Stumble off idle -

[RET_COP]

Good point Gary, you should squirt gas at the slightest throttle movement. Bring the rpm's up in the 750 range, at least for better oiling. Do this by using the mostly primary throttle because you said you had .015 slot showing, more might help to get the transition from idle to main circuit. .015 t-slot showings under the secondaries is plenty. If you have to go more than an eighth of a turn on pri. to get 750-800 rpm, Stop, use a little secondary. "Balance"
Lou

[patrickt]

Originally Posted by Grubby

Best vac was about 7 at 550 rpm idle. Screws are pretty close to 1 turn out. My vac gauge is old and beat up - it may not be accurate.

John, I think you should really double check this with a new vacuum gauge before you start doing any more monkeying with your carb. With your build, if you're down in the single digits with your vacuum, at a normal idle, then I'm going to call "foul" and point to something like an intake manifold leak. You said you double checked your timing and adjusted your valves, those are two other typical causes for low vacuum.... Brent & Bobcat, you don't think his vac is a little low?

[Bobcat]

Patrick .... depending on the cam and idle speed , 7" of vacuum may not be too low . My 482 has a long cam ( 250 @ .050" ) and has about 7" to 8" at 1000 rpm . Having said that , if the vacuum is really that low , then the PV should be in the 3.5 " range ..... and a check for vacuum leaks should definitely be done .
Grubby , you said the timing was ok , but what is your initial and max ? I run 22 degrees initial and 35 degrees max . Ignition/timing should always be checked out before working on the carb .... eliminates one variable . Most carb/ignition people recommend something in the 14 to 18 degree range for initial advance .
550 rpm , for me , is way too low an idle ... I like between 900 to 1000 on my engine .
BTW , as Brent and others have pointed out .... only change one thing at a time or you may have one change cancel out the other . Yes , it`s time consuming , but it`s the best way .

[patrickt]

Originally Posted by Bobcat

Patrick .... depending on the cam and idle speed , 7" of vacuum may not be too low . My 482 has a long cam ( 250 @ .050" ) and has about 7" to 8" at 1000 rpm . Having said that , if the vacuum is really that low , then the PV should be in the 3.5 " range ..... and a check for vacuum leaks should definitely be done .

I vaguely remember his build specs -- and his cam is smaller than mine (mine is 245 @ .050, but with a 114 LSA) and I'm over 12" at idle. If he really is at 7", and I bet he still has the stock PV of 6.5, then it might be easing open and causing problemos. Brent's just spending too much time with that new bratty bundle of joy... he should be on here chasing the important things in life, like carbs.

[decooney]

My cam is 252 @ .050 and I get 12lbs of vacuum all day long on my 428 CJ with only 12 degrees of initial timing. I've tried every discharge nozzle and .031s are fine on most FEs with these cam specs. 7 lbs of vacuum is too low if you ask me, and Idle at 550rpms? just seems too low. A simple (quick test) is to crank up the idle to 900 or so and fatten up the idle mixture screws as others have suggested to see if something in transition circuit is not keeping up. If nothing changes, 1) check for vacuum leaks on intake/carb. 2) Clean the entire carb, replace all gaskets if you have not. 3) If you have a Power Valve in this carb, check-replace if needed. If you are not running at least an 850 on this motor, I've seen problems like this before with lean transition circuit Holley 750HPs on large FE 428+ and even SB Chevy 400s. Just had the same problem on a SBC 400 build. Ended up having to richen up the transition circuit in the primary and secondary metering blocks to cure the problem. Carb was too small to begin with. If we started with an 850, would have never been a problem to begin with.

Still have a few questions though -

Exactly what model Holley carb is this (xxxx-x) ?

What size jets do you have in it?

What size Power Valve is in it?

Are the butterflies drilled, if so, front/rear, how big?

are the secondary butterflies cracked open some, if so, how much?

Are you certain all 8 cylinders are firing clean?

What is your initial timing set at?

