I have a HPI 15FE engine that I bought used. I disassembled, inspected, cleaned, reassembled, and oiled it, and today I ran a tank of fuel through it on a very rich setting to flush out any dust that might've been left in the engine. During the "break-in" run, the engine VIBRATED LIKE CRAZY. I've never had a nitro engine vibrate so much. I used all stock parts when reassembling it, and I didn't cut or grind any moving parts so they all weigh the same as they originally did. I'm at a loss as to what might be wrong. Is it normal for the 15FE engine to vibrate a lot, or is there something wrong with my engine?

The only thing I can think of that *might* make a difference is this engine is mounted sideways instead of front-to-back. Any chance that would affect the way the vibrations propagate through the chassis?

Had you run it before you took it apart? Was it me I'd run it on the bench with a prop and see what it did.

No, I didn't run it before taking it apart. It was dirty, so the first order of business was to take it apart and clean it.

I don't have a bench that I could mount the engine to for testing. I live in an apartment.

Computer's messing up.

Bench can be an apple box in the parking lot. You wouldn't run it in the house anyway. My nitro motors all sit sideways in the car and don't vibrate much so I wouldn't think it was that. Flywheel hole too big and not centered? You'd have seen a broken piston skirt when you had it apart. Almost has to be a balance issue I would think. Motor tight in the mount. clutch shoe missing. Just trying to think of what all would cause a big vibration. You got me curious now.

Yeah, it's a mystery to me. I've never run a Toki-manufactured engine before so I have no idea how much vibration to expect from them.

The motor is firmly attached to the car, the bearings are tight in the crankcase, the crankshaft has no slop when fitted into the bearings, and I don't see any blurriness around the edges of the flywheel when the engine is running, so the crankshaft isn't bent and the flywheel is centered. I switched to a clutch-shoe setup that I've used on several other cars without issue, and that didn't make a difference either.

It honestly feels like the counterweight on the crankshaft is either too heavy or too light, but I'm using the stock piston so it doesn't make sense that this would be a problem unless ALL 15FE engines have this problem. (it's an old engine, so who knows? I can't find any other reports of excessive vibration for this engine, but it dates back to the late 90's/early 2000's, so maybe nobody ever thought to post it online back then.)

The counterweight is round, unlike my newer engines. It's the type that just has a thicker area opposite the conrod journal. I was looking at my Losi 3.4 engine, which has a similar counterweight and it rotates nice and smooth, and there's a flat spot that's cut off the counterweight right next to the conrod journal. I'm contemplating taking a complete shot in the dark and trying that with the 15FE's crankshaft, to see if that helps it run more balanced too. The problem with that, though, is I have no idea whether the counterweight is too heavy or too light, so cutting a piece off could end up making the vibration even worse.

I pulled the crankshaft out and set it on the edge of a glass table. It slowly rolled back and forth until the conrod journal was at the top, but then when I hung the conrod off the journal it immediately inverted. Considering it didn't even have the weight of the piston and wrist pin hanging off the end of the conrod, it seemed to me the counterweight was much too light.

I tried hacking a piece off the crankshaft next to the conrod journal (i.e. the side opposite the counterweight), like was done at the factory on my Losi 3.4. The engine still runs, I think it runs a little smoother, but I'll have to wait until I can run it properly during the daytime to be sure. Call me crazy but I didn't want to rev a nitro engine on my back porch at 1am.

Ran it today. It will need some tuning, but the important thing is the vibration is indeed reduced. Also, that old engine revs like a champ with a modern carburetor installed.

I modified the counterweight some more, to convert some of the vertical vibrations into horizontal vibrations, and that seems to have helped:



However, it's still breaking clutch springs like crazy, and I got curious and compared the weights of the flywheel I have installed vs. the original flywheel intended for use with this engine:

12g vs. 40g.

Considering this engine is basically a repurposed airplane engine, as evidenced by the single-needle carb and the propeller thrust plate that came with it, it seems pretty likely that it was never designed to idle as low as car engines require, hence why it originally included a heavy flywheel to help it idle smoothly. That would explain the excessive vibration and the breaking clutch springs. But I really don't want to use the 40g flywheel that came with the engine, because that's just way too damn heavy and it will significantly affect the responsiveness of the engine. I think I've found a happy medium and I'm waiting for it to arrive in the mail, so we'll see if the engine likes it better once it arrives.