Don’t Ask: Off-Road Dirt Bike Technical Q&A

1. Do not write your email to me IN CAPS. If you do so, I will print out your question and do terrible things to it.

2. Do not request a personal e-mail response. Since I get thousands of questions each month, trying to answer them all would cut deeply into my leisure time, which I value more than your current state of confusion.

3. Try to spell at least in a semi-correct fashion. If you choose to mangle the English language, expect no mercy from this quarter. You might be mocked severely.

4. Do not ask for me to send you copies of my many manuals and literature. I am not in the library business, nor do I want to spend the bulk of my day at the copy machine just because you're too lazy to ask your dealer,  or look around a bit.

5. Don't bother me with truly stupid questions, like how to get 50 more horsepower for a buck and a half

6. Now that you know the rules, think carefully and have at it!
 
Oh yes … I’ll leave your e-mail unedited, for what it’s worth.

Other Don’t Ask Columns:

December 2011

November 2011

October 2011
                                               
                                                         
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SUBJECT: 78 MAICO 440
Dear Mr. Hunky,
I semi restored a 78 440 Maico to race ahrma and I put a Mikuni on it. I can't get the jetting right to save my life. Please give some sort of idea including correct needle, jets , etc. I live in Southern Ohio. Thanks for any help you can give me.
               
Jeff Anthony
 

 
For a 38 mm Mikuni, give this a try:
Pilot 50
Slide 2.5
Needle 6FJ6
Needle Jet Q8
Main Jet 350
 
 
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SUBJECT: 1980 YAMAHA JETTING QUESTION
hi guy's
i have restored a 1980 yam dt-175 enduro. it's basically stock with the exception of a 2nd over wiseco piston kit. i'm running 92 octane supreme pump gas mixed with amsoil interceptor 2 stroke oil at 32:1 and live about 50 feet above sea level. elevation would range between 50 - 300 ft above sea level depending where you are riding. the carb is the stock mikuni with the non adjustable mixture screw and fixed needle, i think a vm 24ss. while it runs decent and makes good power, it's nowhere near as smooth and chrisp as it should be. the plug is clean and dry but indicates slightly lean jetting. is there any way to adjust this carb, or should it be replaced? what jetting would you recomend?

thanks,
mike mckinley

Judging by your description of the plug, it seems the jetting is just a little off. First, try fiddling with the slide needle, and raise it one notch. Then go up one size on the pilot jet, followed by the main. You didn't say what jet sizes you are running, so I couldn't advise you on the correct jet. To be safe buy about five pilot and main jets in increasing sizes. Also, get rid of the non-adjusting slide needed and raise the slide needle one notch. A motorcycle salvage shop should have a good carb from which you can cannibalize parts.

You can also re-adjust the idle speed air correction jet (that's covered with a zinc slug you can pry out with a scratch awl) that affects the mixture up to about 1/4 throttle. After that the pilot jet then the main jet takes over (the main jet is only used at the last 1/4 of throttle at high engine speed).
Good luck and let us know how it turns out.

Regards,
Matt Cuddy
 
 
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SUBJECT:  WHICH DT1 IS IT?
Hi Rick,
 
Congratulations on new book, and it’s a coincidence, but I located a 1969 DT1S MX.  I am being told it is extremely rare. I am told only 1200 produced.
I have to go pick it up next week. Do you have any history on this model?
Rob Phillips
 

 
Your 1969 DT1 MX I take it has a white or copper-colored gas tank, right? Anyway, the DT1MX was a stripped GYT kitted (Genuine Yamaha Tuning) 250 Enduro. The GYT kit comprised of:
 
A single ring racing piston
A high-compression cylinder head with a center plug hole
A special ported cylinder with cleaned and matched ports done by hand at the Yamaha factory.
A 32mm Mikuni flange carb & special intake manifold & air cleaner boot. An expansion chamber with a six-inch-long stinger (no silencer)
The 250MX, with suspension mods was a fast, reliable dirt bike that embarrassed many trick, European-mounted riders back in the day. If you stretched and lowered the frame it became a real handler, and in the right hands nothing could touch it cross-country (desert races, hare and hounds, poker runs, etc).
 
Good luck with yours. Speed & Sport in Philadelphia is the place for parts btw.

Cheers,
Matt Cuddy
 
Rick and Matt,

Thanks for your help.
I finally got the DT1 and turns out the owner left out the “F” in frame number.
It is a 1971 DT1MX  after all. Someone stuck a tail light mirror on it making it confusing. We do have an enigma.  The engine number although has number matching frame the “F” is missing.

