Dan86Z28

Ok guys.. I'm at my wits end.

Here's my combo:

350 .030 over
Edelbrock Performer RPM cam
Edelbrock Performer RPM Intake
Holley 670 Street Avenger Carb
World Products Sportsman II heads
headers, etc etc blah blah
Acell HEI Distributor with MSD 6al box

Ok here's my problem..

Car runs great accellerating. When I'm cruising on the highway, at VERY VERY slight throttle, it seems like it surges/misses/stumbles or something.
VERY light, but still noticeable..

SO I tried retarding my timing, turned back my vacuum advance, and went from 65 main jets to 68, and nothing helped.

soooooooooo yeah... HELP!

radiateu2

What is the fuel pressure at cruise? what are the float levels at? sounds like a fuel delivery issue? Is the accelerator pump adjust correctly?

Dan86Z28

Dunno about fuel pressure..
As far as the floats go, that's an interesting question.... I tried to adjust the primary float, per instructions, but I could never see fuel out the sight plug. i even took it out, and adjusted more, and STILL couldnt see any.

And asfar as the accelerator pump, I havent messed with it.

Thanks for the help so far.

I've got a fuel pressure gague sitting around here somewhere, I'll throw it on and see if I can get some readings.. I won't be able to tell at cruise, but I can pull it up to that RPM and see what it's like sitting there.. I know I know it isnt the same, but maybe it will shead some light on the problem.


Thanks again!
Dan

radiateu2

I bet your fuel bowls are running dry....you should see the fuel just below the threads of the site holes...leave the pump shot alone and adjust your fuel levels first...this should be done with the car running
but be careful

Dan86Z28

Someone else told me to do that. But I tried to adjust it, and I couldnt ever get it to come to the bottom of the sight plug.

I'm gonna try to re-plumb my fuel system today and see if that may help.. Otherwise I'm gonna take the fuel bowl off and see what's up with it.


Thanks!
Dan

Barry85Iroc

If your front jets are too small, the car will stumble upon acceleration....at any speed. Go up 2 jet sizes in the front, and the problem should go away.

Dan86Z28

It runs fine at speed, and at accelleration, and everywhere else, it's JUST at light LIGHT throttle in cruise..

But I did find out something, the front float was adjusted all the way DOWN...so I fixed that, and we'll see tomorrow how it runs

65panhed

Try put this link www.bob2000.com/carb.htm........maybe this will help........... also this link www.mortec.com

xpndbl3

iTrader: (5)

i'm far from a holley tuning expert, but how about fattening the idle mixture screws 1/4 turn each. the way i understand holley's idle circuit is that under very slight throttle you're using the idle circuit. the worst that could happen is my advice doesn't work and it's a 10 second change regardless.

Chickenman35

That will have a huge affect on tip in response. Will probably fix things. You will probably have to reset the idle mix and jet back down to the 65's. Change only one thing at a time though.

Dan86Z28

OK update---

It STILL misses at cruise LIGHT throttle.

And today, I was going up a hill in 5th gear, about 2k rpm, and punched it, and the car started to miss..


soooooooooo that means I have to jet up more, right?

Chickenman35

At LIGHT, Light throttle as you've been mentioning you may be still in the transition circuit. IE: the mains are not flowing fully. Yhe Avenger series uses a cheap Booster in the venturi. It is not very efficient.

One thing you have not mentioned is adjusting the idle mixture. Most Holley's are what is called a two circuit carb. The transition circuit metering is shared with the idle circuit. Changing main jets will have minimal effect on the the Transition circuit ( it will have some...but very slight ).

Try richening the idle mix a 1\2 to 3\4 turn...see if that helps the situation.

BTW.... no carbed car like ours will pull clean from 5th gear, at 2,000rpm.... going uphill. That's really lugging the engine down. A Fuel injected car could do it...but not a carbed setup such as yours. Lack of main booster signal. Booster signal is the most forgotten item in carb tuning...it is also one of the most important.

kd5boc

That's why most carbed setups act like garbage at WOT under 2k...hmmm...that explains why mine acts the way it does...keep forgetting about that.

My Edelbrock does the same thing when in cruise and I press the gas just a little more...my rod spring is not strong enough, but I don't have enough vacuum at idle to keep it down so I will just have to deal with the double-edged sword I made for myself.

Dan86Z28

I tried that, to no avail.

I swapped in 71 jets today, and no perceptable change. But when I take off the vacuum cap for full manifold vacuum, the engine speeds up at idle, like it's running rich?

hmm.. wierd.

Any other ideas?

Chickenman35

Ok...lets get to the nitty gritty. Need some more info.

What are your cam specs? How much initial advance are you running. How much total? Are you running a vacuum advance( you should be) is it hooked up to Ported or Manifold vacuum?

I suspect that you have a slight problem with the accelerator pump circuit. You have a " Tip in " problem.....correct?

