sb50

iTrader: (1)

So I just fired up my engine Friday with a used 850 Holley dp and it wouldn't idle. My specs are as follows lq4 bottom end, ported 243s,eps cam 232/246 .607/615, vic jr carbed intake. My buddy and I had problems with this carb on a afr headed 350 a couple of months ago. So I know it's the carb. I swapped it out for a 770 vac secondary Holley street avenger and idled fine but upon revving it wasn't as responsive as 850. I do not know how to tune a carb but plan on buying a wideband soon. Should I just rebuild 850 dp,use the street avenger or call pro systems and let them sort it out? I would rather not buy a new carb because every one will need to be tuned. I want to know what the masses think because it seems the consensus around here for carbed applications is 750 dp. Does any one have a used one for sale to get me started in right direction? Both of these carbs came off of the 350 sbc due to our lack of knowledge and a pro systems carb now resides on it. So what should I do?

Pop N Wood

iTrader: (1)

Pro systems will get you close, no doubt about that, but they usually need tuning to get them just right.

There is a place called Karb Kings in Pittsburgh that offers a similar service. They will actually rebuild your existing carb. If you sent it to them and tell them in detail what it is doing they could make sure it is mechanically OK.

With your cam and intake an 850 isn't out of the question. General consensus is smaller carbs are easier to tune. Your choice on the 750 vs. 850.

The problem with your 850 could be something like a bad gasket, blown power valve, blocked jet/air bleed, warped base/metering block etc. Something like that could be fixed by rebuilding and checking everything carefully. But it just might be too far out of tune for your motor also.

I agree with your logic on the new carb. You might get lucky and get one closer to being correct right out of the box, but chances are anything you get will need some tweaking. Might as well work with what you have.

ZONES89RS

iTrader: (25)

Surely sounds like the carb needs to be freshened up. Probably trash in there or as mentioned. Bad power valve. Ect. Time to start learning!!

sb50

iTrader: (1)

Would I be better with street avenger or 850? It seems the vac secondary carb stumbles off of idle and is that typical of a vac secondary carb? I'm confident I can rebuild both but what about a used 750 dp and rebuild that?

ZONES89RS

iTrader: (25)

750 will get a ever so slight better low speed signal. Not enough to talk about on a 6.0. The vac sec probably has a smaller squirter so it will stumble till it is tuned with a 50cc acc pump and or larger squatters and pump cam.

speedtigger

iTrader: (16)

This pretty much sums up what you will need to do, but I agree that it will be far easier for you to start with a new carb simply because it eliminates variables. As for the Pro-Systems deal, you would not be any further ahead than if you started with an out of the box Holley 750 HP. Pro Systems usually gets the main jet pretty close, but most of the time, a Holley is pretty close from the factory. As for the Pro Systems carbs being dead on for idle, part throttle cruise and such, not likely. Myself and many others have found that you will simply need to learn to tune your carburetor to get the fine points right.

Once you learn that you will need to make some fine tuning changes to get the carb they way you want it, then you will need some adjustability that the Pro-Systems carbs and the baseline Holley HP carbs just don't have. At that point you will be wishing you had bought a Quick Fuel Q-Series carb. They have all the good parts, replaceable bleeds, emulsion ports and orifices your need to truly tune your carb.

I say get right carb to start with. Get a Quick Fuel Q series and you will have everything you need going forward. I have many say they wish they would have bought one to start with. I have never heard anyone say they wish they wouldn't have bought it.

Doug G

When the secondary comes in...does it stumble right away or goes then stumble...could be a simple pump shot or squirter size swap ?
I like the DP for max effort cars Vs. a VS for more street friendly stuff.

speedtigger

iTrader: (16)

I agree with Doug here, don't waste time on a vacuum secondary carb for a build like yours. You need a mechanical secondary for sure. How much converter do you have? Also, what are the rest of the specs on that cam that Geoff spec'd for you?

sb50

iTrader: (1)

I will check when I get home for rest of cam specs. Car has 3400 to 3800 coan converter and 3.73 gear. I originally wanted a 4l80e but tired of car sitting. It has been a long process with swap but really put my head down last couple of weeks to get this thing rolling.

sb50

iTrader: (1)

I made mistake on cam specs they are as follows 230/246 .600/.615 279/298 111lobe separation 111+4

speedtigger

iTrader: (16)

Those are some good looking specs for what you are doing. Geoff definitely knows what is up.