[Mikes66]

Hi there, taking in everyones advice over the past few days. I spent a lot of time tring different experiments. Limited sucsess.

My inital cry for help was at part throttle cruze condition ( before main circuit activation low 20 - 30 MPH street driving) I have a constant lean stumble in the engine. My conclustion was the transfer slots are not suppling enough air/fuel to keep the gas starved motor happy. My intial question to this forum was, how to increase the volume of transfer slot air/fuel by drilling out the Idle feed restricors in the metering block. How much of an increase in hole size do i need? I do beleive I have the right air / fuel ratio, now that I have adjustable air bleeds for the idle circuit and I was able to get the idle adjustment screws down to 2 full turns from 3 1/2 full turns. I was very happy with that fix. It tells me the idle circuit was too lean.

First let me detail my engine, I have not done this in the previous thread.

1980 302, .05 over. Windsor Senior heads 2.02 1.60 58 cc (had them cut down a little) 1:1.6 Roller tips, Flycut forged,
Edlebrock Perform RPM 6500 RPM Dual Plain, with matching Perform RPM Cam. .494 intake .520 exhaust 110 degree center lobe 234 duration @.05 6500 RPM redline, Holley HP Main body double pumper (750+ cfm), New Acell Bilit Dizzy with MSD. 1966 Mustang Coupe, C4 and 2800 RPM stall TCI. Auburn limited, and 3.00 gears.

12 HG in gear at Idle. 6.5 Power Valve. Currently set to 24 Initial mechanical advance with 36 total advance.

Running low 13.0 1/4 right now, but is inconsistant. Has power potential to reach Mid 12's. Estimate HP near 360, I know there is another 20 - 30 hp in there somewhere....but that is for fine tuning on a dyno.

OK, for the folks that have giving me all the advice , here is what I have done and the results.

Timing:

First I attempted to see how happy the dizzy is. Started with 24 initial mechanical with a 12 degree advance ending up at 36 degrees by about 2000 RPM's. Took me weeks to make the dizzy/engine happy at these setting with my old 650 DBP. Just verified before starting any experiments.

Then changed the curve to be more in line with recomendation. Same initial and total but did not hit 36 degrees till 3000 RPMs.

Result, lots of acceleration backfire (carb spitting) anytime off idle throttle movement. Indicates the air /fuel was not igniting. No power below 2000 RPM's at all.

Just to make sure, I increased the mechanical to 16 degrees total advance starting at 19 BTDC, and it was a hell of a lot worse. Then went back to my fast curve of full advance by 2000 RPM, and it was a lot better, but still an ocasional spit.

Went back to 12 degrees total advance (24 BTDC) and I was back to normal, great speed and no spitting.

My conclusion is the dizzy was set up correctly for this engine. The carburator does not matter on this engine for dizzy performance. I have had three a 750 Edlebrock Perform carb, a 650 DBP and the new HP 750 DBP.

The Carburator:

The main Circuit airbleeds are 36, the Idle Circuit Airbleeds are at 71. Initiallyt they were at 75, but found much better idle quality with 71 bleeds. They will be staying just where they are for now.

The main circuit... Yikes... Went from 36 to 31 airbleeds and I will never do that again. It would seem there is a siphoning effect if the main circuit dose not get enough air. Once the main circuit was activated, ( after a few miles of warm up driving) the engine flooded out and stalled. Luckely I had the 36's in my pocket and put them back in to get home. Did a lot of swearing....

Then one other sugstion I heard from this forum, was to increase the float level a little bit. When I started, the fuel level was just below the port. I have to bump the car to get a little spill out. Then I raised the level but 3/4 of a turn, till the fuel just starts to seep out when opening the site plug.

It would seem to run a lot better right now. Still have transfer slot stumble, but not as bad now. Still have WOT subble till about 1500 RPM ( the range of the manifold is 1500 to 6500 RPM)

What is my next step.... anyone? I will try all reasonable suggestions, as I am desprit to make this carb run like it is a fuelie....with a computer...

