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Northy

911SC bucking at 1500 revs

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I've been trying to get a resolution on a bucking that I get when trundling along through a 30 zone in my SC. 

Always at 2nd and 3rd gear, at 1500 revs the car will start to buck, sometimes mildly but it can get to the point where you have to dip the clutch.  Seems to be worse when the engine is warm. We suggests it's fuel mixture related.  Other than this, the car runs well, starts well hot and cold, pulls hard all the way and makes decent enough mpg.

I've now changed cap, rotor arm, leads, fitted a CDI+ box, checked the timing, checked idle mixtures when warm and reset to 2% (seems to be consensus on here of what's best for an SC).

If I take the oil cap off the revs drop by 100 - 200. If I wind in the throttle bypass screw (aka idle speed screw) it will make the engine turn over really slowly.  So I'm not 100% convinced it's an air leak. I've tried to test for vacuum leaks using some carb cleaner - maybe I need to go a bit more nuts with that, I was probably a bit over cautious for fear of starting a fire. 

Any other ideas? worth just changing the injectors and seals? they've been on here since I've had the car (nearly 20 years) and never been cleaned or anything.  After listening to the guy at JZ - he basically said change them rather than trying to clean them. 

I think I'll have another go at detecting a vacuum leak and if I can't maybe take it somewhere who can properly test that. 

Anyone got any other ideas?

 

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Have you performed a fuel pressure test (system pressure, cold control pressure, warm control pressure etc)?

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2 minutes ago, Lesworth said:

Have you performed a fuel pressure test (system pressure, cold control pressure, warm control pressure etc)?

No sir. Easy enough to do?  I've got a Bosch 044 about to go in (perhaps this week). Do the test before or after (or both). 

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Its easy once you know how! But its ages since I did it. You plumb the test kit into the line between the WUR and Fuel Metering Head I think and then jump the fuel pump relay to run it. System pressure read dead cold without engine running. With the pump still running you open the valve in the kit to then read cold control pressure. Then you switch on the engine and monitor the pressure reading as the car warms up until you take the warm control pressure. Finally you switch off the engine and read the residual pressure x minutes after switch off.

So you check four readings against spec: System pressure, cold control pressure, warm control pressure and residual pressure.

Sounds complex but once you get your head around it, it does make sense. I had a test kit made up for the 924 which also works on the SC, you're welcome to borrow it. I also made up a fused switch on a long lead so I could activate the pump from the jumped relay while at the engine bay. Welcome to borrow both. Bentley covers this.

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Thanks @lesworth that’d be great 👍 I’ll get my new fuel pump fitted and take you up on that.

I’m tempted to bung the injectors in anyway just to know they’ve been done.

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I would also consider minor tweaks to your fuelling/mix as I had the same problem on mine after a service at GCR.  Chris quickly sorted it with a minor adjustment

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I suspect air leak but you might richen it up a bit.  

How are your airbox screws?  Tight?

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I was having a look last night and was tweaking fuelling. I’ve not driven it yet but it sounded very smooth.  

@Jonny Hart Last time I checked the air is screws they were tight, but I’ll check again. I’ll also get a bit more generous with the brake cleaner test. 

 

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Isn’t this just too low a revs Northy? Really should be aiming for 2k plus rpm when on the move? Does it do it if you drop a cog and go above 2k rpm? 

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1 hour ago, Busybee said:

Isn’t this just too low a revs Northy? Really should be aiming for 2k plus rpm when on the move? Does it do it if you drop a cog and go above 2k rpm? 

Mine doesn't buck at 1500 rpm and I don't do this.

I had a stumble rather than a buck at about 1800 which turned out to be control pressure related. I tested the pressures and adjust the WUR to bring cold control pressure back into spec. Oddly this rectified the stumble which used to happen warm or cold. A fuel pressure test can be completed in about 10 minutes and provides a lot of key information for diagnosing starting and running issues.

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1 hour ago, Busybee said:

Isn’t this just too low a revs Northy? Really should be aiming for 2k plus rpm when on the move? Does it do it if you drop a cog and go above 2k rpm? 

 To be fair bucking/kangarooing shouldn't happen at all and if it is there is something wrong. I, like a lot of people, just put up with it but it's not right. 

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Best to start off doing all the control pressure measurements and work from there.

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1 hour ago, Phill said:

To be fair bucking/kangarooing shouldn't happen at all and if it is there is something wrong.

+1 on this...

1 hour ago, Lesworth said:

Mine doesn't buck at 1500 rpm and I don't do this.

...and this.

I can move off from a standstill in second without any issues in my SC and it will pull smoothly from below 1500rpm in third (not that I normally do/recommend this).

