new car with carburetor questions, I'm terrible with them (Page 1/3)
GreenPlatypus OCT 17, 02:59 PM
I am not very good with carburetors, in fact this is the first carbed car I have worked on, so explaining carburetors to me is like explaining string theory to a cheesecake factory waitress.

Car bogs heavily or stalls at WOT throttle. Depends how fast you catch it. Drives great everywhere else. If you ease into it like a human being it hauls ass. If you stomp on it, it will bog heavily or just flat out stall. Wife tried a burn out, her first, and this is how we found out about it and now I have to fix it. Haven't driven it too much as I still need to get plates for it.

Engine is an otherwise stock 96 LT1 350 2 bolt from a Caprice lumped into an 89 Camaro with 700r4 trans - sold her 1990 LT4 Camaro, for this It has a 650 Edelbrock carb with vacuum secondaries and electric choke. It idles good, well awesome, I have 20 inHg at idle, idles a touch rich, and runs a bit richer off idle. Timing is set to 20ish advance at idle, 850 rpm. fuel pressure is 5.5 psi at idle. Haven't checked timing at 3000 yet, didn't think there was an issue till now, but will do it tomorrow. Need to check off idle fuel pressure as well. When she got the car it barely ran, I put on the GMPP LT1 4 barrel intake and a new dizzy and that sorted most of it out, just have this small issue with the carb.

I just need a place to start, I know there isn't many adjustments to an edelbrock, but it just runs soooo good with it otherwise i'd like to keep it. Everyone here tells me to drop a Holley on it. Don't have money for that stupid camaros

I'd ask on thirdgen but don't have an account, yet. besides, I trust this place long before that place.
squisher86SE OCT 17, 03:30 PM
Too rich, you've said as much already that it runs rich. Depending on the model it likely has an accelerator pump that squirts a bit more fuel than the main jet would normally do when you trounce the throttle, and since it sounds like you're borderline rich already it probably just pushes the mixture the rest of the way over into being just way too rich. If you put the model of carb you have I can look it up to see if it even has an accelerator pump (probably does).

It's been FOREVER since I had a carb'ed car/truck, but the theory is the same between cars and motorcycles.

It sounds like you have jets that are too big - if it was a bike I would say your main jet in particular needs to be dropped down a size or two. Based on being rich at idle indicates your pilot jets (if there are any, my bikes have always had them) are too big.

EDIT:
Since you said 650CFM electric choke w/ vacuum secondary I'm going to guess that it's an Edelbrock 1806 AVS series carb. You most certainly can buy different sized fuel metering jets, and in reality that's the adjustable part to a carburetor. You'll want to find out what size you have now so you know what to order, unless you end up finding and ordering an assortment.

Once you have other jet sizes to try it's down to making an SWAG (Scientific Wild-A$$ guess) to how much of a change to try, and see how it runs. Smaller increments are going to be safer. Learn how to "read" your spark plugs. You want a tan/brown color on the elctrode. Black = too rich, grey/white=too lean.

[This message has been edited by squisher86SE (edited 10-17-2012).]

RWDPLZ OCT 17, 03:57 PM
If you can hook up and read a vacuum gauge, you can tune a carburetor. Changing the jets on an Edelbrock is kind of a PITA compared to a Holley, but not too difficult.



Check the float levels, and try 2 sizes smaller secondary jets
carnut122 OCT 17, 10:17 PM
I'll play the other side of the fence. When was the last time the fuel filter was changed?
squisher86SE OCT 18, 09:36 AM

quote
Originally posted by carnut122:

I'll play the other side of the fence. When was the last time the fuel filter was changed?



Good thought - though wouldn't that starve it out after a couple seconds of WOT instead of stumbling as soon as you mash it?
Granted it is a cheap thing to check.

Also: I'm not sure how hard it is to pull a sparkplug, but you might want to try (in a safe location and at low speed of course) mashing the throttle (so that it stumbles) and stopping and turning off the engine ASAP and then pulling a plug - (MC circles call it a throttle chop or somesuch) so you can get a glimpse of what is going on in the combustion chamber at the time of the symptoms. If it's rich enough for it to stumble and choke out the spark plug is likely to even be wet!

