Cuyuna Madness!!

[ddub]

I think these engines are more of a pain in the ass than they are worth but hey it isn't my money so I will give it one more go. The base studs aren't black they are a normal grade 5 looking bolt color(not implying they are grade 5). I have read of the case twist under load. There are just so many inherent flaws working against someone with one of these engines it sucks.

Thanks for all of the help.

Other people are very successful with these engines.............

[Vintagesleds.com]

[79intruder440]

Other people are very successful with these engines.............

That's why I am asking questions to people who have more experience assembling these engines than I do........................

[stinger440]

The biggest issue with these engines is there is no "close enough"  Everything needs to be spot on. I have an angled plug engine from a 77 that I pulled out of a salvage sled. I have no clue how long it had sat before I got it. I have trail rode it, raced it, its been in 4 different sleds, tilly walbro and mikuni carbs,  and still starts second pull. I have not even replaced a crank seal. So yes, they can be great engines.

Sooo for your last attempt.  Take that powerjet out of the equation and install a 350+ main jet. ( your piston pics looked pretty lean to me)  Fire the pipe, add some vents to the hood/bellypan to move air. You might want to try running some 95+ octane fuel.  Then do everything else you said you did before and you should be good to go. 

[scorp11]

This might be lengthy, but I'll try to get you my thoughts in one post.  You have a lot of good thoughts from others so far.  clearly yours was hot an lean on the exhaust side.

From what you have posted, I suspect you are way to lean with the powerjet carb.  First, they need the airbox like the stings / TK's had to work properly.  No airbox and you will need to be even richer than what I list below.  Below are known safe jetting points if the motor has the correct airbox, is sealed, timed and running good gas.

From a running 1980 TK

Main Jet : 290
Pilot Jet : 40
Power Jet : 155
Needle : 6DH21 - e-Clip was in the 2rd groove from the top

From a running 1979 Sting

This was my original sled and I am pretty sure these jets were "stock" in the 1979 Sting.  Thing always ran good, but this seems to be the leanest of all that I found.

Main Jet : 280
Pilot Jet : 35
Power Jet : 125
Needle : 6DH4 - e-Clip was in the 3rd groove (middle)

From what I've read yours is something like below with no airbox.

Main Jet : 250 - 300
Pilot Jet : 35 ?
Power Jet : 125
Needle : ? - e-Clip position ?

I just think you might be too lean without an airbox.   You might try plugging the powerjet circuit and sticking a 360 main jet in it.  Make sure it has a 6dh4 needle or at least see what it has.

We run ours at 40-1 to 50-1 oil mixture so you don't need more oil.  Cooling you might need to work on.  I didn't see what the outside temp you were running at, but that will affect jetting obviously.   You want to get it cooled and super fat and see if it lives at this point.

If you used something other than stock head gaskets, they may be thinner and thus raising compression.  Same with base gaskets.  If you are using a winderosa kit, they come with 4 base gaskets, stick 2 in the PTO side or both.  That will give you roughly .030 to .040 thickness.  Make sure the piston doesn't protrude from the deck more than .008.  I would not recommend installing 3/32" base gaskets.  Sinking the piston .094 below the top of the deck is a lot and would surely mess up port timing.

Run good fuel  95 octane min.  These motors like octane period.   Even with the hemi heads.  They will run with less, but you'll be closer to the edge.  timing no more than .112 BTDC.  we set Mag .110, pto .106

I can't say what not running the insulators would do.  We always run them.  With a Mikuni you may be fine with it not heat soaking the carb because of the rubber carb boot.  Walbro, I wouldn't do it.  Make sure the carb boot is good and not cracked and thus leaking.

Go here for some common things with these motors, most have already been posted here.  http://www.vintagescorpion.com/Documents/CuyunaNotes.aspx    I'll happily do what I can to help you get the thing working, but I know from experience finding the cause sometimes is a pain. 

[ddub]

A question for you guys talking about running too lean.

I guess since the mag side didn't lean out and it is a single carb, I didn't lean that way (pun intended)   

I realize the PTO side runs a tad warmer, but I guess if I had this happen to my engine, i'd be looking at making sure it was sealed big time.

So, my question for you guys, does the pto side run THAT much hotter? 

I have also heard that powerjets NEED an airbox.

[scorp11]

Yes, the PTO Side runs that much hotter.   I bought a dual cylinder head temp setup I'm going to do some testing with, but it is way, way hotter. 

That is why that is almost always the cylinder that has issues.  The Mag side can take being run leaner since it runs cooler.  It is also why I suspect these are so susceptible to air leaks.  No room for error on the PTO side.

I would suspect from a "life of the motor" standpoint, dual carbs would help so you can jet the PTO a little fatter.

[ddub]

Has anyone run them with dual carbs?  Are there any dual carb manifolds?

If the two cylinders run that much different in temp, it would be a good way to tune the engine better by adjusting jet sizes.

I know some of my 90's doo's ran different jets in the different carbs.

As I recall, there really isn't room for dual carbs though?  At least in the stock chassis.

[JLOpower]

Are there any dual carb manifolds?

Not from the factory. The secondary is very close and it would require canting the carbs to the fan side of the motor. Cuyuna did produce a set of manifolds that would mount dual Mikunis for their ultralight engine line. Unfortunately, the thickness of this set up makes it impossible to use in Scorpion's stock configuration. Any dual set up will need to be custom made.

[JLOpower]

[PKBRUT]

One must ask, which state are you in? I know that a few states went to 15% ethonal blend which is causing all kinds of problems. Had a customer with that issue and started using fuel additives which resolved the problem. Just a thought.