 

[Motorhead]

Sounds like you've been busy! Glad to hear you made some improvement at least. I still can't figure out why it won't run right with less initial timing, but I guess we'll leave that alone for now, sounds like you've got it where it likes it.

I know this was mentioned on the last thread, but I still think you should try going a little smaller on the idle air bleeds. I know you've heard that going more than 5 sizes means something is wrong... but running a 750dp on a 350hp 302 is a little bit on the large side, so you've got to expect to go a little outside of the normal tuning procedures to get the big carb to work on the small engine. Also, what are you running for the secondary idle air bleeds? When running 2-corner idle, the secondary idle air bleeds should be down around 36... the Proform main bodies include 74 idle air bleeds in the secondaries, which doesn't work well at all when running 2-corner idle from what I've heard.

As far as stumbling at WOT at 1500rpm, I would totally expect it to stumble at that low of RPM WOT! With a mechanical secondary carb, you can't just floor it and expect the engine to handle all four barrels wide open... you have to wait to get into the secondaries until you've got enough RPM up that it can pull enough air to supply a decent vacuum signal to all four venturi. My 289 doesn't like to be floored below 2500rpm, and at 1500rpm I'd expect it to choke and die from fuel starvation!

 

[Mikes66]

Makes sense about the secondary idle bleeds. I know the idle port is very, very small on the throttle plate. Can't hurt, they are so small, it must be just a small percent of total idle circuit.

I will try knocking the edle bleeds down to 68 and see how that goes.

What will a 1 inch carb spacer do for me? I know I have the room for it. I would think I want a spacer that keeps the two different plains of the maifold seperated as they are now...right?

I guess I did not mean flooring from a stop, when I say wot, I do lean into it, but sometimes too quickly.

This guy dies when I slam the breaks on. Will the tubes in the front solve this problem?

 

[My427stang]

I am chasing a similar stumble on the transition circuit of my 489. Almost identical in symptoms and attempted cure so far.

You were spot on in going toward the IFR and air bleed combo. If it got better with the idle air bleeds smaller (doesnt really richen, it sorta gives the idle mixture more "authority" after emulsion, but same effect) AND it got better with the higher float level (idle circuit needs to work less to get the circuit energized by siphon) then you need a bigger IFR

Here's the deal though, you need to do some math on the area of a circle. If you have .039 idle restrictions in there .001, 1 THOUSANDTH, will be 5% more fuel (find the area of a .039 circle, then see how it changes by making it a .040)

My point is, dont drill much! LOL .001 / .002 at the most

However some other things, with my 489, I did richen in, and the stumble moved as I thought it might, but as I daignose I think I may have a vac leak, very minor, on one cylinder and I am really masking that vac leak.

I'd either pull all the plugs and really look close at them compared to each other, or get it to a dyno, or use a temp gun to see the differences in EGT from cyl to cyl to see if the a/f mixture isnt already too rich

Good observation on the air bleeds, especially the high side.

On the idle side, the IFR (idle fuel restriction) meters fuel, its then mixed with the Idle air bled (called emulsfication, think of making it bubbly like soda)

That mixture goes two places, the idle mixture screw for some additional adjustment, and the transition circuit

You may find if you go too far on an idle bleed, the mixture becomes "solid" like you did on the high speed air bleed

But, .001 increase in IFR wont do that, just remember, what you do to the IFR and IAB for transition, you also do for the idle circuit. However at still 2 turns out, looks like you could use .001 more IFR

Hope this helps

 

[Motorhead]

Some people like to run open carb spacers on a dual plane to give more effective carb flow, but at the expense of loss of low-end power because you loose vacuum signal and the strong pulse waves of a dual plane. I prefer four-hole spacers, supposedly they help build torque through the entire power curve. And if you want to get real fancy, you can buy a cheap 4-hole 1" aluminum spacer, and grind/file it into one of those expensive tapered 4-hole spacers. This is what I did on my 289, at the same time as the 750dp main body swap (knocking off the choke horn gained me 1" hood clearance, so I had room for the spacer). I can't help but think that the spacer probably helped negate any loss in low-speed power that I would have noticed just by going with the bigger main body, but since I swapped both at the same time I don't really have anything to compare.

