I have a Holley 0-4777c 650 cfm double pumper carb on my 90 rs camaro (1985 350 engine was rebuilt and dropped in her). I'm trying to tune the carb and have realized that I have blown power valve (the car does not die when the idle mixture screws are bottomed out, the engine backfired through the carb a few times when we dropped the engine in) HOWEVER, I am trying to figure out what power valve I need to put in it, without having to take the whole thing apart. I know that this all depends on the vacuum reading, but I can't get an accurate one without having a good valve in it (one that isn't blown) Can anyone suggest what to put in there and then I can replace it after we do the tuning and figure out which one i need exactly....I would greatly appreciate the help!

-G. Martinez

 

how big is the cam?? I'd start with a 3.5 if I just had to guess

 

alright i'll see what I can get, thanks for the advice.....

On a side note I wanna know if I'm doing all of this right. I have looked and read some of the other power valve threads and they all say pretty much the same thing, but unless i get my questions answered I don't know that I'll REALLY know that I'm doing it right...

Lately my car has been running pretty rich (she smokes, not a bunch but enough, and smells God awful) Today I finally got the chance to work on it. I have the Holley Tuning DVD that they send so I've pretty much just been using that. We checked the float/fuel level and the front seems to be fine but we can't get the rear to adjust where it is up to the 'line' where the screw is to check the fuel level. According to the video we were supposed to loosen the screw on top of the bowl and turn the nut underneath the screw counterclockwise (this didn't do much).... is this right?

Next, in order to check the PV we turned in both idle mixture screws and the car stayed running (once again according to the video this suggested that the PV was blown, is this correct?)

I'm guessing that I am unable to set the proper idle mixture with the blown valve, is this correct?

I know that this was long but I'd really appreciate some help, and not some smart *** comment, it would be much easier on both of us

Thanks for the help
-G. Martinez

 

also, whats the difference between a high flow and a standard flow power valve, and which one should I use?

 

Yes you turn until the window is half full. If you're able to turn it to the point the needle comes out, you have a problem. You may have debris or dirt clogging the fuel inlet, not letting fuel into the bowl. Or a stuck needle.

Quick fuel needle p/n 18-2 is what my father uses on his cars and he's never had a problem.

Correct. If the engine backfired or caught fire(through the horn) they will pop/break.

I've only used the high flow, never compared between the two.

quick fuel p/n 25-35 for the 3.5
or p/n 25-55 for the 5.5

I'm guessing you have a radical cam, which would warrant a 4.0 or 3.5.

 

Thanks for the insight.... just one more quick question for the fuel level.... this procedure should be done on both the primary and secondary fuel bowl? I know I should just be happy with knowing that it works, but how exactly does turning he nut counter clockwise change the level inside the bowl? Isn't the nut loose on the screw anyway? When we were working on the secondary bowl, it seemed that we turned that nut quite a bit but nothing really happened..... the fuel level BARELY moved.....just curious as so how it all works. I've looked at the "exploded view" for the 4150 model carbs but still can't get an idea of how it works....

Thanks again for the info tho!

 

One other thing....according to Holley the vacuum is supposed to be between 16"-22", when I'm tuning my carb, should I try to get it as close to this as possible? OR is it ok to have it set at less than this? Also where should the RPMs be set? I know these are amateur questions, but lets face it, I'm an amateur here and this is my first time. I just wanna make sure that I'm setting everything up right for the optimal performance of my car....

Thanks again

 

Rather than trying to explin how to chose a power valve in my words, here is what Holley says on chosing a power valve for your car.

"An accurate vacuum gauge, such as Holley P/N
26-501, should be used when determining the correct power
valve to use. A competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmission,
take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAMPLE:
8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5"

 

the needle has 2 flat spots and the nut has matching flat spots. So, as the nut turns, it moves the needle up or down. This allows the float to stay open longer or close quickly.

16'-22' based on a stock cam. Any specs on your cam??

 

Thanks for this info, but I already knew this.... the only reason I was looking for possibilities for a PV is because my valve(s) is/are blown and I don't think I'm getting the proper reading on the gauge because of this. I was able to get the vacuum reading up to 16 but the idle was really high and she was smoking like a freight train..... I'm having problems getting the mixture right and I'm thinking it has a lot to do with the fact that my valve is blown....if i'm wrong let me know but thats what I'm thinking

 

I honestly don't have any information on the cam.... The guy (machine shop) who rebuilt my engine DID replace the stock cam, but I'll have to see if I can get some information from him on it because he didn't really give me anything at the time I picked the engine up....hopefully he'll remember

 

Your right, you are not getting a proper reading with a blown valve. Unfortunately you will have to replace the valve to get a clean idle before you can measure the vacuum and get a correct reading. Kind of a catch 22 so to speak. Unfortunately no other way to do it that I know of.

