On my Z which has a Speed Demon 750 mech. secondaries,I have one hell of a dead spot when cruising and giving her just a little bit of throttle to get going.As soon as I go to give it a little more than cruising it hesitates,actually more like a dead spot.I have a vacuum advance HEI with the lightest springs in it,38 degrees total timing at 2,800-3,000.the carb has 70 primary and 75 secondary jets,#28 squirters on both sides and the stock power valve(6.5).I have pump cams but really haven't played with those yet.Fuel pressure is set at 6 psi and floats are a little below half way on both bowls.The plugs look ok and no black clouds when i romp on it,looking for some ideas on what to chase next.What do you suggest?Any help appreciated.

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Pump cam, pump adjustment, pump check valve, pump squirters...

Did I mention the accelerator pump?

If the car idles, cruises, and WOTs properly, I'd leave the jets and PV alone unitl the pump works right.


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"So many Mustangs, so little time..."
ICON Motorsports

[This message has been edited by RB83L69 (edited October 17, 2001).]

 

A hesitation with no black clounds means it's going lean. Accelerator pump is the first place to look. It can be out of adjustment, squirter size can be inadequate, etc. But lean primary jetting and/or too low a float level can cause a bog even if the acc pump is working correctly.

 

So should I concentrate on adjusting the primary side of the carb first(jets,accelerator pump adjustment,squirters,etc)till the bog is gone then move on to the secondary side,or try to tune both at the same time?Thanks RB83L69 and Damon for your advice so far,much appreciation!

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Primary first, like the name implies. In fact, it's often easier to get them right by disconnecting the sec linkage, if it's possible on your particular carb, so that you can work on one thing at a time; then once the primaries are perfect, add the secs back in, and dial them in at WOT. They won't change things like idle, part-throttle, etc.; only WOT more or less.

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"So many Mustangs, so little time..."
ICON Motorsports

[This message has been edited by RB83L69 (edited October 18, 2001).]

 

For all of this Secondary talk, are you talking mech ao vac? Since all of mine open at the same time, should I still tune the primaries first or adjest them evenly?

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92 RS carb'd 350 5-speed T-tops
650cfm double pumper, 3.73's, and a massive cam

 

As RB83L69 said above..tune the primaries first then secondaries.I have mechanical secondaries on my particular carb so I will see if I can disconnect them from one another and tune each individual side.Thanks for everyone's help on this,much appreciation.

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1982 Z28:355,4 Speed,Lots of bolt ons

 

What intake manifold are you using.
A big single plane may need more accelerator
pump volume and more initial timing.
Check where you're picking up the vaccumm
signal for vacumm advance (should be "ported
vacumm".) Check for a vaccum leak. (manifold,
powerbrake booster, etc). try 72 primary jet
and 83 secondary and add secondary jet extentions. 38 deg total timing is probabily too much for your motor, should be 32 to 36.
12 to 16 initial. try the medium advance springs( total in by 3500) they are more stable at idle.

 

Yeah, the intake style is a big factor. A large open plennum with cause an average vac sec carb to run lean at idle. The fix for that is to open the idle air bleeds. Anyway, the lean idle means that when you hit the throttle, there is a big fuel starvation problem, resulting in a bog or lean backfire. You obviously don't have the vac sec problem. Try pumping the the throttle a bit to get more fuel from the acc pumps to see if the bog is as severe. Make sure it isn't lean at idle. Before you get a 50cc acc pump try a few more things like tuning or changing the acc pump cam.

 

I'd really avoid messing with the idle air bleeds. Way too easy to get lost in the forest. Won't opening the air bleed make
the idle and transition leaner? Sounds like
more like the primary and secondary butterflies need to be equally referenced
to the idle transition slots. The secondaries
are probibily closed off and not sharing the idle. causing a flow delay when they crack open. If after eliminating the chance of a
vacuum leak, readjusting the base timing and curve there's still a stumble I'd first play with the accelerator pump delivery and finally increase the idle feed restriction (fuel)a few thousands. like .002". Vacuum advance should be conected to ported vacuum with no vacuum advance at idle. Demon carbs are real close out of the box so I would leave messing with the idle feed restrictions till last.
http://photos.yahoo.com/russ_q



[This message has been edited by F-BIRD'88 (edited October 28, 2001).]

 

I am running a Weiand Stealth intake on the motor with a double pumper carb.The secondary butterflies are totally closed off,only the primaries have the idle transfer slots exposed,could this be my problem? I have reset base timing at 14 degrees,with vacuum advance hooked up to a ported source off the carb.I have medium springs in the mechanical advance right now.The hesitation is still there but not half as bad as before i played with the distributor,I have yet to even touch the carb.I think i have my timing very close to where I should now I want to get the carb a little better.i have to say driveability has improved 1,000% though just with the distributor tuning.

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1982 Z28:355,4 Speed,Lots of bolt ons

 

The wieand stealth is a very responsive manifold so barring a crack or something in the casting, that ain't it. Try balancing the throttle blade opening on the carb.
temperairly remove all vacuum hoses from the carb and plug. Check the accelerator pump linkage for slop or a bent arm. The one going from the cam to the pump lever.
Fuel should come out the shooter with even the slightest movement of the throttle.
Check and clean the air bleeds and metering block passages. Don't modify yet. Are the metering blocks, the right ones for that carb?

