Modified Predator 212 - breaking valve springs

#1
For the 2nd time in a week I have broken my exhaust valve spring and bent the rod. Guessing my rod length is too long given my mods, but could use some input from the more experienced builders before I keep throwing money at the wrong solution.

Here is the motor setup in my Baja Warrior:

- Predator 212cc Hemi
- .356 lift cam
- .010 thin head gasket (no other head milling has been done)
- Stainless steel valves
- valve lash at 0.003
- chromoly push rods (stock length 5.54" long at .187" diameter)
- 26# springs with billet retainers
- stock rockers
- polished & ported head
- billet piston rod, gov removed, billet flywheel @ 32 degrees (I still need to install the 4 degree offset)
- Genuine Mikuni carb + header pipe
- peak RPM on last run was 6,100, highest RPM achieved under load is 6,440 RPM

Both times the valve spring broke while I was well under 6K RPM however. In fact yesterday I was just riding a woods trail with a few other guys when it happened.

Also, I know I should have 36# springs but the ones shipped to me are bigger diameter and so they don't seat properly in the head. I would have to ground out the head a bit and I don't have the tooling to do that. 26# springs are the biggest ones I can find that fit. At 6K RPM I figure I'm no where near valve float issues anyway.

EDIT: And FWIW, I have a friend that has built nearly the same motor but is running the .308 CAM, and yesterday he also broke the 26# valve spring on the exhaust port.
 
#2
When you remove your broken valves, does it feel hard to push or even slightly restricted? How does the rocker to camshaft seating look? Is it too steep due to the length of the push rod? Cam lobe height could possibly be too much ? Jus kicking some ideas out. Ive been into a number of these motors.
 
#3
Which 356 cam, dyno or isky? What is your coil clearance at full lift? Need to be able to fit a .060 feeler between coils. If you are floating the valves the exhaust will be first to hit as piston is chasing closing ex valve. If it is valve float piston will be marked.
 
#4
When you remove your broken valves, does it feel hard to push or even slightly restricted? How does the rocker to camshaft seating look? Is it too steep due to the length of the push rod? Cam lobe height could possibly be too much ? Jus kicking some ideas out. Ive been into a number of these motors.
Valves are fine and move up / down no issue. The valve spring is breaking right about in the center, and bending the rod a bit

I will have to take it apart a bit to check steepness

FWIW, another buddy has almost the exact same build (same cams, valves, and springs) with two exceptions: He has milled his head down 0.030, and he has no idea what push rods he is running. All I know is he has 6 hours on it w/o issue, but I can't make it 20 mins w/ breaking down.
 
#5
Which 356 cam, dyno or isky? What is your coil clearance at full lift? Need to be able to fit a .060 feeler between coils. If you are floating the valves the exhaust will be first to hit as piston is chasing closing ex valve. If it is valve float piston will be marked.
Dyno cam. What do you mean by coil clearance? Do you mean the coil pickup on the flywheel? Or the gap between the valve spring coils at full compression?

My coil pickup on the flywheel is 0.060, but I'm guessing that's not what you meant : ) I will have to measure the valve spring coil gaps once I get a new set.

I will pull the head off today, but last time this happened I did not notice any marks on the piston.

The friend running the .308 did not have any marks on his piston.
 

delray

Well-Known Member
#8
sounds like you need to install the correct valve spring and get the correct valve spring height when installing them and if it's on a hemi you will need to machine the seat pocket or your going to keep breaking things. for rod I would go with a thicker head gasket and maybe look into milling the head it self(.050)
I believe some of this we talk about in the past when it came to installing that cam.
https://www.oldminibikes.com/forum/index.php?threads/baja-warrior-build.165993/page-2
 
#9
sounds like you need to install the correct valve spring and get the correct valve spring height when installing them and if it's on a hemi you will need to machine the seat pocket or your going to keep breaking things. for rod I would go with a thicker head gasket and maybe look into milling the head it self(.050)
I believe some of this we talk about in the past when it came to installing that cam.
https://www.oldminibikes.com/forum/index.php?threads/baja-warrior-build.165993/page-2
Yes you did mention a few things to watch out for, and here I am after not looking out for them lol.

