Off-Idle Hesitation

[CWizard]

Could someone please help with this problem: When I pop the throttle open from the idle position, I get a momentary hesitation, then it goes like stink. My research says this is most likely a momentary lean condition which increasing the accelerator pump shot should fix. So I have increased the primary nozzle size from .031 to .035 and now .037 with maybe a small improvement. I don't think I'm into the secondaries when this problem occurs, so I haven't messed with that nozzle. So should I just keep increasing the primary shooter size until it gags on too much fuel, or is there another culprit that I need to consider here? Timing? Engine has 6600 miles and timing has not been touched since initial dyno. Carb is a Holley 4150 HP double pumper with mech. secondary. Any suggestions will be appreciated, even one to get a Demon.
Thanks.

[Hotfingrs]

From an article in Carcraft....just a thought in the last paragraph

Tuning for Power
An earlier test drive revealed a disproportionate surge of power at low rpm, so Patterson figured the power valve was opening too soon and dumping excess fuel in before the mild-cammed motor needed it. He hooked up a vacuum gauge to full manifold vacuum, gunned the motor a couple of times in neutral, and found a hesitation right around 6-inches of mercury (in Hg). Removing the primary bowl and metering block revealed a 6.5–in Hg power valve, which Patterson promptly swapped for one that opened at a 3.5–in Hg vacuum signal. He noted that correct power-valve sizing should help throttle response at tip-in.

But one thing often leads to another, and installing the new, later-opening power valve meant we'd lost low-speed enrichment and developed an off-idle hesitation. Off came the metering block once again, and the stock 65 jets were swapped for larger 66s. Problem solved.

[wtm442]

I had almost the same problem .... but a tune up (plugs, cap and rotor) fixed 95 % of my hesitation.

[Carnut427]

Another possibility is the gasket (or "O" ring, depending on age of carb) between the metering block and carb is leaking, not allowing all of the pump shot to go through the squirters...or a bad accellerator pump/check valve.

Dan

[davidlee]

I have a 750 dbl pumper Holley that had a hesitation at 2200rpm. After chasing a lot of rabbits the problem turned out to the the transition circuit was not allowing any fuel at that point. The fix was to replace the metering block. Problem solved.
David

[Tongue Pirate]

Most obvious first! With the motor on or off when you blip the carb can you see a healthy stream of fuel shoot out from the squirter nozzle? Second, is this a newly developed problem or has it always been there?

[Zoom This]

These hesitation problems seem to follow us from one degree to another. I have a 418 with an 850 Holley that was modified by Jet Performance a few years ago. Once I got the motor past 1500 rpm or so it had incredible throttle response. But, I seemed to have this very slight off idle stumble, directly off idle. The Jet carb had the throttle blades drilled 1/8" which helped keep the motor inside the idle mixture range so I could smooth out the idle using the mixture screws. But, the holes were a little too big and the transfer slot was no where near the throttle blades when cracking the throttle quickly off idle. Changing squirters from 28 - 37 did not help at all, and only screwed up city driving with huge squirters by dumping too much gas in the motor. With a vacuum of 13-14", I swapped out the 3.5 power valve for a stock 6.5, lowered my floats, which were too high anyway, replaced the Holley baseplate with a Proform unit and gave it all a try on my brand new rebuilt 418. I have stopped chasing the off idle stumble and all is well now when cracking the throttle. The transfer slots, by the way, on the new base plate are exposed just a tad, and I think that may have been the final modification to set my particular off idle stumble to bed.

Bill

[CWizard]

ALL: Thanks for the responses. If I ever solve this I'll post the answer.
Mr. TP: Yes, there is a very healthy shot of gas coming from the primary squirters on a throttle blip. Nothing from the secondary nozzles, though, but I think that's appropriate because the blip isn't enough to open the secondaries. When I do a WOT (engine off!) I also see gas comong from the secondary nozzles. And the problem has been there ever since I got the car 6 months ago. Just been living with it.

[Tongue Pirate]

Did you purchase the engine/carb new? Was it set up by someone? Do you know if it ever ran correctly? The reason I ask is because it may be something like a worn ACC. Pump or diaphram. If this is a problem that has always existed it could be that your acc nozzle is still incorrectly sized or that your shot needs to be adjusted. How does the motor react when you slowly accelerate from an idle?

