zliminator Posted March 7, 2006 Share Posted March 7, 2006 I had a 350 built that was supposedly barely streetable - 500 lift cam, oversized valves, 1 5/8 headers - and its been running really rich - flooding out easy, black spark plugs, fumes. The carb thats on it is a Holley 750 DP with mech 2ndary and someone told me that all I needed was a 600cfm. Then I called another mechanic I know and he said that I needed the 750. The list # is 4779-5. It takes a #71 jet in the primary and a #80 jet in the 2ndary and a #65 power valve. I'm going to put the right jets in it and drive it again, but I just wanted to get some opinions. zliminator Quote Link to comment Share on other sites More sharing options...
2126 Posted March 7, 2006 Share Posted March 7, 2006 Let me see if I have this correct....you have a 350 cu in motor thats barely streetable with a 750 DP and you drive it on the street! If this is correct, its no wonder you have fouled plugs and its running rich. The set up you have is best run at or near WOT. Your dumping a load of fuel into the engine at lower RPMs, and the lower RPMs in not where this engine and carb is itended to be happy. Maybe with smaller jets and possibly delaying the opening on the secondaries, you could make it a bit more streetable at the expense of possibly loosing a little on top. Quote Link to comment Share on other sites More sharing options...
280Zone Posted March 8, 2006 Share Posted March 8, 2006 I'd try borrowing a 650 from someone and try that. Next I would modify the mechanical secondaries so they are not kicking in too soon. Most common mistake of most people is to over carburate their engine. Quote Link to comment Share on other sites More sharing options...
fastzcars Posted March 8, 2006 Share Posted March 8, 2006 Try this site . It has some great info on how to tune a holleyhttp://www.bob2000.com/carb.htm. Quote Link to comment Share on other sites More sharing options...
Turbo Meister Posted March 8, 2006 Share Posted March 8, 2006 The list # is 4779-5. It takes a #71 jet in the primary and a #80 jet in the 2ndary and a #65 power valve. I'm going to put the right jets in it and drive it again, but I just wanted to get some opinions.zliminator Your carb has basically stock jetting and should be good with your engine combination. Your float level may be set too high or you have some other adjustment problems. My 750 dp performed best with 69/79 jets on my 383 before I turboed it. Hanns Quote Link to comment Share on other sites More sharing options...
ngrimm Posted March 8, 2006 Share Posted March 8, 2006 You might want to take a look here http://www.mortec.com/carbtip1.htm to get some background on carb sizing and pay special attention to power valve selection and how single/dual plane intake manifolds affect carb sizing. Norm Quote Link to comment Share on other sites More sharing options...
spdsk8r Posted March 8, 2006 Share Posted March 8, 2006 I have a 355 sbc with a 510/305 cam. I am running a 600DP with mech. secondaries and have had no problems. Quote Link to comment Share on other sites More sharing options...
Paz8 Posted March 8, 2006 Share Posted March 8, 2006 Greetings to all, new guy to the forum, my first post. I may be missing something here but you failed to mention what kind of trans your running, if it's a manual it's not as bad with a built up sbc made to put out near WOT, BUT, with a auto (700r4 as example) you need some low end power band to make it streetable, back to your 750 CFM ?, try the jets and adjust your accelerator pumps to make them squirt less fuel, I think the quadrajets that Chevy used on 350 were around 750 CFM, don't know for sure. Paz8 Quote Link to comment Share on other sites More sharing options...
zliminator Posted March 8, 2006 Author Share Posted March 8, 2006 I just got a Holley 600 off of ebay for $24 that needs rebuilding. And yes, it does have a 700r4. The next thing I probably need to do (after reading the links you gave me) is build an o2 monitor. I already have the sensor and the bung in my exhaust pipe. zliminator Quote Link to comment Share on other sites More sharing options...
zliminator Posted March 8, 2006 Author Share Posted March 8, 2006 the mechanic that built the motor and did most of the conversion work. He insisted that I needed a 750 DP and he also insisted on the 3" dual exhaust. He advertises a lot in the Houston Z car club and I wonder about what he has done to other customers after seeing some of the stuff he did to me. His name is Kin Chan of Reliable Automotive in Houston, TX. He did a good job building the motor, but I think he was suffering from delusions of granduer for the most part. Quote Link to comment Share on other sites More sharing options...
