Predator?

Mini Bike & Go-Kart Parts

#1
I want to build a hemi. I plan on using the +.020 rod. My question is what’s the biggest cam I can run without running into P2V clearance issues ? I don’t want to have to go crazy on the head. Mod2 ? I’m not opposed to using the hemi bottom end and non hemi head. Thoughts ? Thanks for any advice.
 
#3
if your just looking for some extra power and fun on your bike. all day with a mod2 cam. i wouldn't drink the cool-aid and go crazy. with that cam it should clear everything. 26 pound spring and 1.3 stamp rockers and 22mm china/mikuni carb with a 120 jet to start with and don't forget the good billet parts too..flywheel,rod...etc.and a good header..3-stage possible and try to put some gear in the bike and run a stall at least 3000 for a clutch setup and a 30 series unit also. if you do have a 30 series ? take the overdrive out.install a ring spacer on the driver unit or just use a juggernaut unit,but that will cost more. got to remember all these little things are what going to make your engine come alive. if you skip any of these things it can make things feel little more slower.
 
#4
if your just looking for some extra power and fun on your bike. all day with a mod2 cam. i wouldn't drink the cool-aid and go crazy. with that cam it should clear everything. 26 pound spring and 1.3 stamp rockers and 22mm china/mikuni carb with a 120 jet to start with and don't forget the good billet parts too..flywheel,rod...etc.and a good header..3-stage possible and try to put some gear in the bike and run a stall at least 3000 for a clutch setup and a 30 series unit also. if you do have a 30 series ? take the overdrive out.install a ring spacer on the driver unit or just use a juggernaut unit,but that will cost more. got to remember all these little things are what going to make your engine come alive. if you skip any of these things it can make things feel little more slower.
Sorry this is for a stock appearing racing go kart. I’ve already measured for the longer rod. I want to go to a bigger cam 275 or 308. Will I run into P2V clearance issues ? Coil bind ?
 
#5
Sorry this is for a stock appearing racing go kart. I’ve already measured for the longer rod. I want to go to a bigger cam 275 or 308. Will I run into P2V clearance issues ? Coil bind ?
oh....gokart stuff. i figured it was a minibike since we were on a minibike forum...:scooter:
275 lift your safe with 26# springs. might want to run a 32# springs on a 308 and yes you need to double check cam clearance with both of them.
 
#6
oh....gokart stuff. i figured it was a minibike since we were on a minibike forum...:scooter:
275 lift your safe with 26# springs. might want to run a 32# springs on a 308 and yes you need to double check cam clearance with both of them.
Ah yes but you guys build some badddd stuff too !!! And you know your stuff !
 
#7
you need to always check for Piston to valve clearance. Duration and LSA are what you need to look at on the cam you want to use. Longer duration and tight LSA will cause V to P issues. The place where the V to P clearance is critical is during overlap. From 15 degrees BTDC to 15 degrees after is where you need to check if using a dial indicator, or you can just use clay on top of the piston. Generally the exhaust is more critical than the intake as the exhaust is still closing as the piston comes up to TDC. To be safe you want .060 on the intake and .080 on the exhaust if you plan on high revs. The hemi head flows well stock but has a couple of shortcomings. The angled intake causes the valve to be closer to the piston than a stock bathtub head. If you get a bigger intake that also causes less clearance on the hemi unlike the non hemi head. The spring pockets on the hemi will not take a larger spring like the 36lb spring or the dual springs without cutting the spring pocket, and the rocker towers have broken on hemi's running a lot of spring pressure and aggressive lobes. The installed height stock is less than a non hemi. usually around .810. Using longer valves with aftermarket roller rockers will affect valveside geometry as well. A non hemi head does not have these issues but other than the latest predator rtc 8 or 9 heads do not flow as well stock.
 
#8
you need to always check for Piston to valve clearance. Duration and LSA are what you need to look at on the cam you want to use. Longer duration and tight LSA will cause V to P issues. The place where the V to P clearance is critical is during overlap. From 15 degrees BTDC to 15 degrees after is where you need to check if using a dial indicator, or you can just use clay on top of the piston. Generally the exhaust is more critical than the intake as the exhaust is still closing as the piston comes up to TDC. To be safe you want .060 on the intake and .080 on the exhaust if you plan on high revs. The hemi head flows well stock but has a couple of shortcomings. The angled intake causes the valve to be closer to the piston than a stock bathtub head. If you get a bigger intake that also causes less clearance on the hemi unlike the non hemi head. The spring pockets on the hemi will not take a larger spring like the 36lb spring or the dual springs without cutting the spring pocket, and the rocker towers have broken on hemi's running a lot of spring pressure and aggressive lobes. The installed height stock is less than a non hemi. usually around .810. Using longer valves with aftermarket roller rockers will affect valveside geometry as well. A non hemi head does not have these issues but other than the latest predator rtc 8 or 9 heads do not flow as well stock.
What am I look for on duration ? LSA?
 
