Hesitation when you go WOT, anyone not have it? - Page 5 - Chevelle Tech
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post #61 of 85 (permalink) Old May 30th, 19, 3:49 PM Thread Starter
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Re: Hesitation when you go WOT, anyone not have it?

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Originally Posted by fred's old chevelle View Post
Good suggestions by Levon, one thing I have been thinking about is that carb may actually be a little small for your setup. As i said before I tried vac secondary 750, and the car ran good in the low 13's but when I went to 800 dp instant power and smoke and 12's. Your cam is bigger than mine and you have more compression I think an 800-850 cfm carb would wake that up even more.


I was thinking the same thing that the carb could be a little small for the application.. I am trying to get something set up with the local shop to have them go over a lot of the possible issues discussed.
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post #62 of 85 (permalink) Old Jun 6th, 19, 7:23 PM
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Re: Hesitation when you go WOT, anyone not have it?

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I am running a Quick Fuel 780. Had it to one of the areas best carb guys and there is still a hesitation if I go all out WOT. He could not get rid of it. If you feed it has gradually it is fine, but if you go ar it hard it stumbles then goes, will do it every gear when rowing through the gears. Maybe it is just a typical issue, but it frustrates me.Need to hit up some people to see if they have a carb I could put on to try.
I have this problem for years with my 292H cammed 402 in my SS396 Chevelle with 750 double pumper carbs. I tried everything, HP carb body, air bleeds - up and down, squirters - up and down, squirt cams. The only way I could fix the problem is to get both butterfly transition slots squared up, or less than square - closer to closed, than square-- on both the secondaries and primaries of the double pumper. This is the only way to get enough signal to the carb to avoid a lean stumble. The only way to get the butterflys almost closed on a 292h cam and keep the car idling is to run over 30 deg of timing at idle. In my case I run 38 degrees of timing at idle. I run 22 deg. initial, plus 16 vacuum advance with an idle of 850 rpm in a stick shift Chevelle. Total without vacuum advance is still 38, so, I have 16 vacuum advance, and 16 mechanical advance in the HEI MSD distributer. But, with 9 in hg. of vacuum at idle at 850 rpm, you need a vacuum advance can that is mostly all in by 9 in hg. I have tested the VC1862 and the the Accel adjustable vacuum advance can. With the Accel adjustment screw turned all the way clockwise the Accel unit is quicker to vacuum than even the famous VC1862 can. With the Accel unit and the MSD vacuum advance limiter, you can turn the adjustment screw all the way clockwise and get full vaccuum advance by 5 in. hg... Put the MSD vacuum advance limiter (MSD 84281) on the 16 deg. setting and you are good to go. Gas mileage will also go way up, stink will go way down, and the driving experience will also be much more pleasurable. Learn from my mistakes, try this, you will not regret it.

Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers
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post #63 of 85 (permalink) Old Jun 6th, 19, 9:30 PM
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Re: Hesitation when you go WOT, anyone not have it?

Quote:
Originally Posted by hjdca View Post
I have this problem for years with my 292H cammed 402 in my SS396 Chevelle with 750 double pumper carbs. I tried everything, HP carb body, air bleeds - up and down, squirters - up and down, squirt cams. The only way I could fix the problem is to get both butterfly transition slots squared up, or less than square - closer to closed, than square-- on both the secondaries and primaries of the double pumper. This is the only way to get enough signal to the carb to avoid a lean stumble. The only way to get the butterflys almost closed on a 292h cam and keep the car idling is to run over 30 deg of timing at idle. In my case I run 38 degrees of timing at idle. I run 22 deg. initial, plus 16 vacuum advance with an idle of 850 rpm in a stick shift Chevelle. Total without vacuum advance is still 38, so, I have 16 vacuum advance, and 16 mechanical advance in the HEI MSD distributer. But, with 9 in hg. of vacuum at idle at 850 rpm, you need a vacuum advance can that is mostly all in by 9 in hg. I have tested the VC1862 and the the Accel adjustable vacuum advance can. With the Accel adjustment screw turned all the way clockwise the Accel unit is quicker to vacuum than even the famous VC1862 can. With the Accel unit and the MSD vacuum advance limiter, you can turn the adjustment screw all the way clockwise and get full vaccuum advance by 5 in. hg... Put the MSD vacuum advance limiter (MSD 84281) on the 16 deg. setting and you are good to go. Gas mileage will also go way up, stink will go way down, and the driving experience will also be much more pleasurable. Learn from my mistakes, try this, you will not regret it.
My 750 DP was the same way until I took off the air horn, made it a 4 corner idle and got rid of the power valve, after that it worked perfect until I stripped out the squirter screw hole, now it's a shelf item. What intakes are you guys running when your having hesitation troubles?
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post #64 of 85 (permalink) Old Jun 7th, 19, 1:02 AM
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Re: Hesitation when you go WOT, anyone not have it?

