broken carb

[babi_hrse]

i got a second hand traxxas t-maxx off some lad for 150 quid. she runs but i was informed the carb is cracked and sure enough there is a decent sized hole along the crack. its plastic i was thinking of getting a metal replacement but i don't know if that is going to completely mess up the tuning.
and I'm seeing ructions on the traxxas forums flaming anyone who disagrees with traxxas' plastic carb.
this is my third car but I've never had to change a carb. what will i need? silicone? orings? also what carbs are a good fit for the 2.5 engine
I'm worried of buyin a carb to find it won't fit into the engine block.
direct links to something that'd work on ebay would be a nice touch
this is the model that comes with a reverse gear.(not sure if that's even relevant)

on a side note; the steering servo seems weak. ie it won't properly turn unless on the move.
should that be that way or is an upgrade be nessacary?

and finally i have a 1:8 losi engine could i fit that onto the traxxas without having to change drivetrain and basically rebuilding the entire truck?

if not would a 3.3 engine upgrade just be a straight swap?

 

[olds97_lss]

If you have the 2.5 or 2.5R, a 3.3 will be a direct swap. If your leaning that route, you should be able to trade your old engine in for credit.

For me, i'd just find an entire composite/stock carb on ebay for the engine as they do work ok if you don't overtighten things and are fairly cheap on ebay, then buy the 3.3 outright if the 2.5 runs well. Use the 2.5 in the colder months with the 3.3 in the summer. I find it best to use engines I really don't care about in the winter, old tired engines fit that category for me.

As for the servo, it's a personal preference. The stock servo won't turn the tires while stationary. Depending on your rig, it could be partly the servo saver (if it's mounted on the servo) or just a weak servo. In any case, if it's an older rig, they didn't have very strong servos in them. Most people replaced the steering servo with something better and moved the steering servo to throttle/brake and threw the t/b servo in a drawer as it's not very strong at all.

 

[babi_hrse]

id be of the opinion that this is an older version. na id have to keep the engine I'm based in ireland and we dont have any traxxas or hpi or... anything for that matter based out here. no lhs is gonna take an old engine and sell me a new one at a discount in this recession lol. na ill be buyin off ebay. when my bank account settles. mine is the trx version so i take it I'm gonna need other parts changed too.
so i get a stronger servo for steering and use steering servo for throttle and the throttle can go in the box of spares i have that right?
any specific model of servo i should go for?

 

[olds97_lss]

I'd go for something that has at least 140oz/inch of torque. The hitec 645MG was a popular one. If you run in wet conditions a lot, maybe try Hitec HS-5646WP. I just snagged one off ebay for $47 to give it a try in a 1/8 buggy I also bought.

Not sure what you mean by

mine is the trx version

Is it the old 2.5 with slide carb/rear exhaust or .15 pro with rotary carb/side exhaust? If it's the 2.5, then the 3.3 will drop in and carbs can be found fairly cheap for the 2.5 on ebay. I assumed you had at least the TRX2.5 since the carb was composite. The carb is aluminum on the old 15 pro engine.

The 3.3/2.5 carbs are the same. If you don't have the $ for the whole carb, you can get the carb body and take your bits off yours to put on the new body.

 

[hotrodrob]

Trx is just an abbreviation for traxxas rook.

 

[olds97_lss]

Well... I'm assuming he means TRX 2.5 as that is what the 2.5 has stamped on the side of the engine, unlike the old Traxxas Pro 15.

 

[babi_hrse]

yep it's a 2.5 trx composite slide carb with an ez start system mounted
is there any difference between the pro 15 and the 2.5 in size. i assume the .15 in cubic inches while 2.5 is cubic centimeters for the same.

 

[2revo1maxx]

The pro.15 and the 2.5 are both .15 engines, and will bolt to the same mount, but that's where the similarities end.

 

[babi_hrse]

righto so after upgrading to a 3.3 will i need to change out any other parts flywheel clutch gears? driveshaft? gearbox? like is there anything suspectible to breaking with the added power or is it all good for both? I'm a bit leery of the reverse transmission as I'm afraid ill accidentally trip the switch into reverse and eat the teeth off the gears while the truck is bombin it at WOT?
thoughts?
and then i think i have no more questions

 

[olds97_lss]

The 3.3 isn't overly powerful, but it would be a good idea to have spare center axles, at least one entire wheel axle/slider and a spare diff laying around. That's a good assortment to have anyway for any t-maxx owner.

I would also suggest removing reverse and installing a forward only gear/output shaft. The gear is much thicker which adds durability. It removes a decent amount of rotating mass out of the trans. It removes the lurching when your rev'ing up to get it to go into gear. Makes for a much smoother power transition which in turn lessens shock to all the driveline parts as a side effect.

