I replaced my HP950 DP on my 396 BBC with a 750 DP for street duty. I returned the 750 DP to stock configuration as far as the squirters, jets, pump cam, etc. The card has a nasty off-idle hesitation.

I put the next size larger squirter in it to see if that would get rid of it, but it didn't. I put the next size larger from that one in, with no changes. I then went down one size from stock (.25, as I recall), and that didn't help. I fattened the jets up six sizes, in increments, from 70 to 76, and that didn't help. I then changed the pump cam position to #2. That didn't help either.

The only thing I have left is the power valve. Refresh my memory as to power valve tuning, if you would.

As you can tell, I'm no carb guru. Someone give me a clue here, as to how I can get this thing dialed in...

Thanks.

Try changing the pump cam(also while looking at it make sure the arm is riding right on the cam with no slack-right when you move throttle you should see squirts from the discharge nozzles). Changing the squiter size doesn't give a bigger shot of fuel, it either gives a longer or shorter duration, depending on which way you go. Only changing the pump cam changes the actual amount of shot-try putting on an orange pump cam in the #1 position and see if it does any better.

For PV-theres two ways to do it but i've had good luck with the take vacuum reading in gear or at idle, and subrtact 2-3 inches and pick that PV(10.5 is the highest they make however). Thats what I have in mine which gets 14" of vacuum idle

PV won't fix an off idle lean spot. Tried it on a vac leak I later found.
Is the Acc pump arm adjusted correctly.

Make sure everything else is correct on the motor first. Plugs, wires,cap rotor etc.

A 750dp with that size cam will probably need a couple small holes in the primary plates to be correctly set up.
Make sure your static timing is set first. With a 296 hyd, you'll probably need at least 14* static.
Then adjust your carb so that the secondary plates just barely show the transfer slot and the primary slot is exposed approximately .040-.060. Note: .020 is about 1/2 turn on a 750. Or mine at least.
If the idle is too low, then you need some bleed holes. Start small, .060-.070 or so. One in each plate on the slot side. A lot of people look down on this idea, whatever. A lot depends on the whole setup. 4.11's can cover a lot of low speed problems. Even relatively big cams and low compression.

I had a similar cammed 408 that ran just fine on an all stock 750dp, and ran just fine with some .125 holes too. But it had 12-1 CR too.

The initial timing is set at 12*, all in at 2,400 rpm's for a total of 36*. I adjusted the pump cam set screws prior to installing the carb, and they are good.

As far as the pump cam, like I said, I had the stock pink pump cam in there at #1, position, and then tried #2 position, but that didn't work.

Try the orange cam at the #1 position now? I'll give that a shot.

As far as drilling holes in the plates, it was my understanding that if you have the motor idling, and are not able to kill the car by turning the idle mixture screws in, then you need to drill the holes. I have the mixtures screws set so it will kill the car if turned in, and it idles nicely at about 1000 rpm's.

My 296 cammed 350 need a 37 squirter with a hollow screw in the front and a 35 in the rear. I'm using orange cams in both positions with the front in the #2 hole. As said, make sure the pump is activated the instant the throttle is moved (you may want a very tiny air space if you leave the line off idle). I run 18º initial and 38 total. My carb has holes in all four throttle plates and has a four corner idle system. I don't know how much of this carries over to your 396 but these are my settings and they work well for me.

I would bump the timing up to 16* base

Couple pieces of information for you first - the pump cam changes not only the the AMOUNT of fuel you get but also WHEN you get the fuel. Position #1 gives an earlier shot than Position #2 if using the same cam, and tends to give slightly more gas also.

The "shooters" deliver MORE gas the larger they are - I know it was stated differently above, but I think that's not quite true. There are 2 types of shooters also - ones with little nozzles sticking out the holes and ones without the nozzles. If my memory is correct, the nozzled shooters a bit more effective, as the gas tends to shoot further into the intake.

Holley has some real good reading on all this stuff at www.holley.com. (http://www.holley.com.) The more familiar you are with how different aspects of the carb works the better off you will be.

Since you have such a heavy car you need an instant shot of gas that you seem to be missing, (it's worse with an automatic) and with a DP you are giving the car even more air than you would with a VS. I'm assuming your secondaries will fully open if you step on the gas fully - on a VS they won't open as quickly.

Based on what I've done in the past with my VS before it started to go bad on me, you will probably need a much bigger shooter than you have. If I had to guess, you will probably need a shooter somewhere in the mid .30's - if you are at .25 it's almost certainly too low.

