Here we go again, new thoughts

F-Type

Well-Known Member
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265
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Gothenburg - Center of the Universe
Hi

After some time not touching my octane lemon, as it still wouldn't run properly even though I thought I had finally found the problem in a mismatched carb, I had another go the other day as I had an idea I wanted to try out, kind of "fault tracing" idea.

Me, and a lots of others, have the problem that the car runs for a short while, then stops. When restarted, it'll run for a second or two, and stop. If it starts at all.
If you wait a while, it will run a short while, and then again same thing.

So, I figured, it's some kind of heat issue.

Also, one thing I have noticed is that when I heat the engine up with a hair dryer or a heat gun. I only need to heat the egnine up a very short time, before the sleeve gets so loose I can easily pick it up with my fingers, it's completely loose in the cylinder.

So, here's what i did.

Measured compression, 10 pulls = 150 psi
Started the engine.
Let it run for maybe 20 seconds or so until it stopped. Then same thing as always, starts and stops within 1-2 seconds.
Now I heated the engine with a heat gun, to get it quite hot.
Tried to start it, and now, even hotter, it will not start at all, not even for a second or two.
So heating it up, makes it not work. Not really a surprise.

So, with engine hot, I tested compression, 10 pulls = 100 psi.
Hence, I loose a lot of compression just like that, when the engine gets hot.

Now I am thinking this loss of compression is related to the cylinder sleeve coming loose when the engine gets hot.
However, I can't really see how the combustion chamber compression (to be precise) should drop due to a loose sleeve. But clearly it does.

What I am thinking though, is that the crankcase compression gets ruined.

If the cylinder sleeve isn't snug tight to the cylinder, I am thinking you can have the gas sneak out in between the sleeve and the cylinder, loosing crankcase compression. If this happens, you won't get any gas into the combustion chamber, and the engine dies.

Any thoughts on this?

Also, any thoughts on why the combustion chamber compression drops when the engine is hot?
 

ajdragon

Active Member
Messages
84
Hi

As the aluminum block heats up it will expand faster then the steel sleeve, and the aluminum piston will also expand before the steel sleeve. It sounds to me as if the piston ring is bad or the sleeve is not being held down tight enuff by the head. Also are you still using the original spark plug?
 

F-Type

Well-Known Member
Messages
265
Location
Gothenburg - Center of the Universe
Brand new piston ring, old ones about broken in, no difference.
Have several plugs, no difference.

Like you say, aluminium expands faster, that's why I figure compression should not drop, rather increase as the piston will expand in the sleeve.
 

F-Type

Well-Known Member
Messages
265
Location
Gothenburg - Center of the Universe
I have tried adding an extra seal between the cylinder and the head, No difference. Also there is a bit of margin between the head and the cylinder, so it is definitely resting on the sleeve, and all the bolts are tight, and once again, when the aluminum heats up, the joint should become even tighter.
 

F-Type

Well-Known Member
Messages
265
Location
Gothenburg - Center of the Universe
One thought that just struck me, if there is no or little crankcase compression, then there might be less gas/air-mixture when the compression cycle begins, i.e. a lower starting pressure, ending up with a lower compression.
 

F-Type

Well-Known Member
Messages
265
Location
Gothenburg - Center of the Universe
Read up a bit about sleeves and cylinders today, and about materials and temperature expansions.
When I heat the cylinder up just a bit with a hair dryer, the sleeve can easily be removed. This indicates that at room temperature, the sleeve outer diameter is probably very close to the cylinder inner diameter, likely slightly below, as it requires so little extra heat to remove it easily.

The difference in thermal expansions of aluminum and steel gives that, assuming the same diameter at room temperature, an increase in temperature of 100C/212F creates a gap between the cylinder and the sleeve of as much as 0,1mm/0,004".

What I've learned is that a symptom of having an "over-bored" cylinder vs the sleeve, can be loss of compression. Well, what do you know, perhpas I am finally on to something here.

Also, it turns out "industry standard" for an interference fit of a sleeve in an aluminum cylinder is no less than, yupp, 0.1mm/0,004"
I have no clue if the same is valid for an engine this small, but looking at the forumlas implies that the difference i diameter from temperature is independent of the absolute diameter, and hence, the standard would be the same regardless of cylinder size.

I don't think I have any interference fit on my cylinder-sleeve fit, and there should be one. I can measure the sleeve diameter accurately, but I lack the tool to measre the inside diameter of the cylinder. I'll see if I can get hold of the tools needed.

In any case, the next step in testing, would thus be to find a way to "seal" the sleeve to the cylinder.

Any suggestions on how to do that?
I am considering trying a piece of thin paper.

Even someting that might just hold for 5-10 minutes would be a good start, as long as the engine isn't ruined.
 
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