Grey Motor Head overhaul

[ardiesse]

A box appeared at the front door today. Size, weight and sender suggest it's a BXOV-1.

Rob

[ardiesse]

Here is what the Interstate Stromberg Fettling Service found -

1. The fast idle rod was in the wrong hole in the choke lever. My bad from when I was in Queensland.
2. The throttle was sticky. The throttle valve was off-centre in the bore. I separated the throttle body, loosened the throttle shaft screws and worked the throttle until the valve was centred in the bore and the stickiness went away, then re-tightened the screws. Which, by the way, looked like stainless steel 4-40 UNC round heads. I checked centring of the throttle valve by holding the throttle body up to a light.
3. The fast idle rod is from a red motor Stromberg, because extra hole for a split pin at the choke lever end.
4. The fast idle rod wouldn't go into the correct hole in the choke lever. Oversize. The reconditioner was a good shot with a welder, but not quite so good with a file. I filed the high spots down until the fast idle rod went smoothly in the hole in the choke lever.
5. Same problem at the fast idle lever. Lever was bent. Fast idle rod was oversize. More work with a file to make the rod go through the hole. Then some work with pliers to unbend the fast idle lever.
6. I reassembled the main body to the throttle body, verified smooth operation of choke and fast idle cam. I put the "R" clip back in the fast idle rod.
7. I put the accelerator pump link and clip back, and - the throttle was sticky. Not good. I pulled the clip off, and the throttle action was still a bit sticky. I reversed the link and the throttle action improved. I put the clip back, and the stickiness returned. I inspected the clip: it was bent. I straightened it and put it back. Now there's smooth throttle operation. There's some lost motion in the accelerator pump, but I don't think it's a serious problem.
8. I wound the idle speed screw in until it contacted the fast idle cam (choke fully open), then went in another 1-1/2 turns.
9. I thought I'd inspect the idle mixture screw. It's looking a little second-hand at the pointy end, but will probably be OK. I screwed it in all the way, then out one full turn.

I checked that the idle transfer ports were clear by blocking the idle port with a finger and blowing through the idle passage in the throttle body.

This weekend, all going well, I'll try out this carburettor on the FC.

Rob

[Blacky]

I really should make a point of learning more about carbies and how to diagnose/fettle them

[ardiesse]

I put Steve's carburettor on my FC. A normally straightforward task ended up taking over an hour. You know the nut under the idle mixture screw? I dropped it trying to put it on. It didn't reach ground level. After a lot of work with a torch and fingers, followed by a magnet-on-a-stick, I located it. Wedged between a spring coil and the LH lower control arm.

aaand - the engine almost runs better with Steve's carby than with mine. There's no trace of the off-idle flat spot. Light-throttle and part-throttle performance is good. It's only when you put your boot in that it sags. It's good up to a certain throttle opening. Above that it sags and hunts. And when you take your foot off slowly, you reach a point where it comes good again and accelerates harder than at full throttle.

Suggests to me that the power system isn't working properly. I'll have to check the operation of the power valve vacuum piston and the valve itself to see that it's not blocked.

Rob

[Errol62]

Why I love the SUs Rob


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[Harv]

Does it lean-ping under load?

Could well be the piston sticking, or the power bypass jet blocked.

Perhaps the power bypass jet and pump bypass jet are swapped. There would likely be enough accelerator pump pressure to force open the misplaced power bypass jet (even without the pin depressed) so that you would get somewhat of a squirt on acceleration. The spring in the misplaced pump bypass jet might be strong enough to not fully open under the weak pressure differential at WOT, given leanout under load.

Cheers,
Harv

[ardiesse]

I've never heard a grey motor ping. No, the full-throttle conditions were about 35 mph in second gear uphill on a nearby street, so about 3000 rpm. Same symptoms occurred, but less noticeably, on WOT at 30 mph on the level in top.

I suppose it's not unknown for the vacuum piston to stick in the "up" position, but I'm suspecting the power bypass jet, or the passage leading from there to the main discharge jet.

