carb tuning ???

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carb tuning ???

Post  stanggt on May 29th 2012, 12:01 pm

I got the 351C up and running and it does make some good power. Drug some 434sbc and 406sbc down the track with it a week ago saturday. Here is the issue, when cruising at any speed, this thing is just really rich. The engine runs rough and stumbles at speed unless u give it some throttle. Also semms to be rich at top end also, when I would get off the gas at the 1/8th mile there is a blue fame rolling out the exaust like u see the nascars when they come off the gas going into the corner. Thinkin thats raw fuel in exaust pipes. It has a holley 4150 1000cfm carb on it. It still has powervalves in it also. I need to ck the vaccum on the engine and see if its the powervalve staying open and causing this condition. When I pull the plugs they are black. should i take out the powervalve and jet or should I drop down to an 850 cfm? engine is a .040 over, 4V heads no porting done, 12.8 compression with a 663 lift roller cam and a funnal web intake.
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Re: carb tuning ???

Post  cturk1010 on May 30th 2012, 6:49 pm

sounds like to much carb for that motor i would use a smaller one

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Re: carb tuning ???

Post  460bronco on May 30th 2012, 9:38 pm

Sounds like she's super fat... I'd try jetting it down before you just take it off.
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Re: carb tuning ???

Post  LivermoreDave on May 30th 2012, 9:54 pm

Both responders before I write valid points. The carburetor is (sounds) rich. As has been mentioned, jet down and check power valves. I'm not sure there is a need for a secondary power valve within your application. I too believe the carburetor is big as to your engine's requirements. Before anything else, check your oil for gasoline.

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Re: carb tuning ???

Post  cturk1010 on May 31st 2012, 5:55 am

problem with jetting down is i have seen to much and then run lean and destroy pistons but most of them were small blocks and chevy guys thinkin they need big carbs to go fast to me i would set the 850 on and it should be a good set up if you jet down it will stumble of the line imo

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Re: carb tuning ???

Post  dfree383 on May 31st 2012, 6:17 am

Probably a little to much carb for 351 CID..... If you have something smaller give it try.
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Re: carb tuning ???

Post  4thHorseman on May 31st 2012, 3:42 pm

cturk1010 wrote:problem with jetting down is i have seen to much and then run lean and destroy pistons but most of them were small blocks and chevy guys thinkin they need big carbs to go fast to me i would set the 850 on and it should be a good set up if you jet down it will stumble of the line imo

Too fat is easily just as bad as too lean. Excess fuel stitting on your ring lands when the plug lights it off and cylinder washing can kill an engine quick.

I'd just try backing off the jetting some. Firstly actually I'd ditch the PV's but that's just me. With that done I'd raise the jetting maybe 2 sizes and make an 1/8th mile pass to see if it cleaned up and pulled harder. Adjust jetting from there.
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Re: carb tuning ???

Post  ianxy351 on June 22nd 2012, 10:10 am

I run a 351C with a solid cam but not as much lift as you. I'm running 650cfm with 6.5PV front and rear and 79/84 jets. You are way over carbed on a 351C with the domi. You will most likely be glazing your bores given what you say. Check your vac first to establish the right range for your PVs, but if you are drag only consider running no PVs and bigger jets. Your problem will be where to start though. Normally I would go 8 - 10 jet sizes up if deleting PVs, but you sound like you are already way too rich anyway. I would run the 850 with the factory setup to set a baseline and then tune from there. Shut down at the end of the quarter and check your plugs. Pain in the ar$e to do each pass but your plugs will give you the best indication of your tune. Also look at your MPH - if you are reducing jet size and not loosing MPH you are on the right track. Once you lose MPH go back up a couple of jet sizes and you should be good. One or two MPH is all you are looking for on a full noise pass to give you an indication.

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