I have a 68 with a rebuilt 350 with only 5K miles on it. The engine ran great but I decided to replace the stock Qjet carb with a brand new Holley 4150 ultra 750cfm with mechanical secondaries. The problem is that the engine will not idle. It runs great at higher rpm but it surges between idle to about 3000 rpm every 5 seconds or so with the throttloe closed to idle. The only change I made to the 4 corner idle was to open the secondary idle screw a little as suggested to give better resonse to idle mixture screws.

This new carb was supposedly setup at the factory but the mixture scews were only open 1/2 turn and the float levels(glass windows) have the fuel at the top of the window. The manual with the carb says middle of the window. Where should the floats be and whats the typical mixture setting?

Help before I put the old qjet back on

 

if your 350 is stock then that may be your problem right there . a holley 750 dp can be tuned for a stock 350 but in most cases its probably on the big side. on the other hand if your motor has some work done and has the hp/torque to suit a 750cfm then im sure you can get it working.the 750 ultra is a very nice piece and should perform very well if its matched right.

first thing to do before you even bolt that carb to your motor is to roll it over and set your transission slots. see how in the picture mine look like small squares below the throttle blades, thats what you want. once you have that set you dont mess with that again...ever.



next step is to set the float level and certainly half way up the site glass is where it should be. i even have my secondary bowl slightly lower so it doesnt bog under hardbraking.

next i would start with 1/2 turn out on the mixture screws . use a vacum gauge and adjust your mixture screws no more than 1/4 at a time keeping in mind if you adjust one you must do all the same to the other 3 being a 4 corner idle.you will be looking for your highest vacum reading .

once you have a that set and you see your highest vacum reading ( and hopefully by now it can idle)then select your power valve . there are different ways to make your choice but for me my vacum at idle for example is only 11 so i choose a 5 PV . rule of thumb is to halve your vacum reading and thats your power valve size. in some circumstances for example if you have a vacum reading of 12 you would not choose a 6 PV ..you would choose a 5.5 because if your vacum reading can be divided in half evenly than you always go to one size lower. the ultra is meant to have protection against backfire like my demon but i dont think thats bulletproof. if you had a few backfires while trying to dial in your motor certainly replace your power valve even if it looks ok.



try those basic things for starter and see how it goes.

 

I have a regular 600 double pumper and can tell you for these it's the very bottom of the window.

I don't have the clear windows but the brass plugs. You adjust the floats so it barely dribbles out when the plugs are out. Anything more than that is too much and my car stalls when coming to a stop due to fuel spill-over.

 

I agree with Gingerbread man except on the power valve...

At idle there is not enough air flow (pressure drop) to pull the fuel through the main jets to the main venturis, so, by the same analysis, there is not enough pressure drop to pull the fuel up through the power valve channel to the level of the main venturis either. It doesn't matter one bit what the power valve vacuum setting is at idle. In fact you could put in a 105 or 125 PV (10.5" Hg or 12.5" Hg) in a carb with an idle vacuum of 5" Hg and it won't make a bit of difference.

What the power valve does do is enrich the flow to the main circuit through the PVCR up to the venturis. This is necessary to make the A/F mixture be proper at higher power settings. To change the power valve rating only changes the vacuum at which the enrichment occurs such as going up a hill or in a passing situation - both definitely not idle situations. If the carb manufacturer found that the enrichment was needed at 6" Hg vacuum, they would specify a 65 power valve, but if one puts in a 45 PV (4.5" Hg vacuum), you will create a flat spot until the PV opens and restores necessary enrichment for higher power levels.


__________________
1965 Convertible
NCCC 2B National Drag Record Holder
11.34 second 1/4 Mile

 

I've worked on Holleys for 'bout 45 years, now, and you adjust that fuel level to just barely trickle over the bottom edge of the sight hole. Any more and you'll have problems.
Duane

 

Just reread your post, and if your surging is happening at idle, I'd also check to make sure your accel. pump arms aren't adjusted too tightly. The arms should just barely touch the bottoms of the pump diaphrams at idle. If they're too tight, you'll get extra fuel dribbling out of the squirters at idle and that will effect the idle quality.
Duane

 

omega has the ultra 4150 which is site glasses and not the brass plugs.the float level is adjusted differently so going to the bottom of the glass is not going to cut the mustard.the gas must be slightly below half way on the site glass...just like a demon and the more i think about it this could be all or some of your problem, so any talk of the fuel just dribbling over the holes is wrong with this model.

larrywalk when i touched on the powervalve it was just for general tuning. i did say i select my power valve size using my idle vacum but i didnt say anywhere that it had anything to do with the omegas idle problem but certainly something worth tuning .

duane i like what you wrote about the pump shot but not the gas level on this carb, even though omega seems to have a surge and not a stumble a common cause of a stumble is not having an adequate accelerator pump shot.look at the discharge nozzle and be sure a pump shot is coming out. If not then the pump diaphram might have a tear. Be sure there is no crap caught in there. First you will need to check the adjustment on the pump. To do this you will open the throttle all the way to wide open throttle. Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever. If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size

 

Thanx for all of the suggestions. I will try them and let you know what happens.

 

I'd still take the level to the bottom of the sight plugs just to see its effect. It would only take a moment to do and is completely reversable.

 

I was reading my original post and I left a few things out. The engine is actually a 355 with dart 2 ported and polished heads, weihand intake, and a cam that is mild with just a little idle lope. I read something about drilling holes in the throttle plates to let more air in at idle if you have changed the cam?

 

-dude you just bought a brand new ultra..i wouldnt drill holes in it. your 750 is letting more than enough in as it is. i am curious though how much initial and total timing your running.

-toddalin go for it but on a 4150 ultra you will surely run out of gas on a hard run with the fuel adjusted to the bottom of the bowl

- Holley carbs are fairly susceptible to flooding if dirt is introduced into the carburetor. This sediment gets stuck between the needle and seat which can stop the needle from seating and shutting off the fuel flow. When this happens, the fuel in the float bowl rises and pours out through the main metering jets. i would also look into that omega as it might explain why your floats are so high.