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Discussion Starter #1
Hello everyone,

I’m knew here and tried the search feature but nothing was specific to my issue. So, I have a 01 Honda 400ex. I purchased it a few months back and it rode fine.

After a few weeks of sitting, the OEM carb started to leak fuel. I tried cleaning it up and went through it. It continued to leak and bogged if I tried to smash the throttle.

I opted for an aftermarket carb in the hopes that I would momentarily alleviate the problem while I rebuild the OEM carb. Now, I installed the carb and it runs and idles great (no choke needed).

However, it boggs the same as before. If I smash the throttle, it boggs and dies. I replaced the spark plug and it was black (rich?). I took of the air filter and lid and it still does it.

I’ve had great luck with aftermarket carbs in that they are essentially ready to run right out of the box. I don’t see or hear any air leaks and sprayed some carb cleaner around the airbox and nothing.

Anything else I should do or check? Thanks!
 

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Most generally starting without choke indicates one or both of two things; float level is too high or the idle circuit is jetted too rich.

Bogging at full throttle can be either too rich or too lean provided you are not experiencing some sort of ignition problem. The 400EX has an AC excited ignition - it does not need a battery to run - with what you describe, it's possible the ignition is becoming weak and spark goes away at high RPM or when under load. The fact the engine performs normally at low speed means the CDI unit is good and the pulse generator which signals when spark should occur is working, that leaves the coil, spark plug, plug cap, stator and flywheel.

Usually when a flywheel goes bad, it works better at high speed than at low speed. Coils can go bad in many ways, but the coil is just a low amperage/high voltage transformer and it's pretty durable - it's usually the last thing that goes bad spark plugs fail frequently, plug caps fail more frequently than coils and stators fail at a relatively high rate.

If you determine the problem is electrical and not carburation, install a new stator first.

Since the carb started leaking while setting, attack the carb as the primary source of the problem. Question is why did the carb start leaking while setting? Did the petcock fail due to alcohol damage to the seal? The only other way the carb could start leaking while setting would be if the fuel was not shut off. Not shutting off the fuel will lead to carb damage from water (carried in with alcohol), from Algae (grows in the water), petroleum distillates (what is left after gasoline evaporates) and corrosion caused by oxygen and all the other stuff in the carb float bowl. Sometimes if the carb is over rich, you can shut the fuel supply off and as the level in the float bowl is depleted, it will start running better and runs best just before it is completely out of fuel. If it's lean, you might try putting it on choke to get it to run better. If putting on the choke makes it immediately worse, then it's probably over rich.

If you have had luck with aftermarket carbs, you are part of the minority - I have scrapped new after market carbs in favor of rebuilt OEM carbs when the POS Chinese copy failed to perform properly and the jets were unmarked and had odd threads making rejetting nearly impossible - I have decimal metric drills for rejetting Chinese carbs, but many times I end up junking them and reverting to an OEM even it I have to get it used.
 

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Discussion Starter #3
I appreciate your reply. I agree in that’s this issue may be ignition/spark related. I say this because my OEM carb and non OEM carb performed exactly the same minus the OEM carb leaking.

You are also correct in pointing out why the OEM carb leaked. I mistakenly left the valve open and it caused the issue (I know better now). I plan on rebuilding the OEM carb, but want to sort this bogging issue first.

Any tips of testing the ignition components you mentioned? Also, say we rule out ignition and carb, anything else I need to look for? Thanks!
 

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Discussion Starter #5
The saga continues... My buddy came over and tinkered with this nightmare of a situation (I am easily frustrated 😉). Starts and idles fine.

Thing he discovered was that leaving the carb one click or position up on the choke lever will not cause the quad to bogg down.

He checked over the boot and all relevant air leak items. Nothing found. I’ve sprayed carb cleaner to find a change in idle and nothing.

I doubt it’s the stator or electrical issue as the issue goes away with the choke lever up (air fuel related?). Now, given this new information what should I look for in order to NOT run with the one click of choke on?

Put another way, what would be the next thing to examine since I can’t find any air leaks and the problem is obviously fuel/air related. Recall, that the carb is new and is aftermarket, but I don’t think the problem is carb related. Thanks!
 

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First try going up 2 sizes on the main jet - one thing I failed to mention is with alcohol blended fuel the carb needs to jetted richer - especially is you are running 91 -93 octane as recommended by Honda. If going up on the jet does not eliminate the problem, but improves it, go up another 2 sizes and try again. You might try raising the slide needle one notch, but raising the needle will richen up everything (idle will need reset and main jet might need decreased in size). If you are running 91-93 octane pure gas (non-ethanol) you probably still need to jet up. Changes to the exhaust and intake require changing jets. Just going from a stock air filter to a K&N will cause what you are experiencing.
 
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