still trying to get the new roller motor running well but i'm struggling with this holley. it's a 750vs and every time i drive it, i get this bog on accelerator tip in. this happens regardless of engine RPM although, it is less noticable at or above 3000rpm. the motor is still really fresh, less than 10 miles so i'm not really hammering on it but even just rolling into the throttle, i get this bog and then she takes off like i've been rear ended.
i looked through the holley web site and checked the set up of the accelerator pump and it hits as soon as the throttle moves so i think i'm o.k. there, but i am new to holleys so tips and tricks are welcome.
they also say the accelerator pump/nozzle may be too small. the carb came with a .031 nozzle which i drilled out to .040 with no discernable change in characteristics. i also drilled out the screw to feed it more fuel but i haven't replaced the pump with a 50cc, as recommended, since this was only a test.
other than this, it feels really strong but i do seem to have a miss i haven't been able to track down yet.
i'm still hobbling around on the bad leg so i'm trying to do more e-troubleshooting so your help is greatly appreciated. i'm hoping to go out this weekend and have a plan so i can finally enjoy this thing.

for those new to the saga, the motor is a 355 with ~9.2:1 SCR, roller cam 270*/.495 intake, 276*/.502 exhaust, cam is timed "straight up" and initial timing is set at 10*BTDC. i'm ballparking timing since i really have no idea where it should be on this motor. the L81 called for 6* and the L82 calls for 12*, i figure this is a good split until i figure everything else out. the dizzy is from a ZZ4 so i'm not sure where the curve is on this distributor.

 

You need to get the initial timing up to at least 12 to 16 degrees BTDC. Do you have a recurve kit? You need to map the curve up to 3000 RPM. Like Lars saids if it is not up to about 36 degrees BTDC by 3000 RPM it is too slow.

BigBlockk

Later.....

 

What kind of intake? Dual plane or single? If it's single then that is contributing to the issue due to a lower signal to the carb. What is your setting on the primary throttle blades? If they are open too far to get the idle you want you will expose the transition slots too much which can cause your problem. If I remember correctly it should be no more than .060" of the slot exposed below the throttle blades at idle. You may still need a bigger squirter. You don't need a 50cc accel. pump until you feel the engine pick up on tip in then fall on its face and pick up again. Also, where is you vac. advance connected on the carb?

 

You drilled out your shooters???? I hope you didn't ruin them, those are pretty precise measurements. I would personally just go down to the parts store and by sizes 2 and 4 size larger and tried that...

But anyways you can always change that cam on the side of the carb. Assuming you have a holley or equivalent there are different color pump cams available that will dictate how quick the shot comes in. Also there are two or three settings, you will see a couple small holes on the side of the cam, that do the same thing.

Also you can raise up the float bowls and the primary side and that will help the main flow to come in earlier.

Also you increase your initial timing and increase your curve. Burning that lean mixture a little later will probably help.

Also you can open your secondaries a little bit at idle and that will allow you to close your primaries and not expose so much of that idle passage.

Also, you can try the shotgun method and just increase jets.

So...lots of ways, just experiment carefully and change one thing at a time. Only drill as a last resort.

 

Go up in jet size 2 or 3 and see if that helps.

 

I was having the exact same problem with a 750ms i checked the idle mixture screws and 3 of them were set at 3/4 turns the other one was at 1-3/4 . i put them all at 1-1/2 and the problem is (almost) gone away. I"m sure you allready checked the mixture screws, was just what worked for me.

 

look at the plugs if it is lean than I would go up on the primary jets a couple sizes. There is a good post on the innovate site forums (general tuning) about tuning the holley 750, its long but very good info from very knowledgable people.

Neal

 

If the engine isn't surging at cruise rpm, then jets are not going to help fix the tip in stumble.

The usual suspects with tip in stumble are the power valve and the transition slot. Make sure no more that .020" of the transfer slot is exposed. If it is, then take it down and crank in some idle speed in your secondary to get both throttle blades roughtly equal opening.

If that helps, but doesn't get rid of the stumble, then up the power valve by two.

You should not need a 50cc pump on a 750 unless you go too large on the discharge size. A .040 shooter is really far too big. I would get another set of nozzles and return the shooters to stock. You may be compounding your problem.

