WB1 61x/62t-760 Tuning assistance request

[DCRocks]

I just built a new engine over the winter and am looking for some assistance with the baseline carb setting for tuning.

Engine details:

61x case modded for 760 sleeves
Hot Rods Crank
760 62T Cyl, freestyle port, 64x pistons .5 over
Single SBN 44, 2.5 Needle Seat & Valve
Factory B Pipe Limited
RIVA Girdled head with 32cc domes, around 160psi
Rear Exhaust

I am looking for advice for the jets, and spring size I should start with, and as well as turns of the carb screws.

 

[Matt_E]

Shoot for approx. 30psi popoff, 130 pilot, 150 main.
Would be a good starting point.

Are you sure you got 32cc domes??? Are they 84mm domes? You should have FAR more pressure than 160psi on a fs-ported 84mm T cylinder with 32cc domes.

 

[DCRocks]

Are you sure you got 32cc domes??? Are they 84mm domes?

Yes, and yes

You should have FAR more pressure than 160psi on a fs-ported 84mm T cylinder with 32cc domes.

The engine was sitting out on a cart and I just turned it over with an old battery, so once I get it fully installed, it might register more.

My gauge might be off as well. At that point I was more concerned they were of equal PSI, and at least registered a decent amount of PSI when I tested them.

I thought it was a bit low, but figured it was probably the gauge and/or a weak battery and I didnt really want to crank the engine over much, as it has not has any fuel/oil run through it yet, just the oil/grease I added during th ebuild.

 

[Matt_E]

A 62T/64X cylinder is going to blow a bit less pressure than a 61X cylinder of the same bore due to the higher ports.
Porting - even freestyle porting - can lower that even more.
However: 32cc domes on that engine are most likely screaming for race gas.
It might be worth your while (and your topend) to figure out your compression ratio and go from there.
Better yet, talk to a reputable builder and see what they say.

 

[DCRocks]

Thanks!

 

[sneaky]

those domes came out of the limited i bought, and it was a 701 but it had 205psi compression! 160 sounds low even for 760! :ugh2: ran on race gas too. .

 

[Matt_E]

those domes came out of the limited i bought, and it was a 701 but it had 205psi compression! 160 sounds low even for 760! :ugh2: ran on race gas too. .

What are you talking about? DCRocks' domes came out of a limited 701? In that case, they're not 84mm domes.
Other than that, you can't really compare pressure numbers between the two motors.

 

[DCRocks]

The domes say 84mm 32cc on them, I just double checked.

It is possible my guage is reading low, and I didnt turn the engine over very long to when I took the reading, 5 seconds maybe, as I was concerned over lack of lube due to everything being brand new.

 

[Matt_E]

Have carbs & exhaust manifold on, and pull throttle wide open.
A good battery and starter are essential

 

[sneaky]

What are you talking about? DCRocks' domes came out of a limited 701? In that case, they're not 84mm domes.
Other than that, you can't really compare pressure numbers between the two motors.

i had the same confusion when i tore it apart but the domes fit either way. i have 81.5mm bore. same head now with 35cc x 81 domes

 

[Matt_E]

They fit, but I guarantee you'll run into trouble trying to run 81mm domes on an 84mm bore.

 

[sneaky]

They fit, but I guarantee you'll run into trouble trying to run 81mm domes on an 84mm bore.

i believe you, but how about vis versa 84 domes on 81 bores? thats how mine WAS setup. you think that would cause issues too? i dont know how it ran, but i know thats how it was setup. now it will be 81 on 81.

 

[Matt_E]

Yes, that would not cause trouble. Probably wouldn't run quite right, but it wouldn't destroy anything.