Mark

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I think we are confusing the issue here chaps. Holding the revs at 1500rpm to trundle through traffic is what I'm saying shouldn't be done. Pulling through 1500rpm whilst accelerating is a different matter. I can pull off in 2nd too but I'd drop a cog rather than hang at 1500rpm if in traffic (before I blast past em of course) 

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Just now, Busybee said:

I think we are confusing the issue here chaps. Holding the revs at 1500rpm to trundle through traffic is what I'm saying shouldn't be done. Pulling through 1500rpm whilst accelerating is a different matter. I can pull off in 2nd too but I'd drop a cog rather than hang at 1500rpm if in traffic (before I blast past em of course) 

So, what you're saying is you always leave it in second whilst going through a 30 zone?  

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If you're doing 20 then I guess yes. If the traffic frees up and you're doing 30 then try 3rd. I guess just try to keep the revs a little higher to ditch the very low 1500rpm area you're kangarooing at. 

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The point is, you shouldn't have to do that. You should be able to bimble along in traffic at 1500 without the car behaving badly.

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In my opinion, 30mph in 3rd whilst trundling through a village barely on the throttle, is not the problem. I think the bucking is just highlighting a problem with the car, not that it's a bad driving habit. I'm sure others will disagree, but that's what the internet is for.

I was going to take tomorrow off to do the fuel pump swap, so I'll investigate this a bit further then too. 

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Assuming all parts are working as they should all bucking behaviour can be adjusted out.

Fresh fuel pump sounds like a reasonable start since you already have it on the shelf - after that follow one of the well documented tuning guides to get it running bob-on. Once you get it close then its only really small adjustments to get it just right so dont be too hasty turning adjusters too far either direction as an overshoot in either direction can sometimes create similar symptoms and make you feel its not doing anything.

Good luck buddy.

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4 minutes ago, Phill said:

The point is, you shouldn't have to do that. You should be able to bimble along in traffic at 1500 without the car behaving badly.

Its barely above tick over and you're moving quite a mass with virtually no power at those revs in 3rd gear.

But hey, I'm a lucky fella. 60mph limits on my doorstep and the "towns" around me are one street affairs where you're through in 2 minutes with barely any traffic. Hope the pump and tune sort it @Northy

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2 hours ago, Busybee said:

Its barely above tick over and you're moving quite a mass with virtually no power at those revs in 3rd gear.

I respectfully disagree with you there. I have no problems with bimbling at 1500 rpm and its not barely above tickover, which is 850-900 or so.

18 hours ago, Northy said:

Thanks @lesworth that’d be great 👍 I’ll get my new fuel pump fitted and take you up on that.

I’m tempted to bung the injectors in anyway just to know they’ve been done.

Just let me know when you are ready and I'll post it to you.

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Les is right you know. You need fuel pressure kit to do this.  Check the system pressure (which you can adjust with shims in the side of the metering head) . Make sure the wur is disconnected when you do this. Then flip the valve to the control pressure check the cold reading is in spec- plug it in the wur will begin warming up with the ignition on as it has a live feed. Wait for it to get to temp check the warm control pressure is in spec. If all correct then adjust your mixtures. Should be smooth from idle.  

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21 hours ago, Lesworth said:

I respectfully disagree with you there. I have no problems with bimbling at 1500 rpm and its not barely above tickover, which is 850-900 or so.

Think we'll have to agree to disagree on this Les. To me, 1500rpm and 3rd gear is on the low side. Drop it down a cog and the problem vanishes.

If there is a tweak to smooth this out, I'm all for it. Guess I spend very little of my time in the IB's in town so it's never been an issue for me in particular. 

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OK so it turns out, I'm a 🔔🔚

When I swapped out the CDI box I reset the timing to TDC at idle, not 5 degrees BTDC. 

Changed that this morning, reset the idle and mixtures and it's spot on now.  No bucking and is happy to pull from idle without a problem. 

I also checked the airbox screws, all tight and good. 

Now just got to decide whether to attempt the fuel pump swap before we set off to Nurburgring and Spa... (the current one is fine, but just wanted to swap when fitting the under tray).

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1 hour ago, Northy said:

OK so it turns out, I'm a 🔔🔚

When I swapped out the CDI box I reset the timing to TDC at idle, not 5 degrees BTDC. 

Changed that this morning, reset the idle and mixtures and it's spot on now.  No bucking and is happy to pull from idle without a problem. 

I also checked the airbox screws, all tight and good. 

Now just got to decide whether to attempt the fuel pump swap before we set off to Nurburgring and Spa... (the current one is fine, but just wanted to swap when fitting the under tray).

Ha, good work :)

How long till you leave? I'd suggest leaving orig pump in place and take the new one as a spare.

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