Fuel system issues, old mechanical type and Qjet

oldsmobile
cutlass

#1

I have asked this on Oldsmobile specific boards and I haven’t gotten a response yet since those boards are slowing down lately.
I took my 72 Olds for a trip down the 1/8 mile drag strip recently. Now, the times are not great and I believe the problem is fuel related. About 1/2 way through 2nd gear the car feels like it runs out of fuel, RPMs drop off. Let off the gas and then kick again it pulls on through the rest of the track. This motor has 5Kmiles on its rebuild. The carb has about 500 miles since it was last rebuilt following a book from Cliff Roggles (probably spelled that wrong) specifically about Quadrajet carburetors. The one thing I cannot remember is fuel inlet seat size being measured.
The engine is in a completely restored car. New fuel tank, new hard and soft (fuel rated) hoses from tank to carb.

4 areas of concern for this:

  1. fuel pump to weak to provide the flow required due to possible damage from having the fulcrum under pressure against the cam button for about 6 years before the engine was ever started or from just a too low GPH rate for this motor
  2. the inlet diameter on the needle and seat not large enough to support the amount of power this car makes. Some gains over stock but by no means a race car, the Qjet book talks about this being a big restriction on these cars
  3. I run a 71 fuel filter style system. This has a diverter valve for pressure relief in the fuel filter that dumps back to the tank. I am concerned this is dumping too much pressure under load, suggestions have been made to clamp off the return line but I am not sure if it will blow the needle off the seat because of straight pump pressure. I can try this soon
  4. Sock in the tank getting uncovered and just sucking air under acceleration. I have done hard acceleration testing in this car with varying levels of gas and I don’t remember this making a difference. This weekend it did feel like it fell off sooner in RPM on the second run than the first with a better 60 ft time leaning towards a tank pick-up issue possibly.

I want to fix this because it is not a good idea to run a motor out lean under hard acceleration. That is what this car was build to do and needs to be able to do it. Anyone seen and fixed something like this problem?


#2

Good lord, there’s no where near enough information to answer a question like this.
You mention that the engine was built to drag. That needs specifics. Displacement, cam details, head/valve details, ignition system details, stuff like that. And any modification such as a ram air system that would allow more air to be drawn in.

Also, there are a numbber of different size quadrajet carbs, and we’d need more details along with changes made during rebuild. Things like larger jets.

At least with this detail we might be able to estimate the maximum volume of air going in and estimate how much gas needs to be added… From that you can estimate if the carb size is appropriate and what you need for a fuel pump.

Oh, and is this an automatic? From your description of the symptoms, it almost sounds like the tranny may be upshifting too soon.

Perhaps the best place to start would b eto just test the fuel pressure. Or perhaps take it to a speed shop.


#3

Not a purpose built drag car. Slightly modified stock W-30 car… 455 cu in. 8.9 to 1 calculated compression, .488/.512 cam lift intake and exhaust. @50 duration is 216/226 so most believe this is a mild cam. Stock casting heads with 2.072/1.710 valves pocket ported with standard 3 angle valve job. Pertronix under the cap electronic pick-up ignition system. Stock exhaust manifolds to 2.5 inch pipes all the way to back bumper. Old style Edelbrock aftermarket intake O4B part number been told sames as Performer with lower deck.
Qjet is the part number that belongs to this engine for this exact year, has factory main jets, smaller primary rods (only affecting cruise). Secondary air flap is looser than stock and the secondary rods have a smaller taper. No other drilling modifications and such have been done to the carb, paper element in the carb removed, float height set to this engine spec.

Trans is an auto, 60 ft times were 2.15 seconds which is really slow by most standards.

Car pulls exellent in first gear, shifts nice and firm to second with no issues. As acceleration continues about half way (sorry no tach in car) through this gear it noses over like you take you foot off the gas. That is what I am chasing.


#4

It sounds ilke you’re either running out of fuel or the valves are floating. Fule delivery you’ll have to test for. Valve floating you could determine with a vacuum gage. Both should be tested in aplication.


#5

I hadn’t put any thought into valve float because the car will pull way past the RPM that I am seeing this thing fall over. If I jump on the throttle when cruising it will pull right through these RPM in second no problem.