 

[Mikes66]

Mr 427, thank you for validating my issue. I really was starting to think I was being over pickey. I know once I solve this issue, I may also help solve my off idle, hard acceleration bobble too. Sometimes it bobbles and sometimes it screams. I am starting to think that problem can be patched with an accelerator pump, but that will be the last tweek I do.

About the drilling of the IFR. Have you done this, or has anyone compedent you know have done this and I would guess that the air bleeds would have to be recalibrated to the new volume of raw gas comming into the circuit. So would the air bleed also have to increase, and by the same percentage, or by the air fuel ratio percentage? Guessing the new air bleed could be difficult, how would I know if I have too much transfer slot fuel? If there is too much transfer slot fuel, that just means the throttle position is closed a little more to decrease draw on the circuit?

I am sure I will not have access to drill bits with such fine precision, so how would I be able to increase this size by another method? I would also assume a hand driven tool, not electric. Slow is good....right?

The airbleeds, and if I may start a new referance to how they work.... if you make them too small, you get a siphon efect, and the best way to visualize it is, it works like your toilet. Once the water (fuel) is pulled into the main shaft, then the entire bowl will empty, becuase of the siphon effect. In this case, you could litterally drain the entire fuel bowl, if there was no air-bleedoff to stop the siphon effect.
I think this actually happened to me in last weeks experiment.
(laughs)

So as the warning goes, better know your stuff before playing with airbleeds.

I think before drilling my metering block, I will reduce the size of the idle bleeds to maybe 68, and see what happens iin the transfer slot.

Have you solved your's yet?

About the carb spacer, I want as much velocity at idle as I can get. I am also thinking heat issolation interleaved gaskets/spacers, but I would guess it would be a ruff ride for the gas, with all those uneven survaces on the way down.... Lots of turbulance...Right?

 

[My427stang]

I have done it, and the airbleeds arent too dangerous especially if they are replaceable, play around a bit. If they arent replaceable, its a bit tougher to undo

The IFR is where you want to be thinking though for your idle and transition issue, think of it this way, The IFR determines the fuel amount (with some influence from the float level), the air bleed then determines authority it will be pulled through the circuit. Think of a hole in a soda straw, a small hole you get soda and bubbles, but add enough hole, you get nothing. The bottom of the straw is the IFR, the hole is the idle air bleed, but in a carb, some bubbles are good, you need emulsification for the circuit to provide atomized fuel instead of a solid "shot" of fuel through the circuit

So its not really ratio when you change one, basically you need to richen the circuit, and backing out the screws wont help, AND tweaking it the minor amount the air bleeds wont help, you have to give it more fuel through the IFR

The key here was raising the float level and cutting down the air to the circuit helped but not enough, now you are in IFR territory

Its tough to buy drill sets in .001 increments, but if you can find a set, its done by hand

Next you'll open the IFR by .001 or .002 and you'll find you'll have to turn the idle screws in, because it'll be too rich. Well, lets say they end up 1/2 turn out and the screws are too sensitive (makes it hard to adjust cleanly), well, add a little air bleed and the idle mixture wont come on as hard because of the air killing the signal a bit more as it emulsifies, you'll probably end up 3/4-1 turn out and all will be well

In any case, the air bleeds may stop a siphon effect, but thats not what they are there for, what they are there for is to introduce air into the circuit to allow an emulsified mixture, the reason is, that mixture is more readily atomized at the booster, transition slot, or idle feed depending on the circuit you are talking about

I recommend you put the air bleeds back to whatever they delivered as. Then measure the IFR and see what it is. If its anywhere under .040, open it up .001, reset your float level and see if it gets better. If its already .040 or bigger (thats HUGE BTW but some Holley HP series do that now, I'll talk about that at the end) then look for a vac leak somewhere

Everyone loves air bleeds because they are easy to swap, but IMHO, start with the air bleed the carb delivered with, if you need more fuel, open the IFR, if the idle screws arent doing what you expect then, start playing with the bleed to kill the signal a bit, or add authority to the screws.

