Blaster Powered Go Kart

[RatedM]

Hey all, couple of friends and I are having a race in a few weeks using home made off-road go karts. We're limited to 250cc air cooled engines, so I choose to use a Blaster motor. Picked up an older ATV, and am building the thing for the race at the end of June. Frame is almost done, we just need to finish triangulating and bracing everything.

I could use your expertise on the motor:

The bottom end is in spec, clutch is good, tranny goes through the gears fine. I had a local guy clean up/bore/port the cylinder with a trail port(he's been doing this for 25 years and knew blasters well). New piston, top end gaskets. Oil injection has been blocked off. The previous owner appears to have shaved the head for more compression. I put a new piston, bought some Boyesen power reeds. Shopping for a FMF fatty exhaust.

I haven't done a leak down test yet, will build a tester tomorrow night. But when kicking it over dry last week to test the compression (without exhaust or reed cage installed), I only came back with 80psi. Could this be because the engine hasn't seated the rings yet? Could the fact that I didn't have reeds/exhaust installed lower my result? I assume I should be somewhere in the 150+psi range...

Thanks!

 

[Larry's Shee]

What was your ring end gap? Are they right side up? Are ports chamfered correctly?

Unless you did major head mod you will only be in the 125-130 range.
In fact, depending on how aggressive the porting is you will actually have less kicking compression because the ex port is open longer and trapped volume is less.

Would only see maybe 5+ psi after rings seat.
Intake and ex will have no bearing on comp test.

 

[Blaaster]

80psi on a fresh re build is a cause for concern, something is just not right.

When you said you kicked it over dry, I sure hope there was plenty of assembly added when it was assembled!

 

[RatedM]

What was your ring end gap? Are they right side up? Are ports chamfered correctly?

Unless you did major head mod you will only be in the 125-130 range.
In fact, depending on how aggressive the porting is you will actually have less kicking compression because the ex port is open longer and trapped volume is less.

Would only see maybe 5+ psi after rings seat.
Intake and ex will have no bearing on comp test.

Thanks for the response. I'm going to tear it back down and check the ring gap. I can't tell you that they're chamfered correctly, but I could see the markings showing that he did chamfer the ports. As for the piston ring gap, I'm using a ProX piston, so I understand I should shoot for .012-.017? And I should use the tighter of the rings on top?

The ring ends are slanted and come to a V where they meet with the alignment pin on the piston. The alignment pin is on the upper portion of the piston ring housing. I figured I should have had the V of the rings facing up(as a V, not upside down)as to fit flush with the alignment pin. Is my thinking correct?

80psi on a fresh re build is a cause for concern, something is just not right.

When you said you kicked it over dry, I sure hope there was plenty of assembly added when it was assembled!

Yes, I used a lot of assembly lube on the crank and the wrist pin. Should I have put some premix in the bottom end for the next test?

 

[MercMenace]

before you tear it apart. put a little bit of oil in the sparkplug hole then test your compression. Also see if you can get another compression tester to see if you have a consistent reading. The compression testers are not all created equal.

 

[Blaaster]

Should I have put some premix in the bottom end for the next test?

No do not dump pre mix in the crankcase, as fluid It contains no lubricational properties.

Put 2 or 3 tablespoons of 2 stroke oil in the bottom of the crankcase and a little down the holes that feeds the main crank bearings.

 

[Larry's Shee]

If you haven't torn it apart yet, do a leak test first.
Never used Namuras, sounds as if they only go one way. If gap was within suggested range that should be good.
When testing did you hold throttle WFO and kick until gage stopped moving?
X2 on different gage. Craftsman is the cheapest "good" one you can get, the rest come off a tool truck.

 

[RatedM]

Good news - I ran the compression test using a Matco gauge. Completely different result. I got 190psi! So this means that the head is milled down much more than I had thought and race gas is in my future. Does everyone run straight to VP or is aviation fuel a possibility? I come from the car world, and to reach timing targets we'll tune for e85? Or Torco race gas concentrate additive?

No do not dump pre mix in the crankcase, as fluid It contains no lubricational properties.

Put 2 or 3 tablespoons of 2 stroke oil in the bottom of the crankcase and a little down the holes that feeds the main crank bearings.

Thanks for the tip, I did as you recommended.

If you haven't torn it apart yet, do a leak test first.
Never used Namuras, sounds as if they only go one way. If gap was within suggested range that should be good.
When testing did you hold throttle WFO and kick until gage stopped moving?
X2 on different gage. Craftsman is the cheapest "good" one you can get, the rest come off a tool truck.

Yeah I was using a friend's gauge, turns out it was from Harbor Freight. Nothing on the intake or exhaust side so WOT doesn't matter. Quality gauges tell the truth.

 

[RatedM]

After some research online, seems 100LL AvGas is the cheapest solution. Any downsides to this?
PS here's a good sight to find AvGas near you: http://www.airnav.com/fuel/local.html
And here's a good sight to find Race Gas near you: http://www.racegas.com/fuelfinder

 

[RatedM]

Any recommendations on what octane i'll need to run on 190psi of compression?

Attached a pic of my intake port.

 

[Awk08]

Any recommendations on what octane i'll need to run on 190psi of compression?

Attached a pic of my intake port.

full 100+ octane race gas for that compression.
lets see the dome and cut on this 190 psi. head ?

 

[RatedM]

full 100+ octane race gas for that compression.
lets see the dome and cut on this 190 psi. head ?

Unfortunately, this is the only pic that I have showing the head. I'm starting to wonder if I should buy a new head on eBay.

Also attached a pic of the go kart as it currently sits. Still needs a lot of triangulation and bracing. Lots of work left to do.

 

[RatedM]

I should also note that this head wasn't modified by my engine builder, it was sold to me with the quad originally.

 

[RatedM]

Any word on this head?

http://pages.ebay.com/link/?nav=item.view&alt=web&id=191585090460

I need something very soon.

 

[Awk08]

Any word on this head?

http://pages.ebay.com/link/?nav=item.view&alt=web&id=191585090460

I need something very soon.

that'd make a good ash tray

 

[bansheedave]

that'd make a good ash tray

Same seller that is selling the copies of kens shock bushings

 

[Bluesman]

I see fail in so many ways, but the most obvious one is your version of a leakdown test. You need to read up a bit on 2 stroke leakdown test. Or leak test as some people call it.

 

[RatedM]

I see fail in so many ways, but the most obvious one is your version of a leakdown test. You need to read up a bit on 2 stroke leakdown test. Or leak test as some people call it.

I wasn't using the leaks own tester attached in the pic if that's what you're suggesting. Care to enlighten me on further failures or do you just want to see me fail?

 

[Bluesman]

OK, here's a few.
Mounting the engine to the suspension is not ideal.
With the engine mounted above the axle, I'm pretty sure the chain is going to hit the case.
Those heim joints for the rear suspension don't look big enough.
How are you going to attach the sprocket hub to the axle?
Are you going to mount the gas tank beside your head or are you going to use a fuel pump?

 

[Herpderplol]

Call KOR and ask him for head. I know he bought a lot of them last year so he might be able to sell you a cut one

although that head you have should be fine, just the very high comp is sometime hard to find gas for
im wondering why it is so high. The head doesnt look like it was even cut that much