Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

I have a fresh from the dead 1980 express

I have done a lower rebuild on the crank seals and am intimately familiar with this engine this being said I have wasted too much money on heat seizing my jug and piston beyond repair I asked Benji over at treatland and he said it was most certainly My main jet causing the heat seizing.

I have already gone through 2 Dr 70 kits and do not wish to kill a fourth. I believe the main jet is 180

Do I upjet? Do I downjet? Howbl do I even know if I have fixed the issue short of blowing up a third kit?

Help would be steller. The best answer that fixes my problem will get a treatland reward. 50$ ish

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Charlie Mccharles /

First I'd get a trail tech temp gauge if you haven't already, they're very helpful with tuning and knowing how far you are from heat seizing. Under 400°F and over 350° for an air cooled cylinder+head is a good range.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Murray Rothbard /

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

My VM 20 had a 180 main jet I ordered another kit, jets and sparks spreadsheet says 90 main I also am picking up a TT temp gauge

Also getting the mlm exhaust.

Does 90 sound right?

How much does the idle jet matter?

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Dirty30 Dillon /

The 90 jet is recommended in a vm18, which is way different.

Everything I just peered into say the bike should run with 180, but be optimally tuned around the 140-155 range. I'm glad this thread came up because I'm about to start my DR/vm20 setup today.

If you've been running a 180 and seizing, you have something else going wrong. Are they seizing while riding or on the bench?

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

Probably getting way too hot.

You need a temp gauge

What oil ratio are you running?

It's not a matter of just picking a jet, gotta experiment.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

Pics of carnage? That will help diagnose what's happening.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

180 is a truly massive jet for that setup. On a larger carb, with a cylinder making roughly double the power I am happy at a 150. On an Athena, with the same larger carb I ranged from 120-135. If 180 is where the bike send to run best you likely have an air leak. Also where is your needle position? A rich main and a lean needle can still seize you when cruising at high rpm and light throttle.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Probably Fred /

Lower timing shrink point gap

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

JBOT, Bro Wrote:

-------------------------------------------------------

> Pics of carnage? That will help diagnose what's

> happening.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

YEAH, Pics of Carnage...Please...!

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Here is the damage the jugs are bad too

IMG_20160520_110107.jpg
IMG_20160520_105933.jpg
IMG_20160520_105939.jpg

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

0.68 to what?

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

The temp is good to know I am ordering a ttt gauge right now

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

High rpm 30+ on the road hours after rebuild

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

30 1 first tank 40 1 second tank 50 thereafter

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Pics posted

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Hours after rebuild is too soon. That bike is a single speed with no available gearing changes. It is going to be screaming it's brains out at 35+. I'm not saying that is fully to blame but it's a contributing factor.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

From what I can see looks kinda 4 corner seize.

Just got too damn hot.

Run a b8 or b9 plug,

Maybe mod a puch head to fit, jet as rich as possible.

Retard timing too if possible.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

Btw, your cyls are probably still fine.

Just gotta get the aluminum off the walls.

Sand a bit and just replace piston.

Matter of fact, your piston looks runable still.

Just gotta sand and replace rings.

Did you chamfer all the ports? I do that to every kit with a 45deg dremel cut,

then sand with 220.

Every edge on the piston and bore should be smooth like butter.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

I don't know what it is.. but for some reason my express devoured cylinders, even the same exact ones that had been run on my Hobbit for a long time. No air leaks, jetted pretty fat.. they just got hot. I wonder if the foot peg arrangement screwed with airflow.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

Wc 77 head would fit, ya?

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

I don't know how large it is physically. I would say yes, but it actually may hit the peg "frame". Also different years have more or less clearance. If it is roughly the size of the cylinder then hell yeah it would. It would also be completely overkill on any non belt driven express.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

What, you don't overkill bro?

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

I'd just like to comment that the OP's ring gaps look questionably small as well.

Cast iron forever. Seize it over and over forever.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

That's just cuz the ring is locked in the ringland.

I've never had to gap DR rings ever, or the replacement rings.

Always spot on or a touch large

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Can u post a pic of the inside of the cylinder on the case with the piston at bdc?

I was seizing a kit on my hobbit mult times until I raised it by a .8 mm mlm

Base plate to increase the transfer duration. That reversed a too lean condition that was refractory upjetting.

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Totally normal JBOT /

Yeah, that's why you were seizing...

You nut

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

Did you have the Honda dr kit?

Re: Nc50 vm 20 jetting. Grenaded 2 Dr 70 kits need help!

So I just pulled out a 249 jet out of my carb...... wtf?

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