#9
To see a example go to isky’s site smallenginecams.com and look at the cams for a gx200 under the custom cams you can select durations from 220 to 280 and lsa from 104 to 110. When the duration is 230 or less your v to p should not be an issue. As durations get longer the overlap period can grow. The lsa also will increase overlap when you tighten it up. Your post asked what the biggest cam you can run without v to p issues. That depends on what your trying to do. Are you racing on dirt or pavement? You mentioned SA engine so are you using a small block carb or a 390 carb? A small block carb modified by a top carb builder will flow high 60’s cfm a stock carb in the low 40,s if I remember correctly. That limits hp and in a case like that more duration can help. Also Barry young sells modified hemi pistons with valve notches very reasonably priced ( I think 40 bucks) should you need more clearance. Are you looking for a torque motor good off corners or a top end motor?
 
#12
To see a example go to isky’s site smallenginecams.com and look at the cams for a gx200 under the custom cams you can select durations from 220 to 280 and lsa from 104 to 110. When the duration is 230 or less your v to p should not be an issue. As durations get longer the overlap period can grow. The lsa also will increase overlap when you tighten it up. Your post asked what the biggest cam you can run without v to p issues. That depends on what your trying to do. Are you racing on dirt or pavement? You mentioned SA engine so are you using a small block carb or a 390 carb? A small block carb modified by a top carb builder will flow high 60’s cfm a stock carb in the low 40,s if I remember correctly. That limits hp and in a case like that more duration can help. Also Barry young sells modified hemi pistons with valve notches very reasonably priced ( I think 40 bucks) should you need more clearance. Are you looking for a torque motor good off corners or a top end motor?
I know some are mod2 all the way to a 310 cam. I want something in between.
 
#13
are you under any type of rules for your gokart or is this for fun only? i believe ole4 was asking. this can sometimes dictate what direction you might need to go. for example a 308 may not work good with restriction style carb's and the two guys next to you on the track are running mod2 or 275 dyno cam and have the correct head and porting done. creating good velocity response allowing there engines to accelerating much faster then you. when i said go kart stuff earlier in my comment i new it might lead to something like this. go kart racing..rules...rules and rules. i still like going over to the karts sites and reading alot of good info on engine builds and all. information over there help me greatly on building my flattrack bike for circle track racing only. if you are under certain type of rules, ole4 can help you out. he has run many years of go karts stuff and can get you down the right path... thumb1.PNG
 
#14
are you under any type of rules for your gokart or is this for fun only? i believe ole4 was asking. this can sometimes dictate what direction you might need to go. for example a 308 may not work good with restriction style carb's and the two guys next to you on the track are running mod2 or 275 dyno cam and have the correct head and porting done. creating good velocity response allowing there engines to accelerating much faster then you. when i said go kart stuff earlier in my comment i new it might lead to something like this. go kart racing..rules...rules and rules. i still like going over to the karts sites and reading alot of good info on engine builds and all. information over there help me greatly on building my flattrack bike for circle track racing only. if you are under certain type of rules, ole4 can help you out. he has run many years of go karts stuff and can get you down the right path... View attachment 271348
Has to be stock appearing from 5 feet away
 
#17
I have no experience with the 390 carbs. That being said I know of a couple of carb guys that modify them for methanol and they work fine but they do not do gas as they have difficulty get them to run correctly. I saw a post from a guy who said he gets them to run well on gas but when he was on this board most of what he posted was dead wrong so I doubt that he really had any idea how to get them to run. I would bet If you just got a stock one and tried it I would at least run and then go from there. They are much bigger and have enough flow to not restrict the engine like a stock gx200 sized one would. one would. The mod 2 cam is a long duration small lift cam. If you are looking for a good midrange and good top end power the Isky BM JR is a good cam or if top end power is your thing then the Black mamba would work good or a dyno 308 cam.
 

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