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Old 07-17-2015, 12:00 AM
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Wacker Mechanic Wacker Mechanic is offline
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Changed rings, still low compression

I'm working on a RedMax string trimmer with compression of only 70psi.
Needless to say, it is nearly impossible to start. If it starts, it only runs for a few seconds and dies. I took it apart and found that the piston and cylinder both look perfect. There is no scoring. Thinking that the rings must be worn, I replaced them with new RedMax rings. I aligned the ends of the rings with the divots on the piston. I also changed the base gasket while I was at it, and put a small amount of 30 weight in the cylinder. Then spun it a few times to get it worked in, and retested the compression.
It had 70psi. It seems that every time I try to restore compression, I get no results. Does anyone have success with it? Is it something that improves with practice and technique? Thanks
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Old 07-17-2015, 12:56 AM
herler herler is offline
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It is very easy to get an improper seal on the spark plug hole when doing a standard compression test, resulting in erroneously low readings.

I don't question that your machine is hard to start and won't stay running, however at 70 psi it shouldn't start at all, and it would not run.
Thus I believe your problem lies elsewhere.

Not sure what it is with some folks here, they just want to tear high performance engines apart...
That's insane if you ask me, I've only torn into one Briggs in 14 years and it had seized so I had nothing left to lose...
That's where I stand when I head inside an engine, I've got nothing left to lose at that stage.

You checked out the carburetion, the coil, the fuel supply, you tested for spark, you tested for fuel or air leaks outside the engine?
All the gaskets are good, your exhaust port is clean, the fuel cap seal is tight, the tank vent is good and its seal is good?
All that and more should all have been looked into and tested long before tearing into the engine.
Hopefully you didn't ruin anything...

Last edited by herler; 07-17-2015 at 01:05 AM.
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Old 07-17-2015, 07:32 PM
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Breezmister Breezmister is online now
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Quote:
Originally Posted by Wacker Mechanic View Post
put a small amount of 30 weight in the cylinder. Then spun it a few times to get it worked in, and retested the compression.
It had 70psi. It seems that every time I try to restore compression, I get no results. Does anyone have success with it?
There was a time when I would mic the piston and cyclinder. Not any more.

You need to hunt down a mechanic guide for redmax. I'm sure they are out there, spec out the cyclinder, make sure it's with in tolerances, other wise, just putting rings in with out know the cylinder is good, is just a waste of your time......

Every time I do a rebuild on a 2 cycle, I replace both rings, piston and cylinder. While doing the rebuild, I coat the piston in 2 cycle oil, so I know the oil gets in behind the rings. I also coat the in side of the cylinder. Not every time, but most of the time, I'll even replace the 2 seals. I got a 100 % success rate.

