1976 Nitro Grass Drag Set Ups

[midniterupper]

Okay, I just pulled the CDI box off the sled. I think someone has been there before because I found two different size washers on the mounting bolts. Usually a sign that it didn't come out of the factory that way. Anyway the unit is very rusty, the silver oval decal is very faded but I might be reading "GU11" or "GUV1" in a small box on the left side of the decal, can barely read anything on it. It has 6 wires total coming out of it, a phillips screw on the side with the wires coming out. It measures 2.5"long x 2.25" wide x 1.4" high. On the back side it is black on the black plastic stuff though it has a bubble and a crack on the back.
Not sure if this is stock or not. Is a new one still available?

[midniterupper]

I called Pipestone and spoke with Brad. He thinks we have the correct CDI unit for the sled and mentioned the units typically either work 100% or not at all so I think we are good with this one for now.
I was reviewing my archives from throughout the summer and I did have a note in there stating I did get the motor up to 8300 rpm using the old Comet primary when I installed really light weights (C-2M arms @ 37.6 grams). However, when I ran the sled it only topped out at 39 mph. So the motor is capable of actually spinning at 8300, I just haven't found the happy heavier arm combination yet.
It was weird when I tested that particular combination (Comet primary, yellow/green spring & C-2M arms using the Rupp secondary w/30 degree helix, 11 pounds preload). The clutch engaged at 4500 rpm and immediately climbed to 8300. The holeshot was almost non-existent and the sled fully shifted out at 31 mph at the 200 foot mark on our test track. I didn't know what was going on with the run so I shut it down assuming the arms were just way too light and not in the right direction according to the top speed. Pretty obvious that I'm still learning eh?
So I think I will reuse this cdi unit to continue the testing. So much to try out yet!

[Ruppsforever]

So does the tach you are using have different settings for different pulses ?? I'm not familiar with that tach. Brad probably isn't familiar with the different CDI boxes. Kevin is the one you need to talk to. Kevin should have CDI boxes available but they don't look like stock. I agree either they work or they don't work but the 73 CDI doesn't have a timing curve. The 76 CDI does. Have you had a chance to check you're engine timing ?? Probably wouldn't be a bad idea to check it if you had the whole motor apart.

[midniterupper]

Good question, the tach I'm using is a VDO extreme that I purchased back in 2000 when we raced skidoo 700 & 800 stockers with MSDRA. Back then the tachs were pretty rugged and reliable, they record up to 4 runs by using a little 12 volt battery.
The tach has 4 dip switches inside the cover and they provide a combination of different pole settings to accommodate a variety of engines. It took me a while to figure out which ones worked with this sled but after a few days (told you I am a slow learner!) I finally got it right where it's accurate. Turned out to be a setting for single cylinder Dukati engines produced back in the 70's, a 12 pole something or other but it appears to work nicely. I just went off the existing tach lead for the source. Shows idle at approx. 1800, cranks up to 7400-7500 rpm on the stand with no trouble then gives a great reading on playback (slo-mo & real time) nice and steady throughout the run. I know there must be something else out there, digital would be really cool I just don't know what's available. We've been out of the game for so long now and to top it off, I changed brands! Plus gone backwards by racing sleds 44 years old! Oh well, I wouldn't have it any other way, so much to learn yet!
I haven't checked the timing yet either, do I check it with a dial indicator set up with #1 cylinder or can I check it dynamically? I've never performed that procedure on a two stroke, would it be simular to checking timing on a car? That is a question I had: can the timing be advanced on these motors? if so, by how much? What octane fuel would have to be used to accommodate?
I know I really need to focus on my clutching issues and get a handle on that before I move onto something else but you know how it is, just can't stop thinking.....
(BTW, thanks again!)

