Don't Ask: Dirt Bike Tech and Maintenance Questions

Nov. 05, 2012 By Rick Sieman
If you choose to email a question to this forum, then you must conduct yourself accordingly. Therefore, the following rules are in order:

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.

Have a question for Rick “Super Hunky” Sieman? E-mail questions to editor@off-road.com, Attn: Don’t Ask.

Previous Don’t Ask Columns
October 2012

September 2012

August 2012


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SUBJECT: NEW BIKE RELIABILITY?
Rick,

I have been reading you since the first issue of Dirt Bike Magazine. Even bought your books and the map of Barstow to Vegas, as a friend and I rode parts of it years ago when we made a motorcycle trip out West.

QUESTION: I have read several times where you state that the new four-stroke MX bike engines are expensive time bombs just waiting to go off. But what about the enduro trail bikes like the Yamaha and the the Honda with head and tail lights. I know that they are based on the MX bikes but are the engines that closely related that they will have the same problems?       

I bought a new 1985 Honda XR350 and rode it for 15 years with minimal repairs except for a transmission replacement. If I remember right I replaced the piston and rings and then replaced the piston and rings again with a valve job on the second go around. The transmission split one of the gears and the pieces destroyed the rest. Couldn’t afford the new parts so I found a used  XL350 transmission and put it in. This changed the gear ratios and the output shaft but really didn’t have much effect on the performance. Could you get this kind of life out of one of the newer bikes?

Ken Dobbins

The sad fact of the matter is that the new racing bikes turn almost 14,000 RPM to make their power. This is right near the limit of piston speed. The older bikes you mentioned are nowhere near that kind of RPM that the newer bikes do.  As far as the new bikes with the lights go, they are a bit less stressed than the pure racers but are still highly stressed. For example, a Yamaha WR four-stroke makes almost as much horsepower as a YZ four-stroke. Yes, they are more reliable, but not that much.  I’d  guess and say you could count on about 25% more reliability.  To me, that's not enough.  In my garage right now, I have a 1983 490 Maico and it's my race bike.  I'll change the rings about every two years just because I feel guilty. Could you say the same about a brand-new 450F whatever?  I doubt it and doubt it seriously.
 

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SUBJECT:  BULTACO PURSANG MODS
Hi Rick,

I have a 1975 360 Pursang model 136 and have finally got it running with some work on the engine – repaired coil has brought back the spark, and the engine was checked- cleaned stator, and dry side seal replaced. In addition the piston was checked for correct positioning. So with this work I was able to start the bike and rev it up with success but the idle is floating up and down and does not return  to idle right away after a rev.

It have installed a brand new carb 36 Mikuni to eliminate any carb problems and is still idles poorly – my analysis at this point is leading to a air leak somewhere – it could be the manifold intake or – is it possible that the oil side seal is sucking air – this seal might not have been replaced- however all the signs of this engine show that the engine cases were split and – so I would think they have changed out the seals.

Last question is – if I keep the bike to race I would consider moving the lower rear shock mounts all the way back on the swing arm and install longer shocks – would this provide better power to the ground and handling ? and if so – would the left shock mount bolt nut interfere with the rear sprocket?
Thanks – appreciate your thoughts
Steve  

We turned the answering job of this e-mail over to Matt Cuddy, a man who knows more about the old Pursangs than is healthy. Here are his thoughts.
 
I owned a '76 JPR 360 for about 15 years, was one of the best dirt bikes I ever had. Here's what you might want to check about the air leak/idle:

#1: Check the base gasket, especially around the transfer ports. Use a lot of RTV blue, the gasket is REAL THIN and air leaks result if you don't use a good sealer like Gasgacinch or RTV. Also make sure the head is square, since it doesn't use a head gasket. Also, does your bike have dual ignition? You might want to check the resistance on both top coils, and the mounts (that break real easy).

#2: Check the intake manifold for cracks, and of course the gasket. Rub RTV Black over the rubber part if there are signs of cracks. Why did you get rid of the square Amal? One of the best carbs for a Bultaco for my money.

