715 Holley Le Mans Carb set-up adjustment help needed to cure hesitation

[Brad Griffith]

Here is the kind of great information for which I love this forum so very much, so thanks in advance for your help.

When I was the caretaker of 5s287, my Holley 715 was SPOT on. It was that way when I bought the car from Brett.  That car was a tad stronger than stock, and had what I thought was a slightly more aggressive solid cam than a stock hipo camshaft.  My 66 GT350 was supercharged, so it had a mechanical secondary Holley. My 67 Gt350 had a perfectly set up Le Mans carb also.  I have good luck with them so far.

Today, my black 65' K code is the project at hand. The engine has a .030 over bore, with correct TRW pistons, aluminum heads (painted black) with 53cc combustion chambers. I had the combustion chambers cut to that size so as to be about 10.5 to 1 as to compression ratio. Intake ports are matched to an original 66' Shelby dual plane intake manifold. Competition Cams Nostalgia Plus series 271s solid cam, (more aggressive than a Hipo cam, and with modern tight lash of .012 hot) and roller tipped rockers. They fit under the standard hipo valve covers after I modified one of the baffles . Standard Hipo distributor with Petronix components, probably 14 degrees initial timing and another 20 mechanical advance for 34-36 total. No pinging. Jim Cowle's beautiful coated Try Y Headers, T style Cobra oil pan and an Arvinode exhaust.  I would like for them to exit on the side like a 65 GT350, but being a factory GT I needed to exit the rear valance. Car is light.  Again, aluminum heads, edelbrock painted black aluminum water pump, standard GT non-pony interior, (no console etc.) 66' package tray, no spare inside car, aluminum cragar reproductions, aluminum water pump, radio delete, I pulled the heater, fan, heater core, etc.  Detroit locker with 3.89's.  Car runs very strong.  Wheel hop initially was terrible, so I went with the Maier Racing springs that cured my traction problems.  These springs are probably about equal in weight to standard Hipo springs plus Shelby traction bars (or a bit less) as they are 4 1/2 leaf springs vs. 4 leaf factory springs. Problem solved. ( as an aside, the original Shelby traction bars on my 5s287 really DID work! That car just kind of squatted and jumped up.  I had 235 60's in back and that helped also). This car makes lots of power.  More than did 5s287 did under my care.  I am guessing about 360 horses at the flywheel. Maybe a tad more, but I think that is close.

Back to my carb problem.....My reproduction 3259 Carb was purchased new, recently, by me at Branda. Confirmed primary idle transfer slots exposed a small "square". Engine making 12"(varied a bit from 11-13") or so of vacuum at idle. Idle set at about 750-800 RPMs... Had stock 68 & 78 jetting. Seemed to have a lean bog from a stop and every time gears were shifted when full throttle applied. With the 68 primaries, I had the idle mixture screws turned out 1 1/2  to 1 3/4 turns or so. I bumped up the primary jets to 70's from 68's. That seemed to help reduce the bog some. Now, best vacuum is about 1/2 turn out on the idle mixture screws, as the 70's are allowing more fuel. I bought a Holley Vacuum quick change spring kit, and an Accelerator Pump Cam kit. Currently has the stock white cam in position one. Next we changed the stock 25 Squirter nozzle to a 28, and changed to a white (lightest) vacuum secondary spring from the stock yellow spring.  Other than the slight hesitation at throttle application, car ran great and pulled hard. VERY.

We thought we might have a vacuum leak given the variations as to idle vacuum. We checked, none found. We pulled the carb to check a couple of things and to use a slightly thicker carb baseplate gasket between it and the intake manifold which we preferred to the thin paper one we were using.  We tried 69's as primary jets, as with the 70's installed, and we could bottom the mixture screw on the driver's side and it would still run. Car did not run better, and now we could bottom both idle mixture screws and the car would continue to run. Car had backfired once as we were working on it (it never has in other situations) and I suspected we blew the power valve, which is a failure condition that would allow the engine to continue to run with the idle mixture screws bottomed out. Power valve was the stock 8.5.  We tried a 6.5, as depending on which set of Holley instructions you follow, either can be called for.  Of course a 6.5 delays the secondary fuel delivery, and the car clearly ran worse. We went back to the stock 8.5 valve, and back to the 70 jets.  Adjusted mixture screws again to about 1/2 turn out. Peak vacuum at idle now approaches 15", which is very good given this cam with just under .5" of lift. Car runs great, but still has a slight hesitation on throttle application, but less than before. I have been checking to see that our accelerator pump lever is adjusted properly also.
Our plan is to now maybe A) try a different color accelerator pump cam?, and or maybe B) a slightly bigger squirter from the 28 we are now using. Going from a 25 to a 28 was a definite improvement before. Any other ideas? It seemed to run better when we increased the pump squirter before, so we thought we might try another bump up maybe from a 28 to a 30?
Also, The engine will now idle with the idle screws bottomed. It is my understanding that this is caused by either 1) an idle that is so high as to over ride the idle circuit or 2) a blown power valve or 3) improperly set float levels or 4) improperly adjusted butterflies vs.the transfer slots.  Front float level is set at 1/2" from the factory and that is my understanding to be correct. So I don't know what is causing this condition.  We did not pull the rear float bowl and check the rear float adjustment.  The secondaries do seem to be closed entirely at idle, and appear to be as they should be.  No visible signs of fuel dribbling in.

