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Old 05-07-2006, 06:07 PM
frankenvette's Avatar
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Hesitation!

O.K. been chasing a hesitation in my acceleration since I replaced my Holley 3310-2 with a new 3310-10. If I hammer the accelerator at any speed, the engine hesitates for a split second, then feels like I dropped the clutch, and build's rpm's normally. When shifting, no hesitation between gears. Off idle, same hesitation. From a stop, I have to keep my foot in the accelerator to keep the engine from near stalling. I have been searching and reading forums, web sites and all answers seem like shots in the dark, covering every aspect of the carb. I would like to try to isolate this problem to avoid repalcing everything and not having fixed the problem.

Fron information, mainly at the prompting of Docvette and Gerald (www.bob2000.com), I went through the following test to hopefully isolate the source of the hesitation.

Engine:

383 Stroker
Mild Cam
Double Hump Iron Heads
63 Corvette Roadster (3000lbs)
4 Speed M-21
3:08 Rear Ratio
Primary use - Street Rod (no racing, cruise ins and fun romps around town)

Timing:

Mallory Unilite Mechanical Advance

Idle/Initial 15* (850rpm’s)
1500 rpm 20*
2000 rpm 30*
2500 rpm 32*
3000 rpm 36*
3500 rpm 37*
4000 rpm + 38*

Spark Plugs:

Autolite 85 (set at .035)
Dark & Sooty

Carburetor Holley 3310-10:

Primary Jets 70
Power Valve 65
Secondary Diaphragm Spring Purple (1915-6950)
Pump Discharge Nozzle .031
Adjusted float levels to leak slightly at sight hole
Manual choke (installed rod to insure it never closes)
Adjusted Idle Mixture, leaned out till vacuum started to drop, then richened another ¼ turn (ended up at about 1 ½ turns out)

Mechanical Fuel Pump:

Reads 7 lbs continuously

Vacuum Check:

Pulled from manifold vacuum port
Idle @ 15 lbs (meter shows insufficient vacuum advance?)

Driving Conditions

1800 rpm 20 lbs
2500 rpm 22lbs
2250 rpm 21 lbs
Always stayed in the 17 – 22 lb range while cruising

With Quick acceleration, engine stumbles for a split second, vacuum starts going down, seems to hesitate at about 10lbs, then the car accelerates quickly and vacuum drops to 1-3 lbs quickly, never reached “0” lbs

NO 02 Sensor

Any suggestions based on real experience would be greatly appreciated.

Thank you Doc for the effort and leading me to this point, I learned allot about my carb, vacuum and timing today!

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Old 05-07-2006, 06:21 PM
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Dude.If your acceleration pump circuit has been checked,free all the belts and grab the damper with both hands.Turn back and forth looking for play at the cam chain.That could be the reason.Plugs should not be dark though.The ideal way to check would be a rolling dyno and a labscope with wide band oxy sensor.You would find out in a minute.Good luck.
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Old 05-07-2006, 07:45 PM
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have you checked for vacuum leaks. Had a stroker in my last car same problem found a vacuum leak at the rear of the intake manifold. Spray around manifold and carb mounting flange with carb cleaner if idle picks up you have a leak. Joe
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Old 05-08-2006, 07:26 AM
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Quote:
Originally Posted by jsdailey


Timing:

Mallory Unilite Mechanical Advance

Idle/Initial 15* (850rpm�s)
1500 rpm 20*
2000 rpm 30*
2500 rpm 32*
3000 rpm 36*
3500 rpm 37*
4000 rpm + 38*

Spark Plugs:

Autolite 85 (set at .035)
Dark & Sooty

Carburetor Holley 3310-10:

Primary Jets 70
Power Valve 65
Secondary Diaphragm Spring Purple (1915-6950)
Pump Discharge Nozzle .031
Adjusted float levels to leak slightly at sight hole
Manual choke (installed rod to insure it never closes)
Adjusted Idle Mixture, leaned out till vacuum started to drop, then richened another � turn (ended up at about 1 � turns out)
The first thing that I notice is that your distributor does not have a vacuum advance. This should not make that much difference for performance but will affect drive-ability on the street.

Your initial timing seems to be OK but I would suggest that you try bumping it up to as much as 18 degrees at the idle RPM you show above.

Your Total Mechanical timing should not increase after about 3000 RPM (I prefer in the 2500-3000 RPM range). You show that at the present it continues to increase above 3000 RPM. Changing the mechanical advance springs to have less tension will enable the mechanical advance to reach the maximum at a lower RPM. Try to get in the 36-38 degree range for the Total Mechanical Advance.

