throttle hesitation

Jim Powers · #1 (**permalink**) 04-12-2008, 06:24 PM

Does anyone actually install a Holly Avenger 670 carb on a Ford 302 crate and see if it runs correctly before selling the damned things? Backdraft #324 was ordered with that set up and from day one it has hesitated when given throttle. I did every possible adjustment, from float levels to increasing jet sizes and nothing helped. Finally out of desperation, I had it dyno checked. Sure enough it was'nt getting fuel. We upped both primary and secondary jets (way up, 6 numbers on the secondaries) installed a new metering block, and power valve spring, and changed the vac advance spring. $800.00 later it feels like a different car, and is actually fun to drive. Needless to say, someone dropped the ball somewhere, and I feel there ought to be at least some effort spent on testing these problems before the cars are sold.

cbreez · #2 (**permalink**) 04-13-2008, 11:13 AM

agreed my man...you shouldn't have to put a dime in a new car to fix it!

jhv48 · #3 (**permalink**) 04-13-2008, 12:41 PM

Very typical of carbs with vacuum secondaries.

Cashburn · #4 (**permalink**) 04-13-2008, 03:43 PM

Yes, that is the responsibility of the dealer to verify in that engine install since the motor is not shipped from Ford Racing with an intake manifold, let alone a carb.

If the engine is built and dynoed by a shop then it should arrive off the dyno with the proper jets (another reason we are in favor of true crate engines as opposed to crate long blocks). But ultimately the dealer should verify before delivery.

We had a spell of issues with the Holley Avengers as delivered on the Roush motors for a period last year... this has since be resolved (for those in the know we went from red dot to blue dot to green dot etc.) But happy to report things are as they should be now.

Bottom line is your dealer should have sorted that.

ekrupa2 · #5 (**permalink**) 04-13-2008, 04:30 PM

Hi Jay - I just contacted Roush about mine (402SR TW with a Holley Street Avenger 770). I have about 140 test miles on mine after my self install. Severe stumble/hesitation during mild to moderate acceleration (no issues when really gassing it). What are they doing to remedy the issue? I tried everything to get rid of it - I have verified the timing, tried tweaking the mixture, verified the float levels, changed the Accelerator Pump Arm Play, changed the Accelerator Pump Cams, and changed the secondary springs (parts at my own expense). Just curious what your resolutions were.

Thanks!

madmaxx · #6 (**permalink**) 04-13-2008, 05:26 PM

roush has a 2 year 24 month warranty, have them fix it

Cashburn · #7 (**permalink**) 04-13-2008, 05:41 PM

Quote:

Originally Posted by ekrupa2

Hi Jay - I just contacted Roush about mine (402SR TW with a Holley Street Avenger 770). I have about 140 test miles on mine after my self install. Severe stumble/hesitation during mild to moderate acceleration (no issues when really gassing it). What are they doing to remedy the issue? I tried everything to get rid of it - I have verified the timing, tried tweaking the mixture, verified the float levels, changed the Accelerator Pump Arm Play, changed the Accelerator Pump Cams, and changed the secondary springs (parts at my own expense). Just curious what your resolutions were.

Thanks!

What color dot is on your carb?

ekrupa2 · #8 (**permalink**) 04-13-2008, 05:49 PM

Hi Jay - I just documented all of my spec's for Roush as they asked the same question. There is no dot on the carb at all (and I looked all over). The motor was built in October 07. Cold Idle = 1600 Hot Idle = 1000 Initial Timing=14 Deg Timing @ 4000 = 31 Deg Manifold Vacuum at idle = 13-14. Altitude = 700 MSL

Edit - I am fairly certain the root cause is the accelerator pump as the hestiation is immediate when giving it just a bit of throttle. After the stumble it runs great.

madmaxx · #9 (**permalink**) 04-13-2008, 05:53 PM

Oct of 07, why didn't roush send you a new carb if under warranty? or instruct you to take it to a shop? What am I missing?

ekrupa2 · #10 (**permalink**) 04-13-2008, 05:57 PM

MadMaxx - I did my install over the winter...first fire up in December. Had a major oil leak after initial start up. Roush ended up having to take the motor back to fix the leak. First true test miles on my SPF within the last month (I am in Michigan). So hence the delay.

Jim Powers · #11 (**permalink**) 04-13-2008, 06:34 PM

Jay, since I did not specify the 67o Avenger, and ordered the 302 expecting the builder would be professional enough to know what carb worked properly, then it's their fault. So this would be Fanatix at Backdraft. This is only one of several issues on that engine. They installed originally an 870 carb, valve covers for a fuel injected engine ( no breather valve on one side). Both of these issues were corrected in due course, but all of this just footnotes some very careless engine knowledge.

jdean · #12 (**permalink**) 04-13-2008, 06:41 PM

I installed a new 670 Avenger on a 302 in an FIA Cobra replica a month ago. It continues to have a very slight hesitation at light and mid-throttle acceleration. Once the half-second of hesitation is over, it catches and runs very well.

Tried adjusting the pump, idle air, vacuum springs, timing etc. and none of these have made any difference.