[mrrupp1]

This is just my opinion. I use 91 octane in my liquid trail sleds and they run pretty good, with no issues. But in my race sleds I run VP110, the only real difference in the engine is I run mod pipes and no air box. Porting is all stock. I do run more main jets, but that's about it. What kinda confuses me is the manual says they have 13.4:1 compression, and the manual says use minimum octane 92. I been a mechanic for 38 years and I my own auto shop. We drag race a 79 Foxbody Mustang with a big block Ford. Our engine has 13:1 compression and it has to run 110 because we have so much compression. But Rupp says at least 92, I've heard the compression is different on a two stroke. That's what confuses me. I guess I'm done rambling. If it was me I would try both and see which one works best. Clint

[midniterupper]

Well Clint, your opinion is certainly one to be respected! I know what you mean regarding compression ratios somehow being different with a 4 stroke verses a two stroke, direct/indirect readings ect. Enough to make you want to consume alcohol one way or the other! I know when we use to race the skidoo's back in the late 90's, we had to run spec fuels which they would verify. They made us use C-110, 112 or 114 all defining their octane content by their numbers. Us stockers could choose between the 110 or 112. Our ego dictated the 112 but in testing it actually slowed us down at tad so we stayed with the C-110.
I used to work with a gentleman named Steve Sanchez, he owned a shop called Total Flow Products where he modified and certified top fuel heads for NHRA. They ran on nitro being cut with alcohol, octane numbers are through the roof with that stuff but the rule of thumb back then was to never exceed the minimum octane mixture to prevent detonation. He always said the flame front will slow down from the desired 3300 feet/sec when under the specified compression for that motor. With that in mind along with your input, I will do individual testing using 91 octane pump fuel and with the 110 race fuel once I start getting closer to the rpm we want. Consistency is key eh?
That topic does make me wonder about our combustion chamber squish tolerance. I hate to push things to their mechanical limits and begin sacrificing reliability but I didn't know where you racers maintain your squish readings at. I'm at .072" and my compression reading is 143/145 psi. I checked newer skidoo mxzx 440's they're running compression readings at 160 and I was told they maintain squish at .050" If we did the math right, increasing our compression up to around the 160 range will only gain approximately 1.25-1.5 h.p. on average. But then again that could equate to pulling about a 10 pound weight advantage, it all helps but I don't want to sacrifice the reliability of this motor. I know you guys will have the right answer!
Thanks again Clint for your help!

[mrrupp1]

It sounds like your on top things, you know way more about the different fuels and octanes then I do. I have heard guys running stock liquid nitro say 91 runs better then 110 race fuel. My mod piped sled definitely runs better on 110. I can’t wait to here what you come up with. Have a good one. Clint

[Ruppsforever]

Here is something for you guys to think about. Back in 2009 when I was doing a lot of dyno work with my 440 I experimented with a couple different fuels. When I went from 110 Turbo Blue to 114 Sunoco I lost 8 HP !! Too much octane is no good too.

[midniterupper]

Excellent point! That certainly helps us dial in the fuel selections. I think we will begin our performance and evaluation testing using pump gas @ 93 octane (that's what our local Amoco carries). Once we get the engagement where we want and get her to spin at 8300, we can make enough runs to establish a performance baseline for that fuel (with that set up). Then we can drain that blend and test again burning C-110, make several runs again establishing a perf baseline for that fuel also. It would be interesting to compare the results in actual loads and conditions. I'm sure the weather will affect some factors but in general, the readings should dictate which fuel to use. We know of course jetting changes will have to occur between the fuels. Then at that point, we know the motor ought to be producing consistent power with both cylinders contributing equally for further development in the drive train by trying different arm profiles in the primary along with secondary helix angles.
We had winter arrive here early in Michigan so I have to quickly get all my Fall chores around our farm done. Can't wait to get caught up and begin trying all the suggestions and ideas for the sled! Please keep your thoughts and ideas coming, with all this help and support I know we are going to make a winning name for Rupp once again. Thanks to all!

[midniterupper]

Oh yeah, I've got another question here. I was re-reading the posts to refresh my memory and that got me thinking about the CDI units on our liquids. I gotta check with Kevin to see about getting another CDI, it sounds like he may have aftermarket units or something other than the OEM. My question is, has anyone checked into the program (if there is one) within the CDI that maps out the spark curve? Back in the day, we used to send our ignition units to Bill Cudney where he could massage the units and we would gain 3 hp over the stock programming. I didn't know if anyone played in that area or not?