Now I thought that the accelerator pump check valve and the power bypass valve were of different sizes. They are, but there's also the accelerator pump bypass valve, which looks very similar to the power bypass valve. You might be onto something here. Never seen them swapped before, but there's always a first time.

Rob

[ardiesse]

Executive Summary: I fixed it.

TLDR -

I took the air horn off, and even though the power valve vacuum piston wasn't stuck, I decided to take it out and check the vacuum passage. Yeesh. I thought I would break the piston assembly taking it out, or rip the threads out of the air horn. It was tight. It even "sang" as I unscrewed it. So I cleaned up the threads on both parts and polished off some varnish and light scoring on the piston, then trial-fitted it until it ran freely in the threaded hole. This time the piston's action was much smoother. I cut off about 6" of air-shock tube, wrapped a bit of paper towel around and shoved the tube in the vacuum passage, then sucked. The vacuum piston retracted and sprang back easily.

Power bypass valve next. I had trouble unscrewing it too, but that may be because it was already a bit loose. Once I extracted it, I found that the pin operated freely. I found the metering hole (in the side of the brass body) and put a wire from a wire brush into the hole to confirm it was clear.

Is the discharge passage free? I got the air-shock tube and stuck it down the main discharge jet, then blew. I was rewarded with bubbling sounds out of the power valve well.

OK. So all the components of the power system work individually.

I reassembled the carburettor, started the car and went for a drive. Full-throttle performance is smooth. Don't know what I did to fix the power system, but it's fixed.

Moral of Story: Beware of "exchange, reconditioned" carburettors. They look good. But.

Rob

[Harv]

Great outcome . Probably the varnish on the piston, as there is not much force driving them. The sticky threads tell a story... never been removed before.

I've seen some horror stories of "professionally rebuilt" carbs. Passages full of grit, acid soaked so long the press-fit bowl vent tubes fall out, wrong gaskets....

Cheers,
Harv

[Errol62]

Good result to your indomitable patience Rob


FB ute fixer upper, EK van on rotisserie

[smclaren]

Hi Rob
Just got back from my 3 weeks away in the UK.
Have just read your updates on my carby saga .... some incredible work there, Sir. It's WAY beyond my capabilities, so I am very appreciative of your efforts to restore it back to functional.
I'll give you a call tomorrow (Sunday) and have a chat about getting it back to your convenience.
Thanks again, Rob ... you're an absolute champion mate.
Steve

[ardiesse]

Update:

Carburettor runs well on my FC. Mind you, the FC is somewhat "flat-spotty" off-idle anyway. I can fix that with a 52 main jet . . .

On the dippy, up'n'down streets in West Pymble, I notice a bit of hesitation just when I feed in throttle to go up the hills. Tomorrow (all things going well) I'm going to do my patent nail-punch cure on the accelerator pump pull rod to get rid of the free play. And maybe file a slight chamfer on the underside of the throttle butterfly next to the idle transfer ports - the idea being that the ports will be uncovered very slightly sooner than normal on opening the throttle. (This last idea is a bit experimental . . .)

Rob

[ardiesse]

I "ensmallened" the hole in the accelerator pump pull rod with a nail punch and carefully worked it up with a round needle file until the link just went through the hole. No more lost motion in the accelerator pump.

And now I've made the FC run better with Steve's carburettor than with its own. No more off-idle sag. No more hesitation at the bottom of a dip.

It took a little while for the accelerator pump to pump. I think that the leather in the plunger shrinks when it dries out.
And there's a chance the float valve needle may be sticking in its seat. I got to the West Pymble shops and the car leaned out and died. I checked that the fuel pump bowl was full of fuel and then rapped my knuckles on the float bowl. The car then started and idled as if nothing had been wrong.

Rob

[smclaren]

Hi Rob
Pretty sure I have both a NOS accelerator pump and float needle & seat.
I'll pop them in the post down to you tomorrow.
Cheers
Steve
**EDIT: Goods on the way. 1 x Accelerator pump and 2 x needles & seats to choose from. **