Still not fixed

Looking for more help. I posted before about severe backfiring and a bog/stall when I whack the throttle off idle. The bike also won't idle properly. It stays high and slows down after 20-30 seconds, usually. This all came about suddenly with no changes at all. 2001 bike that had 30-40 hrs already. I have narrowed it to the carb by swapping my carb with someone else. On my carb I have checked for leaks, cleaned 6-8 times, checked accelerator pump, rejetted who knows how many ways, verified float height and operation, verified hot start not open/leaking, did an ohm test on the TPS, it was on the high end of spec on both idle and open throttle, but given how wide the spec is I don't think it would matter(did not do the idle adjust). Can someone explain how the TPS actually works. One thing I have found is if I open the choke and jet it to run with the choke open things are okay, but I can't get the same effect by richening through jetting. Your help is appreciated.


This is caused when the accelerator pump does not deliver fast enough, then continues to deliver fuel after the throttle is closed. Hence the bog, followed by the high idle (until that extra fuel burns off).

You can verify this by watching the stream when you're opening and have closed the throttle.

Keep in mind that the push rod is sprung against the throttle cam, so when things go wrong, it will keep on pumping after the throttle is closed.

The cause? You either have a clogged nozzle (doubt it) or you have some binding in the linkage, (maybe) or the diaphram is bad.

If you shot the carb with carb clean, (without the black cover on) the carb clean runs down the push rod then onto the diaphram which causes the rubber to swell - and you can't tell it's bad simply by looking at it. Same thing happens if you spray it when disassembled.

The good news if that if you let it sit out over night, the carb cleaner evaporates from the rubber and it shrinks back to proper size. Or you can just purchase a new one.

I'd be surprised if this isn't your problem, so pay close attention to what your doing and let me know what you find.


I discovered by accident how carb cleaner makes the diaphram shrivel up like Holyfield's ear. The quick fix for getting it back to it's normal shape is to wash it in medium-hot water and dishwashing liquid. It will return to normal quickly.

[This message has been edited by Boit (edited 04-11-2001).]

Thanks for the ideas. I thought it might be the diaphram as well sorry swapped that out with a buddy. Still no change. I inspected for binding but did not see anything. Will keep looking. I also checked to make sure the acc. pump was delivering fuel. Looked okay. I can't understand why all probs go away when I run with choke out. What changed??? It was suggested I remove the boot and look at what happens when the bike is running. What do you guys think?

Basic things first-

The jetting sounds like a lean problem.

How far out have you turned the pilot screw? 3 turns?

Did you try a #45 and #48 pilot? #45 with 2 turns out, #48 with 1 turn.

The hot start is most suspect, but you seem sure that it's not the problem.

Cable slack OK?


Digging deeper-

The carb body can be split on the lower half by removing the 4 painted hex screws. This will allow access to clean the internal passages. There could be something in a pilot circuit passage causing you problems. This would be a worst case. Be VERY careful if you do this. The seals are not shown as replacement parts!! I have had mine apart before with no problems.



Typically, when the choke is required to make the engine run, the pilot jet is clogged...or partially clogged. The FCR carb is a little more complicated than this, but this is generally a good starting point to cure this particular symptom. Since the choke is only used during starting, this problem points to a slow speed circuit glitch within the carb....ala the pilot jet.

As an addendum; like James explained, the carb can be separated and cleaned thoroughly. The pilot circuit is a very small orifice tunnel and may get fouled before the jet. Every time I disassemble my carb, I'm amazed at just how filthy it is.....especially at the bottom of the float bowl AND the accelerator pump plate. No doubt, crud gets down into the accelerator pump plate via the little rubber sleeve that covers the pump plunger.

[This message has been edited by Boit (edited 04-13-2001).]


My main concern with this is that the bike will idle fast after he closes the throttle, then settles to a normal idle after 20 to 30 seconds. 20 to 30 seconds!!!

The only way this can happen is if a lot of additional fuel is being dumped into the engine. There's no debating that.

Now, the second issue. The bike is bogging when he goes to hit the gas. That's caused by not getting enough fuel when the engine needs it.

Put these two clues together and it's really an issue of not enough fuel when you need it, and too much fuel when you don't. Aka, bad fuel timing.

Sorry Boit, but I can't see how a pilot jet could cause such double trouble. Correct me if I'm missing something.

This is why I'm still suspecting the accel pump.

So Rod, not to question your testing ability, but let's chat about this a bit further.

Understand that just because you see a stream of fuel coming out of the nozzle, does not mean the system is working.

Here's what you need to do to shut me up on this issue.

Remove the air box by just pulling the sub frame off.

Then get a stop watch and time how long the stream of fuel runs out of the nozzle counting from closed throttle to when it stops at wide open throttle. If it's more than 2.5 seconds or less than .5 seconds, you have a bad accel pump, (or a very poorly adjusted one).

Then time how much longer the nozzle discharges fuel after you quickly open and close the throttle. If you get any fuel after the throttle is snapped closed, you have a bad accel pump.

If I'm wrong on the accel pump, you still have to look at the obvious issues around fuel timing. Fuel is restricted, then it is over flowing, and it's a lot of fuel.

Let me know what you find and in the mean time,I'll take a look at a few other things.


Please read the quote from a FCR tuning manual...

"adjust the fuel screw so that when you rev the bike in neutral and release the throttle, the revs quickly drop to exactly idle speed again. If, when you release the throttle, the revs hang up a few hundred rpm above idle speed, then drop to idle, you are too lean. Turn the fuel screws out. In extremely lean cases the idle will hunt between the proper speed and something above it. If, when you release the throttle, the revs drop below idle speed then pick up, the idle mixture is too rich."

Guys, Thanks for all the input. I do not believe it is a jetting issue for two reasons. I had 30-40+ hrs. on the bike, changed nothing, and then ran into this problem. I have tried different mains, different needle positions, and gone from 1/4 out to 3.5 out on the fuel screw. I have not changed pilots, only because the one I ordered has not come in, and because I feel I am just covering up the real problem(given that the everything was okay at the current jetting) BTW - my jetting was stock except for a 168 main - 1.5 turns out on fuel - clip in 4. I am heading out on vacation. I plan to try every suggestion posted when I return. I will post the results. Thanks to everyone for taking the time to post your ideas. It is very appreciated!!!!

I ran into something similar on my '01. After a few minutes of riding the bike would run strangely: popping/backfiring and no idle. A few more minutes and the problem would go away. A new CDI cured the problem.


I'm now having the exact same problem with my machine, and have partially confirmed that it's not the accel pump.

I'm suspecting that it has something to do with the fact that I went into no-man's land when I separated the two halves of the carb when I was hunting down an accel pump problem. My have a slight internal leak.

At the moment, the bike is almost impossible to kick start, and seems to have more of a kick back than normal. This may be an issue of timing, so I'm also suspecting that I may have somehow disturbed the TPS when I had the carb apart.

In either event, I may have additional info for you before you return, so let's chat before you get into it again.

And perhaps we should take this off-line as well so to hash out the details before posting a resolution. If not, I'll post what I found.


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