Gas engine question

[bufferoo]

I've been having a little trouble with a gas engine I have. I "think" it's an echo but no way to be sure. It does have a walbro carb. My question is this:

Are these type (chainsaw\weedwacker) of engine a reed valve type? The reason I ask is that I have a few 50cc moped engines and they are ALL reed valve. I don't see any reeds on my smaller gas engines (homelites and craftsman chainsaw engines) and unlike my nitro engines there are no rotary valves. How is the intake charge prevented from shooting back out of the carb? I'm wondering if this is why I can't start the bloody thing.

I've put the carb on another engine that's almost identical and tuned it up nice. Moved it back and I get nothing but some burbling as I turn it over. It's got GREAT spark (found that out the hard way :rolleyes: ) and it's turning over very fast. I'm using a airplane spinner and a 1/2" drive, 5000rpm hammer drill (hammer turned off) and yes, I am positive I'm turning it the right way around.

Any ideas here?

I have made sure that the hole on the side of the bottom of the carb is lined up with the hole on the intake and it's not blocked at all. Since I KNOW that this carb is good I can't figure it out. :mad: :mad: Even with a shot of either I get just a few burps from the exhaust. Not even a pop.

I KNEW that I should've stuck with nitro. :D

Bufferoo

[Mike T]

Gas weeder engines usually start very easily. They don't have a reed or rotary valve; they are piston-timed. That means as the piston approaches top center, the piston uncovers the intake port that allows fuel/air to be sucked into the crankcase. As the piston comes down, it covers the port again. If the carb ran ok on another engine, I'd suspect the trouble is either a leaky crankshaft seal (resulting in poor crankcase compression) or a scored piston (resulting in poor top end compression). When I used to race Echos, I found a lot of the Echo weeders I got as freebies had been overheated and the piston and cylinder were scored. They would pop and sometimes start very weak, but not pull any power. The cure was about $120 in parts.

Not to start the gas vs. nitro argument, but when a gas boat is right, it's fast, dependable, and very easy to operate.

[bufferoo]

You see...that is just what I had thought. :D I have fully disassembled this engine and the pistona nd cylinder are in great shape. Very nice compression. The reason I was wondering about the reed valves is that I couldn't see how the crankcase pressure would stay up if there was nothing keeping it from being blown back out the carb. Are these engines only relying on the vacuum from the piston moving up? I would think that they would need some sort of crankcase pressure to force the air\fuel into the cylinder.

I am quite aware of how bad seals will affect the running of an engine. Had to do that on one of my mopeds and it was not a fun search for the parts. This thing isn't acting the same way. It just won't start. Each time I've come accross bad seal they will start but once it get's warmed up then it won't idle and runs really poorly.

So I am sure the carb is good. New plug gapped correctly (can't remember off the top of my head what the gap is set at). NICE spark. Hurt's like a bugger :eek: . Fresh fuel mixed at 32:1 as per the sticker on the engine. Turning over at 5krpm in the correct direction. Points are set at .015" as per engine sticker.

What else can I look at? I'm getting stumped on this one and may have to shelve it for the winter. Getting too cold in the garage.

Bufferoo

[RickE]

Is the engine running a tuned pipe or the can-style muffler? In the past I've seen the stock mufflers on Echos cause headaches due to the "spark arrestor" clogging-up inside the muffler. It's only a piece of flat metal screen that you can remove for R/C boat usage. If you're using a tuned pipe this won't help you but I'm stumped too on what's causing your engine to not start.
RickE

[bufferoo]

It is a can style but it it fully open inside. No sparke arrestor here. I know about those things. Had to pull it off my homelite weed wacker to get it to run right. I'm glad I'm not the only one getting baffled by this. :)

So...just what does prevent the crankcase pressure from being blown back out of the carb if there are no reeds or valves? I'm thinking along this line as I saw nothing that would keep the pressure in and I hear the "occasional" huffing sound from the carb on the downstroke if I have the choke on. Now of course I can only hear this when I turn it over slowly by hand but I'm wondering if that's my problem here.

I figure that if I can't keep the air\fuel mix in the crankcase then I'm definitely going to have problems.

I hate to say it but in this case...nitro is easier. :D :D :D

I'll keep at it though.

Bufferoo

[Mike T]

The pressure just starts to build about the time the port gets closed. The piston-timed intake is probably the oldest 2-stroke design there is. It's not perfect due to the compromised closing thing but it works. If you check the timing numbers on a rotary valved nitro engine you'll find they close at some time after top dead center as well. That's why the K&B .21 outborads run backwards sometimes - the long timing.