Losing power at WOT in high gear

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Trevor B

Trevor B

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Location
Novato, CA
Here's the scenario:
Pedal to the floor, shifts from 2nd to 3rd around 75-80 mph at over 4000 rpm (sounds like)
Pedal still on floor, accelerating in 3rd until around 90-92 mph, then it stops accelerating (but maintains speed) and feels to me like it's slightly starved for fuel, pulsing. It reminds me of a much less extreme version of what it feels like when my motorcycle tank needs to be switched over to reserve. Or if I was very, very lightly feathering the gas pedal.

I have replicated this twice now, both on warm but not super hot days on a flat stretch of highway.

Here's the set-up:
1973 Duster with 904 Torqueflite recent rebuild and mild shift kit, factory hi-stall converter, 2.76 gears
318 block bored .030 over, fresh rebuild
Initial timing is 10 BDC - total timing unknown but stock distributor. Vacuum advance.
Comp Cams XE262H (w/ springs from kit): Duration @ .050" 218/224
Carb: Edelbrock 1406 (3/8" fuel inlet using 5/16" - 3/8" adapter at fuel pump). Carb was running great when new last year, did sit for a 6 months or so during rebuild. Seems to work fine except for this one situation.
Edelbrock Performer intake and gasket-matched 302 casting heads (heads otherwise stock).
Fuel pressure tester showed 5 lbs between fuel filter and carb. Filter is clean.
Other info: charcoal canister is disconnected and has cheapo aftermarket gas cap (not vented), gas tank new last year and at the time cleaned out fuel lines with carb cleaner back then and haven't messed with any of it since.

If anyone has an idea about what is going on, please help. I really want to run this car at Sonoma raceway this Wednesday night and see what it can really do!
 
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Mechanical fuel pump?
Could be getting weak.
How much load was the engine under for the
pressure test?
 
how about venting the tank? Does it have alternate venting besides the cap? If the psi says 5, I think thats a good indicator. What is timing at
 
With 10 initial and a stock distributor I'll bet your timing is going too high in the top end.
You need to get that limited.
 
ssba said:
Mechanical fuel pump?
Could be getting weak.
How much load was the engine under for the
pressure test?
Click to expand...

Mechanical fuel pump, yes.
No load with fuel pressure gauge hooked up (in my driveway).
Gonna throw on a new fuel pump I have lying around just to rule it out.

pishta said:
how about venting the tank? Does it have alternate venting besides the cap? If the psi says 5, I think thats a good indicator. What is timing at
Click to expand...

Don't know about tank venting. Where could I look to rule that out?
Also, the second time it happened there was a lot less gas in the tank and the loss of power was more pronounced. I would think that more air in the tank would cause less of a problem as it would have more elasticity.

TrailBeast said:
With 10 initial and a stock distributor I'll bet your timing is going too high in the top end.
You need to get that limited.
Click to expand...

Does the timing change whether the engine is under load or not? As in, can I rev up to 4000 in the driveway and check the total timing? What number should it be?
And then is there something basic I can do to limit it if it is too high?

Also, is it weird that the stumbling didn't happen in 2nd gear? I believe the rpms were higher in 2nd than they got to in 3rd.
 
Trevor B said:
Does the timing change whether the engine is under load or not? As in, can I rev up to 4000 in the driveway and check the total timing? What number should it be?
And then is there something basic I can do to limit it if it is too high?

Also, is it weird that the stumbling didn't happen in 2nd gear? I believe the rpms were higher in 2nd than they got to in 3rd.
Click to expand...

Mechanical timing changes from centrifugal force and not from load, so yes you could do it in your driveway.
www.4secondsflat.com has a limiting plate made just for limiting total timing and J685S is the part number, and they also have installation instructions on the website.
32-34 is an average max without vacuum connected, but every engine is a bit different so no one can/should try to tell you what it should be for your engine with your fuel and your elevation.
All we can do is give an average general starting point.

The load on the motor is lower in second gear so it could stand more advance there, but once the RPM's are lower and the engine load is higher (3rd) it could possibly be too much total.

One test you could do would be to pull your timing back to zero initial, turn your idle up to compensate and try the WOT in 3rd test again.
If the problem is gone on that run, then the total mechanical timing is going too high with the 10 initial and needs to be limited.
 
And don't "negate" the simple but not often thought of. Partly plugged fuel line / filter sock in the tank. Your fuel pump "might be fine" but the filter sock might not allow enough FLOW. If you can't find it, "rig" a fuel pressure gauge up front where you can see the pressure under real..speed...and load conditions.

