opinion on carb issue

[STEVE-O]

Trying to educate myself on carbs. I have a holley 770 ( vacuum secondaries, electric choke), on a Roush 402. Seems to idle fine and if i accelerate evenly, engine and car accelerate OK. That is , seems OK in the range that I dare to accelerate.
However if I'm around 2000-3000 rpm and accelerate more quickly, as if to pass for example, I get a bog/hesitation (?). I can hear some uneveness in the sidepipes.
From reading and research at the Holley site and elsewhere on the internet, I believe it might be that i need a different spring in the carb secondary diaphram.
Before I screw something up that doesnt need changing, does this sound like the issue?? If so, would I go with the next stiffer spring, for slower opening of the secondaries?
thanks

[xlr8or]

I would guess it's likely the opposite. The secondaries may not be opening soon enough and causing a lean condition.

[Got the Bug]

Steve-O,

How long have you owned your Roush 402? If it's still under warranty, go back to Roush or the dealer you bought the engine from. The hesitation is known problem on Roush motors built during a certain period. My motor had the same issue. They replaced the carb with a Holley 750 at no charge and it resolved the hesitation issue.

[wtm442]

The bog/hesitation could be due to a lean condition or a rich condition when you mash the pedals. Putting the car on a dyno would tell you if its a rich or lean condition. Also, get the RPM's up to about 3500 RPM (while the car is moving) and stomp on the pedal. If you do not get the hesitation, the problem is most likely in the accelerator pump circuit.

First thing is too loosen the acc pump spring until clearance is observed under the lever and then tighten until clearance is removed. Then check to make sure you get gas out of the squirter nozzle as soon as the throttle is touched.

Second, change to a larger squirter nozzle assuming you have a lean condition. If that is no help, try putting the existing acc pump cam in the #2 position. Third is to try a different acc pump cam with a more aggressive cam assuming you have a lean condition.

Good luck.

[ANB]

I had the exact same problem with the same engine/carb. Had the jets replaced and the problem went away entirely. Even the "backfiring" you probably get when yiu decelerate.

Al

[greg schroeder]

Get a mechanical secondary.

Maybe a Pro-Systems double pumper. Patrick will send it to you dialed in and ready to hammer. You'll have linear throttle response and it will hit like flipping a light switch when you stomp on it. You'll pick up 20 or 30 HP too.

[Flyin_Freddie]

I'm in agreement with a couple these responses... One, the accelerator pump cam may not be agressive enough to provide enough fuel when the throttle snaps open quickly. This will cause an over lean condition when the throttle is suddenly snapped open...too much air, not enough fuel...and a noticeable nose drop during hard acceleration. Holley sells a series of plastic cams, each with a different color to represent a different amount of gas injected via the squirters. Easy to replace, and certainly cheap enough. But, before you do that...check to make sure your squirters are working properly...like WTM442 suggested. I've seen much the same problem as you described occur when the screw plunger on the accel pump arm just wasn't long enough to fully depress the diaphram. A simple adjustment to the screw length was all that was needed.

Conversely, if you're going too rich when snapping open the throttle, you may be getting TOO much gas through the squirters....you may need to go to a less aggressive accel pump cam. If this is the case, you're going too rich...which, again could cause a noticeable bog under hard acceleration. I think the Holley kits come with 5 different plastic cams...Summit and Jegs both carry them.

And...Two.... I've never been a fan of vacuum operated secondaries. I'm running a Jet Performance tuned Holley 750 cfm 4150 series...their famous "Double Pumper"...no choke, mechanical secondaries. I've been very pleased with performance, both for street and track use. I can open the mechanical throttle linkage and butterflies by hand and verify that all four throttle plates are perfectly verticle when running WFO!!!

But...the best way to tell without wasting a lot of time, money, and effort buying and swapping carburetor parts is to go to a good chassis dyno operator and pay them for their diagnostic time. You'll be real glad you did...


PS...Here ya go....click the link and look at part number HLY-20-12... an assortment of 8 different cams for $21.95, plus more on the page to help confuse ya...

http://store.summitracing.com/egnsea...+115&y=13&x=22

[Bob In Ct]

Steve-o:
I don't know much about carbs but I do know this -

When you open the throttle plates quickly the air (not being very dense) rushes in almost immediately. The fuel however (being very dense) takes a while to catch up. That is why carburetor have accelerator pumps. They supply the fuel needed until everything else reaches equilibrium. I would make sure the APs are working properly before I did anything else.

