RPM Drop

Technical discussion, questions, answers and information
Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

RPM Drop

Postby Craig Hudson » Sun Aug 02, 2015 9:01 pm

Had a two header autocross event this weekend. Aside from a handful of other frustrating problems, the Sunday course was much faster than Saturday. Quite a few nice straights. About halfway through the faster straights (the ones with fast sweepers leading pu to them) my rpm would drop form 8500 to 7500. It was very repeatable and happened in the exact same place on course with a high speed straight.

Happened to both myself and my codriver. Plugs look fine, car pulled hard, quickly letting off the gas and then getting back on made it stop. The secondary clutch has marks going about 1/2 way down the helix so it wasn't going into overdrive or anything. Brand new belt before this weekend, same size as the old one. Formula X1 secondary, polaris primary with 911 cover.

Mel Winnie
Posts: 32
Joined: Sat Apr 26, 2008 10:45 am
Location: United States, Michigan, S. Boardman

Re: RPM Drop

Postby Mel Winnie » Sun Aug 02, 2015 10:05 pm

You have a weight that produces too much squeeze as it shifts into higher speed ratios. What weights are you using?
Mel Winnie

Craig Hudson
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Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Sun Aug 02, 2015 11:16 pm

I honestly dont know. Bought the clutches already setup from Fred Edwards when I purchased his 493. I'm still very new to clutching. Are you saying the primary starts to squeeze faster than the secondary opens at some point? Is this a function of weight profile?

Mel Winnie
Posts: 32
Joined: Sat Apr 26, 2008 10:45 am
Location: United States, Michigan, S. Boardman

Re: RPM Drop

Postby Mel Winnie » Mon Aug 03, 2015 8:02 pm

You are thinking right. Think about the clutches as the drive creates the force and the secondary resists the force. If the secondary resists more than what the drive clutch produces, the clutches wont shift. Inspect the secondary first. If the helix is marked "MWX1" or "X1CT", there should be a outer green spring with the spring tab in the middle hole and there should be either a blue or black spring inside the green spring. The center spring should be just captured but not compressed. If everything is in order in the secondary, then inspect the weights in the drive clutch. Let me know what weights they are (pictures will help) and I can help solve the problem.
Mel Winnie

Jim Murphy
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Location: United States, Georgia, Winder

Re: RPM Drop

Postby Jim Murphy » Tue Aug 04, 2015 2:29 pm

Fred Edwards is a road racer and never autoxed so his setup would be for road racing and not suitable for solo. What do you have for the solo setup as far as final drive ratio?

HTH,
Jim

Craig Hudson
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Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Wed Aug 05, 2015 7:04 am

I'm traveling for work this week but from memory its the mwx-1 helix, green and blue springs as described. Have to check weights friday when I return.

Final drive is a secret ;) but it's pretty high. Higher than 4.5:1. Sundays course was one of the fastest I've seen at Grissom in the 4 years I've had the car.

Jim Murphy
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Joined: Fri Mar 16, 2007 2:13 pm
Location: United States, Georgia, Winder

Re: RPM Drop

Postby Jim Murphy » Thu Aug 06, 2015 4:30 pm

When I changed the final drive for solo, I had to completely re-tune my clutches.

Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Sat Aug 08, 2015 11:46 pm

This is the first I've had a problem in my third season with this setup. Short straight with a full throttle sweeper into another longer straight caused it. It was long enough that I had time to look down at my mychron and see the rpm and give a "what gives? " motion to corner workers. Lol. I'm concerned nationals will have similar sections.

Pulled a weight. 10/M5, 49.5 grams. Secondary is indeed setup as described above.

I have a test n tune on the 22nd to prep for nationals.

Mel Winnie
Posts: 32
Joined: Sat Apr 26, 2008 10:45 am
Location: United States, Michigan, S. Boardman

Re: RPM Drop

Postby Mel Winnie » Mon Aug 10, 2015 7:52 am

That arm profile will build RPM with our secondary when the weight is correct, as long as the motor is producing constant power. Can you tell if the motor is going rich when at high speed? If so, route all of the carb vent tubes into the airbox. Otherwise just install a 1 gram lighter weight. then fine tune from there.

