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Old 08-24-2011, 07:03 AM   #1
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99 ZRT 600 Max RPM & clutch tuning ?'s

Hello all, I am currently reworking a 99 ZRT 600 that I bought last year. Its had an unfortunate history of "excellent" tuning which led to a grand combination of a highly rich mag cylinder combined with a burned down pto cylinder which caused a muffler fire! I am a big fan of the ZRT's and chose to resurrect the beast.

For the engine: Stock stroke and bore, planning on SPI pistons (i hear these should hold up fine) with teflon skirt coating and ceramic on the domes, possibly shaving the heads 10-20 thousandths, light jug porting (mostly to "bell" out the ports to allow better air transfer as sharp edges dont promote airflow, and to more closely match the port size to the opening in the cylinder, no port timing changes), Vforce reeds, 38mm carbs (UFO's have been thought about), Black Magic Pipes, stock crank (replaced bad bearings, trued, welded at the press joints as my dads 98 came apart at the mag end), 2 degree timing key, stock 9 post primary, roller secondary, and 15Wide 20-39 gears in the chaincase.

First question is: what is the concensus on fairly reliable engine rpm? From my research it looks like the ZRT600 is the pretty much the shortest stroke of the Cat engines which should lend readily to higher rpm without problems. Also from research it seems that the Black Magic pipes and PSI pipes are tuned to ~9300 rpm. I was thinking on dialing the sled in to engage at ~6-6500 rpm and peak ~9300 rpm... Ideas??

Second is in regard to clutching: which is an obvious battle, i was going to break the engine in on stock 99-2000 clutching and pipes. But i thought due to the higher rpm demands of the Black Magic pipes i may switch to a setup similar to the ZR440 Sno Pro's for a baseline... Ideas??

Any help to get some bearings with regards to this would be greatly appreciated as i would hate to burn down a fresh engine becuase i didnt ask the knowledge bank!!

Last edited by 99zrt600mod; 08-24-2011 at 08:59 AM.
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Old 08-25-2011, 04:05 PM   #2
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Why not break it in mod trim? I do it all the time run the bag off them right after there built, my 720 last season help up great besides the wiseco pistons(didnt more heat) but didnt have a problem all season, same goes for all the race sleds I have built. I find the sleds are faster with a hard break in and everything seems to seat right like rings and stuff. So will tell you different dont go pass half gas ride slow for two tanks. Like i dont hold the gas open across the lake just like 5-10 sperts WOT then do brapping to dump fuel in to cool cylinders. But you could take it easy on it. As for clutching what every RPMS the pipes call for i would set up for that, I set up one of my sleds like that last season and it was a dog as the pipes were not run to the proper RPM. 6000-6500rpm is a little high for a launch I would be in the 4800-5200 range but thats me, But personnally i have not owned a Cat 600 triple, couple polaris/yamaha and Cat 8/900s triples. Iam just giving a base off what worked for me for years drag racing as my sleds did both and trail ride only had a couple dedacated to full on racing.
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Old 08-26-2011, 06:43 AM   #3
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Thanks for the input firecat. Yeah, had a slight brain fart on the engagement rpm, meant 5-5500, which would be more realistic for a 600 on a trail. As for the peak rpms, I do plan on having the engine run around 9300 under WOT, just have some concerns over what a reliable max rpm is as the sled may see 9500 here and there, though i guess i can assume that since black magic worked closely with cat for a while, 9300-9500 should be reliable enough.
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Old 10-16-2012, 12:31 PM   #4
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Status update: Unfortunately a run of events have kept this project at bay (waiting on parts, lack of time due to house projects, almost no snow in michigan through the 11-12 season), but it is all together and ready for tuning.
Currently the engine mods are Black Majic pipes (everything i have read states they are tuned for ~9300 rpm, unfortunately the manufacturer is no longer with us ), 3 degree timing key, v-force reeds, light port work, exhaust port/flange matching, trued and welded crank, new Tstat, SPI pistons (see how they hold up), 38mm carbs from a ZRT800.