The plugs being dark and sooty show that your idle mix is too rich (depends on how you are actually checking the appearance of the plugs). I assume you are letting the engine idle, then shutting it off and pulling the plugs.

Increase the pump discharge nozzle to two sizes larger. (I suggested this to you in another post you have). Be sure that the nozzles begin to squirt immediately when the throttle has begun to move. Adjust to achieve this.

When adjusting the air / idle screws; do so to achieve the highest vacuum possible at idle and then do not turn them any further.
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Old 05-08-2006, 08:20 AM
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Try the silver secondary diaphragm spring and see if it gets better. Even though you have a light car, the secondaries may be coming in too soon with the tall 3.08 gears. There is no accelerator pump shot when vacuum secondaries begin to open, so if they open to quick its bog city until fuel flow catches up.

What are your cam specs? 72 size jets may be too large with a mild cam.

My 383 likes 74 primary jets and my cam is 236-242 @.050.
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Old 05-08-2006, 07:58 PM
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71C10, I tried a brown spring, no difference, and don't believe it's in the secondaries. Unfortunately, I have no cam info, bought the 383 with no specs, like the motor allot, love the performance, just got a couple hick ups.

Frisco, I agree with your assesment. I was running 16* which helps with the off idle, but was getting as much as 40* total, may have to change the springs, and I have the kit. I would like to have a max of 36*, just need to find the right spring combination. I changed the carbs no more than a couple hundred miles ago, and think the color will change when I get the new carb set up right, time will tell. I did reset the idle mixture and got over a 3/4 turn before a drop in pressure or idle speed, so maybe on the right track there. Curious, if I turned the idle mixture all the way in, it would kill the engine. Have always read that this indicates too large a carb, any ideas about this? As for pump discharge nozzles, the carb came with 6.5's and from my research and my findings on the vacuum readings, I would want to be in the 10 range? Do I have this right or am I confusing parts? Pretty certain the nozzles are squirting immediatley, will check when I get home from my road trip. Thanks for the attentive post, very helpful and you obviously read my findings before posting.
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Old 05-08-2006, 09:21 PM
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Quote:
Originally Posted by jsdailey
... I was running 16* which helps with the off idle, but was getting as much as 40* total, may have to change the springs, and I have the kit. I would like to have a max of 36...
Quote:
Originally Posted by Frisco
.....Your initial timing seems to be OK but I would suggest that you try bumping it up to as much as 18 degrees at the idle RPM you show above.

Your Total Mechanical timing should not increase after about 3000 RPM (I prefer in the 2500-3000 RPM range). You show that at the present it continues to increase above 3000 RPM. Changing the mechanical advance springs to have less tension will enable the mechanical advance to reach the maximum at a lower RPM. Try to get in the 36-38 degree range for the Total Mechanical Advance.....

Frisco has you on the right track, limit the centrifugal advance (all in by 3000) and add initial advance to improve throttle response. Run the idle mixture lean to help keep the plugs clean.

Last edited by automotive breath; 05-08-2006 at 09:30 PM.
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Old 05-09-2006, 07:15 AM
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Quote:
Originally Posted by jsdailey
Curious, if I turned the idle mixture all the way in, it would kill the engine.

As for pump discharge nozzles, the carb came with 6.5's and from my research and my findings on the vacuum readings, I would want to be in the 10 range? Do I have this right or am I confusing parts?
Turning the air / idle screws all the way in should kill the engine if the throttle blades are not opened up too far. This is 'normal'.

The 6.5 you are referring to is the Power Valve. Leave this size in at the present time. The vacuum readings that you have posted show that this size should be OK.

What I was talking about is the pump discharge nozzle; also called 'squirter's'. You show that the size you have at the present is .031. Go to squirter's about .035. This will give more of a fuel 'shot' when you go quickly to WOT. This will or should eliminate the hesitation. There are also different 'cams' for the accelerator pump that enable the 'shot' to occur quicker, slower, etc. Before going to that trial and error phase; I still feel you should try the larger 'squirter's'. Easy install. Just don't drop the mounting screw down the carb.
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Last edited by Frisco; 05-09-2006 at 07:45 AM. Reason: added photo
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Old 05-09-2006, 07:51 AM
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Richen up the idle mixture a little. I had a bog and this made it go away. This fix was recommended by a Holley carb tune web site.

You need a higher number power valve, at least a 8.5 with than much vacuum.

