Need some help

[bosefus]

My son has a OFNA Ultra MBX with an OS Max .30 VG engine. Sometimes the car will go full throttle for a short time with no way to stop it then the engine dies down again back to normal idle. Originally I thought it was his servo not responding so I bought a new servo. That wasn't the problem. I realigned his carb as I thought the angle the servo pulling on the throttle may have been causing it to stick but that wasn't the case either. It's too sporadic to pinpoint the cause. Has anyone else experienced this problem or know what might cause it? Any help is greatly appreciated. Thanks,
Eric

 

[Coach Z]

It sounds like a radio glitch to me, check all of the wires on the car for a cut in the insulation or fraying and make sure all the connections are in good shape. Might be worth while getting a fail safe so it doesn't destroy itself.

 

[Rolex]

With the air cleaner off and the engine NOT running, have him pull and release the throttle trigger as he walks away from you. Make sure he tells you when he pulls it, and when he lets go. Keep watching the carb to see if it opens and closes when he says it should. Make sure there's no binding on the linkage either.
It might be a glitch in the Rx or Tx, so this will eliminate the first step.
If everything works, we'll try something else.

 

[bosefus]

Cool. Thanks for the tips. I'll give that a shot. Also, do you think it would be worthwhile then if you think it may be the radio to check continuity with a multimeter? I'll try your suggestions then check with a multimeter. Thanks.
Eric

 

[Rolex]

I don't know what you'll find with the meter. Intermittent glitches are hard to locate.
Also, have your son move the radio around a lot and see if the power indicator is going out, or if a bad antenna connection is causing it to not send signal.
Either way, get a failsafe installed. I've NEVER run without one since I learned a hard lesson one time.

 

[olds97_lss]

Do you have an electronic fail safe? Also, what's the transmitter/receiver your using? What are you using for a battery for the receiver? AA's in a holder or a 6V NiMH pack?

 

[bosefus]

No failsafe. Going to get one though as it sounds like it would be good to have. Transmitter and receiver are the original ones that came with the car, airtronics. AA batteries in a holder.

 

[swarmjt]

had the same problem.. never solved it.. ended up tossing the truck for parts

 

[olds97_lss]

ended up tossing the truck for parts

That's a bit excessive... lol!

I had issues back in the day when I used AA holders. They would pinch the cells and wouldn't keep constant contact with the battery and the contact terminal on the + side of the cells. It would be fine until I started it up, then the batteries would vibrate/slide in the holder and I'd loose control periodically.

I'd try getting an appropriate NiMH pack and charger. They work better and are more robust than an AA pack. NiMH cells give a lot more current flow which makes your servos work faster and stronger.

 

[jon2]

Do you have a spare set of crystals? or even a different RX and TX? If you have tried all that was suggested by the other members and stillhave problems, try my suggestions. Crystals do go bad if they are handled improperly or if the RX is bumped around in the box often it can go bad too.

Please let us know what you find.

 

[HeartBreak]

Touching on Rolex's suggestion of the linkage binding; did you make sure that the linkage that connects the servo to the carburetor is level?

 

[hvymetalalchmst]

i had that exact problem with my car (which i just fixed and got it working). i spent so many hours wondering what the problem was. i checked everything, including all the solutions posted here.

from that experience, i highly doubt its electrical - it shouldnt be the servo, and an efs should have nothing to do with it (although it is a good idea to have one).

the problem that i had was that the carby was a little gummed up, and all i had to do was strip it down, clean it in metho (i think the US calls it denatured alcohol) and half the problem was fixed. it only kept revving for less than a sec. the other half lies in the engine itself, which you may need to strip down, or if your lazy, dunk the whole thing in metho to hope it ungums the inlet manifold. the reason it revvs like that is because either your car is run quite often without any cleaning of the carb, or that like me, youve stored it for a while without ensuring the carby is clean and the result is that fuel is stored after the carb, feeding the excess fuel into the engine without the carb doing anything

after a good wash, youll come to realise what a huge fool you are, buying a new servo.

 

[olds97_lss]

My digital servo was really twitchy when connected to a FS. Every time the light would blink, the servo would twitch. I ended up putting that servo on steering duties and put a good analog servo in instead for t/b.

The servo was a JR with the OFNA micro failsafe.

 

[bosefus]

Well that's a lot to digest and try. I'll work on it over the next couple of days and see what I find. I'll let everyone know what the final solution is. I really appreciate everyones help. We are new to nitro rc and i don't want sean to get discouraged which he is starting to do. Although we have had the cars for a long time it's just in the past couple of months we have really gotten into it. Hopefully once we can get them running consistently we can take them up to the sugar bowl if it still exists here in atlanta to run them. Thanks again.
Eric

 

[rckid91]

yeah be sure to check the AAs in the car mine jump loose in my rc10gt all the time

 

[bosefus]

Well we got a failsafe, put it on the car and started to put all of the wires and such back into the holder when the failsafe got tripped. I reproduced it consistently but couldn't figure out what the problem was so I pulled everything back out and couldn't reproduce it. So I started putting stuff back in and it tripped again. I am having one heck of a time trying to resolve this issue. I believe it's either the battery holder or the receiver but not sure which at this time. I may try the less expensive to see if it is the cause. Anyways, thought I would give an update.

 

[HeartBreak]

If the failsafe is tripping, it sounds like the unit itself might have a broken signal wire. Try wiggling the wires from the failsafe to the receiver. See if you get it to trip.

 

[olds97_lss]

Look for any frayed wires or wires with a cut in the insulation. If you are touching anything aluminum with a frayed/cut wire, it may cause your problem as well.