QJ major stumble at high RPM

Here's the situation: JW470 with Robbmc 550 fuel pump and 1/2" sending unit. Newly built Level 3 QJ (1973 Buick 800cfm) from Quadrajet Power. -8AN fuel line all the way from tank to carb. It made 582HP on the dyno.

It craps out about 5300 rpm at WOT; bit of a crap out in 1st, especially noticeable in 2nd gear, and haven't gotten there in 3rd gear as I'd be well over 100mph. I suspect its running out of fuel (did the exact same thing with the chevy QJ that Jim built for it originally).

Any concurrence here? I know to really be sure I'd have to buy and install a fuel pressure gauge, which is expensive and if a mechanical gauge, I'd have to keep it outside the car. To do this and still see it when making a WOT run, I'd have to run a hose and tape the gauge to windshield. How do I do this and get it through the hood with it closed?

Would I be better off just upgrading the Robbmc 550 pump to the 1100hp version? Thanks for any input!

 

Thanks Larry. I'm gonna have to bite the bullet and install an electronic fuel pressure gauge. Its unbelievable how expensive they are, but then I guess this hobby isn't for those who aren't willing to pry their wallet open.

I'm gonna go with this one...I've been impressed with the quality of other Longacre products:
http://www.longacreracing.com/products.aspx?itemid=1730&prodid=7403

 

You may be emptying the bowl , one of the down sides to a qjet

Robb will upgrade your current pump fwiw, $125 I believe

 

A cheap rout is to buy an oil filled fuel pressure gauge, and plum it in line near your fuel pressure regulator. Then you can adjust your fuel pressure without any plumbing and rigging.

 

If 8 psi is what you have at the inlet side Of the regulator at the motor , then that's not enough, you need atleast 14 psi.
A common 2 G force launch made by your motors power level will back out .3 psi per every foot of fuel line assuming the pump being at the tank.
Have you checked the Carbs seat, is it the .136" feed hole with 2 windows at the bottom?
I have found that as you get above 580 hp with these Carbs that on the dyno there getting iffy due to that small fuel bowl and that is as seen in stationary dyno testing!
One guy I know took to grinding out those fuel bowl walls as thin as he could, and the same with trimming down the seat!

 

He's got an engine mounted mechanical pump..

Definately get a fuel pressure gauge on it, ASAP.

I assume that your new Q-jet has the big needle and seat in it, the one I built for you did, and we had no issues feeding the motor on the dyno.

JW

 

Yes its got a "high flow needle and seat". I'll install the pressure gauge this weekend and see what its putting out. I went to a Buick carb cause I just didn't like that side inlet on the Chev carb, and the vacuum pull-off interfered with a fitting on my particular intake (and a friend was coveting it for his '72 truck build!).

I suspect the fuel pump just isn't keeping up on the top end...we'll find out soon enough!

 

"You may be emptying the bowl , one of the down sides to a qjet"

Not really, just another "myth" associated with the engine making more power and fuel delivery to it is inadequate or you don't have a big enough inlet seat, or a little of both.

Think about this scenario for a minute.......with any carburetor you are either keeping the thing full on a hard run or you aren't. IF fuel is leaving faster than it is coming in, you'll simply see issues a little sooner if using a carburetor with a small fuel bowl.

Anyhow, some questions on this subject came up recently on my Forum, so I copied/pasted my responses below. A bit lengthy in reading but worth the time/effort if you really want to know what's going on with these things:


Stock fuel pumps and fuel systems were NOT designed for 500hp, or anywhere close to that.

The truth is that most stock systems are hard pressed to keep up with much past about 350hp and we start to see all sorts of issues depending on the vehicle, tank location, routing of the factory lines, etc when folks install powerful engines and start taxing the fuel system harder. Heat can be a player here as well as lines often run too close to exhaust pipes and pick up too much heat on the suction side of the mechanical pump.

I went thru this with my own vehicle over 20 years ago and quickly learned that a stock fuel system on a 1973 Ventura is good for about 350hp with no issues anyplace.