For a "quick check" on your distributor.......find out the total advance without vacuum, at high RPM, then back that down if necessary to about 35 degrees. This of course will skew your initial, but it will be fine for a quick test. Then go out an "make a run" see if things improve.

I'm going with fuel supply.
 
Mine leaned out at 5600 rpm on the dyno. made more power for a bit lean but then stumbled. Found that the float bowls were running dry at that load. Went to a better mechanical fuel pump with a regulator and the problem was solved...runs at 7 psi now all the way up. Could be too that sitting for a long time with E10 gas clogged things up.
 

Thanks for all of the good information.
I put on a new fuel pump last night and got the tachometer working. Haven't messed with timing yet but tried another run, this time, it got to about 98 mph around 4000 rpm before the problem happened again, then I slowed down, then floored it and this time it got to about 103 or so, then started feeling starved again. Perhaps a little hotter today but same stretch of road.

Tomorrow I'll play with the timing a little. This is not my strongest suit.

Worst case scenario, the problem persists. But if I'm lucky, the power loss will stay more in the 100+ range, which I will be happy to hit in the 1/4 mile!
 
Well... Sonoma Raceway decided not to have drag races tonight so I've got one more week to work out problems!

It appears that my mechanical timing advances to well over 40 BTC (using the hillbilly timing tape method) at 3800 - 4000 rpm in the driveway. I'm guessing that with the addition of the vacuum advance, it is getting too much timing at hi revs under load. Will go back tomorrow and back it down a bit and do another test run.

Trailbeast: will look into the timing limiter. Or maybe I should just get one of your kits as I could easily see myself messing up the distributor...
 
40 BTC is probably a bit much but the vacum advance is zero at WOT, you don´t have any (or minimum) vacum at WOT=no vacum advance.
Try TrailBeast suggestion by zero the initial timing and start from there.
 
I had a similar problem with my 360, not as much of a performance engine as yours, but the solution was running a vent line from the fuel filter to the tank. I accomplished this by putting a 33041 Wix Fuel Filter in it. (see below) It has a 1/4" hole for the vent and the two 3/8th" inlet and outlet. Mount it so that the 1/4 vent line is at the top of the installation so the vapor can escape. Note, fuel goes through the vent line when it is not hot, so keep that in mind when hooking up the vent line from front to rear. I had to put two steel lines together because I couldn't get a one piece line to install from front to rear.

Running the vent line to the tank sucked, but it was worth it when it stopped sputtering. It would also only do it in 3rd, at WOT.

The fuel has been modified some how, pump gas. It vaporizes better, quicker, but too quick to for these old cars. It was a "relatively" simple fix. I also put a tube of heat shielding where the fuel and now vent line go by the header on the driver's side. It just slips on and was about $25 at the local parts store.

Between these two "fixes" It has gotten up to 100 degrees and no skip, or "Starvation of Fuel" at all. Cheaper than a pump, but I did have to make brackets to keep the fuel line away from the block and alternator, and the vent line away from the alt and header.

I will take a pick of my installation if you want me to.

s-l225.jpg
 
Well, I took the initial timing from 10 BTC to 0 and ran it again - same problem, same places. Time to start looking at fuel?

lomchivok: what pump and regulator did you get? I'm running a standard Carter mechanical pump (brand new).

70DusterBob - this is very interesting. I would like to see a picture of what your setup looks like.

Is there a better way to clean up any mess in the fuel line besides just spraying carb cleaner through it?
 
Trevor B said:
Well, I took the initial timing from 10 BTC to 0 and ran it again - same problem, same places. Time to start looking at fuel?

lomchivok: what pump and regulator did you get? I'm running a standard Carter mechanical pump (brand new).

70DusterBob - this is very interesting. I would like to see a picture of what your setup looks like.

Is there a better way to clean up any mess in the fuel line besides just spraying carb cleaner through it?
Click to expand...

I will take a pic of the vent system in the am some time. It may have to wait till Sunday though. I'm up late and I have to be somewhere tomorrow. The best way to get the lines clean that I can think of is to take the tank out, go ahead and dump what is in it out to check for rust, water, or anything else. Then flush it if it is necessary. There is a gasoline pickup service in our county. I don't know what number you would use for yours, but you can google "Gasoline pickup in _______ county" and see if that brings anything up.