Make sense?

OK, now some HP series talk

In some, not all, HP series, they run a very big IFR and IAB (idle air bleed) combo. My 1000 HP series on my 489 runs a .043 IFR and a .067 IAB (but it delievered with a .041 IFR and a .073 IAB)

You'd think it would be rich as hell, but if you look in the main body, there are two side by side holes in every barrel on the metering surface. One feeds the transition slot from the idle channel, the other feeds the idlefromt he same well, but passes through the idle a/f adjuster screw for metering. In most Holleys, these holes are unmetered in the body but in some of the HP series, they bush the idle hole in the body (limits flow of emulsified fuel to the idle)

This is a wierd deal, they do that so that they can have this big transition flow good for low vacuum/big cams, but the idle is secondarily leaned by the bushed hole. The goofy thing is the idle screw already does the same job

Good, bad? I dont know, just different. But if you need a leaner idle and richer transition I guess it works. Just remember, you may get so rich on transition that you go pig rich on tip-in, but the carb still wont richen at idle. Odds are you have the bushed idle feeds.

Measure your IFR size if you can (unless you know it) and we can go from there. My guess is by going with the larger body, you need a bit more IFR (maybe .002 at the most) and all will be well

_________________
70 Sportsroof, 427 FE/489 cid, 10.4:1, solids, Stage 2 Edelbrock heads, RHP RPM intake, 1000 Holley HP, TKO-600, 3.70's, A/C
71 F100 4x4 shortbed 390 hot rod
02 F250 Powerstroke 4x4, chipped, exhaust, etc trailer puller



<font size=-1>[ This Message was edited by: My427stang on 5/22/06 11:34am ]</font>

 

[My427stang]

One other thing, small pin dril sets generally go to .039. You do it by hand, and they are cheap. The only thing you prbably should do, is check them with at least a set of good calipers and make sure they measure right.

Then you drill by hand, it cuts like butter, and blow them out with air. Easy easy stuff............tougher is going back if you drill too far LOL but thats why you only go .001 at a time

 

[Mikes66]

I think your suggestion is my next step, not in a rush to do it just yet. I think I will see if I can get a beter metering block with adjustable IFR. I think Proform has one.

With all the messing around I have done, I am going to drive it a bit, and get a baseline before doing any more changes... except a 1 inch spacer. I will get the type with four seperate throttle bores. That should keep velocity high near idle...right?

Thanks for your help....

 

[My427stang]

I dont think a spacer is going to do a lot, but I guess its worth a try

You may want to consider even more initial timing, I know lots of SBF guys that run 15-16 initial, even 18, and then just limit the total

That would increase the signal to the carb a bit

 

[Motorhead]

My427stang: He's already running 24 degrees initial advance I believe, and also the vacuum advance running at full manifold vacuum (not ported).

Mike: One of the claims of the 4-hole spacers is that they 'increase vacuum signal'. I have no idea if that's true... you having a vacuum gauge and knowing your engine's behavior fairly well, I'd think that you would be well equipped to test that theory. In my mind, you are increasing the plenum volume slightly, which would smooth out the vacuum pulses somewhat and therefore decrease peak vacuum signal. However, there is the affect of straightening out the airflow directly at the base of the carb (where your idle/transition mix enters the airstream) so that would have an affect that I wouldn't begin to guess at. Give it a shot! I've never ever heard of a case where a carb spacer decreased performance/driveability.

