Hesitation when hitting throttle...where do i start? - Team Camaro Tech
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post #1 of 12 (permalink) Old Oct 18th, 14, 08:54 PM Thread Starter
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Hesitation when hitting throttle...where do i start?



406 SBC.

Big stumble when I hit the gas. I figure it's one of two things: carb or timing.

1. Timing - I've messed with the timing at various settings. Doesn't seem to have an effect on the stumble. While I'm pretty sure I'm not setting the timing right, the vast # of settings I've tried leads me to believe it's not solely a timing issue. (FWIW - I disconnected the vacuum advance from the stock distributor, set the RPM at 1000, the set 15 deg advance. Plugged the vacuum line back in, and RPMs shot up to 1250 RPM. I then reset RPM to 1000 and checked where the timing was with a timing light. It was at 35 deg. At idle. What am I missing?). Points are a bit dark on the dizzy...a buddy recommended I change them.

2. Carb - edelbrock 1406 (600 cfm). Bought it used and rebuilt it. Seemed straightforward, but I've been know to screw up the simplest of tasks. Vacuum advance is hooked to port on right (which feels like it has good vacuum). Other two are plugged. Mixture screw are about 2.5 turns out from all the way in. Adjusting them seems to have very little effect on the stumble.

Someone send me down a path! Change points? Recurve kit? Carb?
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post #2 of 12 (permalink) Old Oct 18th, 14, 09:42 PM
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Re: Hesitation when hitting throttle...where do i start?

Sounds like carb, accelerator pump to be precise. What's happening is your suspended mixture is falling flat when the throttle blades are abruptly opened. The accelerator pump circuit fills that gap.

I think the pump diaphragm rod has an adjustment, might want to check it out.

Your vacuum advance is correct, keep in mind when the throttle plates open the vacuum drops and so does the vacuum advance, then the centrifugal picks up. Mine is at 34 at idle too.

Another thought is to recheck the accelerator pump circuit, you might have inadvertently put a gasket on backwards maybe covering up the fuel passage to the pump diaphragm.

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post #3 of 12 (permalink) Old Oct 18th, 14, 10:45 PM
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Re: Hesitation when hitting throttle...where do i start?

I agree with Bobby. Make sure your squirters are working when you open the throttle. You should see a good stream going to both sides.

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post #4 of 12 (permalink) Old Nov 3rd, 14, 09:18 PM Thread Starter
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Re: Hesitation when hitting throttle...where do i start?

Pulled the carb apart and checked the accelerator pump. All looked okay. Any other thoughts? Guess I could bring it to a carb shop for an official look, but I didn't see anything that stood out.
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post #5 of 12 (permalink) Old Nov 3rd, 14, 09:52 PM
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Re: Hesitation when hitting throttle...where do i start?

Non Carb stuff.

Timing at idle should not be in the 30's. Get it down to the low 20's with a limiter plate for the vacuum advance. The vacuum to the VA-Can should be taken from manifold vacuum and not ported vacuum.

1. Set points dwell (30 new, 28 used)
2. Then timing
3. Then adjust the idle

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post #6 of 12 (permalink) Old Nov 3rd, 14, 10:02 PM
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Re: Hesitation when hitting throttle...where do i start?

Edelbrock has a good series on Youtube for tuning their carbs. They recommend using ported vacuum on the 1406.

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post #7 of 12 (permalink) Old Nov 3rd, 14, 10:04 PM
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Re: Hesitation when hitting throttle...where do i start?

Sounds like not enough accelerator pump shot to me.
Better ears will know what to do right away.
What hole is your accel pump rod hooked to now? If not in the hole closest to the pivot that's where I'd move it. (Front driver's corner of the carb is the pump and rod going from base plate to top) This should give you a bigger shot of fuel. The hesitation in your vid sounds like it needs more fuel.