Mel
Mel Winnie

Craig Hudson
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Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Mon Aug 10, 2015 10:52 am

Can't tell if it's going rich or not, but it didn't seem to be. All the other times it runs at 8500 rpms and around 1100 EGT's, it was only in the really fast sections that it did this. I don't have an airbox, just carbs hanging off the motor.

One thing I did notice is that the weight has a wear pattern from a couple seasons use (not so much wear, but a shiny spot). After Sunday's event (the faster event, Saturday was slower) that wear pattern was extended another 1/4" and nearing the end of the weight so it definitely was a faster course and a new area in the clutch that I hadn't previously been in.

I'm a noob when it comes to weights. What weight would I order to get the same profile with less weight?

Jim Murphy
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Joined: Fri Mar 16, 2007 2:13 pm
Location: United States, Georgia, Winder

Re: RPM Drop

Postby Jim Murphy » Mon Aug 10, 2015 2:27 pm

1100 degrees EGT is too rich - somewhere just over 1200 to 1250 is better and more power.
You can achieve some increase in EGT by going to a hotter spark plug. NGK 8's and 9's are cold and are for
road racing so I use a NGK 7 mostly for solo; IF I have flooded it then I will swap in some hotter NGK 6's
to clear out the excess fuel quicker and then back to the 7's for the runs. Otherwise one step at a time
leaner main jets until you get to 1200 BUT this all depends on the air density at the time that you are running
so an air density gauge is called for.
If you have a gram scale you can take one gram off each clutch weight - I still have two scales,
the old triple beam version and electronic version. Go buy Aaen Performance Clutch Tuning Handbook
AND his Carb Tuning Handbook so that you can learn what to do correctly.

Welcome to tuning a two stroke with carbs. Several years I ago I submitted a rule change to allow some form of
auto-tuning the carbs for air density changes so that the drivers will not have to suffer changing jets all the time.
I got outvoted on this. Soon after we went with 600cc EFI MC drivetrains and it has an auto-tuner.

Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Mon Aug 10, 2015 8:14 pm

I have both books. If I take off weight, won't that affect rpm at every wheel speed? The rpm has been a perfect 8500 except when we approached the really fast section. As it neared the top of the weight was when the rpms dropped.

I've stepped down the mains a couple times and the egts don't move much. Running 310s right now. Air density gauge shows about the same.

Mel Winnie
Posts: 32
Joined: Sat Apr 26, 2008 10:45 am
Location: United States, Michigan, S. Boardman

Re: RPM Drop

Postby Mel Winnie » Mon Aug 10, 2015 10:05 pm

What engine are you running? What drive clutch spring are you running? Yeah there is horsepower in higher EGT's. I like to see north of 1375 but not everyone can run a motor that high. You shouldn't go over 1250. If the EGT's are dropping when hitting high speed, then something is causing a rich condition. If the EGT's are staying about the same, then I would lighten up the tip of the weight or go with a spring that has a higher finish poundage. I like using adjustable weights, something with the same profile as a Polaris weight. They are not cheep, about $220.00 but it will be the last set of weights you will buy. I may have a set in stock.
Mel Winnie

Craig Hudson
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Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Wed Aug 12, 2015 3:43 pm

Blue polaris primary, rotax 493. One concern with higher EGT's is wondering whether or not the sensor react fast enough to read spikes that may occur in autocross when I don't have long straights but short bursts. I also don't change jets out regularly. I'd rather leave a little power on the table and be safe.

Mel Winnie
Posts: 32
Joined: Sat Apr 26, 2008 10:45 am
Location: United States, Michigan, S. Boardman

Re: RPM Drop

Postby Mel Winnie » Wed Aug 12, 2015 11:26 pm

Depending on what pipe you have, the 493's make power around 8700. I would try a one gram lighter weight. Turn off the EGT's and jet it by the spark plug, an NGK BR9ES. After a run, inspect the spark plugs and look for a thermal line across the ground strap. If the line is near the end, it's too rich. If it's near the bend, it's lean. About halfway is good. Don't worry about getting a false reading when driving back to the pits, the line will still be there. Just wash the oil off the ground electrode with starting fluid to reveal the line. Once you get the jetting right, look at the EGT's to get a baseline. Just remember that a track with longer straights may require bigger jets.
Mel Winnie

Craig Hudson
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Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Fri Aug 14, 2015 9:15 am

I'll try that at the test and tune. I don't road race at all, just AX, so our straights are usually shorter.