 

[My427stang]

I read that backwards earlier.

Here's my little issue with the comment LOL 24 initial, and he retarded it, it backfired.....

Do you think maybe TDC isnt correct on your combo?

It may run better with more initial, but I NEVER heard of a motor backfiring at 12 initial. Any chance your marks are off and you are later that you think?

Any motor that needed that much timing to run would have a very inefficent chamber, and thst doesnt seem to be the case with yours

_________________
70 Sportsroof, 427 FE/489 cid, 10.4:1, solids, Stage 2 Edelbrock heads, RHP RPM intake, 1000 Holley HP, TKO-600, 3.70's, A/C
71 F100 4x4 shortbed 390 hot rod
02 F250 Powerstroke 4x4, chipped, exhaust, etc trailer puller

<font size=-1>[ This Message was edited by: My427stang on 5/23/06 8:20am ]</font>

 

[Mikes66]

427 Stang, Yes, I have lost sleep of this issue many times myself.

Depending on the Carb, ( the edlebrock for instance) I was able to get down to near 17 BTDC and run fine, with very limited spitting. ONce warmed up it goes away. But that carb did not have 1/4 mile acceleration the Holleys do.

I also had night and day difference between the 650 DBP, and the 750 HP upgrade that I did.

I hear many folks with slightly badder engines run with the distributor fixed..(no advance). Is this the reason why, or is there another reason?

My dizzy timing tape is nuts on. The cam calls for 36 degrees at the top. If I adjust to 34 or to 38, I loose top end power above 5500 rpms. So this tells me my tape is dead on. I am still a little lean at 6000 RPMs' too. This 302 is running 72 and 84 jets. The 72 plus ten (power valve). I need to go to 86's in the back. On a 302....I would have never belived it. Plugs are bone white.

The cast iron Windsor seniors I have at 10 years old, but brand new. They have the heart shaped quencher chambers, and are supposed to be very good for my application.

The problem is, there is such a short stroke on a 302, that low end vacuum is low. I could re-tune everything to work at less initial, but I will loose tons of torque off the gate. The way I am configured, I am getting full air/fuel in the cyclinders very early in the RPM's. by the time I hit 2000 RPms, this motor is screaming, with very high velocity going into the higher RPMs. My cam has a lot of overlap. But, I have a very smooth idle in gear at 650 RPM's, starts with 1/2 a crank of the starter... everything tells me this motor is tuned corerectly.

I am hitting almost 70 MPH in first gear, in just 6 seconds....from a dead stop. Like I said tons fo tourque. I like sprint cars better the 1/4 mile racers. I actually don't like going over 100 mph...

 

[My427stang]

You cant say that your car runs best at 36, that your tape is dead on.

So many factors affect the burn in a chamber and timing requirments, that just saying your car ran best doesnt mean a whole lot (thats being polite, it doesnt mean anything).

The same cam is a fast burn head will need FAR less advance the piston shape, the dish shape, the surface prep, materials etc, all change flame speeds, the cam mfgr can give you a guesstimate, but until you check it with a piston stop, you dont know where you are, and surely cant say "I need 36 due to the cam".

Matter of fact, with the chamber you describe, the motor should want less total timing than a stock head. But the key here isnt necessarily total timing, I think that due to the tape being off, your initial is too low.

I am willing to bet that if you check TDC with a piston stop, your numbers are off. Your motor is a nice hot street motor, althought SBFs love advance, it shouldnt need it as badly just to run as yours looks like it does by the numbers

You do have a healthy cam for a 302, but its hardly a "wild man" type of cam. 110 LSA and 230-ish is a healthy cam for a 302, but really its not a LOT of overlap. Thats proved by a 650 rpm idle too

The reason racers run locked advance is they launch above the full advance rpm and the motor never goes below that RPM, so why have additional moving parts. If the motor is always above 3000 rpm from launch to shut down, why have a curve?