Bob

[Randy Rosenberg]

Steve,

1. Follow Doug's advice - you paid a lot of money for your Roush - it should run properly, especially since there's a known problem with the tip-in hesitation at around 2K RPM.

2. Tuning carbs and ignition systems are an art. I gather from your post that you don't have much experience in this area. My advice is (after you exhaust #1 above) to take your car to a dyno and have an expert dial it in for you. This will be cheaper, quicker and most satisfying than trying to fix it (ie: screw it up further) yourself. This will only take a few hours, and then it will be done right and you will not have to mess with it again.

Good luck. Keep up posted.

Regards,
Randy...

[Jim Zagg]

Steve,

Sounds like you have a driveability issue. I have done a lot of Holley carb tuning lately. I think you may have a pump cam that is incorrect for your style of driving. I frequent the Cabin and Wegman's in Canandaigua cruise nights. I would be interested in helping you. My engine shop is in Canandaigua. Let me know if you would like some help.

[greg schroeder]

I had a Holley 870 Avenger vacuum secondary on my Roush 427R at one point. I thought it was ok to do as the Superformance dealer was putting them on many of the Roush engines they were installing. Boy, is that wrong. I took quite a bit of time to make adjustments using all springs options and assorted jets. The graph below represents a casual start in 1st gear, casual shift to 2nd gear, then nailing it to test response time.(It is not an acceleration test) It was very frustrating having 1 to 2 seconds of delay at the punch. I did many, many passes, but the three in the graph are about as good as it got. A simple call to Holley tech may confirm that a vacuum secondary carb is not the best application for a high HP engine in a 2400lb manual transmission car.


I learned quite a bit over the next couple years after getting my first Cobra and I'm sure I have much, much more to learn, but looking back at the vacuum secondary ordeal I wish I'd of just tossed the Avenger in the junk heap and bought the mechanical secondary Pro-Systems carb from the get go. In my opinion just do it yourself and get it done.

The drag video in my signature is a start from idle with a Pro-Systems carb.

[emberglo66]

I had a similar issue with the Tip-in found in the Holley carbs. I went with several custom carbs and ended up with a DiVinci Performance Carb. Dan DaVinci (Smith) is the guy who made the carb for OCCs fireman's chopper. He just so happens to be about a mile from me in Houston. He made the carb and it was a bolt on and go. I am waiting for the weather to cool a bit before I put my SPF and 402 on the dyno. I plan to reinstall each carb and get the results. But I am very pleased with the job DiVinci did, my guess is 20 to 30 HP increase.

http://www.davinciperformance.com/

[wtm442]

Steve-O

Here is a nice graph of the various acc pump cams.

http://www.holley.com/BrowseCatalogs...Holley&Page=65

Also, if you have one of the best Holley books ever published (Holley Carbs by Dave Emanuel), there are some really informative charts on page 49.

[native1]

Everyone with that carb on a Roush has trouble with hesitation. I had the same problem with my 427 srtw. We played with the 770 street avenger for a long time trying to solve the problem. Got better but always had a hesitation. I would recommend using the 770 as paper weight and buy a 750 or 850 cfm holley 4150hp double pumper. Make sure to get it without the choke, it is for sissies. It was well worth the 900$ to me. Zero hesitation, very smooth. Should also be good for a few ponies.

[wtm442]

Originally Posted by native1

buy a 750 or 850 cfm holley 4150hp double pumper. It was well worth the 900$ to me.

I would suggest that you should try making adjustments to the existing carb before buying a new one. No matter what Holley carb you buy, its performance can be improved with a little tuning. If you do not want to learn some of the basic tuning steps, find someone that will do it for you. If the tuning goes nowhere other than pulling hair out of your head, its time for a new carb.

Of course, you could always switch to an EFI setup and make adjustments using a computer.

I would definitely go back to Roush and see what carb they are presently supplying/suggesting with your motor in a very light car.

On a side note ... regarding the $900 carb ... I remember buying my first Holley carb ... an 850 double pumper, and was it expensive. $175.00
Boy, that was a lot of money in 1970.