base timing keeps advancing by itself...

Phreakish

Well-Known Member
Joined
May 28, 2015
Messages
1,969
Reaction score
2,075
Location
N. Idaho
Thanks for the tips. After confirming TDC, I went back in to take a look at the carb. Did a full throttle pull, killed engine and pulled plug, it showed almost completely clean. Ordered needles (Edelbrock avs) to go richer. Will see how that effects things.

I'll admit, my post is probably overly cautious/alarming. The thing is, it's too easy to miss something small and have things go horribly wrong as a result. It's unlikely, but the possibility does exist. If a carb is dumping fuel, or isn't flowing enough, then damage can definitely happen. Fuel wash of the rings can happen quick, and undetected knock can crack a piston or burn things if it persists long enough.

Sounds like you're on the right track though, and with your new observations and checks, you're likely to find the issue quickly.

If the car did run better/different in the past though, needles and jets may help but may not be the root cause. Edelbrock carbs are easy to pull the tops off and you could pull the booster assemblies. If I remember right, there's a lot of small holes on the elumsifiers which can clog up when left in storage for a winter. Might be worth checking those out, as well as ensuring the inside of the carb is in good shape.

Be careful of the accel pump check ball and weight though. Easy to lose them into the manifold if you're not familiar with them. I've also had the accel check ball stick in the seat and cause issues while accelerating. At the same time, I had lots of varnish in the idle transition slots which caused the idle mix to be non functional, and caused a lean surge at cruise. I'm not saying these are or could be your issues, but the same thing could affect various circuits in the carb so it wouldn't be bad to do a thorough check and clean.

I'd changing needles, jets, and springs help it could be that they're the fix, or they may just lead you to the real problem. Personally, I think it's a great path to start down. I'd follow the others suggestions since they're far better versed in tuning than I'll ever be (I gave up and went efi, lol).
 

512Stroker

We are all here because we are not all there.
Joined
May 30, 2016
Messages
3,472
Reaction score
2,866
Location
Freedom, MO
Yeah I have been eyeballing the 650 avs2 for a long time, comes quite a bit richer out of the box. If my needle change (2 stages richer in power mode) is a step in the right direction I might just go ahead and pull the trigger on the new carb and start tuning that.



Reading the Edelbrock manual.. it says to go to a stronger step-up spring to cure a lean spot. Am I misreading it? Currently has the yellow (4") spring which is one step weaker than factory, put in after the new cam which produces a bit less vac at idle than stock. Also suggests this spring is just for transitions, my detonation rattle is only at 3/4 to WOT throttle and is is sustained if I hold the pedal down.

And I told you I was going to check TDC, just had to wait till the weekend when I was off work.
Cool TDC is good
The lean spot that Edelbrock refers to should really be difined as an off idle stumble. A dead spot right at the hit of the throttle.
I have tried tuning with every spring Edelbrock has and I always end up with the orange spring having the best performance. My 340 makes around 10 inches of vaccum.
Good luck keep posting
 

RustyRatRod

I was born on a Monday. Not last Monday.
FABO Gold Member
Joined
Jun 7, 2010
Messages
91,898
Reaction score
79,435
Location
Georgia
How long has this engine been together? How often do you do WOT bursts with it? Or....do you baby it a lot?
 

smokinnjokin

Well-Known Member
Joined
Jan 20, 2019
Messages
237
Reaction score
62
Location
Hopkinsville, KY
It has been together around 2 years, and i romp on it continually. I have around 15” vacuum at idle. I put the pink springs in and they helped. Which seems weird, there was no off-idle stumble issue. Smooth transition to partial or WOT. So far, rattling is gone. I think the carb is very lean still. We will see what the 2 stages richer needle does to plug readings.
 

Bewy

Well-Known Member
Joined
Jan 1, 2021
Messages
3,022
Reaction score
1,927
Location
Australia
Using just a plug check for checking rich/lean is fraught with danger. Even with the old leaded fuels, but worse today with unleaded & the variety of blends.
If you suspect the WOT mixture is lean, go 3 sizes bigger on the jets. With such a large increase, it would cure a leanness problem. If it still detonates, then you have other problems. Has fuel delivery been checked, float level etc?
 
Top