Holley carb problem

[CowtownCobra]

Do you have the kit with different springs? I'm sure MSD has one. Then its just trial and error. Its sometimes hard to tell whether its a fuel or timing problem. But trying to sort out that kind of problem is what makes these cars so fun and satisfying. The street avenger has several accel springs, which adds even more variables. But the street avenger is MUCH easier to work with than the 750 CFM 4150, which I gave up on. Want to buy one?

Good luck.

[Jack21]

If you're going to continue to tinker with the Holley, replace the secondary idle adjustment screw with one that can be adjusted on the car whth the engine running (idling) with an allen screw adjuster. You want to see both the primary, and secondary idle transfer slots as small squares above the butterflies (.020" - .035"). It is preferable if you have idle richness mixture screws to adjust on both primary, and secondary side. If not, purchase secondary metering block that allows richness mixture to be adjusted.

One would think that after 50 years of dealing with these issues that Holley would have sorted this all out. Well, sorry to say they haven't, but Barry Grant has.

If your cam has over 220 deg intake duration @.050, replace your Holley with a Demon. (!!!) As one of the huddling, and unwashed masses who had given up all hope, I sprung for the Demon to repalce the Holley. All I can saay is "Holy Sh!t, I don't believe this!" The car has a real motor in it!

No, I don't work for Barry Grant, or Holley...but! In a recent engine build up mag, all the Holleys were replaced by Demons for their dyno floggings. Chevy, Ford, Buick, Pontiac, Olds...all dyno'd with Demons instead of tried and true Holleys?

I couldn't believe the difference in drivability, and you won't either. I'm still downjetting and plug reading from out-of-the- box, to get the best A/F for street driving, but the responsiveness between the Holley, and Demon must be felt to be appreciated.

[BEB]

I tried a Barry Grant Speed Demon 650 vac. sec. but it suffered from a bog/stumble when the secondaries were opening. I tried every different spring with particular emphasis on the heavy, slow, black springs to slow the opening of the secondaries until later, but it still bogged. I even had it set up on a chassis dyno.

Going back to my old 750 Street Avenger immediately cured that but the problem I now have (as described in my first post) is still there.

I think I shall try playing with the accelerator pump system and check it is working immediately instead having a slight delay.

Any views?

[niles]

Beb:
reread first post. There are two kinds of bogs; both lean and rich, both will cause a "bog" A rich bog will be a mushy drop in power but no real miss. A lean bog will be "hard" almost a feeling of miss or drop-out in power reduction for a moment.
The accel pump must squirt fuel from the "FIRST" movement of the throttle plates because as the plates moves, it initially allows a flow of air, this air has not sphoned any fuel yet, this air has to richened from the squirter. Go back to a basic orange cam and std 28 or 30 squirter and get the squirter adjusted. It's the timing of the squirt with the initial surge of air that cause the F/A go go either lean or rich.

gn

gn

[mr0077]

BEB,
I also reread your first post, and thought of a couple of things for you to check out or consider...
I was trying to dial in my carbs a while back and was interrupted...the car sat for a while, and when I next drove it, I had a similar problem, tried all sorts of things (on a 2x4 setup, so it was somewhat miserable)...I suddenly remembered my earlier tinkering, checked, and sure enough, I had jetted down to a lean condition (probably near 14:1). I got very good mileage, but I had a lean tip-in problem that no amount of accelerator pump and/or cam fiddling could compensate for. Went back to the previous jets and...you guessed it, no more bog. You may have your jetting a bit too lean, try jetting up one, or even better two sizes and see how it feels.
The second thing: be sure that you have ZERO clearance between the accelerator pump arm and the lever at the idle setting, but ALSO be sure you have maybe 0.010" or so clearance at WOT (not sure of the last setting, can someone help on that?), as the pump MUST NOT bottom out at full throttle or something will get bent or damaged.
Let us know what you find.

[Chaplin]

0077-
The clearance on the accelerator pump arm is supposed to be .015"

[Rick Parker]

The Holley is overall a good piece to work with, if not it wouldn't have been used as a pattern for other knockoffs. It has some shortcomings that can be rectified with a little work. As Jack 21 said, the idle transfer slots have to be set properly as a baseline. If they are holding the throttle plate open too much the main sytem will be fuctioning was you attempt to adjust it with the idle system, (not good, little effect). If you have a very aggressive cam a 4 corner idle sytem will be required to get an idle that you will be satisfied with because the engine will have low vacuum thus creating a weak signal in the idle circuit. A standard double pumper can be converted to 4 corner idle with the existing parts "IF" you understand how the carburetor functions, where the fuel needs to flow and if you have the right drill sizes, otherwise buy the parts to convert your existing carb. It smooths out the idle considerably and makes for a crisper transition off of idle.
From the description of the problem I think the timing of the fuel delivery is questionable. The set of cams that are available for the Holley's will help a lot. They are colored differently for identification. Try choosing a carburetor cam that has a "Faster Delivery" injecting the fuel earlier to "cover up the hole" created when you depress the throttle.