You need to read plugs to determine jet size. However, all my 3310's seem to run best with 68's or 69's in the primary's. And 70 to 72 in the secondaries to stop black smoke from the exhaust during hard acceleration. I always convert my 3310 to allow replacement of secondary jets. Stock secondary metering plate is equal to a #74 jet and has always been too rich for all my applications (from mild to wild setup).

With no vacuum advance, use 20 initial timing and 40 total.
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Old 05-09-2006, 11:33 AM
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Quote:
Originally Posted by 454C10
Richen up the idle mixture a little. I had a bog and this made it go away. This fix was recommended by a Holley carb tune web site....
Although adding more fuel might eliminate the hesitation, this should be done at last resort. Jsdailey reported dark and sooty plugs, more fuel will add to the problem fouling the plugs, in addition fuel is getting rather expensive.

Jsdailey, The Mallory Unilite Mechanical Advance distributor has a procedure for limiting the centrifugal advance. Reduce the centrifugal advance to 18 – 20 degrees and set the initial advance higher. This should solve the problem.
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Old 05-09-2006, 12:12 PM
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Quote:
Originally Posted by automotive breath
Although adding more fuel might eliminate the hesitation, this should be done at last resort. Jsdailey reported dark and sooty plugs, more fuel will add to the problem fouling the plugs, in addition fuel is getting rather expensive.

Jsdailey, The Mallory Unilite Mechanical Advance distributor has a procedure for limiting the centrifugal advance. Reduce the centrifugal advance to 18 – 20 degrees and set the initial advance higher. This should solve the problem.

I AGREE !!!!
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Old 05-09-2006, 04:46 PM
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Frisco, got a new Power Valve 6.5 just in case the old one is blown (did have one backfire thru the carb after install, can't hurt). I did not get the new squirter's, and thank you for clariying that for me. Will get the .035's coming in before the weekend. Did buy the quick change secondary spring kit, should have done that before now!

454C10, I tend to agree with the 8.5 Power Valve after reading the Holley Tuning book. I will try this after I have complete a couple other things first. Initial at 20 and full at 40 seems like allot, but could do with the existing springs. Again, will keep this for later!

automotive breath, I agree with no more fuel. I downloaded a tuning proceedure from Mallory and will be tuning it this weekend. Just hope I can get the springs (if required) in without removing the dizzy. Have you done this?

Appreciate everyone's help on this one. It's pretty amazing, after going through all the testing last weekend, then re-reading the Holley Tuning book Sunday evening, it all started making sense, duh. I may actually be able to tune a Holley when I'm through! Something else I read today, larger cams make less vacuum, and at 15 lbs, my cam is probably mild.

Will do more tuning this weekend and will post the results, thanks again!
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Old 05-10-2006, 04:36 PM
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Quote:
Originally Posted by jsdailey
…automotive breath, I agree with no more fuel. I downloaded a tuning procedure from Mallory and will be tuning it this weekend. Just hope I can get the springs (if required) in without removing the dizzy. Have you done this?...
A remember having a Mallory Unilite back in the 80’s. There’s an adjustable slide stop for the mechanical advance. You don’t have to pull the distributor or change the springs to adjust it. Limit the centrifugal advance to about 15 to 20 degrees and bump up the initial advance. This should fix your problem.
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Old 05-17-2006, 07:06 PM
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Here's the latest. . . got the carb off, installed the quick change vacuum secondary spring kit (nice), installed the 8.5 power valve, installed the Russell fuel line (very nice), and reset the timing to 20* initial.

Off idle, with slow constant increase in throttle, it definatley runs better, but a quick stab on the pedal produces a hesitation, same as before, but doesn't seem to fall flat as long (hope this makes sense).

Checked the timing curve and got this:

Idle/Initial 20* (850rpm’s)
1500 rpm 25*
3000 rpm 43*
Didn't go any further

So, I will address the springs next (lighter) to get 20* initial all in at 3000 at 38*(?). If this doesn't do it, then I will change the squirters to .35!

Any more tips on the springs and squirters?

Last edited by jsdailey; 05-17-2006 at 07:26 PM.
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Old 05-17-2006, 07:35 PM
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Quote:
Originally Posted by jsdailey
...Idle/Initial 20* (850rpm’s)
1500 rpm 25*
3000 rpm 43*
...
Limit the centrifugal advance and add more initial. Make sure the adjustment is correct on the squirter pump activation. Check the spring on the secondary butterfly activation, too weak opens the secondary too soon.
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