1976 Nitro Grass Drag Set Ups

[nitrofa340]

That is want you want to do,....ride it like a jet ski, not steer with the handle bars. It indicates you arent scrubbing the skis into the ground.

[Ruppsforever]

Paul , when you say the carbs aren't opening all the way are you meaning where you can feel it with your finger ?? Or where the carb is closest to the motor ?? You want the end closest to the motor open all the way. If this makes sense. Hard to describe. I've already ground a chunk off hand grip to get throttle open more. With stock Rupp cables I never had an issue. You can grind some off the throttle lever too. Anxious to hear if the new carbs help. My Mod motors certainly loved 39.5's. My 440 and my 340 ran 39.5's.

[midniterupper]

Actually, regarding the carb slides both sides of the slide is sticking down into the horizontal bore of the carb. That would be towards the motor side and the air intake side. There is a lot of slop with this aftermarket cable, at least 1/4" freeplay. If I adjust the adjuster screws on top of the carb any further out, it falls out of the threaded hole on the cap. Never had this problem before with any other sled, I'm sure I have the wrong type cable. I ordered a new cable from Wahl bro's this morning, they claim theirs has more adjuster thread and tighter tolerances than any other after market cable. We will see....

Now I did play with the 39.5's today trying to dial them in. I installed a 250 main jet and have the bowl adjusters set at about 1/8 turn out. There were #50 pilots in it and it seemed too lean with those. Had to set the air screws at 1/4 turn out. I had another set of 50's and they measured a .024" diameter hole so I drilled them out with a #70 drill (.026") and it helped a little to eliminate the low end bog I have with these carbs so far. So thinking bigger was better, I drilled them out again to .028" and finally ended up at .039". No idea what size the pilot would be at that diameter but it seemed to finally help with the idle and slightly improves the low end bog but it's still there. I'm kinda guessing around a 60 pilot size. I have moved my needle clip from the middle to the top and back down to the middle position again to help address the bog but like I said it's still there though improved.

I'm very glad to hear you also run with the 39.5 carb. The motor responds stronger once I get past the bog and the rpm's get to 8000 like right now so I'm sure it's worth figuring out. I can't really test until the bog issue is addressed and solved.

Let me know what you think I ought to try, sure appreciate it.

Until the next time,
-Paul

[mutt]

If your other carbs full pulled. Swap the tops to your new carb check if it makes a difference. If you have bored carbs. Like when Crankshop bores a carb they cut the slide to match the bigger bore maybe your in that range. If its oval bore or straight bore.

[Ruppsforever]

So with the bog with new carbs that tells me something is different between the old carbs and new ones. I never had to change pilots because of a bog. All the same jets in the 38's worked great with the 39.5's. Make sure your needles are the same and main jet tubes too. Are the cutaways different too ?? That doesn't sound right that a bog came with just going to 39.5's. That shouldn't happen.

[midniterupper]

Thanks Mutt, good info to consider for sure. I believe they are straight bored to 39.5 but the slides are still the 38mm (1.550") dimension. I always thought both bores got bored out but from what I've been reading primarily only the horizontal gets bored more commonly. So much to learn!

Today (in between farm chores) I was able to work on the carbs more. Quite funny and true story, about 3:30am I woke up thinking about the bog situation. It bothered me because the motor never did that before but once it got past the bog, the motor really woke up which is very exciting. Just as I was thinking about it, my phone chimed with a text from Clint saying he has been thinking about my bog problem too. He thought maybe try his set up and swap out the 3.0 slides with 2.0 or even 1.5 cut aways. I was thinking of changing out the slides this morning just to see and set all other jets back to our normal positions. Warped minds think alike eh?

First tried 2.5s and she ran better. Put in the 2.0 cuts and now we are back in the racing business! No bog or hesitation whatsoever, instant response and even stronger than before.
For grins and giggles, I reinstalled the original carb set using the same throttle cable and sure enough we have been racing and testing at 3/4 throttle only! I am embarrassed to admit it because I should have checked something that basic. Now we really have some dialing in to do! I can't believe it ran that strong. Can't wait until we get the new cable in. May have to do a test pass in the morning just to see the diff between the carb sets. I know everything will change now.

