Off-Idle Hesitation

CWizard · #1 (**permalink**) 04-03-2006, 07:35 PM

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 · #2 (**permalink**) 04-03-2006, 09:02 PM

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 · #3 (**permalink**) 04-04-2006, 06:57 AM

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 · #4 (**permalink**) 04-04-2006, 09:18 PM

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 · #5 (**permalink**) 04-04-2006, 09:30 PM

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.

rdorman · #6 (**permalink**) 04-06-2006, 07:24 AM

Quote:

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 · #7 (**permalink**) 04-06-2006, 08:00 AM

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

Tongue Pirate · #8 (**permalink**) 04-04-2006, 09:41 PM

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 · #9 (**permalink**) 04-05-2006, 08:33 PM

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.