front pistons doesn't work at low RPM.

Couple of questions.

just to confirm cyl 1 and 2 are the two front most cyl.

Do you have separate CHT for each cyl or a standard setup with left and right CHT ? 

does it just run rough at idle or throughout the range ?

have you checked for vacuum leaks ? 

julian van santen wrote:

Couple of questions.

just to confirm cyl 1 and 2 are the two front most cyl.

Do you have separate CHT for each cyl or a standard setup with left and right CHT ? 

does it just run rough at idle or throughout the range ?

have you checked for vacuum leaks ? 

Hi Julian

Yes Cyl 1 & 2 are front cylinders.

I have separate CHT for each cyl, also i can confirm cylinders which are not working are 1 & 2.

run rough at idle, if i increase rpm the cylinders start to work. if i decrease down to idle both cylinders stop work again, i can confirm because run rough and CHT decrease less than 100º C.

Yes I have checked vacuum, and cylinders are ok.

any idea?

Of course this is a guess only, nothing more.

Double check the fit of the spark plug connector and its cable. It may well be that it seems OK but in fact the connector is not screwed on correctly. In that case the resistance from cable to connector pin (inside the spark plug connector) may be too high to supply the spark plug propperly when operated on low revs.

The fact that the problem seems to have deteriorated over time may count for the fact that the pin/cable connection wears off through sparks at that connection (at the inner pin). To eliminate that as a factor you could unscrew the connector from the cable, cut back the  cable by 5 to 10mm and put the spark plug connecor back on. A little spot of grease on the cable may aid screwing the connector in place propperly (to its mechanical end position) - just to ensure that the resistance between the connector rubber and the outer surface of the cable does not hinder completion of the last turn or so.

In case you have spares at hand replace the connector when cutting back the cable.

Peter Kl. wrote:

Of course this is a guess only, nothing more.

Double check the fit of the spark plug connector and its cable. It may well be that it seems OK but in fact the connector is not screwed on correctly. In that case the resistance from cable to connector pin (inside the spark plug connector) may be too high to supply the spark plug propperly when operated on low revs.

The fact that the problem seems to have deteriorated over time may count for the fact that the pin/cable connection wears off through sparks at that connection (at the inner pin). To eliminate that as a factor you could unscrew the connector from the cable, cut back the  cable by 5 to 10mm and put the spark plug connecor back on. A little spot of grease on the cable may aid screwing the connector in place propperly (to its mechanical end position) - just to ensure that the resistance between the connector rubber and the outer surface of the cable does not hinder completion of the last turn or so.

In case you have spares at hand replace the connector when cutting back the cable.

I have done all of that you have told. i have checked resistance on the cables and plug connectors.

Which model of engine, exactly?