THUMPER TALK: The 25 best tips for your 4-stroke

Getting Expert advice...

Thumper Talk

As the riding season winds down, it may be a good time to get your four-stroke ATV running up to its full po­tential. To help you out, we talked with some of the top four-stroke tun­ers in the country and asked them to lay out their best performance tips.

From the Northwest, we contacted vet­eran wrench, Pete Fisher, from Pow­roll Performance up near Bend, Oregon.

For the East Coast, we got shop owner Micky Dunlop from Four-Stroke Tech (  ) to dislodge his Thumper Talk secrets. Tom Carl­son of TC Racing was up next.

Then we headed to Southern Cal­ifornia, to speak with former Team Honda star, Curtis Sparks, of Cur­tis Sparks Racing  (  ).

We asked each one of them to give us their favorite tun­ing tips for four-strokes, and have compiled them into this list of the top 25 four-stroke tuning secrets!

Gentlemen, start your wrenches!

More boost in your roost: Many of the following tuning tips can be applied to utility and utility/sport quads as well. One of the easiest mods one can perform is getting more air to the carb with airbox mods or a better-breathing filter.

Thumper Talk

1. Pipe power: Bolting on a performance exhaust pipe could gain anywhere from 10%–20% horsepower. Un­fortunately, there’s also an in­crease in noise and the really quiet pipes generally don’t work as well. You will probably need to jet one size up on the main jet with the addition of a pipe.

2. Change brake fluid often: Brake flu­id tends to absorb moisture out of the air. It also may get boiled and ru­ined if you use your brakes to the point where they fade. Once brake flu­id has boiled, it will boil easier again. Change the fluid often and use quality high-temperature brake fluid (CT Racing recommends Motul 300C) to resist fading. Make sure you bleed the lines thoroughly (pour in new brake fluid as you bleed out the old) and get any trapped air out. Keep an eye on brake-pad wear and re­place when needed.

3. Filter flow: A lot of mechanics swear that K&N gauze-type filters are the kings of airflow. The downside is that they also pass more dirt than a tra­ditional dual foam filter arrangement. The hot setup is to run a K&N gauze filter with a Uni-wrap foam filter or Outerwears filter on the outside. If you are using an oiled filter, don’t over­oil it. Try to give the air filter a light coat that covers it completely—spray oils work the best.

4. Spark plug gaps: Always check the gap on new plugs; there’s no such thing as a pre-gapped plug. Check your manual or engine tuner for the cor­rect gap.

Calling all megaphones: Bolting on a performance pipe can give the engine a 10%–20% horsepower boost, but be prepared for more noise.

5. Do your research: If you do not ful­ly understand the mechanical task you are about to undertake, don’t be too proud to consult the OEM shop man­ual. There’s plenty of good information in there and it could save your day.


6. Keep a log: Keep a log or journal about your ATV. Good items to note: jetting settings for various climates, gal­lons of gas on the top-end or lower end, any major repair or modifications, and transmission oil changes. This could come in handy in the future.

7. Don’t mix parts: Performance products like pipes, silencers, porting, heads, etc., are in many cases de­signed to work together. Mixing some brands can lead to problems. Your best bet is to stick with one engine tun­er and follow their recommendations since they know what works.

8. Fight resistance: One of the most overlooked gains in performance doesn’t come from the motor but rath­er the friction of all the running gear. Push your quad and see how far it rolls on flat ground. If it doesn’t go very far, lube all the running gear and check for damage and worn parts (wheel bearings, chain, sprockets, brakes, hubs, axle bearings, etc.).

9. Avoid stuck throttles: Always make sure the seal in the carburetor caps is in place and the boot that seals the throttle cable into the carb top is properly installed. It is very im­portant to prevent dirt from getting in through these points, not only to prevent wear but also because dirt could cause the throttle to stick open.

10. ATV salvage: Whenever a part goes bad on your ATV, you don’t al­ways have to buy an expensive OEM replacement from your dealer. There are several ATV salvage companies listed in the back of this magazine ev­ery month that can save you money by buying used parts.


11. Big big-bores: First of all, en­gines do not run hotter just because they have a big-bore kit. To find out about bores larger than .080”-over, check with a performance shop or L.A. Sleeve. Many models have pistons available to .120”-over. You will get quite a horsepower boost from big-bore kits.

