Long story short, after switching from my unmodified 400 last year to a 455, I am finally getting around to making some adjustments to my carb. The carb is not original, it's a 7040240. The car always ran ok, but it seemed to not pull too well at the top, and I was only getting about 100 mph in the 1/4. When I opened it up, I found that it was jetted with 67's and 41 rods! I searched the board and the closest to my combo was running 76/48, so that's what I have in there now. It now jumps pretty well and feels like it wants to go! Should have done this a long time ago... While in there, I also changed the needle and seat, fuel filter, and put on a new secondary pull-off. Also, tinkering with some different secondary rods. The only problem now is a brief stumble when I go from idle and leave from a dead stop. This wasn't there before. once it gets through about 1200 rpm, it's all good. Any ideas on getting this little miss out? thanks, dewey
Is your accellerator pump rod in the second hole, meaning the hole closest to the carb body? That is necessary. If you think that this condition has something to do with your rod / jet change, then swap the primary rods back to the 41's - way richer! See what happens... My $.02
Thanks, Andrew. Yes, the rod is in the closest hole. I'm now thinking that I need to work on optimizing my idle mixture and rpm, as it was idling around 900 rpm when I fired it up and I didn't mess with it. I was just happy that it wasn't puking gas back at me and running good. maybe that will take care of the hesitation.
Generally, a hesitation of this type is caused by too rich an idle setting. Idle mixture screws should be out about 1 1/2 ~ 2 turns. More than that can cause problems. Try adjusting the idle mixture control screws to say 1 3/4 turn, then set the idle RPM with the throttle positioner screw to the correct idle with the engine hot. Jon.
Wow! You may be on to something. that would be way leaner than where it is set. I screwed them in and back out before I fired it up and they were right at 5 turns out. I will turn them in and see what I get. thanks, dewey
better late than never... Finally getting back here for an update. I turned the idle mixture screws in to 2 turns out and got a large improvement. Moving in in 1/2 turn increments and, while much improved, still have a very slight stumble from a dead stop. What else should I be checking? thanks, dewey
I'm still thinking power piston spring. I'll bring my nifty power piston spring tension measuring tool to Temple and we'll see how yours compares to mine.
Thanks, Nick. I was trying to remember what you told me but couldn't find it in my inbox. Are you thinking I have too strong a spring and it's holding the rods down too long or vice-versa? I will pull the air cleaner today and see what is going on in there. i may open up the spare carb and see what that spring looks like if I need to swap them. dewey
Something to consider here is the relationship between the throttle and the openings. If the idle speed and mixtures are optimized, the throttle should be very close to closed. If not, the idle transfer slot gets no fuel. If this slot does not feed fuel , the resluting hesitation can be impossible to troubleshoot. The timing and the idle mixture settings should be spot on to assure the correct opening. The correct procedure to set the mixture is best lean idle, regardless of the number of turns to accomplish that. Ray
My best guess is the spring is too stiff, keeping the rods from seating all the way in the jets at idle. This fits with Jon's (carbking) rich idle analysis, and the fact that it got better when you leaned out the idle mixture. When I had a cam about the size of yours I went to a significantly lighter PP spring.
Warped Q-Jets = Trouble My friend and I bought the tools to unwarp a q-jet. His carb seeped fuel and would never idle properly and was very cold natured. After using the tools to flaten the carb, He was amazed at the difference. It runs perfect, no longer leaks and warms up much faster. If tuning your q-jet is not getting results, this may be the answer. My Buick carb is so warped it looks scarry but runs good. However, it seeps fuel and that alone is enough for me to soon be puting the old carb in the oven for straightening.
Q-jets get warped by overtightning the front two bolts. I have not seen a Q-jet that didn't have some warpage. Try quadrajetcarburetors.com for a solution. Or you can send it to me, I have the repair kit.
One thing I never see mentioned here is float level. The '70's carbs had a low float level setting and raising it to 11/32" will greatly improve driveability if it is low. :TU:
Dewey- your carb is too rich I noticed your carb # and original jetting indicates you have a 750 carb. If so, the 76 jets are for an 800 and should be about 70 or 71 with 40-41 rods for your 750! The 750 carbs start at 67 & 69 stock and jet up to about 70-72 for a 12 sec car. The 800 carbs start at 73 & 75 stock and jet up to about 76-78 for a 12 sec car. Too rich will hurt gas mileage, burn your eyes and nose, and increase ring and bore wear, foul your plugs, slow you down, etc.
new game plan! Thanks, Gary. Since this thread started, I have run about 500 highway miles resulting in zero passes at the track due to bad weather and track operator's indifference. Car still runs like a champ other than the off-idle stumble. Gas mileage appears to be a little better than before, certainly not off drastically. In hindsight, I now realize that I made way too many changes to the carb at one time to be able to assess the effect of any single item. Rather than do a lot of trial and error on the 750cfm, I am going to stick on a working 7041540 that is a 800cfm and try it as-is. Next, I will make small incremental changes so I can figure out what's working and what's not.
