Does your choke open fully..

do you have a PCV valve hooked up?? its a calibrated air leak..

open the secondary idle setting.. done from under the carb or with a J shaped screw driver.. opening the secondary will allow you to close the primary farther to limit the exposure of the idle transition slots..

this will allow the engine to idle only on the idle circuits..

could you have more issue.. yep.. this is where you start to baseline the idle settings..

use a mirror and timing light aimed down the primary carb bores to verify you don't have fuel dribbling out of the boosters at idle.. you should have no fuel from the boosters till around 1900 to 2000 RPM.. everything below that speed is idle and idle transition circuit.. primaries don't fully take over till around 2500 RPM..

 

I think this may be the issue because at idle it's extremely rich. Is there a J bent screwdriver or is this something I'm gonna have to make? Thank you for the info I'm still learning the Holley dp and I can use all the help I can get.

 

Spark plugs

Do you go to a colder plug or stay at the same heat range as before? Cold plugs foul up pretty badly when driven around at street RPM.

Max

 

Do you go to a colder plug or stay at the same heat range as before? Cold plugs foul up pretty badly when driven around at street RPM.

Max

I'm not sure what heat range the plugs are to be honest. They are Accel header plugs, part #8198

 

You didn't say; what's your vacuum reading @ idle w/ vacuum advance disconnected? Also, what is your BTDC timing set at under the same condition as above?

 

I don't have a vacuum gauge yet but I'll grab one after work. My timing is set at 18* initial, vac advance off and plugged and 36* total at 3000 rpm

 

From my experience,I've never had any luck at all with Accel plugs.Not much luck actually with anything Accel for that matter,LOL. If everything else checks out,you may wanna consider a different brand plug.

 

What kind of spark plug do you recommend for use with headers? I only got the Accel ones because they were header plugs and they were out on display and seemed to be good ones. I'm pretty new at this though so a little lesson on spark plugs would be very helpful.

 

If you can buy a set of ngk spark plugs as they are a little shorter then other brands such as ac delco or motorcraft etc. I use the ngk plugs since they are shorter and on my s10 v 8 swaps the other brand plugs are really close to the headers on certain cylinders and I have to use special high temperature resistant socks to put over the boots so they don't burn up but with the ngk plugs they clear enough so I don't have to put any high temp socks to cover my spark plug boots from burning up being to close to my header tubes.

 

X 2 ^^^ Alot of ppl don't like them & depending on your header fit, but, i'v had great service from the Bosch +4 plugs.

 

Well I got a vacuum gauge and hooked it up to the lower port on my Holley 750 dp. I am new at this but I figured since that's the full manifold vacuum port it would work. So in gear with the parking brake on, it's making 6"Hg which is not much but I kindof guessed it would be low due to the cam and the excessive overlap that comes with it.

I got a new set of the same plugs, Accel header plugs, just to try one drive and see what they looked like and this it what I got, black and sooty again.

 

manifold vacuum..

the small fitting on the front of the base plate is ported vacuum.. its just slightly above the throttle blade ..



the J shaped screwdriver is something you would need to make out of bent welding rod..

 

Far too much fuel going into the motor. I would only use a double pumper if it were in front of a manual transmission or in front of a 3000+ stall converter. Describe the cam and give us some numbers please.

 

TH400 with a 3000 rpm converter. The cam is .496"/.496 236/236@.050 and a 106 LSA solid flat tappet by Comp Cams

 

With only 6 inches of mercury at idle yo need a 2.5 power valve Assuming a Holley carb. My bet is that the power valve is opening at idle.

 

With only 6 inches of mercury at idle yo need a 2.5 power valve Assuming a Holley carb. My bet is that the power valve is opening at idle.

If you know exactly how the Holley metering system works, and how the passages in the metering block are laid out, you would realize that this statement is absolutely false. It can never happen...it is a common old wives tale repeated word of mouth and in magazines for years, but it has never been true.

The only way you can get excess fuel at idle from the powervalve is if it's diaphragm is blown, creating a leak straight through it.

What can be happening is the cruise vacuum is too close to the powervalve rating, and that could be the reason the plugs are black, the powervalve providing fuel at low load cruise when it shouldn't, and that would be the reason for needing a lower rated valve....but 2.5" is a little low from a 6" vacuum reading, 3.5" might be more appropriate....it will take some road testing to determine which matches the motor the best.

I never like Accel plugs either, they act too much like Champions, which never seem to work well in an iron head Chevy - they foul too easily.
AC Delco, Autolite, or NGK are my preference.

 

I agree with OldTech. If you're going to stay with your existing DP carburetor, the 2.5 power valve should be installed.
Also, if you do convert to a different Holley carburetor, you'll find that most come with a 6.5 power valve, so a 2.5 will still be needed.

Additionally, what RPM do you have in-gear at idle? With your cam, you may need to increase the RPM setting to increase your vacuum, to counter the cam's possible reversion issues.

 

I got a 4.5 power valve and put that in, but I need to check vacuum again from a manifold port so I can get a good reading. My truck idles in gear at about 750 rpm.

