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Old 07-14-2010, 08:54 PM
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350 carb problems

Well my story goes like this. Bought a holley 750 DP off ebay that was listed as coming off of a race motor. (70 jets in front 76 in back, 4.5 PV in front and a block off in the back, and no choke horn. Not sure on what cc pumps it has) I'm wanting to know what it will take to get this running good on my Chevy 350. My motor is basically stock. It has a mild cam, RPM intake with 1'' spacer and headers with flowmasters. I have had this carb on the motor and it obviously smokes and backfires. Haha Kinda new to the older cars. Did a little research and learned how to take apart and identify everything and kinda have a feel for what i need but just want to get some opinions first. I know I need to ditch the PV block off for the secondaries. Do I need to match it with the primary? Ive read that a 6.5 PV is ok for stock applications? Im thinking this is the reason it was hesatating under sudden acceleration. Should the jets be changed also? Any info is appreciated!

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Old 07-14-2010, 09:33 PM
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Quote:
Originally Posted by 19deuce71
Well my story goes like this. Bought a holley 750 DP off ebay that was listed as coming off of a race motor. (70 jets in front 76 in back, 4.5 PV in front and a block off in the back, and no choke horn. Not sure on what cc pumps it has) I'm wanting to know what it will take to get this running good on my Chevy 350. My motor is basically stock. It has a mild cam, RPM intake with 1'' spacer and headers with flowmasters. I have had this carb on the motor and it obviously smokes and backfires. Haha Kinda new to the older cars. Did a little research and learned how to take apart and identify everything and kinda have a feel for what i need but just want to get some opinions first. I know I need to ditch the PV block off for the secondaries. Do I need to match it with the primary? Ive read that a 6.5 PV is ok for stock applications? Im thinking this is the reason it was hesatating under sudden acceleration. Should the jets be changed also? Any info is appreciated!
The 750 DP is a LOT of carb for a basically stock 350 SBC. It can probably be made to perform OK- but ideally, you'd want something like a Holley 600 w/vacuum secondaries, or even better (IMO) a Q-jet.

Without an airhorn (if it had one originally) the part number of the carb is gone. If it's a HP, the number is still on the carb.

The stock jetting varied among all the 750 4779 carbs, from 75 x 76 to 70 x 80, to 70 x 73 to 71 x 80- all w/a 65 PV (the carbs w/the front and rear jetting very close together uses a rear PV, the carbs w/the jetting 9 or 10 numbers apart used a rear PV block off). Accelerator pump squirters were 28 front, 31 (most often) rear.

As for HP carbs, when the 750 HP first came out, up until sometime during the -1 run, they had only front PV's. After that, the carbs got both front and rear PV's.

I do not know what to tell you as far as how to tune this deal, sorry. It will take someone who has actually done a 750 DP on a stock engine.

Good luck.
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Old 07-15-2010, 09:46 AM
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So i Should probably shelf the holley? I have an edelbrock 600 that ran great on it. Just wasnt exciting. haha What kind of motor will the Holley be best on as far as performance upgrades go? I'd like to achieve 400-450hp range.

Thanks for your response!
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Old 07-15-2010, 09:59 AM
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I agree with Cobalt, a 750 DP for a 350 is pretty big of a carb, HOWEVER! not impossible to run. I'm running a 650 DP on a 289 Ford and it runs great, in fact, better than the 600 Edelbrock I had on it. (I know what you mean by poor performance). I would definitely start by changing the Power valve to a 6.5 and get rid of the 4.5. That will make a big difference. The biggest change on my carb was jetting. I had to go down 4 numbers front and rear. Holley doesn't advise going down more than 6. There are things you can play with on the accelerator as well. You want to try and change things back to stock as much as possible. Modified carbs are tough animals, because whoever modified it, did to the specs of the engine the carb went on. I'd try to bolt it on and see if it even starts and runs. Then do a test run, see how it works and go from there. Reading spark plugs is a big indication on how your carb runs. Another option if your getting too much fuel would be playing the squirters front and rear. In your case, maybe start with .028 squirters. Good luck! That carb probably ran strong on a BBF.. sorry, I had to throw that out there since I'm a Ford guy...
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Old 07-15-2010, 09:59 AM
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Quote:
Originally Posted by 19deuce71
So i Should probably shelf the holley? I have an edelbrock 600 that ran great on it. Just wasnt exciting. haha What kind of motor will the Holley be best on as far as performance upgrades go? I'd like to achieve 400-450hp range.