[JLOpower]

Let's run down the list so far.

Stock carburation: Walbro WF mounting flange leaks air when the rubber isolator fails. In your case, a switch to Mukuni has eliminated that variable. You have introduced a whole new dynamic as far as tuning the new carb to your current conditions. Questions about running an air box or not also enters the picture. The big question that I always ask is: If the carburation is off enough to burn down a PTO piston, why doesn't the MAG side show damage? Heat can't possibly be the only factor.

Intake heat isolators: Stock isolators crack and leak air. I feel eliminating them does not remove them from the equation as far as fixing your air leak problem. As Skiwizzer mentioned, the crankcase moves around on this engine quite a bit. Taking those isolators out places extra stress on this area and I don't think this solves your problem. I would have some spacers made out of phenolic sheet. The two stud mounting system doesn't help.

Cooling air management: Yes, it needs to be done and chopping extra holes in the hood may not be the answer. These engines need cool air in and hot air forced out with ducting to keep under hood tempertures down. Dumping hot air under the hood will not help your problem.

Exhaust: Plugged exhaust can cause problems and should be addressed. The stock set up on the Scorpions is a double edged sword in itself. Scorpion insulated the pipe to wedge it into their compact chassis design and this builds additional heat in the motor. Removing that insulation might help but that would build heat underhood and be certain death to the plastics.

Leakdown: A necessary diagnostic tool and I always recommend it when talk turns to PTO burndowns. However if you have a case that is moving around when the engine is running, it negates your static test results.

Timing: Needs to be right. No arguements here.

Fuel: If you can't find the answer to this question on the internet, you ain't trying. High octane fuel. High octane fuel. High octane fuel. Enough said.

Spark plugs. Coldest plug available in the PTO side. Not sure this transfers much more heat but can't hurt.

Thicker base gaskets: I vote yes.

Base studs: I haven't seen problems here but a valid issue that should be checked and upgraded if you can't reach the proper torque spec. Not yet mentioned, the engine mounting plate. If it is not perfectly flat, bolting it to the bottom of the engine can twist the case and you are back to square one.

Fan housing O-ring. Must be good but why wouldn't this cause problems on the closest cylinder? Blue silicone? Anyone using this miracle in a tube should be subjected to third graders singing opera.

I want to add one other factor to the list.

Clutch imbalance: This has been a problem throughout the entire JLO/Rockwell/Cuyuna timeline. Early steel kidney weight clutches were murder on crank bearings and seals. The later PowerThrust II clutches are still heavy enough to cause problems if they are not in tip top shape.




 

[weverat]

 What size needle jet - P-8, Q0, Q2 ?  If the needle jet is not big enough, you will be lean at 3/4 to full regardles of main jet size.

 I have never used the power jet carb, and never will, I am lacking confidence in the power jet circut. Just saying.

[79intruder440]

Great information guys. Thank you all so much. That's why I asked, everyone has much more info on these types of engines than I ever will haha.

The engine in fact runs the stock air box it just doesn't have the weird TK ducting that pulls air from under the tool box. I don't know which needle is in the carb because that is currently wrapped up hanging on the sled back in my home town and the engine and carb are with me at my place.

So the punch list for this time around as follows:
-Re assemble engine and torque to all proper specs(with thick base gaskets)
-get some insulators from the ultralite place
-re check timing and verify proper adjustment
-re leak test engine
-re jet carb verifying all pieces are properly matched to the rich most spec above(you can always lean it out if it's too rich!!)
-Possibly get rid of the power jet and run proper jetting for that setup(it was just for economy anyways right?)
-fill tank with aviation fuel and mix 40:1 (20:1 for break in procedure?)(also no sustained wot runs in first 5 hours or so of operation?)
-build ducting to exit hot air from engine compartment
-build ducting to pull cool air from outside

Do you think I should make Ryan spend some cash and pick up the flow thru head? The aviation place also has a reverse flow fan that spins the same way. Does that sound worth while? Pulling the air across the "hot" cylinder sounds like a great idea.

I really do feel there is something in the needle or midrange carb adjustment with this engine. Seems like when it burns up its usually coming down from a extended higher throttle run. Slowing down for intersections and such always seems to be when it fails.

I believe it is running the powerthrust clutch. Same as the clutch that was originally on the TK save that the TK flywheel was an electric start clutch(hopefully that will be functional again someday). I wonder who or what type of a machine shop could verify that the clutch was balanced.

Wish most egt gauges weren't more money than the sled they would be used on haha.

I live in Indiana but the last time the sled ran was 4-5 years ago so I don't think ethanol was a problem..

You do raise a valid point as I don't think I have ran my sled with ethanol in the fuel so I will probably go easy on it and watch it closely the first tank. 3 years since it's been ridden for real.

On another note, I finished straightening my skis and re painting tonight. Installed all new hardware and wear bars so its ready to rock. Just a few other little things but it's ready for snow.

My cousin picked up a nice barn fresh 80 Trailfire we will be going through in the next month so I will try to keep you all posted on the Scorpion progress and all of my other sled ventures this winter. Hoping to take a trip to northern Ill and ride with some friends there.

Like I said I appreciate all of your input greatly. I am an avid forum member for the business I am involved in and spend lots of time helping out people just like you guys are doing for me so thanks!!!!

[obj1]

Seen this dual carb set-up on a scorp11 sled

[SCORPION TKX]

The melted center of the piston going towards the exhaust is detonation from timing too hot. I rebuilt my 77 sting 5 times before doing the timing. I set it at .100 btdc at full advance and it never blew up again. Good Luck