[Tracer64]

I'm helping my friend put a blower on his truck. He has it running now ( I believe its a 144 roots type) with a draw through carb, a holley 750 DP. We are going to tune it this weekend. Are there any special mods we need to make to the carb? Right now its running really rich, I think that is just a jetting issue.
Thanks
Tracy

 

[Blown68a]

If it is a draw thru type carb, then that carb is really for a centrifugal w/ a carb enclosure, but they can be made to work w/ a roots. The roots might cause the carb to richen up even when you take jet out of it. You might have to take a couple rounds out to get it to lean out and even then, it might still go rich on you. What kind of boosters are you running? I have found that if you are using a double pumper w/ the truck boosters or one w/o the downleg boosters that can impead the flow of air in the venturis. They will send a strong signal under boost to stay rich so as not to go lean. I had this problem on the dyno w/ my motor recently and that is why I am switching soon to a Mighty Demon 850 which has downleg boosters and open smooth venturis.

 

[Tracer64]

I found out its just a box stock holley like you would by anywhere for a normal car. Does that effect it or do I need a "blower" carb"
Sorry for the confusion
Tracy

 

[Blown68a]

No, you don't need a blower double pumper carb. All they have is a manifold referenced power valve and you can get a regular double pumper to work just as well. You'll just have to take some rounds of jet out until you can get it to lean out.

 

[torino501]

[No message]

 

[torino501]

whoops.....i think centrificals use blow threw carbs,all 3 carbs have somthing diff about them,(boosters,power valves,float design ect

 

[mavman]

I sure have seen quite a few posts lately about Draw-through carbs. Trust me, they're a HUGE PITA unless it's on a roots blower. Centrifugals and turbos with draw throughs...well...they'll work but it adds complexity to the system. With complexity comes $$$$.

 

[ujt389]

For a draw through you need to make sure to boost reference the power valve, that is probibly why you are running rich.

 

[Tracer64]

This is my first supercharger so please bare with me. How do you modify the carb to boost reference the power valve? I read somewhere that eldelbrocks are better for blowers?
Thanks again
Tracy

 

[tonys10sec306]

you can bolt a box stock carb with the roots blower

 

[Motorhead]

The power valve will work fine, you just might have to change the size of it once you are dialing int the cruise/power mode transition. At no load you will still have lots of vacuum under the carb, which will hold the power valve closed just like it should, and at WOT it will open up again just like it should. You just might have to play with the power valve number (stock is 6.5) to get part throttle driving just right. Other than that, just adjust the jetting 'till it's right, and MAKE SURE you don't go too lean.

 

[jeremyl]

draw thru - plain ole carb. carb sits on top of blower, tune it like you would a regular setup.

blow thru - bonnet/box on carb, pressurized bowls, special floats, and boost referenced fuel pressure regulator. It's a bit more work. centrifulgals & turbo setups are usually this way.

Good luck!