Edelbrock manual link
http://www.edelbrock.com/automotive/...ers-manual.pdf
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post #8 of 12 (permalink) Old Nov 4th, 14, 05:17 AM
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Re: Hesitation when hitting throttle...where do i start?

I'll agree with the others here as well that it sure sounds and acts like a lean hesitation from not enough accelerator pump shot. I'd recommend you keep the vacuum advance on manifold vacuum rather than ported and it sounds like you are fairly close if not right on with the ignition timing. New ignition points can't hurt but I don't believe your points are causing your hesitation.

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post #9 of 12 (permalink) Old Nov 4th, 14, 05:31 AM
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Re: Hesitation when hitting throttle...where do i start?

This why a Holley is more popular (notice I didn't say better). I am sure the Edelbrock (Carter AVS) in the hands of an expert tuner familiar with the carb could do the same as can be done with a Holley. I just am not that Carter Expert. I haven't used one in nearly a half century. From memory there are two holes on the accelerator pump rod to change volume of the hit. To change the timing of when it hits you bend the rods holding them with two pairs of pliers (takes more coordination that I posses now due to neuropathy).

Ignore the 20 degree ignition advise. Big blocks love ignition timing and compression: which is an indicator of just how poor the combustion chamber is in design. Just be careful not to get too far advanced as this poorly designed chamber will bite you with detonation occurring in two or three locations in the chamber at the same time as the flame front wanders across the bore. Vacuum should be off the manifold. Ported Vacuum is only useful to pass the sniffer test if you have annual emission testing.

With a Holley you control the volume of the accelerator pump squirt two ways, by changing a cam to a different color or after trying everything else a physically lager accelerator pump. Timing of the shot is controlled by the squirter size. There is no bending of rods which you must admit is an art form. Everything on a Holley is screwed together so you can easily go back to a previous setting if it doesn't work. This is why racers choose Holley.

Carter was on top of every high performance car made back before 1963 When Holley introduced the Dominator carb. Holley soon there after introduced increased cfm rated 4150 series carbs to fill the void above the Cater 600 yet below the 1050 Dominator. Ford and Chevy jumped upon the Holley band wagon with only Chrysler staying with Carter to the bitter end of carburation.

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post #10 of 12 (permalink) Old Nov 4th, 14, 08:43 AM
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Scott
 
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Re: Hesitation when hitting throttle...where do i start?

Been down this same road with both Holley and Edelbrock..... here is some quick stuff:

Timing - set your initial based upon cam duration. Several references on the web for this. I have around a 230 duration cam, and my motor likes 14 to 18 degrees. All in before 3500rpm, 36 degrees total centrifugal + initial (or 32 if fast burn heads). Some motors want it in quicker - I do not know the magic behind this, but changing springs and test driving is easy.

Keep the vac advance modest - 6 to 10 degrees max. It does go away with a pedal smack - so should not be a contributor. BUT.... if you are relying on it for idle rpm (and a lot of it, more than 10 degrees) then when you punch the pedal you are losing more than 10 degrees of initial and it will create a stumble. I actually disconnected mine while dialing in the timing and carb, then reconnected when I had things nice. I only have 6 degrees of vac advance, but I have vortec heads so had to keep a close eye on the upper limit.

Make sure floats are set correctly - Edelbrocks are almost always wrong right out of the box.

Idle mix is important! When you get things better, rock your idle mix a quarter turn lean or rich and check for performance.

Transition slot is important on both Holley and Edelbrock (more so on Holley). Should be a square exposed. This may give you the wrong Idle rpm, which you can change by adjusting the secondary opening (slightly) on a Holley - never did this on a Edel. On the Edel I moved the intial timing slightly to get 750 rpm idle. Transition slot only matters for off idle stumble - if the stumble happens while smacking the pedal while cruising, then it is not a transition slot issue.

