TheSilverBuick's 1967 Thunderbird

[TheSilverBuick]

It's an aftermarket Holley 2bbl, but it's supposed to be bolt on for a GM 4.3L or SBC. I believe the lines are 5/16th in and out, but don't remember for sure.

As for being rich at idle or low throtte, it has not been an issue. I can easily tune 15:1 AFR's at idle and low load.

BUT

I often wonder if I'm tuning around fueling problems without noticing. Example here would be too much fuel pressure, well I tell the injectors to open less on the fuel table, effectively putting a correction factor in the table. Of course that would probably cause other weirdness, and if I'm experiencing side effects from it I don't reconize it due to inexperience and only working on two aftermarket EFI systems. Similarly I have the reverse thought on the Skylark. I'm using a stock Ford pump with a 5/16th fuel line diameter to feed 462cid big block with upgrades. Am I getting fuel pressure drops at high rpm? I don't know, maybe I'm correcting for it by opening the injectors longer to account for a fuel pressure drop? All I know for sure is, at WOT, ~6,000rpm in 4th gear (well over 100mph), I'm running the injectors at 75% duty cycle and have 12:1 AFR. Plenty rich (and safe).

[Bamfster]

For reference, all of the GM TBI trucks that I have worked on (even the BWB) have had a 3/8 feed and a 5/16 return.

[TheSilverBuick]

For reference, all of the GM TBI trucks that I have worked on (even the BWB) have had a 3/8 feed and a 5/16 return.

Then that is probably what it is on mine. The Holley is supposed to be a bolt on deal. Remember my return line from the TBI goes to my surge tank, it's the return line from the surge tank that is 1/4". Of course it has a 3/8th feed line from the mechanical pump going into it =P The TBI has the proper return line diameter going to the surge canister. I wonder if because I'm using a mechanical pump if that the most the surge canister can over pressurize the system is 5-6psi before it effectively "dead-heads" itself.

[Bob Holmes]

Too be safe, match the return and supply, Weldon actually recommends a larger return than supply. Given the marginal increase in cost, larger is better.

[Russell]

Can you put a pressure gauge on the surge tank? if the return line if flowing freely it should be ~ 0 psi?

[TheSilverBuick]

Can you put a pressure gauge on the surge tank? if the return line if flowing freely it should be ~ 0 psi?

Oh I most certainly could, but I don't own a fuel pressure gauge (other than on the fuel regulator on the Skylark), and I'm not curious enough to buy one =P I have thought about installing a ball valve on the return line to see how fast I can drive the car (if at all) off just the mechanical pump with the electric unplugged. Found out before I installed the surge tank that the mechanical fuel pump makes enough pressure to start the car, idle and do some no-load rev'ing of the engine.

[TheSilverBuick]

I've been driving the T-bird the last two days and further refining the tune, which has been surprisingly difficult considering how easy the Skylark is to do, and I think I finally figured out why.

The throttle linkage geometry is poorly ratio'd. It doesn't take much to get to 30%+ throttle, so it's harder to drive the car at low loads (normal cruising is like 15%). It's like driving a car with mechanical secondaries where a slight amount of throttle starts opening the secondaries. Once I realized this driving the car has been nicer and easier to tune, but I think I'm going to try and modify the linkage to slow down the throttle plate opening rate.

Conversely at very low rpm, <1200rpm, WOT kills the air speed in such a way the fuel must be dropping out as the engine refuse to RPM at that point. It's fine with some rpm though. I now get why there are vacuum secondary carb's.

[milner351]

Is there another place on the throttle actuator arm/ bracket to connect the throttle cable?

[Russell]

is their any away to set up progressive linkage? My old Lincoln Mark 8 has a 2bbl TB with progressive linkage...

[TheSilverBuick]

I have to dig up the picture, but as it sits I need to get a linkage kit of some kind and add a mounting ball to the top of the linkage bracket. That will give me more pedal movement and less throttle blade movement. Right now I'm using factory 4bbl linkage on the TBI because it had the Ford kick down lever for the transmission and the throttle cable ball to clip onto.


This isn't a perfect picture, but you can see where the throttle cable clips on to the bracket, you can see where there is a part of the bracket higher up I can attach too, but will have to watch for an "over center" sticking point. And no the throttle return spring isn't located there anymore.

[milner351]

Just be careful that you don't set yourself up for any sticking point, especially not at WOT!

been there, done that, replaced the mufflers....

[Russell]

I bought a package of tc balls when I did my motor swap in the 89 was about $20 thought I was getting ripped off but had to have it now!! (Then)

[Beagle]

take a look at the throttle cable pulleys on some of the other throttle bodies, they aren't circular. They're cam shaped to change the ratio. Maybe something like that might work for you?

[TheSilverBuick]

Here is a better picture. The local parts store doesn't have a holley or edelbrock carb linkage kit, so I'll have to check online for one.

[Russell]

http://www.jegs.com/i/Holley/510/20-...FQ0GnQodAyIA4A