PRNDL

Here is what happened to me on my first (ever) and last 1/4 mile pass at Etown on Saturday:

Once the starter had me lined up, and backed me up so both lights were on (staging lights?) and then the light turned green (one, one thousand, two one thousand, three onethousand..... ok, lets go - j/k)

First gear felt ok, I was just roaring down the track, but at the top of 2nd gear, maybe around 5200 rpm it felt like my foot had been removed from the gas, even tho it was still on the floor. My reaction then was to actually remove my foot from the gas, then put it back down again, thought about pulling over, coasting the rest of the way.... shifted into 3rd, engine picked up again, but by that point I had been screwing around for about 3 seconds.
It is irrelevant, but I know you want to know: 16.3 secs, 83 mph !!

Seems like a fuel delivery problem, right? I have the original L36 bigblock with a new Edlebrock qjet (1901) carb. The fuel pump is 2 yrs old, Zip's $29 replacement mechanical fuel pump. I have an inline cheapo $2.99 purolator fuel filter and no fuel return line (it is capped off).

The only thing besides fuel that comes to mind is that I have a pertronix lobe sensor module and only 8 volts at the + side of the coil. Wonder if 12v would make a difference.

Lars definitely got my attention in the thread about adding an electric fuel pump when he mentioned cars "leaning out in second gear at the dragstrip". That sure sounds like what happened to me.... except a little more than just leaning out - more like running out of fuel.

This happened to me once on a freeway on-ramp, but quite honestly I rarely get a chance to go full throttle from a standstill thru the top of 2nd gear on the street. I may have to return to the strip for test & tune.

So if it is indeed fuel flow, any way to improve short of installing an electric fuel pump (and replumbing the fuel return line) ? Could a better flowing inline fuel filter help? A higher output aftermarket mechanical fuel pump? I still plan to replace the fuel line and fuel filter with an all-steel line per lars' instructions.

Car runs fine in normal driving and occasional short bursts of acceleration but it would be nice to be able to go full blast for 1/4 mile and get into those high 14s that I expect this car and driver are capable of! :)
Comments appreciated. MJ

lars

MN -
You got the classic fuel starvation thing going there - exactly what I was referring to in the post you mentioned. I can't tell you how often I see this happen, and the cure is installing the booster pump back at the tank (after you'rve verified that your fuel filter is fresh). It is so common of a problem that I do it on all of the performance cars I build anymore. It doesn't take much of a pump to cure it - the little Purolator electric boost pump available for about $35 from Checker is all it takes.

73Ken73

There is really no reason that your stock fuel pump and fuel lines shouldn't be able to take you down the track. GM didn't design the Vettes to die half way down the drag strip. I would get a higher pressure mechanical fuel pump and add a fuel regulator. I have an LT-1 fuel pump and I use a regulator to knock the pressure down to 6.5 psi. My engine will crank right into the rev limiter. I have heard bad stories about the pertronix ignition systems lately.

SmokedTires

I had a similar situation happen to me when I first got the motor into my car. Normal driving it ran great, but when I put my foot into it, it would fall onto it's face, nothing more than an idle for a few seconds, then would run fine again. I figured it must be the fuel pump, ordered a new chrome Holley mech., crawled under the car and noticed that one of the rubber fuel hoses on the pump was kinked a bit :bb . I straightened the hose, took it for a drive and problem was gone :) . Went home and put my pretty new chrome pump on anyways :D.

lars

MN -
I just caught the fact that you're running a PerTronix with 8 volts to the coil. This won't work. You have to run a full 12 volts when running the PerTronix in order to get any reasonable performance out of it. Voltage in the 7-8 volt range will cause severe timing scatter at elevated rpm. But my humble opinion is that you ought to just go back to a good ol' set of points....
As stated above, check your fuel suction line out for kinks: it's quite common to get a kink in the line right where the rubber line comes off the steel frame rail line and makes the 180-degree turn to go into the fuel pump. A kinked rubber line will cause your problem.


[Modified by lars, 12:14 PM 11/13/2001]

Turbo-Jet

hey mark, you should look into a dyno session after you think you've solved the voltage problem. the A/F readings should tell you all you need to know, plus the dynographs are cool to have. i wish there was a good dyno shop in my area.

PRNDL

Thanks for the replies guys! I now have a few things to check first. I would really prefer not to install an electric booster pump; and I was thinking along the same lines - that GM factory configuration shouldn't do this, so why now? It may take a while tho to test and tune. I would hate to explain to the police (State police most likely on I-78) "Well officer, I was just trying to see if I fixed my fuel starvation issue..... :( "

MJ

JSB69

Those who suggest that the stock set-up should work are absolutely correct. Find and fix the problem, don't put a patch over it. Don't forget to also check the carb float level and that the vent in the gas cap is OK.

