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HD8 carbs, what is minimum fuel rate to keep bowls full, car is dying.

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jjs64bj8

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Are in final phases of restoration of a 64 bj8 with HD8 carbs. New electronic SU pump with a Faucet type backup pump in line with the SU. Have glass fuel filter between tank and SU pump. Original fuel tank was cleaned and sealed. Gas cap is vented.

During first few miles of test driving with approx 1/4 tank of ethanol free gas (glad we can get that in FLA) car was running fine as we were getting carbs into adjustment. Then put more gas to about 1/2 tank. Did about a 1/2 hour test drive and at the end the car slowly ran out of fuel, yet I could hear the SU still pumping. Switched to the backup pump and same thing, car would not run. After letting it sit for a about 5 minutes, car started and got it home (3 miles away) and pulling into driveway, died again. We confirm that there was no fuel as we pulled of the tops of the float bowls which were empty. Reinstalled and stated back up, but again after a few miles of driving out of fuel once again.

So we measured the amount of fuel the SU was delivering by disconnecting the braided line to the carbs and running the pump for 30 seconds and measured the fuel in a container. I have read that the SU pump should put out about 15-18 gal/hour, or just over a quart/minute. So in 30 seconds we should see about 16 or so ounces of fuel. We are getting just 5oz/30 sec with either pump running.

We have read all the past posts and did all the usual checks. The fuel filter was clean, no debris in the bottom of the float bowls. Then we disconnected the line just before the filter (which is before the SU) and blew compressed air back into the tank. Checked the fuel rate once again, the SU is now at 10 oz/30 seconds and the Faucet type pump is still only getting 5 oz/30 sec. To achieve the 15-18 gal/hour spec, once should get about a pint (16oz) in 30 seconds, or a quart/min.

So what is the minimum fuel rate needed to keep the bowls full running at normal highway speeds? Is 10oz/30 sec enough as we will soon find out with another road test. Apparently the Faucet type backup pump can only deliver the 5oz/30 sec, which seems like it is not enough to let the car run at normal speeds.

Also apparently we do (did) have some debris between the tank and the pump. The line is new so it must be in the tank. We will try to blow air again to see if can improve the rate. Past posts say original tanks did have a screen inside the pickup and they can become clogged when the tank is cleaned and coated.

Any thoughts on what is going on here, how to clean the tank filler tube?

Thanks
jjs64bj8
 
When you say the pumps are in line, do you mean in series or are they parallel? If they are in series, maybe the two pumps are restricting each other. Try to isolate only one pump in line at a time.
 
Pumps are in series as numerous posts have confirmed that by doing so does not cause a problem. But yes I will isolate the SU and then the Faucet and test their individual flow by pumping from a container other than the fuel tank to eliminate the fuel tank pick up as a possible source of the restriction.

Thanks
jjs64bj8
 
I chased a similar problem after rebuilding a 124 Spider, and fitting an electric pump. The P.O. had butchered the fuel system so the tank was not venting, and after about 20mins of driving (it would idle forever), the car would die. The car would restart and run fine after sitting for ~5 minutes (time for the tank's vacuum to leak back to atmosphere), then die again after 15-20 mins. I finally found the trouble by jumping out, and unscrewing the fuel filler cap after it died. There was a great whoosh as air re-entered the tank. The car restarted, and ran. I finally got the tank vent sorted, and the problem disappeared. Not sure this is your trouble, but it drove me nuts, and I thought I should mention it. Good Luck. Jim
 
i also had a similar problem caused by a poor quality petrol filler cap that did not vent properly. Fuel stops flowing and eventually you create a vacuum. If this is happening you will here an inrush of air if you disconnect the fuel line.

What is the history of the fuel tank? If it has been sealed and that is now deteriorating you will have never ending problems. If so its best to get a new aluminium tank
 
Original fuel tank was cleaned and sealed.


I think this is your problem as Derek j pointed out.
 
Gas cap is new and is vented plus I also tested with the old one on with no seal, so the venting (vacuum creating) in the tank is not the problem. I will today disconnect the fuel supply from the tank and substitute with a jug of fuel and measure each pump by themselves to measure the flow rate. if it is where it is suppose to be then the problem must be the pickup in the tank. Really do not want to buy a new tank if not needed. Plus I did read that new ones do not have any screen on the pick up. True?

Thanks for the replies.

jjs64bj8
 
We have done more testing.

New SU electronic fuel pump, pumping from an open jug of gas at same level of the tank we get a flow rate of 22oz/30 seconds or 44oz/min. This is pumping from the jug through the filter into the working SU out and through the faucet type pump (off) and all the way to the outlet of the braided hose by the carbs into a container. For reference 18gal/hr is 36oz/min which is the high end of the 15-18 gal/hr specs I found on the web. I am told that British Car Specialists tech notes says to expect a quart (32oz) per minute. So the SU pump is definitively putting out the require volume when unimpeded.

