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T O P I C    R E V I E W
EdinburghJoePosted - 22 Aug 2017 : 21:05:05
Hi all,
I’ve got a problem with my 1973 96, with the original FoMoCo carb. I’m no way an expert with this stuff, and am not sure what to do next, so I thought I’d put it all down here in the hope someone might have any good ideas or advice.

The car had been running fine up until a couple of months ago when the fuel pump failed. I replaced it with a new Quinton Hazell mechanical direct replacement, and all seemed well.

A couple of weeks ago I was taking the kids to the beach and it suddenly stopped idling – it would just stall. I got it home and started it a few times in the drive. It started fine, but each time it stalled. Then after a couple of tries of this it just returned to normal. Not wanting to just leave it at that, I opened the carb. All looked fine to me, the bowl was full of clean petrol, but I blew air from a bike pump through the main jet just in case. Checked it after and it was definitely clear (carelessly never checked before blowing though to see if it was blocked)

Car then seemed to run fine, so we had the postponed trip to the beach – 20 or so miles. Got there fine – job done, I thought, but on the way back it started to run irregularly at about 50mph or so on the flat. If I pushed the accelerator it went away, the car accelerated fine.

I spoked to John Wyatt (of this forum) and a mechanic colleague of his blew compressed air through the carb channels in case there was a blockage. Again, the car seemed fine, did a 20 mile journey no problem, but on the way back irregular running again.

So – I thought maybe electrics, so I replaced the points and condenser (cheapo generic set for under a tenner) and no difference.
Then I was playing around with it, revving the engine a bit in the drive, and it stopped idling again, but then after a while was fine.

So – I’m casting around for ideas. These problems have roughly coincided with replacing the fuel pump and I’m wondering if it is sending fuel through at a higher pressure than the old one. The fuel filter I’ve noticed runs much fuller than it used to. I think the float must be shutting off properly because I once disconnected the fuel line and got a squirt of petrol out of it, so it must have maintained pressure after the engine had been switched off, but could it be that higher fuel pressure is causing it to shut off with the fuel at a higher level in the bowl?

I’ve cast around on this forum quite a bit and found that these problems can be really hard to track down, but thought it always worth asking for any good ideas / advice that may be out there!

If you've read this far, thanks!

Joe
15   L A T E S T    R E P L I E S    (Newest First)
mellePosted - 05 Sep 2017 : 19:24:30
quote:
Originally posted by EdinburghJoe
The snapping shut when I suck hard is probably not something that ever happens in real life if there's never any positive pressure in the manifold.
The valve should snap shut in case of backfires.

www.saabv4.com
EdinburghJoePosted - 04 Sep 2017 : 12:29:47
That ties in then. There does seem to be a restriction when I blow into the manifold. The snapping shut when I suck hard is probably not something that ever happens in real life if there's never any positive pressure in the manifold.

Anyway, I finally took the car out for a motorway drive at the weekend (ie steady cruising, where the 'lumpiness' had been most pronounced) and it was fine the whole way, so with a bit of luck it was the tightening of the carburettor mounting screws that did it.

Thanks to everyone who contributed to this thread!

Joe
pchristyPosted - 29 Aug 2017 : 18:49:50
Yes, Joe. The idea is that the manifold will suck air through the crankcase and burn the fumes. However, you don't want an air leak into the manifold, so a restrictor valve is fitted which will restrict the flow when the manifold is at low pressure - essentially, at tickover. As the manifold pressure rises with the throttle opening, a light spring will push the valve open allowing more fumes to be drawn into the manifold.

If the valve sticks open, there will be a massive air leak into the manifold, causing lean running at idle, and possibly (probably?) stalling.

Therefore if you *blow* into the pipe that goes to the valve, the valve should restrict the flow (but not cut it off completely). If you suck, there should be little or no restriction.

Hope that makes sense!



--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"
EdinburghJoePosted - 29 Aug 2017 : 13:06:48
I've been trying to understand what this crankcase ventilation is all about. If it's what it says it is, then is it to create a flow of air through the crankcase?

If so, then it'll need to flow from high relative pressure to low relative pressure.
If the pressure in the air filter is at or just below atmospheric pressure, and the pressure in the manifold a varying amount lower still, then air would flow through the crankcase, going *in* to the manifold, which would mean my valve would be doing the right thing?

I'm more than happy to be corrected - I'd not even heard of crankcase ventilation before last week, but I'm just trying to understand it, so please put me right on what is happening and why!

Thanks again,

Joe

EdinburghJoePosted - 29 Aug 2017 : 12:52:42
It certainly looks original, and it can't be the wrong way round as it's screw threaded on one end and lipped on the other for the hose. Also, I've had the car for a year and it has always run fine, the problem I'm having is new.

Has anyone else ever had reason to do what I've done with the sucking/blowing through it in situ? If I'm the only person who's tried it this way (use a long-ish piece of hose to avoid inhaling any gunk) then perhaps I'm just misinterpreting what I've found.

