Thread: HQ 396 Coupe - High Country Targa 2012

Thanks Tim I have done all those. The pump now runs on a relay directly from the battery which is about 600mm away and voltage is good. Fuel pump is below the tank and no filter fitted atm. I have replaced the reg with a new one a couple of weeks ago. Fuel line is shielded from hot areas with alloy sheets and heat shielding over the pipe itself.

I have just fitted a new fuel line from the pump to the regulator , close to the carb... no change.

Just to go over the problem again, when cold the pressure is good and can be adjusted to 7 1/2 lbs, as the car warms up the pressure slowly drops to about 4 1/2 lbs then starts to flick between about 2 1/2 and 4 1/2 then after that it drops to almost no pressure with the Carter but 4 1/2 lbs with the Holley. Strangely the fuel bowls have fuel in them all the time, a bit low but still have fuel.

So I think it is a fuel pressure problem as distinct from a fuel flow problem. If I disconnect it from the carb it fills a coke bottle no problem.

Hi Tim, the gauge is accurate as the car runs s#!t when the pressure drops and of course runs lean and heats up and no power. So it's a fair assumption that the pressure is actualy dropping.

We did a few more tests ...



The first one is self explanatory and takes fuel by gravity directly to the pump. The current system goes over the diff and down to the pump which is below the tank. Result ... no change, pressure dropped to 2 lbs when temp got to operating temperature.

We then checked if any of the components were getting hot ... Results, no the Carter was warm but not hot. The same for the fuel line and regulator, all warm but not hot.

We then tested to see how long it took for the pressure to increase after it dropped off. Result ... about 10 - 15 minutes when the engine temp dropped back to 80 deg. But once we started the car and temperature rose / time passed back it went to two pounds.

I then put my finger over the hose and could feel the pressure but could easly stop it with my thumb. Probably ok but I don't know what 14 - 16 lbs feels like.

Next a flow test and it pumped a litre in about 8 seconds, plenty of flow.

Next we disassembled the pipes and bleed the fuel through avoiding any air locks. Result ... no change.

Ok, for the last test we dropped the fuel line from pump to regulator along the floor so it didn't go up and over the diff and we had a small improvent. The pressure dropped down to about 3 lbs but we were able to adjust the regulator back up to 6 1/2 lbs (as much as the pressure would go) and that was through a 5 m fuel line. It should be noted that a Melbourne storm came through and the temperature dropped from 33 to 17 as it is now. I have been able to do a similar thing with the original setup but it was much more stable in this test. The downside is pressure is up around 11 lbs on cold startup again.

So we haven't solved the problem but it seems that a 'flat' fuel line to the regulator helped. I will see if the pump can be fitted on the other side of the diff but as it has a 9" the offset will move the pump even closer to the exhaust. If this can be done it may add 2 lbs to the minimum fuel pressure. Plus the Holley blue pumps about 3 - 4 lbs more than the Carter at temperature so I will move back to the Holley Blue. On top of that the pump on the other side will shortem the length of the fuel line by at least 500mm reducing the total fuel line lenght to less than 3.5 m. I'm also interested in running a hard fuel line as far as I can to reduce the pressure/flow resistance of a rubber line.

Maybe all of these will add up?

Two questions though for those that have fuel pressure gauges ... Is it normal to have a drop in fuel pressure from cold to operating temperature? And I assume that you set your pressure at orerating temperature ... have you noticed what it is at cold startup?

Interesting, I have a 750 Demon as well, do you only run 5 lbs?

Mine has been setup for 7 1/2 lbs

. . . the carter black is gravity fed and you dont run a restrictive pre-pump filter? . . .There is no chance that air is getting into the feed line? . . . Is your tank vented? When you are pulling fuel from the tank, you are not creating a vaccum are you?

Strange that it's only happening with heat . . . odd.


You run a surge tank dont you?

Is the carter black the feed pump to your surge tank, or the pump from the surge tank to the carb?


My system =
- 1/2 inch line from the bottom of the tank, gravity thorugh a log filter, to a carter black,
- then 1/2 inch line up over the diff,
- along the body,
- up the firewell into the bay, to the carb,
- past the carb to a holley return reg,
- then back via 1/2 inch back under the body,
- up over the diff again to the tank . . .

On a rolling dyno the fuel pressure needle does not move from 6 PSI . . .


Do you have a return line back into the surge tank or does it go in the main tank - you are not introducing aerated fuel in the surge tank are you?



The carter black does not like air in the feed line - they like a consistant flow of unrestricted fuel to the pump inlet, they dont like to 're-prime' . . . where the holley pump will suck fuel and it will self prime if it is exposed to air in the feed line . . . so the holley blue is a little more forgiving in that respect.


Are you pulling fuel from the normal pick-up in the tank, or from a sump in the bottom of the tank?

I had trouble pulling from a sump in the bottom of the tank, for some reason, even with lots of fuel in the tank, the pump did not like it when I went up a hill . . the pump would reduce from 6psi to 2 psi until I turned her off and reprimed the system again . . . (think the pump was cavitating).


If you are having similar probs, it sounds like the carter is being exposed to air or resistance in the feed line and the pump is cavitating . . .


Sorry to hear you have having probs with the fuel system Peter - I had lots of problems with mine too - tried quite a few different pumps and settled on the carter black - it's run really well for me, just needed to ensure the feedline to the pump showed little/no resistance and the pump did not need to re-prime - ie the fuel supply to the pump was continuous.

Thanks for taking the time to reply Bill. The Holley Blue is better with air pockets etc and has better pressure than the Carter so for those reasons alone I will go back to the Blue.

I no longer have two pumps and a surge tank as Brown Davis rebuilt my HQ tank with baffles and an internal surge tank. I also had a second vent fitted and the pickup is now 1/2" placed in the surge tank.

I don't run a return line but never have and it was good for about 2 years???

Fittings are all Speedflow so I assume no leaks. One of the tests was a full prime of the system so there was no bubbles/air locks. I will pull everything out and replace the lines with new and check for air leaks ... there are no fuel leaks.

The other thing is that the fuel bowls in the carb stay at their correct level when idiling in the garage. I think it has flow but no pressure at operating temperature.

I have just ordered a length of solid fuel line and will run it in place of the rubber as far as I can.

Will also strip the Holley and check for contamination but they do have quite a good internal filter, another plus for the Holley.

This is one of the strangest problems I have ever come across.

Yes beeb, Al suggested this as well so I may look into it. Does anyone have a schematic?

I will do the 'other' things first, shorter feed pump to reg, hard line and pump on the other side of the drive shaft and back to Holley Blue.

Another night under the back end of the car