A 1976 340 nitro followed me home today

[mishka]

Greetings,

I picked up a 76 340 Nitro last night.  Its going to be a project for sure.  I’ll be asking questions and looking for parts.  Going to need a track and water reservoir for sure. Sled itself hasn’t been bent or beat to death.  Engine turns freely, has spark, has compression. Speedo show about 2000 miles.  Looking forward to sharing thoughts on the project.

[mrrupp1]

Liquid cooled Rupps are awesome sleds, your going to have a lot of fun with it. There are a lot of parts out there for them. The 340s make good power. My son trail rode one for years. Clint

[mishka]

I need a coolant tank for it. Anyone have an idea where I might look for one. Running into dead ends. Thanks.

[mrrupp1]

The liquid cool surge tanks had a bad habit of cracking. So its kinda hard to find a good one. The fix is take the radiator cap housing out of the old tank. Go to the hard ware store and get a pipe elbow that the radiator cap housing will thread into. Find enough piece you can make into a tee. Cut your existing hoses to fit on each side and your ready to go. If I remember correctly you want your nipple size an inch outside diameter. The only other thing I like to do is change the rubber plug in the head to steel one. I don't remember the size but I can get it for you.
I hope this helps.
Clint

[mishka]

I found a good surge tank! Cleaned the fuel system and pulled the reed valves to check the engine for 💩. Everything looks clean. Had the wrong type of plugs in it. Don’t know if it had been run on them or if they were just screwed in to keep dirt out. The Champion QN19V are hard to get but they cross to NGK BUE. I’ll pick em up in the morning. Should be able to start it. I’ve got two questions:

1. Book says 20:1 mix ratio. What is a good modern oil and mix ratio?
2. Didn’t come with a key. I’m guessing there are only a few different keys. Any way of ordering a replacement for that switch or getting one made?

[mrrupp1]

As far as the plugs go, I like to run NGK BUEs. I have seen NGK BR-9ES in the Rupp liquids and they work fine. Now for the oil I run BG syn 2 stroke oil mixed at 40:1. There are lots of good 2 stroke oils out there, Klotz, Amsoil, just don't run a cheap oil and you will be fine. The keys on Rupps are numbered one through six, but there is no number 5. To tell what key you need just look on the side of the switch the number will be there. Somebody out in the Rupp world will have a key.
I hope this helps.
Clint

[mishka]

Thanks! That helps.

I’ll be running ethanol free 91 octane RV gas with 40:1 Klotz R-50.

Another question. Think 40:1 is too lean in an air cooled 440 Kawasaki 1971 Puma? 32:1 safer?

A buddy of mine lost the key for his Harley. He had the tank off and was busy replacing the ignition switch and a bunch of harness that was part of the switch assembly. I picked to old switch up and put the new key in it. Guess what? 😆

[mishka]

Got it running today. Sounds great with just the expansion chambers after I got the mouse nests cleaned out. Second run and it quit dead. No spark. 😢. Everything ohmed out good and I cleaned up all the connectors. No joy! Engine isn’t plugged into the wire harness. I just put a kill switch to ground the kill wire from the CDI box. I’m guessing the box failed.

[mishka]

CDI box was bad so I ordered a new one from CDIbox.com.  After cleaning the carburetors, pump, tank, and coolant system it started right up.  Confident I’ve got good bones to start a full restoration.  For now I’ve fogged the engine and will put it up until this summer.

[mishka]

I’ve got a high idle problem. Pretty certain I don’t have any air leaks. Put new rubber mounts on. Cleaned the carbs. Verified both slides are closing as far as they will go. Idle stop screws backed out. Still idles about 3000 RPMs. Drops down a little as it warms up.

When I got it the carbs were off and had been disassembled. No idea what problems the PO was chasing. Not sure if jets were changed.

And advice on sorting it out would be appreciated.

Thanks

[midniterupper]

Check throttle freeplay on your carb cables, should have 1/32" on each. Use a Uni-Synch to verify air flow through each carb is the same.

[mishka]

Still chasing the high idle problem. The slides are not closing all the way. Even with the cables disconnected and the idle stop screws removed. It’s got to be hitting something like that brass jet that sticks up from the bottom of the throat. Both are the same. The carbs were off and hanging on the cables when I got it. Screws on float Bowles were finger tight a both gaskets were on one bowl indicating someone gave up on a problem. Any clews? Wrong parts, incorrect seating of the jet barrel…

[mutt]

It probly shouldnt even idle disconnected and screws backed out. Did you get Mikuni mounts,spray a little either on them see if it picks up even more around the fuel pump and main crank seal also. Verify their even the right size carbs size. Throttle cutout on the slide is stock ect?

[mishka]

Back on the project! It appears that the carbs have been swapped out or at least the slides. I pretty sure the slides are wrong. They are smaller diameter be maybe 1/16” the scallop on the slide faces the engine. Both carbs are identical. No left/right.

Could someone with a 1976 LC 340 nitro or book tell me what mikuni carbs this sled I’d supposed to have. Any jetting info would also be appreciated.

Thanks, Dave

[mrrupp1]

The 76 340 nitro's came with 36 mikuni's. They came with 200 main jets, 1.5 slides, 65 pilots, and 6DP5-4 needles. Hope this helps.
Clint

[mishka]

Thanks! Someone swapped the carbs with 38s but used the old slides. They are about 1.4 mm smaller in diameter and we’re in backwards. Left side idle stops vs right. Wouldn’t idle below 4000 RPMs. Could be why they gave up on the project. I’ll order a pair of 36s from Dennis Kirt.

[mishka]

Nov 13, 2023 15:26:51 GMT -6 mrrupp1 said:

The 76 340 nitro's came with 36 mikuni's. They came with 200 main jets, 1.5 slides, 65 pilots, and 6DP5-4 needles. Hope this helps.
Clint

A pair of VM36-4s showed up on the brown truck today along with jets from DK. Installed them and it fired right up.  It’s running rich and getting fuel an bubbles out of the overflow tube. Float arms were parallel when I swapped the jets and needles.  I’m guessing the 3.3 needle and seat are too big for a fuel pump.  Any idea what the correct size would be?  

The old 1.5 slides look good but bind a bit. I didn’t put any force on them and decided to use the new 2.5 slides. Anything signification in running the 2.5s? 


Thanks again.

Dave

[mrrupp1]

The needle and seats in the Rupp 36 mikuni's are 1.5. The 1.5 slides seem to work the best. You won't hurt anything running the 2.5s. I would put them in and see how it runs.
Clint

[mishka]

Nov 17, 2023 14:10:42 GMT -6 mrrupp1 said:

The needle and seats in the Rupp 36 mikuni's are 1.5. The 1.5 slides seem to work the best. You won't hurt anything running the 2.5s. I would put them in and see how it runs.
Clint

Thanks again Clint!  One more quick question.  Do you know the needle jet number that pairs with the 6DP5-4 needles?  The needle jets in the new carbs are 159 Q-5 series.

Got the 1.5 gph float valves on order.  Touched up the 1.5 slides with some emery cloth and they slide smoothly.  

[mrrupp1]

The needle jet is 159 Q-4.