From testing at the track, it appears to be time on acceleration, under heavy throttle driven. I had a 2.21 60 " time and a 2.15 60 ’ time but both yielded enough of an issue that both runs has the exact 9.28 run with some small digit change after that. MPH was 4 slower on the quicker 60 time.

Funny math there. Will start checking the fuel system


#6

Do you have a speed shop in the area that could put it on a dynomometer with the critical parameters monitored and provide a definitive answer? It might be worth the cost.

Meanwhile, I’m gonna openly solicit the input of OK4450 and some other regulars here that are more knowledgable than myself. Meanwhile, if I come up with any ideas I’ll post again


#7

It has been on a dyno and A/F has been measured. Under WOT it was roughly 12.5 to 1 for the entire pull ending up real close to 13 to 1 at the end, 5100 RPM. The issue with a dyno the operators do not like upshifts on their dyno under WOT and the car will not display any loss of power through the entire RPM band during a regular pull at 1 to 1 on the dyno. All other vehicle dynamics cannot be mimiced like fuel slipping off the tank pick-up either.

One time it was acting up like this I changed the secondary rods to a longer taper and it seemed better but that may have just been wishful thinking and more fuel in the tank.

I do appreciate your suggestions here Same. I’m not trying to be argumentative and hadn’t really thought through the other items you had requested earlier.


#8

Please don’t think I’m interpreting you as being argumentative. Truth is, I think it’s going to take a smarter mind than mine to help with this one. You clearly know what you’re doing, and have gone beyond my ability to help.

But let me throw an idea up anyway. Have you considered an in line fuel pump? That would eliminate any possibility of in-tank fluid dynamics being the cause.

The “more fuel in the tank” comment…has the level of fuel been a factor? If so, that would tend to confirm a marginal pump, as the added head pressure would be indicated as a variant.


#9

This car is running the factory style block mounted mechanical fuel pump which I would consider inline.
The fuel level is something that someone else brought up and I have tried to pay attention to it. I have not specifically filled the tank and the returned to beating on it to see if the issue is there. I have seen it act up with as much as 5/8 of a tank by the needle but I cannot remember if it does it with higher fuel levels. Gut says it does.


#10

I had an Oldsmobile that did exactly this very same thing. The problem went away when the fuel pump started leaking and I replaced it.


#11

If it pulls strong all the way through first gear and only lags once it gets to 2nd, I’d say something in the fuel system is letting you down. Possibly the fuel pump is weak and once the engine drinks all the fuel in the lines and fuel bowl, it starts to lag because the pump can’t keep up with demand. It could also be a tank venting issue. I’d check how much volume, not just pressure the fuel pump is delivering. If it’s still running a stock fuel pump, that might be the problem right there.


#12

I don’t mean this in a facetious manner, but are you absolutely sure the secondary plates on the carb are opening at full throttle?

This is mentioned because in the past (memory is hazy here as it’s been a while) I’ve seen a few Q-jet carburetors that would fall flat due to a lock pin in the secondary linkage being dislodged and this would in turn prevent the secondary plates from popping open.
Originally this was to prevent secondary operation until the choke flap was fully open but in some cases a dislodged pin would catch the linkage just by a hair and prevent their opening.

My assumption is that this base is covered, but just pointing it out as a what if… :slight_smile:


#13

QuadraJets use a small fuel filter inside the fuel line to carb connection fitting…They also have a very small float bowl so fuel starvation can happen quickly if fuel delivery is inadequate. I would eliminate the inline filter with the return fitting for drag racing. I would use a large inline filter and remove the small filter in the carb. You can test your fuel pumps flow rate by disconnecting the line from the carb and directing the flow into a clear jug as the engine idles on the fuel in the float bowl…Back in the day, most drag racers would mount a secondary electric boost pump back by the tank to push fuel forward under drag-strip conditions…The pump was controlled by a dash mounted switch. Some just connected them to the ignition switch…

The Q-jets use huge secondary and small primary throttle bores…With a 455, it’s important the secondary’s are opening quickly when you open the throttle…

With adequate fuel pressure, there will be plenty of fuel flow through the needle and seat without any modifications…You want 4 or 5 psi at the carb inlet. Maybe as high as 7 for racing. Some electric pumps have adjustable pressure…


#14

@OK4450 definitely it is opening. That was a big item pointed out in the Qjet book and I made sure to look at all aspects on that, even how far the choke is opening to get out of the way of the secondary.