Once the tank is out and clean, attach about a 3-4 foot (or longer) section of rubber fuel line to the end of the steel line that would normally go into the tank and put the other end in something that compressed air will be able to escape from, like a 5 gal paint can. Someone needs to hold it because blowing compressed air through it "SLOWLY" is what I did to get the crapola out of my line. I don't know of any other way to get it out. I blew the air from front to back, in case there was a clog. Using a slow, controlled amount of compressed air will slowly blow the contents of the lines out. Once there is no fuel blocking the airflow, I picked up the speed of the air a little and blew through it a little harder. You may want to ask someone else if this is safe, but you definitely don't want to be smoking, or have any sparks anywhere near you when you do this, like having a gas water heater nearby is out of the question!!

While we are talking about fuel lines. Do you have a 3/8" fuel line or a 1/4" fuel line on it? If it is a 1/4" that is the problem right there. It cannot supply enough fuel through a 1/4" tube. I had to do that with the Duster when I got it. It came with a /6 and a 1/4" fuel line. I swapped it out with a 3/8" for the 360, and only had problems when the heat got to vapor locking my fuel filter. If the PO took out a smaller engine and put in the 318, they may not have known to put the 3/8" steel fuel line in rear to front.

I will send you a picture of my set up asap.
 
Bob - the gas tank is new as of last year. There was some gas sitting in it for 6 months or so as I rebuilt the engine but there was some Seafoam in there so I don't know if it would have had time to go bad.

The fuel line is 5/16" from the tank to the pump, then I have a 3/8" nipple coming out of the pump going to the fuel filter, which is sitting in between the alternator and the timing cover (zip-tied to the alternator spacer), to the carb. Rubber hose on both sides fuel filter (which is new hose).

Here's something that is bothering me: I can run in 2nd gear at the same RPMs as in 3rd and it doesn't seem to have the same problem.
 
Trevor B said:
Well, I took the initial timing from 10 BTC to 0 and ran it again - same problem, same places. Time to start looking at fuel?

lomchivok: what pump and regulator did you get? I'm running a standard Carter mechanical pump (brand new).

70DusterBob - this is very interesting. I would like to see a picture of what your setup looks like.

Is there a better way to clean up any mess in the fuel line besides just spraying carb cleaner through it?
Click to expand...
For the regulator I used a holley 12-804 and the pump is a Summit QFT-30-360. Above 5000 rpm it now holds 12.5 to 13 AFR and picked up about 20 HP and 15 ft/lbs. The heads and cam (Indy RHS and XE268H) send the peak numbers high in the rpm range. The young Teen really likes to wind up quick. Cost me about $500 bucks for the 2 combined dyno sessions but it was well worth it.
This is the first dyno run showing the lean out at high rpm

dyno sheet.JPG
 
Trevor B said:
Bob - the gas tank is new as of last year. There was some gas sitting in it for 6 months or so as I rebuilt the engine but there was some Seafoam in there so I don't know if it would have had time to go bad.

The fuel line is 5/16" from the tank to the pump, then I have a 3/8" nipple coming out of the pump going to the fuel filter, which is sitting in between the alternator and the timing cover (zip-tied to the alternator spacer), to the carb. Rubber hose on both sides fuel filter (which is new hose).

Here's something that is bothering me: I can run in 2nd gear at the same RPMs as in 3rd and it doesn't seem to have the same problem.
Click to expand...

Trevor, I believe the pump cannot pull enough fuel through the 5/16" line fast enough to keep the fuel supply at top end in third. I'm not positive, but that is what I was told and why I replaced mine. It is a royal pain in the rear gear to swap out the line, but if you buy it in segments, it is easier to manipulate through the frame rail and up around the differential to the tank. They make steel line benders, look like channel locks with a larger wheel on one side and two smaller wheels on the other. It doesn't take long to get the hang of bending. But I don't know how you could test to see if it is indeed able to pull enough fuel through that 5/16" line. I guess if you rebuild the carb and it still does it, that would be the next thing to try.

Fuel sitting 6-8 months with sea foam in it should have lasted that long. Was it full, or close to empty? Because if it was full, it would be best to run the old gas out before putting fresh gas in. If it was near empty, then filling the tank would dissolve any poor quality fuel.

I believe the reason it is at the same RPM in 2nd and not running out of fuel is because in 3rd it has been running longer, thus draining more fuel out of the bowls. So by the time you get to 3rd top end, the bowls are running empty due to a lack of fuel supply.