So that is question 2, what could be causing the mixture screws to bottom and the engine still idle?

Many thanks!

Brad

[greekz]

You could try checking the gap between the accelerator lever and adjusting screw.  Sometimes with too much gap the accelerator pump will not engage soon enough.  Had a small hesitation on my '67 GT-350, once I adjusted the accelerator lever engagement the hesitation went away.  Not saying it will cure your problem, but something to check. 

As for question 2, I have nothing.

Greek

[Brad Griffith]

Greek-

thanks for the reply.  I think I mentioned I did adjust it properly.  .015 clearance if I am remembering correctly.  It is an important adjustment that I have remedied hesitations in other cars by checking for this common problem.  Unfortunately, not the case on this one!  Thanks again!

Best,

Brad

[kram350]

Just some thoughts...

The initial timing may need to be further advanced or is too far advanced and you're chasing a bog thinking it's a carb problem? Based on the cam  and heads you might try setting the timing back to 10-12  BTDC and then tune the carb to that timing setting.  Initial timing setting is effected by many things, but I would set it at 10 BTDC and then set the carb up. Also, to slow of an advance curve can do the same. Get the distributor curved on a machine if you can.   

As the carb sits now, I would take it off to ensure the transfer slot is exposed +/- .035, looking from the bottom of the carb.

While it is off pull both bowls and set the floats; 3/8" primary and 1/2 secondary. Make sure the secondary butter-flys can move and you can adjust the secondary curb idle screw. You want the secondary circuit active. You may need to adjust it later if you need more air and if the screw is stuck its almost impossible to break it loose on the engine.

Check the seat and accelerator check needle under the accelerator nozzle to make sure it is sealing. Also, make sure the check ball or diaphragm in the primary bowl accelerator pump is sealing. I have had them leak. 

Set the idle screws back to 1-1/4 out. With the transfer slot set, do all your idle adjustments with the mixture screws, not the curb idle screw. If you think you need more idle air, adjust the secondary butter-flys, this keeps the primary transfer slot opening set and hence helps with a bog.

You are setting the accelerator pump with the throttle totally open, then measuring the .015"?

If you want, get a power valve plug to ensure you are not getting any fuel from the power circuit into the idle circuit as you get the idle circuit squared away. You can then reinstall a power valve to set the opening to ensure you don't lean bog under load. Put a vacuum gauge on the engine and set the idle screws for max vacuum.

Main jet size has very little effect on the idle circuit. Once you get the engine to idle you have to do other things to get the main jets sized, just as you have to size the power valve based on vacuum and engine RPM, cam size, load, etc.. One system at a time and you will correct it. A note book helps too! 

Check you fuel pressure, 6 psi.

[Brad Griffith]

Thanks so much for the input.  I did set the accelerator pump at .015 at wide open throttle.  That has cured hesitations on other holley equipped cars in the past.  I am going to try one more size up on the accelerator pump nozzles (squirters) first.  This carb came with a #25.  I moved it up three to a #28 per Holley's instructions.  It improved measurably, and I would like to maybe try a couple of points up this time, but the next size I could find here was a #31.  So I will try that.  I do need to check the rear float bowl setting at 1/2".  Thanks for the input.  I will keep you posted as to my progress!

