My engine reached 11,500 rpm and now the spark is week what do I do

what sort of vehicle is a 11,500 automobile

ok, sounds like you covered the basics with spark. I would check in this order; 1-Throttle position sensor, sometimes it wears out and you get poor signal back to computer, 2-clean MAF sensor, 3-check/clean IAT (intake air sensor). If this still doesn't help then I would check your fuel flow....but i'm thinking its your tps. Two screws will change that out quickly.

If you did not have this issue before you changed the plugs then ,yes, plug type and plug gap can cause your complaint.

1. Yamaha Fazer Engine and Transmission
   • The 1998 Fazer is powered by a four-stroke, 16-valve, in-line, four-cylinder engine with displacement measuring 36.55-cubic inches. This motor generates 95 horsepower at 11,500 rpm and 45.0 foot-pounds of torque at 9,500 rpm. It has a six-speed transmission and a liquid cooling system. The 1998 Fazer has a top speed of 139.8 mph.
     
   Yamaha Fazer Brakes, Weight and Fuel Capacity
   • The 1998 Fazer has dual-disc front brakes and single-disc rear brakes. It has a seat height of 31.1 inches at its lowest setting and a dry weight totaling 416.7 lbs. The Fazer has and electric starter and a fuel capacity of 4.75 gallons.
     
   Yamaha Thundercat Engine and Transmission
   • The 1998 Thundercat is powered by a four-stroke, 16-valve, in-line, four-cylinder engine with a displacement of 36.55-cubic inches. It generates 100.00 horsepower at 11,500 rpm and 48.7 foot-pounds of torque at 9,500 rpm. The Thundercat has a six-speed transmission and a liquid cooling system.
     
   Yamaha Thundercat Brakes, Weight and Capacity
   • The Thundercat has dual-disc front brakes and single-disc rear brakes. It has a dry weight of 412.3 lbs. and a seat height of 31.3 inches at its lowest setting. The 1998 Thundercat has an electric starter, a 5.0-gallon fuel capacity and a top speed of 158.4 mph.
     

Knock at low rpm and high rpm" It may be oil pump , After oil heats up it gets thinner and and less lubracated. I would change the oil and add some oil stabalizer, shouls help, if not probaly a rod.

You are hitting the rev limiter top range, which cuts off your ignition.

I have a 2001 Grand Cherokee and the problem drives me crazy, but it is the way the car is designed- as soon as you hit the rev limit, it shuts down ignition and your jeep will "buck" - a real pain when you are trying to accelerate into traffic!

You have the 4.0L straight six engine, yes?

If so, please take my advice and learn to live with the minor annoyance of the rev limiter- that engine is IMHO the best Chrysler ever made (nearly "bulletproof"!) and it was so reliable you cannot even get it in a jeep now! For decades, that engine was used in welders, tractors, generators...and yes, jeeps... and it is a favorite among mechanics for its durability!

go to auto zone and they will run the codes for free

Check the fuel lines for cracks/breaks. If they are o.k., then:

You will need to disassemble the carburetor and spray with carb.cleaner. Make sure the small screen is free of debris.(Should be able to see daylight through it.)

Use a Den Tek Brush, (http://www.drugstore.com/products/prod.asp?pid=165357&catid=1152),to clean the small portals in carb. These portals can become blocked. Reassemble and start.

If the carb. parts are more than 4 years old, you may have to replace them.

If you need further assistance, just let me know.

Jim

Ck or replace your MAF sensor. First sensor in the duct coming out of air filter. It controls the air/fuel ration

I would look for a fault in the ignition system such as a cracked spark plug. Might need a scope check to put your finger on the problem