 

No vacuum leaks from intake,have checked,have already ruled that out.By throttle blade opening are you referring to the idle transfer slot adjustment?Should both primary and secondary be even?I have been told you only set them even on race motors,set only primary with secondary closed totally for the street,true or false?I have not checked accelerator pump linkage yet,should have like .015"-.020" of travel left at WOT right?I have cleaned passages and metering blocks out several times and yes the metering blocks are the right ones(they came with the carb from Barry Grant)Thanks for everyone's help so far,getting closer.

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Yes .015-.020" travel left at wot is good
but make sure there is no slack at idle.
Make sure the float level is right.
Try to get a balance of opening on all four barrels It will idle smoother and cleaner.
Make sure when you check inital timing, the motor is idling slow enough to ensure its not up on the centrifical advance curve. Slow the motor speed, set the timing with a light, re-adjust the idle speed. Put your finger over the ported vacuum source.There should be no vacuum there at idle. If there is, open the secondary barrels a LITTLE and re-adjust the idle speed screw. If you can, check the total centrifical timing (initial+
centrifical advance) Should be 32 to 36 deg
at full advance. Get an adjustable vacuum advance canister for the distributor. One that allows you to set the rate and amount of advance. The curve should be 8 to14 deg inital, 32 to36 deg total @ 3000/3500rpm. And
around 8 to 14 deg vacuum at high vacuum. (highcruise). Inspect the advance curve weights and springs and plate and BUSHINGS for wear and missing parts. Can you give more info on the engine combo, cam, heads,
gear etc. How is it when just getting into the secondaries?

[This message has been edited by F-BIRD'88 (edited October 30, 2001).]

 

I have a Speed Demon 750cfm Mechanical secondaries carb.I have Manley forged 10:1 compression pistons(.030 over),4340 steel crank & rods,AFR 195 (64cc) heads,Weiand Stealth dual plane intake,HEI and MSD 6AL box,Comp xtreme energy 274 cam,1.5 comp pro magnum roller rockers,and SLP 1 3/4 headers into a Flowmaster 3" single exhaust(no cat),3.73 gears and a 4 speed T10.Distributor is a rebuilt Summit Hi-output HEI with around 2,000 miles,so no wear to be found.I have changed the springs to mediums(was lightest) and reset initial timing to 14 degrees,also have the crane adjustable vacuum can.By doing this I have cured the hesitation at part throttle totally(when secondaries open).I only have a hesitation at the frst initial throttle opening now,like when I first hit the gas to take off.Quick flat spot and then she jumps up and goes like a bat out of hell.Floats are Ok.Should I still reset the four barrels idle transfer slots even,could this be my initial hesitation(still only primaries are exposed,secondaries closed)?

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Sounds like an impressive piece to say the least. I would try to balance the primary and secondary butterfly idle position.
First stick your finger over the ported vacuum source, if there is vacuum there
at idle (should be around 800rpm) the primaries are too far open at idle. This
will cause a bad flat spot just off idle.
It would take a book to explain why so just
go with me on this one. You'll need to open the secondary Barrels a "little" then re-ajust the primary idle screws to (750-800rpm)
do this till when you feel the ported vacuum
source, there is no vacuum there at idle.
Finally re-adjust the mixture screws evenly for best idle.
Vacuum should start right off idle when you open the throttle You can select two mounting positions for the accelerator pump cam by removing the small screw and moving the cam over till the other screw hole is lined up and install the screw. This adjustment can make the pump shot
fast/sooner/ more right off idle or slower/later/less. Move it to the position that gives the most agressive pump shot right off idle. Make sure the linkage now has no slop at idle and .015" extra pump travel at WOT. Tell me if that improves the tip in performance.

[This message has been edited by F-BIRD'88 (edited October 30, 2001).]

 

Thanks for all your help so far F-BIRD'88,much appreciation!I am going to try your suggestions tomorrow night and see what they bring and I will update you on her status.Thanks again!

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Wow!! I just happened to stumble onto this site and find out that other people are having problems like the ones I'm having and getting some good info I might add.

I'm having a similiar problem lol with my 750 speed demon mech. sec. and well it has gotten better but not perfect so I'm goin to soak up some of this info...Sorry for the story but I'm happy to have found this site.

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<<Ths Stang Stomper>>

 

Welcome to our board 1QWIKZ!Look around,ask plenty of questions,alot of knowledge here.

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Hey onebad82z

I was wondering if you figured out your carb yet?

 

Hey 1QwikZ...Have set up distributor as mentioned earlier,then went through carb again.I reset all four barrels idle transfer slots equal,set up accelerator pump arms to give the recommended amount of slack(.015") at WOT,and what a difference.From the moment I hit it she comes on like a freight train,no more flat spot or hesitation,just a banshee scream from the motor,man I love a hard running small block!I can honestly say that my Z now runs better ,with better throttle response than my TPI Trans Am had.Thanks F-BIRD'88 for all your help.

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1982 Z28:355,4 Speed,Lots of bolt ons

 

Only too glad to help....