Quote:
Originally Posted by 68Chevele View Post
My 750 DP was the same way until I took off the air horn, made it a 4 corner idle and got rid of the power valve, after that it worked perfect until I stripped out the squirter screw hole, now it's a shelf item. What intakes are you guys running when your having hesitation troubles?
My carb. is also a 750 HP body with the HLY-112-116 aluminum throttle/base plate. I like this base plate because of all the options for a street motor.

I am using a air gap manifold with a 1/2 inch nitrous plate and 1/2 inch phenolic spacer. The problem really surfaces when you try to tune the double pumper for a decent AFR. At 22 degrees advance and no vacuum advance, If I set my double pumper at 11 or 12 to 1 at idle, it only jumps up 1 point AFR during the transition, so, no stumble...., but, if you set a decent AFR at idle, say 14.1 to 1, then, the transition leans out to 15 to 1 plus and you get a lean stumble. If instead, I am able to close the throttle plates a lot more by using more initial timing from the vacuum advance, I don't get a rise in AFR during the transition from idle, instead it gets richer from 14.1 to 1 to 12.5 to 1 -- exactly what I want. That initial opening of the almost closed throttle blades really gets the transition circuit flowing.

BTW, I have a TKO 600 in mine, so, I am mostly in the transition circuit in overdrive on the fwy, so, I need my transition circuit running a great AFR. I also went down on my primary jets, and up on my Power Valve restrictors and I am running a 8.5 power valve with 9 in. hg. of vacuum at idle, and 15 in. hg. best at higher rpm. This lets be lean at cruise, but, instantly richer, when I goose the gas at mid-throttle. I tried a 6.5 power valve, but, my engine got too lean before the power valve kicked in. Try it, it is much better for fuel economy. You need a wideband in your car to play around with this stuff, but, you can get a double pumper to be much more efficient with it.
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Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers

Last edited by hjdca; Jun 7th, 19 at 1:18 AM.
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post #65 of 85 (permalink) Old Jun 7th, 19, 9:22 AM
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post #66 of 85 (permalink) Old Jun 7th, 19, 11:43 AM
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Re: Hesitation when you go WOT, anyone not have it?

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Originally Posted by Reelysalty View Post

Yes, I know this video. Look at the comments from TPV and the replies. I have the same experience. My power Valve is only 1 in. hg. below my idle vacuum, but, it is still about 7 in. hg. below my higher RPM cruise vacuum. My vacuum ranges from approx. 9 in. hg. at idle to 15 in. hg. at higher rpm. With the 8.5 power valve, I can cruise at 13.9 to 14.0 AFR on the primary jets and instantly get 12.5 AFR when I start putting my foot into it. With the 6.5 power valve, it goes too lean before the power valve kicks in. I have never had the problem of the power valve being open at idle.. Of course, all of this is happening because I am trying to get a double pumper to act like an EFI and have a good cruise AFR. If you are OK to have 12.5 AFR all the time, you do not have to worry about the power valve exact rating so much.

Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers
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post #67 of 85 (permalink) Old Jun 7th, 19, 11:56 AM
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Re: Hesitation when you go WOT, anyone not have it?

Quote:
Originally Posted by hjdca View Post
My carb. is also a 750 HP body with the HLY-112-116 aluminum throttle/base plate. I like this base plate because of all the options for a street motor.

I am using a air gap manifold with a 1/2 inch nitrous plate and 1/2 inch phenolic spacer. The problem really surfaces when you try to tune the double pumper for a decent AFR. At 22 degrees advance and no vacuum advance, If I set my double pumper at 11 or 12 to 1 at idle, it only jumps up 1 point AFR during the transition, so, no stumble...., but, if you set a decent AFR at idle, say 14.1 to 1, then, the transition leans out to 15 to 1 plus and you get a lean stumble. If instead, I am able to close the throttle plates a lot more by using more initial timing from the vacuum advance, I don't get a rise in AFR during the transition from idle, instead it gets richer from 14.1 to 1 to 12.5 to 1 -- exactly what I want. That initial opening of the almost closed throttle blades really gets the transition circuit flowing.

BTW, I have a TKO 600 in mine, so, I am mostly in the transition circuit in overdrive on the fwy, so, I need my transition circuit running a great AFR. I also went down on my primary jets, and up on my Power Valve restrictors and I am running a 8.5 power valve with 9 in. hg. of vacuum at idle, and 15 in. hg. best at higher rpm. This lets be lean at cruise, but, instantly richer, when I goose the gas at mid-throttle. I tried a 6.5 power valve, but, my engine got too lean before the power valve kicked in. Try it, it is much better for fuel economy. You need a wideband in your car to play around with this stuff, but, you can get a double pumper to be much more efficient with it.
Try this, get a 1/2 4 hole or a 1" 4 hole space and see if that helps. I had a car in the late 80's that had a lot of drivability issues with the big cam the guy had in the car. It had a 650 Holley DP on it and after we added the 1" 4 hole spacer it drove pretty nice. Also you can change the HSAB to a smaller size to start the mains sooner but you may need to rejet it.
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post #68 of 85 (permalink) Old Jun 7th, 19, 3:10 PM
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http://www.superchevy.com/how-to/add...gnition-timing

This is IMO best article on vacuum advance & timing. I have pasted it often. Parts may apply to OP.
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post #69 of 85 (permalink) Old Jun 7th, 19, 3:15 PM
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“Also you can change the HSAB to a smaller size to start the mains sooner but you may need to rejet it.”