Then tried the faucet type pump on (SU off) through the same series routing. It puts out unimpeded 24oz/min or about 2/3rds the expected quart/min (32oz).

We did not try to check each pump on its own as it appears the SU can deliver more than enough fuel through the routing. The faucet type pump we are using is a cheaper clone of the real deal. Seem to work good enough but the unimpeded flow rate may be a little low. Looks like a real Faucet has the same rate as the SU. Ours just might work as a backup should the SU fail. here is the real Faucet.

https://www.pegasusautoracing.com/productdetails.asp?RecID=7550

So back to the the fuel tank hookup. We gave a couple of good shots of air back through the tank hoping to clear any debris that has apparently been impeding the flow. Now we still see only about 22oz/min from the SU and still only 10oz/min from the faucet. So definitely the is something blocking the full capacity of each of the pumps.

Unless there is a way to fix this, I suppose we are looking at a new tank, just a steel tank, not AL.

Can anyone verify new tanks no longer have a screen on the output?

Also has anyone ever done this type of testing and if so what are your volume rates? Does anyone really know the minimum flow rate required for the HD8 carbs at highway speeds?

Thanks

jjs64bj8
 
British_Recovery said:
I would remove the filter you said you put between the tank and the fuel pump(s).
Bob
Click to expand...

YES:iagree:
Remove glass fuel filter between tank and SU pump. " and connect an electrical jumper cable between the tank line and the input line to the fuel pump
 
When we tested the routing it was through the filter and we got the 44oz/min sucking out of a jug, so we do not think the filter is impeding at all. We removed the tank this afternoon and there is no way to view the fuel pick up tube because of the internal baffles, but you can see just a peak of the bottom of the pick up tube by removing the drain plug on the bottom of the tank. It does appear that there is the sealant coating (pink in this case) on the pick up tube, but is is impossible to determine any thing beyond that. I spoke with a noted Healey restoration proprietor and he claims there is no filter at the bottom of the tube, contrary to many posts saying there is. Thus we are banking on the fact the tube is partially clogged.

Regardless, we have ordered a new steel tank as the prices are not bad right now. So hopefully this will solve the issue. We are at a loss to explain it otherwise based on the testing we have done.

Thanks, but are still wondering what the minimum volume rate to keep the float bowls full.

jjs64bj8
 
So if at 60mph and 20 mpg burns .05 gal/min which is the same as 6.4oz/min. During our initial test we were barely getting 10 oz/min. I estimate our max speed in about a 20 minute drive was 50 mph and I doubt we were getting 20 mpg, so it would appear if John Turneys math is correct then we were indeed on the border line and did empty the float bowls. Plus I am running a little rich at the moment which did not help.

What can one expect in fuel mileage around town? 20 mpg sounds a little high?

Thanks

jjs64bj8
 
What can one expect in fuel mileage around town? 20 mpg sounds a little high?

OH ! About 15 to 17 mpg but depends on traffic and temperature too
 
Anyone have photos of a cut open 3000 fuel tank. I am curious as to the configuration of the pick up tube and of the alleged input screen. I have found photos on the web for MGB tanks but cannot find photos for the Healey.

Thanks

jjs64bj8
 
Update

New tank installed from Moss (no affiliation just informing). Measured the flow from the SU pump and are now getting just shy of a quart and a half per minute. The Faucet clone delivers just over a quart which is supposedly the target rate. Again these pumps are in series and it now appears there is no sizable flow restriction in doing so including the see through filter before the SU pump. We have wired a "secret" switch under the dash which allows either the SU or the Facet or neither to be on. So have a built in anti-theft.

So the best we can determine is the original tank indeed had a restriction, probably due to the screen getting clogged when the tank was sealed. As earlier mentioned looking from the small drain plug one can just slightly see what appears to be the tube hanging down with a plastic looking end (screen?).

Would still like to see a photo of a tank cut open showing the pick up tube.

Hope this exercise helps others diagnose a fuel starvation issue.

Thanks to all who replied.

jjs64bj8

PS I did find (after the new tank was ordered) that it might have been possible to open up the screen by drilling a hole into the top of the right angle fitting on the outlet and poking a rod down the tube hopefully knocking off the screen or at least putting a larger hole in it. Then plugging the drilled hole. Might have worked?
 
Sealed tanks seem to be a jinx - there have been a lot of stories over the years of people with clogging problems due to the sealant.

IMO it was better to go with a new tank than worry about being stranded by yet another tank problem later on even if you fixed it this time.
 
One thing no one has mentioned was fuel pressure, all the discussion seems to be about flow. For SU Carbs it should be about 3psi when the pump stops. This can be easily tested by hooking up a Fuel Pressure guage (Stewart Warner has a good one). If you are not getting at least 3psi it could be a bad pump (new or not). Not sure if that SS pump is adjustable for pressure but I suspect not.

Note that if you are getting pressure significantly over 3psi you may have to put a Fuel Pressure regulator in the line as high pressure will overcome the float bowl valves and they will overflow.

Cheers,
Dan M.
 
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