Engine is a fresh rebuild, only started for a moment. So I will forget the clutching for a moment and ask for some insight into the carbs for a moment, i started with the following setup:
Jet Needle: 6DH8-3
Needle Jet: Q-2
Piston Valve: 3.5
Main: 410
Pilot: 40

Symptoms: Engine will fire choked, but is overly boggy and wont rev up past an idle, and stalls out, backfires on the exhaust side when i attempt to start again. I am planning on swapping in the #35 pilots to see if it helps as the plugs are wet just from the idling. I do understand i am rich, i hoping someone has used 38s on a 600 without UFOs that may be able to lend some info on their setup to give me a headstart and get me somewhere in the ballpark to speed up dialing her in.
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Old 10-16-2012, 01:26 PM   #5
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I run D&D pipes on my 02 along with v force reeds.. I just run the stock carbs, needles and pilot. Needles are in the number 4 position and 340 main jet.

The D&D are 9300 pipes, I run them up to 9250 and engage at 4800. If I remember correctly on the D&D sheet, if running a key with an open air box you run 360's so depending how mild the porting is 380 to 390 should be your main jet.

If your looking for help my guess is to contract straightupperformance near capac. 810.310.6820 is the last number I have for them. He is a long time cat guy that got the tripples to run very good, not sure if they are even still around. If no luck there then just call Jason Price up at Priceless performance in Gaylord. 989.889.0200
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Old 10-17-2012, 11:50 AM   #6
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First chance i get i will button the carbs back up and see how it runs with the #35 pilots. I will see about getting in touch with these sources, hopefully they will be open to throwing out some information, i usually work with Grace, but they do not like to give advice on performance (not that i blame them in this sue happy world).

With respect to rpms, i figure on going for a ~5k engagement, and considering the operating range of the pipes, was thinking on running the engine to a peak of ~9250-9350. Has anyone heard of any issues with these engines and high rpm? My dads ZRT600 is only run to about 8500 and had an issue with the crank loosening up at the press fit on the mag side which wiped out the stator, not sure if this was a fluke, or if cat had issues with the press fits on the 600s.
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Old 10-18-2012, 12:00 PM   #7
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I ahd a 96' 600 with PSI pipes, engaged at 5500 and ran it up to 9300, had a 2 degree key in it and needed to run GOOd Premium or burn down, took the key out and ran it 6500 miles on this set up. I recall 360's in the jetting, but don't recall the clutching. I also sent out my cylinders & heads to have them ceramic coated, also coated the lightly ported ports as well as my the tops and sides of my pstons,. this took the heat away from the motor quickly and it would run great, even lean.
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Old 10-18-2012, 12:38 PM   #8
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 Quote: Originally Posted by 99zrt600mod View Post
Status update:
Symptoms: Engine will fire choked, but is overly boggy and wont rev up past an idle, and stalls out, backfires on the exhaust side when i attempt to start again. I am planning on swapping in the #35 pilots to see if it helps as the plugs are wet just from the idling. I do understand i am rich, i hoping someone has used 38s on a 600 without UFOs that may be able to lend some info on their setup to give me a headstart and get me somewhere in the ballpark to speed up dialing her in.
I would put the 36mm carbs back on it. The 38 are most likely too much air for the 600. Those carbs worked flawlessly on heavily modded sled than yours. Really no need to go to a bigger carb. Just re jet and change the pilots if your not happy.
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Old 10-18-2012, 02:21 PM   #9
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The sled runs markedly better with properly synced carbs as well.
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Old 10-19-2012, 12:12 AM   #10
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My 96 did the backfire and hardly move thing and it was a bad reed on one of the V-force reeds I had in, put in a new reed and synced the carbs and it never fowled another plug and ran great. I just remebered this, getting old I guess. hard to believe I bought that sled new and sold it over 12 years ago.
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