Nitro car running weird when getting hot

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Shadkillz

RC Newbie
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8
Hi!
I have this HPI Trophy 3.5. It drives really well and with no problem at all. However, when i have used 3/4 of a tank and it's around as hot as it gets, it starts to run really bad.
Everytime i hit the brakes it just dies, it has no trouble standing in neutral, but whenever i drive it and i brake, it just dies. My friends and i have tried tuning it for a whole day without it getting any better.
It's not stable at all when idling, it goes up and down and no matter how we tune the low or high speed, it's the same thing.
so since it's only when accelerating and then braking we were thinking that it must be the low speed needle, so we turned the high speed to default, and then startet messing with the low speed. We couldn't get it to run clean when idling, but it runs with no problem in higher rpm, but when we turn it to the rpm where it sound like it's running clean, it's so high that it needs the brakes on not to move.
If we cool it for 10 min, it runs great, and even by turning both the high speed and low speed 1/8 each way didn't make it run worse on the cold engine.

I'm not sure what the problem is, it has been like this since i got it, we have a lot of other cars which we have no problem turning.
 

1mtx1

RCNT Talkaholic
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333
RC Driving Style
i would look for a air leak check the fuel tank, around the carb, the back plate the fuel lines . I would not rule out you are running to lean and after some speed passes or just some normal passes you are pushing your motor to hard. I would really look in to a air leak. I would go over the normal stuff, glow plug,fuel and your tune. Do not give up keep at it i bet when shes running shes running like a beast lol.
 

Macguyver

Hardcore RCNT User
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1,385
Location
in the woods .Quebec Canada
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ya ..probably your linkage that is not adjusted properly ..so when you hit the brakes your carb closes ..take the filter and look inside the carb ..nothing in there should move when you apply the brakes
 

Shadkillz

RC Newbie
Messages
8
i would look for a air leak check the fuel tank, around the carb, the back plate the fuel lines . I would not rule out you are running to lean and after some speed passes or just some normal passes you are pushing your motor to hard. I would really look in to a air leak. I would go over the normal stuff, glow plug,fuel and your tune. Do not give up keep at it i bet when shes running shes running like a beast lol.
Thanks, i've been thinking about the air
ya ..probably your linkage that is not adjusted properly ..so when you hit the brakes your carb closes ..take the filter and look inside the carb ..nothing in there should move when you apply the brakes

So i took it apart today and found out that there was not just 1 but 2 seals between the motor and the upper cooling part (sorry, english isn't my native)
so we removed one of them, and suddenly it sounded like it should!
then we went out to tune it, but again, when it got hot it started acting weird, but now it doesn't just die when we hit the brakes (we already checked to make sure the casket wasn't blocking when we hit the brakes and that was not the case)
this time, it's running pretty fine in high rpm and a bit better at lower rpm (plus, it's easier to start now)
but it just sounds like there is some resistance, to the engine, that it just can't handle at low rmp.
Everytime the dies, there is this "bling" from the pull starter, I'm not sure if that actually is anything, just making sure.
 

HPIguy

Durability Tester
Staff member
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8,105
Location
Great Smoky Mountains
Those aren't seals, those are head shims that control how much compression the engine has. Removing one is going to bump up the compression, and in my experience, make the engine more finicky to tune. Put it back, check all your fuel lines, carb, etc for air leaks then go from there.
 

cbaker65

Hardcore RCNT User
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3,405
Location
Kern county ,Calif.
Thanks, i've been thinking about the air



So i took it apart today and found out that there was not just 1 but 2 seals between the motor and the upper cooling part (sorry, english isn't my native)
so we removed one of them, and suddenly it sounded like it should!
then we went out to tune it, but again, when it got hot it started acting weird, but now it doesn't just die when we hit the brakes (we already checked to make sure the casket wasn't blocking when we hit the brakes and that was not the case)
this time, it's running pretty fine in high rpm and a bit better at lower rpm (plus, it's easier to start now)
but it just sounds like there is some resistance, to the engine, that it just can't handle at low rmp.
Everytime the dies, there is this "bling" from the pull starter, I'm not sure if that actually is anything, just making sure.
You can try screwing in the idle set screw in alittle just enough to when you apply brakes it doesn't
effect the slide barrel in the carb. closing.
Also ,do you run the truck with the body on it ?If so ,is there a hole cut out in the windshield ?
That helps allow air flow to the engine to help dissipate the heat!
 

Shadkillz

RC Newbie
Messages
8
You can try screwing in the idle set screw in alittle just enough to when you apply brakes it doesn't
effect the slide barrel in the carb. closing.
Also ,do you run the truck with the body on it ?If so ,is there a hole cut out in the windshield ?
That helps allow air flow to the engine to help dissipate the heat!
I'm pretty much running it without the body since it's easier to tune, but yes, there is a whole in the body, it was there when i bought it, so the air flow should be great.

