1976 Nitro Grass Drag Set Ups

[mrrupp1]

A Comet yellow green spring at 2-3/8" is a 134 pounds and a 130 pounds every additional inch compressed. It is solid at an 1-3/16. As far as the secondary the helix faces out and not in like the old ones. That confused me the first time I used one. But it did make a big difference on how my sled came out of the hole. Ruppsforever knows way more about drag racing Rupps then I do by far. I live in Colorado and race around 10000 feet so our set up are a little different then most.

[Ruppsforever]

Oct 27, 2019 18:54:14 GMT -6 midniterupper said:

Well, upon further investigation I found the drive chain had begun to fracture and disintegrate taking the 19 tooth top sprocket with it. Both upper and lower chaincase bearings were not the best also and the lower drive shaft seal was leaking. This along with the drive sprockets being badly worn causing the ratcheting on take off. Thanks to Kevin Haase for having the parts available. Now the drive sprockets and all idlers have been trued, even have a 4 wheel rear axle to reduce rolling resistance. Had to install thicker tunnel protectors due to stud damage evidence, perfect opportunity! Installed 1 extra shim plate for the forward mount on the rear suspension hoping to decrease ski pressure and lighten the front end with weight transfer. Hopefully will button everything back up tomorrow, well worth going through the entire drive train making it good as new. Let the testing begin! Going to try to engage at 5800 rpm and attain 8100-8300 rpm during the run. First starting with the comet yellow/green spring with 1 shim using my comet A-10 arm @ 41.3 grams (P-85 primary) and the reverse arctic secondary using their yellow spring and a 53 degree helix. I already had these arms laying around so I thought I'd give them a try. Also want to try the A-3 arms @ 44.9 grams since they're here too. I will look for the Polaris 10 MR arms @ 44.0 grams to see how they shift out. Pretty cool, exciting to see it coming together! Should make quite an impact, keep you posted...

Oh yes the broken chain and sprocket. Been there done that many times. Take a small piece of flat stock And drill holes in each end and slide it over the tabs where the spring slides on. No more spring is needed with this. With this the chain can't get too much slack where it jumps teeth and breaks them off. You might have to make a few before you get it exactly right. You don't want chain too loose and not too tight. The tensioner pads still move freely but there will be no more loose chain. Good luck with you're testing.

[midniterupper]

To Mr Rupp1, I'm new to these forums so I don't know the ins and outs yet to post you properly, but I wanted to say thanks for your input on getting my Nitro set up for grass drags. Yep, I found out the arctic secondary goes on just the opposite of what I thought it would go. I do have a problem sometimes of thinking bass-ackwards! So your advise certainly helps. Thanks for your research on the comet yellow/green spring tension, I'm still trying different ones (or will be in the next few days) to see the differences between them. I think I have it down to 3 now, the comet gold spring, the comet yellow/green or the polaris gold spring. I ended up using the P-85 for a primary, and I found out that arctic ramps/weights are pretty cool. They define the profile cut and the gram weight right on the weight itself, very helpful. Example one weight I'm going to try has a number on it along with the actual weight of the arm itself. I still have to learn what the profile cuts are but at least we know the weight by just looking at it instead of weighing it separately.
I added the four wheel rear axle set up for my Nitro, we'll see if that helps at all. The track rolling resistance seems to be greatly reduced, trued all idler wheels, drive sprockets (new), made new nylon suspension pads that rivet above the pivot, aligned to rear suspension to eliminate rubbing. With all these mods and using different parts from 3 manufacturers, I think I will name her "The Mongrel American". Thanks again!

[mrrupp1]

I was looking at the specs for the Comet gold spring which are at 2-3/8 its 129 pounds and 123 every additional inch compressed. At an 1-1/8 it is 296. I don't have the specs for the Polaris gold. I hope some of this info helps. Good luck with your sled. I love seeing Rupp being raced. Clint

[midniterupper]

Cool! Thanks for the info, it does certainly help. Tonight I just tried 3 different primary springs, trying to raise my engagement rpm. I tried the Polaris Gold @ 3800rpm. Comet Gold @ 4200rpm. Comet Yellow/Green @ 4500rpm. Remember this is on the P-85 I picked up and I am trying the Arctic arms # 502-44.5 (44.5 grams). Tomorrow I'll try testing with this set up to see what the motor will spin at and what the top speed is. Then try the lighter arms (Comet A-10 @ 41.3 grams) since I have them laying around. My reason for wanting to try lighters arms is because on the stand the engine still wants to peak out at only 7400rpm. The ultimate goal is attaining the very best top speed in 500' which will dictate our set up. I really would like to engage at 5000+ and spin at 8300. Not easy to dial in but a lot of fun! Keep you posted.....

