[smythge]

Can anyone help me with a stupid question. I have basically a stock 454 bored .30 over with a mild cam (sorry don't know the specifics) Edlelbrock performer intake, HEI ignition and a Holley 750 Double pumper manual choke with mechanical secondaries, (4 speed borg & Warner T10) It runs good, but the exhaust smells very rich at idle and almost unbeareble when I really get on it. I tried adjusting the air fuel mixture and found the screws have to be almost all the way in, in fact they're only out about 1/4 of a turn, If they're out any further in fourth gear it wants to die. I checked the timing it is right on. Also when cold I can't use the manual choke if it's on at all the motor immediately dies, and as you can imagine, when it's cold it doesn't want to run without the choke either. Basically I am wondering if the carb is too big, maybe the wrong jets or do I just live with the strong exhaust? Any help is appreciated. Also what suggestions would you have on a replacement carb?

 

[Bob West]

Adjust mixture screws to obtain best idle, mixture screws don't have anything to do with it dying in 4th gear. That might be a float adjustment issue. With most cams you are going to get the eye burning exhaust at idle, I installed the Mallory Hyfire and that helped clean it up immensely. Get the numbers off the carb and post them, should be something like 4779 on the front of the choke tower. We can then look up jet sizes and what power valves it should have in it.


or you can look it up here http://www.mortec.com/carbs.htm

 

[smythge]

I didn't think the air/fuel mixture screws would have an affect in fourth gear, but I checked it 3 times adjusted the screws out a small amount and it stumbles in 4th gear put them back in and it runs fine. Yes it is 4779

 

[Bob West]

Check the jets, it appears most use 70/80's with a 6.5 powervalve,and set float levels properly and try it out. Which 4779 is it? I see they have them listed -1 to -6.

 

[smythge]

4779-1

 

[smythge]

What Mallory Hyfire system do you use?

 

[Mike Feudo]

At light throttle in 4th gear the idle mixtures do have an effect. Sounds like something is out of wack. Floats? leaky needle valves, block to body gaskets.

 

[Bob West]

Hyfire 6al I believe it is, the one with the built in rev limiter, no chips, just dial it in. A while back Jegs had them on sale for 20.00 off.
http://www.jegs.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10002&productId=70532&parent_category_rn=&showValue=1&JegsPopup=ProductDisplayPopup

Mallory is digital, MSD is analog.

 

[ratuned]

i have an early 4779 and it calls for 75-76 jets. i seem to have better luck if i use a rebuild kit and go back to the exact settings of the carb and work from there. my 454 is set up that way and i don't have the gas smell that some seem to have. mike

 

[69-CHVL]

Get a vac sec...dp's have a richer idle mixture from the factory.

 

[smythge]

It was at light throttle in fourth gear, that it wanted to die. When I adjusted the screws to clockwise (lean) the problem went away. Right now all 4 screws are out about 1/4 turn, it runs fine, but the exhaust smells very rich at idle and even worse at WOT, just wondering if that is the best carb to be using.

 

[smythge]

I have a Vac Sec.

 

[69-CHVL]

No offsense...sounds like you dont even know what carb you have, so all this jetting info could be useless to you.

A vac sec is a little different than a double pumper 4779

 

[smythge]

I am familar with Vacuum Secondaries as opposed to the mechanical that I currently have, we were talking about ignition, my mind was on Vacuum advance not vaccum secondary, my mistake.

 

[71malibu406]

i definately wouldn't be changing carbs unless it was to check and see if the carb was indeed the problem. it seems like maybe there's some debris in the carb somewhere. Holleys are fairly simple to take apart and clean. might change the powervalve while you have it apart. you could probably find the problem and only have a few bucks in gaskets a powervalve and maybe a new needle and seat vs $300-$400 for another carb. how much fuel pressure are you running? there could be several things causing the problem, and it is usually something simple.

 

[echristie]

I have a milde 454 with a 4779-1 as well. Mine run rich, but is a scalded dog at WOT. I think I can do better. I bought a kit, jets from 72-75 primary, and 82-86 secondary, and 6.5 power valve. I am going to just try systematic tuning to see what seems to work right. I'll post the results when I get them. I removed the carb today and the parts came in a few days ago. Should be a week or so and I can give first impressions.

Oh yeah, I run an Accel HEI and I changed the rotor, cap, and coil. Mileage improved 50% and the idle is much more stable in gear now.

Thanks,
E.

 

[69-CHVL]

smythge,

From 3 sources that I've read, the DP have a richer idle/part throttle AFR's that are built into the carb. This is what makes them unsuitable for street use (for some people of course). The DP's are a more of a competition carb than street. I had a 750 and an 800 DP on my 454/4spd car, and it ran so rich at idle that it would water your eyes and force you to change your clothes after a drive. No matter what I did I couldn't clean it up (transfer slots, vacuum advance to manifold or ported, mixture screws). I also had a full MSD setup too. Car ran super with those carbs, but I have to tell you, bolted up a 770 and and 870 Avenger and guess what - with no screwing around carbs runs cleaner right off the bat, and response is superb.

I confirmed this BTW with my wideband meter that the vac secs runs cleaner.

 

[smythge]

That describes my setup exactly it runs great, but the smell is so bad, I don't want to drive it. What is your recomendation for a replacement?

 

[69-CHVL]

May I suggest either the 770 or 870 Street Avnger carbs, depending on how hot your car is. They are very nice (have electric choke, rear metering block, and quick change secondary spring housing). I have both. Most guys do great with the 770, I feel a little more top-end with the 870. If you go with the 870, you'll need to drop the jetting down a little (I'm actullay going to ask a question on this myself).

I got the 770 new on Ebay for about 300.00 to my door.

 

[smythge]

I was just looking on ebay for avengers, looks like I can find one for around $300.00