Thanks for your response!
A 750 DP will work on a hot 350/383/400 SBC or a real hot 327. Even a BBC as long as it's around 454/468 cid range and not too radical. 496/502 BBC's w/a cam will want more, but if mild enough, a 750 DP might get you by.
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Old 07-15-2010, 07:50 PM
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Quote:
Originally Posted by kleen56
I agree with Cobalt, a 750 DP for a 350 is pretty big of a carb, HOWEVER! not impossible to run. I'm running a 650 DP on a 289 Ford and it runs great, in fact, better than the 600 Edelbrock I had on it. (I know what you mean by poor performance). I would definitely start by changing the Power valve to a 6.5 and get rid of the 4.5. That will make a big difference. The biggest change on my carb was jetting. I had to go down 4 numbers front and rear. Holley doesn't advise going down more than 6. There are things you can play with on the accelerator as well. You want to try and change things back to stock as much as possible. Modified carbs are tough animals, because whoever modified it, did to the specs of the engine the carb went on. I'd try to bolt it on and see if it even starts and runs. Then do a test run, see how it works and go from there. Reading spark plugs is a big indication on how your carb runs. Another option if your getting too much fuel would be playing the squirters front and rear. In your case, maybe start with .028 squirters. Good luck! That carb probably ran strong on a BBF.. sorry, I had to throw that out there since I'm a Ford guy...
I have had it on the car. Runs VERY rich with lots of smoke and backfiring in the secondary. (assuming due to the PV block off) Did alot of tuning and tinkering and couldnt get it to stop smoking. Jets are currently 70 and 76. Car hesitated bad under 2500 and came into it hard past three and seemed like it didnt want to stop. Decided to do something when i'd go to launch it and it would just die. So the info on the carb above is what i found out after takin it all apart. Also when i took the carb off the motor there was fuel on the gasket at the back under the secondary. Is this cause of the PV block?

Anyway i have two 6.5 PVs on the way from summit and already have a set of 66 and 68 jets. should i run these? i also thought about 68s in the pri and 70s in the sec...

And no offense taken to the bbf comment. I like them all if they are fast!
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Old 07-15-2010, 08:09 PM
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Quote:
Originally Posted by 19deuce71
I have had it on the car. Runs VERY rich with lots of smoke and backfiring in the secondary. (assuming due to the PV block off) Did alot of tuning and tinkering and couldnt get it to stop smoking. Jets are currently 70 and 76. Car hesitated bad under 2500 and came into it hard past three and seemed like it didnt want to stop. Decided to do something when i'd go to launch it and it would just die. So the info on the carb above is what i found out after takin it all apart. Also when i took the carb off the motor there was fuel on the gasket at the back under the secondary. Is this cause of the PV block?

Anyway i have two 6.5 PVs on the way from summit and already have a set of 66 and 68 jets. should i run these? i also thought about 68s in the pri and 70s in the sec...

And no offense taken to the bbf comment. I like them all if they are fast!
Like Kleen56 said, Holley says if it takes more than 6 numbers, the carb is supposedly the wrong size. But I wouldn't let that stop me from trying the different jets and PV to see what the results are. You never know, you might hit the sweet spot and have a decent running engine out of it.

For the accelerator pump cam, what colors are on it now, and are the pumps the standard 30cc size? If they are both the same size, I'd imagine they are the 30cc's (what you want), usually the 50cc pump isn't used on both ends. You also want to use the white cam to begin with. The 50cc cams are brown and yellow, IIRC.