Rob Phillips

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SUBJECT:  MIKUNI ON A SNOWMOBILE
Rick,
I have a Mikuni carb on a Rotax 462 snowmobile engine. I took it apart to clean as it had been sitting for several years. Prior to the cleaning, the engine would start and run with the choke on. Now I get a little firing, but it does not run. I do get fuel dripping out the exhaust. Any ideas as what to do?
Thanks,
Ian Garber
 

 
Chances are that you have the leaking float needle in the carburetor. This is a small piece that wedges itself closed when the gas level and the global gets properly high.  If that is worn, you're going to get fuel dripping from the carb and into the engine itself.
 
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SUBJECT: BEST AIR-COOLED TWO-STROKE MOTORS OF ALL TIME?

Rick,

I hope all is well. Its been 12 years since I last posted....the PE400 and TS400 have been traded and swapped. Many things have changed, but my interest in building rat bikes has not.

Reading your 2T into late model chassis series has revived that old feeling.  Thanks for not retiring yet.

Ive got several late model chassis donors...and Id like to know your TOP 10 best air cooled 2T powerplants so I can begin my hunt for engines.

Riding would be in east coast woods, quad width trails if that matters.  Maybe twice a year.

With your response I will begin diving into yellow-jacket poison ivy infested boneyards to begin the journey...
Many thanks.

PS

What is your favorite t-shirt to wear when beer drinking in public?

Did you MIG or TIG weld your pipe together for the 2T into late model 4T chassis?
Yours Truly,

Travis Murphy
 
 
My favorite shirt is a Maico T-shirt in warm weather and a Maico sweatshirt in cold weather. Regarding welding, I've been MIG welding for years.

Your question posed some very interesting thoughts. I talked with Matt Cuddy about this, and he had some ideas of his own on what would make a great air-cooled two-stroke powerplant. We decided to break this down into big motors, midsize motors and even small motors, just to give you a choice.

1. 1983 Maico 490. No doubt about it, the number one motor of all time is the 1983 490 Maico. I have one of those beasts in my garage and it is equipped with a large Mikuni carburetor. This way, it pulls harder down low than the stock Bing carbureted bike and still has all of the midrange and top end that you expect in this motor. It pulls from incredibly low revs, through a staggering midrange and a top end that will curl your socks up. Until you have ridden one of these bikes, don't even talk to me about how fast the new 450 strokes are. I have gone out and drag-raced new Kawasaki 450s and pulled them badly.

2. 1981-82 495 KTM. A close runner up to that ‘83 Maico would be the 495 KTM. It was another brute that had unbelievable low end power and a ferocious mid range. It did not pull as high on the top and as the Maico, but was as fast almost everywhere else. 

 3. 1981 490 Maico.  While not quite as fast as the 83 motor, the 81 490 is still a favorite for vintage motocross racers. It has great low end power, plenty of midrange and revs out nicely.  Most racers prefer the 81 over the 83 just because it's easier to ride and not anywhere near as terrifying as the 83. You can't really go wrong with this motor … if you can find one, as they're hard to come by.

4. 1980 Yamaha YZ 250.  For folks who love mid-size engines, this particular YZ motor is a real eye opener. While it didn't have a tremendous amount of low end power, it had a phenomenal midrange spread of power for a 250. It didn't have all that much on top, but the midrange was wide and if you used the six speed gearbox properly with the power, it was just about unbeatable in its class at that time. Even today, you have to classify it as a fantastic motor.

 
5.  The 1981 YZ 465 was a real brute.  It revved rather quickly and you had to pay attention to the burst of power or you could find yourself in real trouble, but it did have great power throughout the range.  Many of the open class works bikes patterned their power band after this particular motor. With just a few simple mods, there was even more power available and it would rev to the moon.

6.  1973 CZ 400.  The so called radial CZ was a virtually bullet proof motor. We heard about racers who went three or more years on the same set of rings.  While the chassis may have been a bit dated compared to the other bikes available at that time, you could not beat the radial 400 for reliability.  It had good power throughout the range, but not overwhelming power.  Riders who put big Mikuni carbs on the bike and more modern ignition systems were rewarded with even more muscle from that power plant.

