Revo 2.5R Engine - hard to start, never has run right

[tinker1980]

Hi all. I have a Traxxas Revo with the 2.5 nitro engine in it, that I bought a few years back. (I always wanted one) I followed the break-in exactly, but I bought it during a time of year when it was cold, so I don't know if it really got broke in properly.

The little engine hasn't ever, in my opinion, run right. The factory settings are 4 turns out on the high-speed screw, and flush with the surroundings on the low speed screw, and set to these settings it's a glow-plug eating machine. If I set it any leaner, (And I mean changing by less than 1/8th turn leaner, like from 1:00 to 2:00) it will eat plugs even faster. Seems to get really hot too. If I set it richer, it runs better, runs cooler, the plugs will outlast a tank of fuel, but when accelerating, it bogs down but eventually gets up to turf-flinging speeds.

The current problem is it just won't start. The EZ start and I have had a long and sordid history, but it seems to me that the engine should start easier than it does. Lately it's been a matter of crank and crank, won't fire unless I hold my finger over the exhaust, and when it does fire, the EZ start stalls out. crankcrankcrankPOP then nothing.

Sometimes it acts like the carb jets are plugged up. Does it even have jets? Is the carb removable? And should I, after 4-5 years of owning this machine, be looking at exchanging the engine?

 

[Racer 1966]

Changing engines is a personal preference thing IMO. To me it sounds like it needs to be disassembled,cleaned and SEALED. Cold weather doesn't give the best break-in but I think you'll have better luck with it after doing a clean and seal job.

There are definitely better engines available if you want to spend the money.

 

[Rolex]

If your LSN is too lean, it will overheat quickly. Start with a new plug and freshly charged batteries and start it and let it idle till it warms up. Adjust the LSN a little bit at a time while doing short blips on the throttle. You'll probably need to adjust the idle screw at the same time.
When running at higher speeds, that's when you adjust the HSN for performance. If it's bogging, it's most likely too rich.

 

[SteelCityNitro]

could this be an air leak?

 

[Metalhead Matt]

It could be. I'd do exactly what Racer and Rolex said, and take it from there. Start at factory settings, but don't be surprised if you have to go richer from there. Depending on how cold it is. Not saying you will, but if you do, don't be concerned. Normally I wouldn't recommend running nitros in the cold, but I don't think you'll hurt this engine. So take it apart, clean all of the grime on the outside of the engine off while the air filter and exhaust are still in place, take the carb off of the engine, the needles out of the carb and soak them all in fresh fuel over night. Take the boot off of the carb before soaking as well. Take the backplate off of the engine and inspect the internals. If there isn't any old fuel build up or debris of some sort in there, don't worry about soaking/cleaning it. Take the header off and clean it and the mating surfaces real well. Then go read this:https://www.rcnitrotalk.com/forum/showthread.php?t=60832. It explains sealing in detail. Blow everything off with compressed air if your able to when you take the parts out of the fuel. Another thing to look for is a leaky head. If there is brown residue where the head bolts to the block, you need to address it. Pop the head off, clean the mating surfaces and the shim(s), and put it back together. Tighten the head bolts evenly in a star pattern. Snug them all first, then tighten. Other leak prone areas are the pipe coupler, tank cap and fuel lines.

 

[tinker1980]

Thanks.. you guys have told me more in four posts than the LHS has managed to divulge in several visits. )I think they just want me to leave the truck there to be fixed and pay $$ for labor. I'd wager that outside of Nitro engines, I'm more mechanically inclined than the lot of them.) I'll read the link about "sealing" the engine, hadn't ever heard anything about sealing a nitro engine but I've rode two-stroke bikes for years and an air leak makes perfect sense.

I've not taken the engine apart before, but it looks simple enough. I had a Cox .049 BabeBee when I was a kid, and it looks similar to that but larger.

 

[Metalhead Matt]

It's pretty darn easy, even the first time. Every Traxxas engine I've ever dealt with has leaked at some point. I know all this sounds like a lot of work for an old engine, but once you get into it, you'll be done in no time. And no money wasted. Everything you need to seal an engine is cheap and will come in handy for the next mill. If it runs erratic after it's cleaned and sealed, you'll know it's time for a new engine. But for a little encouragement, mine ran just like yours until I got all the leaks fixed. Even 3/4's of the way shot, my engine would run all day without stalling.