Flareside

Mark, my L36 has no problem revving to redline with a completely stock fuel system, so the design of the system isn't your problem. Maybe you've got old, clogged steel fuel lines? I'm still running the stock return line as well.

I'll second Lars' recommendation for points! If you need a hotter spark, add an MSD box or something similar to augment the stock ignition. I just don't get the attraction to Pertonix. Added complication with very little benefit (and a whole lot of debatable negative issues!) I put a new set of Accel points in over a year ago, and they have never even needed an adjustment since. (Every time I check the dwell, I'm dissapointed because I don't get to tinker with them :))

-Joe



[Modified by Flareside, 5:26 PM 11/13/2001]

Chuck Gongloff

My 69 L68...an L36 with 3 x 2 carb set up will pull right to the redline without problems. It's an automatic THM 400 with a completely stock fuel delivery system. My wife just ran a 13.75 ET at Cecil County Dragway last month. Best I could do was a 13.82. Something else is wrong...maybe the Pertronix...maybe bad fuel pump, lines, etc., as others have suggested. Chuck

lars

I agree with the comments here that a stock fuel system, in good condition with a stock mechanical pump, is adequate to run a stock or near-stock car with no high-rpm lean-out problems. The problems I encounter on occasion are not caused by the fuel system itself, but with the fuel currently being used in these old fuel systems.

The stock systems in our pre-'82 Vettes use a mechanical pump on the engine to create a low pressure area up front to draw the fuel out of the tank, up the fuel line, and to the pump. In most cases this is not a problem. However, many areas of the country have gone to oxygenated fuels where MTBE or Ethanol is used as an additive in the fuel. Problem is, these additives have a higher vapor pressure (they boil at a higher pressure) than straight gasoline, and depending on how much of the additive is used and the exact formula, the suction created on the fuel line between the tank and the pump can create vapors at the pump inlet - classic vapor lock under high-flow conditions. The instant the pump gets a gulp of vapor on the inlet side, fuel outlet pressure drops to zero and the car will fall on its face no matter how good the pump is, and no matter how perfect the fuel line and feed system is. I've seen it happen repeatedly up here at our 6000' altitude where we're forced to use these oxygenated fuels. A small boost pump at the tank to keep the fuel line under pressure solves this problem.

The boost pump should not be used as a Band-Aid to mask other system problems. It is imperative that all aspects of the fuel supply system be in good condition, with no kinked hoses, blocked vents, or plugged filters. But depending on the fuel you're running and other conditions, a boost pump may be needed to eliminate the fuel starvation problems caused by high fuel vapor pressure.

Flareside

Lars, I think you've hit on something. Mark has his stock fuel return line disconnected, and I believe it's main function is to reduce the possibility of vapor lock. Mark, maybe you need to reconnect that line after all?

The funtion of the return line is a mystery to me. Why would a Qjet car need it, but not a Holley car? Maybe GM knows something we don't :confused:

-Joe


[Modified by Flareside, 5:32 PM 11/13/2001]

Barry's70LT1

Hi
In the thread about adding an electric fuel pump, I replied that I had
"capped off" my return line and added an electric fuel pump.

This did not work !!!!
I got the exact same symptions. At top end of 2nd, it would fall over.
It never happened until the engine was quite warm.
It was caused by the return line being capped.

I opened it and I never had the problem again.

Barry

lars

Your stock setup uses an in-line filter with a fuel return line coming out the top. If you'd like to avoid the expense of the return-line filter yet maintain the return-line feature, you can install a 1973 fuel pump. This pump has the return line outlet right on the pump, so you can run a short piece of rubber hose from the pump over to the 1/4" return line on your frame rail. The system will function as designed, and you won't need to use the special filter.

spokewrench

Hey MN Jack,
I will be at E-town next weekend for my first trip down the quarter mile, I d'ont have much experience with the trees but everyone I talk to says to leave the staging just when the 3rd amber lights, you mentioned that you left on the green and then waited a few more seconds. if this is what you meant then you lost a lot of time at the start. The theory is that by the time you react and your car starts moving that the light is already green. Like I said I d'ont have much experience but you may check some bracket racing web sites.
I also had a similar problem when I would hit 5000 rpms the car felt like it ran
of fuel, thought it was the fuel pump, so I replaced it with a Holley, that was"nt it, anyway it turned out to be my rebuilt points dist. I installed a stock G.M. HEi from a junkyard and it revs to 6000 no problem! I did have to buy a elec. tach though, however Eklers has a HEI w tach drive for about $400.
Hope to see you running next weekend, also I heard that vapor lock only occurs @ idle when the car does'nt move.

