bad compression.. how to fix it

casalejuan Sat Feb 18, 2012 4:20 pm

Hello,

I bought 3 Pee Wee 0.20 engines Babe Bee .049

but only one of them works Sad

. Before start, I changed all the venturi gasket, and little parts like needles, etc.
I saw that the engine that works, has a good compression and the others has not a good compression Huh...

. Somebody can tell me how can I fix this problem? all the engines are new and they came in unopened blisters. So never were started.

I wait for your answers Reading

Thanks!!

Juan. bad compression.. how to fix it Pee_we12

WingingIt74 Sat Feb 18, 2012 4:22 pm

your prop is a little big I believe. How many head gaskets do you have installed?

nitroairplane Sat Feb 18, 2012 4:47 pm

I have had pee wees swing 7x5 props .
Well one thing I did for a low on compression pee wee was to remove the piston and bake it in the kiln for about 30 mins, this swells it somewhat giving a tighter fit. After this the engine had to be run in again.
Indra

proctor Sat Feb 18, 2012 4:52 pm

If they are new engines the piston cylinder fit should be OK.
I would look at whether the cylinder is properly screwed down
into crankcase and remove head and make sure just one gasket
fitted. Check the glow then screw head down carefully and make
sure it is snugged down properly.
The engine should then at least fire on the prime.
Not one of the more expert member but I have wasted a lot of
flying time trying to start a peewee with a head that had slackened off
after a flight.
John

**Cribbs74** Sat Feb 18, 2012 5:01 pm

I only can add one thing to the comment above.

Lap the head on some very fine grit sandpaper to ensure it is level. Install a new gasket or flip the one already there if you don't have spares. I have corrected many compression issues doing this.

Surfer_kris Sat Feb 18, 2012 5:02 pm

Check where they leak first. Put a bit of detergent around the head and turn the engine over slowly and check for bubbles. Some heads have a poor sealing surface, use fine grinding paper in a flat surface, like a piece of glass. If that is not enough you might have to lap the head to the cylinder, remove the washer and lap with fine grinding paste.

If there are bubbles around the piston there is nothing you can do (I've never had any success with growing piston using heat). But the oil they come with is very thin and they can seal better once they are running on full castor fuel.

casalejuan Sat Feb 18, 2012 7:15 pm

WingingIt74 wrote:your prop is a little big I believe. How many head gaskets do you have installed?

Hi, the prop is not the problem. I have a lot of props and is the same thing. I have one gasket installed in the head.

**Cribbs74** Sat Feb 18, 2012 7:18 pm

Yeah, the prop is not the problem.

All the suggestions above are good ones. How's the compression now?

casalejuan Sat Feb 18, 2012 7:26 pm

Surfer_kris wrote:Check where they leak first. Put a bit of detergent around the head and turn the engine over slowly and check for bubbles. Some heads have a poor sealing surface, use fine grinding paper in a flat surface, like a piece of glass. If that is not enough you might have to lap the head to the cylinder, remove the washer and lap with fine grinding paste.

If there are bubbles around the piston there is nothing you can do (I've never had any success with growing piston using heat). But the oil they come with is very thin and they can seal better once they are running on full castor fuel.

Hi! I´m going to try this first. Mabe that´s the problem and is a simple way to check it.

Thanks!

casalejuan Sat Feb 18, 2012 7:29 pm

cribbs74 wrote:Yeah, the prop is not the problem.

All the suggestions above are good ones. How's the compression now?

Compression is very slight. however one of the two engines that does not work occasionally makes an explosion but does not start at all.

John Goddard Sat Feb 18, 2012 7:44 pm

If it's firing but not running it's not getting fuel.
If it were me because its a new engine I'd start looking at the reed valve first. Has
It stuck or is there some dirt underneath it.
I've several new in boxes that don't seem to have a lot of compression
Whilst others of the same type and age do. I was so concerned with
A late 1960's 010 that I took it from box (because I thought it was a dud)
And fired it up, I ran it in as per instructions and it seems to have
A bit more comp now but it runs like a champ.
What's your glow like is it bright yellow orange or dull orange?

Mike1484 Sat Feb 18, 2012 8:27 pm

Maybe your problem is sticking your nice motor in a vise . A complete lack of respect for any motor.

Mike1484

andrew Sat Feb 18, 2012 8:32 pm

nitroairplane wrote:Well one thing I did for a low on compression pee wee was to remove the piston and bake it in the kiln for about 30 mins, this swells it somewhat giving a tighter fit.

Indra --

I've heard for some time that heat soaking a piston can cause it to swell, but I've never tried it. What kind of temperatures are we talking about here?

andrew

proctor Sun Feb 19, 2012 6:55 am

Hi, back in the day we Brits had a technique called ' Cherry Bombing'
which entailed heating your piston to red heat and dropping into
brake fluid. This did work but entailed lapping piston to cylinder to
get our wee diesels working again. Mostly practiced by the team
race people. Most of us never used our engines enough to have
to resort to this fairly desperate measure.
john
ps cast iron pistons of course, running in a steel liner.

nitroairplane Sun Feb 19, 2012 6:59 am

proctor wrote:Hi, back in the day we Brits had a technique called ' Cherry Bombing'
which entailed heating your piston to red heat and dropping into
brake fluid. This did work but entailed lapping piston to cylinder to
get our wee diesels working again. Mostly practiced by the team
race people. Most of us never used our engines enough to have
to resort to this fairly desperate measure.
john

You wanna see a really wee diesel?
bad compression.. how to fix it Img_1110

Thats piston thing is what I was talking about but I never used the break fluid I must try that one day!

casalejuan Sun Feb 19, 2012 7:12 am

Mike1484 wrote:Maybe your problem is sticking your nice motor in a vise . A complete lack of respect for any motor.