12. Good lubrication: Always use high-quality oil and change it often (about every 20 hours). Lack of lubrication (not enough or worn-out oil) is one of the most common mistakes that will damage any engine, particularly a four-stroke.

13. Rev limiters: Some utility and 4×4 quads won’t go any faster with a modified engine because of the rev limiter kicks in. Changing to a larger-di­ameter set of tires will increase your speed without having to overrev the engine.

14. Faster big-bores: Modified large-displacement engines should be geared up (larger countershaft sprocket or smaller rear sprocket), so your big-bore engine “pulls” instead of revving too high. One or two teeth larger on the countershaft is the most cost-effective way to accomplish this.

15. Breathe in, breathe out: An ex­haust system combined with a vented airbox lid (or simply removed lid) and re-jetted carb can yield a decent horsepower gain. Gains in the 5% to 10% range are common, with 7% be­ing about average.


16. Modest compression increases: Most utility and 4×4 quads re­spond very favorably to increases in low-end torque. This can be achieved with a modest compression big-bore kit, stroker crank, and/or torque cam. The compression ratio should be kept down to help prevent overheating at low speeds. Stroker cranks add a lot of low-end power but require a complete engine teardown, while torque cams yield a more modest increase and can be in­stalled without major hassles.

17. Stronger clutch springs: Any time you modify an engine there is more load put on the clutch. Use strong­er clutch springs anytime you exceed the stock power by more than 20%.

We try harder: It’s no secret that it takes more effort to build a top four-stroke racing quad than a two-stroker, but once you get there, they fly! We asked some of the top racing tuners what the average guy can do to get some extra horsepower.

18. Equalize tires: Even if your two rear tires have the same exact psi, slight differences in size (from manufacturing) or composition will make one tire better than the other. At speed, this can cause the quad to pull to one side and work against itself as the rider tries to keep it straight. Check your rear tire sizes by doing a wheelie on a straight, smooth surface to see if the quad veers to one side. Then make slight changes in air pressure to get them equal.

19. Valve adjustment: If you are rac­ing, make sure you adjust the valves on your machine every race. Hon­da 250Xs, 300EXs and Mojaves have a tendency to tighten up in the valve train area after hard use. Loos­en them to .004” on the intake side and .005” on the exhaust side for most racing cams. Trailriders should consult their owner’s manual for a recommended valve adjustment schedule.

20. Choosing a cam: Cams control the opening and closing points of the intake and exhaust system. To get more fuel into the chamber, change cams to get more lift and duration to pull more air in. There are low-end, mid­range and high-rpm cams available. The type we would get de­pends on the type of riding you like most and how your engine is built.


21. Pick a good engine oil: Rods and wrist pins are where oil wear shows up first in your engine. Ex­am­ine these areas closely and check to see what kind of wear you get here with the type of oil you are us­ing. There are petroleum, synthetic and cas­tor or bean oils available. Each has its own benefits and drawbacks. Pick an oil that suits your requirements and stick with it. Changing back and forth is usually not a good idea.

22. Plug reading for four-strokes: Since four-strokes don’t have oil in the gas, they will read lighter than a two-stroke plug would. You generally run a four-stroke leaner than you would a two-stroke. On a four-stroke, when the power starts falling off and surging, that is an indication that the mixture is too lean.

23. Oil coolers: For some reason, most ATV manufacturers haven’t ta­ken the precaution of equipping their four-stroke quads with oil coolers. The addition of an accessory oil cooler on your ATV will generally lower the oil temperature of 20°–40°. Be­sides helping your engine live long­er, it also doubles the life of your oil.

24. Stroker cranks: A stroker crank will usually make 10%–20% more torque than a big-bore motor of the same cubic-centimeter range.  For us, this had the added benefit of long­er engine life because the rpm’s are kept lower than a high-compression, big-bore-kitted motor would be turning.

25. Port and flow: One of the best-kept secrets to more power is in the cyl­inder head. When porting a four-stroke cylinder head, you want to keep good cylinder pressure for the most torque you can make. A lot of peo­ple will put in a big cam, which tends to lower cylinder pressure, and you end up losing low-end power. By in­stalling a shorter-duration cam and flowing the head, you don’t lose the bottom end and you build quicker re­sponse from the bottom to the top.

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