SOHC.co.uk Forums > Project Board
New (to me!) '77 CB400F - US model
mattsz:
Thanks for the input, guys. I guess I can only try it and see what happens.
The history is as follows: I rode the bike in the summer of 2015. It seemed to run ok then, decent power up to the red-line at least, anyway. The PO rode it a couple hundred miles next summer, 2016. It was stored in an unheated garage from the fall of 2016 through fall of 2018. The guy who stored the bike said he drained the fuel tank, but not the carbs, and that he "kicked it over a few times" during that period, but I have no idea what that really means. When I got the bike in 2018, I examined the dipstick oil and found it clean and clear, removed the plugs and squirted some oil in the cylinders, then kicked it over myself and found it to spin freely. I buttoned it up and put it away. It spent the winters of 2018 and 2019 in two different heated garages.
Stuck rings are certainly a possibility, the cause being rust also a possibility. Just giving some recent background. Wondering... how often does an engine's compression fail this badly, but also uniformly across all four cylinders simultaneously?
Bryanj:
One big problem is the legth of flexible hose on the compression tester, i know you tried 2 testers but if they had about the same hose legth it could be your problem. The space left in head when piston at tdc is not much and could easily be doubled.
Thats one reason i say try it and see
mattsz:
--- Quote from: Bryanj on June 02, 2020, 02:21:02 PM ---One big problem is the legth of flexible hose on the compression tester, i know you tried 2 testers but if they had about the same hose legth it could be your problem. The space left in head when piston at tdc is not much and could easily be doubled.
Thats one reason i say try it and see
--- End quote ---
Yes, that thought entered my head as well. Both the testers I tried had similar 12-inch-ish length rubber hoses. I've read posts espousing that effect on the US site, but I've heard other smart people reject that theory with arguments that sounded very convincing to me, partially ignorant as I am to the finer points of internal combustion engines. I do find the uniform nature of my test numbers to be suspicious.
Is it a linear relationship? If the tester is doubling the "compressed" chamber volume, would that imply a reading of one-half the normal value could be expected?
EDIT: I just tried the "thumb test" on cylinder #4 - did my best to get a good seal, cranked it over... it blows right by my thumb. I folded up a "latex" glove to help get a better seal, and tried again with enough force to cause discomfort to my thumb... I may have been able hold against the compression, but not sure. Does this suggest anything?
K2-K6:
Where is the release valve on the test gauge?
As Brian says the tube will be part of the volume ( and give problems of reading) if the control valve is away at the gauge end of the tube as the tube volume is then evacuated at each exhaust stroke.
If valve is located right where the spark plug location is, then the tube remains charged and should then report peak psi, which should be close to specified data for the engine.
mattsz:
--- Quote from: K2-K6 on June 02, 2020, 03:15:48 PM ---Where is the release valve on the test gauge?
As Brian says the tube will be part of the volume ( and give problems of reading) if the control valve is away at the gauge end of the tube as the tube volume is then evacuated at each exhaust stroke.
If valve is located right where the spark plug location is, then the tube remains charged and should then report peak psi, which should be close to specified data for the engine.
--- End quote ---
The release valve on both the testers I used is located at the base of the gauge, not at the engine end. I guess the volume of the gauge should be considered as well...
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version