[mrmotors]

Just curious, when the octane has been increased in the fuel, does the base timing get advanced to compensate for the slower burning fuel?

[midniterupper]

Yes, that is exactly the gain from my understanding. The explosion can begin to occur earlier (by advancing the spark timing) due to the higher octane ability to burn at a slower rate with a more complete burn cycle. There are still several factors that can influence the burn process such a air temp & density (colder temps = more air molecules), what additives they may blend in like alcohol, methanol, even detergents. It's amazing how an engine can breath in, produce a controlled explosion and breath out harmlessly at 100 times per second and we think nothing of it!
I'd like to learn more of setting the timing of our motors. I believe there is a formula for setting/advancing timing on a 2 stroke but I have to research it more. Something like so many thousands BTDC equates to a degree. I think that's called static timing because the engine is not running (dynamic timing with it running). I gotta go look at my motor to see if there are markings around the flywheel area, I don't remember offhand.
Well, off to do more research. Thanks again! -Paul

[Ruppsforever]

Nov 16, 2019 23:03:50 GMT -6 mrmotors said:

Just curious, when the octane has been increased in the fuel, does the base timing get advanced to compensate for the slower burning fuel?

I wasn't sure who you were asking this to ?? If it is me , I have my timing advanced as much as I can get away with already.

[mrmotors]

I was just asking a general question. High octane fuel burns slower so you need to light the fire sooner. Timing needs to be advanced or you will lose power.

[midniterupper]

Excellent! I believe there is power to be gained from advancing the timing on these motors. How far did you advance your timing and how did you set it? I haven't played with the stator at all yet. Thanks for your input!

[Ruppsforever]

I'm not sure how much can be gained on a stock engine. I run mine at .100 BTDC. You need to have a dial indicator that rides on top of piston that reads in thousandths not mm. You need to make a stiff piece of wire and bolt it by the primary clutch pointing to the outside of the clutch. Find top dead center make a mark on clutch spinning motor backwards make a mark every 10 or 20 thousandths I make marks all the way out to 120 thousandths. Then with a timing light hooked to cylinder one spark plug shine it where you're marks are on clutch. You will need to have someone rev motor over 6000 rpms. You will see the timing jump at 6000 rpm if you have the right CDI on it.

[midniterupper]

Wow! What a wealth of information on that reply! Fantastic and very helpful. I've been wondering how you performed that timing procedure, it makes perfect sense. I want to try marking the clutch as you stated and try the CDI that came with the sled. I'm purchasing another CDI from another gentleman here in Michigan to try also. He said he removed it from his nitro a couple of years ago, it has the number CU11 on it as well. I'd like to witness the timing advance actually moving with the RPM. We'll see if there is a difference in my sled's rpm and performance, mainly to hit that elusive 8300 for me.
Also, I was poking around on line the other day and came across something interesting in regards to our CDI units. I went to CDIbox.com and found that the Fireplug units are becoming popular with the surge of interest in vintage sleds. They are manufacturing several replacement units for older bikes and sleds, sounds like they may be onto something good. They mentioned the Prestolite units and specifically mentioned Kevin Haase for lending the company his personal sled (Rupp) for the development of a new CDI for the Rupps. Hmmmm, maybe a programmable spark curve? We'll have to give Kevin a call and see what he predicts, maybe be able to test with that box? It's gonna be a great race season!
Thanks again Josh for sharing your wisdom, can't wait to try some of your ideas!
-Paul

[Ruppsforever]

The programmable ignitions Adam Hood builds. They ae quite pricey but probably worth the $$$$ I always wanted to try them but for drag racing I just couldn't justify it. Oval racing would be a better fit with his systems. I just noticed that I put a decimal point in the wrong spot on my last post. I run mine at .100 BTDC.

[mrmotors]

Hey Josh, what is the diameter of the clutch that you are making the marks on? Thanks, Mike

[Ruppsforever]

Where the marks are and where the wire pointer is pointing probably around 6 inches. I make the marks down the sheave where the belt rides.