[Canadian Charlie]

I just replaced my carb on my truck with a new one. Went from a Holley 750DP to another Holley 750 DP. Only difference is the new one doesn't have a choke. From a Holley 4150 0-4779 to a Holley 0-82751 HP series

Now today I went to fire up the beast and its idleing at 2000rpm's, went to turn the idleing screw down and its still the same. Now all I did was bolt it on and didn't tamper with anything,

What could be my problem?

Thanks

[adantessr]

First thing to check is the linkage . You may try unhooking the linkage from the carburetor to make sure it is not holding the throttle open . Also check to make sure the secondary linkage is adjusted properly and that the secondaries are closed completely . .....Allan

[Frisco]

You state that it doesn't have a choke. Does that mean it isn't hooked up or that no choke or choke linkage is present?

If the choke linkage is present, look to see if the idle stop screw is on the 'high idle cam'. If it is drop the idle stop screw so that it is off the high idle cam and re-check / set the idle after engine is fully warmed up.

If no choke linkage is present, look for vacuum leaks. Check for any vacuum fittings on the carb around the base that are either unplugged or the hose has come off. A vacuum leak can cause high idle as well as a lean condition.

Also check that the carb linkage is fully returning and not sticking or in a bind.

[Canadian Charlie]

As in "secondary linkage " you mean the return springs?

[adantessr]

Originally Posted by Canadian Charlie

As in "secondary linkage " you mean the return springs?

I mean if the secondary throttle plates are open just a little , hung up , stuck , etc .

[bubbahotep]

I agree check secondaries not closed fully?

How's the float levels? Any difference in the jet sizes of the new carb? Possible power valve difference between the old and new carb?

[bigdog7373]

Make sure all the vacuum ports are connected or blocked off. Some carbs have a large one in the back that you may miss. Make sure the return spring is strong enough to bring it back to idle. Make sure secondaries are closed.

[T-bucket23]

Is the new carb actually new or just a different one. Did you use a new base gasket. As mentioned above the most obvious is either binging trhottle blades or a huge vacuum leak

[dmx6]

I've read there is a connection between the settings of the primary and secondary as too openings. Secondaries can be open too much and idle will not come down--close up secondaries and put them in relation to the primaries and you can set idle. I put a new Holley on and had that problem. Closed secondaries up but not closed and I could set idle like it needed to be. I left them open enough to just let light through--.010 or so.

I think it all depends on the engine and mods as to the needs of the system.

My 2 cents--

[adantessr]

I haven't had my hands on a Holley in many years but i do know that a Q-jet has an adjusment to make sure that the secondaries are closed tight when the engine is at idle . It is about the last of 14 required adjustments when properly setting up a Q-jet . Also that adjustment can be made incorrectly so that it holds the primaries open. We need a Holley expert to chime in here . Please ?

[timothale]

When changing carbs I always check to see how the throttle plates fit in the bores. Back off the idle screw, close the throttle and hold it up to see if you can see daylight around the edges. If so, loosen the screws tap the plates with a screwdriver handle while holding closed, then retighten and recheck, etc... unequal air flow at idle can cause problems,

[adantessr]

Originally Posted by timothale

When changing carbs I always check to see how the throttle plates fit in the bores. Back off the idle screw, close the throttle and hold it up to see if you can see daylight around the edges. If so, loosen the screws tap the plates with a screwdriver handle while holding closed, then retighten and recheck, etc... unequal air flow at idle can cause problems,

Excellent tip there . Some things seem so basic to me that I fail to mention them . I have a bad habit of assuming that everyone knows as much or more than I do , unless they ask a specific question .

[dmx6]

Kinda long post but ran accross this--good to read helped me alot on my carb.

1. Bench-Set the Idle Speed
Even though most “street” Holley carbs only have idle mixture screws on the primary metering block, almost all Holleys have an idle fuel transfer passage that discharges idle fuel on the secondary side as well as on the primary side. This means that the carb meters idle fuel and idle air through all 4 of the throttles – primary and secondary. For this system to work properly, it is absolutely critical that all 4 of the throttle blades ALWAYS be set at the same setting (NEVER set idle speed by only adjusting the primary idle speed screw). This will assure that the carb is balanced and working right from the beginning.

Before installing the carb to the engine (if you have installed it, yank it off), turn the carb upside-down on your workbench. The first thing you should notice is the recessed secondary throttle idle speed screw located on the passenger side of the carb: it’s recessed into the throttle plate near the secondary shaft. Next, if you look at the throttle bores just below the edge of the throttle plates (“butterflies”), you will see a vertical slot. Open the throttles a little to see the whole slot. This slot is called the “transfer slot,” and it provides a fuel discharge transition circuit between the idle circuit (which discharges fuel out of the round idle discharge holes below the throttle plates) and the main metering circuit (which discharges fuel out of the main discharge nozzles once airflow through the ventures' is high enough to pull the fuel through the nozzles). The transition slot receives its fuel from the idle metering supply circuit.

With the throttles fully closed against their idle stop screws (not on the fast idle cam), noting that the secondary's and primaries both have separate idle stop screws, there should be exactly .020” of the transition slot exposed below the primary throttle plates. Use a .020” feeler gauge to measure this: Place the feeler gauge on the throttle plate up against the transition slot and adjust the idle speed screw so that the slot JUST BARELY disappears behind the feeler gauge. At .020” slot exposure, the slot will appear to be a perfectly square hole. Adjust the primary idle speed screw so that the primary blades are in position.

On Holley carbs that have been in service for a while, the secondary throttle adjustment screw is often partially seized, so you have to use a small screwdriver with a pair of vice grips attached to the shank of the screwdriver to turn the screw. It’s best to “rock” the screw back and forth to get it loose enough to turn. The next thing to note is that the secondary transition slots on a Holley are often located higher in the throttle bores than on the primary side: If you try to obtain the .020” transition exposure on the secondary side of a Holley, you will often end up with the secondary's opened significantly more than the primaries. To avoid this, use the following secondary throttle adjustment on the Holley carbs: After adjusting and setting up the primary throttle blade angle as described above (.020” transition slot exposure), hold the carb up to a light and look up the bottom of the carb (with the choke blade wide open) – note the amount of “light gap” around the primary throttle blades. Now, adjust the secondary idle speed screw to duplicate the primary “light gap” on the secondary side. This will usually get the secondary throttle blades close to the transition slot, but you may not have .020” exposure. Idea is to get the same blade angle and airflow through all 4 corners of the carb – the “comparative light gap” method allows you to get this very close.

Once the primary and secondary throttles have been set to this initial idle speed setting (which should make your car idle very close to the correct idle rpm range), it is your job as a tuner to assure that any further idle speed changes occur by adjusting both of the screws equally from this point on. Never adjust the idle speed by only adjusting the primary screw: if you adjust the primary idle speed by ¼ turn, you MUST adjust the secondary idle speed screw ¼ turn as well. Keep the two throttles adjusted the same.

[adantessr]

After reading the last post , I am glad that I have never owned a Holley . I'll stick with my Q-jets . Just my preference .

[bubbahotep]

Originally Posted by adantessr

After reading the last post , I am glad that I have never owned a Holley . I'll stick with my Q-jets . Just my preference .

lol. Its a lot of words for a simple setup. 10 minutes at most if you've done it once before. usually its just 1 screw for the primary blades (the idle screw) and check the secondaries are not too open