and exhaust port size. Also, they brought the rear cylinder projection around much further, which I assume was to reduce case volume. I didn't like that flat triangle between the transfers. I wish that I had measured the stock timing but I did not.

Here is a shot of the case & piston:

The piston is domed.

My first attempt at porting was to widen both intake and exhaust, and the exhaust was raised and the intake lowered:


I tried to smooth that block between the transfers too:
The base gasket was removed but the squish was still 0.040". That ran OK, but nowhere near as well as my little ported Poulan clamshells do, not to mention the GZ4000s.

So I went back in to raise the transfers, which I really struggled with. I was able to raise the transfers closest to the intake, but I left the exhaust side ones alone as I was concerned about damaging the cylinder. That is not optimal but I don't have the tools - those transfer slots are narrow.


Here is the timing at that point:

It still was just OK.

 

[Warped5]

Love the graphic.

Continue the fine work!

 

[Chris-PA]

Next I decided to try to open up the lower transfers to match the piston side cutout. I cut back the rear cylinder extension with a 4" grinder, and blended the lowers:


I thought it looked a lot better - but I believe it was an error, and it certainly didn't run any better. It was OK, but kind of sluggish and didn't want to rev.

I tried different ignition modules & flywheels (from a Ryobi/GZ400) and a different carb, but that made no difference. So I decided to go after the squish, and turned the base on the lathe at work. Plus a little more shaping on the transfers:

The timing at this point was:


Squish ended up right at 0.020", and it was somewhat improved, but not where I thought it should be. More in a bit...

 

[Chris-PA]

I forgot the muffler and other systems. The muffler mod is my usual on this family of saws - I grind an opening in the back and use a die grinder to hog out the baffle. I use a small sheet of stainless steel from an old microwave housing to cover it, sealed with RTV. Then I grind open the outlet:




Here is the carb that came on it - it's a Walbro WT clone, but it has a fixed H jet in addition to the H screw, which makes adjustment nicer. It's a decently made carb, but the check valve was leaking. I was able to repair the check valve and it is fine now (sorry for the blurry picture of the jets):

 

[Chris-PA]

After all that I tried did not seem to wake it up I tried to figure out what I'd done wrong. I think messing with the lower transfers beyond my first small work on that triangular block between the transfers was an error. It only added to case volume, which on a 38cc engine is a problem. Most larger displacement engines seem to be increased mostly by larger bore, which means that the extra crankcase volume is a bigger problem on small displacement engines. I don't think I went too crazy with the blending but it wasn't helpful.

Also, my ports are very rounded on the lower intake and the upper exhaust.

Looking at the port timing again:

There is a lot of case compression. That's good considering the lost case volume, but I thought it was excessive. I didn't want to raise the exhaust or reduce the blowdown, so that left the intake. I reasoned that having a greater volume of air pulled into the case would mean more case pressure, even if the case compression angle was reduced. So I went back in.

I broke the cardinal rule of troubleshooting here - I changed more than one thing at a time, so I won't be able to tell what any improvements are due to, but I didn't want to take it apart again.

New intake, much more square (thanks to Mastermind's comments):

Exhaust timing was not changed much (I gained a degree), but the roof is more square than it was:

Here's the shape of the lower intake compared to the piston edge (the intake is not symmetrical on these):

Here is how the timing came out - with the new port shapes the time-area should be improved more than just from the timing:


The muffler outlet got some additional attention:


Then it was assembly time.

These have hot air doors, which I set for heat as it was cold today, but you must remove the top cover to change it:

It feeds air into the air box through the opening under the filter support bumper:

Gratuitous assembly shot:

 

[Chris-PA]

So - how did it work out? I'm finally happy with it! Here is video of it noodling white oak before the last round of porting:



Here it is today noodling the same stuff, only a little wider and frozen:


And cutting a cookie of the same frozen oak:


It feels much better, and I think it's pretty decent for 38cc pulling a full 16" bar.

 

[Tractorsaw1]

Way to stay on it!

 

[Poleman]

Very interesting with what you all did, your thoughts and feeling on how things were turning out. Your tenasity with sticking to it and trying new things was AWESOME. This what you did is how we all learn and can use and apply for process of mods.