 

On my new 350 dart shp small block chevy which is now just done and I have to pick it up the carburetor I have on mine with a cam specs of 282/290 231/239 @50 535/550 lift and a lobe separation of 110 is a holley double pumper 650 and in park I have around 12 inches of vacuum and once I shift into drive it goes down to about 9 inches of vacuum and I have a 3000 rpm stall so I had it on with my 6.5 pv and went with a 4.5 which was down from what came size wise in my carb as most in general come with a 6.5. Only if ruptured or a bad power valve gasket letting the fuel past. Can't hurt to try as that is part of tuning.

 

Can any of you guys tell me what the sight window on the sides of the carb are supposed to look like, I've heard different stories. Right now, when I look at the sight window the fuel goes halfway up the glass, is this right?

 

This is all really great news guys. I was so bummed because I thought me rings were toast letting oil in the cylinder. I'm not in the clear yet but I know you guys know your stuff so I'm hopin. I got a compression tester so when I get back home from work I'm gonna test all 8 cylinders and I'll post a pic of the paper I log it all on and see what you guys think

 

Here's the other engine specs TechInspector just in case ya need them. Also I've seen you calculate an estimated hp/tq and power curve chart with your computer, if it's not to much trouble id love to know what the program makes of my combo.

355
10.3:1 compression
RHS 170cc heads 2.02/1.60 67cc
Edelbrock Perf RPM manifold
Holley 750 dp w/ 1" 4 hole spacer
Long tubes
3" exhaust Flowmasters
MSD pro billet HEI
Comp .496" 236@.050 106lsa solid
Comp Ultra Gold 1.5 roller rockers

 

RHS 170cc heads 2.02/1.60 67cc, are these heads cast iron? vortec heads?

 

Cast iron but not vortecs

 

If your wanting a set of aluminum sight glass window fuel bowls you can buy from a place where I shop called allcarbs.com Normal fuel bowls with the site plugs you take out to check the fuel level to where it just trickles if you bump the side of the vehicle and if it has sight glass windows on it then yes about the half way on front and back. If your look at the fuel bowls you will see that the front is about 1/16 higher then the rear bowl. Now if you do have the standard fuel bowls and you take out the site plugs and yes there are clear plastic plugs you can use but they are just for checking only and you take the brass plugs out and then screw in a set of clear plastic plugs and then adjust it then when done it will still have to be set to where it is showing the fuel level on the clear plug by bumping the vehicle and it will show but you have to really look at it but be careful as they do crack easy and if you leave them in there over time the plastic gets brittle and one could break and fall off and then fuel will be shooting everywhere and could cause a fire. I get most of my stuff from all carbs as they have a lot of the same things that others like jegs and summit sell but they sell it for sometimes only half of the cost as summit and jegs has to have there big markup. Below is an aluminum sight glass bowl. They have pretty much everything for carb stuff especially holley and edelbrock and a quadrajet.

HOLLEY FUEL BOWL ALUMINUM PRIMARY

 

The sight windows are the glass ones that came on the Holley carb. So you should be able to see fuel halfway up the glass then? So my float level is okay?

 

ok so you would want to have the fuel level about half way in the sight glass window then.

 

I wonder why it's so dang rich then. My plugs get totally fouled after 5 minutes. I think I need to try a different brand. But even if the plugs don't foul it's running so rich there's a haze around the whole truck while it's idling

 

How much of the transfer slot is showing on the primary side of the carburetor??

How far out are the mixture screws turned right now??
Two corner or Four corner idle carb??

 

How much of the transfer slots is showing, is something I'll need you to help me with please because this is my first time tuning a carb. I've had a couple Edelbrock carbs but never touched them.

Mixture screws are 2 turns out, and I believe it's a 4 corner. It's a brand new Holley 750 double pumper if that helps. I just a vacuum gauge so I'm trying to learn how to tune mixture screws with it

 

Read this Wiki article it tells a little about the transfer slot and the adjustment. http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor#Holley_idle_transfer_slot With the big camshafts this is why you increase the initial timing to keep the transition slot from being overexposed.

 

So there is a mixture screw at each barrel, both front and rear metering blocks have mixture screws??

Reason I ask is Holley DP's are available both 2 and 4 corner idle mixture screws.

Info on transfer slot exposure, it is viewed from the underside of the carb, carb must be removed to see it well. It is a very important setting to control mixture at idle.
http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor#Holley_idle_transfer_slot

2 turns out on a 4 corner idle carb should be awful rich, try turning each one in at 1/4 turn increments until idle gets rough. You want to try to keep all 4 adjusted the same.

 

Yep it definitely has 4 mixture screws. Good to hear that 2 turns is very rich because that could be a big part of my issue. Thanks for the help I'll get on that and report back

 

Here is a video that will help you in adjusting the idle mixture on your 4150 750 DP.

 

I can't click on the link

 

 

This is the link to Youtube.com If it don't work try typing in How To Adjust The Idle Mixture Screws On Holley Carburetors in a search block. Everytime I try to send you the web address that video shows up again without the link I typed.