When I installed my first 428 engine making about 400hp I didn't have any issues at all until I took steps to improve traction (DOT tires or slicks) and installed 3.42 gears and a good aftermarket converter to get the car to leave harder. The car ran fine in all scenarios until I found good traction and left hard, then it would start to loose power right up near the shift point (5500rpms). I could actually "feel" the engines power falling off around 5000rpms but it never ran completely out of fuel, most likely just sucking the fuel bowl pretty low.

I tried an HP Carter mechanical pump, pusher electric pump, and they really didn't correct the issues, the car still felt like it was loosing power past 5000rpms but only at the track or on the street when it hooked up really hard.

Next move was to install a more powerful 455 engine which INSTANTLY resulted in fuel delivery issues much more pronounced than with the 428 engine. The engine would pull like a freight train in low gear to about 4500-4800rpm's then nose over, quickly recover and pull right on up to the shift point.

It ONLY did this on hard runs with good traction. I could do "John Force" burnouts with zero issues, or blast thru the gears without any problems, etc. Any attempts to heat up the tires and launch hard and it would nose right over.

At this point I was perplexed by the problem and had tried several different things to correct it and each time I thought I had it beat then did a hard run with traction and the problems came right back.

The last attempt I did was to remove the gas tank and install a larger line and relocate the pick-up more to the rear of the tank as it was right in the front. I put a Holley Blue pump just outside the tank, and 3/8" steel lines to a regulator in front of the carb dead headed. Low and behold the problems still existed and I was completely stumped at this point. I even tried pushing to the Carter HP mechanical pump which wasn't quite as good as just re-routing around it.

The car by now was being drag raced couple of times a week weather permitting and running solid mid 12's with best ET of 12.43 at 109MPH. I settled on just short shifting the engine at 5000rpms for all runs right before where it would suck the fuel bowl low and start loosing power. At the suggestion of several well meaning folks I even built and installed a Holley 4781-2 850 DP carb and it didn't do much better, power still fell off at high rpm's. I would have thought that the much larger fuel bowls and 2 N/S assemblies would have certainly helped, but really no improvement as the car still ran mid 12's at 109mph.

So I just dealt with this problem for several years and enjoyed the car on the street and track, and logged over 1000 track runs with it without any noticeable problems. Over one Winter I decided to upgrade the fuel system one last time and pulled the tank and welded on a CE rear sump, upgraded to a Comp 140GPH pump, and 8AN lines/fittings everyplace.

First track outings the next Spring resulted in 12.0's at 112mph!!!!!!! I was simply amazed with the improvements and the engine pulled so hard past 5500rpms on the first two runs that I didn't get it shifted quick enough and actually ran a little slower than the runs to follow shifting at 5500rpm's. I ended up with a best run for the car that day of 12.02 at just under 113mph!

I would also mention here that I carried the big Holley 850 to the track that day and it also ran 12.0's at 112mph, showing that it was also not staying full on hard runs with the previous fuel delivery system.

Lessons learned in fuel delivery with these cars for sure. I went on to install a bypass system back to the tank to take some load off of the electric pump as I "fried" the brushes in the first one in about 2 months running it "dead head".

All of this happened in 1999-2000 and to date the current system has been FLAWLESs clear down to 7.18 @ 96mph in the 1/8th mile and nearly into the 10's in the 1/4 mile with a newer and more powerful 455 engine.

By this point in the long response folks have probably figured out that the pick-up location in the factory tank was the culprit and I was uncovering it on really hard runs. Installing a rear sump and moving the 140GPH pump behind the tank, and larger 8an lines everyplace also works for us on hard launches as fuel rushes back to the pump not being sucked away from it.

With this topic you will see a wide variety of threads on different websites where some folks run deep into the 10's with "stock" fuel systems where others can't get out of the 14's trying to do the same thing.