#3: Check the primary side crank seal, replace it. You can also build a compression tester for the lower end, but that involves splitting the cases and removing the rod, so that's a bit much.

Now about the shock mounts. The hot setup for my 360 was straight up-and-down 13-inch Gas Curnutts. Since you can't get Curnutts anymore, get with Clark Jones at Noleen Racing in Hesperia.  He's got every Ohlins shock known to man; check our website (superhunky.com) for the phone number and address. You don't want to lay the shocks down, since you'll get too much travel, and the chain will come off. Have to be careful with that chromemoly tubing too; get a good welder if you want to modify the shock mounts.

Now the front forks work real good if you replace them with 'Zokes off a '77 VR370 Montesa. Plush forks, along with the shocks mounted upside down, and straight up-and-down from the top mount. I could hit giant square wash outs and giant sandy whoops with no issues.

Good luck!

Matt Cuddy
Somewhere in Burbank
Drunk

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SUBJECT:  WHAT’S WRONG WITH KAWASAKI KX 125s?
I've got an ‘88 kx125 I was wandering if it would be eligible for vintage racing? I've also noticed the kx's arnt very popular is there a reason for this?
Gary Lewis

There's nothing really horribly wrong with the KX 125s, except that the Honda CR, the Yamaha YZ and the RM125 are all far superior bikes.  You put them all together on the starting line and if the riders are of equal skill, the KX will come in dead last. That's the hard fact.
 

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SUBJECT:  AT1 MX FEEDBACK NEEDED
Hi Rick,

You may or may not remeber me. Quite a few years ago I sent you an 8mm film of you riding your 501 on the Inernational track at Indian Dunes. You copied it on a disc and sent me a copy. Anyway, I would like to know if you have any information available on the first year (candy white) 1969 Yamaha AT-1mx. I`m looking for it`s release date, price, technical data on the motor such as carb size, chrome or iron bore cylinder, etc. It would also be kind of cool if you have an actual test/ tests on it. Of course I would be happy to pay for it.
Thanks
Marc Biro



Once again, we turn to the memory banks of Matt Cuddy, who owned an AT1-MX back in the day.  His reflections lend valuable insight to the bike.

I started riding in the dirt around 1968, when I was 10 years old, on a bed pipe Briggs & Stratton wonder, following the lead of my Uncle, Richard. Around this time, my Uncle Rich had been riding dirt bikes for about 10 years, and the bike to have back then was the mighty English 650 twin. Didn’t matter if it was a Triumph, BSA, Norton or Royal Enfield, if you wanted to go fast, you got a big Brit twin, and tricked it out with all sorts of stuff, like lightweight wheels, go-fast goodies such as bigger carbs, cams, lightweight pistons, the works. But you still ended up with a 300 lb. dirt bike that put out an honest 40 plus horses to the rear wheel.

Take my Uncle’s 1962 A-10 Super Rocket. Unfortunately, the A-10 had a big weak spot, especially if you hopped it up beyond its ability to run at racing speeds, without blowing up like a trick cigar. That weak spot was the timing side of the crankshaft that ran in a sintered bronze bushing, with an oil groove in it. The crankshaft was about as big around as your pinky finger on that side, and under the stress of racing, (with Heppolite pistons, racing cams, lightened flywheels and big valves), the crankshaft would usually snap off right at the bronze bushing, with alarming regularity. So after that happened about five times over the course of a year, my Uncle went looking for a new dirt bike.

All the magazines of that era had been testing the latest bunch of lightweight two strokes from Europe, and raving about things like handling, power and more power. So Uncle Rich went out and bought a used 1968 Bultaco El Bandito 360, with an up-pipe, and giant skid plate. Turned out that skid plate was good for an impromptu work bench, since it was big enough to hold all kinds of tools and broken Bultaco parts. The El Bandito was a fast bike that of course depended on a lot of factors, like if it was running or not. And most of the time it wasn’t running.

