Scarab Rotax 250 SC one engine surging at cruise. 2018/255 ID

[danny4255]

I've owned it since new. 140 hours, over-maintain Rotax oil and filter every fall, new plugs. same time but not last season on plugs might have 40 hours on them, coil boots, and rocker cover after season 2. High-performance coolant after 2 seasons, complete flush/fill. STA-BIL 360 Marine Ethanol Treatment & Fuel Stabilizer and Boostane up to 91/92 octane ever fuel up. The only issue ever was cold misfire codes in the cold spring waters in Michigan. Resolved by running true octane booster
BOOSTane if anybody's interested. So running great all season except yesterday on the way back in for the day, at cruise 20/25-ish/ 5000 RPM's 2-3 ft waves were pretty rough. The port engine began the repeatedly surging between 5000 and 7500 (approximate) RPMs. Could obviously hear regular supercharge noise with surge. Both engines in Sync, no check engine light (didn't check for codes yet though). Shut off/restarted still present. Limped back. On trailer inspected, no weeds, prop, and wear rings look fine and same side to side. Called 2 local service departments today, never hear of concern, bring it in next week. Wondering if anybody with the same setup ever had a similar issue. Also, it always has had a cavitation-type sound on WOT hole shots, almost none with 1/2 to 3/4 throttle from a stop. Has trim tabs as well but this is with tabs up. Thanks if anyone can assist, Danny

 

[Dachshund Dad Bob]

I've owned it since new. 140 hours, over-maintain Rotax oil and filter every fall, new plugs. same time but not last season on plugs might have 40 hours on them, coil boots, and rocker cover after season 2. High-performance coolant after 2 seasons, complete flush/fill. STA-BIL 360 Marine Ethanol Treatment & Fuel Stabilizer and Boostane up to 91/92 octane ever fuel up. The only issue ever was cold misfire codes in the cold spring waters in Michigan. Resolved by running true octane booster
BOOSTane if anybody's interested. So running great all season except yesterday on the way back in for the day, at cruise 20/25-ish/ 5000 RPM's 2-3 ft waves were pretty rough. The port engine began the repeatedly surging between 5000 and 7500 (approximate) RPMs. Could obviously hear regular supercharge noise with surge. Both engines in Sync, no check engine light (didn't check for codes yet though). Shut off/restarted still present. Limped back. On trailer inspected, no weeds, prop, and wear rings look fine and same side to side. Called 2 local service departments today, never hear of concern, bring it in next week. Wondering if anybody with the same setup ever had a similar issue. Also, it always has had a cavitation-type sound on WOT hole shots, almost none with 1/2 to 3/4 throttle from a stop. Has trim tabs as well but this is with tabs up. Thanks if anyone can assist, Danny

Hello Danny: I own a 2020 215ID with twin 300's and have been experiencing the surging issue as well, not certain it is one or both engines however. No engine codes on the dash as well. I thought someone posted something against using octane boosters on Super Charged engines. Another guy advised 91 octane fuel even though it doesn't call for it. Let me know if you are able to solve this problem. Bob

 

[Thermobrett]

I have a SC Yamaha SVHO and it specifically calls out for high octane fuel. 91 or higher. I have heard similar issues with the Yamaha engines when not using high octane, but this may just be a myth.