Still has a stumble/hesitiation

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71Duster

71Duster

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750 Mighty Demon on my 340 with 3.55 gear and a 3000rpm stall and heres the issue.

Pullng about 9" of vacuum in gear warmed up so I swapped to a 4.5 power valve 65 Jets in the front 75 in the rear this was checked on an exhaust gas analyzer while driving.

31 shooters front and rear and red pump cams. WOT of the line from idle the car if warm while pick up slowly not a full on bog and its trying to squak the tires but not much else.

Cruiseing around 2000rpm with quarter throttle tip in the engine will pick up rpm and fall back down then pick up and go again you can even watch it on the tach when it does this.

I've tried up to a 35 shooter with no change and when running the 6.5 power valve it was also no different.

I've tried resetting the idle speed screws but is it possible I have exposed the transfer slots too much still? Starting to feel really lost and eyeing a 3310 Holley new in the box for sale locally.
 
Well...

Initially I would try a 7.5 PV, 37-42 shooter w/corresponding pump cam.


Just to make sure, does it bog and recover... or cut out and then go?

What in gear idle rpm are u at?
another thing is timing.
 
The one while cruiseing is in 3rd around 2000rpm. Timing initial is 16 and 32 all in at 2500rpm.

I've tried more timing but at WOT it was pinging a little at 18 initial.

When you stab 1/4 to half throttle it will pick up and very quickly cut out then pick up and go hard you can actually see the tach bounce about 500rpm when it does it.

Never played with pump cams at all yet, how do I know which is the corresponding?
 
Just for sh*ts and giggles, I'd try a smaller carb, say ,600 Holley or 625 AFB. I think 750 is just too much carb for a 340, unless you've got a huge cam,pistons, convertor and gear to go with. I see some guys w/ 408's running 625/670 carbs...(shrug).

Just my thoughts....maybe worth what you're paying for 'em.....
 
like wild said id try the 7.5 pv.but id only make one change at a time.try the pv first.also your primary jetting seems a little lean.a 600 cfm holley comes with 66 jets in the primary.
 
sounds lean on the shot, 'if the jetting is right'.

It's not enough initial volume or timed late enough, with the 35 shooter also try the orange pump cam.

Then if thats not enough still, try a 37 shooter or he 2nd position on the pump cam.
basically go bigger till bogs then go's...then go back till it's just right.

there are 2 to 3 adjusting holes on the cams, the 1st provides the full sweep, the 2nd will provide a shorter duration of the 1st but more volume initially 'basically starting at mid ramp' and the 3rd even further into the transition.
 
The demon I last ran came ootb jetted 76/83, I ended up running 73/83.

When I run holley's...it's usually the other way around, I go up 1-2 jets in the primary.lol
 
With the 67 jets in the front that it came with the A/F ratios where 12.73 to 12.85 at WOT and 13.56 to 13.78 at cruise. It was recomended I could go down 2 jet izes and if it didn't ping it was good to go. So I dropped down on the front to a 65 with no noticeable change.

The reason it ended up with a 750 Mighty Demon with annular boosters is I had a 650 speed demon and had bog issues off the line I couldn't get sorted out so the engine shop that built it loaned me a 750 mighty demon they had and mid range and top it where way better the bog was a little less but no worse so I picked up a new/used 750. I am starting to think vacuum secondary as my ability to tune the Demon's hasn;t been that great. I see a lot of guys runnign and liking the Holley 3310 750

I'll try the power valve this weekend but when I was on holleys website they said divide your vacuum in gear by 2 which is where I got the 4.5 from.
 
I'll hit the local speed shop this weekend and pick up apower valve, 37 shooter and the orange pump cam. Do I ned the 37 on the rear too or just the primary?
 
Put the 7.5 power valve in and went for a drive almost cures the hesitation it will only do it once and awhile now. Before I checked the forum I just put the 37 shooter in the primary but no pump cams as they didn't have any and I didn't go to the other place.

Half throttle from a stand still or from a roll the engine just lays over and will almost stall, WOT and the engine will just cut right out.

Just to try it I put a 25 in that I had pretty much identical result. Put the 31 back in a cruised for the night. Stomp it off the line it has no snap but no bog it just slowly picks up rpm. Sadest 340 out there in that I can;t even get it to do a burnout on nice fresh pavement. It'll just sit there holding the brakes and listen to the engine grunt.


Engine shop tells me my 3.55 gears and 2800-3000rpm stall isn't enough and off the line performance will always be that way.
 
71Duster said:
Put the 7.5 power valve in and went for a drive almost cures the hesitation it will only do it once and awhile now. Before I checked the forum I just put the 37 shooter in the primary but no pump cams as they didn't have any and I didn't go to the other place.

Half throttle from a stand still or from a roll the engine just lays over and will almost stall, WOT and the engine will just cut right out.

Just to try it I put a 25 in that I had pretty much identical result. Put the 31 back in a cruised for the night. Stomp it off the line it has no snap but no bog it just slowly picks up rpm. Sadest 340 out there in that I can;t even get it to do a burnout on nice fresh pavement. It'll just sit there holding the brakes and listen to the engine grunt.