[Grubby]

I will attempt to answer as many questions as I know;

timing is set a 16 deg @ 800rpm and 36 deg alll in before 3000rpm. No vac advance.

The cam is mild but I don't know the specs. The engine dynoed at 480hp. I would say it is a pretty typical Joe Lapine build for ERA.

I checked my vac gauge on my 2003 GMC Sierra. 17 inches at idle.

With the Cobra at 800rpm it has 10 inches.

Confirmed that there is .015 clearance after WOT on the accel pump. It actually had maybe .020 so I adjusted to .015. And the moment the linkage moves I get fuel from the accel pump.

The carb is a Holley 3310-11 1045 - 750cfm.

Last summer I determined the secondaries weren't opening and went through the excercise of finding the right spring. I replaced the Black with a Purple. The secondaries come open cleanly after 3000 rpm.

I did try richening the idle screws 1/2 turn after best idle. No change was noticeable. They are now set back to best idle.

The car will die if the idle screws are turned all the way in.

Throttle plates do not have holes drilled and secondary plates look like they are completely shut.

I will place an order for a 31 squirter. That was clearly not the problem and I will go back.

Don't know the jetting. The plugs and side pipes are a deep tan color. I assume primary jetting is correct.

I installed new lifters last fall to get rid of some valve train tick. They are hydraulics and were adjusted at that time. Installed without removing the intake.

The stumble is not new. I am just working through my list of items to make the car perfect. The stumple has been there since day one.

Back in the mid 90s I was ASE certified master tech. I was a driveability engineer with GMC truck. I have always worked on my on stuff, but have little holley experience and no stock of Holley parts.

John

[decooney]

John,

Not knowing your cam and heads and flow, but...based on this info you may be lean on the primary and transition circuit chasing a less tunable situation with that particular carb until doing some additional work on it. I found a few references that indicate the 3310-11 came with 70 or 72 primary jets, a 6.5 power valve, and 76 secondary jets stock. Also, the transition circuit in that carb might be lean too. Some guys run a softer spring on the Vac Sec unit to get it to open quicker too. I ran my FE with 78/78 jets once with a 6.5 pv and it was still lean. I settled on 82/82 squared with 5.0 PVs in my Holley HP carb, and 80/80 is probably more spot on with my particular cam and heads-flow and exhaust setup. ERA sidepipes flow pretty decent. Ran mine 78/78 initially with my original ERA sidepipes until I changed them out for something even more freeflow. Had to jet up again after that.

Before its all said and done, you could end up trying all sorts of different changes with that particular 3310-11 carb and not totally cure the lean spot 100% without some drilling and changes to the metering blocks along with jetting changes, etc. I tried a non-HP standard Holley 750 on my motor once for 15 minutes and it did not run as well as my current Holley HP 830 reconfigured some. Sort of guessing without knowing more about your cam/heads setup. This just based on several other single Holley / FE 428+ setups I've seen guys run.

Do you know anyone near you with a Quick Fuel or Holley HP 830 or HP 850 you could pop on there real quick and try it out before making several changes on your carb?

[patrickt]

I know his carb -- I know it pretty darn well and it should be plenty, if it isn't broken and there's nothing else wrong. Out of the box, it's 750 CFM, the primary jets are 70, his secondary metering plate (not a block) is 21, which is pretty much the same as 75 jets, it came with a .031 nozzle, and a 6.5 power valve. I have the same carb and it serves me quite well. It's either broken, or the problem is elsewhere....

[decooney]

Originally Posted by patrickt

I know his carb -- I know it pretty darn well and it should be plenty, if it isn't broken and there's nothing else wrong. Out of the box, it's 750 CFM, the primary jets are 70, his secondary metering plate (not a block) is 21, which is pretty much the same as 75 jets, it came with a .031 nozzle, and a 6.5 power valve. I have the same carb and it serves me quite well. It's either broken, or the problem is elsewhere....

Well, if its not anything else - you can always put an AF meter to it and find out for sure if its running lean or not right as the hesitation or problem occurs. Good Luck!