Edelbrock air valve - wire it shut (temporarily) - easy to get a lean bog. I have a AVS and can adjust the air valve - I have tightened it and it still opens when revving at idle! I need to tighten a little more. I believe folks end up adding a weight to the 1406 to keep the air valve closed longer.

Edelbrock pump squirt is pretty limited in adjustment. Magic to the arm height (can be found on the web), hole to adjust squirt length and pump shooter nozzle.

In the end, the edelbrock has a needle enrichening system controlled by vacuum and springs - adust the springs to match engine vac, then jets and rods to control cruise and power mixture.

AFR meter works wonders. I used an LM1 on mine, and it helped me dial in pump shot and mixture. Do not use the AFR to set idle - my experience on about a dozen motors is that the idle mix wants to be on the rich side of life for good off pedal performance. Nothing crazy rich, but 12.8 to 13.5 instead of the magic 14.7.


Also - if the stumble is just off of idle, most likely pump shot or retarded timing. If the stumble also occurs from pedal smack while crusing (2000 to 3000 rpm) then it is most likely the air valve opening prematurely.
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post #11 of 12 (permalink) Old Nov 4th, 14, 09:14 AM
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Re: Hesitation when hitting throttle...where do i start?

Sorry!!! I don't know why I thought this was a Big Block. You clearly spelled out SBC. I was wondering why the others were offering advise on small blocks when the big block is a different beast.

You won't have detonation problems introduced by too much ignition lead as a SBC does need any where near that much as your heads are much better. If you have fast burn heads (the heart or kidney shaped chamber) then you can pull out even more timing.

There is one other factor that I haven't seen mentioned and that is your cam choice. You have a smallish carb for a 400 cube motor (Chevy installed a 720 cfm Rochester or a 780 Holley on top of their 396 427 motors). I would be interested in a vacuum gauge reading at what idle RPM.

Everything every one has stated so far makes sense (Including the 20 degree ignition statement). I am just wondering if your engine has a small vacuum leak somewhere that is supplementing your cfm overall just driving you very lean in the process. The Edelbrock has a tuning kit with stiffer springs to keep the rear barrels from popping open when not needed. You can read about tuning the carb here:

http://www.thecarburetorshop.com/AFBtuningtips.htm

Big Dave
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post #12 of 12 (permalink) Old Nov 16th, 14, 04:36 PM Thread Starter
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Re: Hesitation when hitting throttle...where do i start?

I found a guy with my exact same carb. (1406). Swapped his to mine and it works perfectly...no hesitation.

I tried matching all the rod bends on my original (which I bought used off eBay) to exactly match his. Did this, put mine back on, and the stumble....is still there. Doh!

Only thing I can think of is that the guy who rebuilt my carb is an inexperienced clown (aka "me"). I thought part of rebuilding the carb was to take EVERYTHING apart...including the rods holding the butterfly valves on. I had a hell of a time drilling out a couple of the screws holding the valves onto the rod. BECAUSE THEY WERE STAMPED IN PLACE! To a normal American...this would have said, "don't remove me!" I blasted past the obvious and drilled them out anyway. Wait...it gets better. After removing and cleaning the rods and valves, it was time to put it back together. I had a hell of a time getting the rods back in, but they were easily moved into place with the use of a heavy duty hammer. Then it was time to tap new holes in the rod for new screws on the butterflies. The rods probably came in and out of the carb a half dozen times during the process. To finish it off, I tried to "stamp" the screws once installed.

None of this seemed out of the ordinary to me until the guy I got the carb from said, "Careful of setting/dropping this carb on a flat surface since doing so can bend the cross-rod (because the drivers side cams sit lower than the carb body) and create a vacuum leak."

If a slight drop can cause a vacuum leak, how bout my hammering, tapping, etc? (Btw - a dremmel was involved too).

I think I'll pay the guy $175 for a perfectly running 1406 and be done with it.

Let me know if a potential vacuum leak caused by my Neanderthal tactics is likely the cause. If so...I hope you enjoyed the laugh.
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