Predator Hemi Rpm issue

[Slo4sho]

I've got a predator 212 hemi and it doesn't seem to turn the Rpms I think it should. I've tried jetting up and down but doesn't improve. Max rpm I've seen is about 6,700rpms. Motor is on a mini bike with gear ratio of 5.00. 14.5in tires, 75t axle gear, 15t clutch.


Mod list: Arc flywheel 8*, Arc billet rod, .010 head gasket, head shaved .030, 28lb valve springs, stainless valves, Arc J2227 carb, 140E tube, intake, exhaust, governer removed, custom gas tank, walbro fuel pump.

I'm out of ideas and scratching my head.



Thanks

 

[karl]

camshaft, muffler? Trying to push 60mph on a minibike?

 

[Slo4sho]

camshaft, muffler?

Had a mod 2 cam installed when I hit the 6,700 since then I removed it. I'm going to reinstall it now with the head shaved and see if there's a difference. I ordered a new muffler to replace the lawn mower one.

 

[chancer]

Usually it is a gearing issue when your engine doesn't hit the RPMS that you expected or that it was built to hit. The Rpms are there it is just gearing not allowing them out.

 

[Joe-405]

More timing.

 

[rickou812]

What fuel are you running?
What is the compression ratio?
What is the piston to head clearance? How far in the hole is the piston at TDC?
Have you ran it without the muffler, just straight pipe?
How does the plug look, is it rich or lean after a WOT run?

In simple terms, an engine is a air pump, you have to get air in and air out as efficiently as possible. Any bottle neck in the air flow path will be the limiting factor. Then of course you have to have ample fuel at the appropriate air/fuel ratio for ultimate performance.

In your case, you may see a significant improvement by removing the muffler...may not, just depends on where the other restrictions are...
But first, you must ensure you have the right fuel (right octane for CR) at an adequate amount.

 

[KartFab]

okay, so the mod 2 cam in race engine maxes out at around 7k rpms, so you are only 300 rpm off...

I dont think this is a problem really. Im sure you could figure out a few minor things to do to get those few extra rpms. What about porting? I also noticed you said you put in 28 lb springs? The cam card only calls for 18 lb springs... so that is a tiny bit of power loss just by using excessively stiff springs. The right pound springs will give a miniscule boost in power right before they float... I also noticed that you changed the compression ratio. Did you calculate the static compression ratio, and what fuel is required for that ratio? The wrong octane gas can cause problems.

what a lot of people fail to understand is that engines have a somewhat narrow power band which has a lot to do with the cam profile (compression changes are also great, but it looks like you have already done that)....


If you want to run above 7k rpms, you might consider a forged piston, and a cam with higher lift, and ditch that carb for a mikuni. If you ended up with a higher lift cam, you would for sure need to check valve to piston clearance, as well so you dont end up wrecking your engine. I do remember reading something about .020" being a good clearance for the piston to head as the piston to head clearance does change when the engine gets hot.

Also, revisiting the cam. If you look at the dynocams website, it will spell out for you what you need to set your timing to, what valve lash you need, what springs you need, and the target max rpms. Keep in mind that if you get a cam designed for 'maximum rpms' you will have to trade off some low end torque and sometimes the ability to idle at lower rpms for that shift in the power band to higher rpms.

Just a few thoughts.

 

[rickou812]

Another thing to note, he is running a hemi head, any cam with more duration or lift than stock will require checking piston to valve clearance. Most hemi headed predators will require clearancing. Also must check for clearance at the crankshaft and cam. Some hemi crankshafts has additional material around the edge of the rod journal that has to be addressed with higher lift cams.

 

[Slo4sho]

Running 93 octane fuel also tried a 1:1 with 110 but didn't change anything. Piston to valve clearance is at .130. When I installed the cam I clearanced the crankshaft and lapped the valves. Spark plug is a racing AR3910X.

 

[Bbqjoe]

Well, just for beginners, you have your recoil start cover on wrong.
That muffler might not be helping any either.

 

[rickou812]

Running 93 octane fuel also tried a 1:1 with 110 but didn't change anything. Piston to valve clearance is at .130. When I installed the cam I clearanced the crankshaft and lapped the valves. Spark plug is a racing AR3910X.

Looks like you have done your homework. I would agree with Kartfab and say you've found the capability of the cam you're running in this particular setup.

However, if the air to fuel ratio is off, your not going to see the RPM's. If it is fat, it will hit a wall. If it is too lean, you risk overheating and damaging parts. Until you know that the tune is correct, everything else is just guessing.

To step up from here would require starting over to an extent. Just throwing another cam in it will not help if the engine isn't built to take advantage of the new cam.
A new cam may need:
Increased compression
Head porting
Larger valves
notched pistons
crankcase ventilation
Supporting valve-train, hemi heads sometimes require work in the valve pocket for higher lift.
proper fuel and fuel pump.

Of course all this is dependent on what you are wanting to do and how much you are willing to do to get there

There is always a trade off that must be considered as well. The more you move your power band up in RPM, the more low end torque is lost requiring higher clutch/converter lock up RPMs. This leads to drivability issues if the bike is intended for backyard play.