Also, when you change something on a carb make sure you do one thing at a time - everything plays a part in different ways and it can get out of hand quickly if you make too many changes at once. But if it idles well, cruises well, and only has an off-idle bog, the only change you may need is the shooter size.

Hope that helps-

Originally posted by Xtreme70SS396:

The "shooters" deliver MORE gas the larger they are - I know it was stated differently above, but I think that's not quite true. Not be rude but- Holley Rebuilding and Modifying(A guide to Holley Modular Carburetors) -pg.112-113-"The discharge nozzle, located in the airhorn area between the throttle bores, containes orifices that determine how long the fuel shot will last"

And I've read it many other places also. The pump diagfram(be it 30 or 50cc) and the pump cam are the only things that control how much volume of fuel is actually given. The hole in the squiters simple dictates if the fuel gets splashed in real quick, or slower. There is too much pressure in the system for the orifice size to create a big enough restriction to prevent or block the actual amount of fuel coming from the squiters. Have you ever taken off a fuel bowl and then pushed the arm down on the diagfram? Fuel squirts out like 10 feet.

I would try bigger squiters(.32s or so) to dump the fuel in quick, and then switch to the orange pump cam in the #1 position to give it a larger volume of shot- if memory serves me right the Pink cam you are using now is rather small and conservative and doesn't give a very big shot.

EDIT- I guess the Pink cam is bigger then the orange cam already-are you sure it is not red and just faded over time? Get a pump cam tuning kit($10) and compare them and try to get one that will give a bigger shot.

Not rude at all - just trying to explain everything as best we both can, and different thoughts help us ALL learn more. To put my thought another way:

Edit - I'm just repeating what was said above.

My understanding on going to the larger shooter on a Chevelle is that the heavier cars need more gas right away, which is why he needs to go to a larger shooter size. All else the same, a larger size will give him the larger amount of gas he needs when he needs it (even though the total shot is the same).

We're really both saying the same thing, and I totally agree with your solution as described above - move up in shooter size first, then pump cam if that doesn't fix the problem. Switching pump cams changes 2 factors at once (size of shot and duration), which I try to avoid.

Pump cams determine inital flow/duration of the shot.

Something is troubling me Sean. When I tuned both my 750 and 850 dps, almost every change was noticeable. And I was learning on them, not some pro, so let's eliminate any assumed prior n\knowledge of Holleys. You're changing things way more and you are not noticing anything. Are you CERTAIN you don;t have a vaccum leak. Are you CERTAIN that the fuel bowls are filing properly, i.e., float level is up there when hot. I can't imagine the pump diaphram being shot without gas leaking out of the housing. Is that check valve thingy installed in the pump housing? Are all the internal passages clean? ( blow some carb cleaner, and then air down all four air bleeds).

You should have felt quite a difference with as much adjustment as you've made. Something simple is really wrong here. It's not metering correctly.

I have to agree with the post above that it is just not metering correctly. With all these changes you should have felt some kind of a change, good or bad. When I put my new 750HP DP on I had the same problem you are having. No matter what I did I had a severe off idle bog. I ended up noticing that the gaskets that go under the nozzles were leaking. When the pump pressurized the system the fuel would sort of lazily come out of the nozzle and also come out from under the nozzle. I replaced the gaskets with new gaskets and that fixed the problem completely. The nozzle gaskets that came on the carb were red in color and when they got wet with fuel it seemed as if they started desintegrating. I replaced them with some gaskets that I had laying around which were black in color (the ones I have always used). Is this carb a new out of the box carb or has it been laying around for a while.

My 750HP DP is on a 402 BBC. I run 75 jets in the front with a 6.5 PV and 85 in the rear with no PV. This is set up for consistent bracket racing conditions and I have run as lean as 72/80 with no problems as well. I use #31 pump nozzles with the stock pump cam which I believe is a pink cam in the #1 position.

As I said, initially I went from a stock .28 to a .30, and then a .35. The hestitation got worse. That's when I went to a .25, with no change.

I then bumped the primary jets to 75, and then 76, from a stock 70 I think it is. No change, still have a pretty serious hesitation. I have to ride the clutch through off the line to get past the hesitation.

I then changed the pink cam from #1 to #2 position. Now, I don't know if this cam is stock, but I assumed it was. I have a pump cam kit with all kinds of colors. Red is stock, isn't it? I don't think there's such a thing as a pink cam, as I recall, so I assumed the pink cam was faded red.