[Grubby]

I put things pretty much back to the baseline. A .031 squirter is on order and I will install. It should show up on Friday.

The idle mixture screws are out 1/4 richer than best idle. The idle speed screw is in 1/4 turn more than .015 t slots showing. It is needed so I can drive the car until I find the real issue.

I am not convinced it is a carb problem, so I bumped the timing 3 degrees to see what happens. The car actually runs noticeably stronger, but still has the stumble. Timing is now back to where it was.

The stumble may occur off idle to, but it is most noticeable between 1500 to 3000.

I called Holley tech line - you just never know. They suggested going two sizes richer on the main metering jets. I am not sure at this point.

John

[decooney]

If you hold the throttle steady in 2nd gear at 2200 rpms does it stumble and bumble along? Then, when you push further in to the secondaries it clears up?

[patrickt]

Originally Posted by Grubby

... I bumped the timing 3 degrees to see what happens. The car actually runs noticeably stronger, but still has the stumble. Timing is now back to where it was.

John, it couldn't hurt to just double check your TDC. Chances are the balancer hasn't slipped, but it only takes five minutes to check it with a two dollar piston stop. Remember, my balancer came apart....

[Grubby]

The engine runs steady and pulls smooth in all gears down to about 1500rpm. No indications of lean accept during the noted condition. Side pipes and plugs indicate correct mixture and if anything leaning towards rich. A nice dark tan color.

I think I have a piston stop in my tool box. It may be a good time to pull it out and check things.


John

[decooney]

Grubby,
After checking my notes from my last 428 Cobra with a "mild" cam, and looking back at some threads on this on network54 and mortec comparing the 3310 series to 4779 series 750 carbs it brought back a few memories. Quote "Any way you cut it, the later '750' carbs are lean, especially in a performance application with free-flowing exhaust sytem...much more so with open headers". Just went through this with a friends carb on a 400ci motor and 750 Holley. It took a really good local tuner to help us solve it. Exact same problem. Today's blend of ethanol into fuels is not helping either and having to compensate in some carb circuits is not uncommon these past two years. Kind of a pain really.

If you run out of options and confirm your electrical and ignition system is firing clean, you still have the option of doing what the Holley tec recommended by going up on the primary jetting some. Looking at my notes after lots of testing on my last 428 Cobra with moderate flowing sidepipes, the last jetting I end up with on a standard 3310-2 was 74 primary, 76 secondary with a 6.5 PV. As I recall, there were several of us who used 3310s around here with 74 primary jetting but all later switched to Holley HP or QF-Q carbs for more granular carb circuit tuning. I believe your -11 carb is a little different in overall design and secondary plate as compared to the early 3310s, but the primary circuit is similar enough to compare to for some basic testing. IMO: reading tan plugs gives a generic idea across the board but its hard to diagnose a part-throttle circuit this way without using an AF meter to diagnose it. And, if you don't have a brand new set of plugs and do a clean run and shutoff at a static rpm and using a good targeted beam (medical grade type pen light) light to look down inside the insulator. Even so, its still difficult to troubleshoot a lean-stumble in one circuit this way. The idle circuit can be rich and transition circuit can still be lean. If all else fails and before you give up on that particular carb, you can still try and pull out those 70 primary jets and replace with 74s and a new 6.5 power valve up front (if you have not blocked off the PV). Also, I'm making an assumption you have pulled the entire carb apart and thoroughly cleaned it first. As others pointed out previously, a dirty and gooey carb or poor fitting gasket and vacuum leak will cause the symptoms you are experiencing. If that fails, bring it to a reputable engine tuner with a meter who will find the root cause quickly. Good Luck.

[Grubby]

Duane,

I am going to think about this a few more days before I act. After bumping timing with no success, I believe the problem is a momentary lean condition or possibly a vac leak that could cause the same. There are certainly no leaks around the carb or on top of the intake. The bottom of the intake is tougher if not impossible to check.

