Skip the stock carb completely. Use the 3.3 carb. The idle screw will likely need to be all the way in to idle. The HSN will be the only adjustment since the LSN will be completely pulled from the seat.
HOWEVER there is some strange conicidence of science or magic that makes the Traxxas carb work perfect for 4 strokes Ive seen it used on.
As for a heavy flywheel... have you already cut the crankshaft??

I chose poor wording for my previous post. Airplane engines aren't tuned absolutely lean, they're tuned relatively leaner than car engines. Airplane engines are tuned much closer to an ideal air:fuel ratio than car engines are -- something they can get away with because of the adequate cooling provided by the propeller. Car engines are tuned richer than ideal so the excess fuel (which is mostly alcohol and evaporates very easily) can absorb heat from the engine internals, evaporate, and carry that heat away through the exhaust. (and truck engines are tuned even richer than car engines, to account for the lower speeds, increased drag, and faster heat buildup caused by running off-road.) The large heatsinks attached to car engines are not enough to prevent the engines from overheating; they mostly just prevent premature detonation of the air:fuel mixture during the compression stroke.

Anyway, my point remains: regardless of why airplane engines use a leaner fuel mixture than car engines do, it is nonetheless true that they do run leaner, and so they require a higher percentage of oil mixed into the fuel to provide adequate lubrication. A higher percentage of oil + a leaner fuel mixture = an equivalent volume of oil entering the engine. So running an airplane engine with the carburetor tuned richer for use in a car, with high-oil-content airplane fuel, will cause the engine to be excessively lubricated. Granted, no engine ever wore-out from having too much oil in it, but there's nothing to be gained from it either -- and it may even cause higher running temperatures since there's less alcohol in the fuel to remove heat through evaporation.

The loading of the engine is not a significant factor in determining how much lubrication is required; a heavily-loaded engine may require thicker oil to avoid viscosity breakdown at the higher temperature caused by the heavy load, but it won't require more oil.

No, there is no relatively leaner anything here.
Where are you getting this information? Have you specifically measured air fuel ratios on various airplane and car engines? Stuck an oxygen sensor in the mufflers?
Or are you assuming that airplane engines use higher oil concentrations so that must be because they run leaner ratios? If that were the case (which it is not) manufacturers would design engines to run "normal" ratios and spec a lesser oil content.
The statement that airplane engines run leaner is completely unsubstantiated.

Airplane engines (2 strokes with ball bearings) doesn't need much oil at all to stay safe, it's just something that lives on from really old days.
Whole flying clubs here that run 5% oil, some take it a bit safer and run 10% especially if they run 4 strokes as well.
Though there is a difference in how much heat a car engine will generate depending on surface (for instance), you can't offset all of that by drowning it in fuel.

No, with both engines to get the best they do run the same air/fuel mixture ratio but because an airplaine engine has a smalle carburator (+/- 4 to 5mm instead of 8 to 9mm) they will get a total less mixture and so with that less oil if you use a car fuel.
And how the rod and bearings are lubricated is the way how the piston ring is shaped, acting as a kind of oneway valve it will push some oil on the sleeve down into the crank case.

Some food for thoughts...
One thing to remember is that a larger engine needs less oil to cover the internal surfaces based on that the volume increases quicker than area.
There is only so much space available between parts so more oil might not help much at all, but instead an oil with stronger film might be needed, or making sure it's not diluted with too much fuel.
As the power goes up you just can't compensate enough, take an example of say lawn equipment or a moped engine that can run happy for many years on 2% mineral oil, and then you have something like a gokart engine that runs maybe +5% of the best synthetic oils and still wears out in no time.
At some point it just gets ridiculous to try and compensate and you just have to realize that nothing last for ever, or at least spend the money where it matters.

Regarding propeller cooling, if the prop is 100% efficient it wouldn't cool the engine, the speed of the airplane would, but that's just theoretical.

https://cimg6.ibsrv.net/gimg/www.rct...e160fe0ef5.jpg

Ten t
https://cimg5.ibsrv.net/gimg/www.rct...8a1e0924b0.jpg
So much to update on but ill make this quick. Got a chance to get my .91 thumper into my 2.0 truggy. Clutch still needs work, new shoes are on the way. Also forgot I put buggy diffs in truck so gearing is all funky at the moment.

Also got a chance to get MINI truggy started. This is a losi ten t with a .30 thumper conversion. Engine, servos, clutch are done. Still needs custom intake, fan and throttle linkage set up yet. These have been some really awesome projects to work on. There is a shit ton of machining into these trucks. It's all worth it in the end to see them running around the track.

Very first drive of mini truggy with .30 thumper. Still a good amount of work to do yet

https://youtu.be/Y924DvyNSM4

https://youtu.be/Y924DvyNSM4

I wish I had time to work on my 4-stroke Revo. I barely have time to take a dump between work and chores and taking care of the baby. I wish he'd get to where he can sit up and crawl around so I wouldn't have to stop what I'm doing every 10 minutes to move him around so he doesn't get bored. People say babies start getting into way more trouble when they can move, but at least I could sit in one spot and work on stuff while I keep an eye on him. I've got a puppy training collar all ready to go so I can discipline him with the push of a button whenever he gets near something he shouldn't touch. If that doesn't work I'll upgrade to an invisible fence.