@Caddyman, I will look at a higher flow pump and look for one with a built in return, the car is set-up to have a return and many people on the Olds forum run a return. I had no idea there were some many different options on pumps for flow. Paper element is removed from the already. The Qjet book talked extensively about varying diameters on the needle and seat inlet hole. It claimed for 300-350 HP applications you would need a 0.125" but some rebuild kits have come with as small as 0.110 inch in their research. I will have to see what I have in there. I have no problem removing the inline filter and replacing with hardline along with installing a different type of filter somewhere else in the system.

@oblivion tank venting? How would I check this? The tank has a charcoal canister hooked up in the factory location but how can I tell it is working. Conversely, can I use this line as a positive pressure point (air compresser regulated way down) to check for leaks?

Good thoughts in here guys keep them coming.


#15

MB:
I’m curious how valve float can be tested with a vacuum gauge.
Don’t the conditions that result in valve float (high rpm) imply wide open throttle - and hence low vacuum?
Is it that if the valves begin floating, then the you’d see an unexpected rise in vacuum? If that’s what you were thinking, I had never considered that.


#16

A 455?? I would put a tach on it and never take it over 45-4600 RPM…These are not high RPM engines. You would KNOW if you floated the valves…You will think you blew the motor if that happens…Where did the cam you have installed come from?

Fuel delivery…With a suction pump mounted on the engine, under hard acceleration, G-forces acting on the fuel in the line back to the tank can be considerable. These forces are acting in the opposite direction to what you want…For drag racing, an electric pump installed so it pushes the fuel forward works best…


#17

Crane Cam is used in this motor.numbers quoted are from memory but still very close to what the cam card claims. Cam it straight up in relation to the crank, not degreeing offset.

Are electric fuel pumps any quieter than they used to be.

And I realize these are not high RPM engines but man they have the torque to plant you in the seat. :slight_smile:


#18

Back when I was drag racing, one of the first modifications was to install a fuel pressure gauge that could be seen from the driver’s seat. Often could be sneaked up under the hood, viewed out the windshield with a quick glance. Once it was dialed in, the gauge was moved to the rail just as a periodic diagnostic check. I didn’t run any mechanical pumps, everything converted to electric. You can hear it whir when you first turn the key. If you can hear it with the engine running, your motor is too small :wink:


#19

Good points, JoeMario. If the valves start floating, the vacuum reading will become vary unstable. There are other possible causes of unstable vacuum when getting into high rpms, such as an intake leak, but with no other symptoms it’s worth checking. But, as pointed out to me, I’d expect valve float to not be affected by relasing and then punching the throttle.

Truth is that with the exception of a tranny shifting too late, I can’t think of anything that would cause those symptoms. If it were me I’d definitely replace the fuel pump, but even insufficient fuel doesn’t seem to quite fit.

I’m having a hard time seperating the shift points of the tranny from the dropoff and recovery of the power.

All of that is why I solicited more help. This one is one that I’m more likely to learn from than to help solve.


#20

Plan of action is to verify the needle and seat tonight. If I have the higher volume diameter in there then I will probably move to this:


Even though private testing shows only 70 GPH but that is still far from the 25-30 that the remans may be able to support.

Rough math, my car is only supporting 0.0667 Gallon of fuel estimating that the 8 second mark it fell on its face and assuming a 30 GPH pump is used. I guess there is some hot rod method of deciding if that is enough for the 455 cu in motor. If I did my math right, this pump can product roughly 8.5 oz of fuel for that 1/8 mile blast when it shut down partially. If it was a 23 GPH degraded pump number could be 6.54 oz of fuel. That does not seem like much but let me calculate some more and get back. I know the fuel bowl has fuel in it at the start also so that has to be factored into how much was used.

Thoughts on this?