Odd issue... Engine dies when coming down from big jump a certain way...

[WA2FAST]

Just as the title says... If I am jumping the buggy (1/8 Power Racing Products Swift with stock motor) with pretty large jumps... probibly about 7-8ft in the air, when it comes down, if I land it even slightly on the rear wheels first, the motor just dies instantly. I have to make sure I hit front wheels first every time, otherwise it just simply dies. No air in the lines... I just slap the glow igniter on it, give her one quick pull and I'm on my way again... it's just wierd. I seriously can't figure it out. Nothing is damaged, bent, or anything like that... it's just if the landing is biased towards the rear even slightly, it is like somebody flips the switch off immediately uppon landing. Any suggestions? :rolleyes:

[CorradoPsi]

what are you landing on? if its something soft, it could be enough to touch the flywheel. have you landed rear wheels first in the gas a little bit? if you keep the rpm up a little as it lands it might not shut off.

[WA2FAST]

Hard grass/dirt surface... not soft at all. I thought that maybe something was hitting the flywheel, but there is no way, cause it happened too many times for that to be a coincidence... the flywheel is in the stock location which is not through the frame by any means... it's all right there recessed with the access slot as it should be. Unfortunately it happened 2 times, then I walked to the other side of the ramp where it was landing and repeated it two times to see if I could figure out what was going on, but the tank ran out and I was done for the day. My last landing was on the right front wheel and sheered the CVJ bone clean off anyway. I didn't get enough time to play around with it while on the throttle to see if it still did it. It was just weird cause I can repeat it like clockwork. The ground is by no means even, but there is NOTHING protruding enough to be able to poke through the flywheel slot... and even if so, what would the difference be from landing front to rear, or dead flat vs. slightly rear to front. Just odd...

[CorradoPsi]

well i was just tossing ideas out there. my onroad car does this every now and then. we tend to race pretty rough and its not uncommon for a car to end up airborn. ive landed on my wheels already and would have continued racing however my car did exactly what yours is doing. need less to say i didnt try to repeat the act though. by any chance could your backplate be lose or pullstart need attention? again just an idea. but if you backplate is lose the crank may be walking allowing the flywheel to contact the front of the motor, or binding on the pullstart shaft. thats about all i can think of right now. ill ponder it as i take mine apart for a good cleaning and assessment of damage from bashing yesterday and today. so far all that i know of is the broken front support, and a missing washer.

[WA2FAST]

Yeah, I know... I already thought that maybe something was hitting the flywheel... but after the second time and then the fact that I could repeat it over and over again... there is no way. As far as the backplate and pullstart... good call, I will definitely check that. I was also tossing around the fact that since it was at idle or very low RPM while landing, it could possibly be that the piston was towards the BDC and maybe on it's way back up and the inertia was just enough at the right time to stop it dead... but again, it shouldn't matter how I land at that point... you would think if that was the case, I would be more prone to that happening if landing on my front because of how it "slams" the rear down once the front wheels contact because of the weight transfer and inertia. In either case, it is definitely strange and I am going to take that pull start and back plate off to see if I can tell anything from that. I will try to play around with it more this weekend to see if I can get it to repeat. So strange.

[FastEddy]

Brake this down to 3 issues that will stop an engine and go from there.
Fuel
Air
Binding

Binding.
Chassis flex will cause binding. broken motor mount, cracked chassis. Look at all the parts that may cause flexing. Start the engine and try to recreate the issue on the work bench. This will help you isolate the issue.

Fuel / Air
Is something pinching the line when you land? Re-rough the lines.
Is the idle screw not set high enough. The servo will keep linkage in place so the idle is fine. When you land the chassis flexes just enough to shut the carb and die. Take the linkage off the carb and adjust the idle screw.

There is an issue, take your time on the bench and you will find it. Just think look and think again.

[dgrobe2112]

WOW!!! weird one.. and those are all good idea.. however.. i wouldnt think that inertia would play much role in this.. have you tried it on another jump.. maybe.. just maybe.. when you are 8 feet in the air.. im sure you are on the gas.. or off the gas.. while in air.. controling the car.. if you are on the throttle to pull the nose up.. you could possibly be lean bogging the motor.. and when you get that instant load on the motor from landing.. just killing it..

also the low idle thing comes to mind. but im sure your car is not idled too low.. some guys that race with me.. when they race..t he car would just die after 3 minutes.. turned out.. that when the motor warmed up.. and the Piston and sleeve expanded.. that the low idle wasnt enough to keep it running when on full brake.. you know.. just some ideas..

and the backplate../pullstart thing is a good one too.. with some play in the crank rod.. moving back and forth.. and plastic housing on the pull start.. maybe when you land that hard on the rear.. that there is some flex in everything.. the crank coming to the back of the motor.. and the pull start flexing.. allowing more movement of the crank.. could be touching the front bearing on the motor.. or something.. to kill it

[cbreaker]

Originally Posted by FastEddy

The servo will keep linkage in place so the idle is fine. When you land the chassis flexes just enough to shut the carb and die. Take the linkage off the carb and adjust the idle screw.