7.  Most any Yamaha 250 DT1.  After much talking, Matt Cuddy and I agreed that the Yamaha DT1 250 motor deserved a slot in this lineup.  While it was not the fastest motor available at the time, it lent itself to all manner of cheap hop-ups and still retained a phenomenal amount of reliability. All of this at a low cost.  Truly a remarkable all-around motor.
 

 
8.  Just about any 100 cc Hodaka/Fuji motor.  If you're looking for small motors that fit your requirements, you have to think about the Fuji motors that went into the late 60s and early 70s Hodakas.  These inexpensive little powerplants were simple, reliable and very easy to hop up.  
 

 
9.     1983 Honda CR 480.  The last of the air cooled big bore Honda motocross bikes was the 480.  This bike was very fast, although it was a little bit hard to ride, as the power came on very abruptly.  It was a fairly reliable engine and it too responded to modifications that turned it into a genuine brute.
 

10. 1975 Bultaco 360 Pursang. Matt Cuddy absolutely demanded that this motor be included in the list. While I never spent a lot of time on that particular bike, Matt raced it for years and never had a spot of trouble with it.  He swears by that particular motor, so here it sits in that list of all-time great powerplants.
 

 
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SUBJECT: WHICH 450?

Hey Rick,

Been a big fan of yours and Dirt Bike for years. I cherish my, autographed by you, hard back copy of Monkey Butt I got from you back in the 90's at the Adelanto Grand Prix. Also have the B to V poster I got from you hanging on my garage wall.

Earlier this year my 01 XR650R bit me good; all kinds of broken bones, collapsed lung, etc resulting with a hospital stay.. Having had lots of time to ponder my riding addiction, I'm thinking its time to get something a little lighter, more nimble and something with that special little button on the bars. Don't get me wrong, I love my BRP and am not getting rid of it; it's plated and loaded with all kinds of goodies, from XRs Only, Baja Designs, Scotts and ESP. That said, I got the itch.

On to my question: I know nothing of the new 450's and wanted to get your thoughts. If you were in the market for a new 450 which would you choose? I am a fairly experienced rider on the wrong side of 40, 6'3 at 215 lbs. No interest in motocross, just wanna chase my buddies across the Mojave Desert and trail ride with my kids.

Thanks in advance for your response and keep up the good work!

Pat
Saugus, Ca

I'm not a fan of the new four-strokes because they're nothing more than hand grenades with a short fuse. However, I found that the KTMs last longer than the average Japanese bike.
 
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SUBJECT:  MAICO 501 PORTING
Hi Rick,
How's life treating you down there in sunny Arizona? Still racing that 1983 Maico 490? Kind of wish we still lived down there in Phoenix, instead of up here in Seattle. Just had a quick question for ya though. Can you provide accurate porting diagrams, timing specs, and compression specs, to make a 1973 501 run like you describe in your recent article? Or maybe recomend a shop that could accurately do the job? You kind of created a bit of , how should I put it,  " bench racing discussion", with that one! But me being the stubborn type I am, want to give this a try if possible. Will do a before and after dyno run, on a modern Mustang eddie current dyno.

Thanks,
Joe Nelson
 

 
I regret to say that I don't have those specs anymore and the years have made the memory a bit fuzzy on what was done. I do recall, however, that the magnesium type 55 Bing carburetor made huge gains in horsepower. I also seem to recall that we bumped the compression up a bit, took about 10mm out of the head pipe, added three quarters of an inch to the belly and made the stinger a bit longer. Sorry I can't be more specific.
 
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SUBJECT: CR 125 AND 250 M ELSINORE
Hi i have both bikes from 1973/4 they have a GEM inlet is this from the
period? if so can you send me some info as  have a scruiteneer here that
needs proof before i can ride
hpe u can help
mark
 

 
No problem at all. In the November 1974 Dirt Bike magazine on pages 21 and 23, there are ads for GEM inlets for Elsinore's listed. That should take care of any questions you have.
 
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SHAMELESS PLUG
My new book, THE LAST RIDE, is at now out. It's fiction and starts in 1969, when an 18-year-old kid just out of high school gets a chance to ride his Yamaha 250 DT1 from Pennsylvania to Los Angeles … all off-road.  His adventures are truly amazing.

The book then jumps 40+ years where the same person, now in his 60s, wants to get that old Yamaha back in his possession and return it home by riding it all off-road across the country again. 

The book is $15 plus $2.75 for mail anywhere in the US (or $5 for Priority Rush mail) and for more information, the email is:  superhunky@gmail.com.