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- This topic has 9 replies, 7 voices, and was last updated 2 months, 1 week ago by jeff-register.
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September 11, 2024 at 8:44 pm #290738
Would anyone care to offer some thoughts on a couple of (low Compression) motors i have please ?
1955 Evinrude 7.5 Fleetwin Whisper Power both cyls. @ 50 / 55
1956 Johnson 10 hp. QD-17 both cyls. @ 50 / 55
Using a cheap Amazon compression tester.
They both seem to run fine but………..? Any thoughts , Thank you !
September 11, 2024 at 9:04 pm #290739If they work don’t fix them.
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September 11, 2024 at 9:15 pm #290740OK , TBH Just wondering how low is low. They seem to work fine. Thanks
September 11, 2024 at 9:19 pm #290741If they start easily, you are golden! Hard starting and poor idle can be a sigh of troublesome low compression.
Joe
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September 11, 2024 at 11:19 pm #290742I don’t think you can get a motor to start if the compression is below 60.lbs. I’m positive you cant at 50 lbs. I would suspect your gauge is off. There are plenty of motors out there that don’t start and don’t run well. Get one of them to fix and just run the others. Tubs
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September 12, 2024 at 1:54 pm #290750Cheap testers have made many people unhappy for no reason. If they run OK and idle, quit worrying. If they haven’t been run in a long time, there’s a chance they will get better compression after being run and getting up to temperature a few times.
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September 12, 2024 at 2:33 pm #290753Rkymtn, Were these motors sitting for a long period of time?. If so, that will make for low compression. At least the compression is close on each cylinder, that’s a good sign. If you can start that engine, let it run for awhile, so the rings reset themselves, just make sure the engine doesn’t overheat, or take out for a boat ride. When done testing motor, check compression, should be better than 50psi. Let it sit for awhile try starting when motor cools off. Compression should be as high as it’s going to be. If it stays at 50 or below, then it’s time to check rings or overhaul. I agree with Tubs, compression should be at least 60psi. If you are going to buy some gauges, or tools, pay the extra dollars and buy name brand stuff, you only buy them once, just my 2 cents worth. Gene.
September 12, 2024 at 5:18 pm #290757Thank you all very much. They both seem to start easly and idle fine.
Im not going to worry about them right now. Ive just been barrel testing so far but will put them on the water and get some more time on them asap.
I appreciate the info. These are the first two i have rebuilt and have learned alot with your help. Thanks D
September 13, 2024 at 11:18 am #290761Suggest at your next meet that your Chapter have a “compression gauge comparison”. All you need is one motor for consistency….along with a prescribed number of pulls. Everyone hook up your testers and see how they compare.
Wile “poundage” is certainly an important figure….consistency between cylinders has greater significance.
September 13, 2024 at 9:59 pm #290773I had a 3.5hp Scott that had no compression. Took it apart & the rings were glueed to the pistons from bad oil. Soaked them c;ran, reassembled & it ran fine.
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