Hemi build questions

Tim Curry

Member
I'm wondering about push-rods for a hemi build. I'll be using a Dynocams CM grind super-x with 22# springs, Arc rod, flywheel, stock rocker arms, carb, header, etc. My question is about push-rod length. I took about 0.030" off of the head to raise the compression & did some mild porting, so the push-rod length is now about 0.030" too long. My fly cutter did an ok job, but I had to grind & lap the head to make sure it was flat. Is this ok with stock length push rods, or should I get the trim to length ones & cut them accordingly?
HeadCut.jpg


Next question is the clearance at the piston to head. The piston is 0.015" below the deck with a standard length rod, can I get away with a .010" head gasket, or will I need a thicker one if my target RPM is about 6500-7000?

Thanks in advance, this is the second engine build I've done & a bit different than the first one.
 
Pushrods length will probably be ok. Valve train geometry is one of those things you always have to check. Gx160 lifters are shorter and work good when milling head. At .015 in the hole I would run a .020 head gasket or stack two .010.
 
Engine back together. Not started yet as I need a couple of things for the bike still (catch-can, fuel shut-off valve, clutch cover) prior to running it.

One thing I'm curious about, how many rpm's is the stock piston good for? I nicked the top of the piston for valve clearance on the mill, so it should be good there, but how fast will a stock piston run & for how long? Planning on 6500 rpm at this point with the stock valves & 22mm Chi-kuni.

Right.jpg
 
Quick update: It's running now, but not very well. The Chi-kuni is not jetted properly from the factory, so I'm waiting for idle jets to start with the tuning. It's stumbling off of idle & not running smoothly. It's way off at this point so there's no point in going past where I am now.

The air mixture screw on the bottom of the carb is turned all the way in & it still isn't idling correctly. On a Mikuni, this indicates a pilot jet which is just too lean. The air mixture screw is used to allow more air into the carb to lean out the mixture at idle as it is opened, so with the full amount of fuel going into the motor at idle, it's still too lean. If you don't have the idle jet working properly, you can't tune the carb at the next step, the main jet.

Once I have an assortment of idle jets to get this mixture correct & the air screw out about 1 1/2 to 2 turns, I can do the main jet size. When the idle jet is the correct one, the air screw should be set to the 1 1/2 to 2 turns out position for highest rpm's at idle, this is what is needed for a correct idle mixture. Then the main jet is next after the idle jet is right, to get the WOT position pulling properly & running maximum rpm's & power at the top end. I'll play with the different main sizes, which I have in the shop already. Once the idle & main are right, the last thing to do is the needle position. This regulates the throttle response between 1/4 & 3/4 to prevent bogs or flat spots as the engine is pulling up from idle. Lean enough for power & rich enough to make a smooth transition is the correct spot for the needle clip.

A lot of people bad-mouth these carbs, but if you're running a Mikuni, this sequence needs to be right or the tune is wrong & you aren't getting the best performance out of the motor. I'm having to wait for the idle jets to arrive & will report back next month to give the feedback on this motor build.
 
Sounds good . Stock piston should be good for your rpm range without issue . Unless you get a lot of detnation or physical contact it should last long enough :) or till you pull it apart again .
 
I'm running a mixture of AV gas & unleaded regular, so the octane should be plenty high enough to prevent pinging. There's a small strip near me where I can buy their hose purges pretty cheap for high octane.

Should be OK with the piston relief-cut on top for valves & the stacked head gaskets with the total being 0.035" between the head & piston. As long as I don't suck a valve, bend a push-rod, bend a crank (etc. etc. etc.) it should run well enough & hold together for a while.
 
Finally got the idle jets & have tweaked mixture. Strange, but it seems to like the 20 idle jet & 90 main jet combo. It's 6500' up here, so I guess that's why, but it's different than the other setups I have that run well. Need to adjust the E-clip again, but I've made nough noise today & don't want a visit from the Popo at the 'hood. I have a nice frontage road to use to test now that things are working better. More break-in needed as well, but things are working & it's alive!

I'm watering the AV gas a bit now with regular, to reduce the octane rating. Pretty sure it was too rich in octane for this build. Thanks for the tip Mike.
 
With things running now & the carb tuned a bit better, this combo runs up to 7,000 rpm's, but sustained is 6,500 rpm at our thin air elevation of 6500'. Top speed on the mini bike is 64 mph now & so far the push rods are holding up well enough, but there aren't too many hours on the motor yet.

Only way to do better would be to dump in more money for a taller cam, bigger valves, bigger carb, better piston & more compression ($$$). Happy with it now, for a cheap motor setup it's pretty good. Here's the bike as it stands now, a steering damper was added, along with an 1/8" brake disc for better stopping.

RS-Finished.jpg
 
I did a mild port of the head, just smoothed it a bit & took a little out of the short side to open the flow gently, nothing major or radical. It's enough to let it flow better, which really only helps on the top end. As I said, it's a cheap build but seems to pull well enough with the mods I did. Anything is better than a stock cam, springs & carb!
 
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