Chat soon, Enjoy this Holiday!
-Paul

[Ruppsforever]

So with your 38mm carbs were you running 2.0's ?? I never ran anything but 1.5's. I could see with warm weather that the 2.0's could be better though. And yes you need full throttle LOL !! I bet your main jets are too big once full throttle is reached now. I guess you will find out soon !!

[mutt]

like they say 1000 words

[midniterupper]

Wow! Excellent video! How did you do that?
Thanks for sharing!

[midniterupper]

Okay, time for a quick update. Got the Hy-Lex throttle cable in from Wahl. It is a very high quality cable with very little friction. It opens the slides fully now and has plenty of adjustment room. What a difference!

Also, the F/A curved springs arrived. They installed nicely and look like they make the front end lighter. I first installed them without any shims, seemed like it wouldn't be a problem to slip in a couple of shims so I did as an ultimate effort to get that front end as light as possible.

We did a little testing with the sled and new set up this morning, had interesting results. Right off the bat with the first pass wasn't my best. Had the sled all lined up and packed down for a good holeshot. The clutch is still engaging low at 5,000 rpm so I didn't expect much. The sled launched hard lifting the skis about a foot off the ground for the first 50 feet. I had to let off and straighten out then get back on the throttle to complete the pass. The clutches shifted out and I couldn't recover to make a good run so that was that.

Further on, I was able to make complete runs but only topped out at 60 mph max and had a slight miss. I think that may have been a carb setting problem. The sled really pulled hard from launch to mid track but then just tapered off on the power. Both exhaust temps are good now at 1195/1145. Tach reads steady hitting 7600 climbing to 8000 so we have a little tuning to do there yet.

Now when I pulled the plugs after the last pass, they appeared overall on the lean side. I have adjusted the carb bowl screws to the half way position, was almost closed prior to that. Hopefully that should richen the top end mixture and help that out. Sounds better to me, a more deeper tone.

Going over the sled I noticed that the track was rather tight. That may have also caused the sled to slow down. Got that adjusted in properly tonight too.

Should be able to make another test or two tomorrow.
Take care, keep you posted!
-Paul

[midniterupper]

This morning's test went well though still cannot attain more than 60 mph with this set up. The carb adjustment improved the motor's performance, no more top end miss. The plugs appeared a bit lean still so I turned out the bowl adjustment screws to 3/4 turns out to richen the mixture up. Piston wash looked dry also. Kinda cool just turning a screw, what a great concept. Not spilling as much fuel.

The sled is pulling holeshots like never before, lifting the skis a good foot off the ground at launch. Aligned the track better and now she lifts straight. For some reason the motor just flattens out and will not gain any rpm's at half track. It sounds strong like always just not able to get into the power band. It may be the Thundershift kit and not having it dialed in correctly quite yet so just for stablity in mind, I switched to the A-2's until I get things figured out and get our speed back up.

I looked at the rear suspension using the F/A springs which I had installed the 2 shims at first. It looked like it may have stressed the springs by using the shims so I removed them today for the next test session. The springs appear much more relaxed in this position, maybe it was causing some type of additional rolling resistance.

The exhaust temps are much improved, both running today around 1194/1190 degrees. Within 4 degrees for the recorded high temp is encouraging. Such a nice motor.

That's the latest, keep you posted
-Paul

[midniterupper]

Welp, we got another chance to test tonight. What a night! I made the changes that I mentioned in my previous post an hour or two ago and now we are back in business. The sled ran flawlessly this time, great holeshot and strong all the way through.

69 MPH! That's our best speed yet! Exhaust temps are 1190's and our rpm with the A-2's is 7600 climbing to 8050 max. If I can get the r's to stay right at 8000 the entire run we would really be there! I think it's doable.

Okay, we're off to celebrate! Thanks again to EVERYONE for your help and support!
Chat soon,
-Paul

[Ruppsforever]

And that my friend is why good note keeping is very necessary. You can always get back to where things were !! Congratulations on the gains made !!

[midniterupper]

Well, it's been a while since we gave any updates on our end here. Life is good, been busy getting more stuff done around the farm while we can. Appears the Covid canteen may be getting more contained and some of our favorite activities can resume. I see swap meets are scheduled and so are our races for next month. Hope they will resume and we can demonstrate our collective efforts! I know it's gonna make an impact for the Red Team.

