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911SC bucking at 1500 revs


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1 hour ago, PeterK said:

I might try raising CO, maybe as high as 2.5%

When you check control pressure, does the WUR hold the applied vacuum (mine leaks) ?
Any air leak around the deceleration valve (the bit behind the left side of the engine when looking in from the rear)

From memory the WUR will hold a vacuum over a longish period. I’ll check that again. 
 

Haven’t located any leaks around decel valve. Large vacuum hoses etc.  Should get the smoke test kit tomorrow, so will give that a try too. 
 

I’ll try tweaking the mixture up a bit and see what happens. 

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I just tried richening it up and it didn’t make any notable difference.  

I think the next logical thing is to complete the smoke test and completely rule out air leaks. 

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Often what feels like fuel can be ignition.

Have you got a spare coil you can check? Maybe check timing? Dizzy cap and rotor? Plug lead interference?

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Agree with John on this, I had a similar problem and sorted it with a new dizzy

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See previous page - I’ve been through quite an extensive process so far. Timing is correct, tried new cap and leads. Checked rotor arm has correct resistance.  I do have a coil I can check against (old black jobby).  I might try locking the dizzy and setting the advance using the CDI+ : just in case it’s dizzy/advance related. 
 

I have tried going richer, so I will first try going leaner on the mixture and set it at 1%, which is still in spec. 
 

Smoke test kit should be here in a few days so will rule out any leaks. 

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On 28/04/2024 at 22:39, Northy said:

 

It sounds like an air leak, here is a page from the workshop manual they may help with some checks.

I would have said 2% co was a bit lean, but you would feel it right through the rev range.

Screenshot_20240505_121940_OneDrive.thumb.jpg.74a42eaa2327f5aa7189e741d7cd2927.jpg

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It’s interesting. I have had a play with the timing. If I set the timing at idle to 5 degrees, the stumble at 1400 revs is pretty much gone. The problem is that even with the vacuum disconnected, it’s pulling over 25 degrees advance at 4k, perhaps more like 27. With the vacuum on, it’s pulling something like 30 degrees total.  Previously I’d prioritised the high revs timing and set that to 25 degrees, which put the idle timing at 2-3 degrees. 

So the good news is the stumble has gone (5 degrees and 3% mixture at idle). 

My question is do we think that much advance is a problem (largely standard SC + SSIs). 

I suspect the answer is put a custom map into the CDI+ box to control both ends of the scale. 

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Sounds like your going in the right direction.

The early engines could run 35 degrees at 6000 rpm. I used to set my 2.4 to that and leave the idle to be whatever.

One thing I never got to the bottom of with a vw engine was you would set it 32 max with vacuum off....which gave 40 with vac on. Apparently I was told that was completely normal and something about when driving the vac would not be above 32. I never figured that bit out.

I couldn't find a curve for a sc....but im sure Johnny will have one for the cdi+

I'm sure you've got this but some extra bits for reference.

 

20240505_164841.jpg

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Thanks buddy, @Strictly. That’s interesting, mine is an 82-83. Not shown on that chart. The 1981 showing not above 25 degrees, is a bit of a concern.

@Jonny Hart any thoughts on max advance on an 82-83? 

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I use the curve from Jonny’s SC (it’s on the CRF website) as a complete overlay.  My dissie advance is locked off.

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4 minutes ago, PeterK said:

I use the curve from Jonny’s SC (it’s on the CRF website) as a complete overlay.  My dissie advance is locked off.

What are the key numbers Peter? Advance at idle and 4k rpm? (They are plus 5 degrees aren’t they?). 

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I’m away sailing (for a change 🤣), but IIRC, it’s almost zero at tickover rising to about 24’ max advance between 3-6k, all plus 5’ static.

 

Initial curve is similar to Porsche, although much flatter (& higher) in the mid to upper rev ranges.

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10 hours ago, PeterK said:

I use the curve from Jonny’s SC (it’s on the CRF website) as a complete overlay.  My dissie advance is locked off.

Same here on my '83 SC.

So long ago now (I have CDI #1!) I can't recall the exact numbers.

Mark

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Posted (edited)

@Northy  Just grab the software from here (CDI 1.38), put it on a Windows machine and my SC profile is in the list.  You don't need to actually connect to the box to see it.

What's probably happening is that your springs/weights are either tired or sticky so the slope of the advance from idle upwards isn't right.

Edited by Jonny Hart
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IMG_2169.thumb.jpeg.a43f7923c532a02a9030d2ea068db86a.jpegIMG_2168.thumb.jpeg.b1b649b6c7c08f5382d5d950f83d6593.jpeg
 

Thanks for the advice. I’ve locked the dizzy and applied a slight variation of the SC map. I dropped the advance back a few degrees higher up the range, mainly to keep the advance from going too high (it’s still pulling past 25 degrees on the lamp). 

I haven’t had a chance to try the smoke test yet. Still going to do that, just to eliminate it from the puzzle. 

 

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Injectors 1 and 4 smoked like Dot Cotton (RIP).  Plus a whisp of smoke around the left side of the airbox seam.  

I had a waggle of the injectors (which are new 2 years ago, as are seals) and the sleeves are moving pretty freely. Clearly they need to be changed. 

The airbox might soldier on for a bit, but it’s almost inevitable it will need doing at some point. 

So I have some decisions to make.  

 

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