I know my set up, in the photo below, looks kind of squirrely but it is the only thing I could figure out in a pinch. I had to get some testing done because it was already scheduled for the dyno and it started crapping out in third at top end 2 days before the test The line goes from the pump over the alternator, to avoid the hot spot between the timing chain cover and the alternator. I put thread lock on that bracket's mounting bolt in the alt. The return line goes over the valve cover to avoid the header and I ran it right along side the fuel supply line back to the tank. The tank had a vent line fitting already for me to connect a rubber hose to it and the steel vent line, I don't know if yours does, some do and some don't depending on the year. Since I also put a heat shield sleeve on the vent line and the fuel supply line where they go 2" from the header along the frame, I may be able to put the supply line back between the alt and timing chain cover and it would look a lot better. I just need time to try it. If it starts vapor locking, I suppose I could put a heat sleeve around the fuel supply line that goes between the pump and the fuel filter.

The second pic shows the vent line under the mid section of the body, then going through the same frame rail hole that the fuel line goes through. You can also see where the connection is holding the two separate lines together. The snakey 3/8" line is my fuel line. I couldn't get the vent line to bend like that so I just made it straight through that section.

Let me know how it goes.

IMG_1726.JPG


IMG_1729.JPG
 
using the stock 5/16 line from the tank (5/16 pickup) to the pump..pump being a HV Holley mechanical pump...and then AN -6 line from the pump to the 750 dp....73 Duster ran 12.68 @ 106...shifting at 6000 rpm.....
 
70aarcuda said:
using the stock 5/16 line from the tank (5/16 pickup) to the pump..pump being a HV Holley mechanical pump...and then AN -6 line from the pump to the 750 dp....73 Duster ran 12.68 @ 106...shifting at 6000 rpm.....
Click to expand...

Do you think Trevor may need a High Volume Pump then? Maybe the regular mechanical isn't strong enough?? It would be a LOT easier than replacing the line!!
 
There is a simple test for the pump called a volume test. You simply disconnect the output from the pump, plumb in a gas hose and run it into an alternate container. There is (or should be)enough fuel in the floatbowl to do this test. You simply run the engine, at idle for 15 seconds,shut off the engine and measure the fuel volume. After that you can refill the bowl, and repeat the test at say 2000,and 3000. The outputted volume should be adequate to feed 1/2 pound per horsepower per hour. For example;200 hp x 1/2 = 100lbs per hour or 100lbs/(60minutes x 60seconds x15sec) =.417 pounds per 15 seconds. Since 1 pound of fuel is 132/6 =22ounces,Converted to liquid ounces per 15 seconds, this is .417 x 22 = 9.174 ounces liquid.
So you could estimate hp/21.8 = ounces per 15 seconds.Thus 200/21.8=9.174 ounces/15 seconds. And 300/21.8 =13.76 ounces per 15 seconds. Etc. So that would be what you are looking for,as to pump output.
As to the running out at 98 mph, this represents probably 15 seconds or more of time. To prove a venting issue or a restricted line/sock issue, it should simple be an output test over that time period. If you divide 15seconds by 3 say, then during each 5 second period, the pump should put out at least 1/3 of that fuel requirement, or 1/3 of 9.174ounces, in the case of 200 hp. So if the pump output remains linear at 9.174/3 =3.058 ounces per 5 seconds,for 15 seconds, then the fuel system supply side is good. Then you might suspect a problem in the float valves, or the line from the pump to the carb.
I have found this simple test to be a real time saver,at times.
FWIW
I have seen the return spring on the pump arm get weak and fail to return the arm onto the eccentric. At highrpm the output test failed
 
Last edited:
Sounds pretty complicated to me. How can you get an accurate measurement of the fuel because the engine turns over a few extra times before it stops pumping fuel? Also, the fuel would continue to bleed out of the fuel line into the container after the engine is shut off as well.

Otherwise it sounds like a highly accurate test.
 
70DusterBob said:
Sounds pretty complicated to me. How can you get an accurate measurement of the fuel because the engine turns over a few extra times before it stops pumping fuel? Also, the fuel would continue to bleed out of the fuel line into the container after the engine is shut off as well.

Otherwise it sounds like a highly accurate test.
Click to expand...
5/16 line, fuel bowls empty in the last of high gear, ring any bells ?
 
TrailBeast said:
With 10 initial and a stock distributor I'll bet your timing is going too high in the top end.
You need to get that limited.
Click to expand...
You were so right! For a variety of reasons (one of which was extremely dumb on my part and too embarrassing to post here), total timing was way too high. Now it's back in range and running strong. Keeps pulling all the way through.

For one thing, the stock (so 43 year old) springs in the distributor are pretty weak.
 
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