I agree with you. After a hall way conversation today at work we determined and confirmed that 65 and 66 Ford 427 equipped cars had 850 double pumpers. The 750 is probably being stretched a bit for this application.

I bought the car with 800 miles and it has always had this stumble, so I have not cleaned the carb yet. I may do a complete strip and clean and change jets pure the Holley suggestion.

My other option is to go to a local engine tuner for an opinion.

I thought I had a line on a 800 cfm Holley for a test run but that petered out. It would be nice to try a 850 double pumper, but I am not confident enough to purchase one yet.

John

[Gaz64]

My bet is the carb idle circuit is borderline lean for the engine.

It WILL idle, but the transition circuit won't supply enough idle emulsion above idle.

With the fuel we have today, the whole carb including the idle circuit needs to be richer.

[decooney]

Originally Posted by Gaz64

My bet is the carb idle circuit is borderline lean for the engine. It WILL idle, but the transition circuit won't supply enough idle emulsion above idle. With the fuel we have today, the whole carb including the idle circuit needs to be richer.

EXACTLY. Transition is lean. Also, one other consideration I thought about is the older and earlier ERA Cobras had slightly more restrictive sidepipes too. I believe the newer ones flow a little better. With the slightly more freeflow sidepipes combined with today's standard (non-HP) 750 Holley, and more blended fuel - I'm betting this is the case; too lean. Especially with 70 primary jets he is using today. Most guys here run 850s/950s on our 428+ FE engines here now days and we don't see these problems like we did before using a borderline 750. I think he could make the 750 work though with some tweaks. Or, throw a Quickfuel 850 on it and be done with it.

[patrickt]

Alright. Grubby, order an 850 of your choosing and put it on there, run it, and if the problem is still there quickly clean it up and send it back for a refund. I hadn't thought of the sidepipe thing... I'm not sure about it, but maybe.

[decooney]

Originally Posted by patrickt

Alright. Grubby, order an 850 of your choosing and put it on there, run it, and if the problem is still there quickly clean it up and send it back for a refund. I hadn't thought of the sidepipe thing... I'm not sure about it, but maybe.

If he can get it to someone with a meter who can check the AF ratio (before buying another carb), might be money well spent. Would be interesting to know if the problem is better or worse in the summer heat vs. cold winter. Just recently came across a brand new Holley HP 750 that had casting flash on the base plate partially blocking one of the circuits; i.e. if everything is not perfect, little issues like this can pop up. A good carb blueprinter can find and fix his 750 and make it work better than it does now but its all time any money.

Note: Phil at Quickfuel is a good resource. I've used Phil a few times to validate my own assumptions. If Grubby provides all the engine/car specs, Phil at QFT can get him pretty close. They have spec'd out QFT carbs from 780 to 850 for FE engines set up like Grubby's. I like my own Holley HP 830 but may move to a Quickfuel 850 next. Super adjustable and better quality.

[Grubby]

Duane, Patrick and Gary-

OK guys I am feeling the love.

I used the Holley tool to determine correct carb size. If I choose "mildly modified" and 6000 rpm redline I get 750cfm. If I choose "highly modified" I get 850cfm. Clearly this engine is on the edge and I suspect Holley would error on the small side to make the car the most drivable for your average guy that just bolts it on.

Holley Interactive Carburetor Selector

With the definitions below my engine could fit into highly modified except my compression is 10.45. That means Holley suggest a 850cfm.

"Mildly modified - performance camshaft, performance intake manifold, headers and ignition upgrade

Highly modified - aggressive camshaft, performance intake manifold, headers and ignition upgrade, aluminum/ported iron cylinder heads and 11:1 or higher compression"

There is a engine tuner locally that comes highly rec. I am going to drive over and see him this week and see what he thinks. I would gladly spend $100 to "fix" my 750 vs $600+ for a new 850.

Duane - I would love to call your QF guy, but I don't have cam specs to give him. Do you think he would be much help with the information I have given you guys? Engine build sheet shows ultra Dyne cam with 108 deg cam ICL. No idea on lift.

John