holley carb problem on 340

[coupe32]

I can't seem to get the Holley carb tuned the way it should be. I'm thinking about going to a smaller size carb. The carburetor is a Holley 700 4778 double pumper with mechanical secondaries. I bought this carb new and ran it on another 340 engine with (5 speed manual tranny) and it ran a little rich but ran good so I didn't mess with it.
Since I have changed the motor it is running too rich. The tail pipes are black and sooty. I also put a Holley kit in it and that did not help any. I went down two sizes on the primary jets and that didn't help either. I have the idle screws backed out about 1 1/2 turns and used a vacuum gauge to adjust them. The engine runs good at wot and from kickdown but after you go back to idle the rpm's drop below idle speed and engine will quit if you don't feather the accelerator. Once you give it a little gas to keep it running it will come up to proper idle speed again which I have set at about 800-850 RPM. The carb was removed from the old 340 and ran good. It was removed and stored in a box for 3 months and reinstalled on the rebuilt 340 and now I have the problem. If I change carburetors I thought about going one size smaller maybe to a 650 (4777)
Any ideas on the problem or a new carburetor choice? Thanks

[crisser]

Is there a - and number after the 4778?

[coupe32]

Its a 4778-3 When I put the kit in it I changed the Power valve to. I put the stock one back in which is a 65.

[coupe32]

I have been looking at the 650 AVS Edelbrock carb also. I thought I would try and find someone that was good a tuning holleys around here before I bought another carb. (I'm apparently no good at it.)

[mhenesian]

Hi Coupe32,

The 700 cfm Holley DP should be a great carb on any 340 !

Try snugging down the fuel bowl screws in a criss-cross pattern. A reasonable torque, not excessive. Sounds like your Holley has a fuel leak between the main fuel wells and the idle fuel wells in the metering block where it mates up to the main body. The main body will warp over time (and many heat cycles) causing a sealing problem with the metering blocks. If this tightening doesn't improve the idle performance, then it's probably time to change the fuel bowl and metering block gaskets (yet again !) Quickfuel has a great kit for this and reasonably priced. Last I checked it was $28 (http://www.summitracing.com/parts/QFT-3-210/), has everything except power valves.

If new gaskets don't fix the problem, then the next step is to take a 8 ~ 10" long medium finish "flat file" and take a few careful passes across the main body surfaces that mate with the metering blocks. These surfaces need to have less than 0.010" warpage, otherwise even new gaskets will have trouble sealing. Double check the flatness with the straight edge of a machinists metal ruler or a machined "flat". If you want to run slightly on the "lean" side, the jetting would be 66's in the front and 76's in the back,

Mark

[crisser]

Originally Posted by coupe32

Its a 4778-3 When I put the kit in it I changed the Power valve to. I put the stock one back in which is a 65.

69 and 78 are stock jets in it. So, i`d start there. Then, i`d check both float levels with the engine running. Make sure they are not too high. Then, set the REAR float a little lower than the "book" calls for. Lastly, get a small wire and some carb cleaner and check to see if all the air bleeds are clear. Idles on the outside and mains are inner and smaller. Make sure to check the rear bleeds also. They can effect the idle even though they are on the secondary.

[Beast D]

If it were me I would get rid of the DP and get a 750 Vacuum secondary if you are running a single plane intake. If you are running a duel plane I would run a 650 VS you will be much happier.

[rampage_82]

Well what vacuum is your engine pulling? If you put a cam with more duration you could have a problem with the power valve opening at idle. I've always had good luck by going by half of what the engine pulls at it's lowest point at idle. I.E. 13" or above = 6.5" 10" = 5" 8" = 4" and so on... Also make sure to double check your float levels that is a good one to check. When the vehicle is idling look down the main body, and see if fuel is coming from the venturies. Also you might also try to lean down the idle circuit, even though it isn't set by the vacuum method, if it fixes your problem, then who cares. Your engine will tell you what it wants. Also I'm assuming that your double pumper doesn't have 4 corner idle circuits. Another problem I found one time, was that the air cleaner came down too far and closed the choke, although by you saying that it runs good at WOT, then I don't think it would be your issue.

Also while it is idling take your hand and wave over the top of the carb about an inch or so over the top... if it kills it you are rich, if it picks up rpm you are too lean, if no change you are in the ballpark...

[rampage_82]

Also if you are running a single plane or open plenum manifold, if you have some room get a 1" 4 hole spacer, it will make carb tuning immensely easier... It improves the signal to the carb. It works well for dummies like me.... lol

[crisser]

I can tell you, if your air bleeds are clogged, you will NEVER get any Holley to run right. No matter what cfm or what jets or power valve you put. It will always go rich because it doesn`t have any air entering the circut to mix with the fuel and, the "pull" on the circut will be stronger also fattening up the engine.

Remember, the bleeds are not just the brass holes. It`s in the metering block passage also. So, a blast of air down the holes with the blocks still on can be a help. Once they are known clear, then proceed with any tuning if needed. That 700 should run fine on that 340. The Page is also right on the power valve. It needs to be based on idle vacumn so it doesn`t open at low speeds.

Once again, float levels and air bleeds 1`st before going further or you will be banging your head against the wall not being able to tune.

[440roadrunner]

Originally Posted by rampage_82

Well what vacuum is your engine pulling? If you put a cam with more duration you could have a problem with the power valve opening at idle. I've always had good luck by going by half of what the engine pulls at it's lowest point at idle. I.E. 13" or above = 6.5" 10" = 5" 8" = 4" and so on... Also make sure to double check your float levels that is a good one to check. When the vehicle is idling look down the main body, and see if fuel is coming from the venturies. Also you might also try to lean down the idle circuit, even though it isn't set by the vacuum method, if it fixes your problem, then who cares. Your engine will tell you what it wants. Also I'm assuming that your double pumper doesn't have 4 corner idle circuits. Another problem I found one time, was that the air cleaner came down too far and closed the choke, although by you saying that it runs good at WOT, then I don't think it would be your issue.

Also while it is idling take your hand and wave over the top of the carb about an inch or so over the top... if it kills it you are rich, if it picks up rpm you are too lean, if no change you are in the ballpark...

[rampage_82]

If you get tired of that 700, and go with the Eddybrick, you could always package it up and send it to me... I will give it a good home... LOL

[cageman]

Also, on a double pumper, you set the idle by the secondaries, not the primaries. You will uncover the idle slots if you set the primary side only, and no matter what you do to the idle screws wont matter, as they arent working. With todays crap gas, I find myself cleaning the idle air bleeds all the time with carb cleaner, I have a can in every holley vehicle I have. If it doesnt have it, I would put a check ball in the power valve hole in the base plate to prevent a back fire from blowing out the power valve, this could be why you are rich at idle too, and why it runs great at wide open.
I love my holleys.

[coupe32]

Well thanks everybody for all the info. One thing I found was that I set the float level with the engine not runnning and the electric fuel pump running only. This corrected the problem of the rpm dropping below idle speed and quitting after you would give it gas. Actually it runs pretty well now that this dummy set the floats right.
I have already had it apart and cleaned it, blew it out good and put a kit in it. I am still going to double check vacuum at idle and see if I need to change the power valve from the stock one which is 6.5. It is amazing the difference the right float settings can have. I think it is still running a bit rich though. Im going to play around with it some more . Might go a little more lean on the jet size. Right now I have 68's and 78's. Thanks again

[cudabob496]

Float levels!