What is the cause of the blowback?

Aggressive camshaft profile. It's a non-issue.

I machine all my flywheels, timjs will be using a buku setup I believe here soon. Thing is with buku clutch you have to use there flywheel also. Flywheel is not a direct fit on these thumpers.

After a year and a half of occasionally working on my Revo FS but mostly leaving it sitting on the shelf, I finally finished fitting the engine and all necessary ancillaries tonight:





Now I just need to charge the receiver pack and wait for a warm-ish day to give the engine a bench test and establish an initial tune with the HPI 2-needle carburetor I'm using.

Nice man! Do I see some sort of a rotostart on that thing?

Nah, just the stock pullstart. Maybe someday I'll pay a machinist to make an adapter plate so I can install the Traxxas EZ-Start system, but the pullstart works well enough for now.

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Nothing major happening in the following video, just a quick test run to prove to myself that the engine actually works and can hold an acceptable idle with the BuKu brass flywheel:


It started on the first try with a flywheel and carburetor both designed for totally different engines, and my guesstimate for the initial carb settings was pretty close to correct. I like it when things work.

I finished setting up the throttle/brake linkages so they have the necessary throw, and I'm about to run the engine a second time. What's the recommended break-in process for a ringed 4-stroke engine, anyway?

Well, crap. My flexible exhaust pipe cracked on the first test drive. I bought another one, but if this happens again I'm going to have to figure out how to thread and bend some rigid tubing to make a rigid exhaust pipe. I've never done that before and I don't know how.

Cool! Looking and sounding good from here. I also have some of that flex pipe. Haven't tried it though and sounds like ill be passing on that from your experience. Get a "die" and you can make your own pipe. Or have a machine shop do it, probably not that costly. O you'll need a lathe to turn down pipe to correct diameter also.

Its funny as I haven't been on here in forever it seems. But started messing with my .91 losi thumper today and thought I'd check in. Have you thought about a fan-cooling setup. You'll need one to get the max out of that engine. They run 🔥 as hell!!

Yeah, I'm considering a cooling fan, but the first order of business is to get the exhaust sorted out. Maybe I bent the flex pipe too much, or maybe I got unlucky and mine was defective. If it happens again I'll have to come up with a different solution; maybe some straight tubing held together with silicone bends, or something like that. I dunno yet.

I've got my engine geared-up pretty high, so I should be able to make use of the engine's massive torque to get good speed. Maybe I can get away with running it rich enough to not need a cooling fan, or maybe not.

These thumpers are tricky little bastards for sure. To rich and not enough rpm the clutch will fry. To much rpm and they become extremely hot. Silicone won't hold up to the exhaust temps, been there. Teflon was used by timjs but I think that also failed. Best bet is to thread and bend your own pipe. Of course iv had a exhaust port on the head break off with to much weight. I was out jumping the Crap of of it though.

You broke a cylinder head?? Or you broke the rigid exhaust pipe you made?

The exhaust gases do seem to be extremely hot compared to what comes out of my 2-stroke engines. Not sure why.

Broke the threaded exhaust port off the head. Combo of being to hot and maybe header not being supported well enough. It was just a thin straight tube pipe no muffler so not much weight at all.

Still, that meant you had to replace the entire cylinder head, right? That's a pretty serious breakage for a discontinued engine.