Anyway, I need to re-check my intake and carb base for vacuum leaks. No sense in trying to tune a carb with a leak. I bought the carb used on ebay, so I don't KNOW for sure whether it's been modified at all. However, the carb had VERY low use on it, damned near brand new, so I assumed I'd be able to slap it on without a rebuild.

At this point I think I'll take the carb back to stock, check for leaks, and start over.

And make sure the fuel level is correct in the bolws. That's the first step when dealing with carbs.

The diaphram is the maximum amount of fuel the pump can deliver in one shot.

The cam tells how much to deliver and at what curve.

The squirter tells how long the duration of that shot should last (smaller squirter = longer shot, larger squirter = quicker shot).

Is there any smoke out the exhaust when this happens? What color?

Originally posted by Fried_Guy:
And make sure the fuel level is correct in the bolws. That's the first step when dealing with carbs.

Already did this. It's the first step I can after installing a carb.

It's also possible you're getting a RICH bog - as you said, when you went up in shooter size the problem got worse. I'm not as familiar with the DP's, but you should have secondary shooters as well that may need to correspond to the primary ones.

As said, you might have other issues. Does it ONLY bog off idle, or at any speed?

Yea, give us a little more to go on as in the description of the bog. Does it happen when you are accelerating lightly or only when you go WOT from idle? Does it do it while cruising if you accelerate? At what rpm does it 'catch-up'? Does it accelerate fine for a second, then bog, then catch up? Or just bog right off the bat?

Scott_68_SS
I had a similar cammed 408 that ran just fine on an all stock 750dp, snip... The cam was very close. like a few degrees, it was the first time I'd checked on so probably some error. I mean all stock...

Sean, work on the primary side only. Forget about the secondaries for now. Assume you are getting the bog during *light* acceleration, not cracking the secondaries.

Wonder if there is something terribly funky inside that carb, like someone hogged out the power valve channels, which are generally too big anyway. You want them around .055" is all. They can be restricted using small pieces of guitar strings.

Easy check if you have a vaccum guage, read the vaccum drop when you let off the clutch and see if its at or below the PV rating.

BTW, the way the carb is responding to the richer settings doesnt lead me to infer there is a vacuum leak, unless its off the part throttle vacuum port. Make sure your rubber plug is good on that one.

Hope its something we can find and not another Ebay junker. I do have a known good primary metering block for a 750 if you want to swap it in and see. Only trouble is I'll need it back.

Originally posted by Xtreme70SS396:
It's also possible you're getting a RICH bog - as you said, when you went up in shooter size the problem got worse. I'm not as familiar with the DP's, but you should have secondary shooters as well that may need to correspond to the primary ones.

As said, you might have other issues. Does it ONLY bog off idle, or at any speed? Off idle only. Hold off on suggestions for now...you're starting to overload my little brain. I'm going to take carb back to stock config, recheck vacuum signal, recheck how much of the slot is showing in the venturi on the secondary side, and see where I'm at. I suspect my power valve is too big at 6.5. I'll post results after tuning. Thanks, guys!!

do yourself a favor and before going ANY further, check for vacuum leaks.

Especially in the accessories and lines.

If you have a Mityvac hand vacuum pump, take the fittings off the carb one and a time and pull a vacuum on them with the hand pump.

If any of them dont hold the vacuum (bleed down) then you have a vacuum leak in the line, or in the part (modulator, brake booster, ect).

After you're finished with that, then set the idle mixture screws with a vacuum gauge.
I start at 1 turn and work out slowly until i find the max amount of vacuum possible.

After which, reset the idle to where you want it.

BTW, make sure you're timing is where you want it to be, any changes in timing will effect where the carb wants to idle mixture screws to be, as well as the idle speed.

I try to go for as much initial timing as possible (whichever bushing allows for it). In my case, since i have a CSI starter i locked out the timing to 39 degrees.

What the reason for this is, that since you have alot of inital timing, you can back off on the idle plates and close up on the idle transistion slot, so that its not too far exposed.

Please check your float levels. When i remove my site plug i can rock the car and fuel just barely dribbles out.

Too high of a float level will put excessive weight and therefore pressure on the circuits and cause the car to have a rich condition.

Check your boosters to make sure they arent leaking at idle, causing a rich condition.


It may be worthwhile for you to remove the carb, buy a gasket set and dissassemble the carb, clean all the passages, check the powervalves with a vacuum pump, and reassemble.

I finally got the motivation to get out there and do it...

