Loosing compression just rebuilt enginge and now it wont start. have fuel and spark but not holding compression

if spark and good compression ..just leaves fuel

will it start with fuel down the carb ?
not flooding
jets adjusted correctly

would say the old carby has had it.. worn out

needs a new one

If you see a spark most likely you have enough energy to fire it. If it wont start more than likely your carb needs to be rebuilt. Kits are cheap, just remove carb, clean with carb cleaner, clean all orifices with a piece of brass or steel from a brush. Try not to enlarge the holes, blow out with air if you have it, be careful to put it back together. Clean out tank an replace fuel lines. If you have compression it should start after rebuild. If you want you can just purchase a new carb an make it easy on yourself. Somewhere between $30-50 to replace. That should do it.

Have you tried spraying carb cleaner or straight fuel in carb and try to start. this will show if it is a fuel issue. You also may have to adjust the carb if it has a high and low or idle screw. Also could be the kill switch. How about the fuel lines are they connected correctly and is the fuel filter connected correctly.

sounds like,yes, Might try pulling the spark plugs and try it then,
if the intake gasket leaked or even fuel leaked into a cyl. when
the compression stroke comes up liquid won't compress so the
engine won't turn over.

some bikes need a certain amperage from battery for the pickups to work, is battery good, and correct type

Its likely the diaghrams in the carb are stiff and holding the needle open causing the unit to flood. Remove the spark plug and turn the unit upside down over a towel and pull the start rope a few times. This will help remove all the excess fuel from the crankcase. Once the crankcase has been purged of excess fuel perform a compression test and check for spark. If its over 90 psi and you have spark rebuild the carb with new diaghrams.

the rule of thumb is spark, fuel, compression. I found I gad a plugged air filter. It would not start until I removed it and tried to start.

Check the fuel pressure and fuel regulator. You can have fuel, but not enough pressure to push threw the injectors.

pull out the plugs and check for compression,the tappets will be loud if not set but it should start

You'll lneed to test for 3 things:
Spark at the plug
Fuel, making it to the cyliinder
Compression, above the minimum spec.

Sometimes when engines have been rebuilt, they have really poor compression as they try to start for a few minutes of cranking. If you have good oil pressure, check compression, if low add a tablespoon of oil to each cylinder to build compression, as soon as it starts it will begin to seat and seal itself, as well as provide ample lubrication.

For Fuel and Spark, just normal diagnostics, based upon your description, I won't mention doublechecking now that it has been cranked a bit.

WD40 can be sprayed into the intake while cranking to get one lubed a bit and to start to idle, it will stall when you stop spraying, indicating a shortage of fuel.

good luck