Topic: 915 motor

[kezz32]

So im building a 915 motor , have ported the head but , I’m using 31mm smooth bores would I be best advised to open the the inlet manifold to suit the carbs ,
Thanks guys

[bwaller]

I'm not a porting expert but for good velocity a reasonable rule of thumb is engine side of slide S/B largest diameter of the port considering seat throat 88-90% of valve head diameter. So to maintain velocity follow above. Depending on intake valve size, you could even use larger carbs.

[livefast_dieold]

I'm not a porting expert but for good velocity a reasonable rule of thumb is engine side of slide S/B largest diameter of the port considering seat throat 88-90% of valve head diameter. So to maintain velocity follow above. Depending on intake valve size, you could even use larger carbs.

Can you elaborate a bit more? I haven't really got this concept :-)

[Sam Green Racing]

Watching this one.

[bwaller]

Watch or comment Sam?

I realize my comment is pretty simplistic, but we didn't get too much info. What cam, valves sizes, what is the engine intended for, street or race? Our old heads are side draft and as such very few guys in this crowd understand best port shapes. This head was self ported?

Always the best advice is to match a seamless passage through the carb, manifold into the port. My point without knowing much more is don't go hog wild once into the port until some homework is done.

[MRieck]

 I believe this fella has an old Clymer manual with the hop up parts section. It talked about heli arcing the spigots and increasing the ID to the carb throat.

[PeWe]

When I bought my Mikuni VM29 to my ported CB750 head the shop mechanic adapted the carb boots to the carbs. Grinded them off a little for a smooth passage carb- carb boot-head spigot.

[Sam Green Racing]

Watch or comment Sam?

I realize my comment is pretty simplistic, but we didn't get too much info. What cam, valves sizes, what is the engine intended for, street or race? Our old heads are side draft and as such very few guys in this crowd understand best port shapes. This head was self ported?

Always the best advice is to match a seamless passage through the carb, manifold into the port. My point without knowing much more is don't go hog wild once into the port until some homework is done.

OK Brent, I'll comment.

Speak to Mike Rieck, 

[bwaller]

Yes sir Mr. Green, I knew exactly what you were thinking..... a wink says so much. 

[MRieck]

Are you using a K head or F2 head