The down side of doing it this way is the cost. If the cost of doing it my way is 1/2 or more then a new unit, buy a new machine, we do. That's why I only do blowers and hedge trimmers or the high end trimmers any more. It's not cost affective unless you know what you are doing is going to work. No guessing allowed
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Old 07-17-2015, 09:47 PM
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Wacker Mechanic Wacker Mechanic is offline
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Thank you for both of your replies. I realize that it isn't really financially worth rebuilding a trimmer like this one. I'm doing it more for the learning experience. And I am definitely learning by doing it! I hope to apply the knowledge to RedMax blowers so that the time and money are more worthwhile.
I believe I have a good seal at the spark plug hole with the compression tester. I've been through a number of $20 chinese testers, and they have all wound up in the trash. Since I got the Mityvac, I will never go back to a cheap tester. With the same tester, I get readings of up to 150psi on other trimmers, so there is definitely something to the low reading. I should also mention that I've had a number of RedMax trimmers that were fairly easy to start and ran strong, with compression readings of only 85psi.
I did check everything else very thoroughly, with the possible exception of a true coil test. I get spark with the inline spark tester and with the "jump the adjustable gap" tester. I also get spark at the plug itself when it is grounded against the block. Although there is definitely spark (hence the running for a few seconds), I'm not sure if the spark is really strong enough. A new coil may do the trick.
But I still do think that compression is very low, given that my tester gives me readings anywhere from 40psi (on an Echo blower) to 150psi on another trimmer. It seems to be working fine.
I do realize that replacing the piston, rings and cylinder would restore compression completely. I may just do that in order to satisfy my curiosity. Like you said, though, the cost is really prohibitive unless you are doing it for the learning experience or if you're working on a very high-end unit.
I also have a RedMax EB7001 blower with about 75psi. I have ruled out everything but compression at this point, because I had another working EB7001 and was able to swap all parts (carb, coil, plug, etc).
I am going to change the rings and gasket on it to see if I can get it running again via this "cheapo" fix.
If that fails, I am looking at getting a rebuild kit (cylinder, piston, rings and gasket) for $127.
I will post again if / when there is any progress. Thanks again!
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Old 07-18-2015, 11:32 AM
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milo milo is offline
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id say change the seals in the bottom end. if they are leaking it would lower compression. it has to be either that or your cylinder is worn out.
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Old 07-18-2015, 02:56 PM
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Wacker Mechanic Wacker Mechanic is offline
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Milo, thanks, I am working on getting a setup where I will be able to check crankcase pressure and vac. I'm not quite there yet, but I believe it may be the missing piece of the puzzle for many of my machines.
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Old 07-18-2015, 03:14 PM
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Wacker Mechanic Wacker Mechanic is offline
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[QUOTE=herler;5288014]Not sure what it is with some folks here, they just want to tear high performance engines apart...
That's insane if you ask me, I've only torn into one Briggs in 14 years and it had seized so I had nothing left to lose...QUOTE]

Herler, just in the past year, I've had about 15 machines that didn't have enough compression to start. Correct me if I'm wrong, but my understanding of compression is that it's like the mileage on a car. It starts out at roughly 145 - 150 psi, and decreases over time as the motor (rings, piston, cylinder) wears down. The rate at which it decreases is due to usage (mileage) of course, but also due to how well a machine is maintained, particularly the quantity and quality of the oil used. Too much or too little oil leads to faster compression loss. The way it is tuned (lean vs. rich), also contributes to the rate of wear of the internal parts of the motor.
The way I see it, compression loss is not a rare occurrence. It is the inevitable decline of all motors.
I do agree with you that you should only tear into motors when you have nothing to lose. Only do it when all other possible problems have been ruled out. However, I do believe that low compression is very common, and that you can trust the compression tester. If it reads anywhere from 100 to 150 psi, your motor should run. And who cares if the tester is off by 10psi, it wouldn't matter. But if you get a reading below 80psi, then your spidey senses need to kick in. The problem is compression. You should still work your way through ruling everything else out, but odds are you have an internal problem.
I tore apart my RedMax EB7001 blower with the 70psi reading. And despite it being a high performance motor, I found at least one reason why its compression is so low despite a nearly perfect looking cylinder and piston. Good thing I did take it apart, because this broken ring would ultimately score the cylinder.
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Old 07-18-2015, 05:20 PM
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BigFish BigFish is offline
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You see anything wrong with this plug???

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How 'bout this exhaust port??
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Both came from the same redmax blower.

As I've said a slew of times, the first thing I do when workin' on a hand held is pull the muffler and check the exh. port, cyl, piston/rings.

Now this unit had excellent compression...

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Not !
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Last edited by BigFish; 07-18-2015 at 05:26 PM.
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  #9  
Old 07-18-2015, 10:14 PM
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Wacker Mechanic Wacker Mechanic is offline
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I've seen exhaust ports as bad as that, but I've never seen a spark plug like that, and have never seen a piston quite that bad. It's amazing that thing ran long enough to do that amount of damage. Were you able to get it rebuilt again without replacing the whole block? From the looks of it, I would imagine you'd need nearly $150 in parts alone.
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Old 07-18-2015, 10:15 PM
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Wacker Mechanic Wacker Mechanic is offline
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With that amount of carbon, do you scrape it, Chem-dip it, soak it in Seafoam, put it on the grille (lol)? What's the best approach?
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