FOC: http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCNA8&P=7

 

[babi_hrse]

ok new composite carb added. reverse forward shifter getz stuck in gear. it will move fine when the engine is not running but once the engine is idling the shifter gets stuck so if it was in reverse when i start the engine it will likely remain that way for the duration of the engjne being active and vice versa for forward. the servo moves but the stick is just jammed. to make things even more confusing i applied the brakes while switching to stop the transmission but it is still prevalent?
a foc is on the way but id like to solve this issue as I'm reluctant to walk away from this problem without learning the cause?;

 

[T-MAXX BASHER]

Is your engine idle too high because that will stop tranny changing from forward to reverse (i had same problem myself)

 

[Snook Man]

a foc is on the way but id like to solve this issue as I'm reluctant to walk away from this problem without learning the cause?;

The cause is most likely a high idle speed. The way the forward/reverse gears and inner clutch bell are designed in that transmission, it works as a safety feature so as to not be able to switch between forward and reverse if one of the gear clutch pawls are open inside the rim of the clutch bell.
When open the pawl will hit the inside lip of the bell and the servo will not be able to move the shift fork or bell over to the other gear.
If the pawl is open inside the bell due to a high idle applying the brake locks the output shaft and subsequently keeps pressure on the pawl against the pin in the bell and prevents the pawl from retracting back into the gear and you’re stuck in that gear until the pressure is taken off the pawl by either stopping the engine or rolling the truck back and forth.
One way to prevent the issue is the no buck mod.
The pawl is removed from the forward/reverse gear(s) and a axle grub screw pin is screwed into the gear. The pin sticks out of the gear and engages one of the holes in the bell instead of the pawl engaging one of the pins in the bell.

Some would also remove the lip of the bell with a dremel and keep the pawl. Either method works but there is no safety feature then and the tranny can be switched from forward to reverse while in motion.

 

[babi_hrse]

i shall try lowering the idle
then moving onto the no bump mod. i must take apart my transmission again and have a look at what your talking about i only understood half of what you said and my mind fabricated something to accomodate the female end of the pawls like a groove or track in some circular object to lock the drivetrain from shifting (like i said. ill have to open my tranny and have a look). the grub screw idea sounds like it eats at the bell until it slots into a hole? will that not cause excessive shock damage?

 

[Snook Man]

the grub screw idea sounds like it eats at the bell until it slots into a hole? will that not cause excessive shock damage?

If the idle speed is too high and the spur is spinning quickly when you try to engage/switch the forward or reverse gears then yes, the pin would have a little trouble engaging one of the holes in he bell...

But, the spur should not be spinning at idle speed when you attempt the gear change. Thats one of the biggest problems people have had with that model.
The spur should not spin at idle speed and if it does it should only spin just a little due to the friction created between the crank and clutch bell bearings. If it spins considerably the idle is too high or the engine clutch is engaging at a low rpm due to a spring thats too weak or another clutch related issue.

 

[Nitro4910]

i shall try lowering the idle
then moving onto the no bump mod. i must take apart my transmission again and have a look at what your talking about i only understood half of what you said and my mind fabricated something to accomodate the female end of the pawls like a groove or track in some circular object to lock the drivetrain from shifting (like i said. ill have to open my tranny and have a look). the grub screw idea sounds like it eats at the bell until it slots into a hole? will that not cause excessive shock damage?

The shift bell is hardened steel.
If you shift it with the Idle really high you will get a shock load to the gears, if you shift it on throttle then you run the risk of shelling the trany gears.

The pins of the "No Buck mod" engage into the large holes in the shift drum.

Inside the shift bell there are pins on the outer edge & that's what the pawl catches.

The explanation in the pic below was for a stuck in neutral problem, but it's the same if you Idle is too high it not allow the pawl to retract & that "sticks" the trany in either FWD/REV.

 

[babi_hrse]

is it possible that the pawl is loose i mean to say like is there a brass clip that retains it or a spring behind the pawl that may be going faulty due to age causing the pawl not to retract fully at idle?
or are the pawls always loose but only have enough force to engage against the pins rather than rubbing over them when the centrifugal force in high enough?

 

[Nitro4910]

is it possible that the pawl is loose i mean to say like is there a brass clip that retains it or a spring behind the pawl that may be going faulty due to age causing the pawl not to retract fully at idle?
or are the pawls always loose but only have enough force to engage against the pins rather than rubbing over them when the centrifugal force in high enough?

The pawls are held "closed/in" by spring pressure, they do move freely on the pin, once you get enough RPM/centrifugal force to swing the pawl out they should swing all the way out.

It is possible that the set screws got fooled with or the spring got crushed/is bad.

Both the forward & gears screws are set so the are just level with the edge of the hole like in the pic below.

Inside the gear is small egg shaped spacer that the spring pushes against which in turn pushes against the pawl.

 

[2revo1maxx]

A new trans can be had for cheap, just sayin'.