Rick

[BEB]

Thanks Guys;

Ist reply to Niles:
It is definitely a lean bog as I have a air/fuel analyser fitted in the form of a lambda sensor so I can see it momentarily leaning off.

To mr0077:
I may well have to go up 1 or 2 primary jets to compensate. Also, I have checked the accelerator pump arm gap at wide open throttle and it is .015". However, if the arm doesn't move immediately when opening the throttle does it mean I should try a different colour cam, and if so, which one as the current one is Green in the number 1 position.

To Rick:
I'm fairly certain the throttle plates are set correct (the primary one definitely is). The cam is a relatively mild Comp Cams 268 hydraulic and the idle vacuum is around 15/16Hg. The idle is nice and smooth.
I think the problem is caused by the accelerator pump not getting fuel in soon enough. I'm running a Green cam at the moment so which one will give a "faster delivery"?

Your feedback is excellent and I'm very grateful to you all !!!

[Chaplin]

Originally posted by BEB


Thanks Guys;

However, if the arm doesn't move immediately when opening the throttle does it mean I should try a different colour cam, and if so, which one as the current one is Green in the number 1 position.

The arm must move the instanst you begin to open the throttle or you will get a stumble/hesitation. Adjust it so that there is little or no clearance when the throttle is closed. You need the pump to squirt as soon as you begin to open the throttle. But make sure you have the required clearance at WOT.

[mr0077]

Yeah, BEB, I agree with Chaplin, the arm HAS to move instantly when you move the throttle. If you have clearance at idle (that is, if it is "loose" and not in contact with the cam), then there's sumpin "rong", maybe a bent arm?
Easy way to check for clearance is to turn throttle to WOT and then use a screwdriver to push down on the pump lever...make sure it will still go down a wee bit while at WOT, and is not bottomed out.
My Holley book is at the house, check the Holley website and see if they can tell you which cam gives the biggest squirt. Check out this website
http://www.holley.com/HiOctn/TechSer...fo/TI-221.html

[trularin]

Hey Cowtown, got the same dist. Works great.

Guys, the .037 with the 50CC and I think yellow cam( not sure on this one ) seems to work. I am used to the old days where it would stumble and then rock.

Rick, it the garage heated???

Happy holidays to all of you!!!!!!!!!!!!!!

[Jack21]

Here's another one that will drive you nuts with Holleys. Fuel leakage from the accelerator pump discharge transition between the metering block, and the throttle body. If the gasket, or casting is faulty, fuel gets pulled out of the acc pump transfer slot into engine through the power valve vacuum plenum. The engine acts like it's jetted too rich because under high vacuum it's pulling in additional fuel from the accellerator pump transfer feed. But, when you move the throttle, the engine bogs because the fuel that should have been in the accellerator pump isn't there because of the transfer feed leak.

Difficult to diagnose. Z7 epoxy can fix with some work with a file to smooth mating surfaces. Drilled slots, brass tubing, and "O" rings can fix this. Drilling metering block and throttle body to use Holley 4175 type accelerator pump fuel transfers will also work.

Bolting on a Demon 650 DP just solved everything. There have been some posts regarding Demon vacuum secondary carbs opening too early or too late, but big engine - light car, such as a Cobra, really need double pumper, mechanical secondary carbs.

[BEB]

Can anyone tell me which Holley pump cam will give me an "earlier" shot of fuel. I am currently using a Green cam but would like an earlier, quicker shot of fuel. I went onto the Holley website but couldn't find this info.

I have a spare Black and Red and Orange cam - which one should I try next?

[niles]

Of the cams you have:
red - 20cc per stoke
orange- 24cc
black 18cc
green- 30cc.
Now these are in position 2!
position 2 flows about 25% for any cam than the same cam on position 1

gn

[bullseyeprec]

I had same problem! I replaced the squirter and put cam in number 2 position and problem went away. If your car idles at 1000 rpm it seems that position 2 or 3 works best. My squirters are 31's. Good luck

[BEB]

To bullseyeprec & niles:

Thanks for the info.

However I still don't quite understand exactly what that means. You are talking about the amount of fuel each different cam pumps. But; which one pumps the fuel in quickest/soonest/earliest?

I have tried position 1 and 2 and my engine certainly prefers pos. 1 (possibly because it idles smoothly at 750 rpm). In pos. 2 there is a noticeable stumble, but in pos. 1 there isn't.

Please bear with me.....