 

[tinker1980]

Sealed the back of crankcase, around the carb, and cleaned the carb out. I'm waiting for the sealant to dry before i try to start it up. I had to take the engine with me to the parts store to get the right size socket to take the carb off, it was a big hit with the guys at O'Reilly's. They said it was the first time anybody had carried an engine in the store in one hand. (I'm on a first name basis with those guys after resurrecting my '88 Isuzu Trooper.)

The engine feels like it has good compression still, if I remember right I've only run a total of a gallon and a half of 20% through it. The bottom end had a lot of fuel in it, I cleaned it all out. looks like it was a leaker, there was oil all over the back and bottom of the engine.

 

[Metalhead Matt]

Let us know how it runs. Should be a lot better.

 

[tinker1980]

It's acting exactly the same as before, unfortunately. Have to hold your finger over the tailpipe to even get it to fire, and if you stop doing that, it just sputters a second or two more and dies. Then it won't make an effort to fire again until you plug the exhaust again.

The hoses going from the exhaust to the tank and from the tank to the carb are clear and connected, no cracks in the silicone coupler between the header and the muffler. It's acting exactly like a gasoline engine does when it's starving for fuel. I hate to go buy ANOTHER $35 can of fuel to find out that isn't the problem either, maybe I can get a quart of the stuff. Could the carburetor itself just be plugged up? is there a nice easy way to clear it out?

 

[Metalhead Matt]

I explained in post #5 what to do with the carb. Take the needles out, soak them and the carb body in fresh fuel over night, and hit everything with compressed air to finish it off. Putting your finger over the exhaust is a normal way to prime an engine. You should have to do this to get fuel to the carb. But once the engine fires, the fuel system should stay pressurized. I'm willing to bet your tank cap isn't sealing. It should take a little bit of pressure to push the cap into the tank all of the way. If it just falls in, you have to tighten the 2.5mm hex that's on the bottom of the cap. Check for cracks in the o-ring as well.

 

[tinker1980]

I wondered about the tank cap. On my revo, it's a spring loaded round part that seems to just rest over the opening in the tank - doesn't go into the tank at all.

However... I could get a rubber stopper and test that quickly and cheapy, couldn't I?

EDIT: the tank is sealing a-OK. A closer inspection of the fuel line while trying to start the engine, shows fuel moving very, very slowly in the line from the tank to the carb, as if there were pressure in the line. As if it were clogged. Pull the line from the carb, and fuel of course pours out. So... what, exactly, is the trick to get the carb off? I took the clamp nut off, and the other side of the bolt it attached to was simply flat, no hex no screw head. when I removed the nut, the carb wouldn't turn loose, and I didn't want to risk breaking it. I cleaned it out as well as I could with it installed, by removing the adjusting screws and blowing a bit of canned air and fuel through them.

Grr... half the problem with this truck has always been the @#$% starter. There doesn't seem to be a battery made that will hold a charge long enough to get it started.

 

[deanwrx]

How did you verify the tank is sealing ok? There are 2 hoses attached to the tank. Blow into one while placing your finger over the end of the other. You should not be able to blow into the tank. If the cap leaks, well, you have a leak. Both my Revo and Rustler had this problem. The rubber gaskets lose ability to seal after time.

To remove the carb, you only need to remove the 2 pinch bolts on the sides of the carb base - they need to be removed, not just loosened for the carb to come out. Not sure what you meant by clamp nut. Then you should be able to pull it off or twist it slightly. Also a good idea to put Green Slime (or similar grease) on needles before you reinstall them.

 

[Metalhead Matt]

Carb removal and installation: The nut has to slide out of the bore to let the carb come off. Take the bolt off, then thread it back in 2-3 turns. Give it a tap, and it will push the nut out. Take the bolt back off, finish popping the nut off, and the carb will be free. Apply some high-temp, copper rtv to the o-ring when you reassemble. Just a light coating on the o-ring does the trick. Push the carb FIRMLY into the bore with your thumb while tightening the bolt. A little rtv around the nut helps it seal.