We've changed shooters on the accelerator pump...but what color of cam is on the Primary side? Edit: just noted that you haven't messed with the accelerator pump. Missed that.

Check the accelerator pump shot with the engine off. As soon as you move the throttle lever, even a small amount, you should get fuel squirting from the nozzle. I suspect it is dribbling out. Make absolutely sure that you have no freeplay between the accelerator pump lever and cam at idle. Proper method is to adjust pump lever clearnce to zero, then add 1\2 to 3\4 turn pre-load to spring.

For some strange reason, Holley doesn't supply the technical specifications for the Avenger series. What pump cam and shooters are in it now? I'm suspecting a fairly small shooter such as a #25 or #28 and a white or red cam. This is a standard baseline setup from Holley....and it needs to be tweaked.

Get a #31 tube type shooter ( #121-31 ) from Holley...install that. Also, get a pump cam assortment from Holley ( #20-12 ). Install the Blue cam in the #2 hole. I think that you will find that this will solve your " Tip in" problem. Depending on your engine cam shaft spec...you may have to go up to a #35. But that is usually really only necessary on engines with big camshafts.

Pump cams differ greatly in profile. While many deliver the same overall amount...it is the timing of when the fuel is delivered that is critical. SBC's like a sharp hard initial shot . The blue cam delivers this. White cams and Red cams have a very even profile. They do not give enough fuel delivery in the early degree's of throttle movement.

You can likely go back to the stock jetting...as that seems to have had no affect on this problem. Keep an eye on your plugs. Make sure that they haven't fouled with all the mucking about.

Remember to change only one thing at a time....and record the changes as you do them. IE: Keep a written log.

kd5boc

Not sure what you mean by this...if it is the Ported vs. manifold and you aren't worried about emissions, got a great link for you!

Tom

Dan86Z28

Ok, back to basics..
My setup:
Performer RPM intake
Performer RPM cam
Holley 670 Street Avenger carb
Accel HEI dist.
MSD 6al ign. box

and the problem isn't in tip-in.

it's... well.. ok. let me describe it.


I get in the interstate
I shift through the gears, end up in 5th at 60mph at around 1800-2000 rpm (If I decide to speed or not :P )

When I get on a straight streatch of road, and am at a consistant, very light throttle position, the engine will 'buck' or 'miss' or something.. it doesn't run smooth. Accellerating from a dead stop is smooth, as well as accellerating from that 2k rpm, as soon as I push down further on the gas, it runs ok.

ALSO, when I downshift to engine-brake, the car bucks pretty bad. Dunno if that's related or not. But when I give it gas, it seems to pick itself back up.


Any more ideas?

Dan86Z28

no... what I meant, is when I was hooking up the vacuum gague to the full manifold vacuum port on the bottom of the carb, I took the cap off with the engine running, and the engine increased in RPM by around 2-300 rpm.

Maybe it's timing related.. but I tried that too. I tried disconnecting the vac. advance, and the car ran WORSE. I've adjusted the vacuum advance both ways, to no difference.


Also when I hook the vacuum advance up to full manifold vacuum, the car seem to run better?

And as far as what my timing is set at, I'm not entirely sure. It's around 14 degrees initial, but I don't have a timing tape to see what it's doing otherwise.

I'm kinda stumped..

kd5boc

As far as the manifold vacuum goes...it is a "however you like it" type of argument...for non-emissions folks, manifold is best...just read the TIMING 101 post....

http://www.hotrodders.com/t47495.html

Summary: When an engine is lean (idle and cruise) it runs best with more advanced timing. when rich (WOT) it runs best with a not-so-advanced timing.

Running on Manifold vacuum raises your timing a lot at idle (20+degrees BTDC) but it runs cooler because the fire isn't still burning going out the exhaust...it burns more in the cylinder...and runs better (it smoothed out my lope by quite a bit and idles much better). During cruise, manifold and ported act the same...WOT they are both cut out by low manifold vacuum.

You set your timing the same way. Disconnect vac on distributor, hold 3500rpm, set timing to about 33-35*. idle should be around 10* or so...mine's at 14*. This will vary from vehicle to vehicle...Then connect vacuum to manifold (this will raise idle some). Lower idle screw to proper idle rpm and drive away. Those with lopy cams may have a problem at idle...get an adjustable vacuum can, or go back to ported.

Tom

Chickenman35

Again...

What are your cam specs? How much initial advance are you running. How much total? Are you running a vacuum advance( you should be) is it hooked up to Ported or Manifold vacuum?

Camshaft specs are crucial. If you have a big cam you simply may never be able to solve this.

If you cruise in 4th gear at 2,000 RPM or higher do you have this problem?

How much of the Transition slot is uncovered. If more than .040" of the transition slot is uncovered then that could be the problem.

