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BT 2X4 intake/jetting front carb differently

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Post  the tree man May 12th 2024, 12:27 pm

Curious if anybody has done this. With the carbs reversed the front carb primaries look to be centered over the 6 forward ports and the back one is over the rear 2. Looks like the front one is doing most of the work when cruising around. I have the engine running really well now but I've wondered about the positioning vs carb tuning and will probably try screwing around a bit. The pump shot in particular has been a bit of a chase. I'm up to a .042 on both carbs and it's very close but could still use a little more. I was going to do a 50cc pump on the front carb only with a longer cam to help crutch it over that flat spot until rpm's catch up. Cruise/wide open AFR's are good and when I temp gun the headers they're all close but every once in a while I get the lean spike when I pedal too hard from very low rpm. I know this engine is not really built for that type of city driving but if I can eliminate that I'd be really happy. Any thoughts or input appreciated.

BT 2X4 intake/jetting front carb differently 20200311
the tree man
the tree man

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Post  supervel45 May 12th 2024, 1:39 pm

50cc Pump/Cam Crutch or Stiffer Secondary Springs, Bandaids Balancing act kinda.

Aftermarket metering blocks with changeable IFR jets and main body's with removable air bleeds. More pain in the ass.

^ Problem with more transfer slot fuel is you will be fighting your cruise AFR's where the main booster overlap and both circuits or flowing. Crying or Very sad

Or a Looser torque convertor.

50cc Pump/Cam easiest route I suppose.

I would not get too picky if it is running well most all of the time and an isolated stumble.


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Post  rmcomprandy May 12th 2024, 11:53 pm

YEP ... but, I don't remember what they were.

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Post  the tree man May 13th 2024, 12:32 am

I'm going to start with the bigger pump shot then try some other changes (IFR,IAB,etc.). I'm curious how it affects EGT's also. I keep good notes when I'm making changes so I can always go back to this if things don't work out. Thanks for a little reinforcement.
the tree man
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Post  BBFTorino May 13th 2024, 3:59 am

A friend had that same setup on a 514 crate motor, and he experienced much of the same conditions. I remember him mounting the carbs so that the primary's on both were in the middle. Yeah, the linkage and fuel line routing was a head scratcher, but it did work quite well.

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Post  supervel45 May 13th 2024, 10:08 am

the tree man wrote:I'm going to start with the bigger pump shot then try some other changes (IFR,IAB,etc.). I'm curious how it affects EGT's also. I keep good notes when I'm making changes so I can always go back to this if things don't work out. Thanks for a little reinforcement.

Just an idea if you get real deep into them.

Lock out the secondary's with a micro clamp/s on the vacuum pod rod/s, or do it my other means, to see what the affect is on your suspected primary stumble tuning.

Don't forget secondary metering plates have IFR holes along with secondary transfer slots in the baseplate. Sad

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