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DR350 & DRZ400 FAQ - Engine

Q:

How difficult to convert my DR350 from kick start only to electric also?

How difficult is it to convert a DR350 that is kick-start only to electric start?

A:

You would have to replace the bottom-end cases, the cylinder and then purchase the automatic compression release that the electric starter needs. You would also need the wiring harness, rectifier/regulator and the CDI box. In the long run, it would be much simpler and cheaper to trade up to a bike with an electric starter.

Q:

Working on DR480 big bore kit?

I have seen on some forums that you are working on a BIG bore kit for the DR350, a 480 I believe. Any news on that project?

A:

I am just getting the motor back together for the 2nd time. I blew the first version up on the dyno! That was after seeing 40 HP though! It was definitely detonation that caused the internal explosion though and that is correctable. It toasted the piston but didn't hurt anything else. It took several runs to get the jetting in the ballpark and I am pretty sure that those early poorly jetted runs took their toll on the piston I hate to say. To help avoid it the next time, I changed the shape of the top of the new piston to give it a little more squish area - that should help. The thing that is most amazing to me is this motor was breathing through the stock TM33 carb, the ports had not been touched and the valves, seats and guides were all worn out! With a bigger carburetor and fresh motor this thing should really run!
DR480 hp chartDR480 torque chart

Except for a White Bros. cam and aftermarket valve springs & retainers, most everything else is pretty much stock. I bought the bike to use as a test mule and when I pulled the motor apart the first time I was pleasantly surprised to find the cam, springs and retainers! The owner said nothing about that. Other than that the ports and carb are stock. The airbox is wide open and it has a Pro Circuit exhaust system. The chassis is completely stock but improvements there will come later - after the 480 kit development is done.

Q:

How does the auto-decompression on the DR350 work?

As I understand it, when you are trying to start an automatic decompression release equipped DR350 (using the kickstarter or the electric one) the cam slightly opens one intake and one exhaust valve, is that true?

A:

No, the compression release slightly opens both exhaust valves. The intake valves aren't affected at all.

Q:

What is the proper break-in procedure for a new top end?

Should I follow the stock, recommended break in for a new top-end or is there a better way?

A:

Yes there is a better way! Go to mototuneusa.com and follow his break-in method. I started doing this a few years ago and have never had a problem. It just makes sense. The factory does us all a disservice recommending a slow break-in. I think the real reason for this comes from their legal department. They want you to ride your new bike cautiously until you get familiar with it. The break-in period is for the new owners brain - not the mechanical parts in the motor.

I was recently watching a documentary showing sportbikes coming off of a Kawasaki assembly line. Every bike was gently warmed up then went right onto a dyno and did a 'full throttle through the gears' dyno run. Then went off to have the oil drained and get crated up. They wouldn't be doing that if there was some harm to be done to the engine without a slow break-in.

Q:

Are electric start and kickstart cylinders the same?

I'm looking at getting a used big bore kit for my 1998 DR350. The kit includes the cylinder. Are all the year models interchangeable? What about the electric vs. kick vs. street legal models, are their cylinders and pistons interchangeable as well?

A:

The right hand end of the electric starter motor bolts directly to the cylinder. If your bike has e-start then you need an e-start cylinder. If your bike is kickstart only either cylinder will work.

Q:

Do I need a big bore head gasket?

I'm rebuilding my 1997 Suzuki DR350 with a big bore kit. When I installed everything and attempted to find Top Dead Center (TDC), it would not go. It froze up just before getting to TDC because the piston is hitting the head gasket. How do I solve this?

A:

You need a big bore head gasket. We recommend Cometic gaskets. Be sure you tell them it is big-bore and the size of the bore! Contact us if you need further assistance.

Q:

Big bore questions!

I am interested in a big bore kit for my DR350. What modifications are needed for the 385cc kit? Do I need to take my cylinder jug to a machine shop and have them bore it? I read for the 440 kit that I need a "big bore sleeve installed in your cylinder and bored to fit the larger piston." Will a regular old machine shop do this for me?

Another question is: I need a new front dirt rim and spokes for my 1991 DR350S. Do you sell those? Can you lace the new rims for me? Could you install the big bore kit as well? If you don't do installs on the big bore kit, I understand.

Last question is do you sell individual jets for the TM carb. All I saw was 54 piece kits.

A:

With the 385 kit, the stock cylinder has to be bored out to fit the new piston which is 4mm larger than stock. Your machine shop will have to have the piston in hand in order to bore it precisely to the correct size. Most good machine shops can do this but some may not be set up to mount motorcycle cylinders in their boring machine. Installing the bigger sleeve for the 440 kit is a similar job but we prefer to have re-sleeving done by a company that specializes in sleeves, like Northwest Sleeve. You ship them the cylinder, sleeve and piston and they install the new sleeve, bore it to match the piston and return it to you ready to install. Sorry to say that we don't do any client motor work any longer. We just work on our own performance development motors. The DR350 is one of the easiest motors to build though.

A front rim and spokes for your DR350 sells for $240. You would want the 1.60x21 Excel dimpled rim. Click here to take a look at that rim. We do lots of wheel lacing and we can lace and true your wheel for $80 per wheel.

Individual jets are $5 each, just let me know the sizes you need.

Q:

More re-sleeving questions

I want to install a big-bore kit in my motor. How do I put the bigger cylinder sleeve into my motor?

A:

The new big-bore sleeve is a 'shrink fit' in the cylinder. This means that first the old sleeve is bored out of your cylinder. Then the cylinder is machined to an inside diameter slightly smaller than the outside diameter of the new sleeve. The cylinder is then put in an oven and heated up to expand it. Once the cylinder is fully heated up, the new sleeve is dropped into the cylinder. As the cylinder cools, it contracts to form a very tight fit onto the new sleeve.
After the new sleeve is installed it is then bored and honed to the proper size to fit the big-bore piston. Some folks are puzzled about why the sleeve isn’t sized to fit the piston to start with. The reason is that the stresses of the shrink fit would distort the final size of the cylinder bore, making it unfit for use requiring machining anyway. The only way to fit the bore precisely to the piston is to do the bore and hone last. Naturally, this is all precision work and should be done by an experienced machine shop that fully understands the process. Though most good automotive machine shops will understand all of this, if you have any questions, you can always send your cylinder, new sleeve and new piston to Northwest Sleeve in Oregon and let them do their magic on it.

Q:

Can you lighten the fly wheel on my LTZ400?

Can you lighten the fly wheel on a LTZ400? What will this do for my ATV? Does it hurt the life of the motor? How long does it take to return?

A:

Lightening the flywheel will make the motor more responsive. It will rev quicker and have a bit more 'snap'. Lightening the flywheel will have no effect on reliability or engine life. We normally have a 1-2 day turnaround on flywheels.

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