I like this idea the best. If you're using the radio to adjust the EPA for the idle, because the idle screw isn't holding the carb open enough on it's own, this would make perfect sense.

My friend had a problem similar to this and it ended up being some badly worn out clutch bearings; they were binding/tight and the shock of the wheels suddenly stopping would slow down the crankshaft enough to stall the engine. But since it could in no way be your flywheel, this probably isn't it..

[olds97_lss]

I know it isn't a maxx, but I had a simalar situation... I had a cracked engine mount. Whenever I'd take a hard nosed landing, the engine would shift just far enough forward to rub the tip of the crank on my trans housing and kill it instantly. It took me quite awhile to figure out what it was. Considering you couldn't really see the dent it had made in the trans case without taking off either the engine, or the trans... It wasn't until I pulled the engine and one of the little 2.5 engine mount legs stayed on the truck whereas the other three were still part of the engine mount. Then I noticed the nice little hole puched in my trans case.

In my case, it only did it 3 or 4 times, then it went away. My guess is it's because once it punched a hole in the case, it wasn't binding anymore. Just moving, then moving back.

[StaticSag]

I seen this kind of deal myself once and it ended up being the 90* air filter tube bouncing enuff to close the tube and shut air off to the engine killing the motor by simply flooding it. Just a thought.

Saggy

[WA2FAST]

Thank you for all of your suggestions guys, I really appreciate it. I am going to go through it right now. One thing is for sure, it is not a low idle issue, I actually prefer to have my nitro RC on the virge of moving from idling just a tad high... I am not a super low idle fan. Secondly, the idle screw is set on the carb perfect. The EPA's are set perfect, and when the throttle linkage returns to idle, just as it should, I have the spring on the linkage put slight pressure on the carb to ensure that it closes every time the servo returns to idle... so I know it is not that either. I must have something to do with chasis flex, something binding on something else weather it is internally or on the outside of the engine, but I have my fuel lines clipped VERY tight with a very speciffic path from the tank to the carb... they don't move at all, and there is nothing that can touch any of the lines at all under any circumstance. I will check it out and see what I come up with. You guys have given me some great ideas and thigns to look over. I'll give you an update later today.

[Maxx Trooper]

i had a problem similiar to this. when going off jumps, the engine wouldnt want to go. it just sat and idled or crawled around. but after going back to the stock settings and tuning it, havent had a problem since, and im running just a tad rich.

later

[niggle]

Here's something that may have some bearing on your problem.

The front edge of the flywheel on both my Swifts is being worn away. Upon inspection it is obvious that the flywheel is positioned very close to the rearmost brake shoe and is sometimes coming into contact with it. I suspect that contact only occurs when the chassis or diff centre mounts flex sufficiently to allow the engine and centre mounts to move closer together.

Possible solutions include:

1. Move the engine slightly further towards the rear of the car and adjust the clutch bell shims to restore the pinion - spur alignment.

2. Use a different flywheel (Savage? OFNA Hyper?) where the knurled starter ring sits further back/closer to the engine case.

3. Use a larger clutch bell pinion such that the engine must be repositioned further away from the centre diff mounts.

4. Remove one of the rear brake disks and the centre brake pad.

(3) is my preferred solution at the moment because the Swift comes with a 13T clutch bell which is too small for where I run. I think a 15T or larger would be more suitable for the low-profile street tires that WA2FAST runs on his Swift.

[CorradoPsi]

very viable theory. i did notice that the bolts for the rear calipers come pretty close to the flywheel when working on the car, but i didnt think about the chassis flexing so that they actually came into contact with each other. another possible solution would be to machine off a small amout from the front of the flywheel to narrow the knurled section and create more clearance for the shoes. you could also just change the braking system over to dual disk instead of quad and use fiberglass disks and eliminate the need for pads, just use the metal calipers. this would thin down the braking system significantely. from all ive read, dual brakes are plenty effective at hauling down a buggies speed.

[WA2FAST]

Originally Posted by niggle

The front edge of the flywheel on both my Swifts is being worn away. Upon inspection it is obvious that the flywheel is positioned very close to the rearmost brake shoe and is sometimes coming into contact with it. I suspect that contact only occurs when the chassis or diff centre mounts flex sufficiently to allow the engine and centre mounts to move closer together.

Well, you are on the money... I checked it out and that was EXACTLY what I found. I was checking everything over and noticed that the front of the flywheel was worn away quite a bit and gouged in some places... I noticed that the flywheel sits damn close to the bolts for the rear caliper just as you say. I am going to look into changing something later today to correct this issue. BTW, I don't use my Swift on road mainly... only to tune and screw around a bit here and there... she's mainly an off road beast.