So Uncle Rich, seeing the writing on the wall with his latest purchase, dumped the Bultaco as quickly as possible, and bought a new, off-the-showroom-floor AT1MX Yamaha 125, white tank and all.

My Uncle’s riding buddies were all taken aback, as it were, when my Uncle unloaded his new dirt bike from the back of the Willys trailer. Next to the big six fifties of his riding buddies, the AT1MX looked like a toy, with its 18-inch wheels, shod with trials tires, and big square number plates that seemed to hide the bike beneath them, with funky little MX bars and controls. And when my Uncle fired up the 125, all his buddies were thrown into fits of laughter, as the little 125 made the classic “ring-a-ding ding ding” noise so despised by the six fifty riders. That was, until they went riding.

Our riding area back then was at El Mirage dry lake, and the big loop for the weekend ride was to ride to Edwards AFB then back through Hi Vista for a pitcher of suds, then up the giant sand wash that ran to the aqueduct, down the fire road next to the aqueduct to Phelan, then back to El Mirage across open desert. All the big double knockers looked down at my Uncle on this tiny white Yamaha and shook their heads, wondering who was going to tow Rich back after it blew up. But that wasn’t the case, hardly.
 
As it turned out, my Uncle and his new AT1MX blew all the six fifties into the weeds, as my Uncle worked like a dog to keep the 125 on the pipe and moving forward through the sand wash. Bill Heywood on his Hindall framed six fifty Triumph seized it solid trying to keep up with my uncle and the little “ring ding.”  When they finally all got back to camp, there was my uncle, sitting on a milk crate, nursing a tall Coors. The AT1 heeled over on its kickstand, surprisingly clean, since nothing leaked from it like the Brit bikes. In the late afternoon of the desert, it looked like some sepia-toned photograph of a man and his machine. That scene is burned into my memory banks like a brand.

My Uncle loaded up my mini bike and his AT1MX as darkness settled across El Mirage, and the time-honored bench racing session started around the campfire, talking and laughing about the day’s ride. No one mentioned the little ring ding, and how it blew past all the big six fifties, but we all knew it to be a fact. The king was dead; long live the king (the Yamaha dirt bike).

My Uncle Rich, not to be content with a stock dirt bike, immediately started modifying the little white AT1 into a real desert weapon. First the frame got stretched and lowered by C&G Engineering in Lawndale; some Curnutts got bolted on the back, along with a No 1. Products Fork kit for the front forks. The 18-inch front wheel got traded for a 21-incher, complete with Nitto knobbies, front and back.

Since a 175 top end bolted right on the AT1’s cases, that got the nod along with a 30mm Mikuni flange mount carb. A GYT KIT chamber along with a Webco air filter, and the bike became a fast and forgiving sled,

After riding the bike for well over a decade, Rich found out about Precision Cycle in Lawndale, California, run by the guru of two-stroke performance, E.C. Birt. EC took the 175 top end and punched it out to 200cc’s, with a cold-forged single-ring piston, and another two boost ports machined into the barrel right next to the intake ports. Reed valves weren’t the hot set up yet, so it was a piston port setup, now sporting a 32 mm Mikuni flange mount carb on a hogged out Webco intake manifold. A Pacer head rounded out the package with a re-shaped squish band. E.C. also welded up a super trick through-the-frame pipe, with a built-in silencer. The bike hauled ass now, even faster than it was before. And was still as reliable as the day is long.

Lucky for me, it was 1973, and my Uncle purchased a Honda Elsinore 250, and gave me the old AT1! Man, that was one nice gift, an AT1MX with about five grand worth of modifications done to it. So now the super-duper 200cc AT1MX was mine. I went to the local bike shop and got a Lucas-style brake light, a dental mirror for the rear view, and a squeeze horn. After checking that everything worked, I boogied over to the local DMV and got my license plate.

See, back in those days, the safety Nazis weren’t into every facet of our daily lives, and the rules for turning an MX bike into a street legal motorcycle were simple: An operational brake light, a horn, and a rear view mirror. Many the street bike was embarrassed as some strange looking long and low dirt bike smoked them off a start (I always used 2nd gear when drag racing another bike).

