Jetting setup on big bore

CARLOS_ABRAHAM188
Edited Date/Time 3/20/2018 11:56am
Hi guys. Im sorry this is not about my dirt bike but its bugging me, ive put a 3mm bigger size piston on this thing. But i dont think its running like it should...
Ive already changed main and pilot jets one size up but still feel it not ok. What do you guys think? What do you suggest? Its my fist time installing a big bore. And ive always struggled to tell from the spark splugs

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Paw Paw 271
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Paw Paw 271 3/14/2018 11:32am
3/14/2018 11:32am
I have almost never seen many big bore kits require a jetting change as all you are doing is increasing the amount of air fuel mix that is entering the engine to be increased at the same jetting as it were before. Now this can change if the compression ratio was altered via the additional engine size or there is an intake leak or if the engine is running hotter. In those cases the jetting change would be helpful, but only as a band aid fix for the real problem at hand.

Paw Paw
fullysicmate 3/14/2018 1:30pm
3/14/2018 1:30pm
Thats interesting, I am about to fit a 490 kit to my KTM 450 and was assuming mapping changes would be required.
Paw Paw 271
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Paw Paw 271 3/14/2018 3:02pm
3/14/2018 3:02pm
fullysicmate wrote:
Thats interesting, I am about to fit a 490 kit to my KTM 450 and was assuming mapping changes would be required.
Now remapping may be needed, but that is EFI not a carb.

Paw Paw

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fullysicmate 3/14/2018 3:06pm
3/14/2018 3:06pm
fullysicmate wrote:
Thats interesting, I am about to fit a 490 kit to my KTM 450 and was assuming mapping changes would be required.
Paw Paw 271 wrote:
Now remapping may be needed, but that is EFI not a carb.

Paw Paw
Righto, cheers.
BobPA
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BobPA 3/15/2018 7:39pm
3/15/2018 7:39pm
Getting the jetting correct with a pod filter is sometimes tough. What is it doing?
crc245
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crc245 3/15/2018 9:24pm
3/15/2018 9:24pm
Typically you have to lean out the jetting on big-bores. More draw so smaller jets required. On our 2008 CRF450R to CRF488, we went down one on both the main and pilot (back to stock 42P and a 175/178M). This was with a +4mm kit, for reference. Best of luck and please keep us updated.
speed_racer
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speed_racer 3/15/2018 10:10pm
3/15/2018 10:10pm
Paw Paw 271 wrote:
I have almost never seen many big bore kits require a jetting change as all you are doing is increasing the amount of air fuel mix...
I have almost never seen many big bore kits require a jetting change as all you are doing is increasing the amount of air fuel mix that is entering the engine to be increased at the same jetting as it were before. Now this can change if the compression ratio was altered via the additional engine size or there is an intake leak or if the engine is running hotter. In those cases the jetting change would be helpful, but only as a band aid fix for the real problem at hand.

Paw Paw
I concur with the above statement. I use an EGA to test jetting on the big bore's and they often do not need changes.

Now the bike in the picture needs major jetting changes because Honda made it with an air box that was closed off for the most part, also the exhaust is wrong, but for now we won't go into back pressure.

This is a good start for your jetting.

Airbox missing and modified exhaust, go 3 main jet sizes for the airbox and two for the exhaust. 5 sizes larger
We count sizes in 2.5 increments. Example= stock main jet 100, modified size 112.5
The pilot jet could need 2 size jump. Those bike came lean, remove the airbox and you will need fuel.
Test, take a rag and wrap in around that air filter, see if it runs better with less air.
CARLOS_ABRAHAM188 3/20/2018 11:56am
3/20/2018 11:56am
thanks for the support guys i think i got it running good now, i tried 1 step down on pilot jet and it felt lean right away, it backfired and the engine felt way hotter, so i went back to stock and it feels good now, but dumb me put a sound restrictor on the exhaust tip and that was causing the problem, it felt flat at 5th and 6th..... i removed that and noticed the difference right away! im very pleased with it now!!
THANKS

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