[midniterupper]

Ok, today we did our testing. Sloppy conditions with snow/rain mix for a while, temps holding at 37 degrees. Not the best but do-able. Made runs with the Yellow/green primary spring, can't get engagement up above 4500 rpm for some reason, even with swapping weights. Tried the comet a-14 arms @ 40.9 grams, got rpm's up to 7400 but the primary didn't shift out all the way and lowered the top speed down to 51 mph. Previous test set up was at 60 mph.
Tried the Arctic arms #502-44.5 (44.5 grams) got lower rpms at around 6900 but the top speed came back up to 60.
I think the low engagement issue can be addressed by using a higher tension spring like the Polaris almond/gold with a rate of 150/290. Just my guess, what do you guys think? I am at a loss of how to gain top rpm, need to spin at 8300 but I just can't seem to unlock the magic combination yet. Just for giggles, I'm going to try a pass with the Polaris 10 MW arms (44.9 grams) but maybe the profile will shift out more aggressively. Gotta wait for the mud to dry out a bit
Any ideas on how to figure out the profiles and effects on the primary clutch weights?

[Ruppsforever]

Sounds to me that either you're tach is garbage or your helix is too steep in the secondary. Did you try stiffening up you're secondary spring to see if that helps ?? Try taking belt off and rev motor up over 7400 rpms. If it won't move past 7400 tach is junk. I had one once that wouldn't go over 8000. Chased clutching for a few weeks.

[midniterupper]

Yep, that is certainly a possibility. I have always used a VDO tach when we raced our skidoo's 20+ years ago and actually that is the very same tach, so maybe it's time to replace it. One thing I did notice while I had the drivetrain tore down was that one of the stock exhaust cans has a rattle to it when you shake it. I'm wondering if the exhaust is becoming blocked somehow due to that rattle. Has anyone else run into this exhaust issue? I couldn't see any loose metal plates in there that was blocking the outlet port, but it does rattle bad. I'm thinking of trying to run the engine without the silencing cans to see if the rpm's will get above 7400. Sure seems odd when reviewing all my notes that I have never been able to get above that rpm...

[midniterupper]

I did take a look at the secondary. I noticed witness marks on the helix ramps showing the secondary is only going about half way up the ramp, no further travel or shifting beyond that. I played with the different spring holes, it's a lot different than the skidoo secondary. The spring was originally in the center hole, tension measured at 13 pounds. Placed the spring in one hole clockwise, tension increased to 18 pounds. One hole counter clockwise from center, tension decreased to 11 pounds. I didn't try the two other positions yet, I left it at the 11 pound preload to see if it will shift further up the ramp. I haven't made a pass with it yet, hopefully it will dry up enough today to do it. Final race is next Saturday, would love to wake this thing up. The top speed on vintage in our area (Michigan) using our radar gun has only been 67mph. It doesn't seem to unrealistic to be able to achieve that speed does it? Thanks again for the help!

[midniterupper]

Well, this morning gave an opportunity to make a couple of test passes again primarily to eliminate the track ratcheting problem the sled has always had. I replaced the original track drivers with new ones because of the problem, even with the track excessively tight. The problem still exists with the new ones and I have the track tension at 1" @ 10# down force. Alignment is perfect. Not sure how to tackle this one, still engaging at 4500 rpm so it's not from slamming the drive. Using only (48) .875" studs. How do you oval guys do it?

[mrrupp1]

Not sure what to tell you to do on the track ratcheting. My sleds have updated tracks which lets us run 4 drivers. Are you measuring your track tension in the middle of the slider?

[mrrupp1]

I would make sure your not getting to much flex in the suspension and I would take the track tension to a half inch measured in the middle of the sliders. I do have one sled I race with the Rupp track. It has 96 .875 picks in it and it doesn’t ratchets, but I do run it pretty tight. Most our tracks are snow covered ice so we don’t hook up that hard. My clutch comes in around 5300 and runs the motor up to 8200. I use a white spring in the secondary on the tightest setting. I can’t remember what helix I’m running. My primary clutch has a yellow green spring with c cam arms. Hope this helps. Clint

[midniterupper]

Thanks for your help Clint, I greatly appreciate it. I'm measuring the track slack at the middle of the slide suspension, pulling down on the track I get about 3/4"-1" clearance. I believe the track that came with the sled is a replacement by Goodyear, using just two track drivers. I played around last night with the chaincase chain tensioner. It appears the new chain and sprockets were slipping so I made that little bar with 2 1/4" holes spaced 3.75" on center. Maybe the ratcheting is actually my chain rolling over the sprockets? We'll see if this helps, looks much more solid now. Also cut into my one silencing can because of the rattling noise it has. I found 1 baffle plate broke loose sorta blocking off the exhaust, found a spiral baffle loose too so I welded that back into place also. I will let you know how the sled does asap! Thanks again for your input!

[midniterupper]

Well! We are making progress here in Northern Michigan! I repaired the exhaust can this morning, cut it open and welded all the baffles back into position so no more restricted flow there. Also studied the drive chain tensioner set up. Made the support bracket that Ruppsforever recommended, drilling two .250" holes 3.75" apart on center. Still floats when torque is applied but it doesn't back off the tension allowing the chain to roll over the top drive gear. I think that is where the problem was originating, at least after replacing the drive cogs.
I still can't get above 7500 rpm with my current set up, still engaging at 4500, but it's the fastest pass the sled has made yet, a whopping 64 mph in 500'. Good enough speed to at least have some fun at the races on Saturday. Running out of weather, got to get caught up on my Fall chores! Maybe for next year I'll get the rpm's up to where they should be, we'll keep trying! Thanks again to everyone that has pitched in, really do appreciate it!