This was about 1974, and I rode that old AT1 to school, the burger joint, District 37 events, and night MX at Indian Dunes. Play riding, poker runs, road racing street bikes over Angeles Crest (and usually winning). It was one reliable, fast motorcycle, something unheard of a few years before. You can see now why the Japanese motorcycle factories took over so quickly; why wrench when you can ride?

But alas, like so many motorcycles in Los Angeles, right before I went off to the Navy, the AT1 got stolen out of my Mom’s garage and I never saw it again. That hurt, because we had done so many neat things together. After the bike got ripped off, my Uncle felt real bad for me and lent me the money to buy another Yamaha Enduro, this time an RT3 360, another unbreakable dirt bike.

But that’s another story for another time. I still have dreams about the old AT1 and wonder where it is today. Hope it’s still running, stolen or not. I hope.

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SUBJECT:  STREET LEGAL MAICOS
There have been all sorts of rumors about various street legal Maicos throughout the years.  We received this information and photos from Mr. Russell and would like to share them with you.

In various countries, the laws mandated that road-registered machines have lights/horn/blinkers/stop lights/mirrors in various permutations. In some countries, this led to unusually large rear taillight assemblies and round red reflex safety reflectors on some models, as Suzuki ensured their compliance with local design rules. All PE's came fitted with effective mufflers, with restrictors and double-walled expansion chambers (with mesh) to meet noise and spark restrictor laws where applicable.

The USA machines usually came standard with a trip-meter and minimal AC lighting, with a speedometer as an option, whereas PE's in Australia and Belgium (for example) had speedometers, stop lights, horn, 6V DC battery and charging system to suit, by law.

The UK machines did not have indicators or battery; however these did come with an AC horn. The lighting requirements varied across years and countries, so a PE in the UK, South Africa or France might have substantially different rear taillights and mounting assemblies to those in the USA. The French PE's (for example) continued to use the PE250B style headlamp shell, with a parking light and a yellow globe insert, due to local regulations.

Also some countries (UK for example) required an alloy fuel tank rather than plastic, which is almost identical to the PE250B tank, but with altered front mounting-strap tabs. The capacity of the plastic fuel tanks varied from 12 litres to 10.6 litres on various models, without any obvious visual differences, possibly due to mould thickness. Suzuki usually supplied the PE with one of three different brands of tire, depending on local requirements. The standard tire was the IRC, however some years and countries saw other brands such as Bridgestone or Dunlop fitted (run at 5-11PSI).
 

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SUBJECT:  360 YAMAHA ENDURO WON’T FIRE UP
I have a 1970 yamaha 360 enduro.and I cant seem to get it to fire up
Donny

Remember, your 360 needs only three things to run: spark, compression and gas. Since I don't know what condition your 360 is in, let's first check the points; they're behind the flywheel on the right side of the bike, a good gap is to use a business card. Then check the carb's main and pilot jets for being clogged; if so, clean out the whole carb. Then check to see if you have compression, and check the crankshaft seals for being there. The early 360's had a bad habit of blowing the crank seals right out of their groove, and then you have no crankcase compression for the two-stroke cycle to work. Check those things and your 360 (one of the strongest bikes Yamaha ever built) should run fine. Watch out for the vicious kickback a 360 puts out; move the kick start lever counter clockwise two notches, that helps. Good luck.
 

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SUBJECT:  750 HONDA DIRT BIKE?
Ken, in the Sept. "Don't Ask!" said that when he removed the head from his Honda 750, he found "...the big end bearing" was the problem. Would that be the con rod to crank bearing? if so, man, talk about grenading an engine!

