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weak throttle response - will a carb spacer help?
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aharris05
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Joined: 07 Sep 2011
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1968 Chevrolet Camaro

PostPosted: Tue Apr 21, 2015 10:27 am    Post subject: weak throttle response - will a carb spacer help? Reply with quote

Hey guys - I recently changed the carb on my 383 from a 600 edelbrock to a 750 holley with vacuum secondaries. I was hoping more flow would give me better numbers on the big end. Well, I lost a full second, although MPH is about the same. With the 600, best time was 12.50 @ 106, 1.7 to 1.8 60 fts. The 750 lagged off the line, 13.55 @ 105 mph, 2.0 to 2.1 60 ft.

motor - 383, 10:1, Hydraulic cam, Torker open plenum intake, Sportsman 2 heads, hydraulic cam, 232/243 @ 0.050, 0.523 lift, 9" vacuum at idle, 4.5 PV, #31 pump nozzle.

What can I do to improve throttle response? Can I tune this thing to work, or should I go back to a smaller CFM carb? Maybe a 4-hole spacer would improve signal to the carb?
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Big Dave
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PostPosted: Tue Apr 21, 2015 10:46 am    Post subject: Reply with quote

A four hole under a smaller carb might help, but it depends upon the manifold, in your case a mis-named Torquer single plane. Wih a hydraulic cam you are not reving high enough to need a single plane manifold. Your motor will make more over all power with the Air Gap Performer RPM dual plane.

You use an open carb spacer to increase the plenum volume to compensated for a carb that is too small for WOT pulls. The four hole is used to straighten out the air flow (along with turtles) in a single plane manifold in the hopes of improving bottom and mid range torque when your cam is to aggressive for the application. Aside from the cost to buy the two parts, it can't hurt the motor one way or the other, so I would try both of them at the track to see if either hurts or hinders your motor's performance with your current gearing and tires.

Big Dave
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Tue Apr 21, 2015 11:34 pm    Post subject: Reply with quote

that 750 should work great. i dont like a vac secondary though. youre probably lean when you go to WOT. that vac secondary only has one pump nozzle. 31 is what nozzle i usually see on a double pumper....x2. might need to go up on pump shot. i would try a 41. where is your timing ? is it locked ? vac advance ? mechanical? its an entire combination. not just a carb thing. and what Dave said.... phenolic or wood. you want something to insulate from the heat.
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aharris05
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1968 Chevrolet Camaro

PostPosted: Wed Apr 22, 2015 7:42 am    Post subject: Reply with quote

The timing is set at 16 deg advance, mechanical advance only. Someone suggested to me that I disable all advance and set it at 30 deg since its not a street car.

I have a 37 nozzle I can try next. Have the white pump cam set in position 2. Also have an open spacer (aluminum) I could slip in too. That would increase plenum volume, but might help flow because the intake is so short?
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Wed Apr 22, 2015 2:06 pm    Post subject: Reply with quote

i run my timing locked but without a retard on startup, cranking may be a bit difficult when its hot. i broke my starter like that.

i have a set of drills i use to open up the pump nozzles. its easier than buying a bunch of different ones. you can slowly adjust your size. i got mine from SnapOn with the Pin Vice to go with them.

i would change the pump cam too. http://www.quickfueltechnology.com/Instruction_Sheets/99-41141-100.pdf

while i agree the spacer is a good idea, i think you need to work on that pump shot first.

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Paul P
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PostPosted: Thu Apr 23, 2015 4:40 pm    Post subject: Reply with quote

This is purely tuning. Get a trick kit and some jets for the Holley and try changing things. Make note of each change. Probably a spring change in the vacuum secondary along with a jet change will get you close. If the performance was there with the other carb it should still be there no matter what the fuel source.
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clay
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1972 Chevrolet Nova

PostPosted: Thu Apr 23, 2015 5:52 pm    Post subject: Reply with quote

I'm with Paul - it's a tuning issue. Secondary spring is a definate area to look. Don't just look at it as controlling the opening rate - it basically sets the vacuum level in the intake at w.o.t. The spring works against vacuum trying to open the secondaries. Say for instance the stiffest spring lets the secondaries open at 4" Hg. It will try to never let intake vacuum get lower than that. As rpm's climb the increased vacuum will open the secondaries farther until they're fully open assuming the engine can pull that hard. Your 383 might be able to get them fully open but a DP carb would be wide open at the hit. I would swap to the lightest spring in the kit. This will let the secondaries open faster and farther. After this you'll most likely have to go up in pump shot to something like a mid 30 and maybe a different / quicker cam. If you don't have a secondary setup you can jet it would be helpful to do that. Seems like they are 72 pri and about 76 sec for jetting and I've always ran larger than that. Not much - maybe 72 - 74 pri and 80 or so sec. Depending on your converter I would run locked timing also - even if it was a street car. If it'll flash to over 3000 or so all your timing should be in by then anyway and the converter won't allow you to load the engine hard below that rpm. However 30* doesn't sound like enough timing. I also run Sportsman II heads and have always wound up at 36* for best results. As a matter of fact my current combo is similar to yours. 383, 10 - 1, ported Sportsman II's, Victor Jr., mech. flat tappet, 750 vac. sec. Tried to get to the track a few weeks ago but we didn't find out they were closed until we were in the parking lot Evil or Very Mad . Hopefully in another few weeks. Clay
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Thu Apr 23, 2015 9:54 pm    Post subject: Reply with quote

why use the vac secondary? i never could get one tuned where i was happy with it. what am i missing? i switched to a double pumper back in the 90s and havent run a vac secondary since. learn me sumthin Smile
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clay
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PostPosted: Thu Apr 23, 2015 11:08 pm    Post subject: Reply with quote

