My 1987 Force 125 has a consistant miss that causes the motor to jump or 'buck'. I have replaced or checked the electrical. All 4 plugs are firing. What can cause the miss?

The computer usually picks up a miss fire way before you will notice it. If your check engine light is telling you that you have a cylinder miss fire then there could be several reasons, but starting with the plugs is as good a place as any. Remove them from the engine and see if they are worn away. The electrode can get worn and the gap get too large for the spark to jump consistently. Or it could be a bad plug wire, or if your vehicle is coil fired, possible a bad coil. It might not be in the ignition at all, might be a bad injector.

Go to youtube for instruction on testing the coil packs as well as the fuel system and compression!

Coolant getting into #1cylinder could cause this.

engine miss caused by many things,weak ignition from bad ignition coil, ignition timing off if you have high mileage engine low compression from worn burned valves.could have something minor like stopped up air filter, clogged dirty fuel injectors, through the years i have seen many things cause engine to miss.

Start with plug and plug wires, This is the most common problem with a missfire. Can check by pulling plug wire off the plug with a pair of plyers and see if it is fireing, If it is fireing of it will cause the motor to boug down when you remove, then replace and will start back running the way it was , Do this on all 4 cly's and see witch one is your problem and replace. Good luck to ya.

Usually "bucking" is caused by ignition. Timing or faulty spark plug wires. They will ildle fine, but when you start driving , the miss firing starts.
First, try aiming a timing light at the crank and see if the marks are jumping around, or staying in one spot. If they are jumping, the timing belt, or chain, depending on what we're dealing with, has jumped a cog.
If not, then try changing the wires.
As for the up and down idling, i would think you still have a vacuum leak. Try dumping some soapy water along the intake gasket while it's running, and see if it bubbles.

Check timing chain. Aim a timing light at the crankshaft marks. If the mark is jumping all over the place, your chain has jumped, and will need to be replaced.

You might have a small leak in your vacuum system, check all of the hoses and plugged vacuum connections. Check your plugs and wires, distributor cap and rotor. Sometimes a crack in the distributor cap will cause carbon to build up. Since carbon is an electrical conductor it might be jumping from one cylinder to another that is not in line with the firing order.

Sounds like a load miss. Chk plug wires for deterioration and or cross firing. Wires should not cross over each other. Worn plugs load wires causing increase spark out put increasing cross firing potential

Did you replace the boot that connected to the coil the touches the spark plug. If you didn't then the boot could be bad and instead of the park always going to the spark plug it hits outside the boot on the head. Good Luck