Ok.. My frustration level is through the roof at this point (Mainly because i am supposed to go to the track tonight).

Anyway, longstory short... Had purchased a new intake (Edlebrock Performer RPM) and found out that my carb was warped (base plate), so i bit the bullet and purchased a Holley 750 double pumper.

So, now i am in the tuning stage and i am taking everyone's advice from my buddy who has a mid 8 sec AMX to the local speed shop owner who run's high 7's in a station wagon.. Not that there is any relevance but i figured they have to have some good idea being their car's run pretty quick and consistant.

The carb is pretty much stock and i have the metering screw's out about 1 1/4 turn's each and the car idle's great but i cannot get the car to stop bogging off the line. I tried increasing the shooter size for the main accel pump from 28 to 31 and it seemed to help a little but i cannot get rid of the bog.

The fuel level is right on, the fuel pressure is at 6psi, the accel pump's are set at .010 and the timing it at 36 total.

the specs in my sig are pretty much right on except for the new addition of the intake and carb but it is just driving me nut's at this point, the two guy's i talk to seem to think the stock jet size's should be fine for now and they are not creating the bogging. others seem to think i need to bring the shooter size down on the accel pump. at this point i do not have a clue.

Suggestions needed... what should i look at./change.

Thanks

John

 

S'up, Olds? I'll give it a crack here.

You say the accel pump is set at .010"? You mean that there is .010" travel left with the throttle wide open, right? There should be ZERO slack when the throttle is at idle- the slightest movement should cause the accelerator pump to start shooting. Make sure this is true of the secondary-side accelerator pump as well. Just measuring clearance sometimes isn't enough. It's better to bottom the pump than to have slack in the pump arm at idle. BTW- can you actually see both the primary and secondary side squirters shoot out fuel as you move the throttle- I know it's basic but if something's blocking the fuel from getting to one of the squirters.....

Stock jetting should be close enough to prevent a bog, I'll agree with that.

When it bogs is it only when you nail it from a stop or just getting into it a little tooling around town?

Is it an immediate bog and then it goes or does it go for an instant, lay down and then go again?

Stock pumps, shooters, etc, are usually plenty close if you have a well matched combo (you do). The high stall converter should do a lot to prevent a bog- it reduces the amount of pump shot required to prevent a bog, so I wouldn't think the stock calibration should be too far out.

 

You have to tune these things in the correct order. #1 float height, 2 powervalve, 3 main metering (jets), 4 get idle screws adjusted, now you can (finally) start to play with your accelerator circuit. Then after all that you can start to play with when your secondaries open DOing this in the inccorect order is how to get yourself mixed up. When you change the powervalve you have to do the steps that come after all over again. The beauty of Holley is the fine tuning although it can get real ugly and tedious sometimes. You have the correct powervalve? (2" below idle vacuum). Also what rpm is it idleing at? With a big cam you sometimes end up with the throttle plates touching into the transfer ports while idling because the engine needs a lot more air than can go past in the idle circuit. This can cause a stumble as well. Does it run better when cool? less bog I mean.
Check out
http://www.bob2000.com/carb.htm
and
http://www.corvetteforum.com/techtip...=108&TopicID=3
for tuning tips.
If it bogs more when hot try lowering your float bowl a little, the gas may be too hot and be boiling (it will surge while cruising if its boiling as well). Good Luck and keep us posted.
CoryM

 

I might also suggest the pump cam. You can change them out to modify the amount of gas you get from a full shot from the pump. Then you use the squirter size to determine how fast the fuel gets put into the carb.

Biger cam, more fuel
bigger squirter size fuel gets dumped in faster (shorter duration of the pump squirt)

 

I have almost the exact same combo(see sig) and have the same problem. I have tried adjusting the float levels and changing the discharge nozzle sizes. Some one told me to decrease the discharge nozzle size and try that. The pump arm should have .015" clearance and that is right from holley. Please let me know if you get this fixed as my car is driven only a few times per year(drag car)

 

When you say "bogging", what exactly is it doing? Falling on it's face and stumbling, or hesitating and then taking off? How are you launching it? Off idle, or power braking it? There are a bunch of different issues involved to try and fix a "bog"
Another thing, you say your old baseplate was warped, what kind of carb did you have on it before? What is different?
How much timing are you running, and what kind of curve?
To start to diagnose, have someone watch your exhaust when you launch, look for black smoke indicating an over rich condition when it bogs, if there is no black smoke, and it is more of a hesitation than a stumble, that would be a lean problem, very easy to do with a double pumper if you are just flooring it off idle. (that is why they recommend a vacuum sec with an auto trans)
It can be dialed in, just needs tuning and TLC! I had a 406 SBC with an 850 DBLPMP that I could stomp off idle without a flutter in my camaro; BUT it is also hard to tune one over the internet!!

