How To Compression Test Motorcycle Engine | Honda CB750 Resto Project Bike | Part 13
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- Опубліковано 28 бер 2021
- How To Compression Test Motorcycle Engine | Honda CB750 Resto Project Bike | Part 13
In this episode, I compression test my Honda CB750 project bike. I have spent some time fixing the carburettors and chasing leaking float bowls. Now that I have managed to have it running on 4 cylinders it was time to see if is sound.
Here's what I used in this video:
Compression tester: ebay.us/pbjIYp
Compressor: ebay.us/Q1yndj
Fire Extinguisher: ebay.us/Mks7nC
When an engine isn’t run for a number of years it is always likely that either there will be sticky piston rings, or corrosion around one or two of the valves. This will cause some of the cylinders to be down on compression.
Frequently when you run an engine that hasn’t been run for a long time the sticky rings free up, and the corrosion around some of the valves burns off. Sometimes it doesn’t!
To do the compression test, one spark plug is removed at a time, so that a line to a pressure gauge can be fitted. Then the engine is turned over a number of times so that the pressure gauge shows the peak pressure the cylinder is achieving.
The pressure for each cylinder is recorded so that they can be compared against each other.
The wisdom is that the pressure should be above 100psi, the higher the better, but rarely above 140 or so. There should also be little difference between each cylinder, in that they should be within 10% of each other.
You will have to watch the video to see what result I got!
For more helpful how-to guides and restoration project logs, visit our blog: www.spannerrash.com/
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I've started watching your videos last week and loved them. You are a very patient man with so much energy and motivation toward your job.
Thanks, patience isn't easy :) And I really do love the tinkering and spannering. Hope you keep on watching, I am having fun with my latest project, and it is requiring of a lot of patience.
Two years later and this video is still helping people. Thanks for great content!
Glad to help!
Another enjoyable video! Great seeing your progress! You deserve a cuppa tea!! : )
Hello from South Africa . Great Video sir , I have just got my hands on a 1980 cb750F which needs work , I have ordered a complete engine and carb Kit off ebay , I am considering getting the compression tester now . Keep up the good work .
Thanks Clifford. Sounds like you're taking on a great project bike there. I can definitely recommend the compression tester. They're a great tool and can really tell you a lot about the health of an engine, wouldn't be without one now. :)
The pressure in the cylinders is perfect, the problem may be that the fuel does not draw, try to install the air filter or set a higher fuel level in the float chamber :) Kris.
Cheers Kris, I think you may be right with that, installing the airbox should be an easy one to try. :)
Hi from Ireland 🇨🇮
I've just become subscriber 651.
Love your content and clear camera work!
Your logo is very cool...will you have tshirts for sale anytime soon?
Hi Steve, thanks for the kind words! :) Merch of some description is definitely a possibility down the line, but not quite sure how soon. 👍
I'm about to embark on a compression test on a 75 cb750 SS. Last night I noticed blue'ish smoke and mates told me that it was most likely piston rings. THank you for this clear cut and precise demonstration. I noticed you kept the kill switch off. Does that mean I don't need to fabricate a hanging fuel tank?
Hi mate, you don't need a fuel tank for compression testing luckily. 😄
To find out whether it's rings or valves, compression test it dry initially, then add a little bit of oil through the spark plug holes and test again.
If the pressure goes up a lot with the oil, it indicates it's more likely to be rings.
I'll cross my fingers its nothing major for you, hope that helps! 🤞
Hi, thanks for the help on the solenoid (another video) so today i did a compressison test myself and i have 90 psi on 1 & 4, like 75 on 2, and nothing registering on 3 - the bike has not been run in 30 years including now, i have the exhaust fully off and the carbs too while i wait for rebuild kits...before taking a long break between working on the bike i had rebuilt the engine with new rings so long ago that while i think it was right maybe not...any thoughts short of taking it back out and tearing it down? Oh, and it had no oil until last month - with nothing going on either, just sitting.Thanks - jim
I would start by checking the cam timing and the valve clearances, making sure that all of the valves will open, and aren't stuck open. If they are all OK and the readings don't change, Try putting a squirt of oil in each cyclinder to see if it makes a difference to the readings. If it does then it indicates that it is a problem with the rings or bores. If they don't change, then maybe head gasket or valves. If you do get a reading with the oil and the cam timing and valves are ok I would be tempted to do the carbs and see if I could get it to run at all. I bit of heat through the cylinders might just free up the piston rings. You could also try using a bore scope, the aren't too expensive to try to have a look at the bores. Good luck.
Great results! If you were performing this test on a cold cb750 that didn't run what kind of numbers would you deem as satisfactory?
It isn't a question that can be easily answered. A cold engine that hasn't run for a time isn't going to give great results. I would look to see if there was some sort of consistency between the cylinders, within 10psi or so. This would show that there isn't a specific issue with one cylinder. You would also look for 100psi as a minimum. Although slightly under isn't always the kiss of death. When an engine runs and warms through the valves seat better and the rings seal better, this deteriorates over time when they don't run so the figures go down. As long as there isn't too much corrosion or deterioration most of it can be redeemed by running the engine. I hope this helps.
@@SPANNERRASH Thank you! I'm going to look at a 72 CB750 tonight and will be performing a comp test. The bike doesn't run (but turns over) and was wondering where I should draw the line between good and bad comp which will determine how aggressively I will negotiate the price. Subbed!!
@@brazoon1 Nice, hope your haggling goes well and you pick up a good bike! 😃
@@SPANNERRASH Thank you, Sir!!