SPOKE-

Gordonm

Hey spokewrench, the clock does not start until the car starts to move s oyour time has no effect on your reaction time. The reaction time is measured from the time the light turns green until your tires break the beams of light. Thisis when the clock or elapsed time starts. Yo ucould sit on the line for 2 seconds and not start the clocks. I'd love to see your car run but last weekend I broke the rear at Etown. Might have to take a ride just to see everybody run.
I agree with every one else here. I told you at the race I thought you had fuel problems. Bud Snyder had the same problem and when he got home he found some junk on his needle and seat of his Holley. He cleaned it out and it ran fine after that. I never did like the pertronix. I had it on mine for a short while. I have a Crane HI6 that is triggered by a set of heavy duty ponts and I don't miss a beat to 6500+ rpm.

PRNDL

The description of my start was a little bit of a joke. The starter did have to back me up first to get both lights lit, but I tried to launch on the final yellow; didnt wait for the green. Nevertheless, my reaction time was about .89 secs !! lots of room for improvement.

I don't see why the lack of a fuel return line would cause fuel starvation. I also don't understand why Qjets need the fuel return line but holleys dont. I have been given 2 explanations for the fuel return line: 1. The mechanical fuel pump flows more fuel than the carb needs AT IDLE, and the fuel return line really only comes into play at idle. and 2. i cant recall what the other explantion was, but it was different from the first. Maybe it was to protect the fuel pump from pressure buildup. but.... if Barry cured the exact same problem by reconnecting the fuel return line I will have to consider this.

First thing I am gonna do is check the fuel line for kinks/crimps. Am I supposed to have an "S" shaped hose connected to the pump? I have a feeling there might be a problem there. I can easily go back to points. I will try putting 12v on it first.

Test & tune at Island dragway sunday.... anybody? :) MJ




[Modified by MNJack, 9:02 PM 11/13/2001]

Barry's70LT1

Hi MNJack...

Here is what I understand the problem to be:
First... Q-Jet vs Holley -- The bowl (gas reserve in the carb.) in a Q-jet is
very small compared to a Holley.

When the engine is at idle or very low rpm, very little fuel is being used.
This lets the "waiting" fuel in the fuel pump/fuel line to get quite hot.
In fact as Lars has stated, fuel can boil quite easily. Boiling caused bubbles.

Now, when you get to the line, and blast thru first gear, your burning the
reserve fuel from the carb first. Carbs feed fuel from the bottom of the bowl.
(Cooler fuel)..
As this is happening, fresh fuel from the pump/line enters the carb (top). This fuel, if hot enough, has been boiling, and has a lot of bubbles and air.
Since the carb. now has a mixture of air an fuel, there is not enough new
fuel entering the carb. to satisfy wide open throttle.
This all seems to catch up in second gear.

For the lack of a better description, it is fuel starvation.

When the return line is connected, cool fuel is constantly being circulated
from the tank. This helps prevent the fuel from getting hot and boiling.

Holleys don't suffer from this mainly because of their large fuel reserve in the carb. Just a guess, a Holley has approx. 6 times the reserve.
If using the bowl sizes that come on a LT-1..

When I had the problem, I had a Q-jet installed at the time.

As what I said above, it really ends up being a starvation problem.
So,.. any starvation of gas will cause the same symptom.

Only difference with this is you only seem to get it in second gear.
After the hot fuel is used, fresh/cooler fuel has made it's way to the carb.
Other causes like, bad pump, kinked line, partially blocked filter will
usually cause problems at the top end of each gear, not just second.

Hope this helps some.
It's very difficult from your end, when you get a dozen people telling you
what's wrong with your car and we haven't even seen it.
Keep it simple, check the obvious things first.

Good luck.
Barry

lou

Some other things to think of is to make sure your float level is set correctly. If It's to low, your already behind the fuel curve at the start and quads do have a small bowl. Also my brother had the same problem you described last year. We check all the things mentioned above, but nothing worked, finally we check the filter sock on the pick up tube in the fuel tank. Bingo! Not glogged enough for daily driving, but at way out throttle through second gear, it would run out of gas and fall on its face. Changed sock, problem solved. The best way to check if its gas ralated is to duplicate the problem, stop and shut the car of emmediatey. Check the float bowl. If the flaot bowl is emty(As in my brothers case) or low then you know it's a fuel delivery problem. Just a thought. lou