VC 625 rpm range

[oldphart]

What's a safe max rpm for a bone-stock VC 625? I just sat the (hideously ugly) rear body and the base of the rear seat on it and blasted it down the road a quarter-mile or so and back. Nail it and it IMMEDIATELY runs 4K+.... Goes like stink, BTW. Now I gotta get to work on paint.

Thanks for any input!

Jim

[raydhd]

Did you get your carb issue worked out ?

[sho305]

Most all lawn based engines are 3600 and you can usually run them 4K, though I don't know about non stop 4K. Most of them float the valves at 4400-4600 and wont rev more. That can cause damage after a while if you run them floating. With some valvetrain parts you can usually hit 5K no problem, but I would not recommend keeping it there a long time, it depends on the engine a vanguard for example would be fine, a clone is fine long as you don't do it constantly and its been broken in. But the clone might puke the rod at 5K if run there long enough, short bursts I ran mine to 5100 but that was not normal use for me only once in a while for short distance. IIRC the 625 does have pressure lube which is good, but its also larger that is more weight moving.

So generally on trails where you nail it down a straight and then slow for the next corner you can rev higher. If you run roads at constant high speeds you can't run it as high you should get HS gears and/or larger diameter tires.

[oldphart]

I haven't done any more with the carb yet....it's still back to the stock setup. I'm gonna' get the body & paint done so it's actually bolted together and then get serious about tuning it when it's driveable. I'm kinda' at the mercy of the weather....COPD has negated me making dust in the shop, so I gotta' sand outdoors. Booth has a fresh-air system and fans, so I can spray smaller stuff in there (even though I'm not supposed to...).

I wished the clutching was like the Polaris stuff I used racing....I'd guess it needs a little more weight in the primary, and a more aggressive/higher angle helix in a perfect world, but we'll see.

Thanks!
Jim

[slonomo]

What did the paperwork from VC say? They usually include paperwork on how to break the engine in, max rpm etc.

[oldphart]

Originally Posted by slonomo

What did the paperwork from VC say? They usually include paperwork on how to break the engine in, max rpm etc.

Nope.....looked through it all again yesterday. There's nothing in there on max. rpm. I wished I knew what was in the primary in terms of weights as VC ships these kits out....because I think it needs a little more.

I miss my Polaris clutches in racing sleds....

Jim

[slonomo]

Originally Posted by oldphart

Nope.....looked through it all again yesterday. There's nothing in there on max. rpm. I wished I knew what was in the primary in terms of weights as VC ships these kits out....because I think it needs a little more.

I miss my Polaris clutches in racing sleds....

Jim

Might send Aaron or Geoff an email, see if they can send you the paperwork.

[superpro56]

Here is a nice chart for the Comet 780 primary:

http://forums.bajasae.net/forum/uplo...SAECatalog.pdf

I ended up swapping my hard parts for a 1700 rpm engagement in the 780. I have been happy with the results thus far.

As far as RPM is concerned, the stock 625 regulator limits max rpm to 4000 rpm. If running downhill you can get a little over that, but only 500 or so rpm. Supposedly, billet rods are designed and awaiting full production. That should safeguard from over reving these engines. In order to get a usable powerband above 4k rpm, we will need a revised valve train.

[slonomo]

Originally Posted by oldphart

Nope.....looked through it all again yesterday. There's nothing in there on max. rpm. I wished I knew what was in the primary in terms of weights as VC ships these kits out....because I think it needs a little more.

Needs a little more? Please explain?

[oldphart]

Originally Posted by slonomo

Needs a little more? Please explain?

The way I always clutched a drag sled, the primary is simply a governor. If engagement is where you want it, and max rpm is where you want it, leave it alone. I think this is revving higher than it should....but I'm not sure. More weight = lower rpm.

I also may be injecting a lot more science than necessary. 23 hp & 4000 rpm is a lot different than 140+ and 11K rpm.....

Jim