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Post by olyman on Oct 25, 2020 13:30:00 GMT
Good detective work Mark. Its good that you found out a 260 will make it run good as it sits. Have you confirmed all of the other bits and pieces are correct for that carb? Assuming that's true then two things IMO. 1) Its not a bad idea to sand off that area you pointed out is all rough. That's where the fuel gets pulled out of the bowl and it could bring extra with it making it run too rich. And hence too small of small main jet (260) makes it work right, as you are compensating. 2) In warmer weather you need less fuel to run the same as the difference in the air density. Although that's true it wont make as much of a difference that you are seeing. It will just add to it. So I would do what you can to sand that as flat as possible to seal it up. You wont burn this thing down unless you leave it lean and wide open for a long time under load. You are already paying attention to the plugs, which is the main thing.
You are definitely going to be busy in the pits of team 77 this winter!
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Post by Doo640ER on Oct 26, 2020 15:29:31 GMT
Cool beans Olyman, thanks for the advice. Ok, the carb will come back off and I'll smooth out that rough edge. Then give it another good cleaning. Oh yeah, I did confirm that all of the other components of the carb are correct. I still want to try a 270 main and see what that does. Unfortunately when I ordered my jets I only got 270 and 280. Didn't think to get a 260 so I may need to pick one up. Guess I'll see what happened after doing the Mikuni surgery.
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Post by bones on Oct 26, 2020 15:59:23 GMT
Doo640ER.....I am doubting that the main jet is the issue ?? As you are likely aware the main jet has very little affect until about 1/2 or 5/8 throttle open, there is a little cross over affect maybe before that but very, very little. In theory you could install a 500 main, keep throttle below 1/2 open & engine should run fine. I think it more like something you fellas are discussing about not sealing at the main jet etc. I am not sure....When you put the Citation carb on and it ran nice....Did you still have all the Citation jetting or had you switched the TnT jetting into the Citation carb ? We also have to remember when dealing with some of these old carbs, sometimes in the past a previous owner was rater aggressive with jet cleaning, using a drill bit in a drill !! Bones
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Post by Doo640ER on Oct 26, 2020 22:47:30 GMT
Hi Bones, When I put the Citation carb on the TNT, it was plug and play. Took it off the Citation and installed it as is on the TNT. Hmmmm, but installing the 300 main on the Citation carb would be an interesting experiment. That may help prove or disprove the theory that there is gas getting through the rough edge at the main jet. I'm going to take a few days off from working on this so I'll report back likely this weekend. I am getting buried in leaves so I have a date with a rake for a few days. I really appreciate you guys chiming-in on this little mystery. We'll figure it out.
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Post by Doo640ER on Nov 1, 2020 12:40:15 GMT
OK so I got a couple hrs. of fun time in the garage yesterday with the goal of smoothing down that rough and sharp area of the carb where the main jet seals. I took a small fine file and evened it out, then hit it with some fine sandpaper, maybe 600 or 800 grit. Cleaned it up and reinstalled it with the 300 main jet inside. This is the best the sled has run with the 300 main, so I would have to say we were correct on that rough edge not sealing properly and letting extra gas flow through. I'm not sure it is running optimum yet, the throttle response does not seem as crisp as when I had the citation carb installed. I'm a little worried I took a little to much material off when I filed it, this carb is quite soft metal and it filed down a little too easily and it might not be perfectly level this allowing a little bit of gas to get past. I will likely take it back apart, and try putting a gasket on that surface. The way it is currently running, when I put the throttle to the bar, there is some mist/fuel cloud exiting the carb intake, but quickly gets sucked back in when it revs-up within a second. Is this normal? I don't recall seeing this when the Citation carb was installed.
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Post by olyman on Nov 3, 2020 22:53:22 GMT
Sounds like it made a decent improvement though. That's good. Adding a thin gasket can put the idea of leaking to bed. Also it might be set up pretty good for cold weather now? If its rich at the bottom end then it makes me wonder about the pilot jet letting too much in. Also make sure the float is sitting at the right level and inlet needle closes at the right level, but I think you already did that.
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Post by Doo640ER on Nov 8, 2020 13:40:24 GMT
Hey Olyman, thanks! I've added a small, home made gasket a few days ago, but have not started it yet to give it a try. Regardless of that result, I'm going to leave it alone and try running it when I finally get some snow. I tend to agree and hope that its just running rich and will lean-out once it gets frigid. It doesn't seem to be rich at the bottom-end and as best I can tell and the needle, seat and floats are all set and functioning correctly. Looking forward to giving this sled a minor workout to check it out completely. Right now I've got the seat off for some surgery. There were some staples that had rusted-out and let go and the rear hold-down studs need to be replaced. The seat wood is warped also so I'm going to cut a new board. Best I can tell the ground wire for the tail light where it attaches to the hardware that is buried inside the seat is pooched also so there is fun work to do. I've never been inside a seat like this so this is all new to me where the tail light mounts to this plastic housing. What in the heck are these mounting hardware. Some sort of rivet? On the opposite side of the board these rivets are ground down smooth so the board lays flat on the tunnel.
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Post by 1970tnt on Nov 11, 2020 22:50:08 GMT
HAVE A 78 TNT THAT HAD THE SAME PROBLEM. CLEANED THE CARB 3 TIMES FINALLY GOT IT CLEAN THE THRID TIME. I DID NOT HAVE A SONIC CLEANER THEN BUT DOO NOW. IF YA HAVE ONE OR CAN USE ONE I THINKTHAT MAY SOLVE YOUR PROBLEM. ON THAT CARB GOT A VERY VERY SMALL SPEC OUT OF ONE OF THE HOLES. YETI
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Post by olyman on Nov 12, 2020 0:25:51 GMT
I think those are basically old split rivets. They are actually split and you hammer them into wood. I've taken those seats apart too and you can replace them with something else if you want to - in fact likely far easier unless you find some of those rivets. Check the info on split rivets here
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Post by Doo640ER on Nov 15, 2020 15:25:18 GMT
Hi Joe, I suppose it's possible that the carb could use one more cleaning, ya never know. I've had it apart at least 3 times already, maybe one more time just for kicks and to put to rest the possibility that I missed something. I do have an untrasonic cleaner at work in the Lab so maybe I'll take it in there and give her a good sonication.
