I have noticable carbon buildup on my plugs and in my exhaust system. I have decided to lower the jet size. So, how should I start, just go down 3 jet sizes? I don't want to get to lean so how can I prevent from doing so?

 

You can't expect to make one change and have everything right with the world. Get an asortment of tuning parts and spend some time working on it.

BigBlockk

Later.....

 

I figured it would take a bunch of trials. Instead of getting the ones I need seperate would I just be better off getting this jet assortment?
http://store.summitracing.com/partde...5&autoview=sku
Is there anything special I have to do for a dual inlet, or is it just changing out the primary jets like normal.

 

If you don't do all kinds of things on your Vette you really don't need a big kit. Just pull off the front bowl and see what you have and then try maybe two sizes smaller. Never make big changes on timing or carbs. You just get confused with the results

 

I have 70's in there now, so should I just go down to a 68 to start?

 

make sure the egr system is working well - if it isn't, seems like it will result in same symptoms (if you still have one)

there are good sources for tuning here by search

 

You might want to post your carb CFM and some particulars about your motor. Then somebody could get you in the ball park on jetting and power valves. single planes and dual planes use different jetting on the same motor

Just by you saying 70 primary I would guess that you have a 750 CFM DBL pumper

 

As was stated by others,buy the kit,and plan on a long inch along process. I've been chasing an idle problem for months now,making small changes to both timing,and jets/air bleeds.

As Gkull said,dual planes vs. single plane intakes on the same engine makes for a whole new ball of wax. My 406 ran a RPM intake for years and the plugs were always sooty black. When I swapped to the Vic Jr.,with a cam change,they went albino white. I rejetted 4 sizes BIGGER,changed my power valve from 6.5 to 4.5,drilled my idle feed restrictors from .031 to .036,and am now playing with idle air bleeds.

Every change I made was run for a week and plugs were pulled to guage the progress at the end of a week. You can't just wave a magic wand and expect the right combination of parts to jump out of a hat.

 

Did you ever think of changing the power valves to about a 2.5,?
I think you have an open circuit in the carb because of not enough vacuum to hold the power valve closed until needed.
This makes holley run very rich.

 

I have a 750 dual inlet. On top of a 350 with alum heads a lunati 235°/240° duration at .050, .490/.490 in. lift, 112° lobe sepration cam, and a dual plane holley street dominator intake. I'm thinking I should change my power vlave also, the stock one should be a 6.5 right? I don't have much vac at idle.

 

Alright, I have a question. I pulled off one of my headers to wrap it and pulled the plugs to get it out. The all of the plugs look like I'm not getting enough fuel they are white. And one tube of the header has white in it. The rest are sooty with buildup. What is the deal here? I will post pictures later.

 

Here are pics of the plugs:

Passanger side:



Driver Side:



I decided that while I was wrapping the header I would replace the plugs and wires so I did that too.

 

It depends on which plugs came out white and which ones came out sooty. You refuse to state which list Holley (there are many varieties of 750cfm dual feed Holleys and it does make a difference) you have so we're just going to go with a garden variety 3310. You should have 72s in the primary. The secondary uses a metering plate, but the jet size comes out to a 78.

If the plugs follow the path of the carburetor to the intake's plenum (the cylinders that are joined on one side of the carb), then you probably have a carburetor issue on that one side. It may be a dirty air bleed, a bent butterfly, bad gasket, or anything else that would allow more fuel on one side of the carb. You just have to go through the carb. I doubt that you'll solve your problem with a power valve. A 6.5 works very well in most setups. You would look to the power valve if you had a very dirty cruise (power valve opening too soon and making cruise A/F ratio too rich) or a stumble on a heavy pedal(too late PV making the tip in lean).

Also, the 3310-2 and beyond were intended for the big inch engines of the day (the 454s) and have a notoriously rich idle circuit. It helps on smaller engines to restrict the idle fuel channel. A .010-.012 wire works well on 383-400 inch engines. A 350 will probably do better with a .015 wire. Getting a cleaner idle A/F ratio will smooth out the engine and give the idle mixture screws some sensitivity.

Your issue is further compounded by a big cam and a lot of reversion in the intake. This upsets the carb signal. And you'll probably notice that the carb gets sooty. It pays to keep it clean.

 

It does stumble when I lay into the pedal hard at low rpms. And the inside of the carb does get dirty. None of the plugs really seemed sooty to me there are a couple that were darker that the others. the pictures are from left to right in order. pass 2-4-6-8 driver 1-3-5-7
I'm pretty sure this is the one:
http://holley.com/0-80508S.asp

 

I took it out a few minutes ago and it was running very smooth, one of my spark plug boots was melted from the header and was arching. That was the reason for the one header tube not being built up with carbon. That still doesn't tell me why my plugs were not very dark but my exhaust is full of crap.

 

Does anyone what to by a high performance small block that needs to be tuned. I quit I can't get the damn thing figured out if my life depended on it.