Heus

New 350 ATK crate engine with Edelbrock 650 AVS 1806 carb. .498/.501 cam, Dart vortec style heads. It runs great at idle and under hard acceleration. However it has an annoying lean surge at steady cruising speed between about 1500 and 2300 rpm. I have tried several combinations of metering rods and jets, according to Edelbrock's chart, in an attempt to richen up the cruise mode. I also played with many timing variations with no improvement. Another footenoot that may or may not be related is that it backfires through the side pipes when I let off the gas. Vacuum is 12" at idle.

chevymans 77

first thing that comes to mind is a vacuum leak,

Do you have the duration on the cam to post,

Neal

Heus

220/224 @.050
112 lsa
Lift is actually .495/.502
Its a roller cam in a 97 vortec block

claysmoker

Probably a timing issue. Post up all the timing numbers.

jnb5101

Lars sells a distributor advance limiter that when installed limits the total vacuum advance to about 12*. The total vacuum advance with which these cars were designed will cause this "lean surge" or "trailer hitching" with modern fuels. As a test, disconnect the vacuum advance hose and drive the car in the range that causes the problem(the engine will run hot and idle poorly, but ignore that for the test). If that solves the issue, contact Lars for his distributor part.

Heus

I should have mentioned that it still does this even with the vacuum advance disconnected.

Jebbysan

What year is the car sir?

Jebby

Heus

Its a 78

454Luvr

Does the intake match the carb (squarebore/squarebore), or are you using an adapter? Was there any difference in driveability when you richened the jets/rods?

Heus

No adapter plate. Its a squarebore Weiand street warrior intake.

chevymans 77

have you pulled the pugs and to see what they look like after a long cruise?

MotorHead

A lean surge is just, the engine is lean at a certain RPM/Speed range. First check for any obvious vacuum leak. If no leaks then it is a fine line between cruising RPM and idle, part throttle, full throttle.

One of these is obvious, your cruising lean condition. However it could be lean across the board, just not so obvious as you probably will not notice the idle and WOT lean conditions.

Do you have an A/F meter ? Wold you be opposed to buying a Holley carb?

You are most liely lean everywhere so you have to figure out how to richen up the carb you have.

454Luvr

The deceleration backfiring tends to confirm this. The 1806 is calibrated for standard street motors and is probably set a little lean. I would start by installing the jets and rods used in the 1826 performance version.

cardo0

X3 on a A/F monitor. This is the modern way to tune a carb and a good wide band is getting cheaper by the day.

ddawson

I never would have known how far off I was without a wideband.

Idle was fine but once I was moving I was super rich.

http://www.summitracing.com/parts/inn-3837

chevymans 77

I agree that a good AFR meter with data logging is very hard to beat but you can get close with the plugs and a little effort.

Go for a cruise in the range that is surging, stay in that range for a little while then pull a couple plugs and see what they look like.

Neal

Heus

Its been a couple of months and I still have this lean surge! No vacuum leaks. Plugs are a nice tan color. Have continued to play with different initial timing numbers, primary jet/rod/spring combinations, and idle mixture settings. One thing that I now notice is that the drivers side idle mixture screw, when adjusted has no effect on the idle. I can even pull it all the way out when running and their is no difference. Im almost ready to ditch the edelbrock and try a Holley 670 Street Avenger which is what ATK recommends for this crate engine anyways.

diehrd

Vac is low .. Exhaust back fire can be a simple gasket leak on the header.

billschroeder5842

I NEVER had any luck with a Eddy. I had lean issues every which way but loose.

I switched to a Demon. I put it on and had no (Zero) issues right out of the box. Pulled through all ranges nicely and was a breeze to tune. Plus, it has the same footprint as a Eddy so all throttle and gas lines matched from the git go.

So, ditch the Eddy and be happy.

MotorHead

Two months with the same condition, move to a Holley and I can almost guarantee your lean surge will go away. if not then I am much more experienced in tuning a Holley and would help you there.