Brad

[kram350]

Two additional thoughts, if the engine is idling with the mixture screws closed, the idle circuit is way too rich. That being said, if the idle feed restrictions are to small based upon your engines requirements (or they are clogged with debris ) you will have a hesitation and bog. Make sure the idle air bleeds and idle feed restrictions are open and no dirt is present. It really sounds like your timing is off and you are trying to fix a symptom and not the cause. #31 nozzles are way to big for what you have. I have a pretty radical engine and #25's do the trick. Good Luck

[Brad Griffith]

Two additional thoughts, if the engine is idling with the mixture screws closed, the idle circuit is way too rich. That being said, if the idle feed restrictions are to small based upon your engines requirements (or they are clogged with debris ) you will have a hesitation and bog. Make sure the idle air bleeds and idle feed restrictions are open and no dirt is present. It really sounds like your timing is off and you are trying to fix a symptom and not the cause. #31 nozzles are way to big for what you have. I have a pretty radical engine and #25's do the trick. Good Luck

Thanks so much for your input.  I will re-check timing, but it had been spot on.  Will do so again. It ran much better with 70 mains than the 68's to be sure.  I think the first thing I need to do is check the rear float setting. I have not yet installed the 31 nozzles.  The 28's are in there now. Update soon!

[kram350]

When you check the timing does that mean the initial, advance curve and of course, the total? A distributor machine can be your friend and once the timing is correct, the carb is usually pretty easy to sort. Best of luck. 

[Brad Griffith]

I checked rear float bowl setting.  I found one post that said front should be 3/8" and the rear 1/2" and another post that said the reverse! (From a previous post http://www.shelbyforums.com/threads/holley-lemans-715-problems.7111/ vs. one in this thread). Mine is set at 1/2" and seems ok.  I reconfirmed ignition timing. It is set at 12 degrees BTDC.  I can bottom the idle mixture screws and it still runs, all be it poorly. When I look down into the carb, it appears as if the rear Venturis are not fully closed....The blades are not parallel with the ground so to speak. I am wondering if maybe I am pulling a little fuel through there. That is the only thing I can think of since the power valve is good and not blown. I will pull the carb and check the rear venturis closing before I do anything else.  Should the secondaries be fully closed, i.e. parallel to ground level, at idle?

My hesitation is very slight, but since my idle seems a bit rich, I am thinking this is my most likely issue at this point.  It may be any 289 based engine is going to be a bit rich at idle with a 715 CFM carb.  Thanks again for all the help!

[shelbydoug]

I checked rear float bowl setting.  I found one post that said front should be 3/8" and the rear 1/2" and another post that said the reverse! (From a previous post http://www.shelbyforums.com/threads/holley-lemans-715-problems.7111/ vs. one in this thread). Mine is set at 1/2" and seems ok.  I reconfirmed ignition timing. It is set at 12 degrees BTDC.  I can bottom the idle mixture screws and it still runs, all be it poorly. When I look down into the carb, it appears as if the rear Venturis are not fully closed....The blades are not parallel with the ground so to speak. I am wondering if maybe I am pulling a little fuel through there. That is the only thing I can think of since the power valve is good and not blown. I will pull the carb and check the rear venturis closing before I do anything else.  Should the secondaries be fully closed, i.e. parallel to ground level, at idle?

My hesitation is very slight, but since my idle seems a bit rich, I am thinking this is my most likely issue at this point.  It may be any 289 based engine is going to be a bit rich at idle with a 715 CFM carb.  Thanks again for all the help!

The secondary throttle plates should be open a crack. There is a spec for the settings for that. That is done with a set screw.

It's very difficult these days  with any Holley to get the float levels right. That's because of the ethanol in it.

Typically the floats need to be set down about 1/8" from the factory specs to keep them from flooding when the engine is hot or even warm.

The Lemans bowls don't help the situation.

The 715 IS a 427 carb. It really doesn't belong on any street 289. A 600 will idle better for sure.

The engine is capable of 18-20 mpg but not with a 715. Try 10-12 with that. I'm not even sure it's better for racing use on a 289?

[sg66]

If I'm reading this right, your idle screws used be set around 1 1/2  turns and after opening the carb a few times, now they bottom out? I would go back and double/triple check that you have the right metering block gaskets and pay particular attention to the hole above the power valve which transfers the accelerator pump shot. I have seen the wrong gasket used which allows fuel to leak down through the body where the power valve receives its vacuum signal. There should also be a small ridge around that hole on the metering block and/or main body that helps to seal up that passage. Sometimes that ridge gets damaged from scraping off old gaskets.