 

First off, Thanks for all the reply's..

It happens whether it is hot or cold, seems worse as it warms up. It is Off-Idle, i slam the gas, the car falls on it's face, Pop's through the carb and then takes off like a bat out of hell.

If i load up on the converter a little the problem goes away.

I adjusted the float level's down a little but it still seems to be sputtering when cruising a little, it did help, but it still does it. I do not know how low i can go before it's too low.

I tried having the accel pumps at .010, .015 and right now it is at .000

with the 3,000 stall, does the advance curve really matter? i have the mid and light screw in there right now, so it advance's pretty quick. oh, it is at 36 right now.

I took the car down cecil on friday but the conditions sucked because it was like 100% humidity, but i was able to squeeze a 13.7 out of it, and that is with my other problem (for those of you keeping track) with the hesitation above 4,000 (Some sort of trans heat related issue??)(not related to this issue).

I had the 750 vacuum sec on there but i had nothing but problems with it, it was rebuilt twice and we could never get it to run right, just did not seem that there was enough vacuum to fully open the secondaries. ?? At idle the car has about 5-5 1/2" of vacuum with that cam, i had it as high as 6 1/2" but it was running quite rich. I also have a 2.5" powervalve in there as well. I had a plug but i heard many say that plugs are no good.

 

So does the bog go away if you load up the converter and then stomp on it? At what rpm does this happen 2000,3000,what?

 

I only ran the car a few times... but it seems if i load the converter up to 2k and stomp on it that it is fine, but if i try to load it up around 25-2800 then it still bogs a little out of the hole.

 

When you say pops thru the carb, that tends to indicate a lean condition, and not enough timing. What is your timing at idle? Play with the bushings in the dist to get 18-20 initial, with 36-38 total
Leave your accel pumps at 00, set your floats to where it just trickles out of the site plug when you move the car...
Does it have a power valve in the secondary? If you haven't changed that, I think they come stock with a 6.5, which would explain your rich idle. If you put a plug in the sec, jet it up 8-10 jets sizes to compensate
If it idles at 5.5 vacuum in gear, try a 3.5 or 4.5 to get the enrichment to come in a little quicker, ..
If you can drive the car on the street, cruise down the road at a steady speed in 3rd gear, and just "barely" touch the gas.. if it hesitates, go up 2 steps on the jets on the primaries until it is smooth, this will help the bog also..
On your vac sec carb, you needed to go to a lighter secondary spring, probably a purple, to get the sec to open better..
Sounds like it needs some good tuning!!

 

Thanks, i will definatly give that a try.

I did not think the double pumper had a power valve in the secondaries as well?.. hmm.. should the primaries match the secondaries?..

I agree the car needs some major tuning, but i am learning slowly... I tried the whole book thing but they do not really go into the fine tuning, i know because every car is different it would be impossible anyway.

i'm pretty sure it is at 12 initial right now, if i bump it up to 18 wont i have a hard time starting?

Thanks again for all the help.

 

If it has a power valve in the secondary, they need to match, it pulls out of both bowls at idle
On the timing, subtract your initial from your total to figure what you have in the distributor, yours is 24 deg., which is what the standard bushing in the MSD distributor is. You should have gotten a bag of springs and bushing with the dist., and a chart, you need to change the bushing to one for 18 deg. (18 initial + 18 in dist. = 36 total) I want to say it is black, but don't quote me!

 

If my initial timing is 18, wont i have a really hard time starting this beast?

I have not had a chance to try any of these suggestions as all of my stuff is in my old garage still. that and the weather has been crappy lately and have not had the car down.

 

I have my timing set at 18 initial and 36 total. that works perfect for me. my eng. turns over easily, but I have a late model LT1 starter which I think is a gear reduction (high torque) starter. I dont have any bog, but my carb. is a stage II carb and Im not sure what they did to it exactly. I know i have the holes drilled in the main throttle plates to let more air in so that the idle mixture screws are more responsive.
I think the holleys come w/no sec. main valve. that is why they are jetted bigger in sec. then on the primarys. have you tried to change the jets to see if that helps yet? cant hurt and its easy enough.

chris