Technical Dripping Stromberg 97s after motor shutdown

Can anyone point me in the right direction with an issue I am having with my Stromberg 97s.

They are pretty much dry during operation but drip from the throttle shaft after the motor has been shut down, almost only when the motor is fully hot and especially on hot/sunny days. There has been speculation that percolation might be the cause and so phenolic spacers were installed but no change. A fuel shut off valve has been installed immediately before the motor and that does seem to resolve the issue apart from a recent event when it dripped even with the valve shut off.

I pulled the tops to see what was going on and discovered that in a couple of the carbs the fuel level (which stays static whilst the motor is running, creeps higher when it is shut off. I suspect a bad inlet valve but am open to suggestions.

The interesting thing is that it creeps higher for @3 mins and then stops.

I recorded this and the video below is sped up 6 times normal speed so the rise in fuel level lasts @30 secs. The video commences afew seconds after the motor was shut off. In this instance the motor was not much more than warm and the fuel itself in the bowl cool to the touch.

You will see that the fuel rises despite the float not moving, hence my suspicion that the inlet valve is maybe faulty.

I am hoping that this in some way links to the weeping when the hot motor is shut off.

Any help much appreciated. I have reached out to Clive over at Stromberg and he is being very helpful. With the car undertaking a 3k mile drive in a week time is somewhat pressing on getting this resolved, hence this post.

 

Inlet valve leak seems like a very likely diagnosis!
What kind is in there...steel needle to brass, rubber tipped needle, or Grose ball type? if steel, I would take it out. examine for dirt or grooving, and reseat...I hold the seat part in one hand, give needle a light smack with a very light hammer, rotate and repeat a few times.

 

Kee-rect as usual, Bruce.

(the Igniters' club of San Jose's 'club-within-a-club', Hernando's Racing Team) adopted me at a young age. Fascinated with carburetors, I was the 'electee' for performing figure-8 glass honing on float bowel surfaces, re-jetting of carbs (four of them!) with my small hands...
and tapping the steel ball seat with the jeweler's hammer. (Alphonse insisted I perform this op on a piece of leather he had nailed to the bench!)
I have faithfully repeated this process for 60 years. (must be the proven method!)
DO check your fuel pressure, Wex...

 

What are you running for a fuel pump and what pressure? Gary

 

I have always been a big fan of the Gross Jets. Never a problem

 

Running gross jets. Will confirm fuel pressure in the next day or so but it was set at @2PSI previously. Mechanical fuel pump/regulator.

 

Wex
Maybe a problem with the regulator?
Type?
Mfg?

 

Following up on some guidance from Clive (Stromberg) and will report back once we find the solution in the event t helps others.

His suspicion right now is high residual fuel pressure after shutoff and we are trying to pinpoint whether that is indeed the cause. Might be looking at a return line to lessen the issue.

 

Can't see the Vid!

 

I will be interested to hear the solution to this problem. It is one of the key reasons I did not put strombergs on my flathead. I asked a local guy how he handled his leaky carb problem. He said he shuts the fuel off before he enters the garage and then leaves the engine idling until it runs out of gas. Probably not the best solution...... but it works for him .

 

Interesting, I'd say heat soak too. I know the fuel wells trap heat in them, the gas expands and pushes it out the booster. Maybe this expansion is what you are seeing in the video and thinking it is more fuel entering.
Just as a thought, is your gas tank vented?
I have 3 psi poppets to use as bypass regulators if you need one give me a shout. They return fuel.
What you might try is swap the needle & seat inlet of a carb without dribbles with the one in the film.

 

Don't remember where I learnt this, but I was told to make sure the fuel system all the way from the pump to carb should be uphill. Do you have a block that sits higher than your carbs?

 

Floater taking in petrol ?

 

If you have the little round regulator with attached little gauge, be aware that gauge and adjustments have only a random possibility of correlating with reality. Ge a real gauge and see what's happening.
I have one of oj's return type regulators (not on anything yet, though). It is a nice piece, and I think the return type makes a lot of sense. Especially if heat soak is a problem!

 

I have had a number of emails back and forth with Clive from Stromberg as well as Norm from Comp Fuel Systems and I have performed a number of tests.

First, the regulated fuel pressure to the carbs is a perfect 1.5-1.75 PSI so well within spec. Second, this issue impacts multiple carbs and so a leaking float is not the primary suspect piece here.

The hotter the engine/day the more likelihood that more carbs will display a drip from the throttle shaft.

I ran a test this afternoon and on this test found two carbs suffering from the raised fuel level in the 2-3 mins after motor shut off. Both were rear carbs... NOTE THAT THIS WAS WITH A COOL ENGINE AND COLD FUEL. i.e. heat/percolation is not the issue.

Based upon input from Norm it seems that this is likely due to residual fuel pressure in the line which causes seepage past the inlet valve into the bowl.