Mike C Posted March 8, 2006 Share Posted March 8, 2006 600 is a waste of your time IMO. Better to have spent the $ on a book on Holleys. I have a stage III BG 750 that flows 1040 cfm on my mild 355 and it idles perfectly at 700 rpm. Even if theoretical cfm of a 600 is sufficient, real world tests prove over and over more power is made with a bigger carb (up to the point of diminishing returns, obviously) If you want to spend some money, get a Holley or Proform 750 cfm replacement body with removed choke horn, replaceable air bleeds, and convert to 4 corner idle. What is your idle set at? If you are in the 1000 rpm range, you mave have started to uncover the transition slots which can flow fuel from the main circuit. If this is the case, remove the carburetor from the engine, flip it over, and increase the air flow through the secondary throttle blades at idle by screwing in the set screw on the rear throttle shaft. Then re-install the carb and close the primary throttle blades by the same amount you opened the rear. This will increase airflow without affecting the main circuit. Re-set idle mixture screws and this should help dramatically. You can experiment with a 45 power valve as well. What shooter and pump cam are you using? What position? Does it bog when you hammer it off idle? Seriously consider purchasing a book that explains how to tune Holleys, it's well worth the $. Quote Link to comment Share on other sites More sharing options...
jt1 Posted March 8, 2006 Share Posted March 8, 2006 Read MikeC's post about 3 times, it's good advice. The problem isn't with the main jets, it's the idle, transition, and PV circuit. Most likely the idle circuit isn't responding because you have the primary blades open enough to engage the transition circuit. Open the secondary blades enough to get the idle circuit air bleeds responding, clean the idle up, then tune the PV and then the jets and pumps. jt Quote Link to comment Share on other sites More sharing options...
desert dog Posted March 8, 2006 Share Posted March 8, 2006 Good advice all... And check to make sure you have no vacuum leaks. I had a seriously modified 350 SBC and used a Holley 4777 650 DP. It was plenty of fuel for the engine's 7100 RPM redline... Quote Link to comment Share on other sites More sharing options...
staledale Posted March 8, 2006 Share Posted March 8, 2006 Zliminator, Check your power value. If you had a backfire and blew it out it will leak. Turn your idle mixture screws all the way in, and if it still idles it's getting fuel somewhere else. I ran many 750, 780's on 350's with no problems, a good rebuild might fix ya up. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted March 8, 2006 Share Posted March 8, 2006 adjusting the secondary butterflies applies only if he has a newer carb, the old ones didn't have that adjustment. Quote Link to comment Share on other sites More sharing options...
Alf Posted March 8, 2006 Share Posted March 8, 2006 i think your main problem is your camshaft and transmission choises. as was said earlier this combo would be more streetable with a manual transmission because you control the RPMs and when the car shifts. your combination is a mid to upper RPM engine. whats the compression ratio? it should be at least 9.5.1, if not that may be part of the problem. the holley 750 DP is a good carb for a 350, i have one on my 355 in my Camaro with a slightly smaller cam(.490 lift) with a muncie 4 speed. this is a much heavier car than a Z but it is fairly streetable, MSD box helps alot. if i was you i would either swap to a manual transmission or change the cam. many people think peek horsepower numbers mean everything, but how often are you running at the RPM peak hp is produced? the fact is if you have more average hp you car will be faster, more streetable, and more fun. you will get better fuel milleage too. try to grab a book on holleys, the knowledge learned is worth every penney. Quote Link to comment Share on other sites More sharing options...
zliminator Posted March 8, 2006 Author Share Posted March 8, 2006 to what it should be (71 on primary - 80 on 2ndary) and I did put a #65 power valve in. It seems to be more reasonable now, but like you said, I will do some more homework. BTW, the CR is 10.5:1, I think. The tranny is a 700r4 and the rear end is a LSD out of a 300ZX. Thanks. Quote Link to comment Share on other sites More sharing options...
Alf Posted March 9, 2006 Share Posted March 9, 2006 what kind of vacuum is this engine pulling? Quote Link to comment Share on other sites More sharing options...
GAZRNR Posted March 9, 2006 Share Posted March 9, 2006 After reading the posts I didn't see if you had checked your fuel pressure and set the float level on both bowls. Next check your timing and set it 12-14 make, then make sure the firing order is correct. If you get it to idle check the vacume and your may need to see what's in there for a power valve. The engine will run with a 750 DP and I noticed a number of other carb recomendations. For what it's worth on my 350 I tried a 600 Edelbrock, Holley 650 DP and spent time tuning each carb for my application after all was said & done I ended up with a 750 CFM stage 2 Jet Quadrajet on my 350. Many will say the Quadrajet is the best carberated way to mix your fuel & air for the street...to beat the Quad you need to step up to EFI. I not trying to get into a whats the best...argument, look into it and make your decision. Rusty Quote Link to comment Share on other sites More sharing options...
zliminator Posted March 11, 2006 Author Share Posted March 11, 2006 good question Alf, I have a vac meter and I will check it today. I will also check the timing and firing order and get back to you. zliminator Quote Link to comment Share on other sites More sharing options...
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