12 replies to this topic

[alpder]

1275 with HIF38. HIF38 was professionally rebuilt. Mechanical fuel pump was new. As was the tank and all the fuel lines.

Boring cam. Original 65D distributor.

And was all set up on a rolling road early last year.

 

3000 miles of perfect behaviour since then. It ran fine yesterday, but not today.

 

Just today started bogging when lots of power is demanded, at any significant rpm.

 

Feels like fuel starvation, because if you ease off for a few seconds and press the pedal again, then it'll pull fine for a couple or three seconds, and then start bogging again. My guess is there's only a dribble of fuel getting in, so asking for lots of power drains the float-chamber, but easing-off allows the float chamber to fill up again. That realistic?

 

So far have checked:

 - Oil level and smell of fuel in the oil (don't think so).

 - Fuel leaking externally from the pump (no).

 - Plugs (look ok - bit pale maybe).

 - Carb piston has oil and is free (yep).

 - Fuel in the tank ( ).

 

Could be a weak fuel pump. Is a stuck valve at all common on these? Is a split diaphragm really obvious?

 

But there's no filter between pump and carb (one is on order now). So that seems a more likely cause. Where in the carb am I most likely to find dirt and how much of the car do I take apart to access it?

[ACDodd]

What fuel pump has been fitted? (Picture)

Ac

[Thomas_11]

Yes, it sounds like a fuel supply problem

[alpder]

One of these. Have I done something daft (like too fat a spacer on the block...?)

 IMG_20250227_134512.jpg   41.61K   3 downloads

 

(The wrinkly look to the outlet pipe is a glassfibre insulated sleeve on it)

 

 

[alpder]

 IMG_20250227_151813.jpg   50.13K   1 downloads

 

BZX1062 ?

[ACDodd]

The usual test is to fit a pressure gauge in the deliver line to the float bowl and measure what pressure you have when the car is displaying the fault.

Ac

[bpirie1000]

Would the petrol hard pipe have been damaged at any point restricting the flow at a pinch point?

Or a loose item such as rust in petrol tank limiting flow on higher demand hence the delay?

Edited by bpirie1000, 27 February 2025 - 07:02 PM.

[alpder]

Thanks everyone.

 

That pump number, though: BZX1062... has hardly any mentions on google. Is it even suitable for the engine? If it isn't right, which one should I buy for a 1275 (for 1/4" in and 1/4" out pipes)?

[NLinPEN]

What I would do first is to remove the fuel hose that connects to the carburetor and point it into a bottle. Then switch on the fuel pump and see how fast the fuel flows.

[alpder]

Fuel flow looks fine - half a litre in twenty seconds or so, at fast idle (engine running on contents of the float chamber).

 

Fuel pressure is almost non-existent at all rpms - the needle of the gauge (which is a cheapie 0-7 bar previously used last week to check a 1980's EFi) just pulses fractionally up-and-down on the zero-stop. Maybe bouncing as high as the first tick-mark (~2psi) with each pulse of the pump, then bumping back down to zero on the stop, repeatedly.

 

Are these numbers normal-ish for a mechanical diaphragm pump?

 

https://youtu.be/0APNt0SHNYI

 

Edited by alpder, 01 March 2025 - 11:15 AM.

[alpder]

Looks like dirt in the carb is the problem. Half a dozen very thin and light flakes of a soft non-ferrous found in the float chamber. Just the right size to be sucked up the jet. Can't have come through the inlet mesh - and wouldn't have fitted through the needle-valve anyway.

 IMG_20250301_201358x.jpg   90.61K   0 downloads

 

Inlet mesh was about 1/3rd blocked with non-metallic debris - even though the tank, the pump, and all the lines are new, and the tank has a mesh on the pickup. Not the cause of today's issues, but seems it's always worth putting a filter between pump and carb - even on a new install.

 IMG_20250301_201827x.jpg   51.49K   0 downloads

 

Thanks everyone for the help. Hopefully, on test tomorrow, this'll now be sorted.

[NLinPEN]

Fuel flow looks fine - half a litre in twenty seconds or so, at fast idle (engine running on contents of the float chamber).

 

Fuel pressure is almost non-existent at all rpms - the needle of the gauge (which is a cheapie 0-7 bar previously used last week to check a 1980's EFi) just pulses fractionally up-and-down on the zero-stop. Maybe bouncing as high as the first tick-mark (~2psi) with each pulse of the pump, then bumping back down to zero on the stop, repeatedly.

 

Are these numbers normal-ish for a mechanical diaphragm pump?

 

https://youtu.be/0APNt0SHNYI

An SU carburetor does not depend on fuel pressure, only on fuel flow. The fuel flow ensures that there is sufficient fuel present in the carburetor bowl, from which it is led into the carburetor (and then "sucked into" and vaporized in the air stream). The carburetor's fuel bowl thus acts as a local reservoir. Cars that have fuel injection need the fuel to be pressurized in order to push it into the cylinder.

Edited by NLinPEN, 02 March 2025 - 12:44 PM.

[alpder]

Thanks everyone.

 

The issue did turn out to be the dirt. Although the fuel pump number is an oddity, and there's very little online info about it, the pipe-off-carb test showed it'll provide enough fuel.

On the first test run - before cleaning the carb but after fixing the piston spring - it remained intermittently bog-y.
After cleaning the carb, on a long run around the borders of the county (plenty hilly too - this is Wales), it ran perfectly.

 

How did the metal flakes get in there? Can only put it down to when the carb was "professionally reconditioned" before I bought it. See also the thread about the bent spring which suggests the reconditioner was out-of-stock of professionalism that day and decided just to make do without it. Unless someone sneakily pulled the top off my carb, bent the spring, and put it back together again... but AFAIK I haven't upset the neighbours quite badly enough to push them into such a class act of vandalism.

 

I guess the reason that the car would pull uphill OK for the first couple of seconds after closing and re-opening the throttle (even with dirt in the jet) is that the piston would've then been doing it's thing of enriching the mixture briefly, until the piston fully opened against the drag of the damper. And, maybe, the act of dropping the needle into the jet and then lifting it again would've temporarily knocked the dirt back down into the float chamber, too.

Edited by alpder, 02 March 2025 - 01:58 PM.