I actually don't mind vacuum secondaries - and no I don't put the screw in the slot Laughing . They're like blow throughs - you can't go fast with that - they don't work. However I will say they aren't perfect either. Using my setup above sometimes on a slow roll on you can feel the secondary transition. Mainly because with a quick pump cam I've pretty much used the pump shot by the time they start to open so there can be a little sogginess followed by a good hit. To solve that problem - don't roll on slow - STAB IT! The only thing I had to work on is if the engine is pretty cammed up getting a good idle can take some playing around. I guess I could convert it to a 4 corner idle but it's doing pretty decent after some air bleed / IFR tuning. What specific problems have you had? Clay
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Thu Apr 23, 2015 11:36 pm    Post subject: Reply with quote

i dont really remember from back in the day. however, i had one recently at work that felt like it needed more pump shot. it had a terrible bog on tip-in cold and got better as it warmed up. it was a 750 and came out of the box with a 42 pump nozzle. it had the pink cam though. idle was tricky too. it would idle fine in park but was struggling in gear. this was on a GM crate motor. nothing fancy. i messed with it and got it running pretty good but, i think it could have been better. its sort of like the EZ EFI.... just never get one to run great.
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Knarley Darley
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PostPosted: Tue May 05, 2015 11:07 pm    Post subject: Reply with quote

This is an interesting topic. The thing about vacuum secondaries on a drag car is that if you have any measurable vacuum at WOT, the carb is too small in theory. A bigger carb would remove the restriction, and net more power. Also if the carb is big enough to not have vacuum at WOT than what opens the secondaries? I wonder if his secondaries are open at all? I would check the little gasket that seals the vacuum port out to the secondary diaphragm, and has been previously stated put a really aggressive pump cam and a large squirter, plus get rid of the power valve, and lock the timing at whatever the car likes, probably at least 35 degrees, and as stated put the lightest spring in the secondaries. My engine likes 37, but it is a big block. I like to hear the feed back on what was tried, and what the result was. (what worked , and what didn't) If possible I would get one of those cheap hand held vacuum gauges, tape it to your windshield, go out and drive it, and see what the vacuum is at WOT as close to redline as possible. I think your car will work better though with mechanical secondaries.
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aharris05
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1968 Chevrolet Camaro

PostPosted: Thu May 07, 2015 10:50 am    Post subject: Reply with quote

I am sure the secondaries opened, because I put a zip tie on the actuator rod, just snug enough to slide easily. before the run I push it to the top, and after the run it is at the bottom, telling me that the diaphragm pulled the rod up. But I don't know exactly when it opens. I will try getting the real light spring for it, and check on the vac readings. (I just got a panel mount vac gauge to install). If I block the power valve, I assume I need to increase jet size? I know I should get a mechanical secondary, but I'm trying to work with the parts on hand right now. In fact, that may be why it had a better launch with the Edelbrock 600. Thats mechanical secondaries, isn't it?

As for the ignition, I haven't played much yet with it, but many people have also told me to lock it out. My concern is with starting it if I do that. Can I set it around 20deg base, put the lightest springs on the mech. adv, and limit the mechanical to 15 deg, giving me 35 total? It should come in early, but still have a little retard for start up?
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clay
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1972 Chevrolet Nova

PostPosted: Thu May 07, 2015 4:05 pm    Post subject: Reply with quote

Your 600 Edelbrock has vacuum secondaries also. It does open the secondary butterflies mechanically but it has an air door on the scondaries that works against a counterweight to regulate intake vacuum/airflow. Think kinda sorta Quadrajetish but basically a Carter AFB from what I understand. I personally wouldn't remove a primary power valve. Secondary ones are another story though. The problem with the secondary power valves in a drag car is trying to keep them covered with fuel. Of course there are jet extensions but the power valve intake is located higher up and will get uncovered with a decent launch. Try what you are talking about with the advance - that should be pretty good. You should wind up in the 34 - 38 total degree range on timing depending on your combination / altitude / temperature, etc. Clay
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Knarley Darley
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PostPosted: Fri May 08, 2015 1:42 am    Post subject: Reply with quote

I only like removing the PV because it is a little easier to get the jetting right, but it is a little trickier to get it to not bog, or have a flat spot when you stab it using just pump shot. If you have a separate switch for your ignition so you can crank the engine, and then flip the ignition on, you shouldn't have a problem starting it with the locked out ignition timing. I like the zip tie on the rod trick. Just make sure it is not vibrating itself down.
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aharris05
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1968 Chevrolet Camaro

PostPosted: Fri May 22, 2015 1:14 pm    Post subject: Reply with quote

I sorta took your advice on the ignition advance. New billet distributor with a magnetic pickup hooked to a 6AL. I put a bushing in it to allow only 12 degrees mechanical advance. Plugged the vac advance canister inlet, and set initial timing to 22 degrees. Put one light and one medium spring on the weights, and now all the advance is in by about 2000 rpm (34 deg. Total).
It is definitely snappier off the footbrake.

Carb:
I put the lightest spring in the secondary diaphraghm. So now it starts breaking up around 3000 / 3500 rpm. If I stay in it, power picks up, and it smooths out around 4500 /5000. I am thinking that the power valve is opening too late, it goes lean and starts to miss, then when the secondaries open it richens up & runs better. Or the PV opens and then it runs better. Does either sound like a possibility to you? Itís a 4.5 power valve, I am gonna try a 6.5 this weekend. Maybe I should increase the primary jets? (I should get a vacuum reading at the point where the miss starts, maybe that would indicate if itís the PV.)
Or possibly run it with the secondaries disconnected and see if it acts the same.
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