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 Intermittant uneven running when warm
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EdinburghJoe
Starting Member

United Kingdom
24 Posts

Posted - 22 Aug 2017 :  21:05:05 Show Profile Reply with Quote
Hi all,
Ive got a problem with my 1973 96, with the original FoMoCo carb. Im no way an expert with this stuff, and am not sure what to do next, so I thought Id 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 Im casting around for ideas. These problems have roughly coincided with replacing the fuel pump and Im wondering if it is sending fuel through at a higher pressure than the old one. The fuel filter Ive 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?

Ive 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

pchristy
V4 Guru

United Kingdom
1737 Posts

Posted - 22 Aug 2017 :  22:07:48 Show Profile Reply with Quote
Have a look at the crankcase breather valve. Its a small non-return type valve that sits between the rocker cover and manifold, location depending on year and any mods done! If these jam open, or stick intermittently, they can cause poor or non-existent idling and part throttle performance.

Remove, clean with carb cleaner, shake to check that it rattles (it should!), and replace.

A simple job to do, so easy and worth checking!

One other thing to check is that the fuel pump pushrod is clean and not sticking. When you changed the pump, it should have dropped out. If it didn't, it might be sticking and causing reduced fuel flow.

Remove, clean off any congealed oil and replace. Make sure you put it back the same way it came out! One end is case hardened and this end should be the end that touches the camshaft!

--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"

Edited by - pchristy on 22 Aug 2017 22:10:51
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UK_Sub
Moderator

United Kingdom
2109 Posts

Posted - 23 Aug 2017 :  08:14:53 Show Profile Reply with Quote
This bit?


Don't think I've ever cleaned mine - seems like a fairly easy thing to do (until I come to actually take it off)!
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UK_Sub
Moderator

United Kingdom
2109 Posts

Posted - 23 Aug 2017 :  08:25:38 Show Profile Reply with Quote
Surprised no-one has mentioned ditching the Fomoco carb and replacing it with a Weber 34ICH.
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Derek
V4 Guru

United Kingdom
1782 Posts

Posted - 23 Aug 2017 :  10:09:25 Show Profile Reply with Quote
Did you pop the carb top off immediately after it stalled? It might just have run out of juice but then get topped off again after you attempt to start it again.
There is a tiny filter in the carb that rarely gets a mention. Remove lid and float. Carefully remove the needle valve. Under it you will find a small nylon filter. Check for debris. The fact that the level in your main filter is showing a bit higher might be due to the pump having to work harder to get fuel past this internal filter.
The idle circuit does get its fuel from the main jet drilling so any blockages there does affect idle. Blowing out may work but is a bit unpredictable.
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EdinburghJoe
Starting Member

United Kingdom
24 Posts

Posted - 23 Aug 2017 :  11:56:08 Show Profile Reply with Quote
Thanks for replies so far. I'll give Peter's suggestion a try (the breather valve) - as he says, there's not much to lose (assuming I can get it out!) As for the fuel pump, I'm convinced it is working fine (and the pump rod did fall most of the way out when I changed the pump)

To answer Derek's question - I've had the carb top off twice and both times the bowl has been pretty much full of petrol. However, I don't think it has ever overflowed as (touch wood) I have NEVER smelled petrol off this engine in the year or so I've had the car. That would make me think that the needle valve is working (no overflowing) and the filter clear (bowl full). I can check the needle valve though...

How full should the carb bowl be? Mine does look pretty full - maybe 1cm off the top. Is there a correct fuel level that can be measured once the lid is off and the float is out?

My main problem is the uneven running when cruising. The failure to idle has only happened twice - I mentioned it though to make sure I gave a complete picture.

No one has followed up on the electrics. Is this reckoned to be unlikely to be the problem? Forgot to mention I replaced the plugs and leads earlier this year with parts from Malbrad.

Joe
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pchristy
V4 Guru

United Kingdom
1737 Posts

Posted - 23 Aug 2017 :  17:19:14 Show Profile Reply with Quote
Simon, yours looks different to a standard one, but yes, that looks like it. Usually they are a straight bit of tube, threaded at one end to go into the manifold / adapter, and a lip on the other for the hose. I know when I fitted a 34ich to my 95 I had to get mine modified to clear one of the linkages, a bit like yours, but only a 45 degree bend!

Can't find a photo, I'm afraid, but I'm sure Melle will be along in a minute with an illustration from the manual.....!

When I had my original 96 back in - er - '69, or shortly thereafter, one day it started stalling at idle. For some strange reason, loosening the top cover on the air filter seemed to cure it, but my friendly local mechanic diagnosed a clogged breather valve. Pulling it out and cleaning it thoroughly provided a complete cure.

If it sticks open, its like having a hole in the inlet manifold, and bearing in mind what goes through there, its not surprising if they clag up occasionally!

--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!"

Edited by - pchristy on 23 Aug 2017 17:19:53
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melle
V4 Guru

United Kingdom
2283 Posts

Posted - 23 Aug 2017 :  21:13:53 Show Profile Visit melle's Homepage Reply with Quote
This can be a tricky one to diagnose indeed... Many good suggestions already; two other issues that haven't been mentioned so far are a leaking inlet manifold (check/ tighten nuts and bolts) and a faulty coil.

Pete, here you go. ;)



Well worth cleaning the pcv valve, it must be one of the least understood and most neglected engine parts. The one in Simon's picture has been modified (and I doubt it works properly that way).

www.saabv4.com
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EdinburghJoe
Starting Member

United Kingdom
24 Posts

Posted - 24 Aug 2017 :  07:30:09 Show Profile Reply with Quote
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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pchristy
V4 Guru

United Kingdom
1737 Posts

Posted - 24 Aug 2017 :  08:24:07 Show Profile Reply with Quote
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!"
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Derek
V4 Guru

United Kingdom
1782 Posts

Posted - 24 Aug 2017 :  10:11:49 Show Profile Reply with Quote
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.
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Borstlap
V4 Fanatic

Norway
269 Posts

Posted - 24 Aug 2017 :  14:12:17 Show Profile Reply with Quote
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
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melle
V4 Guru

United Kingdom
2283 Posts

Posted - 24 Aug 2017 :  20:27:03 Show Profile Visit melle's Homepage Reply with Quote
I think only MY75 FoMoCos have a vent hose?

www.saabv4.com
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pchristy
V4 Guru

United Kingdom
1737 Posts

Posted - 25 Aug 2017 :  07:52:39 Show Profile Reply with Quote
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!"
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Derek
V4 Guru

United Kingdom
1782 Posts

Posted - 25 Aug 2017 :  11:20:44 Show Profile Reply with Quote
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!
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EdinburghJoe
Starting Member

United Kingdom
24 Posts

Posted - 26 Aug 2017 :  09:17:47 Show Profile Reply with Quote
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
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