After our last test session where we obtained our best mph I decided to send the head out to Pipestone to have the spark plug threads repaired as they were getting sloppy and I didn't want then to blow at a race. I measured the squish using Kevin's new steel gaskets and we are at .075", compression is at 145/147 so I thought we could bump that up a bit. Talked with Kevin and he agreed that shaving .015" off to bring the squish to .060". We will see how it affects the engine performance. It may produce a better bang burning the C12 mixture.

Speaking of racing fuels, this Covid thing has affected the availability of at least VP fuels. I wanted to test using their VP113 fuel which is oxygenated for engines running 15:1 or less compression. None in the state of Michigan though there is a higher compression blend (19:1+) but that won't be good for me. Still picked a couple of pails of the C12 for this season. Sure smells good in the morning!

I did install the UFO's on the 2.0 slides I was using. I machined matched the slides to blend with the UFO inserts and installed the recommended 1/2 size pilot jets. It took a little adjusting and I had to go up 2 sizes on my mains and 2 sizes on the pilot jets. They made a definite difference in response, mostly mid-range. It idles nice and crisp without any drowning or hanging idle.

We tested with them installed (only two passes) and the top speed has not increased yet but I think it's due to still getting the main jets right. When playing back the testing video, the motor sounds stronger through the entire run so I think we may be onto a power gain by using them. After the installation is complete with the blending of the slides to match the UFO insert, it certainly appears to improve the air flow under the slide. We will see, I'm gonna pickup another set of 2.0 & 1.5 slides when I visit the salvage yard next time just in case.

Okay, I think that is it for now, just waiting for my head to return. Remeasure the squish, install the B10EV plugs and resume to testing. The first race is September 12, I think we are at least going to place in the top 3 in all classes. But ya never know, that's what racing is all about right?

Take care, stay safe and have fun!!
-Paul

[Ruppsforever]

I once did that mod to slides with the UFOs. I was too reluctant to try them though and never did. I sold them. When we used to race I had a hard time testing. It was pretty difficult to get pond cleared off and everything. That head should help. Are you just getting it shaved or redomed ??

[midniterupper]

Hey, Thanks for your reply, you always have great input. Right now I just had the head shaved the .015" to bring my squish down to .060" so it's nothing too radical but should help bring the motor to a good snap.
Back in the old days with Rotax motors, .050" for squish used to be the hot set up. Bill Cudney was always a good source for power seekers, he had a lot of resources, contacts and experience he would share. He was a good racer and he built his engines maintaining .050" with no reliability issues.

Now with these Xenoah's it sounds like a little different ballgame. There are very respectable Rupp racers on this site that know how to build these motors and they know what they are doing so I'm just going off their advice. But so far this sled is exceeding my expectations abundantly! So many great ideas to try, and they are working out well. Thanks again to everyone who contributes their support, ideas and even parts! Our Project sled meets the competition on September 12, only a couple weeks away!

The UFO's on the carbs are kinda scary to carve and blend the slides to match the UFO inserts but with salvage yards not too far, we can always go back if needed. They do change the appearance of the slide but you would have to pull the slides out from the carb to notice them. Great response though! We'll see if they improve the top end, wide-open air flow. One review from Snow-Tech magazine stated an 8% increase in top end flow rates. That's why I figured I would experiment with them. At our races, they don't care what we run which makes it more fun for all of us. Gotta love the Vintage relaxed rules!

I forgot to mention the slippery shims installed in the primary. They bumped up my engagement rpm of course, but only by about 100 rpm so it is adjustable, especially if you use the additional plastic shims. I'm not using them at first. The slippery shims affect the clutch shifting pattern in a way that is difficult to explain. It sorta makes the clutch feel like it's free-wheeling or something. Seems to shift out quicker. We will see if it actually reduces spring resistance by contributing to a faster top speed. Lot's to do yet.

The head should be here by Tuesday, Kevin's shop in Eau Claire got hit by a bad storm and lost power for a few days causing my delay. No biggie, we still have time to test.

That's it for now, as soon as we resume our testing, we will give an update. So Exciting!
Stay safe and cool now,
-Paul

[Ruppsforever]

Yeah that was why I ditched the UFO's. I didn't have time to test with them. I had carburetion perfect so I didn't want to start over and loose testing time. I forgot to ask what pistons you are using ?? Are you using the higher domed pistons that Kevin made or just regular domes ?? That head should help a little but having one redomed is best. Also it would add some HP. One rule I always went by is .045 for drag racing and .052 for oval racing. Many engine builders build to those squish numbers. For you to get .045 on a 440 Xenoah you will probably have to shave the top of cylinders off too. Piston doesn't even come close to the top of cylinder ATDC.

