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 Engine ‘bogging’ under hard acceleration
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Betsy67
V4 Fanatic

United Kingdom
409 Posts

Posted - 25 Aug 2019 :  19:08:30 Show Profile Reply with Quote
Just had a grand day out up around Swaledale and Tanhill. Can’t say that this problem has just occurred today, because it’s been there awhile.
1500, Webber 28/36 set to Cortina GT spec. Electronic ignition. Jetex exhaust.
Will happy pull throughout the Rev range when engine is cold. Once warm it’s a different story! Accelerate steady and it’s ok (ish, still not 100%) Put your foot down hard and ‘big bad bogging’
Any ideas chaps ?

Dirtbiker
V4 Guru

United Kingdom
1060 Posts

Posted - 25 Aug 2019 :  20:46:54 Show Profile Reply with Quote
I had something similar with my 96. Condenser cured it as was failing when hot. I have an electronic ignition that still uses points and condenser. Maybe coil also as can be affected by heat.
Cheers
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Betsy67
V4 Fanatic

United Kingdom
409 Posts

Posted - 26 Aug 2019 :  11:00:42 Show Profile Reply with Quote
Mmmm, never thought about the coil as it only occurs if you put your foot down hard. Pulls absolutely fine the rest of the time.
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Derek
V4 Guru

United Kingdom
2187 Posts

Posted - 26 Aug 2019 :  11:26:25 Show Profile Reply with Quote
Carb accelerator pump not working? New diaphragm needed?
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melle
V4 Guru

United Kingdom
3830 Posts

Posted - 26 Aug 2019 :  11:29:48 Show Profile Reply with Quote
What's the story with the engine and carb, have they recently been rebuilt or what is their mileage? Did this set-up work before and did the issue start suddenly? Has it gotten worse over time? When did you last adjust the valves? Always tricky to diagnose from behind a keyboard...

It could be ignition related as Dirtbiker suggests. However, I would start by checking the vacuum hose from the carb to the dizzy (split ends?) and re-torquing the inlet manifold bolts/ nuts and the carb nuts (do not over-tighten!) to make sure there are no vacuum leaks there. Unfortunately it's not uncommon for old carbs to leak around the spindles, you could try and test that by squirting brake cleaner onto the carb and seeing if that alters the revs. This is not a watertight method by any means though, i.e. there may be no perceptible change yet there can still be a leak.

www.saabv4.com
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melle
V4 Guru

United Kingdom
3830 Posts

Posted - 26 Aug 2019 :  11:32:51 Show Profile Reply with Quote
quote:
Originally posted by Derek
New diaphragm needed?

The DCD doesn't have a diaphragm.

By the way, does the 2nd stage spindle move freely? Take the air filter off and move the accelerator manually to check. They tend to bind occasionally.

www.saabv4.com
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EdinburghJoe
V4 Fan

United Kingdom
162 Posts

Posted - 27 Aug 2019 :  16:09:45 Show Profile Reply with Quote
I'm still very much learning all this, but I'd have thought that if the problem was an air leak then putting your foot down should hide it, as the open throttle would reduce the vacuum in the manifold. That was my diagnostic test with two running issues I had at different points this year: if the problem goes away under hard acceleration that points to air leak, if it gets worse then something else (in my case the points were sooted up, I think down to the condenser or poor earth maybe)

What do you mean by bogging - misfiring, uneven running, loss of power: that sort of thing?

If what I've said above is wrong then someone please correct me so save me barking up the wrong tree in the future...

Joe
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melle
V4 Guru

United Kingdom
3830 Posts

Posted - 27 Aug 2019 :  16:38:55 Show Profile Reply with Quote
quote:
Originally posted by EdinburghJoe
I'm still very much learning all this, but I'd have thought that if the problem was an air leak then putting your foot down should hide it, as the open throttle would reduce the vacuum in the manifold.
That's correct, in theory. However, it's often possible to mask a vacuum leak at idle/ low rpm (high vacuum) by adjusting the slow running circuit, but it will be more difficult to hide when the main and acceleration systems kick in (the DCD is a bit particular in its stages as the bulk of the running is controlled by the "idle" jets). A vacuum gauge may be a very helpful tool here. The hose between the n/s rocker cover and the PCV valve also is a known cause for leaks by the way.