[rdorman]

Number one problem for off idle hesitation (assuming everything is mechanically sound and working) has to do with the idle settings. Usually, the idle transfer slot is exposed to much. It should only be exposed about .040 to .060. It can be over exposed for many reasons. But again assuming that the carb is properly selected for the motor, then my first guess would be initial timing. As a rule of thumb, the hotter the cam the more initial timing the motor will like. The more initial timing, with in reason, the more you can close down the butterflys. Check your initial and total timing and see where you are. With a 230 at .050 cam I have about 18 degress initial. Pull the carb and see how much of your transfer slot is explosed. If over or under exposed, open or close the secondary by the same number of turns you have to open or close the primary. Idealy, you will have little or no secondary transfer slot exposed. Once set up, I adjust my idle speed with the secondaries. Holley has a cheap little kit that will allow you to do this with the carb on the car. Of course if you have a very radical cam, drilling of the butterflys is the way to go. You know this is required because you can not get the car to idle with the butterflys properly set. Once you rule that out, how far is to idle mixture screws out from seated when PROPERLY adjusted? 1 to 1.5 is about perfect. Try this little trick, with the car idleing at temp, tap the accelorator pump to squirt a little gas into the motor, NOT blip the throttle. Does the idle get better, worse or stay the same? If better, the car wants more fuel, turn your screws out a bit. I think you get it from there! Forget 'lean best idle practices' with these motors, give it what it wants. More then 1 to 1.5 and you are pulling to much fuel from the transfer 'curcuit'. Less and the transfer 'curcuit' will be rich. If this is the case, then usually a modification to the idle feed restrictors/bleeds is required. Any good Holley book on modifications will guide you through the entire process. Tune with the air cleaner in place. If you can't get to the screws because of a drop base, flip the base over while you are tuning. Don't forget to flip it back over before you close the hood!

If this doesn't work... go have a couple of stiff drinks and call us in the morning!

[CWizard]

Mr. TP:
Answers: "Did you purchase the engine/carb new?" Yes, in June 2005.
"Was it set up by someone?" Yes, by the engine builder.
"Do you know if it ever ran correctly?" This particular problem has been here since day 1. I have just been living with it.
"The reason I ask is because it may be something like a worn ACC. Pump or diaphram. If this is a problem that has always existed it could be that your acc nozzle is still incorrectly sized or that your shot needs to be adjusted. How does the motor react when you slowly accelerate from an idle?" Motor does great with slow acceleration from idle. As for accelerator pumps, etc., I have a pink cam (#330 I believe) and have tried it in both the 1 and 3 positions. Position 1 (if I understand this stuff correctly) gives the most volume quickest. I have changed the primary nozzles from .031 to .035 to .037 to .040 with no effect on the hesitation.
Thanks for your continuing interest.

[CWizard]

Thanks for the info. I have a .576 lift, 235 degree duration cam (at least that's what I think the build sheet says), and I checked the timing today. Initial is 22 degrees, and at about 2500 rpm its 42 degrees. Sound OK? Haven't followed up yet on your other suggestions, but I will. Thanks very much.

[Tongue Pirate]

What size is your carb? The symptoms you suggest indicate that you aren't getting enough fuel down in there when you "hit it". .30may be a good starting point. My 289, when I had the holley on it, a 600 DP, ran perfect with a primary nozzle of .25
-Are all of your unused vaccum lines plugged off?
-May seem odd but did you try going down a size or two on the Primary nozzle?
-Using the carb size calculator (displacement x rpm)/3,456 if you are planning on spinning 7000rpm you could go up to an 850 carb, at 6000rpm you could run 750. For street smaller is always better & throttle response tighter.

I rebuilt a holley at one point & put it on a motor. It ran poorly for a few days on the lower end. About the third day what ever was in one of the passages must have worked it's way out cause it just all of a sudden ran perfectly. Small particlles of dirt can cause a lot of grief. The other part of this story was that this was another 600 DP & it was on a Built Big Block Chevy motor. The car had pretty tall gears so 4500 rpm was probably all I needed out of it but that carb ran so well on that motor. Much tighter than the 750 & 850 before.

[CWizard]

Hyde - The carb is a 750 cfm Holley 4150 Street HP, supporting a 520 hp stroked Windsor. Vacuum lines all plugged. I haven't tried going down on the primary nozzles because I see no puffs of black smoke that would indicate an overly rich mixture when I punch it, not even with the relatively large .040 nozzles. But you raise a good point - I'll try going down and see what happens.
Thanks.

[rdorman]

Originally Posted by CWizard

Thanks for the info. I have a .576 lift, 235 degree duration cam (at least that's what I think the build sheet says), and I checked the timing today. Initial is 22 degrees, and at about 2500 rpm its 42 degrees. Sound OK? Haven't followed up yet on your other suggestions, but I will. Thanks very much.

