I am trying to analyse C3-C7 carboxylic acids (mono,di and also with OH)with LCMS. We bring the sample to pH 2. Using ESI+ and MeOH/H2O (90% to 30%)+HCOOH we got lots of cluster ions and almost no M+H ?? With TFA no linear C1-C6 can be seen and only the diacids give nice M+H ??
Should we stay with formic acid, or perhaps APCI ??

With ESI positive, you're trying to add a proton to an acid at an acidic pH, not an easy task. Try switching to negative ion ESI with the capillary voltage at 2.5 or even 2.0 kV. Then you would be detecting the M-H ion. The reason for the lower voltage is to avoid corona discharge at the ESI probe tip, a situation that would be detrimental to your signal. I would suggest staying with formic acid. TFA can supress the signal starting with concentrations of 0.5% v/v.
As a side note, make sure you're aware of how your LC/MS software does negative ionization. It may not take the tune settings for ESI positive and reverse the appropriate polarities for ESI negative. There may be a completely separate section for ESI negative that the instrument goes to for the settings.