[midniterupper]

Hi Josh, Thanks again for your input. I have Wiseco pistons in the engine right now, I got them from Kevin last year when I first had the cylinders bored. I'm not sure if they are domed or flat, I will have to check on that. I greatly appreciate your sharing your squish values, I know the .060" is rather on the conservative side so it's good to know I can push it harder if we need to.

I'm not sure of a good resource for getting the head dome reshaped. If you know of a shop that can cut our domes, that would be really cool! We may want to go that route eventually of seeking the outer limits of our engines. Would love to massage those ponies out of her. Speaking along those terms, I thought I read that the cylinders out of a '77 engine would be a good combination for the '76 crankcase to get more horsepower? So with that thought in mind, can the '76 cylinders be ported to the '77 specs?

Also, I thought I remember hearing that the cylinders can have thicker base gaskets installed (shaving the head the same amount) and thereby altering the port timing? I'm not sure if I want to go that far for the added performance as that would be getting more into the open mod classes. Would love to have the additional power. We'll have to see what comes to the races.

I wouldn't mind playing with a spare set of pipes that have the modifications already done to them. I'd like to install the new ISR silencers onto the stingers to see how they sound and perform. I'm sure the exhaust has to be getting a little restrictive to our output. I'll have to keep an eye out for a set to play with sometime.

I have interesting side note regarding our competition at our local races here. I got together with one of the vintage Yamaha racers that we will be up against (he wins regularly) and he said he sent his motor out to a performance shop for a "rebuild" and they somehow discovered 14 more horsepower on the dyno. He still is using the stock appearance exhaust and they swapped out the carbs along with the rebuild so it really should be a good season. He mentioned they have been testing downstate with good results so we shall see...

well, that's about it. I will keep track of your squish stats, I sure appreciate your support and ideas. Hopefully someday I can return a favor back.
Take care now Josh, keep you posted.
-Paul

[midniterupper]

Okay, got my facts straight on the differences between the '76 & '77 engines thanks to Clint. I'm glad you guys are around, your expertise helps so many of us. Thanks again!

Anyway, the cylinder head came back in. Kevin did a great job as always. My squish now measures at Pto: .060" and Mag: .057" with the compression readings now at 158#/160# respectively. Fired right up and sounds great. Had to adjust the idle down about 200 rpm or so. Response has also improved.

The first test pass topped out at 68 mph but the main jets (290) were too lean and I had to shut down early, but a good pass nonetheless. 7600 rpm then climbed to 8000, exhaust temps were not too bad at Pto: 1188 degrees, Mag: 1069 degrees.

Second test pass topped out only 66 mph with (320) mains, 7600 rpm with no change up or down just steady. Exhaust temps actually went colder being at Pto: 1114 degrees, Mag: 869 degrees.

Third test pass dropped to 65 mph with 310 main jets, 7600 rpm steady again. Exhaust temps at Pto: 1119 degrees and the Mag: 855 degrees. The Mag side exhaust temps have almost always been quite a bit colder than the Pto side and I'm not sure why. Both cylinders are jetted exactly the same, needle clip position, air screw setting at 1 turn out.

As we all know from testing that our top speeds may vary even when nothing has been changed. Conditions and a lot of other variables come into play that are outside of our control but those variables will affect the other sleds too, which is good.

I'm just not sure why we get different temps between the two cylinders. The plugs and piston wash appear a little lean on the Pto side and just about perfect on the colder Mag side. Kinda odd isn't it? Anybody else out there have a big variance between your cylinder temps?

On the negative side, our 10 year old radar gun died today. Won't give accurate and consistent readings any longer so we had to order another one, should be here Thursday. Hopefully that will give us some time to figure out the temperature difference. I will try swapping carbs again to see if the temp change follows the carb swap.

Until next time, keep seeing Red!
-Paul

[Ruppsforever]

Maybe try swapping carbs around again. Do you have adjustable bowls on the 39.5's ?? Possibly the O ring is shot on the rich side. Or could the lower temp cylinder be reading right and the higher temp one reading wrong because of a air leak ?? I'd say just swap the carbs side to side and see what that does first.