alright thats it this sucks for sure now. I've been trying to get rid of a full throttle bpg for a while now . eeverything pointed to the carb so i had it re-built replaced tps and set it up using the tech articles. Still have same problem full throttle bog doesn't do it at say 75% just from there up. Fuels pressure it idle is about 4 psi when held at 2500 rpm or so its 5-51\2.BTW i know this should make a difference but this tarted when i went to a 4th gen auburn LSD (just coincidence i think) Please help i'm not quite ready to throw in the towel yet but getting close Could it be ignition, vacuum leak, i dont know?! also no bog just normal acceleration when i floor the throtle with the car parked only when under load. Thanks in advance i hope someone has an idea for me

 

what about your float level is it where it should be

4psi sure seems low

normal is usually around 7-9psi with carb

was your car originally carb ?

it has to be something small thats wrong


have you made sure that your timing is advancing correctly and not too little or too much ?

any pinging ?


good luck

 

Hi thanks for replying. The guy I had rebuild the carb is an instructor in our locla Lindsay Hopkins Technical Institute. He said the carb was pretty gunked up and that the float was way off he set it up on the bench as per spec. I agree that 4 psi is low and iwill replace the pump soon but when i rev the motor the pressure goes to 5-5.5 so i dont think thats the problem either. According to the service Chevy service manual for the car it should be 5.5-6.5 Yes the car was an original LG4 However the engine was rebuilt before i bough it and the car on it is from a 87 305 and has CH secondary rods. I have no pinging and the motor ran fine before this happened I dont think my timing would be off since i have touched it since i replaced the intake manifold voer 2 years ago and it was fine .

 

It's not a vacuum leak. There's no vacuum at WOT, so a vacuum leak doesn't affect it much, if at all. Vacuum leaks cause extreme problems at idle and cruise, when a little bitty leak under high vacuum lets in enough air to upset the fuel metering, because it's significant compared to the engine's air demand; but at WOT, with 4 sq in of throttle plates wide open, a little crack somewhere isn't going to cause a problem.

Your fuel pressure is fine. You're chasing a red herring there. Carbs don't work off of fuel PRESSURE anyway, they work off of fuel DELIVERY. 1/2 psi would be perfectly fine, if it kept the bowl full. In any case, a fuel delivery problem usually shows up as it runs fine through 1st and 2nd gear, then falls on its face in 3rd; or some pattern like that. In other words, you floor it, there's plenty of fuel in the bowl, life is good for a while; the engine drains the bowl down faster than the system can replenish it; eventually it runs dry; that's when it screws up. NOT just when you give it more gas.

What is your AV tension set to? Try increasing it in 1/8 turn increments.

See my signature for helpful troubleshooting advice. What it means is, don't go inventing wild theories to try to explain things; stick with the simple stuff.

 

I've set the a/v tension between were the tech article says and the factory spec but remeber i hadn't touched the carb prior to this happening so i havd not modified the a/v tension. i know this sounds stupid but could the rear end in some way be the cause ? as this is what i chaned when this all began (i went from the open diff with powertrax to an auburn unit from a 1994 z28 also replaced the axles to 28 spline from the 26ers i had before) maybe be under full load the diff is locking up or something? stupid i know but its the only thing that i did when this happened

 

No I don't think it's the rear end. Grasping at wild straws like that is right on up there with blaming it on alien abduction or a secret FBI thought-transmitting device implanted in your brain or something.

Again, see my signature for helpful advice.

What's the air valve tension? Set it to about ¾ turn and see if it acts any different.

 

4-5psi is right on the money. If you have the ability to check your fuel pressure as you're driving, i'd find out what it looks like during the bog. Mine did that when the pump was dying, but it showed up as .5psi. A new pump fixed that.

I'd be inclined to agree with AV tension. If the lock screw wasn't tightened up and *something* nudged it, then you could be anywhere.

 

I'm pretty sure i have it setup correctly as noted in the tech article i dont want to warp the spring by tightening it too much. Sofa kingdoms mention of there being enough fuel in the bowl to keep the engine running and operating even if there is not enough flow to keep it full seems to make sense to me (of course until the fuel is gone)BTW if i simply rev the car from under the hood it doesnt bog or hick up or anything just sounds nice and strong. Also if i observe the a/v it only seems to open about 20 or so degrees but i guess this would change under load.

 

Yea, the secondary air valve will only open under load.
Check it for tension, try 1 turn if you want, you won't break it with that.