Joe
pchristyPosted - 29 Aug 2017 : 07:18:39
That's weird Joe! The manifold is normally full of vacuum - er, if you get my drift! At full throttle it will have little or no vacuum, but unless you have a super- or turbo-charger, there should never be positive pressure in the manifold!

Are you sure that's the original valve? Someone hasn't replaced it have they?

As a quick test, you could try removing the pipe from the rocker cover and blocking it off completely (cork or something bunged in the pipe!), and see if that fixes the problem. You will get some oily fumes escaping from the rocker cover into the engine bay, so don't treat this as a permanent solution, but it could help diagnose the problem.

--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"
mellePosted - 26 Aug 2017 : 21:05:51
quote:
Originally posted by EdinburghJoe
according to the crankcase ventilation diagram on page 27 of Haynes it looks like the flow is in through this valve and out through the other rocker cover and up into the air filter
No, the other way around.

www.saabv4.com
EdinburghJoePosted - 26 Aug 2017 : 09:17:47
Interesting... I squirted carb cleaner into the breather valve and re-tried my test sucking through a pipe and it *definitely* snapped shut when I sucked (ie when air rushing *out* of the manifold towards the rocker). I can't see how a malfunction could make it work the wrong way round, and according to the crankcase ventilation diagram on page 27 of Haynes it looks like the flow is in through this valve and out through the other rocker cover and up into the air filter, so maybe it's OK?

Anyway, with the lose carb screws tightened the car seemed fine on a short drive, so fingers crossed that was the problem.

If not then I'll be checking the float level next as Derek suggested.

Final thing about the vent hose: mine is a 1973 car and it had a vent hole but no hose. Having read on this forum about others' experiences of fuel leaking onto hot engines I fitted a hose and fed it into the suspension turret to keep the end out of the way.

Thanks all for your help with this - given me plenty of ideas to go on...

Joe
DerekPosted - 25 Aug 2017 : 11:20:44
If a sunken float is causing flooding it will leak fuel down from the accelerator pump tube into the manifold when the engine is running. If running with freewheel it will then drown the engine on a longish downhill, foot off the gas pedal. You really are coasting then and of course it won't bump start again. You can find yourself in a stick situation but coasting, say onto the hard shoulder and waiting for while for the fuel to evaporate will mean it will fire up as normal, until the next time. Took me ages to find the cause. It get worse, more frequent as the float takes up more fuel. For me the eureka moment came when I was trying to start it at my garage with the air filter off and the carb spat back a puff of smoke. Looked down the carb to see the fuel dripping into the manifold. Fully documented here a while ago. New float cured it. Note that late models, about '76 inc Jubilee had a different float setting and main jet size. Carb has a different number. Not noted in Haynes, surprise, but is in the final supplements in the Works manual. I caused the leak by attempting to change the float position from the correct one to the one in Haynes. Many, many months of grief and a flatbed home the outcome of that!
pchristyPosted - 25 Aug 2017 : 07:52:39
My 95 had one, and that was a '72 car.

--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"
mellePosted - 24 Aug 2017 : 20:27:03
I think only MY75 FoMoCos have a vent hose?

www.saabv4.com
BorstlapPosted - 24 Aug 2017 : 14:12:17
A leaky float will result in petrol squirting out of the vent hose connected at the top of the carb lid that dangles down the generator side of the engine. Guess how I found out....

Alex
DerekPosted - 24 Aug 2017 : 10:11:49
If you have the carb top off again, remove the float and give it a shake to see if has any petrol in it. As a cross check, totally submerge it in VERY hot water and see if any bubbles emerge. A leaky float will be heavy and give you a false fuel level, too high.
pchristyPosted - 24 Aug 2017 : 08:24:07
Joe, was the engine warm when you tried to remove the valve? If so, try it stone cold. If it was cold, try it warm! (The steel valve will expand at a different rate to the aluminium plate with temperature!)

The restriction should occur when there is a high vacuum on the *manifold* end of the valve. If it was only shutting when you sucked on the rocker cover end, something seems not right. But a loose adapter plate could also cause an air-leak which changes with temperature, so you may have found the problem already.

The valves themselves are very simple things, with very little to go wrong - aside from getting clagged up occasionally. A good squirt of carb cleaner down it should help, but let it evaporate before you try and start the engine....!

--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"
EdinburghJoePosted - 24 Aug 2017 : 07:30:09
I had a look last night. When trying to remove the PCV valve I found that the plate it was attached to moved slightly- it was a bit loose.
I gave the two bolts that hold it a half turn tighter and it is now solid.
I guess this could have caused an air leak which would have explained the whole problem? Unfortunately didn't have time to test it yesterday so I don't know what effect it might have had.
As for the PCV valve, I couldn't get that off- the plate it's attached to looks delicate and I didn't want to break it. It looked clean though and I stuck a pipe over it and gave it a good suck and the ball snapped shut.
How about I spray some carb cleaner into the PCV valve, leave it a few minutes and start the engine, then take it for a test?

Joe.

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