Tried to remove my primary clutch. I've done it several times before putting a rod in the primary to stop it from rotating and then using the puller tool. It was turning quite easily (normally requires a breaker and cheater bar sometimes, not this time) and then it popped. Usually that popping noise is the clutch coming off the crank. Instead, the outer sheave now rotates free of the inner sheave. :shock: Don't think that's supposed to happen..... :(

Not my photo, but how I've always done it:

Image

Cory McLeod
Posts: 673
Joined: Mon Mar 19, 2007 11:10 pm

Re: RPM Drop

Postby Cory McLeod » Fri Aug 14, 2015 9:53 am

Using a screwdriver through the clutch to keep it stationary during removal will damage it. You need to get a tool like this. It works great - you will be glad you spent the money.

http://www.snowparthub.com/725_255_SNO_Stuff_Clutch_Holders_Tool_Clutch_Hldr_Arc_pol?src=Google&gclid=CInTz9PbqMcCFYckgQodWb8BoQ
F500 - Can't Get Enough of the 2-Cycle Smoke and CVT!

Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Fri Aug 14, 2015 10:13 am

What does it mean that the outside is spinning separate from the inside sheave?

Cory McLeod
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Joined: Mon Mar 19, 2007 11:10 pm

Re: RPM Drop

Postby Cory McLeod » Fri Aug 14, 2015 12:03 pm

Craig Hudson wrote:What does it mean that the outside is spinning separate from the inside sheave?


If you hold the stationary sheave and rotate the rest of the clutch clockwise, does it come to a stop or rotate freely forever? If it rotates freely forever, the spider is broken.
F500 - Can't Get Enough of the 2-Cycle Smoke and CVT!

Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Fri Aug 14, 2015 12:27 pm

Not sure. When it started to spin (even just a little) I stopped and decided to research it a bit more as I've never had that happen.

Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Fri Aug 14, 2015 6:53 pm

It comes to a stop.

Cory McLeod
Posts: 673
Joined: Mon Mar 19, 2007 11:10 pm

Re: RPM Drop

Postby Cory McLeod » Sat Aug 15, 2015 8:44 pm

That's good! The spider just came loose. I'm not sure how to tighten it enough without special clutch rebuilding tools, but there is probably a way. Hopefully, someone more knowledgeable than me will respond. If not, you will definitely get a good answer at f500.us.
F500 - Can't Get Enough of the 2-Cycle Smoke and CVT!

Craig Hudson
Posts: 404
Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Sun Aug 16, 2015 4:06 pm

I took them to Leon. :) I couldn't get the clutch off the car but Jeremey got it off (with that tool you posted) in 30 seconds. They're going to look at them. I'm a bad F500 owner. I've never taken apart a clutch before and cleaned it. :oops:

jtc161
Site Admin
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Re: RPM Drop

Postby jtc161 » Mon Aug 17, 2015 11:17 am

Craig Hudson wrote:I took them to Leon. :) I couldn't get the clutch off the car but Jeremey got it off (with that tool you posted) in 30 seconds. They're going to look at them. I'm a bad F500 owner. I've never taken apart a clutch before and cleaned it. :oops:


We built a tool out of some flat plate that was welded to a 2" square tube that we inserted into the truck's hitch for taking apart the primary. It worked well for adjusting the shimming etc.
1997 RD
Image

Craig Hudson
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Joined: Thu Aug 26, 2010 8:58 pm

Re: RPM Drop

Postby Craig Hudson » Mon Aug 17, 2015 7:57 pm

The 911 cover adjusts without taking it apart or shimming. :D. I still need to make a tool to take it apart for other reasons.


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