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Help me please with my 2004 CRF 450

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OK guys I need some answers please. I recently had my 2004 CRF 450 rebuilt by an engine builder that has been around for 30 years. I not going to name him, but I’m sure 90% of you guys have heard of him. Anyhow, I elected to have the Kibble valves put in and the Hot cam stage one. He also replaced the exhaust values as well and installed a Wesico high compression 13.1 or 2 piston or something like that. The problems I’m having is that it will stall easy and it is a bitch to restart, I’m taking about kicking it until I feel that I’m going to have a heart attack. Today I raced four classes, the first class I made it for one lap and I stalled it in the whoops and it took me about 15 minutes to restart it. The second class it also stalled, but luckily I pull in the clutch and jump started it without losing a position. The third class I wasn’t able to start the bike on the line and the only thing I did was washed it before the race. Also, I was unable to get the bike started for the fourth class. (We ran one moto formats due to a heavy rain). My main question is that will high compression piston is the bike a lot harder to start? Also, is it possible to flood a four stroke so bad that the plug needs to be replaced? The bike will act like its trying to start, but it wont. Please help me.

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Yep, just put the motor back into the frame last saturday with a brand new spark plug. I never had to kick the hell of my motor before I got it rebuilt.

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I test rode the bike last weekend I though maybe the timing was off to, but I had it checked and it was good. With the race gas, the guy told me that I would get better performance if I ran race gas, but it wasn't necessary.

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Not to sure, with the stock pistion it seemed that it didn't take much effort to kick it over, now its like 2.5 times harder to kick it over. As with the valve-2-piston clearence. I had the whole motor sent off for a complete rebuild, unless the dealer checked it when I had the timing check. What exactly will happen if I run race gas in a four stroke? Will I have to rejet completely?

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Talked to a guy yesterday about cams and he builds mx engines for a living and I asked him about hot cams and starting problems because I've heard about starting problems. He said as long as you don't set the cam off tilt , the decompression lobe should work fine. He said alot of problems are because of this. He has an 04 also. He's a big guy and said the 05 was a little too skinny for him.

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Thumper Troubleshooting

Bike won’t start after a crash

Pilot too lean

Idle set too high

Improper starting procedure

Bike wants hot start button

Bike runs on or won’t idle down when throttle is chopped

Idle set too high

Air leak intake or engine

Pilot too rich (when bike is hot)

Bike won’t start when cold outside

Pilot jet too lean

Air filter is over-oiled

Motor oil too thick for temperature

Bike sputters/won’t clean out at high RPM

Main jet too rich

Air filter is over-oiled

Spark plug has debris on electrode

Bike coughs and stalls in slow turns

Pilot too lean

Idle set too low

Valves set too tight

Decompressor is set too tight, so turning the bars engages release slightly

Bike hesitates or bogs over deep whoops or G-outs

Float level too low

Carb vent tubes blocked

Main jet splash shield not installed

Float level too high, gas is trapped in vent tubes (install T-vents)

Bike starts but won’t take throttle without sputtering

Pilot jet too rich

Water in fuel

Debris in main jet

Bike suddenly starts sputtering/gas flows from vent tubes

Stuck float check valve

Debris in gas or carb

Bike runs hot/feels slow and flat on straights

Main jet is too lean

Fuel octane low, causing detonation

Bike coughs and stalls when you wick open throttle

Needle too lean

Slide cutaway too lean

Pumper circuit blocked or too lean

Your basic thumper carb

Your engine is basically an air pump, and your carb meters how much air and fuel are sucked into that pump. Even though they may differ wildly in size, shape and design, all four-stroke carburetors have the same basic parts or circuits. Your slide cutaway (or throttle valve) needle and needle jet will all affect your bike’s acceleration from one-quarter to three-quarters throttle, and this is the most important area for off-road riders since we spend most the time at these throttle settings. Due to the hassle of making changes to these circuits, these are the most neglected areas of tuning.

Too rich jetting (too much cutaway, needle positions too high, too large a needle jet) can make your bike lunge and hard to control. If it’s too lean in this area, the bike will feel really flat and down on power, but will respond quickly to changes in throttle position. It also may detonate (ping) under a load. Pinging can also be caused by too little octane or winterized fuel (oxygenated, blended with additives), so keep in mind any fuel changes if your bike suddenly starts detonating in otherwise “normal” conditions.

You main jet is probably the most talked-about circuit, and it’s as critical to get right on a four-stroke as with a two-stroke. The main kicks in at half throttle and takes over metering duties as you hit full throttle. If your main is too rich, the bike will sputter and surge as it tries to burn all of that fuel. Too lean, and the bike will run flat or have a flat spot in the powerband. A severely lean main will cause your bike to seize just like a two-stroke. It’s better to be slightly rich on the main than slightly lean, because it will run colder.