68 Chevelle, why would making HSAB leaner start the mains sooner ?

School me on this 😁
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post #70 of 85 (permalink) Old Jun 7th, 19, 3:17 PM Thread Starter
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Re: Hesitation when you go WOT, anyone not have it?

Took the car to the shop today. He got 99% of it out by raising the float level a little bit. He already had it pretty aggressive on squirter so did not want to put more. Stumble is practically gone and it pulls MUCH better.


He is a big fan of double pumpers, I know everyone has an opinion on them, but we are going to do a cam swap this winter and possibly a little work on the heads and may go to a double pumper.


He suggests a Quick Fuel 850, so will start looking around. With everyone going to EFI should be some carbs to be had.
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post #71 of 85 (permalink) Old Jun 7th, 19, 5:34 PM
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Re: Hesitation when you go WOT, anyone not have it?

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Originally Posted by Pats70ss View Post
Took the car to the shop today. He got 99% of it out by raising the float level a little bit. He already had it pretty aggressive on squirter so did not want to put more. Stumble is practically gone and it pulls MUCH better.


He is a big fan of double pumpers, I know everyone has an opinion on them, but we are going to do a cam swap this winter and possibly a little work on the heads and may go to a double pumper.


He suggests a Quick Fuel 850, so will start looking around. With everyone going to EFI should be some carbs to be had.
Cool ! Great news ! I keep my floats pretty high too- they dribble fuel out at idle with the sight screw out. I also love double pumpers. I spend so much time with mine, I should marry her.
BTW, I am running a 27 HSAB and a 68 LSAB. As mentioned above, the relatively small HSAB will allow the mains to come on sooner (help lean transition) and also keep the carb richer on the top end. The smaller LSAB will also give you more fuel during the transition to help with the lean stumble.
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Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers
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post #72 of 85 (permalink) Old Jun 7th, 19, 8:59 PM
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Can’t beat Holley DP 👍

You really should get your timing squared away first. Often, carbs are blamed and mis-tuned to hide ignition issues.
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post #73 of 85 (permalink) Old Jun 7th, 19, 9:09 PM
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Re: Hesitation when you go WOT, anyone not have it?

Quote:
Originally Posted by hjdca View Post
Cool ! Great news ! I keep my floats pretty high too- they dribble fuel out at idle with the sight screw out. I also love double pumpers. I spend so much time with mine, I should marry her.
BTW, I am running a 27 HSAB and a 68 LSAB. As mentioned above, the relatively small HSAB will allow the mains to come on sooner (help lean transition) and also keep the carb richer on the top end. The smaller LSAB will also give you more fuel during the transition to help with the lean stumble.
Spot on, glad to hear you got it fixed. I learned about the HSAB stuff from our dyno guy. He was a great help when it came to those types of problems.
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post #74 of 85 (permalink) Old Jun 8th, 19, 2:19 AM
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Re: Hesitation when you go WOT, anyone not have it?

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Originally Posted by Pats70ss View Post
I am running a Quick Fuel 780. Had it to one of the areas best carb guys and there is still a hesitation if I go all out WOT. He could not get rid of it. If you feed it has gradually it is fine, but if you go ar it hard it stumbles then goes, will do it every gear when rowing through the gears. Maybe it is just a typical issue, but it frustrates me.Need to hit up some people to see if they have a carb I could put on to try.
I had 2 different hesitations with my Holley 770 Avenger. One from dead stop that required moving up in accel pump squirters and another, at around 2000rpm, that required changing the vacuum secondary spring. Not sure how you could ever properly tune these outside of the car because they are both load dependent. Things like car weight and, gearing, and trans type will all come into play.

The dead stop required going from a #25 squirter to a # 33 and the 2000rpm bog required moving down from a white spring to purple. No hesitation anywhere now.

'72, 385 sbc, Voodoo roller, 219/227; .515/,530 lift; AFR 180cc Eliminators, RPM Air-Gap, 770 Avenger, TH350 trans, 12 bolt 3.42's, Eaton Posi, TCI 2000 stall, Pypes 2 1/2" X-pipe system w/Race Pro's, Tri-Y headers.
Vehicle weight: 3390 lbs
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post #75 of 85 (permalink) Old Jun 8th, 19, 2:26 AM
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Re: Hesitation when you go WOT, anyone not have it?

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Originally Posted by Pats70ss View Post
ok, figured it out. With the vacuum unhooked and capped at the distributor

I am at 20 initial and 38 total, Should I put more in and see if that helps the hesitation or is that about right?


In reading through the thread it appears I have enough timing, I think I need to look at a bigger squirter based on the replies.
I think you are on the right track.

I would never run more than 38 degrees total.

Vacuum advance can help your efficiency and gas mileage but has no effect at wide open throttle.

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