No mention was made of how to correctly chose a power valve before I posted other than a quick mention of a vacuum gauge, so I posted the info so either you or someone else may benefit from the information. Most people tend to chose a valve just under the idle vacuum reading and that results in bad mileage and an overly rich condition because they don't understand exactly how the system works.

 

First, I seriously doubt you've got a blown power valve. The newer Holleys (all those built since a lot of you were born) have a check valve in the throttle body to protect the power valve.

2nd, the power valve does not have anything to do with the idle mixture. I don't know how to get that through people's heads. The power valve only enrichens the main metering circuit, which enters the air stream through the booster venturi - there isn't enough air flow at idle to pull fuel into the booster venturi. If your idle mixture screws are not having any effect on your idle, you've got other things maladjusted.

Start by going through the Holley carb tuning sticky. Setting the throttle blades for idle, for both the primaries and secondaries (even if you don't have "4-corner idle), is the first step.

 

I know what you mean about the power valve probably not being blown but according to Holley, if the idle screws are bottomed out then the engine should die (with a GOOD power valve).... when we did this nothing happened, at least the engine didn't die, I can say that much.

I just read the "Holley Tuning" Sticky, which did help out, however, is it talking about the Carb Standard Main Jets??? Like I said I am an amateur here and I know you get tired of questions like this but I really do appreciate the help and would rather get it from experienced ppl like yourself rather than f'ing up my car over all...

 

Alright, I spoke to the guy who rebuilt my engine about the cam.... he doesn't remember the EXACT cam he put in her but he said he KNOWS it was a slight performance cam somewhere around a duration of 262/272 with lift of about .420/.442 and lobe separation of about 112 degrees....... anyone suggest where my vacuum should set with this cam? (like i said i'm an amateur and honestly don't know much about where i need to set it especially with this cam)
Thanks for all the help
-G.Martinez

 

You should be able to get away with a stock 6.5. I wouldn't use that carb, its too much for that cam. Vaccum should be between 18-20 inches.

he probably used p/n mc2201
http://www.goenginepro.com/chevroletsmallblock.html

 

what carb would you suggest?..... Right now i'm stuck with this one, but when I can afford another one I would replace it but would like to know what I should look for

 

Holley p/n 1850s It will work great as a daily driver and handles mild mods(ie-headers, performer/rpm intake and cam under .500 lift).

 

That's assuming the basics like throttle blade adjustment are correct. Most likely yours is not. Go through this:
Take the carb off, flip it over, and look at the transition slot. You want less than .050" of it exposed below the pri throtle blades. If more of it is exposed than that (it will be), open the sec throttles with their idle screw, 1/8 turn; put the carb back on; re-set the idle speed and mixture; then pop the carb back off and look at it. Repeat until you have between .030" and .050" of the transition slot showing. The "slot" should look like a square (the part you can see, the rest covered by the throttle blade) when set properly.

Correct. The "primary" main jets, in the front fuel bowl, and the "secondary" main jets, in the rear fuel bowl.

Remember, main jets don't affect the idle.

Your 4777 is fine. 1850's are for tow trucks.

 

Alright, in need of some more assistance...

I'm looking at the transition slot and according to the information on the tuning sticky i'm supposed to 'open the sec throttle with their idle screw'..... is this just the main idle (green screw) for the over all idle? Or is this supposed to refer to an idle mixture screw? This particular carb DOES NOT have any idle mixture screws for the secondary blades/fuel metering plate....

thanks for the help

-G. Martinez

 

Never mind that, I found the adjustment screw underneath...

 

FYI- those cam specs line up perfectly with the entry level Edlebrock Performer cam (also sold as a Summit house brand cam under part number K-1102). It's definitely a mild cam- just one notch over "stock", exactly as your builder remembers. It shouldn't take any serious work to make a stock Holley DP work with that cam, if it's in good operating condition.

Should pull plenty of vacuum and not require much throttle opening to hold an idle.

In all this discussion about the carb have you ever checked your ignition timing? 90% of all carburetor problems are found in the ignition (old mechanic's saying that's still true today).

 

Well we changed out the accelerator pump cam for the primary (went from pink to black) and got the fuel level set in both bowls.... This took away both the hesitation and the smoking. As soon as I get a chance I'm going to look into the jets and see if I need to change them. We only got about 15" or 16" of vacuum on her, not sure if this is good or not but according to the Holley Tuning DVD it may be a bit lower (than the suggested 16-22) cuz we're higher than 3000 ft (we're around 4500-5000 in elevation)..

All in all tho she's running a HELL of a lot better than before, but I think will just need a bit more turning before she's damn near perfect