All I can say about all of that is that we have learned here to make sure the fuel system is more than adequate for the power level and vehicle performance and since those lessons nearly 20 years ago have had zero issues in that area.......Cliff

 

I can't call it myth , for me that's all it could have been , I had a RobbMC pickup with a 3.5in leg welded on the end going to the rear of tank , 1/2 line, RobbMC pump and a worked Qjet, it would lay over then pick back up , took a new Holley 950 HP to the track in my passenger seat, ran car with qjet,... Layed over, bolted on Holley and pulled perfect, 7.30's 1.5x 60ft can't imagine it being anything else but the seat couldn't pass fuel fast enough to keep up with engine demand. I fought with probably 15 different Qjets on that car and they all did the same, I wanted to use a Qjet because the super stock guys do it, the F.A.S.T racers do it, why can't I ?? And to be honest I still don't know why I couldn't and kinda don't care at this point, but the fuel system was as solid as one could be without electric pump,sump and all that mess. Now of course this was at the track where as you mentioned the car is loaded much more therefore consumes much more fuel, on the street it was fine if you just eased into it, but who wants something that only works some of the time?? Just my experience

 

I've built quite a few Q-jets for the Pure Stock and FAST Class racers and a few are into the 9's with their cars in FAST, and several Pure Stockers knocking on the 10 second door with less fuel delivery than you have available.

Did you ever consider that "laying over" may be something other than sucking the bowl dry. There are other things required on a Q-jet to be successful with 1.5 or quicker 60' times......FWIW.....

 

I'm all ears , because the car would 60' fine with the Qjet then lay over around 200ft then pick up and sometimes not recover at all, so you can see why i would come to that conclusion, why wouldnt i makes perfect sense to me anyway, you can only put so big of a seat in it maybe my engine simply consumed fuel faster under load idk, but clue us in on why a Qjet would lay over if not due to bowl volume. Air door angle wouldn't make sense because most of the time it would pick back up, I assume after the bowl would recover. I don't mess with Qjets anymore all our stuff is 4150 now but still curious

 

If it makes it thru the 60' then noses over out at 200' it's simply going empty. That indicates that you do not have enough fuel delivery to the carburetor, or it may be interrupted for a moment. Could be a few other things or combinations of.

I have ZERO issues in that are and have cars using these carburetors running well into the 9's over 135mph.

We have seen a few folks that couldn't figure out the deal no matter what they tried. As for dedicated drag racing the worst electric race pump is better than the best mechanical pump. Mechanical pumps are dependent on engine speed and have valves that open/close so the fuel flow starts and stops with each stroke of the pump. Drag racing in particular puts a HUGE demand on fuel delivery on the launch, and fuel control is also very important.

I remember one year racing at Norwalk that I had a soda about half full when they called me to the line and I just put it in my lap and made a time trial. I was wearing most of that soda when I got to the end of the track! Probably why I keep having to flip my passengers side floor mat back after every pass as well.

I know, I know, "high performance" mechanical pumps are used successfully in all sorts of racing, but none of them will work to full potential unless they are fed by HUGE fuel lines and very well located sump or even better a sumped tank or fuel cell.

From the testing I've done the very best set up is an in tank pump with baffles and no suction lines at all, next best is to have an electric pump behind a sumped tank or fuel cell so that we use the "G" forces involved with quick acceleration to feed the pump not make it harder for a mechanical pump nearly 14's away to suck it thru bent lines and such.

Pressurizing all the fuel to the carburetor also has other benefits associated with heat soak into the fuel which happens a lot these days and even worse with this new fuel from what I've seen in recent years. On the subject of fuel pressure it is your friend with these things, and due the piss-poor float/hinge pin location of the pre-1969 Q-jets they got a bad rap for handling high fuel pressure and working well in a high performance application from that deal as well. For cars over 500hp or so we run at least 7psi and usually a .145" or larger fuel inlet seat.

There are different schools of thought on that deal and some SS racers run smaller seats and really high fuel pressure, others go to larger seats and less pressure. All of them will have PLENTY of fuel delivery and be using a fuel cell and rear located electric pump or pumps.