As I mentioned before, my old vacuum gauge (POS) finally gave out on me, and I invested in a good Blue Point gauge. It's amazing what good tools will do for you.

I checked for vacuum leaks with a spray bottle of water, and didn't find any, so I ruled that out. I dinked around with the idle mixture screws, timing, and idle until I was pulling between 7.5-8 inches of vacuum, which is the highest I could get with the mixture screws. The screws ended up being 2.5 turns out, which is a good thing. Any more and vacuum dropped off and bounced around more erratically. Any less and same thing, bouncing and erratic.
These adjustments were using 13* initial timing. Mechanical was set when the distributor was rebuilt at all in, 24* @ 2400 rpm's, for a total of 37*. Idle was set at 1000 rpm's.

The exhaust still burns the eyes a little, but not anywhere near as bad as before. I believe that it's running a little lean, but set the mixture screws according to highest (and smoothest) vacuum reading.

The motor is running 110% better than it was. I checked for pinging on a hill running at about 2500 rpm's, and then got on it to create a load. No ping. I will trying advancing it a degree or two and see what it does. I'll do this at a later date.

There is still a slight hesitation coming off idle, but my adjustments were a marked improvement over what it was. One thing I'll have to do is adjust the clutch so I have less travel from engaged to disengaged. The clutch disengages at the top of the peddle, and I like it at about a little lower than the top (if that makes sense).

After a clutch adjustment I'll get into the fine tuning on it. I believe I'll leave the stock size (.028) squirter in there for now, and focus on jetting and pump cam. Can jetting cause a slight hesitation off the line? I realize a pump cam that's too small may cause the hesitation I'm experiencing. I'm currently at pink (stock) position #2, and as I recall, someone mentioned trying orange #1. Does that sound correct.

I've read in my holley tuning book that if idle vacuum is, say, 7.5 you need to go 2 power valve sizes lower, to a 5.5 power valve. Does that sound correct here? I've currently got a stock 6.5 in there.

As far as the big picture goes, this is the best this motor has run in a long time. Good suggestions on the carb size way back last winter, and good suggestions on things to take a look at in tuning.

IIRC, Holley recommends working with the squiters first. They recommend going up (or down) 3 sizes for each change. I use an orange cam in the #2 position. The #2 position is for engines that idle at a higher rpm, like your engine.

I'll give it a shot right after I get the clutch adjusted. Thanks, Pat.

As I said, I'm getting a hestitation right off the line, and also at cruise if I get on it. If I ease into it, it's there, but not as pronounced. That leads me to believe, since it's in the primary circuit off idle and at cruise, that it's a pump cam.

You can disconnect the secondary link so they will notopen and concentrate your tuning entirely on the primary side. 2 1/2 turns out on your idle mixture screws is way out, WOW. She must be burning your eyes big time. 8 inches of vacume isn't much either. I'd concentrate on cleaning up that idle before moving on to the accelerator pump and main circuit. You may have a small vacume leak if the mixture screws like being at 2.5 turns out. Did you try setting the mixture screws for the highest rpm? Also set your accelerator pump clearance with the throttle closed at .005. If the pump arm is tight and you have a rocking big cam in there it can cause fuel to dribble from the squiters at idle really making the idle nasty. Has anyone messed with the idle air bleeds on this carb? If they opened them up that might explain the 2.5 turns out. Getting this ready for street duty, have you changed fuel octane? Fuel hieght within the bowl effects idle mixture and how soon the boosters will flow. The lower the level the slower the boosters will flow fuel because it will be necessary for the fuel to be pulled farther up the main wells. Hope you get her figured out. Bob

Never heard of tuning for highest rpm instead of highest vacuum?

when the ideal idle mixture is found you'll be at the highest rpm in the adjustment range, some people like to find the highest rpm them lean it about 25 rpm. Final rpm adjustment is made with the throttle blade positioning screw. The needle on the vacume guage should also be it's steadiest at that adjustment. JMO

FWIW, I have found that Holley's Power valve recomendation of 2" below Idle vacuum doesn't always work that well. I have much better luck, (especially with anything under about 12") cutting the idle vacuum reading in half. For example: My 355 only makes 8" of vacuum at idle. I swaped out the stock 6.5 for a 4.5, and it's perfect. (New 650 DP)
2 1/2 turns out on the idle circuit does sound extreme...usually somewhere around 1 1/2 turns out.
I agree with Bob, get the idle cleaned up first, then move on.

Thanks, guys. I'll work on that some more and see what I can come up with. graemlins/thumbsup.gif