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 25 BiG BLOCK Carb throat vs 31 Big Block 
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Joined: Sat Nov 18, 2017 7:57 pm
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Post 25 BiG BLOCK Carb throat vs 31 Big Block
Ive been researching the difference in the 25 BB vs the 31 BB which has the same Cubic in motor. Looks as if the camshaft is different and the carb is different. Can anyone tell me the difference in the throat size on the two? Id like to know if its worth buying the stock 31 carb and rejet, vs the mikuni. OR .... if they have the same throat I can just rejet to match the two and save a bunch. Thanks for any info. my thoughts are change the cam and carb to match and I should have a 25 BB converted to a 31 BB exactly.

Also if I buy that 31 carb ifg it has a larger throat will it bolt up the same? THANKS!


Mon Nov 20, 2017 8:57 am
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Location: Rowlett, TX
Post Re: 25 BiG BLOCK Carb throat vs 31 Big Block
Don't know the difference in diameter, but if you're gonna do it, get a 35 carb. The difference in the 31 to 35 carbs was noticeable. Would assume they use the same intake, but really not sure.

Don't invest a bunch of money in it either. If you're spending more than a few bucks, go ahead and get a mikuni instead. If you can get a bigger stock carb for free or something, then go for it.


Thu Nov 23, 2017 8:50 pm
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Post Re: 25 BiG BLOCK Carb throat vs 31 Big Block
Thanks for the help. I did a little bit of research and found that the mikuni has an accelerator pump on it for greater throttle response and you can tune it as well. SO I did purchase one. I will let you know how it works out. The stock carb has no accelerator pump. My motor really boggs down when you give it gas almost falls on its face until super warm engine , even at that time its not great. SO I think the mikuni will do the trick.


Mon Nov 27, 2017 9:53 am
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Location: Tarkington, TX
Post Re: 25 BiG BLOCK Carb throat vs 31 Big Block
the 25 BIG block as a WOT limiter on the carb. Cut that off and your pretty much a 31 carb. Still a single barrel unit. I would get a 35 carb.
Yes the cam is different as well. I have NOT checked it compared to the 35 cam
25 and 29 have the same cam27, 31, 35, 37 have the same cam

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Tue Nov 28, 2017 8:00 am
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Post Re: 25 BiG BLOCK Carb throat vs 31 Big Block
So if you have a new 25 BB, the BPS stage 1 with rapid muffler vs Q muffler. What cam would you recommend? I know nothing past this point. I'm looking to gain a few more mph with a load. Not trying to go too big on mods but at willing to change can because I hear that's what I need to do? Let me know what cam you suggest and if there's a specific one? Also if I'm changing the cam and I have iron mountain heads from bps do I need to change any valves, pushrods, etc? Thanks for the help guys. Hoping this is the last question I have


Tue Nov 28, 2017 8:25 am
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Location: Tarkington, TX
Post Re: 25 BiG BLOCK Carb throat vs 31 Big Block
I will Recommend Bayou Surface Drive for modifications.
They can do it all

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Tarkington, TX 77327


Tue Nov 28, 2017 11:29 am
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