rebuilt engine compression

[gmgbv6]

I recently purchased a basket case '67 305 scrambler with rebuilt engine. The pistons and rings are new. I don't know if the cylinders were bored and if oversized pistons were installed. This being said, the problem i'm experiencing is low compression. Bill Silvers text says that I should be looking for 150-175 psi Both cylinders read @ 100 psi until I add a spoonful of 30 wt. oil. Then the pressure climbs to 150 psi. The motor loses power at higher rpm's and misfires. I've assumed that the low compression is the probable cause Would low readings such as these be typical of a newly rebuilt motor before the rings become seated? The motor has about 50 miles on it since rebuild and I was thinking that the rings should have seated by now.<br /><br />Any thoughts that you might have on the matter would be greatly<br /><br />appreciated.<br /><br />Thanks,<br />Gary Lee

[vintagedude]

<br />Interesting question, and I do have some opinions. In my learnings, if you have an engine with low compression, and add oil, and it comes up radically in compression, its almost always the pistons/rings. If you add oil and it does not, (or comes up very slightly) its almost always the valves.<br /><br /> One thing I have learned with HONDAS and bikes in general over the years is that if they have sat for a long time, they MAY go down in compression due to sticky oil on the rings and guck in general.<br /> MANY TIMES, I have had an engine that reads rather low after sitting 20 years, then after firing it up, changing the oil, it reads normal compression!<br /><br /> But more importantly, I DO NOT RELY on the compression guage to read an engine condition, nor diagnose it. <br /><br /> I use a LEAKDOWN TESTER.....<br />I Put the engine on compression stroke and pressurize the cylinder with it, and read the leakage in a percentage. Usually 1 to 10% is optimum.10-20% there is some wear, and at 30% to 40% there is a serious problem, and at 90-100% there is a hole in the piston the size of a quarter dollar coin, or a valve with a big chunk missing..<br /><br /> So how do I know if there is a problem with rings or valves? Well its very simple! JUST LISTEN!<br /> If its leaking at say...40%, the air is going somewhere.....right? Well, put your ear up to the exhaust outlet, if you hear a loud air rushing, its an exhaust valve.<br />Put your ear up to the carburetors, if you hear air, its the intake valve. Pull the dipstick, do you hear air at the dipstick? Its the RINGS/PISTONS!<br /> If you hear air at all locations, then engine suffers wear everywhere. <br /><br /> In fact I dont even use a compression tester, well yes I do actually, its called my INDEX FINGER. I hold it over the spark plug hole, and crank engine. If it blows my finger off the hole, IT HAS COMPRESSION!<br /> I then use the LEAKDOWN TESTER to ASSESS the ENGINE CONDITION!<br />In closing, use a compression guage for TWO STROKE ENGINES<br />and use a LEAKDOWN TESTER FOR 4 stroke ENGINES.<br /> Mac, Snap-On, Lisle, Cornwell, etc all make leakdown testers, they are quite inexpensive!...You must use a compressed air source to pressurize cylinder however.<br /><br /> Ok then...go &quot;leakdown&quot; that engine and get to the TRUTH, instead of guessing!<br /><br /><br /> Your friend, Wilbur Wiggins<br /><br /><br /><br /><br /><br /><br />

[gmgbv6]

Wilbur,<br />Thank you for the excellent explanation! I love this bike, but was at my wits end until now. I'll post the results of the leak down test as soon as I get them<br /><br />Best wishes,<br />Gary Lee

[jesse]

be sure that you have the throttle all the way open when checking compression.yes the rings should have seated in 50 miles.could be that the cylinder walls were not honed when the rings were installed.if that is the case then they will never seat properly.as for the high speed miss,it is not likely to be caused by low compression.probably more like bad points or carburetor <br />problems.<br />later<br />jesse

[gmgbv6]

Jesse<br />Thanks for your insights. Upon further inspection, I noticed that one of carb spacers had cracked between the outside edge and the o-ring, allowing air to leak into the mixture after the carburetor and causing a lean condition in the fuel mix. I noticed also that one set of points were arcing while the other set were not. I ran a file through them and reset , and will retune using EMO's tune up procedures as soon as the engine is ready.<br />The kick starter does not engage and turn the engine over much of the time, and so I'll be looking to make that repair . I'm guessing that the pawl is worn and needs replacing/repairing. Any more insights?<br />Cheers,<br />Gary

[jesse]

i have several 305 scramblers and would say that you are right about the starter pawl.it's quite a job to replace.also when you work on the carbs be sure not to over tighten the nuts as the slides will bind if you do,and pay close attention to the timing(both sides)when you replace the points.sometimes it's a matter of point gap versus moving the point plate to get both sides in time.but thats why we like these old bikes ,so we can play with them.<br />have fun

[ray]

If your engine passes the leakdown test it could still be OK. my C77 is rebuilt with Honda CB750/4 61mm pistons. these are a popular replacement for Dream engines as they are more readily available than O/S Dream pistons, but are a flat top piston as opposed to &quot;Dome&quot; top pistons fitted originally.<br />There fore your engine has now become a higher capacity, low compression engine. Mine runs GREAT this way and still has Plenty off power. I agree that the backfiring is unrelated and is either air mix or timing related