'98 Mxz670 Slower After 'proven' Mods?

[sureynot]

Hello all,
Last season my 670 ran great, but after finding this forum I convinced myself I need more from it. So I cut my rotary valve to a 501, timed at 146/76, .8 mm base gasket, head milled (squish is about .065-.067), y-pipe mod, shorty boots... basically everything from the dneperformance website.
The problem: Feels slower, and it won't reach the proper operating rpm. Last year on the tach and clicker 1 it was around 7800-7900, now I'm at clicker six and maybe 7600-7700 tops. I know with the above mods I should be around 7900-8000. I've replaced the secondary buttons and spring tension is at 20-22 lbs. Compression is equal @ 135 psi. Clutching is otherwise stock. Also adjusting the clickers up or down hardly seem to have any effect. Carbs are clean and new plugs. Any suggestions?

[idooski]

Check very carefully for air leaks. Check the plugs to be sure you are not running lean. Could be the rotary valve is cut wrong or installed wrong.

Check for lean condition first before you do anything else, including riding it.

[dave583]

Yeah, recheck that rotary valve.

[sureynot]

My jetting/plugs look good; I'm using the stock 310 pto/290 mag. I'm ruling out air leaks since it doesn't look overly lean, plus I've put on 40 or so miles and haven't burned down. Rechecked the rv and it's timed right on, another reason I cut my own so I could get the angles precise.
My thoughts are turning towards clutching. Is it possible that a worn pri spring could cause it not to reach peak rpm, and maybe why the clickers don't seem to change anything??

[idooski]

It could be clutches, but that usually takes place over time. How much did you mill off the head? Any other changes to the head? Stock gasket was .6 on that machine?

[TT670]

Its probably the clutching, Ive done my fair share of 670's and I never liked the Y-pipe mod, especially with the raised port timing. Cutting the y pipe does make more HP but it also narrows the powerband and makes the motor peakier and more finicky. Combine that with raising the ports which also shifts the powerband upwards and you end up with a sled thats harder to clutch and its much more likely to be on eof those sleds that runs good one day and like crap the next if the weather or conditions change. Once youre done you'll be a much better sled tuner, thats for sure. Id clutch it to run about 8200 on your tach, that'll get you 8000-8050 which is where you should find the sweet spot. What are you running for clutching? If its stock then It'll run tons better by just running a 200/320 primary spring and 280 ramps, run the stock secondary with the spring in b-6.

[sureynot]

TT670 thanks for the clutching advise, I'll go ahead and change the spring and ramps and see what happens. It seems like if I can just get the r's up it'll rip. Right now it pulls really good up to 7500 but after that feels like it hits a wall.

As far as what was done to the head, I just had the machine shop take off .013", nothing else. The stock base gasket I believe was .5mm and stock squish was around .070" before the head cut.

Now that it seems I have a more narrow-tuned machine, would something like a Holtzman tempaflow help when conditions change? I've been thinking about getting one for a while and with my mods Iv'e done now would that help the drivability??

[oldyeller]

I had all these little tricks done to my 97 670 last year.I wasn't overly impressed with it to be honest with you.I didn't have any rpm trouble like your having and the sled wasn't finiky or anything,it just didn't seem to have any extra jump.To be fair I didn't do any clutching mods(DNE said it would work ok without them) and I also never really had ideal conditions to check it out (lakes had too much snow and then got rough when they did pack down).I'm looking forward to checking it out again this year but I have to say I was disappointed overall.The last thing I did was get the shortie boots and now the airbox sits right up tight against the hood.I'm afraid if I ride it hard over the bumps it might rip the boots on the intake side or one of the carbs will fall off.If I had to do it over again,I wouldn't

[mxz670]

[quote]As far as what was done to the head, I just had the machine shop take off .013", nothing else. The stock base gasket I believe was .5mm and stock squish was around .070" before the head cut.

If you went from a .5mm gasket to an .8mm you would need to take .013 off just to stay were you were. The target number I have seen for squish for this machine is .060- .063 which means you should have had them taken (.013+. 008=. 021) .020-.021 when you had it machined.

I have machined my head twice and now have it at .060-.063 my starting number I believe was .076 my original gasket was .6 now a .8mm I calculated .008 for the gasket and used .014 to get a theoretical .062. (.076-.062=. 014) The first time I ground the head I took .022 off (.008+. 014=. 022) and than found I need to take off an additional .005 the following year. My compression is 147-152 approx.

I have done all of the mods you have done plus I installed a pair of UFO's took the airbox out and run kickass filters Instead of the 501 rotary valve I bought a carbon fiber with the same duration. I bought a used set off 44mm carbs. I install a Holtzman atacc at the end of the year have not really fully tested but I like the Idea. I also decided to purchase the porting templates from Race Logic and ported the cylinders. This was a very time consuming but and enjoyable experience. I believe the biggest improvement was with the cylinder porting. I also have a stock 02mxzx800 the 97mxz670 now seems a bitt quicker.

Good Luck

[sureynot]

I cleaned the primary, changed the spring to a 240-320, installed 280 ramps and WOW what a huge difference!!! I'm at the proper rpm now, and man does this thing accelerate hard compared to before. Oldyeller I highly suggest these clutch changes you will be much happier, I sure am! Thanks to all; this site is great.