| Author | Message | ||
     Benoit Leus Experienced User Username: benoitleus Post Number: 20 Registered: 6-2009 |
My Shadow II is suffering from a rough idle. All ignition components have been replaced, I have checked for a vacuum leak and the carbs have been adjusted. I took off the carb assembly and checked the floats, butterflies and overrun valves, which all seem to be fine. However something else seems to be wrong. The gasket between the carb assembly and the induction manifold was soaked in petrol and has started to delaminate. Also the underside of the carb assembly is scratched as if someone has attacked it with a screwdriver. The gasket was replaced a few thousand miles ago when the A-bank valve stem seals were replaced, after which it idled much better. Since then it has gradually worsened again. Could the fact that the gasket doesn't seal well be the cause of the rough idle ? Benoit   | ||
| Jan Forrest Prolific User Username: got_one Post Number: 298 Registered: 1-2008 |
I may be wrong, but it seems to me that you have an induction leak at this gasket which has required the carbs to be set over rich to compensate for it. This would explain both the rough idling and the excess fuel which may not be atomising properly. If you can't lap the mating faces flat again to give a clean sealing surface you could always apply a thin smear of liquid gasket to both faces of the new gasket before fitting it. | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 62 Registered: 10-2010 |
Benoit, If air is leaking in at the gasket, then it could definitely cause a poor idle. However, I'd suspect an out-of-flat base on the plenum over those screwdriver gouges -- they don't look terribly deep. Check for flatness with a good straight-edge. I'd also want to know why there's so much petrol down there. (Note: petrol will leak through much smaller spaces than air, so it's possible petrol is wicking through the screwdriver gouges.) Anyway, check the o-rings where the little fuel filter bowl connects to the carb body. Lastly, when idling grab the butterfly shaft and push it to one side and then the other, both horizontally and vertically. If the idle changes depending on what position it's in (careful that you don't rotate it), then your bushings are worn and air is leaking in along the shafts. Jeff. | ||
| Benoit Leus Experienced User Username: benoitleus Post Number: 21 Registered: 6-2009 |
Jan, Jeff, thanks for your help. I checked the flatness of the base on the plenum with a straigh-edge. When I measure it as pictured (the red line) there is a very small (+/- 0,2mm) difference, where the 2 outer holes (red arrows) are lower. Could such a small difference be enough to create an air leak ? If so, do I : - have it skimmed - replace it - use a fuel-resistant sealer Benoit  | ||
| David Lacey Yet to post message Username: dlacey Post Number: 1 Registered: 11-2010 |
Hi Benoit, If you think there's an induction air leak causing a misfire you can search for it using a propane torch (unlit)....squirt the gas at potential leak sites with the car running and if the misfire improves you've found your leak. | ||
| Benoit Leus Experienced User Username: benoitleus Post Number: 22 Registered: 6-2009 |
Hi David, I already tried that but couldn't find any leak. Benoit | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 63 Registered: 10-2010 |
Gasket papers are usually about 0,4 to 0,8mm thick. With a single bolt providing the compression I doubt you'd get even the thicker stuff to compress by 25%, and you're certainly not going to get the thinner stuff to compress by 50% -- so yes, a 0,2mm out-of-flat might indeed cause an air leak. If you've got someone that can skim it, that'd be my first inclination. If not, Hylomar Blue should work fine, if perhaps making assembly/dissassembly a bit more messy. (And it's certainly worth trying the Hylomar to see if it cures the problem.) Jeff. | ||
| Benoit Leus Experienced User Username: benoitleus Post Number: 25 Registered: 6-2009 |
I am in the middle of replacing the valve stem seals on my Shadow and used the opportunity to inspect the spark plugs. I installed NGK Irridium plugs about 2000 miles ago with an electrode gap of 0,6mm. Imagine my surprise when I remeasured them and found the gap to have grown to 0,8mm on all the spark plugs. This kind of wear is of course abnormal, and I suppose it will have contributed to the rough running. What could be the cause ? Regards, Benoit | ||
| Jan Forrest Grand Master Username: got_one Post Number: 308 Registered: 1-2008 |
Which model number of plugs did you fit? From the increase in gap I'd be inclined to suspect that they are running hotter than expected. A cooler running plug might be a good idea as & when you replace them. You can get much the same symptoms of the fuel mixture is set too lean and could be the cause here as the plugs have eroded a lot for the stated mileage. | ||
| Benoit Leus Experienced User Username: benoitleus Post Number: 26 Registered: 6-2009 |
Hi Jan, I fitted NGK Irridium GR4IX plugs. Previously I used NGK BPR5ES plugs, which are indeed cooler, and the car seemed to be idling better with them. I was also very surprised by the amount of wear. |