 

i'll reset timing this weekend when i try and find that miss. like i said, i really had no idea what initial timing on this motor should be.

dual plane, performer. i have it hooked to the port on the very bottom of the base. the one that comes off at a bit of an angle below the bowl. (whoops, just found out from holleys site that's all wrong :bb: i'll fix that tomorrow.)
don't care if i ruined it. like i said, it was an experiment. i figured i would have to buy a new one anyway. i might as well spend a dollar on a .040 drill bit and see if anything changed instead of $18 on a new nozzle.
it's a holley. i haven't noticed the color of the cam but i have seen it (i seem to recall it being white-ish, which is probably stock). i did check the screw hole settings you are referring to and it's set on the first hole.
floats are already set pretty close to right.

i set the primaries to 1 full turn out. i still have to figure out where my vacuum gauge is... when i got injured some friends helped mrs. dust clean up the garage and i have no idea where some things are.
when i checked they were set at 1.5 turns out. i turned them all the way in and it died, of course. next i went to .5 turns and it died but very slowly. i tried .75 turns and it would idle o.k. i went to 1 full turn just to be safe.

pulling plugs this weekend. like i said, there's only about 10 miles on the car now so i'm not sure how much that will even tell me, but we should find out tomorrow.
innovative site forums? is that the name of the site?

there is no surging at cruising RPM. the holley book i have says this symptom is likely caused by the accelerator pump, hence the focus of my efforts thus far. i need some familiarization with the transfer slot as that hasn't really come up in the holley literature i've read so far.
probably am compounding my problem but i'm just following what holley suggests as the likely culprit.
BTW, this is the culprit...

 

Mine just seemed to like 1-1/2 on the mixture screws, i tried 1/8 turn in and 1/8 turn out from the 1-1/2 and the hesitation came back either direction i went. might want to try 1/8 turns at a time on both screws. holley recommends #1 timeing #2 idle mixture screws #3 vacum leak. Then they talked about make sure the accelator pump was shooting fuel as soon as the throtal leaver was moved, then they said try a different shooter size and check the power valve to make sure it isn"t stuck or the wrong size, but before you adjust the transitation slot or secondarys to call the holley technical phone number and they would tell you exactly how to do that.

 

I fixed a similar problem with a power valve change on a spreadbore holley 6210. The stock value on mine was a 5.5" and I went to a 3.5".

 

Here's what I would do:

First, get your initial timing up around 18 degrees while maintaining 36 total. You may have to limit the advance curve length to do this. Are you using vacuum advance? If not, you need about 24 initial timing with 36 total. If you have vacuum advance, make sure it's a very soft unit with about 16 degrees, and run it off manifold vacuum.

Next, that cam probably has your idle speed cranked up a bit. Flip that carb over and take a look at your primary side transition slots. I'll bet you have more than .020" of the slots showing below the throttle blades. Close the throttle so that only .020" is showing, and then open up the secondary idle speed screw so that the secondary blades are cracked open equal to the primary blades (you can hold the carb up to a light and look up the bottom: open the secondary idle so that the "light gap" around the secondaries equals that of the primaries). Then, bump the primary jet size up 2 sizes from stock, set the idle mixture screws at 1 full turn out from lightly seated, and set float levels to the bottom of the sight hole. To fine-tune the idle speed from this point, you need to adjust primary and secondary idle speed screws equally, which will involve pulling the carb off and flipping it over a few times.

Once these settings have been completed, I'll bet your stumble will disappear...

 

Before you go and beat your head against the Carb. You better find the miss first. Otherwise you're chasing your tail.
Make sure you dont have a Ignition problem before you blame the carb.

More often than not a stumble at tip in or cough at accelerator pedal input is cause by a Ignition problem.
You dont mention if your running a HEI or points type Ignition or a point eliminator type of Ignition.
If Electronic Ign(any type) isnt receiving a good clean 12v at the + side of the coil you're going to have issues.
I just went through these types of problems on a C2 turned out the Ballast resistor was killing the voltage to the coil and causing a intermittent miss

I'm not disagreeing with Lars I'm just adding some past lessons learned

 

thanks for the help, lars. i saw one of your write-ups on another members site. i will be following that today.
pmullaly, dizzy is a ZZ4 take off. going to pull plugs this afternoon.

 

one thing that might help is to put a vacuum gauge on your motor to see what level you're pulling at idle (in gear if auto trans). THEN, you can size your power valve appropriately.

If you don't know your vacuum level, your power valve may not be tipping in soon enough, or worse yet, too soon, but you'd know the latter scenario by it always running rich.

your cam selection will affect your idle vacuum level, so your power valve selection HAS TO BE tailored to your cam/motor combo.

this process is detailed in several of the holley books. If your idle vacuum is 11, then you want a 9 power valve (or 9.5). My recollection was that you go 2 below your idle vacuum, but don't trust my memory, check the books.