Engine shop tells me my 3.55 gears and 2800-3000rpm stall isn't enough and off the line performance will always be that way.
Click to expand...
wow...lol.ive got less stall,and less gear,and can burn the tires at a light stab,so i dont agree with the speed shop on this one.unless your way over cammed.is the demon the only carb thats been on it?do you have/or can you get a good working loner just to try?
 
71Duster said:
The reason it ended up with a 750 Mighty Demon with annular boosters is I had a 650 speed demon and had bog issues off the line I couldn't get sorted out so the engine shop that built it loaned me a 750 mighty demon they had and mid range and top it where way better the bog was a little less but no worse so I picked up a new/used 750. I am starting to think vacuum secondary as my ability to tune the Demon's hasn;t been that great. I see a lot of guys runnign and liking the Holley 3310 750
Click to expand...

Speed demon and might demon are 2 very different line of carbs. Instead of make 1 step up you took 3 steps basically. I think that Holley 3310 or a 650 Mighty Demon would be more suited to 340 but I am not a wizard at all when it comes to carbs so take this for what it is.
 
Have you tried "running it" with larger jets? Those 750 Mighty Demons will not run good with a 65-67 jet, Try running 73/74F 82R with the 31 squirters & the 6.5 PV, i'll bet your problems disappear.
 
List all your cam specs here so the people know what you are working with.
 
Running problem is where I am weakest. The more aftermmarket and performance is involved the weaker I am. Having said that...I spent the past 3 weeks trying to cure a similar problem with idle rpm changing, and sintermittent stumble. The real puzzle was watching a see thru fuel filter go almost completely dry then fill to about half way in cycles. Replaced everything from the bottom of the fuel tank to the needle and seat. No change.
Problem and cure was so simple I'm still kicking myself for not finding it sooner. Bowl vent wasn't opening. The only way fuel could enter the bowl was for the the air in there to move back into the line and filter.
My point is dont overlook the obvious or simple stuff. Good luck
 
The carb has a lil larger venturi than a holley 750, it needs more shot to transition into the mains, the high stall only increases the necessity for more fuel......

the change is 5% per jet with a demon, 2 jets is huge change...but he's saying the a/f is bitchin.....whats one to think here...hmmmm

I ran a 750 demon on my 340 '4spd 3000lbs', the holley 750 before it liked 28 shooters/red cam and 71-82 jets...and ran a PV 2'' below the max vac at idle.....now when the demon went on...it wanted 31 shooters and orange cams and went from the ootb 76 primary jets....to 73's or 74's 'if i remember right'.....
now look at his setup and 3000 stall.....see?

it needs more shot.....
 
Sorry had some computer issues. Here are the specs on the car.

Stock J Heads 2.02 intake 1.60 exhaust and Crane Gold 1.5 ratio rocker arms.

3.55 gears and a 727 with 3000 stall from SMR, stage 2 shift kit from transgo.

It runs a Hughes cam #heh2328al

111deg lobe seperation/installed centerline 108deg
intake lift .506/ exhaust .524

[email protected] Intake223deg/ Exhaust 228deg

Cam timing @ .050” Tappet Lift

Intake Opens: 3.5 Deg BTC
Exhaust Opens: 48 Deg BBC

Intake Closes: 39.5 Deg ABC
Exhaust Closes: 0 Deg ATC


Engine is .060 over with Keith Black Hypereutectic pistons on stock rods and a stock forged crank.

Ignition is an FBO box


The Mighty Demon I tried first was tuned for a 408 stroker and was pig rich on the car but aside form some smoke and terrible mileage she ran good.

I'll try the pump cams and some more shot this weekend. The 650 speed demon never ran any better and wasn't as strong mid range/top end. Bottom end was all the same.

I've only ever run the 2 demons on the car which is why I might pick up a 3310 Holley just to see what it's like.
 
Waaaaaay worse with the orange cams, can't even floor it it just about stalls completely. As a side note the Holley pump cams have much less foot print then the Demon ones which is of course all they had. I noticed there are 3 different but marked scew holes on the red pump cams, is it worth trying them?
 
71duster
just a side note i haven't seen yet but if you do try a larger squirter say 37 and up remember to get the different screw for it.i think anything 37 and up has a hole in the screw for the added fuel
terry
 
sounds like your carb is mechanical secondary? I have a 750 mighty demon w/annular boosters, mechanical secondary on my 318...bought new, it came stock with 70/77 jets. when I had a .223/.230 cam I ran it with 68/75.......now with .244 @ .050 running 72/79 jets
no bog issues.....strange that the motor would ping at 34 total unless its lean..34 is usually the recommended timing, although on mine I can go to 38 without ping @ 1300 feet. what is you fuel delivery system? filter? line size? sorry I cant help more..good luck.
 
With all the recommendations on jet sizes from other members, consider the locations. I see you are in Edmonton and have some elevation. some of those recom come from Calif and may not be right for your elevation.
 
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