I looked into the machining tool for the right springs...$140 for that if I were to do it myself. Probably cheaper to find a local machinist that can do it. And possibly mill the head too.

I'm going to see about the spring coil gap and go from there.
 

delray

Well-Known Member
#10
sounds like you need to install the correct valve spring and get the correct valve spring height when installing them and if it's on a hemi you will need to machine the seat pocket or your going to keep breaking things. for rod I would go with a thicker head gasket and maybe look into milling the head it self(.050)
I believe some of this we talk about in the past when it came to installing that cam.
https://www.oldminibikes.com/forum/index.php?threads/baja-warrior-build.165993/page-2
are bending the conn rod or just the push rods?
 
#12
The first time it broke both intake and exhaust valve springs, but only bent the exhaust push rod

Yesterday it broke just the exhaust valve spring and bent the exhaust rod again too

Both times was < 6K RPM and bike had run fine for 15+ mins before happening, for whatever that is worth
 

delray

Well-Known Member
#13
ok,it might spin over fine when the engine is cold ,but when things start to warm up and expand and the clearances are already off(valve spring coil bind)it won't take long for things to break. if I am correct the factory hemi exhaust spring height has been known to be little shorter then the intake side. you can't cut corners when installing a 356 cam.
 
#14
I ordered the spring seat cutter from OldMiniBikes so I can get the 36# springs installed. Might be on back order but I'll update once I get things back together.
 

Cuda54

Active Member
#15
You could just put the 670 on it with no mods and really have fun. Just a idea. But the trail riding could be changing the engine load to fast. That the engine just can not handle the RPM changes for the changing loads. Try riding some back roads to see if it does the same there.
 
#16
You could just put the 670 on it with no mods and really have fun. Just a idea. But the trail riding could be changing the engine load to fast. That the engine just can not handle the RPM changes for the changing loads. Try riding some back roads to see if it does the same there.
Interesting advice, but, I'm building this bike for one reason....first place:

 
#17
Check that...two reasons....my buddy built his w/ the .356 and I'll be damned if I don't match him :)

Edit: He is #207 in that video, just to my right. He came in 2nd, I came in 3rd, so I guess we both have work to do. I was running a Mod2 at the time.
 

delray

Well-Known Member
#19
Check that...two reasons....my buddy built his w/ the .356 and I'll be damned if I don't match him :)

Edit: He is #207 in that video, just to my right. He came in 2nd, I came in 3rd, so I guess we both have work to do. I was running a Mod2 at the time.
i don't think the cam is whats going to make it or brake it when it comes to winning. there's alot of fine tuning just in the bike and drive train that could of easily made you win. for example your probably still running a comet 30 series with that 10 percent overdrive and the stock rear spocket. take all that off and go with a 20 series with a 7inch driven and a larger rear sprocket and use a 3000 stall spring in the driver and go with a good round knobby tire so you can lay it down good in the corners and pull hard out of them without dumping it. you can make that mod2 cam work real good,but there are some tricks that need to be done first to the engine to make it pull hard from 4000- 7000 rpm's and thats kind of power band you really want for that type of racing. to get real good power out of a 356 cam you really need more mod's done to the engine and it needs to spin closer to 4500-9000 rpm's to get max power out of it. also a big part is in the driver(you)
whens the next time you get to do that race again or one like it?video look like a lot of fun racing.
 
#20
You are correct Delray, running a strong race is a lot more than a cam of course. I'm just being a bit of a simple neanderthal / Tim The Tool Man type here..."Big CAM, Big Power, <grunting noises> bwahahaha " :p

I am running the driven springs for 3,300 RPM engagement, and for the most part do need the power in 4-6K range. For the 15 mins it runs bike feels like a 125 cc two stroke. Rear tire brakes loose on demand, and can wheelie out of corners. It's actually a little tricky to drive, but a whole lot of fun.

And I'll add, dealing with broken parts and learning how to build this thing properly is just as much fun. The collective knowledge and experience gained from folks on this forum have been a big help as well!!
 
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