UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 9)*

POWER SOLUTIONS INTERNATIONAL, INC.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

73933G 20 2

(CUSIP Number)

Gary S. Winemaster

c/o Power Solutions International

201 Mittel Drive

Wood Dale, Illinois 60191

(630) 350-9400

Copy to:

Catherine V. Andrews, Esq.

Power Solutions International, Inc.

201 Mittel Drive

Wood Dale, Illinois 60191

(630) 350-9400

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

April 24, 2013

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Item 1. Security and Issuer

This Amendment No. 9 to Schedule 13D (“Amendment No. 9”) relates to the shares of common stock, par value $0.001 per share (“Common Stock”), of Power Solutions International, Inc., a Delaware corporation (the “Issuer”), and amends the Schedule 13D filed on May 9, 2011, as amended by Amendment No. 1 to Schedule 13D, filed on November 14, 2011, as further amended by Amendment No. 2 to Schedule 13D filed on September 24, 2012, as further amended by Amendment No. 3 to Schedule 13D, filed on January 2, 2013, as further amended by Amendment No. 4 to Schedule 13D, filed on January 16, 2013, as further amended by Amendment No. 5 to Schedule 13D filed on February 2, 2013, as further amended by Amendment No. 6 filed on March 14, 2013, as further amended by Amendment No. 7 filed on April 11, 2013, as further amended by Amendment No. 8 filed on April 23, 2013 (as further amended by this Amendment No. 9, the “Schedule 13D”). The principal executive offices of the Issuer are located at 201 Mittel Drive, Wood Dale, Illinois 60191.

This Amendment No. 9 is being filed by Gary S. Winemaster (the “Reporting Person”) to furnish the additional information set forth herein. Except as specifically provided herein, this Amendment No. 9 does not modify any of the information previously reported in the Schedule 13D. Capitalized terms used but not defined herein shall have the meanings ascribed to them in the Schedule 13D.

Item 2. Identity and Background

Except for information provided in Amendment No. 2 to this Schedule 13D, all information pertaining to the identity and background of the Reporting Person remains the same.

Item 4. Purpose of Transaction

Item 4 of the Schedule 13D is hereby amended by adding the following:

On March 13, 2013 the Reporting Person entered into a Programmed Plan of Transactions under Rule 10b5-1 with Stifel, Nicolaus & Company, Incorporated (“Stifel”) pursuant to Rule 10b5-1(c)(1) under the Securities Exchange Act of 1934, as amended (the “10b5-1 Plan”). The description of the 10b5-1 Plan set forth in Item 6 below is incorporated herein by reference in its entirety.

The transactions contemplated by the 10b5-1 Plan will result in the disposition of securities of the Issuer as further described in Item 5 below (Item 4(a) of Schedule 13D).

Item 5. Interest in Securities of the Issuer

Item 5 of the Schedule 13D is hereby amended and restated in its entirety as follows:

(a), (b) 9,216,621 shares of Common Stock are outstanding as of May 9, 2013, the date of this Amendment No. 9. Based on the foregoing, the 4,345,500 shares of Common Stock (the “Reported Shares”) beneficially owned by the Reporting Person represent approximately 47.15% of the Common Stock outstanding as of the date of this Amendment No. 9. Such beneficial ownership percentage is calculated in accordance with Rule 13d-3 under the Securities Exchange Act of 1934, as amended.

The Reporting Person has the sole power to vote or direct the vote of, and the sole power to dispose or direct the disposition of, the Reported Shares, subject to the Purchase and Sale Agreement, as amended by the Purchase and Sale Amendment.

(c) Pursuant to his 10b5-1 Plan with Stifel, the following transactions were effected by the Reporting Person: on April 24, 2013, the Reporting Person sold 1,600 shares of Common Stock for $26.00 per share; on April 25, 2013, the Reporting Person sold 2,313 shares of Common Stock for $26.02 per share; and on April 26, 2013, the Reporting Person sold 33,350 shares of Common Stock at $26.00 per share. Apart from these sales and the entry by the Reporting Person into the 10b5-1 Plan, the Reporting Person has not effected any transaction in the Common Stock during the 60 days immediately prior to the filing of Amendment No. 9.

(d) As of the date of this Amendment No. 9, no person other than the Reporting Person is known to the Reporting Person to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Reported Shares covered by this Amendment No. 9.

(e) Not applicable.

Item 6. Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer

Item 6 of the Schedule 13D is hereby amended by adding the following:

10b5-1 Plan

On March 13, the Reporting Person entered into the 10b5-1 Plan with Stifel. Under the 10b5-1 Plan, beginning on March 18, 2013 and ending on June 16, 2013, Stifel is authorized to sell, and has sold on behalf of the Reporting Person, 90,000 shares of Common Stock, subject to the parameters of the 10b5-1 Plan. All sales under the 10b5-1 Plan are to be made in accordance with the terms, conditions and restrictions of the 10b5-1 Plan and the Reporting Person shall not have any control, influence or authority over sales made pursuant to the 10b5-1 Plan.

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: May 9, 2013