40DCOE-18s installed on 3.1 stroker tends to die upon deceleration

[dawg7]

Hello Gents,

As the title states I am experiencing some issues during deceleration when coming to a stop using the gears. I can put the car in neutral and it stays running. But when I use the gears to help with the stop especially when coming in hot and fast the engine just wants to die. It always dips to 500 RPM and most of the time just stops running. It idles good, it runs good going thru the gears, sounds freaking awesome! but the damn stop.....

Been tuning the carbs for a few weeks and I think it is real close. Idles at 1000. Fuel PSI is 3.5. Fuel return line is capped. I barely have the 2-3K rpm stumble. I can tell it pulls very nice all the way to 5500 but that is as far as my butt is calibrated....anyway I think the cam tops out of 6400???

Timing set to 12 degrees at 1000 rpm and 34 degrees advance at 3000 rpm ( no creep when rpm increased to 4000)

I have read many Weber tuning manuals and haven't come across the "dies upon decel" troubleshooting steps. The carbs were good when they came out of a L24 and have been sitting for a few years. I cleaned them up like a rookie would and did a minor overhaul using the Peirce Manifold kit. Change the chokes and the idle and main jets per Peirce Manifold tech reps guidance.

I don't have a clue what my next step should be other than checking for some restrictions in the carbs.

Here is the specs of the engine:

CARB SPECS

CHOKE 34

AIR CORRECTION 180

MAIN JET 140

IDLE JET 50F9

EMULSION F11

AUX VENT 4.5

PUMP JET 1.75

NEEDLE SEAT 1.75

INLET VALVE .55

ENGINE SPECS

N42 block overbore 3mm

KA24E PISTONS ITM RY6660 CUT .5mm

N42 head port polished gasket matched

ISKY L480 Cam

Cosmetic head gasket 1.2mm

71 240z connecting rods

MSA 6-2-1 header

2 ½ “ Magnaflow glass pack and Magna flow exhaust

Canon Intake

Vacuum plugged

Weber DCOE 40-18

1975 280z distributor

3 ohm Petronix flamethrower

Taylor 8mm spark plug wires

Edited January 24, 2014 by dawg7
wrong engine pic

[madkaw]

I'm guessing no wideband. What do the plugs look like?

I noticed with my wideband in the conditions you stated the AFRs are rich - as in the engine is trying to burn off the extra fuel from running hard to a fast stop. Maybe a bit rich and she's loading up.

I run Mikunis so they are a different animal

[dawg7]

My engine builder looked at them and told me they looked great. The plugs have that milk chocolate appearance of a good burn.

[240260280z]

Can you compare immediate deceleration vs slow gentle deceleration? This will give us two data points.

When you do an immediate deceleration (called overrun), the pistons are going fast and sucking hard (high vacuum in the manifold).

The only air it can get is what is pulled past the closed throttle plate AND the air + fuel from the idle jet.

If it is fuel starvation, just open up the idle jets a turn or more to give it more fuel then try testing again.

If it is too much fuel, you will have to open the throttle plate stops a bit more then re-jet the idle.

(Thanks Dr 240z for the Passini book!)

[240260280z]

What is your fuel level? (This normally is not affected by forward/backward G's too much but it is another data point).

[240260280z]

Opps I should clarify in post #4 that I meant "idle enrichment screw" in place of "idle jet"

Gee I wish the 1hr edit limit could be extended to 24hr or indefinitely.

[dawg7]

Thanks for the replies. The fuel tank level was 3/4 full when I started driving that day. Ended up with a 1/4 tank and experienced the same issue through out the drive. I'll open up the idle mixture a little bit and see what happens. My hope is the RPM doesn't change too much. I feel good that the throttle plate to the first progression hole is set correctly. I did it with the carbs removed. To me it was hard to get the hole covered correctly by shining the flashlight thru the choke with the carb installed. Thanks again!

[siteunseen]

Gee I wish the 1hr edit limit could be extended to 24hr or indefinitely.

I'd never get any sleep!

Noun 1. anal personality - (psychoanalysis) a personality characterized by meticulous neatness and suspicion and reserve

[240260280z]

Thanks for the data.

For fuel level it would be good to also note to the level in the float bowls. It should be identical for all 3 carbs.

The carbs will work better if you first set them up on a bench but fine tune on the car. O2 sensor and air flow tool are highly recommended.

[dawg7]

Wish I could report that I made good progress but........nope can't say that. I only got a few runs before life got in the way. Didn't get much done at all. This weekend is mine right up to the Sunday at 5pm CST!

[Lazeum]

Are the throttle plate closed completely when you're at idle? Is the linkage ok and free to move within their entire range?

Timing at idle seems very low to me. I'm running 20° with 9.5:1 CR, Rebello 276 cam and same 40DCOE-18. That would change total timing (so be careful with knock at high engine load) but you should try to increase advance at idle with engine running and see if rpm rise. Best timing at idle would be when rpm are the highest.

You've got a big overlap cam, you definitely need higher timing.

Have you checked for leaks? It could also lead to some problem.

Edited February 2, 2014 by Lazeum

[gnosez]

Okay, stupid question(s) time.

1) fuel pressure at both idle and WOT

2) why didn't you go to a larger diameter fuel feed line (say 3/8th)

Okay, not so stupid question time.

1) You have brakes so why are you using the gears to stop you? That's what they're there for.

2) Wouldn't a better set of pads, a full on upgrade to the Toyota calipers or Wilwoods, and yes, I'm going there, a better driver make this mostly go away as an issue?