Dont let that confuse the issue here, I promise your motor is not that wild with a 230-ish, low 500's lift, 110 LSA cam with a dual plane. My guess, which you could prove wrong with a piston stop, is that you are really running a lot less intial and total timing.

I would also add, that once you verify that, you may still need to richen up the idle, but maybe not. Residual heat from having the timing retarded will really cook the chamber (not harmful) but will make it act a bit leaner

I do assume you did not degree the cam either? Becuas eif you did, you could have verified TDC then

_________________
70 Sportsroof, 427 FE/489 cid, 10.4:1, solids, Stage 2 Edelbrock heads, RHP RPM intake, 1000 Holley HP, TKO-600, 3.70's, A/C
71 F100 4x4 shortbed 390 hot rod
02 F250 Powerstroke 4x4, chipped, exhaust, etc trailer puller

<font size=-1>[ This Message was edited by: My427stang on 5/24/06 12:23am ]</font>

 

[Tracer64]

What rpm is your 20-30 mph cuize at? Something doesnt seem right, my cam isnt that much different than yours and when its running right it wont idle below 850 in gear and has a definate lope. First it sounds like your timing is way too retarded. I had the same off idle lean and went to quick fuel metering blocks with changable IFR's. I ended up going up 8 "sizes" and it cured the problem. Bought the next 3 sizes of IFR's from stock, kept my originals and drilled the replacements.
I do have a 4 corner idle, but I got rid of my low rpm floor it stumble by adjusting som slack into my seconday squirter so there was do overlap between the front and rear.

 

[Mikes66]

Mr 427stang, At this point I am trying all sugestions. I just spent some time and verified TDC and timing mark tape.

It is a stock 3.75 dampner, with a stock ignition TDC pointer, and the water pump exists on the drivers side. I did that on purpose so I could see the timing marks very clearly.

I also verified the tape. It is the MSD style tape. I have a spare dampner and spare tape, and just verified the tape matches all the marks on the dampner from +10 to - 40.

I built this engine 10 years ago, and it has been sitting till this spring. I clearly remember degreeing the cam ack then. When I checked it, it was dead on. I did not have any adjustments to do.

I am taking this seriously, so as I mentionmed before, I will entertain any reasonable ideas.

Like I said I have had three carbs on this bugger. The edlebrock allowed me to bring the time down to 18 BTDC.

Maybe the transfer slot circuit is so lean, way lean and this is why I can not get lower on this HP Carb? This new writer ( sorry forgot your name) says he expanded is IFR slot by six sizes. Does this mean for example going from .039 to .045? Would that be a 36% increase in raw gas volume coming into the idle circuit? Match this with any airbleed chages? Stock airbleeds are 75, would they stay the same?

 

[Mikes66]

Thats 6.375 dampner.....

 

[Tracer64]

When I get home from work, I will look (I wrote them down) what IFR size I ended up at. Mine are still the stock air bleeds on the proform centerbody. I dont know if it makes a difference, but I'm using a single plane intake.

 

[My427stang]

How did you determine your timing marks were right? Are you using a piston stop and some way to degree it?

I hate to sound like I dont believe it, but it isnt magic, its just a little wedge headed motor, and if it needs that much initial, there is something wrong

Again, hate to sound like a broken record, but that motor should run fine (down on power, but act normal) down at 8 degrees BTDC, maybe lower

Not a hobbyist here, I promise something isnt right. May not be the balancer, but you shouldnt need that much initial.

Tracers cam is significantly more radical at 108 LSA, something is up.

 

[DF66Stang]

I had trouble with mating a Crane ignition and Accel distributor because the instructions called out the wrong polarity for the magnetic trigger wires. This created a situation where it would retard the timing (rather than advance it) with increased RPM, but it would run like this if initial was set high enough and the mechanical advance was limited to a small amount.

Any chance you might have a situation like this? Have you observed the timing advance as your motor increases in revs?

Just thought I'd throw it out there as a possibility.