I hinted at this in some-one elses forum, but thought I'd bite the bullet and start a new one. I have a 400 +0.40, approx 9.5-1 KB pistons, 265 DEH cam, 302CC heads with 4v valves, Holley street dom single plane, and standard cast manifolds with dual pipes (a set of 2nd hand headers that I will eventually "fit" later though). This is topped with a 725 cfm holley. I realise that it is a little bigger than normally suggested but I also run a dual fuel set up (propane) to make it a bit more affordable to run. A bigger carby apparently suits LPG better especially if I go twin mixers at a later date for a bit more go! I'd thought I got a 780 and believed that it was ok as that was a Mustang carb and it would be quite cool, but after having it for a while and checking the numbers found it was only a 725 (still apparently a mustang carby). Not that upset really, but heres the question. What would be a good suggestion regarding jetting? The 725 was only for 289-302 and while the bore is pretty close the swept volume etc is all different. I did originally have some issues down low and changed out the pump squirter to a bigger one I had in a spare carby that a friend gave me (I can't remember the number but was approx 3-4 sizes bigger than the standard list) and that made a big difference.
It is in a 72 gal with really tall (2.73ish gears I believe) and goes quite well but maybe not as good as I'd like. Is there a good base line jetting that may give me an idea to start with or should I fork out $100 aussie for a jet kit and start playing? I live at sea level and there is nowhere I will be driving that goes over 3000ft above sea level. local conditions don't go much below freezing and up to 35-40deg C in the summer. More of a sunday road warrior anyway. Like I wrote it goes ok but maybe not as good as it should.
There is also a part of me that says not to touch the jetting until the headers are on so maybe some ideas for when that happens. I have some issues with the propane mixer spoiling the airflow (only rated at approx 450cfm max) when switched to petrol so I have to make a throttlebody type intake that gives another air source when I switch back to petrol. I was just under the possibly misguided impression that between everybody I may get a good cut and dried answer like "dont go below 7$ on the mains and 8& on the secondaries.
So what I'm trying to say is I'm trying to cheat a little but isn't that what these forums are all about? I look forward to all/any response. Cheers

 

what type carb is it. also on the little booster in the ventura or above the throttle plate? Also on the gearing and probably holding lower rpms in daily driver situation, smaller squiter like a 28 just guessing is where I would start. Also their is a cam behind throttle that are different colors and you can change the agressiveness of pump shot to squiter. several types of cams. yellow blacks blue and tan and a couple others. On your carb the type like doublepumper or vacume secondary is a big question I need to know. if I have the tools to check air fuel I jet down to 12.5%O2 or 13%. If I don't have the tools and have to do by the seat of my pants I jet down till I feel power drop off in jetting then I jet up 2 jets or atleast 1 jet above what I felt. have to know your setup well which will happen once you go through all the tuning. Air temp will drastically change things and could cause pinging or other bad things to happen so 2 jets up is a safe bet usually if timing is decent. need some more info to steer in correct direction.

 

thanks for looking WY'. list number R3259-1 holley vac sec 725cfmprimary jet #68, secondary jet #78 and #85 power valve. Strangley the pump nozzle had no number on it, but when I put a squirter in it from a another carby which from memory was around the 29-31 mark (sorry at the time I knew but I recollect it was at least 4 numbers higher). that made a big difference. Going by the way this carb was rebuilt I'd guess that it was back to book standard on all jettings,valves and cams etc. I also put quite a stiff spring in it to address what I though was the secondaries coming in a bit quick. I believe it was nearly the stiffest available (black from memory) but I think i may have been barking up the wrong tree with that one and may try putting it back to standard.
The rear axle is quite tall in the range of a 2.72 or something like that. It is only registering 3000rpm at 70mph on the speedo. it actually goes ok up top once it clears it's throat, it's more the down low I am after. It does/did have a bit of a stumble down low when I floor it, but it did improve with the bigger squirter so the old girl just may need more gas. The rest of the info is on the other post. See what you make of it. Thanks for the help.

 

On your carb air fuel screws what are you at 1.5 turns out or tighter or bigger? Have you checked with a vacume gage. your 8.5 power valve is kinda big but with your gearing believe it might work. need a vacume reading off manifold to give a better guestimation. The black is the heavy spring and holds off the secondarys like mentioned. If you get a chance check out the quick fuel carbs ss series vacume secondays. They have a different way of going about tuning the secondary. scary simple adjust a screw. The 850 vacume secondary have the standard holley quick tune kit. Also on your carb need to make sure your squiter pump has no delay. make sure your 30cc pump arm is touching but not cheating you out of some fuel. this can cause a hesitation. smaller the squiter longer the spray of fuel during of acceleration. their is a fine line once you go bigger squiter like 35 on up. pump cams change the agressiveness of squiter. the pump cams have 2 different positions you can play with 1 or 2 just a flat head screw you can change this. takes about 2 minutes behind throttle linkage. different colors for different situations. good luck but might have created more questions but we are always learning. Also make sure your choke is in straight up position and not moving around and cheatin you out of some air. Need a vacume gage to really give you a better answer.

 

On your carb I usually run a 8 split on jetting. so like a 68 frt and 76 rear. good place to start. Are you running a or auto. need to make sure your car isn't kicking on to next gear to quick. also do you have a metal line from modulator to carb. vacume line can collapse and do strange things in make tranny change gears to quick depending on throttle acceleration. stuff that has kicked my butt on occasion.

 

Sorry, gave you everything but the trans-C6. I think I need to adjust the kick down as well as it comes on a bit early. I wont be playing with it too much up top as I am pretty happy with that so I may pull the plugs and see if i can jump the primaries a couple of sizes-especially if I follow your rule of thumb about the 8 size difference (sounds logical). I do agree about the vacuum gauge and have been planning about getting one of them for a while. Yes you have suggested all the fiddlies and I am a bit scared about all of that. The trans operation doesn't seem to bad, and I am probably trying to get a mild rebuild on a heavy car with tall gears to do something it shouldn't but the basic upshot is when its on LPG it goes better than on petrol all things being equal and considering there is a nearly 400 cfm difference between the mixer and the carb there should be some difference. I checked the squirter pump back when I played with the squirter and that is fine. to infinity and beyond!!

 

On the modulator they have a adjustable modulator and you can make some minor tuning for your shift points. Also have minimum amount of tubing on signal line from carb to modulator. run as much metal tubing possible. If the tubing flexes if messes wiht the signal to modulator. Just more stuff to keep you up at night. I now run a manual shift c-6 lowgear kit in my truck. shift when I want problem solved. all this little stuff just keeps us up at night.