When you "tickle" the throttle does it buck and miss for a secnd or two...and then run clean ( assuming no further opening of throttle ) or does it keep missing?

Dan86Z28

The performer RPM cam is 234/244 .496/.510 lift

If I cruise in 4th gear, I still have the problem, all be it masked by the torque multiplication. But I can still feel it.

How would I go about checking the transition slot?

as far as timing, I'm not sure, because I don't have timing tape to figure out whats going on. About all I can tell you is I have around 14 degrees at idle.

I've run the vacuum advance off of both ported and full vacuum. It seems to run better off of full, but it doesnt change the stumble.

Dan86Z28

sorry forgot about that one.

I'm not sure what you mean 'tickle' the throttle?

Chickenman35

Light steady throttle application.

Check out this link for a excellant article on setting up a Holley by TMP owner Troy Patterson.

Shows detailed picture of how to set up transfer slot.

http://www.carcraft.com/howto/77958/

Chickenman35

You might be stuck with your stumble. That cam is big enough so that you are just out of it's cruise RPM at 2,000 rpm. Lack of main booster signal. Mike1985 had the same problem. It can be a SOB to cure.

Your CR is also a bit on the low side for that cam. 10.5 to 1 would be better. What is your rear end gear ratio?

Check the Transition slot as mentioned in previous post....but you may have to live with this.

Dan86Z28

My rear gear is 3.23 currently.

Dan86Z28

And what if my idle transfer slots are exposed too much?

what can I do then?

Chickenman35

The secondary throttle blades have an idle adjustment. You have to remove the carb to get to it. It is a small allen screw on the underside of the carb baseplate. Increase the secondary idle adjustment. This will enable you to decrease the Primary setting and put the Primary butterfly back into the proper relationship with the transfer slot.

Looking at your cam specs, I'm betting that you had to crank in the idle screw a fair amount to get it to idle. Transfer slot may be exposed too much.

Those 3.23 gears aren't helping matters. Mike1985 had 3.07 gears and he really couldn't solve the problem till he got 3.73's.

Hopefully the transfer slot is uncovered too far and you may be able to get things better....

Dan86Z28

Ok a few updates... (sorry I've been sick for almost 2 weeks.. bleah)


I removed the carb, and the idle transfer slots WERE exposed too much, fixed that, didn't help..

So I got to playing.
I can reproduce the 'miss' with the car sitting and me SLIGHTLY on the throttle.

Like, it's not load related, and I can reproduce it in the driveway.

It's JUST off-idle.

Any more ideas?

kd5boc

Have you tried richening the idle mixture?

Dan86Z28

yup.. to the point of it burning my eyes..

2.73's Suck

I have the exact same problem with my mustang and its all stock except for the holley. When I get a chance to play with it Ill let you know, hopefully youll figure it out first so I dont have to The first thing Im going to try is ported vs manifold vacuum. I know what you mean about how it does it when you try to go one speed and its unbelievably irratating.

Dan86Z28

TTT

Any other ideas?

Dan86Z28

Also, something interesting.. It seems to get worse when I un-hook the vacuum advance (and plug the port on the carb.)

Maybe I'm not getting enough timing?

Is that possible? To hesitate/miss/stumble because of not ENOUGH timing at cruise?

I was under the impression it was the other way around?

Anyway.. help :P

Chickenman35

As I stated in a previous post....your cam is big enough that you are out of it's cruise range...you just may have to learn to live with this.

Still a couple of things to try. A big cam like that will like lots of vacuum advance, so full manifold vacuum is where you want to connect the vacuum advance can.

However, what are you using for a distributor advance can. If it is a stock GM can that could be part of the problem. The spring tension on these is quite high and you simply may not be getting enough full vacuum advance at cruise. As soon as you step in it a bit the vacuumm afvance may be kicking all the way out instead of remainimg steady ( light throttle application here folks).

If you have not already done so, purchase a Crane vacuum advance can. Set the advance limit to 15degree. Then adjsut the inside diaphram spring to MINIMUM tension, or a tension that fully pulls the Can in by 5" to 7" of HG. This can be checked with a "Mighty Vac" ( Vacuum pump). That should help a bit but may not solve things entirely. It should also stabilise your idle.

I suspect that the main problem is lack of booster signal and the inefficient ( IE: Cheap) booster design used on the Avenger series and you Camshaft specs. One solution would be be have Annular boosters installed on the Primary side. Annular boosters are far superior to anything else and can really help driveability.

BTW, did you ever install the Blue pump cam and get the #31 tube type shooters like I suggested? That should help some.

The end result is that the cruise RPM of that cam is around 2,200 rpm. Anything below that and it will not be happy. You may be able to decrease the problem...but you may not be able to make it go away. Cam is simply not designed to run cleanly at the low RPM,s that you are trying. You have a classic mismatch of cam and gears.

