99 blaster jetting.
- Thread starter 99blasterMan
- Start date
Blaner
Your Friendly South African Ambassador
Use genuine Mikuni jets...jetting is specific for each motor, do a search on the forum. You need to leak test, then plug chop. but ballpark you looking at 270 - 300.
Blaner
Your Friendly South African Ambassador
https://www.blasterforum.com/threads/leakdown-testers.55015/
https://www.blasterforum.com/media/over-built-leak-down-tester.8826/
I cant find the exact link quickly to the thread that explains how to perform the test, but basically you need to determine if your motor is air-tight. Any air entering the motor that is not metered through the carb will mess up jetting. (Crank seals, reed cage, spark plug etc) So you need to take off the exhaust, plug the exhaust port with a rubber bung, put the piston at BDC (take out spark plug to check, then re-install plug) take off carb, insert leak tester in place of carb and slowly pressurise the system to 7psi. Watch the gauge and make sure the motor doesnt loose the presssure. if it does, you have a leak...I use dishwashing liquid/water mix in a spray bottle and spray all over the motor to determine where the leak is. Its ALWAYS advisable to fix the leak before trying jet, you will soon fry your piston otherwise.
Generally, leaks occur around the reed block (easliy fixed with a engine silicone sealer) or around the crank seals if damaged or old. These can only be replaced by splitting cases.
You can easily whip up a tester, I use a small bicycle pump on mine. Dont forget to leak test your leak tester too!
Otherwise, 320 is too big for those mod. If you not ported and do not have a rechambered head, Id go less than 300. Search for plug chops too, they are helpful.
HOWEVER, the fact that your motor wont rev passed idle raises a few concerns, have you checked your reed valves for damage? Also, dont overlook the needle in your carb. The adjustment here plays a massive role in your off-idle mid range engine work, which is whre you operate most of the time. Ensure your needle clip is engaged in its slot and that the needle is seating properly. Your clip should be on the middle groove, move the clip UP to LOWER the needle (Leaner) and move the clip DOWN to RAISE the needle (Richer.).
In summary:
https://www.blasterforum.com/media/over-built-leak-down-tester.8826/
I cant find the exact link quickly to the thread that explains how to perform the test, but basically you need to determine if your motor is air-tight. Any air entering the motor that is not metered through the carb will mess up jetting. (Crank seals, reed cage, spark plug etc) So you need to take off the exhaust, plug the exhaust port with a rubber bung, put the piston at BDC (take out spark plug to check, then re-install plug) take off carb, insert leak tester in place of carb and slowly pressurise the system to 7psi. Watch the gauge and make sure the motor doesnt loose the presssure. if it does, you have a leak...I use dishwashing liquid/water mix in a spray bottle and spray all over the motor to determine where the leak is. Its ALWAYS advisable to fix the leak before trying jet, you will soon fry your piston otherwise.
Generally, leaks occur around the reed block (easliy fixed with a engine silicone sealer) or around the crank seals if damaged or old. These can only be replaced by splitting cases.
You can easily whip up a tester, I use a small bicycle pump on mine. Dont forget to leak test your leak tester too!
Otherwise, 320 is too big for those mod. If you not ported and do not have a rechambered head, Id go less than 300. Search for plug chops too, they are helpful.
HOWEVER, the fact that your motor wont rev passed idle raises a few concerns, have you checked your reed valves for damage? Also, dont overlook the needle in your carb. The adjustment here plays a massive role in your off-idle mid range engine work, which is whre you operate most of the time. Ensure your needle clip is engaged in its slot and that the needle is seating properly. Your clip should be on the middle groove, move the clip UP to LOWER the needle (Leaner) and move the clip DOWN to RAISE the needle (Richer.).
In summary:
- build or source a leak tester
- Leak test motor and repair if necessary.
- Put in a 270/280 main jet.
- put needle on middle clip
- test.
- adjust needle clip one groove at a time down one notch, TEST, then up one notch from middle TEST again.
