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351cj 4300D runs super lean

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Grodiekitty View Drop Down
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    Posted: 11-April-2022 at 1:58PM
  First time post and I am pulling my hair out over this. I have a 72 Gran Torino sport fastback 4 speed with a fresh 351cj and original 4300d and I can hardly get it to even run. Modifications are KB pistons 10.3 compression ratio, comp cams extreme energy camshaft, adjustable valve train, pertronics ignition. The only way it will run is with the idle cranked way to far up and partially choked, even when warm, and even then it seems like it has a lean misfire and runs terrible. When you cover the air horn the idle speed drastically increases. I installed a edelbrock 600 with a adaptor and was able to get it to run fine. It idled about 700 albeit with a very noticeable lope but seemed normal for the cam and had good throttle response. I had about a half turn out of adjustment on the idle screws. But I can’t get the 4300d to run worth a crap on it. I bought another clean original checked it over and tried it and it did the same thing. I have been through the carb a dozen times, understand every circuit, chased and cleaned everything multiple times and put up to 68 jets in it. Idle screws seem to do nothing, but the circuit is sound and I can see plenty of fuel dropping. The secondary’s are fully closed. When you stab the gas it gets a good accelerator shot and the speed raises but then slowly falls like it is running out of gas. I have increased the timing up to the point it labored to start hot. Every cylinder carry’s 160 psi on compression check. It had about 5.5 inches at idle with the edelbrock on it. Valve train has been quadruple checked at a half turn lash. I even loosened all the rockers and sealed off the manifold and it would hold a pound or so for a couple minutes. Any more and it would blow the caps off. There are no open vacuum sources, everything has been plugged including the booster, vacuum advance and pcv. If anything it should be running rich because of the cam. I am going crazy. And also I want to be able to run this in a stock class so I have to use the 4300d. Hoping to get some different view points to try to figure this out. Thanks,  Curt
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Rockatansky View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 11-April-2022 at 2:55PM
check to make sure the float isn't hanging up in the bowl, i 1/2 way seem to think i remember
someone saying they had to trim / shave the sides of the float for clearance?
if the float isn't dropping free, you're not filling the bowl and it'll be lean
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pete rad View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pete rad Quote  Post ReplyReply Direct Link To This Post Posted: 11-April-2022 at 3:31PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Grodiekitty Quote  Post ReplyReply Direct Link To This Post Posted: 13-April-2022 at 1:43PM
So I did double check the float and the level was a little low but after setting up a rig and getting it at close as humanly possible to 13/16 it did not seem to make difference. I did also check side to side clearance and it had enough when properly adjusted. I could also see the fuel level through the accelerator pump hole and it looked nearly level to the gasket. I ended up putting the 4300d on my 69 mustang f code 302 with adaptors and got it to idle fine at about 700 with 19 inches of vacuum. What I did find is that it had a lean transition off idle at about 800-1300, but it did not think that was uncommon for this carburetor. What I’m beginning to believe is that it’s just that there is no way to adjust this carburetor so it will function with a engine that is only drawing 5.5 inches of vacuum at idle. Unless someone has a better idea my next step is to put in a stock replacement camshaft. Thanks for the help, Curt
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 13-April-2022 at 6:00PM
make a change to the appropriate air bleeds slightly?

IDK if the 4300-D is a 2 circuit or 3 circuit carb?

my 1st inclination would be to enlarge the air bleeds but i'm not positive ATM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Grodiekitty Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2022 at 1:06PM
Tore it down today and the new cam goes in next week. I will post update when complete.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2022 at 5:14PM
i think you're on the right track as far as figuring that a very low vacuum camshaft can have a huge effect on carb metering. the Barry Grant Demon line was marketed as being calibrated internally for specific camshaft duration ranges. Holleys have some degree of internal metering variation but it's not widely touted and therefore not many are even aware of the possible disappointing results of choosing the wrong carb. not all carbs within a cfm class are created equal!

so getting to the factory tuned 4300-D, i contend that it had issues from day 1 with the experimenting of fuel blends going on at the time. whatever that is coming out of the nozzle today is even more different than it was then from what the carb was intended to deliver. today's ethanol blend will be lean no matter what unless steps are taken to correct the AFR profile. in this case changing the camshaft to a grind that will act on the metering system hard enough to pull fuel would be on that list


Edited by Rockatansky - 16-April-2022 at 6:09AM
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Grodiekitty View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Grodiekitty Quote  Post ReplyReply Direct Link To This Post Posted: 16-April-2022 at 3:23AM
Thank you for the insights! I really do appreciate it. And as stated I will give a follow up after the cam swap. Another cam for the parts shelf I guess.
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