I didn't even know the '77 Honda CB750 4K (K4?) WAS a dirt bike!
David Fruhling

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SUBJECT:  YAMAHA JETTING CONFUSION
hi i have a 99 yz 125 iv been having carb jetting problems the bike has a fmf pipe v force 2 reeds and stock bore but new top end the main jet was to lean so i keept richen ing it up i ended up having to drill the jet with a 1/4 inch drill bit to get a lite brown on the eltrode with a chop test at wot my piolt circuit is working fine i got that jetted in now my mid rand has good power im confused tho on why my main jet has to be so big ive don a air leak check on the carb boot air box and reed side ive done crank seal checks any ideas im stumbled i dont no if letting that much fuel go through it will hurt anything the spark plu test is a lite brownish tan color so id like to go richer but im already at a 1/4 in and its a 125cc ???
Jody

Your e-mail is the perfect example of how not to communicate with people. Do you realize that the only punctuation here in your scribblings are three question marks?  If you were only about eight years old, I would be much more forgiving of your writings.  But since you are big enough to sling your leg over a YZ 125, I can only assume that you are at least a teenager. And if you are a teenager and still write this way, I can only guess that most the time as was spent picking your nose rather than paying attention to what the teacher was trying to get across.  If you would like to get someone to write your dribble into something a person can understand, I’ll attempt to answer it. If not, please go away and don't bother me anymore.

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SUBJECT:  NAME THAT YAMAHA
Re: Yamaha Vintage ID
hi i'm lookn 4 info on i bike i bought. i'm a 1st tymer. 1M2-004834. can some1 help me please n ty .
Randy

The closest numbers I can find are for the 1977 DT400D that starts with IM2-000101 and the 1978 DT400E that starts with 1M2-100101. By the way, your e-mail was very close to the ramblings of a person above. I suggest that you read it and my answer, taking all this into consideration. This is your only warning.

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SUBJECT:  JETTING ON AN ISLAND NEAR AUSTRALIA
Hello, im trying to figure out wot the right jet setting is to stick in a 94 wr250z 2-stroke after its had v-force installation an fmf exhaust? i live on norfolk island which is near australia an part of the south pacific, in sub tropic area...so the setting i was thinking of trying would be a 370 or 360 main an 45 piolet due to not being able to get ne higher size piolet jet for a mikuni carb...thanx
Jess

I can only assume that you're very close to sea level. Even knowing that, the modifications that you have made to your bike prevent me from doing much more than guessing at your jetting. My best estimate would be to go up one or two on the main jet and possibly one size up on the pilot jet. Of course, the best way to do all this is to install the jets, take a full throttle run, chop the throttle and look at the color of the spark plug tip.  If it's chocolate brown, you're in the ballpark.

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SUBJECT:  390 HUSKY WON’T RUN “WRITE”
i have a 1979 husky 390 that im having a problem getting it to run write it only run like quarter throttle i did everything to cleaning the carb the reeds and still runs like quarter and like s**t now could some one help me out here
Kyle

I can only assume that your Husky is mechanically sound.  If the top end is all worn-out, and it will run much like you described.  Your problem could be something as simple as the throttle cable not pulling the slide all the way open to an air leak. You're not giving me much to go on.
 

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SHAMELESS PLUG

SUZUKI GRAND PRIX FROM 1970 THRU 1973
This DVD covers the glory years of Suzuki, when Joel Robert and Roger DeCoster ruled the motocross world. It covers lots of events in Europe and gives you some inside looks at how dedicated the entire team was, from the racers themselves, to the mechanics. Those early RH and TM racing bikes were worlds ahead of everyone else back then and the result was a whole slew of world championships. Excellent racing footage throughout. The DVD runs over two hours. Cost is a mere $10, which includes postage in the US.

YAMAHA GRAND PRIX FROM 1979 THRU 1983
These were the years when Heikki Mikkola was the top racer on the factory Yamaha open class machines. Yamaha fielded a huge number of motocrossers in Europe and the USA.  Even though Mikkola is the star of this DVD, Danny LaPorte is covered nicely. The emphasis is on racing in Europe and there’s even a good segment on sidecar racing.  The DVD is 1 ½ hours long and well worth having.  Cost is $10, which includes postage in the US.

For ordering info, go to:  www.superhunky.com and click on STORE.

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