Olyman, thanks for that info on the split rivets, never heard of them before. I think when I re-mount that plastic housing. I'll drill holes and use stainless hardware and countersink the screw heads on the underside where it comes in contact on the tunnel. This seat is turning out to be quite a project, LOL. Good thing the seat cover is in good shape, at least for now. Hopefully I don't put my knee through it on it first ride at 20°F. The most fun will be when I re-staple the cover back on, NOT! I think there had to be 200 staples on that original job!! I'm still trying to figure out how I'm going to staple through that plastic around the taillight opening. I don;t have and industrial strength staple gun.
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Post by Old Iron 1978 on Jan 9, 2021 17:09:19 GMT
Just gonna throw my 2 cents into your adventure. Similar carb. vm32. on my 88 Safari 377. Ran for years like a top. End of last season, after a few minutes of riding, would bog out and die. if left alone for 5-10 minutes, it would fire up again and go a bit further...then die. I thought for sure it was electrical... replaced coil, wires, caps, plugs... no change. In fact the symptoms kept changing on me. at one point it wouldn't idle without throttle.... then it would idle great, but did exactly as you described. 3/4 to full throttle, it bogged down terribly. usually my go-to fix is a carb clean. I popped it off and in moments I found my culprit. 2 small bits of plastic debris (most likely from the airbox or disintegrated carb flange to airbox). I gave it a thorough 10 minute cleaning and I haven't looked back. So all of that to say. Carb clean, then again, and again. You have enough sleds that you're no stranger to carbs.... previously I was a huge advocate for the tillys.... but after working with the mikunis... wow. night and day difference... Sooooo easy. crap in the carb
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Post by Doo640ER on Jan 10, 2021 15:04:40 GMT
Hi Old Iron 1978, Welcome to the site! And thanks for the valuable input and your experience. Adventure, that is a good way to describe some of what I've been going through. Since my last update on this sled, I've had it out for a short test run on my property, after changing out the plug wires and caps and installing brand new NGK BR8ES plugs. It ran good momentarily, then bad, then good, then fall on its face as some would say. After 2-3 minutes of this nonsense I brought her back into the garage and checked the plugs. They were as white as new! Yikes! Now, I'm not too surprised at that since I was running a carb that was originally on a spare Citation 4500 I have in storage. It was cleaned well and set up for the with the internals for the TNT carb if I remember correctly except I used the Citation's 260 Main jet. I think I had the needle set in the most lean position also. I tried this set-up because the sled had been running so darn rich in the garage on the stand with the 300 Main. Well, this didn't work so well obviously. So I swapped out the 260 main for the 300 main and set the needle clip on the stock position. Started it up in the garage on the stand and the serious bogging returned. I have installed a 280 main but haven't tried it yet to see if the bog is still there. That's today's project. If it is, then the carb will be removed and cleaned again, for the 3rd time. I actually have my own ultrasonic cleaner that I got from a friend who picked it up in a garage sale for $1.00 from his neighbor! Will try that new toy as it's big enough to hold a carb. One thing I still want to do is change the caps since the ones I just put on there are the NGK 1000 Ohm resistor type. And I want to get some non-resistor plugs. I did check the external coils both cold and hot. I had 6500 and 6700 ohms cold. Installed on the sled they are obviously about 1000 Ohms less with those caps on there if I read it through the plug wires to ground. I still haven't done a leakdown test, but I did try spraying some carb cleaner at the PTO seal and there was no change in rpm. Not sure how much I trust that technique. As Bones mentioned somewhere on this thread, all things being equal, the stock carb set-up should work well on this sled. But there's something going on I haven't figured out yet. I hope it is just a stubborn carb with some hidden dirt!!
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Post by dando127 on Jan 11, 2021 23:57:28 GMT
Get a couple NGK LZFH plug caps before you go too much further. Try the easy stuff first.
I have a 250 twin sitting on the shelf because I couldn't figure it out. I was still new to the vintage stuff. I had installed resistor caps, and looking back, that was probably my issue. One day i'll get back to that engine.
Either way, the plug caps are an easy swap, and one more thing to knock off your list.
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Post by Doo640ER on Jan 12, 2021 17:13:01 GMT
I agree Dando, knock off the low-hanging fruit. I'll doo that soon, I need to order 4 or 6 of those LZFH caps, and some length of plug wire for other projects. I need non-resistor plugs also, but haven't been able to locate B8ES plugs anywhere yet. I'm not feeling too confident that the caps and plugs are significantly playing into my issue since the symptoms before and after changing them didn't improve or get worse. But this should still be done for reliability since everything on the motor looked original before swapping caps and wires.
All, I wonder if this could be a timing issue? Could a stuck timing advance lever cause all of this bogging?
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Post by Doo640ER on Jan 17, 2021 13:56:54 GMT
I still haven't messed with this since my last update. But I did do some shopping this morning in preparation of ruling-out further carb issues. I've got all of these items in my cart at Amazon. For the cost, it certainly couldn't hurt to just replace all of these items, at least then I would know none of them are going to cause any angst in the future. But in your experience, do the needle jets and jet needles actually wear out? Attachments:
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