He states that whilst the fuel pressure is >0PSI then there will be some seepage past the inlet valve into the bowl. This seep rate is not normally an issue as the carb supplies more fuel to the motor than seeps into the bowl.

I was (apparently wrongly) under the impression that the inlet vlave if shut was SHUT and allowed NO fuel past it. I am being advised that most components have SOME seep rate, small as it is.

However, when the residual fuel pressure remains for several minutes then during that time the fuel level will rise slowly due to this seep rate.

I suspect the reason that the rear carbs (and possibly the fronts also) see this issue more than the centers is that if they are left to idle (bearing in mind a progressive linkage) they take gas (seep) but supply almost none unless the foot is buried into the carpet.

Norm's suggestion is a return line to the tank which contains a .024-.028 restrictor. This is large enough to allow sufficient gas back to the tanks to bring the residual fuel pressure down in a matter of seconds but not so large as to reduce the working fuel pressure. By eliminating the residual fuel pressure quickly this will mean VERY little rise in the fuel level in the bowl.

I do find that a couple of the carbs are worse offenders with this issue and for those two I am replacing the inlet valves tomorrow to see if this seep is lessened.

Bear in mind that if you have a progressive setup AND shut the idle screws totally on any of the carbs then their fuel level will rise as fuel seeps in. They need to supply at least the seep rate to the motor to prevent a rising fuel level.

I hope some of this is helpful to others with 97s and I will report progress as I move towards having the return line installed.

My band aid solution in the interim is to shut off the fuel cutoff valve with the engine still running and THEN shut off the engine a few seconds later. This should result in a slightly lowered fuel level in the bowl which even if it then rises a little shouldn't cause the dripping.

 

As long as the needles are not leaking when the engine is running, there is an easy fix.
It takes a 5 inch piece of fuel line and 2 "T" fittings. Solder up one end of the tubing and drill a
.030 hole through the solder. Plumb the line in as a bypass around the pump.
This will quickly bleed off the residual pressure when you shut the engine off but will not affect
normal operating.

A more costly fix is to replace the needles with the nitrile tipped needles that Daytona Parts have.
They will hold 8 lb pressure.

Oh, one more thing. Bruce has it on the bypass type regulator (2 line type). It is the only type that will hold constant pressure at wide open throttle.

 

This test performed this afternoon shows how much the level can rise...you need to view in full screen to see really.

Once I shut off the motor jump to time marker 1:30 or so and you will see me pan to another cab to show how high it is.

 

Thanks, I think a return line with restrictor is on my to do list now.

 

Yup, vented tank.

Check your PMs re the regulator.

 

A quick update. I replaced the inlet valve (grose style) on one of the problematic carbs (nearest carb in video below) and the fuel level which rose on each prior test barely moved once replaced.

I have another very bad carb (far side carb at time marker 1:55 in the video) which I will switch out today as I have two more inlet valves arriving today. If that carb is likewise fixed it looks like the inlet valves might be the issue.

This video commences a couple of seconds after the motor was shut down.

 

To be clear, it is now my opinion (thanks to input from Clive/Norm) that this is a combination of residual fuel pressure and inlet valves not doing their job. This allows seepage until such a time that the fuel pressure falls to 0.

For me the solution will be to switch out the worst offenders (inlet valves) and install a return line.

The return line will ensure that the fuel pressure returns to 0 in a few seconds after motor shutdown, preventing the rise in fuel level in the bowls.

 

Well done! I was watching the distant carb's float bowl variations in the video, it was going thru lots of activity.

 

Thanks to Paul for this post, and others for their input. Something we didn't mention from Norm's input is that, in recognition of this phenomenon, many racers with Holley and other 4-bbl carbs would literally lap in their inlet valve needle and seats like they were lapping in a new intake valve, to get that seat as tight as they could. Personally it sounds like a recipe for them to stick, to me, but I'm not a race carb builder...We are continuing to work with Paul on this issue and will keep everyone up to date.

 

Another thought on this relates to a recent experience with my 56 F-100.
The long and short is that modern replacement gas cap from NAPA has a valve to allow air in but not let air escape. I discovered that with this gas cap, the tank was under considerable pressure when the truck sat in the sun for a while.
This would be a cause for residual pressure in the fuel line that would not be relieved by a return line.

I drilled a small vent hole in the cap to relieve the pressure in the tank, end of problem.

 

FWIW.
My car was almost finished, so I just ran a return line back to the main delivery line, before the pump.

 

Curious as to what a gross valve looks like vs. the needles and seat that I'm familiar with?

 

Here you go...

 

And this is the Stromberg 97 version.

 

This is an old thread however I’m wondering if any of the guys that were in the discussion are still here and could comment on the return line with the restrictor.

I’m curious about @clem s return line back to the supply line rather than the tank and how effective that was.

I’m also wondering if anyone else has any experience with this issue/ topic.

 

Still in a temporary set up, but have used the car and works really well.
Didn’t use a restrictor, but just recently found out that there are after market fuel filters available with a return port in them.