New top end but no compression or very little

Any combustion engine needs 3 things to run...Spark, Fuel, and compression. You need to figure out what you are missing. Easiest thing to check is spark. Pull the spark plug and hold the metal base against metal engine casing, and making sure you are holding onto the insulating rubber of the boot, have someone help crank the engine over while you hold the plug there. Look for a good strong blue to white spark at the plug gap. If you have no spark or weak yellow to orange spark you will need to look into the ignition system. New plug is the cheapest and easiest thing to change and test again. If it is still a weak spark then you will need to look to the coil/ ht lead, igniter box, or pick-up. While the plug is out, you can check for compression in the cylinder. If you end up having good spark and good compression then you look to the fuel system. If you can't find a problem with your fuel system, your fuel, spark, and compression also have to happen at the correct time....so you check your timing marks. Simple steps of engine trouble shooting.

Misfires are caused by one of 3 things-gas, spark, or compression-one of those is the problem. For gas, check the fuel pressure and see if it is at the right pressure for your car. Low fuel pressure will cause misfires. If pressure is good, it could still be an injector problem, like a clogged injector. If low pressure, suspect the fuel pump or the fuel filter.
For an ignition spark problem, check your spark plugs. Pull them out and inspect them for broken or cracked plugs, check the spark plug gap, check for oil fouled plugs. You may be able to isolate the misfire to a cylinder or two by the plugs condition. A misfiring plug or cylinder should show more black, sooty deposits than the other plugs. A normal firing plug will have a tan or brown colored insulator on the end with very little deposits. If the plugs are okay, clean them a bit if needed, and check for spark on the spark plug wire ends with engine cranking. Buy an inexpensive spark tester, or hold each spark plug wire metal end terminal 1/4 inch from a ground on the engine block, while cranking the engine. Wear a heavy glove to avoid getting shocked. Check that every cylinder is getting a strong blue spark to eliminate ignition as a problem. A weak, yellow or orange spark is an ignition problem, possibly a bad coil, or other ignition part.

If spark and gas is good, you may need to check engine compression. Any cylinder that has significantly lower compression than the other cylinders will cause a misfire, and set a misfire code. You should check for any trouble codes in the engine computer. A code reader or scanner would have to be connected to the diagnostic connector under the dash. What is "significantly lower" compression? About 15-20% less compression. Say you checked and had 150 psi compression in most cylinders. If one had 20% lower compression ( 150 X 20%= 30 psi, 150 -30 =120 psi, or same as saying 150 X 80%= 120 psi.), a condition like that can cause a misfire. I hope I have made sense here. Good luck, and post back what you find, or if any questions.

Spark, gas, and compression. And check timing. You've done the major work. Now you have to make sure you have done it right. By getting it running, you know. I'm pretty sure you can rent a fuel pressure tester and a cylinder compression tester at some national parts stores, Autozone or O'reillys, maybe others.
I don't understand your expression "...a little miss from bottom end" -an ignition miss, or a bottom skip or noise?

Start with checking ignition-a blue snapping spark-what you want to see at all cylinders. Then check that gas is getting into the cylinders. Fuel pressure and injector solenoid pulse. Then all you need after that is compression -good even compression in all cylinders-. Timing is right, spark timing is right, it should crank, er start.
Just one thing about compression, on a cold engine you're just looking for gross irregularities, you can't depend on the readings to be very accurate. A good compression test should be done on an engine at normal operating temperature. It gives a more accurate picture of your engine condition. As long as compression is over 100 psi per cylinder on a cold engine might be a general guideline, I suppose, but take that as only opinion, not fact..