[rdorman]

The cams are rated in CC per ten pumps. For instance, if you have a cam that is rated at 22cc then that is 2.2 cc of fuel per full stroke. The cam determines how MUCH fuel.

The cam position determines how much fuel relative to the throttle position. Position 1 is for very low idle speeds say around 600-700. Position 2 or even 3 'delay' the pump action to a 'later' amount of throttle opening. For instance, a car that idles faster. This is of course affected by other things and is a generalization. Think of this as off idle tuning.

Cams also determine when the fuel is delivered relative to the throttle position some cam profiles may deliver all the fuel (think of it as reaching maximum lift on a cam) by say 30 degree of throttle movement while others may go as high as 50 degrees. Think of this like the cams duration.

Here is a chart that shows all the pump cams:
http://www.carbdford.com/tech/pumpcams.htm
There is also a great chart in most holley books and I think with the cam kit.

If you can picture you camshaft for the valve train, you are a long way along to picturing the pump cam.

Squirters. Squirters determine how long, time wise, it takes to deliver the fuel to the engine. The bigger the squirter the faster the fuel comes out. Pour water through a straw or a hose? Over .040 you will need a hollow squirter screw, and over .037 go to the 50cc pump setup, go up and down in about three sizes each time.

As you can see, there are many variables. For some one to say 'use this cam and this squirter' and to have it work on a different car is not a real high percentage shot. Pun intended

Generally, if it stumbles or even backfires give it more fuel. If it hesitates or is not crisp/clean, give it less. Start with the stock cam and pick the right squirter size to give good off-ilde acceleration.

Then move to the cam. Buy the whole lot of them and lay them out side by side and look at the difference. No movement of the arm or not enough off idle. Try position 2 or 3. Off idle good but get a hesitation when you are already at higher speeds, try a longer cam (look at the chart!).

If the car will accelorate smoothly in high gear, then it will in a lower gear. So tune in high gear! Some times you find that low gear is great and high gear is not. Often decreasing the squirter size helps because the engine takes longer to build speed and needs a longer duration shot.

Of course don't forget initial timing, proper idle mixture adjustment and proper throttle blade to transfer slot relationship.

Take the day off for this one, you will need it. Change one thing at a time and take notes.

Rick

[bullseyeprec]

My car ran good with 72 and 80 jets but when the weather warmed up It didnt run quite as well...so I checked plugs...LEAN! everything was white!! I have a 567/576 lift roller 392, single plane, 4150 750 double pumper with orange cam and factory powervalve. I looked at ford motorsports and they said bigger jets at my elevation (almost sea level) so I went with 74/86 jets and plugs look good. In the meantime I developed a hesitation off idle...if accelerating slowly...so I swapped pump nozzles from 25, 28, 31 and I could adjust it to either off idle hesitation or good off idle but would stubble or have a dead spot at 2K at slow acceleration. It was sat night and speed shop was closed...my smallest bit was a .040 so I drilled it from a 31 to a 40 on the primary side.and all problems went away. I hope it runs good in 100 degree weather I am really tired of swapping out parts!!!!! I had a friend tell me that installing adj air bleeds (the screw in type) would help the engine rpm drop when you let of idle faster than it does now but I am afraid to do it since it runs so well now. I thought rpm drop was due mostly to flywheel weight but I may have been wrong. I have never had so much trouble tuning a carb! SO maybe jumping pump shooters a bunch would help..they are easy and cheap to swap. I adjusted the over travle screw so that it shoots fuel as soon as throttle is cracked and made sure I hade clearance at WOT by mashing pump with screw driver with throotle wide open. There ar so mnay variables that effect the carb my recipie mint not work for every car, it just worked well on mine. BTW I have a 30 cc pump. Good luck

[BEB]

I'm back - Cobra still not fixed !

Last night I tried playing with different pump cams in a bid to make sure the throttle plates move the moment I hit the gas - I checked that on all the different colour cams that there was .015" gap at WOT. Funny thing is that on the Green cam, which the engine 'seems' to prefer, the accelerator pump arm does NOT move immediately when I hit the gas, neither does it with a Pink cam.

However, with a Black cam the pump system works immediately but there seemed to be a 'bigger' bog/stumble with the Black cam.

Now I'm totally confused as I'm sure the problem is that there is a delay in the pump system working but the cam that the engine likes best has a delay (I guess because of the cam profile/shape).

Looking at the pump arm it seems to be slightly bent upwards where it rest on the cam. Should I try bending it to give me an immediate pump action whilst maintaining the .015" gap at WOT?

[Mr.Fixit]

Don't bend it, it will have the same effect as adjusting the spring loaded adjustement screw, so just use the adjustement screw. Try to get the pump cam to engage the accel pump the instant there is any throttle movement, no slack, no waiting.