The Avengers are supposed to be "plug and play" for Holley to compete better with the Edelbrock carbs (which if sized correctly for the application are plug and play). False advertising.

madmaxx · #13 (**permalink**) 04-13-2008, 07:46 PM

ekrupa2,

Sorry, I was fishing, I was curious to see how Roush would honor a warranty by an individual install. As Cashburn indicated earlier the hesitation is a well known problem at Roush particulaly around your delivery date. I am a little disappointed knowing you sent the engine back and they did not correct the carb issue even though you did not mention. They have the solution. I would contact them again and they will know how to resolve.

Jac Mac · #14 (**permalink**) 04-13-2008, 07:55 PM

I think you will find your problem is the power valve, its a bit lean for your particular engine combination- this is one part in a carb that should be checked/changed to suit each individual engine combo- in this case being lean means you have to go up on the number to cure it- lets say you have a 6.5 now ( ie starts to open @ 6.5" vac ) , try a 7.5 so that it opens a little bit sooner. Holleys were jetted about 5% rich in the 'good old' days to cover most situations, now things are a bit different in this PC world & if 'you' happen to have a good combo with strong vac signal it will hold the PV shut longer & cause a lean condition. This can be real pain in cruise mode & you find yourself constantly having to slow down /speed up to avoid the stumble.

Jac Mac

BDR427R · #15 (**permalink**) 04-13-2008, 09:33 PM

Jay,
I have the same issue with #501 w/ the 427R. My carb has the green dot on it but it is not a slight hesitation but a definate jump. I have never said anything about it because I thought is was the nature of the beast. Should I contact my BDR dealer or notify Roush? The car and motor is just shy of a year old. 1400 Miles. It also appears to leak fuel down the outside of the carb only at start-ups.
Regards,

Got the Bug · #16 (**permalink**) 04-13-2008, 09:41 PM

Eric - Just curious if you recieved my email response to your question about the carb issue. It's a known issue and they should repair or replace the carb.

wtm442 · #17 (**permalink**) 04-13-2008, 09:59 PM

To anyone posting that mentioned a hesitation, you can check to make sure its is the fault of the accelerator pump circuit. Just bring the revs up to 3500 or 4000 in 2nd or 3rd and punch the throttle. If there is NO stumble, you have confirmed that it is the accelerator pump circuit.

Suggest you change the squirter to a larger size. Also, call the Holley tech line at 270-781-9741

wtm442 · #18 (**permalink**) 04-13-2008, 10:31 PM

Quote:

Originally Posted by Jac Mac

I think you will find your problem is the power valve, its a bit lean for your particular engine combination- this is one part in a carb that should be checked/changed to suit each individual engine combo- in this case being lean means you have to go up on the number to cure it- lets say you have a 6.5 now ( ie starts to open @ 6.5" vac ) , try a 7.5 so that it opens a little bit sooner. Holleys were jetted about 5% rich in the 'good old' days to cover most situations, now things are a bit different in this PC world & if 'you' happen to have a good combo with strong vac signal it will hold the PV shut longer & cause a lean condition. This can be real pain in cruise mode & you find yourself constantly having to slow down /speed up to avoid the stumble.

Jac Mac

Jac Mac - sorry to disagree with you, but the difference of gas getting to any engine with a 65 vs a 75 power valve is very small amount.

The numbers after the dash only determine at what level of manifold vacuum opens the power valve, and never has been an indication of the fuel flow amount.

There are two different "single stage" power valves.

The most common is the standard flow, and a 6.5 "Hg opening is part number 125-65

They also have a high flow unit and and a 6.5 "Hg opening is part number 125-165

If you switched from a 125-65 to a 125-165, then the power valve would open at the same vacuum level, but more flow would go thru the power enrichment circuit.

Before anyone changes the power valve open point (65, 75, 85, 105 etc) you MUST get a handle on the amount of vacuum at an idle, especially with a rude cam! And the vacuum gauge cannot be a cheap one that includes a ton of internal damping. With a cam with a lot of overlap, the vacuum could be going from 15 to 10 "Hg (for example). But a cheap gauge with a lot of damping would stay steady at 12.5"Hg. Therefore, you could install a power valve (10.5) that opens and closes at idle! Been there ... done that. I have a 4 inch diameter vacuum gauge that will follow the vac level of really wild cams.

Holley also builds a 2 stage power valve, but I'd avoid those.

To anyone having tuning problems on their Holley, go out and buy Dave Emanuel's book "Super Tuning and Modifying Holley Carburetors". After reading the book, start over and read it again.

http://www.holley.com/36-136.asp

http://www.buy.com/retail/product.as...01&dcaid=17902

ekrupa2 · #19 (**permalink**) 04-14-2008, 05:16 AM

Doug - yes I did receive your email. I will update you as soon as I hear back of the proposed resolution.

Warren - my stumble only occurs during light throttle situations so I was thinking the same thing - accelerator pump.

jhv48 · #20 (**permalink**) 04-14-2008, 10:03 AM

This same problem has been documented and discussed many times.

Basically, if you have that stumble, you will probably not be happy until you get a mechanical secondary carb.

For a variety of reasons, the vacuum secondaries don't like these light cars.

Some people have gotten them to work, others just replaced them.

Sounds like you have already spent the time and money to make it work.

Might be time to replace it!