Mystery Jerk

My Sunny 5-speed manual jerks a lot. Here's a history of the repair and modification done.

1. Originl E13 engine upgraded to E15 complete with gearbox in 1999.
2. Changed to a new clutch kit including clutch bearing, clutch plate and cover in 2002.
3. Changed to electronic ignition system including ignition coil.
4. Changed to a new carburettor with auto choke in 2005.
5. Did a top overhaul in 2006 (due to blown head gasket and water sipping into the engine). Did a pressure test too.
6. Changed all four shock absorbers and springs, and front right and rear right lower arm bushes.
7. Changed three of five engine mountings (right gearbox and firewall mountings).
8. Changed to new spark plug cables (five cables)
9. Changed to a new fuel filter.
10. Tried Bosch and NGK spark plugs. Now using Bosch.

When I let off the accelerator during slow driver (especially in 1st or 2nd gear) the car jerks two to three times. The problem occurs even when in 3rd, 4th or 5th gears, but the jerks isn't as bad. I tested my friend's Sunny (1.3) and to my surprise there was no jerk at all. The mechanics have no idea what is causing it.

There is another problem. Sometimes when I open the air-filter pod cover, there will be some engine oil inside the pod. I presume this is deposited from the hose connecting the air-filter pod to the engine valce cover. Why does this happen?

Reply:
If I read you right, the jerking is not caused by carburetion but rather there is a 'clunk' or juddering whenever you depress the accelerator. If that is so, I would look at all engine mountings again and the front and rear 'buffers' or stabilisers for the engine/transmission assembly. There should not be any noticeable movement of the engine/transmissiona ssembly if one rocks the engine back and forth and left and right. If there is movement , tha could be reason for your 'clunking' and you need to check all the mountings especially the ones that were replaced. The reason is that these mountings are supposed to be fitted in a certain position to take the weight of the engine and torsional movement of the engine on acceleration and deceleration. If not fitted correctly, it can collapse, resulting in plenty of engine movement. Check the front and rear buffers for any cracks or signs of collapsing. You may have to replace the mountings if they show signs of wear or collapse. You should use original parts and not replacement units as some of the replacement parts do not use the correct rubber and are too soft and collapse after some time or are affected by oil, etc.

2. The hose connected to the valve cover is to recirculate the oil fumes from the engine so that they get burnt up in the combustion process. If there is a lot of oil in the air cleaner housing, it could mean that the pistons and rings are worn, and combustion pressure is blowing past the pistons and coming through from the crankcase.