Yamaha four-strokes have an accelerator-pump circuit. This system squirts a stream of raw fuel into the carb venturi every time you wick the throttle. Think of it as the four-stroke PowerJet carb—it richens the mixture to run best at lower engine speeds, yet allows a leaner top for more over-rev. If you radically modify your engine (flowed head, hot cam, etc.) you may have to richen this circuit slightly, but it’s otherwise not something you mess with for mere weather or altitude changes.

Your pilot jet (or slow jet) controls the idle circuit, or from zero to one-quarter throttle opening. The pilot jet and airscrew control the amount of fuel and air going into the engine at slow engine speeds. It’s very important to tune these circuits because they control throttle response and starting. The pilot circuit has a major affect on how well your four-stroke starts—or refuses to start—after a fall. At every event we attend, there is always some four-stroke rider who comes into the pits with his bike revving wildly. Invariably, this rider will say that his bike is hard to restart after a stall, so he turns up the idle adjuster so it won’t die.

That’s like jumping from the frying pan into the fire. Thumpers are only hard to start when they are jetted poorly or when the wrong technique is used. The rider who turns up his idle is only perpetuating the myth about thumpers being hard to start. Most manuals (and this magazine) will tell you that you should not touch the throttle when you kick a thumper. Well, turning the idle up is mechanically opening the throttle, right? You will make your bike even harder to start. You have to fix the problem, not the symptoms of the problem.

General Jetting Tricks

You bike’s owner’s manual is a great source for recommended jetting and tuning tips. If you bought your bike used and don’t have a manual, get one. Set the idle speed as per your manual. If it won’t start readily using the manual’s technique, your pilot jet is the likely culprit.

Whether your bike is air- or water-cooled, you should start it and get it up to race temperature before tuning the pilot circuit. A hotter engine will run leaner than a cold one, so failure to properly warm the bike will result in a too-rich setting.

With the bike up to temp, adjust the air screw so that the bike runs and responds best to slight throttle movements. Now, kill the engine and see how many turns out you have on the airscrew. Less than one, and it’s too rich. More than two, and it’s too rich. Install the next-sized pilot and repeat the test.

Most off-road bikes are jetted lean to meet emissions standards, soyou will likely want to richen these circuits, especially if you have gone to an aftermarket pipe, air filter or removed OEM baffles (in pipe and/or airbox). If you remove the muffler diffuser, you should toss the airbox stuffer too, or the airbox won’t be able to draw enough air to feed the engine. Most aftermarket companies will give you recommended jetting, so use this as a baseline.

Under most conditions, about the only time you will need to go leaner on an EPA-legal four-stroke is because of altitude. Air is thinner at higher altitudes, so it contains less oxygen and your jetting will be too rich. You will want to go down a size on the pilot, one or two on the main and lower the needle a position (raise the clip).

Cold air is denser than warm air so it holds more oxygen. On cold mornings your jetting will be slightly rich, but thumpers are less susceptible to changes than two-strokes. Where you might change the pilot on a two-stroke when it’s really cold, an airscrew adjustment will suffice on a thumper.

The same is true for barometric pressure. As the barometer rises, the pressure compresses the air, and your jetting will be slightly lean. A falling barometer causes a rich condition, but thumpers don’t care about the weather as much as two-strokes. Overall, the Yamaha thumper is jetted almost perfectly from the factory; however, it is very picky about its air filter. Do not over-oil the filter and do not expect it to start immediately after oiling the filter. Let it sit overnight (and not in the cold) to allow the carriers to evaporate. Better yet, keep spare filters in a plastic bag so that you never put a freshly-oiled filter in the bike on race day. Modifications throw stock jetting out the window, so this troubleshooting guide will apply to most four-strokes.

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Dont forget to check your stator/pulse coil, this could be your problem, or the main coil. I was told general rule, if it backfires out the pipe, its ignition, if it back fires out the carb, its carberation...

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I'm going clean the carb and jets today since it been sitting for almost 5 months during the winter. I notified the builder and he is suppose to go ever some things with me tomorrow.

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I had similar trouble with my stock crf450 after rebuild over the winter. It died in rhythm sections or under hard braking, any time I grabbed some throttle it would bog. I later learned this is not uncommon in 04's. It turned out that I had a loose throttle cable at the carb and I replaced the diaphragm in the accel pump and it's back to normal. The diaphragm kit includes diaphragm, spring and o-rings and retaild for about $18. I had read they rubber diaphragm ages and does not work well. It had nothing to do with the rebuild. For 18 bucks you may take a look at this. Joe

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Hi there, I'd like to ask how does the bike run/operate generally? Also, since your using a high compression piston you should expect kicking it over to be more of a challenge. But as NMdesertRacer said possibly the camshaft chain may have jumped one tooth on the camshaft sprocket by accident in which case the timing is out, not doubting the person who did your engine work, but mistakes do happen even to the best of people.

Regards Honda0001

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