BT 2X4 intake/jetting front carb differently

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.

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.

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.

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

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.

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.

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.

So I did the 50cc pump on the front carb only and it's a huge improvement. I also had my idle mixture screws at 1 1/2 turns on both carbs to get idle AFR's high 13's bouncing into 14's but when I had the intake off only ports 4 and 8 were black. At the time I thought it may have been from the PCV going in at the bung on the back of the intake,even though I have a separator on the PCV line,but now I think it may be from the primaries over the rear 2 ports. I've been able to turn in the rear idle screws a 1/2 turn and open the front carb 1/4 turn. AFR's stayed almost the same and my idle exhaust temps have evened out a bit more. It was kinda stinky too and that's improved. I think once the RPM's get up around 3000,the extra flow/turbulence and pulling out of the boosters negates that problem with the positioning of the rear primaries at low rpm. The 750's were very lean at WFO,AFR in the 14's,so I drilled out the metering plates the equivalent of 6 jet sizes and that brought it down to 12.5 to 12.7. Runs great. We have a couple nostalgia meets coming up in Aug and Sept so I feel the car is sorted enough to take down the track and I'm looking forward to that. I'm a little nervous because we had a guy from Minnesota died in a crash up here in 1965 driving a Thunderbolt. The twisted hulk is still squirreled away locally too. I have a couple buddies that were at the track that day. Creepy.

http://www.motorsportmemorial.org/searchall.php?s=Jack+sund&db=ct&s_into=all&s_db=selecteddb&Search=Submit

Very Nice.

The 3310-2 and latter dash 750's with the number 21 metering plates are pretty lean on the secondary's compared to the 3310 and 3310-1. They are a (3310-2 and Up) 74 jet eqiv. and of course no power valve in the rear, with 72's in the primaries, plus a power valve.

To go up 6 jet sizes the old Holley books show a .093" hole to be an 80 jet eqiv.. That's (80 Jets) what most 750DP's have by the way in the back, with 70's in the front. Also the 750DP's have a slightly smaller venturi's, so there's that.

Did you use the thick Brown REO Pump Cam, at almost full shot on the 50cc pump, just curious?

Be safe and always remember racing is a blood sport, I hate to say.

Looking forward to some good passes and hopefully a singe digit timeslip.

The 50cc kit came with a purple (brown?) and light tan cam. The tan cam must be be for dominators because the notch for the throttle shaft was way too big for my 750.

the tree man wrote:The 50cc kit came with a purple (brown?) and light tan cam. The tan cam must be be for dominators because the notch for the throttle shaft was way too big for my 750.

The pump levers are different between a Dominator and 4150, also.