It's not closing in the carb when i brake, it's not even moving, but it still won't go fluently

Those aren't seals, those are head shims that control how much compression the engine has. Removing one is going to bump up the compression, and in my experience, make the engine more finicky to tune. Put it back, check all your fuel lines, carb, etc for air leaks then go from there.
It just seems like if you put 2 copper rings on top of each other, they will have a problem keeping tight, that's the case on every other motor so why should that be okay?
as i said, it starts better now, and runs better, but it still can't idle at all, going up and down in rpm unless it's so high that it starts moving.

also it just feels weird that almost no matter how much we adjust the low speed when it's in idle, the engine doesn't change tone, unless turn it 1 ½ way around
 

HPIguy

Durability Tester
Staff member
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8,105
Location
Great Smoky Mountains
It just seems like if you put 2 copper rings on top of each other, they will have a problem keeping tight, that's the case on every other motor so why should that be okay?
as i said, it starts better now, and runs better, but it still can't idle at all, going up and down in rpm unless it's so high that it starts moving.

also it just feels weird that almost no matter how much we adjust the low speed when it's in idle, the engine doesn't change tone, unless turn it 1 ½ way around
What do mean by "keeping tight"? The bolts that hold the head on keep them in place and keep them tight against each other. Also, copper is very malleable, so once you tighten things down, the shims mash into each other and will usually be kind of hard to get apart actually. Have you checked for air leaks like has been asked? How's your glow plug, new, or worn?
 

cbaker65

Hardcore RCNT User
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3,405
Location
Kern county ,Calif.
Then it must be an air leak from the cylinder head ,If theres an air leak there ,it effects everything
including the pressure to the fuel tank,an heat dissipation.
 

ninnon

Hardcore RCNT User
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2,281
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England, UK
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It could be that due to the high temps we have recently, especially when running for long periods of time, fuel starts to vapourise in the carb before it gets to the engine. This causes all sorts of running problems, however there isn't much you can do about it. Try turning up the idle and checking linkages, see whether they move when you break (see by taking off the air filter and testing) :)
 

Shadkillz

RC Newbie
Messages
8
So what is the easiest way to check for a leak? I'm pretty sure thats the problem, but I'm not sure how i go about putting some constant air pressure on it
 

Rooky78

RCNT Window Licker
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553
Location
Fishers, IN
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Well... you can the the tank and lines by pulling the fuel line off the pipe and the carb, plug one end and blow thru the other. The engine... I'd just go through and seal it up.
 

bigwe33

RCNT Rookie
Messages
23
Those aren't seals, those are head shims that control how much compression the engine has. Removing one is going to bump up the compression, and in my experience, make the engine more finicky to tune. Put it back, check all your fuel lines, carb, etc for air leaks then go from there.
I think my HSP Boomwheel 1:8 nitro Truck has an air leak. It has similar symptoms. When it starts, it idles OK, then after a while the idle sounds very high pitch all the time. S when you give it some gas it moves away OK but when it stops the high pitch idle sound continues. I switch off the engine and and listen theres seems to be a hissing sound coming from somewhere. This only happens when the engine is still hot. I am also unable to prime it again. When you try blowing fuel into the Carb the fuel just runs back into the fuel tank. What is the best way to find an air leak?
 

1mtx1

RCNT Talkaholic
Messages
333
RC Driving Style
I would advise to go over your hsn/lsn/idle on your carb. I have seen some cases where a bad tune can be a nightmare which will lead to people over thinking and sealing there motor. I never had to seal a motor an i never advice anyone to do so.
 

Shadkillz

RC Newbie
Messages
8
I would advise to go over your hsn/lsn/idle on your carb. I have seen some cases where a bad tune can be a nightmare which will lead to people over thinking and sealing there motor. I never had to seal a motor an i never advice anyone to do so.
the problem is that when it get's hot, like when it reach the temps it's suposed to hit when driving, (i can hold my hand on it for a second or two but not more than that), at that point it doesn't matter how the needles are tuned, not even at the manufactures standard settings will it run properly, it goes from when cold, being a great car, i can tune it as i want with no problem, then after driving a bit it just goes out. Then i go to start it and the whole car is sounding different, kinda like it's getting way too little fuel, but even if we turn both needles to give it more fuel the sound won't change, no matter what way or how much we turn them it won't sound normal. Then we need to cool it down to make it run normal again, we need to cool it for 20-30 min before it's okay, it needs to be totally cool.

I've checked for leakage, and there is nothing, it's completely fine. I'm not sure if it's the carb, but it seems fine
 

Shadkillz

RC Newbie
Messages
8
If everything checks out good ,then it could be a bad carb!
Is that a composite/plastic material type carb?
Yea, it's some Hard plastic, we have some other carbs, but we can't really test them since they aren't big enough. The one on This car is quite big, but I'm not sure if i can test it in Any other way than purchasing a New one
 

cbaker65

Hardcore RCNT User
Messages
3,405
Location
Kern county ,Calif.
Yea, it's some Hard plastic, we have some other carbs, but we can't really test them since they aren't big enough. The one on This car is quite big, but I'm not sure if i can test it in Any other way than purchasing a New one
I hate those type of carbs ,an I really don't trust them!
I had one on my hyper 21 engine an it leaked like a siff were the metal parts
were pressed on.Had issues with the engine an it wouldn't eccept a tune no
matter what settings I did ,so I said what the hell ,I will try a different carb on
there!
Problem solved! It was the carb,I changed it to an Ae 28 all metal carb.
 
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