[mrrupp1]

Good luck, one thing I was thinking is your helix might be a little steep. I was looking at my secondary and I have a 48 and they came with a 30 degree helix. So it might be trying to up shift to quick not giving the engine time to build some RPM. Just something to think about. Have a great time racing. Clint

[midniterupper]

This is what I have got the set up at the moment for the best top speed in 500'. Any suggestions/corrections are greatly appreciated, let me know what you think...
Main Jetting: 320 (non-adjustable mains)
Pilot Screw: 1 1/2 turns out

Primary: P-85
Spring: Comet Yellow/Green (would like to try Polaris Almond/Gold)
Arms: Comet A-2 (48.8 Gr.)

Secondary: Arctic Reverse
Spring: Yellow, Position #4 (1 hole CW from center hole) Preload: 11-12#
Helix: 53 Degree straight

RESULTS: (42 degrees, 80% humidity)
Engage @ 4500 RPM
Max RPM: 7300-7400 (VDO Extreme tach)
500' Top Speed: 64 MPH

Help! Still 1,000 RPM shy of powerband! (Even when on the stand)

Good News Is The Ratcheting Problem Is Eliminated!!

[Ruppsforever]

Glad you got the ratcheting figured out. That spring with tensioner pads is a joke. An old friend on here told me about that trick. Thought I would pass it down. 320 main jets are way too big if you're engine is stock. Is you're engine a 1976 or 1977 ?? 220's are stock on the 1976 and 1977 stock was 230's. Of course they had the adjustable main jet on the bowl. I most of the time left the adjusters shut so I was only running 220 main jet. Did you ever take the belt off and see if it would rev past 7400 ?? If it does go past then jet it down some and see if that helps. Do you have a piston light ??

[midniterupper]

Cool, thanks for your input, you're little trick did the job! I couldn't believe the difference. I wasn't sure about the main jets, the sled is a 1976, pretty much stock other than being bored .030" fitted with Wiseco pistons and using K&N tapered air filters. Squish is at .072". When I bought the sled it came with a 230 in the mag carburetor and a 270 in the pto carb. I will make another pass or two and do some more plug readings while jetting her back down. I do seem to have a low end bog on take off I was going to address also, not bad but it shouldn't be there. I wasn't sure about running the motor without the belt on, didn't want to have any incidents. Is that really okay to do? I'm all for it, it would be cool to get some more RPM. It just seems so weird that the motor just won't rev there. I'll see what I can do today with it, I gotta try to get some of my other chores done around here, the cows are getting hungry! Thanks again for your time and help, I do greatly appreciate it. Not many folks around these days with experience and expertise! It's so exciting to see her wake up!!

[mrrupp1]

It sounds like the carbs that came with the sled aren’t the originals. I would make sure they are 38s. It seems like they run the best with 38 mikuni in stock form. On my stock motors my main jets are 190 both cylinders 50 on the pilots and 1.5 cut out on the round slides. On the needle it seems they like the one that came stock in the 76s I can’t remember the number off hand. I live in Colorado and we race around 8 to 10000 feet in elevation so we do have to lean them down. And that’s with running an air box. Ruppsforever knows way more then I do so I definitely listen to him over me. One more thing I’m running 19 top 39 bottom for gearing. Good luck racing. Clint

[midniterupper]

Hi Clint, Thanks for your response and help. I have the same gearing 19/39, sounds like they also came with 18/39 or at least it was available. So far the ratio seems fine, I don't think I know enough to change that too You're right, I believe the carbs are not original though they are 38mm. When I tore them down the first time, I documented all the jet sizes. I will have to look them up to see how they compare with yours. I forgot to mention that I did put in Boyeson reeds too back when I rebuilt the motor.
I made some small changes this morning to the sled, tightened up the rear suspension preload to stiffen it up a bit. Was actually able to back off the track tension just a little to make it spin easier, no ratcheting whatsoever!. Then I fooled around with the main jetting. I first put in 230's to see how it sounded, had a flat spot on revving it up. I then went to 280's, sounded better but not a full response. I put in some 310's I have (couldn't find any 300's) and she immediately responded better. I played with the air screws to get it tweaked and made a pass. 66 MPH in 500', that's the best it has ever run since I got it! Still not quite shifting out all the way on the primary marks but further than it's ever climbed. It's engaging today at 4700 rpm, still spinning at 7500. Man, if we can ever get her to spin right and engage a tad higher, we should be pushing 70 mph. If we can do that, the sled will dominate the Vintage class here in Michigan! I do think is it achievable, next year for sure. Final race is this Saturday, Thanks again for all your help!