If there's no change, then run it up on the highway until it bogs, then flick off the keys and coast to the side of the road and pop off the top of the carb and check the level of the fuel in the bowl. That's a cheeseball way to check if your fuel pump and float level are good.
I'd check pressure as you're driving before that. Heck, i'd do the A/V tension dead first.

 

heres an interesting observation: i've got my fuel gauge inline tothe carb inlet am i supposed to see the needle oscillate from say 1-8 psi and just average it out?. say this today and was wondering because sometimes it holds steady at 4 psi or so and other times it fluctauates as i noted above. could this be from evaporated fuel in the line and just takes awhile to get cleared out or is this a sign of the pump going bad?

 

That depends, WHAT IS YOUR A/V TENSION AT? WHY DON'T YOU SET IT TO 1 FULL TURN AND SEE HOW IT RUNS? You seem to be abnormally set against checking the easiest part.

Well, does it stay that way as you're driving it?
It shouldn't oscillate that much. Usually between 4-6psi, and hold steady at around 5-7psi as you're giving it throttle. If it dips to 1psi its probably on it's way out. It's $24 pump, that's easy to replace...

 

May be over stating the obvious but sofakingdom is right about the fuel delivery the fuel VOLUME is whats important. With all else being checked and assumed good, replace the fuel filter. I have had more people than I care to count spend boocoo bucks on repairs for the same type of problem, and then fixed it with a $3 part.
A partially plugged fuel filter can cause your pressure guage to read steady, the blockage in the filter will dampen the normal surges the pump produces.

 

hi! well i'mnot unwilling to check the a/v tension as stated earlier i started by adjusting the tension as per the tech article in the tech section of this site.i have since been adjusting it till i'm at about 1 full turn. as far as fuel i went ahead and replaced the pump anyways $15 Carter unit psi still reads the same i used the method in the service manul for my car and it said to remove the line from the carb and install a gauge well since mine is inline i simply removed it from the carb and clamped the end and testeed it still oscillatred but at least it's between 3 and 10 now I dont care if it didnt do anything it was the original pump and the hoses were very old and weak cause i squeeze them and they staay flat for a long time.The fuel filter is clean i removed it and checked its clean only about one year old as well.
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as a follow up: the guy who redid my carb said to adjust the dwell to spec. how do i do this i know i've got to set it to 6 cyl range and connect it to the mc solenoid and the bare green connector by the blower but what do i move to adjust it?

 

i forget exactly how is checked....it involves a dwell meter and turning over the engine... i like to set it at 28 though... dwell is the measurement of degrees the cam rotates from when the points close to when they open again... that is old school why would you run that type of ignition and that would bring about a whole new list of things to check

 

well just for the hell of it checked my timing and it was set to about 12 degrees so i set it to 6 as per spec and the bog ismore pronounced than before

 

Think about what you said yourself the hoses were old and weak, hoses with liquid in them in most cases break down from the inside, when they do where does the materiel go? into a filter. Just because a filter LOOKS good doesn't mean it is. Filters are measured in microns which is the size of particles that can or cannot pass through. At WOT there is a high demand for fuel and the smallest restriction will have a huge effect. How did you check the filter. did you have it FLOW Tested or what most people do? just blow air through it.
After everything you have done with no good results I found that sometimes it pays to verify the basics and not try to over think things.


I had a 70 Camaro which had stalling at half to WOT, it would flat out die and not restart till I pulled the filter and the top of the carb and cleaned it full of dirt. I would have to do that at least twice a week until I put a good inline filter in, then I only had to clean the filter every few months. I found out after I got tired of messing with it and sold it that the guy that bought it replaced all the rubber hoses and never had a problem since.

 

fuel filter replaced no change

 

If your carb is starving for fuel it will start out strong, then fall off. You can rule out the AV pretty much by road testing it under load. Find a good hill, put it in second and give it enough throttle to keep the revs high under load. If it doesn't fall off you're probably getting good fuel flow. AV bog will cause it to immediately stumble when the throttle is mashed then pick up and go. Kind of opposites, really.