You saw turned out good and was easy better in the later videos. It hold its speed better and just sounds better.

EXCELLENT JOB!

 

[Chris-PA]

Very interesting with what you all did, your thoughts and feeling on how things were turning out. Your tenasity with sticking to it and trying new things was AWESOME. This what you did is how we all learn and can use and apply for process of mods.

You saw turned out good and was easy better in the later videos. It hold its speed better and just sounds better.

EXCELLENT JOB!

Thanks! Discussions about case volume, case compression and port shape with Randy and others certainly played a big part.

 

[milkman]

Looks sounds and cuts just like a real saw.

 

[Chris-PA]

Great write up. Did you run the origional bar and chain?

Actually that is the original chain, just a different bar with a larger tip sprocket. Bailey's was blowing out those bars with 56DL Carlton N1 chain for like $13 - I got 3 of them. However the Quake likes 57DL better with that bar - the tail is a little fatter and the 56DL chain does not move as smoothly over the tail.

 

[Chris-PA]

Was the saw you modified one of the "for parts only" super inexpensive ones?

Yup - $30 or $35 delivered, can't remember which.

many of the photo's were lost in the origional thread when the system crashed. It would be nice if they could be reposted.

Yeah, that's another reason I made a separate thread. But even if they were still there, that thread's so long they'd still be lost!

 

[Mastermind]

Very nice thread Chris.

I don't understand the timing numbers though.

Is there any way you could tell me what the opening points are as measured from TDC?

 

[Chris-PA]

Very nice thread Chris.

I don't understand the timing numbers though.

Is there any way you could tell me what the opening points are as measured from TDC?

Thanks - sure:
Exhaust = 106
Intake = 84
Transfer = 127

 

[Mastermind]

I know you said you didn't have the proper tools to do it, but raising the transfers may have given you a larger gain than lowering the intake.

I like the exhaust height......but would like to think that an opening point of 120 - 118 on the transfers would be great.

If you have another jug, I'd like to have you determine how far down from the squish 120 would be, and send me the jug so I could raise them for you.

In the name of science of course.

 

[Chris-PA]

I know you said you didn't have the proper tools to do it, but raising the transfers may have given you a larger gain than lowering the intake.

I like the exhaust height......but would like to think that an opening point of 120 - 118 on the transfers would be great.

If you have another jug, I'd like to have you determine how far down from the squish 120 would be, and send me the jug so I could raise them for you.

In the name of science of course.

Thanks, that is generous, but I'm afraid I don't have any spares. Actually I bought this as a parts mule for my GZ4000's!

 

[Mastermind]

I just would like to help out with the transfer work. I've got more to do now than I need........but these sort of builds interest the hell out me.

 

[Mastermind]

I bet we can find a jug somewhere.

 

[LegDeLimber]

Little booger was tossing some wood strings and yet never looked it even started to foul the clutch cover.
In that 2nd vid, the light is on the stream of noodles and a bit before mid-vid, You'll notice how quick that easter-bunny nest is growing.

As for the transfers? Somewhere else (RC plane?) I saw a fellows cylinder that he had
done by clamping it on the drill press and doing a bit touch-off in the ports
Because of not having a rt angle head.
While you're trying this, You might want to use a notched stick or something
to reach into the bore and support to cutter shank
and help prevent it from kicking out of the port IF it happens to grab a little.
if it swung out, it beat the jug to death before you could even think "aw #@%*"
heck, a hunk of wood fitted just like an arbor for turning the base flange,
Would be nice to have in there.
who knows, might even be able to drill the diagonaly and use something to poke down to and influence the cutter a wee bit
or even run the cutter itself, down the angled hole.

 

[LegDeLimber]

Wasn't there a 50cc~ish engined one somewhere?
[[ I may have to duck a few scored pistons being thrown at me for asking this ]]
But where's the contender to try knocking the Husky 346 off of the throne?

By the time I could try to "filter" a few bucks into the pocket change jar and maybe just bid on a pair of 45cc,
rest of these obsessives had drove the doggoned market back up to retail price on 'em.
oh well, at least the thread made for some entertaining reading