There is a DCD manual and a parts book on my website: http://www.saabv4.com/index.php/project/carburettors/ I can email a Weber Tuning manual pdf if required.

www.saabv4.com
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Betsy67
V4 Fanatic

United Kingdom
409 Posts

Posted - 27 Aug 2019 :  20:16:24 Show Profile Reply with Quote
Definitely a ‘heat’ problem. Ran Betsy to work today and pulled clean and strong in all conditions.
Checked the liquid levels on Sunday and found the coolant to be low. Probably put 1.5 litres or so in. Not checked it recently as every time I had previously checked it no top up was required. Temp gauge never moved away from normal, so no hint that it was low.
Downside is that I reckon the LH head gasket has gone, so a job for the weekend. The law of sod blesses me with 2 spare RH gaskets in my collection! 2 LH ordered today.
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EdinburghJoe
V4 Fan

United Kingdom
162 Posts

Posted - 28 Aug 2019 :  12:49:25 Show Profile Reply with Quote
I read in Haynes that a poor condenser can run fine at start up when the engine is cold, but after a few miles will start to cause problems again. I was a bit sceptical about this, but it did seem to be part of a problem I had earlier this year. I could go between 5 or 10 miles perfectly, but then the engine would misfire and lose power. It would still idle, but could barely move the car, even in first.
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melle
V4 Guru

United Kingdom
3830 Posts

Posted - 28 Aug 2019 :  13:47:48 Show Profile Reply with Quote
quote:
Originally posted by Betsy67
Downside is that I reckon the LH head gasket has gone, so a job for the weekend.
Bummer! Good luck fixing it and don't forget to investigate what caused the HG to fail before putting the head back on. Would be a shame if the head is warped or cracked and a new gasket doesn't solve the issue. I hope you ordered good quality gaskets, I've had issues with "eBay specials" in the past.

quote:
Originally posted by EdinburghJoe
I read in Haynes that a poor condenser can run fine at start up when the engine is cold, but after a few miles will start to cause problems again
I can also confirm this from experience. I always kept a spare condenser in the car (I now have electronic ignition) but gave it away a week before a holiday in the Alps. I forgot to replace the spare and of course the one on the car failed in Austria. Tried to limp to NL, but gave up in southern Germany and the car went home on a flatbed and we in a rental car, as even a local Saab dealer couldn't source a replacement condenser. The insurance insisted on this solution, instead of having a replacement posted from NL which would have been much cheaper...

Be aware of aftermarket condensers and buy any genuine Bosch one you can snap up if you still run points ignition!

www.saabv4.com
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Betsy67
V4 Fanatic

United Kingdom
409 Posts

Posted - 28 Aug 2019 :  19:42:56 Show Profile Reply with Quote
Electronic ignition, so no condenser.
Further inspection today and I reckon the HG is ok. Changed the fuel pump a few weeks back and the appearance of oil has only started since. Everything is tight and I fitted a new gasket. However, there is ‘vapour’ around the rolled edge where the diaphragm lives. Will put the old pump back on and take it from there.
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melle
V4 Guru

United Kingdom
3830 Posts

Posted - 29 Aug 2019 :  12:28:54 Show Profile Reply with Quote
Why did you change the fuel pump? Why did you initially suspect the LH HG and think it's OK now?

www.saabv4.com
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Betsy67
V4 Fanatic

United Kingdom
409 Posts

Posted - 29 Aug 2019 :  19:29:47 Show Profile Reply with Quote
Changed it to rule out one thing on the ‘bogging’ issue. Thinking it was a fuel delivery problem.
Swapped them back over tonight and I’m Quietly confident that the leak will be cured. The mating surface on the pump was definitely not flat!! Just by eye, you could see that it wasn’t flat, but a straight edge confirmed it.
No bogging whatsoever over the days since I put this post on - How very typical [:-)]
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