Interesting 399 Hirth engine in classifieds

[sr88]

It is out of a Ski Daddler XX1400.  Model is 399BP.  It has twin Bendix float carbs with reed valves.  Spark plugs mounted in heads on an angle.  Didn't know Hirth made a reed valve engine back then.  What do you know about these engines?  Are they a special build and different from a stock 399 Hirth engine?  Thanks

[Vintagesleds.com]

[markxx800]

399Bp's are about 34HP(1970) or 37Hp(1971). AMF designed,built by Hirth. They are the only Reed Valve Hirth motors as far as I know. Chrome cylinders with twin carbs. Hard to find original OEM pistons and rings. They also built 439BP's
Cool motors-only used in AMF Ski Daddler XX Racers.

Mark

[sr88]

Thanks for your reply.  AMF must have had some inovative people back then.  Very few, if any reed valve engines in the snowmobile industry in '70-'71.  How did the 399 and 439 XX racers run back then compared to the Polaris, A. Cat, and Ski Doo's? I have  seen some XX's (possibly yours) on this wesite, they are pretty cool.

[markxx800]

Great Idea,poor design. According to Lynn Paxton,AMF's Unofficial head of the Factory team "Those BP motors didn't run worth a Da*". Probably not that bad of course. But they are a very cool and unique motor and are sought after by AMF collectors.

Mark

[sr88]

Do you know of a website where I can learn more about the XX sleds.  If I remember correctly, when AMF quit building Ski-Daddlers you could buy an XX for about $750.  Thanks, Jess

[markxx800]

Your right, I begged my Dad to go to the distributor in Syracuse NY. They had all models of Ski Daddlers for 50 cents on a dollar. I'd  love to have had a new 72 XX340. Ha Ha-Didn't happen
Shoot me a email and I'll give you a few websites to look at. For questions just post here or call me at 607-382-3485 up 10pm eastern. I love to talk sleds!

Mark
elwell9801@acninc.net

[ACCA]

I'll take one 1650 please 

[Sledded]

Speaking of reed valves, OMC had them all along on their opposed engines. You probably know of the 650 XX in the crate if you're a Ski-Daddler guy. It has a red baron in it. Suspension looks like an Arctic Cat suspension.

[markxx800]

No, I don't know about that sled,but I know a lot of New in the Box Ski Daddlers have been found and I bet there are more out there.

Mark

[Sledded]

There's a guy at Waconia that drives around every year with his Ski-Daddler that looks similar to the one in the brochure. I thought it had a 793 in it. I love those 793's. What did they put out for HP? Maybe 55?

[stingerboy]

Was there any bp's that didn't use a reed valve intake? We have a 439bp motor but it does not have the reed valve intake and is a single carb? Just wondering if that is a different year or something?  Stinger

[markxx800]

Sledded, the 793 was rated at 80HP. 800 Ski Daddlers came with Megs out of the box. Here's mine,1 of only 2 that I know of that had the Factory Optional Expansion Chamber kit.
Stingerboy,yes all the BP's had reed Valves. Lynn Paxton told me they would switch out the BP topends for 493 Hirth topends. He claimed they ran better that way. Add that and it was hard to get piston kits for them even back in the day and I'm sure a lot of BP's don't have their correct topends anymore.

Mark

[sr88]

Very nice!  Does yours have the aluminum tunnel and an 18" track?  Can you post any under-hood pictures?

Sledded, the 793 was rated at 80HP. 800 Ski Daddlers came with Megs out of the box. Here's mine,1 of only 2 that I know of that had the Factory Optional Expansion Chamber kit.
Stingerboy,yes all the BP's had reed Valves. Lynn Paxton told me they would switch out the BP topends for 493 Hirth topends. He claimed they ran better that way. Add that and it was hard to get piston kits for them even back in the day and I'm sure a lot of BP's don't have their correct topends anymore.

Mark

Having the benefit of 20/20 hindsight since the early seventies.  The plus side of a reed valve is it allows the intake ports to stay open longer yet still avoid spitback through the intake port which should provide a larger intake charge.  The minus side is  a reed valve is more restrictive than an open port (more so in '70-'71, the reeds were probably not very flexible). I don't think any engine performance will be gained if the intake opening duration is not lengthened when a reed valve is installed.  Just my thoughts of why the BP motors did not run very well with the reed valves.

[markxx800]

17 inch panther track and skid frame-steel chassis. the only XX's with aluminum tunnels are the 72 XX300/340.

Mark

[Sledded]

Very Nice, SR88. 80 HP. I wouldn't have guessed they put out that much. But they sound like they have 200 HP. Hope to see a few this weekend at Waconia.