Mike1484

Sorry Mike but that's not the problem. One of the three engines work well and they were all initiated in the vise. But your observation is good because if I adjust the vise incorrectly, I can break the tank. Sorry! I´m new in the Babe Bee .049

world.

casalejuan Sun Feb 19, 2012 7:23 am

ultimately, you think it would be a good measure to buy a new kit that includes a cylinder, a piston and a glow head? I saw that in E bay we can find those parts.

GermanBeez Sun Feb 19, 2012 7:55 am

juan, pleeease put that engine out of the vise...it's not good for the engine. Neutral

screw it onto a block of wood and put THAT into the vise. Wink

**Cribbs74** Sun Feb 19, 2012 8:08 am

casalejuan wrote:
cribbs74 wrote:Yeah, the prop is not the problem.

All the suggestions above are good ones. How's the compression now?

Compression is very slight. however one of the two engines that does not work occasionally makes an explosion but does not start at all.

Perhaps compression is not the issue. Are all these engines new and never run? You say some were in blister packs. If that is the case did you disassemble them and clean them?

I am going to have to agree that mounting engines directly in a vice is a bad idea. It's not safe and it will marr the finish and distort the steel. You probably spent a lot of money to obtain these engines. You should protect your investment and pick up a correct bench mount from Cox International. Or at least use a block of wood.

Surfer_kris Sun Feb 19, 2012 8:31 am

If they have never been run before I cannot imagine that the piston to cylinder fit would be so bad that they cannot run, that just doesn't sound like a cox engine...

Check the head to cylinder fit, you might have a leak there. Other sources of problems are the tank to case fit and the tank-backplate to ventuiry seal, it is a really tiny o-ring.

I once tried the heating method on the piston of an old Veco .19 engine, didn't work one bit. Biggest change was the loss of the castor build-up, hence giving an even looser fit.

PV Pilot Sun Feb 19, 2012 10:12 am

You know, clamping it in that vise might have caused your loss of compression.

If you do use a piece of 2x4 wood for mounting, use the side of the board, not the end. Side of the board is side grain which holds the mounting screws tighter. I see alot of folks using the end of a 2x4 which is end grain, and conversly a looser grab on the screws. Once doused with castor oil and fuel, those end wood grains swell very rapidly.

casalejuan Sun Feb 19, 2012 6:47 pm

I´m going to clean all the engines. to do that I had to create a tool because the disassembly tool that I purchased in Cox International was bent by the force that I applied in it.

So, if you are agree, the first step is disassembly the engines. Second step, i have to clean it with isopropil alcohol. Third step is to change venturi gaskets, etc. Then I have to assemble the engines and Fifth step is to mount it in a wood block. I´ m going to try this tomorrow and then I will tell you.

**Cribbs74** Sun Feb 19, 2012 7:33 pm

casalejuan wrote:I´m going to clean all the engines. to do that I had to create a tool because the disassembly tool that I purchased in Cox International was bent by the force that I applied in it.

So, if you are agree, the first step is disassembly the engines. Second step, i have to clean it with isopropil alcohol. Third step is to change venturi gaskets, etc. Then I have to assemble the engines and Fifth step is to mount it in a wood block. I´ m going to try this tomorrow and then I will tell you.

Hold on there..... Where did you apply that much force to bend the tool? If on the cylinder then I know why your compression is poor.

Maybe I misunderstood? Hope so.

casalejuan Sun Feb 19, 2012 9:12 pm

cribbs74 wrote:
casalejuan wrote:I´m going to clean all the engines. to do that I had to create a tool because the disassembly tool that I purchased in Cox International was bent by the force that I applied in it.

So, if you are agree, the first step is disassembly the engines. Second step, i have to clean it with isopropil alcohol. Third step is to change venturi gaskets, etc. Then I have to assemble the engines and Fifth step is to mount it in a wood block. I´ m going to try this tomorrow and then I will tell you.

Hold on there..... Where did you apply that much force to bend the tool? If on the cylinder then I know why your compression is poor.

Maybe I misunderstood? Hope so.

It was with other engine.
A few minutes ago I was thinking if the problem is in the Glow head. The engines are new but.... I conected the glow head to a 1.5 V battery and... Affraid or WOW!

surprise!!! 2 of the 3 glow heads doesn´t lights. and coincidentally are the two glow heads of the two engines that don´t start.

So... I change the glow head of the engine that works to another engine and tomorrow
I´m going to start it. If the engine works, the problem is the glow head.

**Cribbs74** Sun Feb 19, 2012 10:50 pm

A functioning glow head does make a difference.

Something to consider..... Nothing on these engines requires enough force to bend a tool. If you find yourself breaking a sweat trying to loosen something it's a good time to step back and reassess....

Hope it fires off for you tomorrow.

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