Amazon: Freshly rebuilt Zenith with some concerns

[142 Guy]

I don't have the specifications for the VN36 carburettor; but, I am thinking that your fuel pressure may be part of the issue if not the issue. Volvo's recommended fuel pressure for the SU carburettors on the B18 P1800 (I don't have an Amazon service manual) right up to the SU and Zenith - Stromberg carburettors in the early '70s was 1.5 psi minimum - 3.5 psi maximum.

[samaron]

Manuals received. Many thanks, Derek!

Having a quick look in the one for the engine, I find a description of the fuel pump. It says the following: "The pump is adjusted for a maximum pressure of 3.5 psi (0.25 kg/cm2)."

Although 6 psi isn't a lot of pressure, it appears to be nearly the double of what it should be. Haven't found any specific numbers regarding what the carburetor can handle, but have seen multiple references saying it is sensitive to fuel pressure issues. I assume the engineers back then must have had a reason to specify the pump to have a maximum of 3.5 psi. Probably a safe bet that anything above this number puts the carburetor at risk of leaking.

Considering that the leak stops shortly after releasing the pressure from the carburetor inlet, and gravity feeding it doesn't cause a leak, I'd say it is a pressure problem and not a float height problem.

I could certainly double check the washer thickness that sets the float height, but want to wait for a response from the company before proceeding.

Also found this service bulletin for a Zenith IV series carburetor. It states the following regarding hot start problems: "Where it is found that petrol floods into the manifold from the outlet of the emulsion block within seconds of switching off the motor in spite of fitting a new needle and seating, check the fuel pump pressure, and if necessary have it reduced to prevent the internal flooding which could explain difficulty in hot starting. An additional washer under the needle seating may be effective in preventing flooding and could be tried before recourse to lowering pump pressure."

The big question now is the fuel pressure. Looking at it in person, and reading in the description, it doesn't appear to have any way to adjust the pressure. It is installed properly with the spacer block and gaskets on both sides. VP Autoparts part number 403269 is the one I have. Perhaps I should get a rebuild kit for the original pump? I had it replaced a year ago because it started spraying fuel when running.

[Derek UK]

The original B16 glass top fuel pump had a priming lever Does yours have that? I doubt it has a different pressure to the later glass tops fitted to the B18 but it is interesting. Some of the early B18's also had the lever. I think they used them until they ran out of stock. There is no adjustment for pressure on these pumps. Doubling up the paper gaskets may reduce it a fraction but I doubt if it would be significant.

[samaron]

It does have a priming lever, yes. Very handy to have if the lines are drained.

Had someone yesterday insist that this is a heat problem, and not a pressure problem. Carburetors can't handle modern fuel and all that. He is certain that the reason for the leak is due to fuel boiling, causing additional pressure and overfilling.

I'm sure it is a real thing at certain ambient temperatures, elevations or situations. The butane mixed with the petrol has a low boiling point, but... I can't see or hear anything indicating boiling in the system.

As an additional test, I started up the car this morning. The weather is windy and cloudy, so it hasn't been baking in the sun. Completely cold, in other words. Turned off the engine just a few seconds after starting it. Looking down the carburetor throat I could see it leaking just as it has been on the previous tests.

This should hopefully confirm that this is indeed a pressure issue, not a heat issue.

[142 Guy]

First off, make sure that your pressure gauge measurement is correct. It looks like you might be using a pressure gauge meant for a fuel injection application which means that 6 psi is close to the bottom or the gauges measuring range. Mechanical gauges are typically least accurate at the very top and bottom of their ranges. A fuel pressure gauge meant for carburettors would typically have a maximum pressure value of 10 psi which means that you would be measuring closer to the middle range. If you are going to chase the fuel pressure demon just make sure the demon is real.

Your boat fuel tank test and the service bulletin for the VN36 does support the hypothesis that it is a fuel pressure problem. The additional washer under the needle valve seems like a no cost; but, Hale Mary attempt to bandage the problem. The most desirable solution would be if the pump could be modified to reduce the output pressure.