30cc PUMP ASSEMBLY


50cc PUMP ASSEMBLY
Notice the different pump arm and the thicker appearance.
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Old 07-15-2010, 08:19 PM
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Oh yea forgot about that! They are 30cc and have the red cams. I havent researched the cam colors yet so i dont know if these would need changed too.
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Old 07-15-2010, 08:28 PM
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Quote:
Originally Posted by 19deuce71
Oh yea forgot about that! They are 30cc and have the red cams. I havent researched the cam colors yet so i dont know if these would need changed too.
It goes:

cam color #1 #2

white 17cc - 19.5cc

blue 18cc - 20cc

red 18.5cc - 20cc

orange 19cc - 24.5cc

black 19cc - 18cc

green 24cc - 30cc

pink 30cc - 37.5cc

Also check the accelerator pump nozzles closely to see if they've been drilled oversize. You may have a hard time telling, if they did a good job so a numbered drill set or a wire spark plug gap measuring feeler gage would help to determine the size.
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Old 07-15-2010, 09:00 PM
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When you use the block off PV your usually as a rule go with much bigger jets. I had .68 and .72 jets on my DP carb and am currently running .64 primaries and .68 in the secondary. I am running 6.5 PV's. The PV function according to the vacuum on the engine. If the engine has a high lift, high duration cam, the vacuum is much lower than that of of stock cam or motor. Therefore, with less vacuum from the engine, you would use a 4.5 or even less PV depending on the vacuum reading of the engine. In your case, your engine is mostly on the stock side. So, 4.5 would definitely cause some smoking issues and a bad running carb. Very important on Holley's. Check the float level. With gas on the gasket on the secondaries, that might be an indication your floats on the secondaries are too high. Remove the screw on the fuel bowl and see if gas comes out. They should be adjusted to where a slight trickle of gas comes out at best. That will cause a rich mixture and flooding of the carb as well. Check the size of the squirters. My guess is they put bigger squirters in the secondaries as well to compensate for the block on the PV. The number can be seen on the squirter. I use a small mirror and you can read the size. I'd check that as well.

Last edited by kleen56; 07-15-2010 at 09:06 PM.
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Old 07-15-2010, 09:31 PM
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Awesome! You guys are great. Lots of good info. Gonna order some parts from summit and try the holley again. Thanks!
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Old 07-15-2010, 09:34 PM
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Quote:
Originally Posted by cobalt327
It goes:

cam color #1 #2

white 17cc - 19.5cc

blue 18cc - 20cc

red 18.5cc - 20cc

orange 19cc - 24.5cc

black 19cc - 18cc

green 24cc - 30cc

pink 30cc - 37.5cc

Also check the accelerator pump nozzles closely to see if they've been drilled oversize. You may have a hard time telling, if they did a good job so a numbered drill set or a wire spark plug gap measuring feeler gage would help to determine the size.
Ill check the nozzle sizes tomorrow. So with these cam numbers the red ones would probably be too much? guess there isnt much different between red and blue though...
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Old 07-15-2010, 10:30 PM
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Quote:
Originally Posted by cobalt327
It goes:

cam color #1 #2

white 17cc - 19.5cc

blue 18cc - 20cc

red 18.5cc - 20cc

orange 19cc - 24.5cc

black 19cc - 18cc

green 24cc - 30cc

pink 30cc - 37.5cc

Also check the accelerator pump nozzles closely to see if they've been drilled oversize. You may have a hard time telling, if they did a good job so a numbered drill set or a wire spark plug gap measuring feeler gage would help to determine the size.
Is the number 2 hole refer to the one on the carb linkage or the second hole in the cam?
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Old 07-16-2010, 09:13 AM
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Quote:
Originally Posted by kyle86
Is the number 2 hole refer to the one on the carb linkage or the second hole in the cam?
Both, actually. Hole numbers on the cam and linkage should match.
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Old 07-16-2010, 04:01 PM
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Have the two 6.5 power valves. Gonna try to give it another run this weekend.
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