Webers bogging on transition, how do I fix?

[cstreit]

Just took the racecar for it's inaugural drive and 10 mile initial break in drive... (More later)

I thought I tuned the carbs well, but whenever I punch the throttle the car bogs for a second (maybe half a second) and then takes off.

Any ideas what I need to do to fix this?

THanks!

finally got one oil leak, top breather cover, no big deal...

[yelcab1]

Is it too lean? did it pop on deceleration? Else, the accelerator pump maybe giving up for ghost.

[obrut]

Hi Chris

Tuning issues aside, here is a couple of things to check:

1. Do you have the adjustable linkages for the acc pumps? If you do, make sure they are not set too aggressive. My car intially had fixed linkages which were dumping too much fuel upon throttle opening and bogging things down.

2. Do you have the long style air velocity stacks - I also had bad flat spotting with the short style.

Fixing the 2 issues above greatly improved driveability but I cannot completely remove the 3500-4000 ish flat spot in my motor (2.7RS spec w/ carbs) under full throttle.

Good luck - Ryan

[cstreit]

The accelerator pumps worked fine before I rebuilt the motor.

Is this generally caused by too rich during transition, or to lean?

[911pcars]

Chris,
Let's make sure you have a squirt upon acceleration. With the engine OFF, peer down each carb throat, then open the throttle. Each throat should get a squirt from the accelerator pump circuit. Too much, too little and you will experience some degree of bog.

In addition, check the fuel level with the screw-in float gauge. If it's too low, there won't be enough venturi signal to pull fuel from the discharge nozzle and the engine will be too lean - momentarily.

How does it shift?

Sherwood

[Chuck Moreland]

Do you have the graduated vile for measuring accelerator pump volume? What is it reading? If you don't have one, it would be a good investment.

Didn't you increase displacement with the rebuild? Did you also change chokes and jets to match?

[cstreit]

Chuck, .5-.6cc.

No displacement increase, just a slight bump in compression and a bigger cam. Went to bigger main jets. (from 150 to 165) and slightly bigger idle jets... Air correction jets number went up (which means smaller from what I recall) . THis setup recommended by Richard at PMO.

I read Waynes 101 book on syncronizing and it says to lean the mixture untill speed slows down, then back out slightly. THe Weber book says to richen untill the speed doesn't increase any more.... From what I recall, this is almost 1/2 - 1 turn difference in the mixture. SInce my carbs are big for the motor, and i increase venturi size from 36 to 40 (again per PMO) I think maybe my draw isn't as strong now, and the transition might be leaning out. So I'm going to try the Weber tuning method and see if that helps...

[911pcars]

Chris,
40mm venturis. Wow, that's some serious hole in your intake tract. I'm going to press my case there's not enough low speed venturi vacuum to pull fuel out of the discharge tubes. Perhaps the tall secondary venturis might help .... or higher rpms.

Sherwood

[cstreit]

Sherwood, I have the tall secondaries...

[keitho64]

Chris

Are you going to be around this Sunday? I was hoping to use your synchrometer and we car work on your carbs as well. It almost sounds like you have an overly rich mixture off idle. I generally attribute an off idle bog to a rich condition. You may want to back down the main jets depending on when the bog occurs. At what RPM does it bog and how does it pull at the upper rpms? 150-165 is a big jump, you may want to try 155 or 160 mains.

I will email you a Weber tuning guide I got off Pelican. You probably have I, but if not it is a great reference.

[cstreit]

I didn't try all RPM's, but did try it at 2k, 3k, and 4k. It occured at all of them. It seems to occur mainly when I stomp the gas. A smooth application of throttle gives it a nice transition...

[john walker's workshop]

kinda hard to give an opinion without complete jetting specs and engine specs.

[cstreit]

C'mon JW... Just take a shot in the dark... LOL

Okay...

Early spec 3.0L SC motor. tall racing manifolds, Mild Webcam (bigger than 20/21 though) Mahle 10:1 RSR p&c. Power expected from 4000-7000RPM

1 5/8" equal length exhaust with Supertrapps.

46mm webers. 40mm venturis with tall secondaries.
180 air corrections, F3 emulsion tubes, 160 air correction jets, 60 idle jets.

Is this what you need? Need more?

[john walker's workshop]

i assume you meant 160 mains. 46 webers are kinda big for that engine, especially with fairly mild cams. that may be some of your problem. you may be able to overcome the bog at lower rpms with a slightly larger idle jet, although above 2500, it's pretty much out of the system. enlarging the lower transition port may do it, but that would be a last resort, as the tiny repair sleeves are about impossible to find if you have to go back. what's the total advance by the way?

[cstreit]

John, totally a race motor remember...

The cam isn't THAT mild... Webcam 120/104. Just not a full RSR sprint cam...

Advance is 8deg start, 29 deg at 3k RPM, 26 deg at 7000. (per Clewett)

Mains are 165. Air correctors 160. (This setup per PMO)

Previous setup had 55 idle, 150 main, 36mm venturis... Was perfect...

[tryan]

try moving the timming up two degrees and see if the bog changes.

how is your steady state/ no acceleration? if it's good i would cut back on the accel pump volume. just a little and note changes.

do you think you ought to break the motor in a little bit. 100 miles or so?

[Wil Ferch]

Guys:
Besides all this good advice....did anyone mention Bruce Anderson's suggested mods for just this sort of behaviour ?...something about drilling passages to improve transition response? It's in both of his Performance books...maybe someone more knowledgable can now chime in? Let's not overlook something that may be more basic.
---Wil Ferch

[cstreit]

Tryan, well I'm actually probably running not nearly that advance now that I think about it... I set it somewhat low because I knew I wouldn't be able to worry about it just yet. I'll measure actuals later, as I've got the E-motive advance curves being recorded in my data logger...

The carbs worked fine untill I changed them and the motor of course... Which leads me to believe that it's not their basic function that's incorrect, it's the setup...

[tryan]

i had trouble moving a holley from one 72 351w to a 77 351w. same bog. it was an excellent excuse to head to the lake every night and play with the carb. momma did not like it one bit.

the air tubes mix air and fuel to get a better transistion. if steady state is good and john 'the man' walker says thats the proper amount for the accel. pump, i would just swap to the 180 air corrections and see what ya get.

[cstreit]

I'm just trying to bend my mind around this stuff now...

So against the weber book, accel pump volume is slightly low, but not much... Meaning it's slightly leaner in transition

THe main jets are a little big perhaps, meaning it goes rich kinda early...

I've got 180 air correctors lying around which would richen it up during transition right?

Not sure which direction to go here...

I'm going to richen up the idle jets a bit and see that that doesn't help... or make it worse...