Well, that won't be a problem with the wider carburetor I'm going to be using with my FS-40S-CX.

Lookin' awesome!!

Thanks for the kind words guys, haven't got any more uodates yet. Ill keep working to improve performance of these engines and vehicles.

https://cimg4.ibsrv.net/gimg/www.rct...cebba2bbf1.jpg
Test fitting body

The spit back out of the carb is normal. My air filters get pretty wet with fuel. Its not a problem.

Same here, fuel soaked air filters.. Just wonder how much better fuel milage we could get without that happening.

Wouldn't an extension on the carb inlet fix that problem?

Anyway, the fuel economy shouldn't be affected, because the fuel evaporates easily so the fuel that soaks into the filter just evaporates and gets pulled back into the engine along with the air.

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:

http://i.imgur.com/jJFtv6D.jpg

http://i.imgur.com/FOzgyY5.jpg

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.

- - -

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.

Yeah that was on my O.S .52 engine, swapped a magnum .52 head onto it.

That sucks. Hopefully I won't have that problem. Whether or not I can successfully use the flexible exhaust pipe without it breaking again, I'll need to make an additional support for the exhaust. Currently it's only supported where it attaches to the engine, and all the way at the other end near the outlet of the resonator. The middle of the pipe is free to wiggle, and I'm pretty sure it vibrated until the metal fatigued and broke apart.

- - -

Since my FS40 is out of commission while I wait for the replacement flexible exhaust pipe to arrive in the mail, I decided to take the time to install crankcase ventilation on the engine. Even though I've only run the engine for half a tank, I already see oil spray on the chassis in a line just behind the flywheel, meaning that oil is being pushed out past the crankshaft bearings by pressure inside the crankcase. This doesn't surprise me, because I've also run a 49-PI rotary engine and I've seen the crankcase ventilation tube slowly siphoning droplets of blackened oil out of the rear counterweight housing on that engine. Anyway, I didn't say "PCV" because the ventilation isn't "positive", meaning there isn't a one-way valve to guarantee that air never flows back into the crankcase. I'll add that later if I determine it would actually be useful, but since I don't need a guaranteed supply of vacuum to operate any accessories like a car engine does, adding a one-way valve to the crankcase ventilation hose is just unnecessary complexity for now.

http://i.imgur.com/UUsL10b.jpg

http://i.imgur.com/TxFXBMa.jpg

http://i.imgur.com/zarHelX.jpg

I bought another FS-40S to use for spare parts in case I ever need to rebuild my engine. (yes, I know some parts are different in the FS-40S-CX, but the parts most likely to wear-out are the same.) While I have the second engine apart to double-check that it is actually brand-new as advertised, I was studying the crankshaft and camshaft, and I had an idea for how to easily advance the valve timing for better operation at high RPMs. When the crankshaft is pulled backwards, as if being removed, the camshaft rotates forwards, which is the same direction it rotates when the engine is running. So if the crankshaft is held in a slightly rearward position, that will cause the camshaft to be rotated slightly forward relative to its normal position, resulting in advanced valve timing. This could be achieved by installing shims between the crankshaft and the crank bearings. Only small adjustments would be possible without moving the connecting rod rearwards enough to cause uneven loading on the upper conrod bushing and the wrist pin, but even small adjustments to valve timing can be beneficial. I made some measurements and did some math, and here's what I found:

The average diameter of the cam gear is 12.47mm. (OD is 13.57mm and ID is 12.47mm.) So the average circumference of the cam gear is 39.1757mm, and each degree of the cam gear's average circumference has a width of 0.1088mm. Installing a 0.2mm shim behind the each of the crank bearings would move the crankshaft rearwards by 0.2mm, which would conversely rotate the cam gear forwards by 0.2mm. That rotation translates to a 1.84° advance in camshaft timing; due to the camshaft's 2:1 gear ratio with the crankshaft, that would result in a 3.68° advance in valve timing with respect to crankshaft rotation.

Unfortunately there's no way to adjust the intake and exhaust valve timings separately without regrinding the camshaft (and if you have the tools to do that, then you could probably just make your own custom camshaft), but even being able to advance both valve timings together should make high RPMs easier to achieve. It's not like the FS-40S can't afford to spare a little of its peak torque in exchange for a wider powerband.

Cool pics of the fs40 you got there. So I haven't really seen a car version of the FS up close. Did you have to install that breather port or are those there factory? All my FS engines have a factory pressure port on them.

Also good idea about crank-cam shaft timing. Cant say I've tried that but its worth a shot for sure! Wish I could grind my own cams. Or at least be able to copy the .40fc cam as it has higher lift.

I had to drill and tap a hole to add the breather port. The pullstart version doesn't come with one. That was a real "I hope I don't mess up" moment when I was drilling a hole into my rare discontinued engine.

EDIT: Ding! Post #2000!