Another thought just occurred to me: Is the choke fully opening after startup? (I'm assuming yes since it runs reasonably well at all).
weaselbeak OCT 18, 06:37 PM
Or you have an accelerator pump that is weak. That will bog it immediately but it should recover. Sounds to me, tho, like a problem with the secondaries.
carnut122 OCT 18, 08:41 PM

quote
Originally posted by squisher86SE:


Good thought - though wouldn't that starve it out after a couple seconds of WOT instead of stumbling as soon as you mash it?
Granted it is a cheap thing to check.
).



It would run OK under little or normal demand for gas, but once the throttle plates open and the fuel supply doesn't match the air supply, the stumble/bog would be pretty immediate (assuming there's no reserve in the bowl). The accelerator pump is also a good place to look. With the engine off and the top of the carb exposed, open the choke plate and pull on the throttle to see if the accelerator pump is squirting a good stream of gas down the throat.
rogergarrison OCT 19, 06:59 PM
I love Edlebrocks myself...and hate Hollys. I never have trouble with them. Like said, all you need is a vacuum gauge keep adjusting both mixture screws till you have the highest vacuum reading. Some things you can do, is go to smaller/ larger jets, change the stroke for the accelerator pump shorter/ longer, and make sure the float level is correct. A lot of people just throw on the biggest carb they can and think it will have more power. Sometimes smaller is better. Im running a 600 cfm Edlebrock on my 413 mopar.

I also think your running a bit too much advance that may be your problem. Is it pinging at all ? It should be 10-12 degrees at idle with vacuum advance off. Should also be around 35 degrees with vacuum, above 2500-3000 rpm.
GreenPlatypus OCT 20, 10:02 AM
Ok here is what I have done.

1. Tossed out my vacuum gauge and tachometer and played it by ear/nose. Leaned out my idle/air screws. A lot. I've got maybe a 1/8 turn before they bottom out, one side is a lighter harder to turn than the other - may have a bent rod/screw, will pull them out later and check them. Still seems a touch rich to me, especially off idle, but it doesn't blow as much smoke while driving around the lot. My eyes no longer singe like they did on first start up The idle quality has suffered and it takes a little longer to start. It runs around 15inHg on the vacuum gauge, not the 20-22 I had before. Plugs look clean but it takes a bit to get some of them out. Didn't notice any wetness and will look again.
2. Checked my timing again, at 850 rpm idle speed with full vacuum advance it is at 20 degrees. At 3000 It is around 40ish. I am guestimating as it is awkward reading the timing while controlling the throttle, when I get a bit of help I'll get some better numbers. I am thinking of backing off the timing a bit, maybe 15-17 at idle, may have too much advance in it. These base numbers where the popular consensus for a Z28 LT1, not a Caprice, so I *have* a less radical cam and don't need as much timing. Thinking out loud here.
3. Was playing with the accelerator pump rod position, no change. Secondaries and accelerator pump look like they are working. Choke works, I think, not sure what to look for really, but if I had an issue with it I would probably know about it (need to readjust my cold start screw, backed it off while trying to get the idle speed down)
4. bumped my idle speed up a bit, it seemed to help, but eventually it went back to bogging real bad. Didn't seem to stall completely so I have temporarily left it there. will set it back when i can check my timing a little more accurately.

If I blip the throttle a couple times, i don't have this issue. Until it is in gear. Anything over 30-40% throttle and it just doesn't want to go. May also check my fuel pressure with a proper gauge, not the one on the fuel line. Maybe i need more flow, not pressure. It is the stock 305 tbi fuel pump afterall.

So I "believe" from guees work I may need to adjust my secondary jets, and will definitely need to fiddle with my timing. Apparently most carb issues are due to incorrect timing settings. Sounds logical to me.
Jake_Dragon OCT 20, 09:09 PM
What is your fuel line pressure?