The Q-jet is actually an EXCELLENT full race carburetor, and shines above many other designs for off road applications, rock climbing, hill running, mud runners and any application where fuel control is important. The fuel bowl is in the center of the carb and jets on the bottom of the bowl, and excellent venting system (some models need minor modifications in that area).

Same for drag racing. One of my long time customers has a 9.90 car and uses a Q-jet. He also has a big Quick Fuel carb at his disposal but prefers the Q-jet instead. He says the q-jet is more user friendly on the return road and drives better in the pits. It also stages better as throttle response at light throttle openings is better than the big Holley making it smoother coming up on the brake. At any track on any day he says there is less than .05 seconds between the carbs. He uses a 250gph pump with 10an lines to the regulator and 8an to the carb with a full bypass. Says that system has been flawless for the 2 decades that I've known him.......Cliff

 

So I've been racing with a guy who has a Olds diesel block thats now a 432 in a 70 Cutlass Convertible thats a low 12 second car. He's got the same thing going on, it noses over at the 1-2 shift at about 5300, but is able to recover and pull through the 2-3 shift and crosses the line at 5300..I would think that it would empty at the big end where the whole pull was at 5K plus with secondaries open for 4-5 seconds?
He's done the Robbmc pump/am line thing, checked the pickup and runs with a full tank (not sumped). If it noses over at 200 ft(?) wouldn't the condition occur again in the long pull?

 

Load is greater on the bottom end of the track , so it consumes more fuel, plus the primary jets are at the front of the bowl so when the nose is up fuel runs from them. That was/is my thinking anyway

 

I'd mention here that my car was stuck in the 12.40-12.60 range for almost a decade and in that time period I tried several different fuel delivery systems including high performance mechanical pumps and "pusher" pumps to them. I also installed a Holley 4781-2 double pumper carburetor and although it didn't "nose over" at the top of any gear the engine felt "flat" on top end and lacking power up around the shift point.

Since the big Holley with more bowl capacity didn't improve top end power I just figured the camshaft I was using at that time wasn't making the grade.

I was building a more powerful 455 to replace the one in the car (455/455hp) so over one Winter removed the fuel tank and sumped it. Move the SAME Comp 140gph pump behind the tank, and 8an lines/fittings everyplace.

With NO OTHER CHANGES the very first runs were 12.o's over 112mph up from the previous very best run of 12.37 @ 109mph! I swapped on the big Holley the same day and it went same ET/MPH as the Q-jet. Best part of all is that the engine now pulled HARD past 5500rpm's and revved so quick to the shift point I actually over-revved it on the first two runs not getting the shift in before 6000rpm's.

BIG lesson learned on my part about fuel delivery for a car that has perfect traction, 3600rpm flash stall converter and decent power. To this day I think all my issues were with the location of the pick-up in the tank, even though I had installed a larger suction line, the front of the tank is only about 8" wide then it slopes a couple of feet up to the back of the tank. Not sure to this day if the fuel was simply all up against the gas cap and there was enough air in the top of the tank to uncover the pick-up, or if the "G" forces involved with a hard launch just hurt suction at the front located pick-up, and it really doesn't matter at this point.

Since installing the sumped tank and moving the pump behind the tank I have had ZERO issues in that area and all that was done nearly 20 years ago. Have had two more powerful 455's in the car since and more improvements to the suspension and fuel delivery has never reared it's ugly head since.

The car currently runs 7.18 @ 97mph in the 1/8th mile with high 1.59-1.61 60' times and never falters once anyplace on the run. Since I use the car for a test platform I've had Edelbrock Performer/Thunder series carbs on it, Thermoquads, Holley's, Quick Fuel, Demons and dozens of Q-jets on it at the track and they all run very close if/when they are spot on for the tune.

The only carburetors I can NOT get to make the grade at the track are those Edelbrock AFB clones (Performer Series). My engine absolutely HATES them, and they "puke" all over themselves on a hard launch no matter what I do for them far as tuning, pump shot, etc, but that's another topic........Cliff