ARE u certain fuel is reaching piston? do a 1/2 sirenge-force' fuel thru carb into the cylinder and try to start if nothing' check compression using a any conventional engine comp gage if compression is 95 or lower entire unit is toast!! most 2-strokes including huska ,echo and stihl will egg-shape cylinder-walls due to wrong mix!!! and happens over two to three hrs use with wrong mix..2 stroke ANTHING MUST be perfect in fresh-fuel with fresh proper oil mis usually 40-1 unlike trimmers most are 32-1 as is chain saws..compression test is always done 1st before i look for da obvious no-spart/fuel da cheapest fix.. for weak or little comp the unit MUST be used for parts and buy an identical unit and use it right!

First question since i am assuming it was just sitting around for some time if i am correct you need to pull the carb remove the float bowl and components if everything was done correctly with your rebuild the likely problem is your carb is gummed up remove everything in the carb low speed jet etc. soak everything in sea foam fuel cleaner overnight and hopefully you have some compressed air and blow out that low speed jet and passage way pay close attention that you have see fluid coming from the other end and check the jet make sure it is unclogged hold it up to the light.

Test the spark plug (gapped at 0.020") for blue, snappy spark. If weak or nothing, remove the left side cover to expose the flywheel and ignition module. Turn the flywheel until the magnets are under the poles of the module, insert a thin business card into the pole gaps, loosen the module screws to allow the module to contact the magnets. Tighten the screws and turn the flywheel somewhat to release the card, then turn the flywheel one full turn to make sure the gaps don't close up. Remove the ignition switch wire from the module body, and try for spark again. If nothing, replace the module using the above procedure. If you did get spark, replace the ignition switch wire and try again--if nothing now, look for a short circuit to ground in the wire or the ignition switch. If you got a good spark in the first test, pour a little fuel mix into the plug hole and try for start--if pops several times, you have fuel delivery problems. Check the fuel filter, air cleaner, and muffler for plugging. Check the fuel lines for decay or other damage. Make sure the primer bulb fills with fuel when pumped several times. Does the engine seem to have good compression? Hope some of this helps!

Make sure you have Spark at the sparkplug and that your sparkplug is in good condition with proper gap.
Makle sure fuel is getting to carbs, by loosesing the carb drain screw - fuel should flow out.
if your getting fuel, tighten the drain screw back and turn the motor over.

make sure key switch is on and on/off switch is on.

if motor doesn't start, spray a very little of carb cleaner spray into air filter box.

if motor starts and dies that means that your carbs are getting gas but the jets are clogged and not allowing fuel to go through the carb, pull your carbs off, disassemble, soak, rinse, blow out with compressed air, make sure you can see through the jets, reassemble and your bike will run.

If your getting fuel through carbs and you have good spark, then pull your spark plugs and do a compession test. You must have at least 100 psi compression, if not your rings are bad, or you have a stuck valve.

If no compression:
You can check and adjust your valve clearance, only when the motor is cold.
Do this and then recheck your compression. If compression comes up. Your motor with run.
If still no compression. Then you must take the top end apart and see why.

Have your head inspected to make sure valves are seating.
Check your piston ring end gap.

You will find it to be one or the other to cause your loss of compression. Fix and repair.Reassemble.

a 2 stroke motor with no compression is usually a blown head gasket.

Timing Reference High Resolution Signal A ( No Pulses) This is a misfire code so try this info. This info is where your mechanic would be following.

The following info comes from a code reference guide for troubleshooting.


Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.
A random misfire code usually indicates a vacuum leak or bad gas.
a misfire in a specific cylinder should lead you to check the spark plug, fuel injector and compression.

something is creating alot of air to pass over the 02 sensor.


Low compression
Big vacuum leak.
Weak ignition
IAC stuck a little open.
TPS that is saying your foot is on the gas when it is not.
EGR stuck open a little.
Too low an idle
Weak spark plugs
Bad fuel pressure regulator
No vacuum to the fuel pressure regulator.
Bad coolant temp sensor.

What are the readings. Need to know the co and the hc reading in order to know if you have a misfire or an over fueling issue. All 5 reading are really needed to diagnose fuel related issues sight unseen

yeah something you wanna check next, compression. if you can get your hands on a compression tester.