I had a similiar problem when I went to a Comp XR288HR ( 236\242 @.050" ), but I had 3.73 gears, so at 60mph my cruise speed was above 2,200. Anything around 2,000 and it did not want to run clean...and I have all the tricks... Annular Boosters, Crane advance cam and a Weber Power Plate ( which makes the car run like it had Fiel injection ) . Above 2,200 rpm no problem anything much below this and the car would buck with light throttle applications. Solution was not to let it run at that low of an RPM. I finally went back to my old standbye ( A custom ground Hyd Roller with less duration and lift ) for the street. Car happy as a Pig in S**t now.

End result may be:

1: Change rear gears to 3.73's and live with low RPM limitations of cam.

2: Change the cam to something milder. ( 218\228 @.050" 112LCA would be ideal IMHO for a street car.

3: Do neither and just live with it if you want to Drag Race.

Dan86Z28

ok cool.. The dist. is an accel HEI with adjustable advance. I'll adjust it all the way down and see if that helps.

As far as the pump cam and shooter, no I havent. I will this friday tho. (if someone in my area has them in stock)

Also the strange thing is that with my Edelbrock, it didn't do this.. Wierd.

Thanks for your help!!!

Chickenman35

The reason that your Edelbrock didn't have this problem is that it has a more efficient booster design and a more sophisticated transition circuit.

Q-Jet's and Edelbrock are what we call three circuit carbs. They have a separate circuits for idle, transition ( sometimes called low speed circuit ) and the main circuit. Most Holley's ( 4150\4160 series ) are what we call two circuit carbs. The transition circuit is combined with the idle circuit. This is cheaper to produce but less efficient. Most Dominator series are three circuit carbs.

If you really want to make your Holley behave like Fuel Injection give Troy Petterson at TMP Carbs a call. He supplies the Weber Power plate, which replaces the stock Holley metering block. This converts the carb to a three circuit design and uses all Weber components. You have separate idle, a fully tunable low speed circuit ( transition ) with Weber fuel and air corrector jets and a fully tuneable main circuit with Weber DCOE air correctors, main jets and emulsion tubes emulsion tubes. It ain't cheap....but man does it work!!

TMP Carbs
10030 Canoga Ave.
Chatsworth, CA 91311
818/885-0410


Here's an article from Car Craft where Troy was doing some extensive tuning on a project car.

http://www.carcraft.com/howto/77958/

Dan86Z28

Or maybe I should look into getting an Edelbrock carb again

Chickenman35

Each has it's advantages and disadvantages. Edelbrocks are useless for Road Racing or Autocross for example. Float chamber design all wrong. They starve out on right hand corners.

Nothing really wrong with the Holley..it just takes a bit more tweaking to get driveability. The Avenger series would not be my first choice however. It is a cheaper line model..to appeal to the masses. The HP series would be my choice.

ProForm also has come out with a very nice line now. Check them out.

http://www.proformparts.com/v2/catalog/carburetors.html

$499 from Summit.

ProForm 750cfm from Summit

Bunker82

Unfortunately Chickenman, Troy has moved and that info is now incorrect. I made an order right when he started moving and went down there in person when it didn't show up, but the place was shut down. Eventually he got back to me and said my order was coming but when it showed up it was missing $60 worth of stuff and I have no way to get a hold of him to sort it out. I really wanted those idle air correctors since they are specific to the power plates. Anyway, I hope I can find out his new contact info someday.

Chickenman35

Bugger...that sucks. Regarding the Idle air correctors. They are Weber 48IDA parts. Readily available from any Weber supplier.

The Idle jets themselves are 70F10's ( Part # 74823.070 ) and the Idle corrector is a 200 ( Part# 77903.200). Note if you can't get a 77903.200 Idle Jet holder, then order a 77903.120 and drill out the hole to 2.00mm. That's what I did.

MIKE 1985

as chickenman stated i had a similar problem. One problem i did have that you haven't mentioned was at idle if i floored the car it had a flat spot. Does yours do that ?

Any how this is how i fixed mine. initial timing 25 total 35 ( i had to modify the dist. slot) and i used 1 medium spring only. I had to limit the vac adv. to 10 deg and hook it up to MANIFOLD vaccume, so, when the car was idleing i had 37 deg timing. I also ended up with the blue cam in #2 pos. and a #37 tube type squirter. It would smoke when floored from idle, but would clean up. This was the only way i could get the stumble out of it.

BTW my cam was smaller 224-234 @.050 112 LSA.

I redid the motor with Canfield heads and a nitrous HP flat tappet 218-230 @ .050 and 113 LSA. The car drives like a stocker and makes 292 RWHP @ 5900 and 421 TQ @ 2100.

Still have the 3.07 gears and just ran 12.63 @ 109 and 11.38 @ 118 on a 100 shot with a 1.55 60'.