Sounds like WAY to much total timing. Check with your builder, set timing to spec, re-set idle and see what happens. If no good, go back to my first post and follow along! Please let us know what your initial timing ends up being.

[392cobra]

I agree with Rick,way too much advance for your 418W.

If you have an MSD,it sounds like you have the blue advance bushing.It allows about 22*.The black cuts it down to about 18* cent..
Your stroker only needs 34*.

I have attached a link that shows what the diffferent bushing and springs do.
http://www.cobralads.com/gear/msd2.gif

[Zoom This]

I'm with rdorman here. Timing should be set at 32-34 degrees on a 351W. By pulling down the timing 10 degrees, your motor will drop in idle speed. Readjusting the idle speed by opening up the throttle blades will expose some/more of the transfer slot and may help cure you off idle stumble. Easy fix if it works.

Bill

[rdorman]

Do check with the builder. Depends on many things. You will probably find the total should be in the range mentioned, 32-34. Maybe as low as 30 but doubtful as high as 36 with a modern alloy head. My lazy old open chamber iron cleveland heads likes bunches! Initial, probably in the 16-20 range. If you bring it back to 8, it will be a slug by comparison off the line and you have to open the plates more which MAY aggrevate the stumble. Get the timing right, adjust the carb and go from there. Many a builder does not take the time to recurve the distributor. So, they set it for total timing, max power and don't worry about the initial. If is starts and idles, fine. They are looking for max power. But, that does not work so well for drivability. I tend to run as much initial as I can. So long as it will start when hot (and heat soaked for a few minutes) and does not rattle, it is good. I crank it in until I can't get it to turn over and back it off two degrees and that is where I leave the initial. Have it curved for total and forget about it. I chased that darned off idle issue until I was blue in the face and it was all about the timing and the effect the timing has on the carb. Happy tuning!

[CWizard]

Hey guys -The title pretty much says it. I can't read the numbers on the damper. Rechecked my timing while totally awake and the real numbers are 22 initial, 34 total. Exactly the same as when the engine was dynoed by the builder. Rdorguy, I tried the small tap on the primary accelerator pump and to my amazement the engine almost died! Recovered immediately, though. So I'm either too rich or right on the hairy edge. Since it idles fine and the idle mixture screws are about 1 1/2 turns out, I'm thinking the latter. But I may be too close to the edge since I tuned it with the air cleaner off.

I think timing is OK so I'm going to retune the carb (this time with the air cleaner on), and maybe try smaller primary pump nozzles. Will probably check the idle transfer slot unless a dramatic change occurs from retuning. It'll be a day or two, since I'm wrapping up another science project - I'm measuring the pressure loss caused by my remote oil filter setup.
Thanks to everyone for all the help.

[rdorman]

Originally Posted by CWizard

Hey guys -The title pretty much says it. I can't read the numbers on the damper. Rechecked my timing while totally awake and the real numbers are 22 initial, 34 total. Exactly the same as when the engine was dynoed by the builder. Rdorguy, I tried the small tap on the primary accelerator pump and to my amazement the engine almost died! Recovered immediately, though. So I'm either too rich or right on the hairy edge. Since it idles fine and the idle mixture screws are about 1 1/2 turns out, I'm thinking the latter. But I may be too close to the edge since I tuned it with the air cleaner off.

I think timing is OK so I'm going to retune the carb (this time with the air cleaner on), and maybe try smaller primary pump nozzles. Will probably check the idle transfer slot unless a dramatic change occurs from retuning. It'll be a day or two, since I'm wrapping up another science project - I'm measuring the pressure loss caused by my remote oil filter setup.
Thanks to everyone for all the help.

Sounds like you are on the right track! Give your air bleeds a little squirt of cleaner. I do it everytime I have my aircleaner off. Check the transfer slots first and get them right. Then use the tap trick and set the idle mixture. You don't tap the pump hard, just enough to get a drop out. If the idle gets better, turn the idle mixture out, worse, in. About 1/8 turn each and try again. When you are close, an 1/8 out and tap will make it worse, 1/8 in and tap will make it better. The difference will be subtle. Four corner idle? Repeat on the secondaries. Have about 2/3 of the air fuel coming from the primaries. Repeat tapping both pumps and turn all four equally about 1/16th of a turn.

I don't recall, but you did check the accelerator pump to be sure that fuel comes out immediately didn't you?

Take a look at a plug. Soot around the top of the body (near the threads) is a good indicator of a rich idle (which leads to a lean transfer curcuit). Ideal is about 1 turn out.

Change the cam and squirter back to stock. Rarely, do they need tuned. If this doesn't do it, we will go from there!