Help! Engine sag when throttle first applied

[wood8835th]

For 75 but with big block installed, A/T

MSD 6000rpm limiter
Holley 670 street avenger
performer 2 intake
650 rpm idle in drive, 800 rpm idle in park

Runs well except when throttle is first applied normally from a stop. Engine sags and almost dies.

changed acclerator pump dicharge nozzle sizes, ensured accelerator pump is working properly. Same result.

What am I missing? Any trick or hints?

 

[Thore826]

I've got the same problem w/ my '75; if I don't ease on the gas, it'll begin choking up and'll stall if I don't let off. I'm thinking its the carburetor, but I'm not sure. btw, I'm all stock, auto trans.

 

[minifridge1138]

I would say you are either getting too much fuel or too little fuel.

I would try adjusting your accelerator pump.

You don't say what kind of carburetor you have, so I can't go into specifics.

Speaking about Holley Carbs, they have different plastic cams that adjust the accelerator pump.
Each cam changes where in the throttle range the fuel gets pumped.

Buy the assortment of cams (about $20) and they play.

Go to bigger one and see if that is better or worse.
If worse, go to a smaller one.
If better but not great, go to the next bigger one.

 

[Crimson Thunder]

What are your initial and total timing set at? You would be suprised how many carb problems are actually ignition problems.

 

[gmjunkie]

What are your initial and total timing set at? You would be suprised how many carb problems are actually ignition problems.

Bump the timing up about 2* and see what happens!!

 

[wood8835th]

Thanks for the replies. Initial 12 deg, total 34 deg. all in at 4500 rpm, at last check. That was a few months back. I'll check again tomorrow evening and tweak a few degrees.

 

[gmjunkie]

Thanks for the replies. Initial 12 deg, total 34 deg. all in at 4500 rpm, at last check. That was a few months back. I'll check again tomorrow evening and tweak a few degrees.

I have a 30 over LS6 in my hauling truck with a low end towing cam and a stock Q-jet set at 14* initial and 38* all in at 2800! 85,000 miles since I built it,Likes gas depending on how big of a sail I put on top!!

 

[baxsom]

Thanks for the replies. Initial 12 deg, total 34 deg. all in at 4500 rpm, at last check. That was a few months back. I'll check again tomorrow evening and tweak a few degrees.

i thought it should be all in by 3000.

 

[Rodney Gold]

An abrupt stumble and backfiring would indicate too small an accel pump shot , a bog and then a slower recovery indicates too much.
You must determine whether its an overrich or overlean condition.
Timing should be all in at 3000 rpm and 36-38 degrees.
First thing to check for is vacuum leaks , 2ndly the 650 carb is too small for a performance 454.
Check the float levels on the carb.
Try the orange cam at position 1 ...also check your spring loaded pump actuation lever is set correctly.
(no play at udle and at least 1mm of travel of the actual pump lever at WOT)

 

[wood8835th]

Thanks all for the help. Looks like I'll be resetting timing, and some springs.

 

[Hib Halverson]

That the engine sags when you drive away from a stop sign in a normal manner begs the question....is the sag just as you step into the gas slowly, like just off idle?

Also, what happens when the engine is idling and you just whack the throttle wide open?

Lastly, what happens if you pull away from a stop sign agressively..like you get the car moving then really roll into the gas quickly?

 

[wood8835th]

My only info this evening is a controlled, progressive (step on gas slowly) push on the throttle from a stop sign, and I get a sag there. If I don't let off and reapply the throttle with a second push I can, most times, kill the motor.

I will take it out tomorrow and get the remaining information. Sounds like some fun testing coming up.

Thanks for the help!

 

[Hib Halverson]

Flip the carb over and see if the edges of the primary throttle plates are positioned such that only the very bottom of the idle transfer ports are exposed.

Also was this "Street Avenger" (sheesh where does Holley think up the stupid names) installed right out of the box or were changes in calibration made before installation?

 

[wood8835th]

Sorry I didn't respond last evening with the test results. That darn work gets in the way of what I want to do sometimes.

Rechecked timing, 15 deg initial at 800 rpm in park, vacuum advance removed and plugged. Changed springs and am now getting 36 deg all in at about 3200rpm.

Still had sag problem from stop with light throttle.

From dead stop to punched throttle. Very minor sag but engine catches quickly, no real threat of killing engine. Much better.

From dead stop, let off brake to car rolling, punch throttle, no sag issue whatsoever.

Tried just turning idle up to between 850 and 900 rpm (about 650 plus a little in drive) and seemed to reduced sag issue noticably but not completely.

The carb was installed direct from box then taken to expert who had to tweak a bunch to pass emission then it was returned to "normal" so it would run properly. Runs well except for this sag issue.

 

[TMPJoe]

I run a 770 on my 383 stroker. that 650 is way too small for a 454 big block. No way that is enough fuel as these are intended to be street friendly and the pumps are not that aggressive... .. i am sure the "sag" is a lack of fuel.. we do not have emissions here but I have to ask the "expert" what he did to reset the carb and then what the definition of normal is?... put at least the 770 on it.... and the lightest spring in the package they sent with it

also the idle adjustment sounds like its out.. it takes a bit more fiddling to get it right as there are two adjustments for it IRC

which springs are you using?