Dwell for your car is the measure of the mixture control solenoid cycles within the carb. The MC cycles at different rates (usually from 10-50 degrees on a dwell meter set at six cylinder) to adjust mixture for the primaries. The Idle Air Bleed set screw in the center of the airhorn (if the factory cover is not on it) is adjusted to vary the amount of air allowed in, more air=leaner mixture. The IAB is normally turned in small, 1/8 turn increments until the dwell reading 'hovers' around 30 degrees at idle in park. No adjustments should be done to the IAB until it is verified that the MC dwell is responding to O2 readings, ie the dwell is constantly 'hunting' or wavering +/- 5 degrees or so and responds to choking the airhorn (with a rag) by increasing dwell (leaning mixture-less fuel). For more info see a recent post started by Nate86 on rebuilding the q-jet.

Dwell adjustment should have little to no affect on your bog. It only affects the primaries and only at partial throttle.

I see where dropping 6 degrees timing had a pronounced impact. I would also verify with a gun that you're getting timing advance. With EST connected you should see 20+ degrees of timing and it should increase as you rev. Without marks you may have to estimate this, but verifying that it is advancing in your case is more important than precisely how much.

 

Ok i'll check the timng as you siad later today or tomorrow(daily driver) also will check the dwell as stated btw : this is really strange but for the hell of it i replaced the pcv valve and so far it doesn't seem to bag as badly

 

Well today was my first experience at running my car on a dyno. This is a lot more interesting and exciting than i thought.I went to a local shop that has an all wheel chassis dyno and for $50 i got three runs. First run was completely stock running thr car as i would when driving down the road (except for an open hood) I secretly was waiting to get about 100 or so hp ! Results:

139 peak hp at 3800 rpm
217 lb/ft at 3150 rpm

Higher than i had expected as i know these LG4's are 145 / 240 stock (measured at the flywheel no accessories etc.) Now for the second pull i removed the air filter lid only and ran it again.Results:

177 peak hp at 4150 rpm
249 lb/ft at 3100 rpm

Wow almost 40 hp from removing the lid alone was incredible I never could have thought that the stock air filter was that restrictive. So for the nest pull I put the lid back on but repositioned the air snorkle hose so that its drawing air from a more "airy" location just sitting on top of carbon canister Results:

150 peak hp at 3700 rpm
240 lb/ft at 3000 rpm

Down 27 from no lid but up 13 from the first run. Well i'm going to clean my K&N air filter and put together a dual snorkle system as detailed in the tech articles here.Also i noticed how rich i was running with the air cleaner on as oppsed to a very nice mixture with the lid off. I will post the dyno results as soon as i can scan and post them here. This is going to get expensive/fun as i change things and run the dyno pulls to verify the changes were positive or negative.

BTW I'm running the following: Rebuilt LG4 (20k miles or so) Performer intake stock q-jet ( with alittle bog still) Summit 11102 headers and flowtech 2 1/2 y-pipe to a flowmaster dual outlet out back (single pipe from y to muffler) as mentioned the k&n air filter fresh set of plugs old wires(replacing soon) stock ccc hei lots of other stuff but not really relavent to this topic. Stay on the look out for the dyno sheets!!!

 

Good deal and interesting numbers, imagine if that was a firebird and got even less air to the tiny snorkle opening.... Almost certainly your 'bog' is in the secondary circuit of the carb either choke pull off or air valve tension. You've verified that it's opening and you're not starving for fuel.

 

Ok as i said earlier i went out and got a cleaming kit for my K&N filter cleaned it last night,let it dry and oiled it this morning. I took my air cleaner assembly and removed the littlte tab on the inside that directs/blocks the flow out of the snorkle and into the filter also removed the thermac valve completely and sanded and rattle canned the whole thing black. This afternoon i put the K&N flter on with the lid upside down so as to let a little more air into that housing till i get the dual intake done went out and drove it. IT FELL ON IT'S FACE.put the lid on as normal same thing removed the cleaner and did another drive wow full power no bog just go! and the rpm's climb much faster than before. After this i went to autozone and got two things a regular paper STP replacement filter and an Edelbrock 1002 air cleaner> first the STP replacement noticeable improvement over my K&N cleaned and oiled as per instructions and was worse flowing than the regular paper filter. Next i put on the Edelbrock 1002 had to get a slightly longer screw stud but easy 1 minute install otherwise, Test drive same power and responce as having no cleaner at all I know some will say that i'll pick up hot air from the engine with it but i only drive relatively short distances and it was only 21.99 so it will suffice till i do my dual setup.

 

well its fixed now and verified my whole problem was my air filter it no longer bogs, has plenty of power and it cost a grand total of $4.50 for the filter who would havethought that that was the cause mf my headache?