There are low pressure non by-pass regulators intended specifically for carburettor applications. However, reliably controlling low pressures with a non by-pass regulator is a challenge. The other problem that can occur with non by-pass regulators is that they need fuel flow to achieve the pressure drop. On shut down, you may get into a 'control race condition' where the fuel flow drops faster than the engine / pump speed drops which means that you might get a pressure rise on shut down. Not what you want. You would really need to do your product solution homework to make sure you get a regulator that does what you want when you most need it.

It is odd that your pump is generating pressures over 6 psi since the Volvo service manual pressures all appear to be in the 1.5 - 3.5 psi range. You could try a different mechanical pump or gaskets / spacers to position the pump farther out from the cam so that the pump stroke is reduced. You could also try a low pressure electric fuel pump. There are a wide range of low pressure electric fuel pumps available, some of which look questionable. Facet offers a range of electric fuel pumps in their 'Cube' line. The nice feature is that they specify rather tight pressure ranges - you can order one with a 2 - 3.5 psi control range. The not so nice feature is that they are more expensive than some of the dirt cheap generic fuel pumps

[samaron]

The gauge actually has a 10 psi range. The left side is for vacuum. Regardless, it is a cheap gauge I bought at an autoparts store. I wouldn't trust the numbers to be super accurate, but at least give an indication of what is going on.

Have cleaned up the old pump, which is an original AC unit. The one installed in my car now is a reproduction unit of unknown branding. Bought from VP Autoparts. I'm suspecting that it might have some slight differences on the cam lever. Eventhough it physically fits, it might not be tuned for this specific configuration. A rebuild kit for the AC pump is on the way. Hopefully I can assemble it and compare this weekend.

Would be nice if I can resolve this without splicing in additional pressure regulators or converting to an electric fuel pump. Might even be as simple as a speck of dirt is stuck in the needle seat, keeping it from sealing properly. Still no response from the company. Will give them a few more days before I start removing the float chamber without their consent.

[Derek UK]

I think the Zenith suggestion is worth a try before you do anything drastic. Yes, I hope the carb company get back to you. A couple of washers in a normal envelope would get through the post OK and would cost them pennies for the goodwill. Good luck.

Also found this service bulletin for a Zenith IV series carburetor. It states the following regarding hot start problems: "Where it is found that petrol floods into the manifold from the outlet of the emulsion block within seconds of switching off the motor in spite of fitting a new needle and seating, check the fuel pump pressure, and if necessary have it reduced to prevent the internal flooding which could explain difficulty in hot starting. An additional washer under the needle seating may be effective in preventing flooding and could be tried before recourse to lowering pump pressure."

[142 Guy]

If full scale on your pressure gauge was 10 psi, then your pressure measurement is probably accurate enough for the purpose at hand.

A fuel pressure regulator or switching to an electric fuel pump would be a definite last resort. If a rebuild kit for the original pump resolves the pressure problem that would be the best outcome.

To be fair to the carburettor company, right now your fuel pressure is grossly out of spec which puts the flooding problem into the 'created by the owner' category. If bringing the fuel pressure down to the spec in the Volvo service manual fails to resolve the problem, then its possibly back in their court. I would be more inclined to complain to VP Auto about the high pressure caused by their reproduction pump. Perhaps they can provide an explanation for the high pressure and how to reduce it or would be prepared to allow you to return it.

[Ron Kwas]

Sam;

I'm following this thread with interest...and I don't know if it was mentioned before (surely I would hope it must have been!), but with a mechanical FuPu driven by the Camshaft, the Spacer is a critical part which may not be omitted...I'd first assure the carb supply pressure was not being exceeded, because 10PSI would surely overwhelm the Bowl Valve...and it certainly sounds like it is...!

Good Hunting

[samaron]

The rebuild kit arrived and assembled the old, original pump.

Removed the new pump I bought last year and had a closer look at both units.

I can observe that the rocker arm has a different angle on the new pump. The surface finish where it contacts the cam shaft is also rough in comparison to the old pump.

The spacer between the engine and pump is more or less identical. The original spacer is 5,2 mm thick, while the new is 5,3 mm.

I installed the rebuilt original pump and hooked up the gauge again. This time I get 3 psi pressure.

Connecting the fuel line again and running the engine and stopping it, I do not observe any leaks like before. The car is also much easier to start. No need to push down the accelerator pedal now.

Here is another video clip.

Hopefully it can be called fixed now!