Slight Jump in RPM on Warm Start - Problem or Paranoia?

537 Views 9 Replies 3 Participants Last post by WintrSol, 3 mo ago

jkwon21

Discussion Starter · 3 mo ago

Hi everyone, as the title suggests I am seeking advice concerning a slight jump in RPMs when I start my bike up when warm.

2005 Yamaha FZ1 - Ivan's Jet Kit

www.youtube.com

For more context, I jetted the carbs for my 2005 Yamaha FZ1, set the floats at the correct height and got the mixture screws the right amount of turns out. Everything seems to be in proper working order except for the slightly high RPMs on that first start.

As you can see, the bike starts right at the proper idle level every time after the initial start. The bike was warm for the initial start. RPMs drop back to idle smoothly when revving in neutral.

Is this something I should be concerned about or am I overthinking because I've removed these carbs around ten times now at this point? Perhaps something that might be fixed with a good carb sync?

Thanks in advance.

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SemiFast

· #2 · 3 mo ago

jkwon21 said:
Perhaps something that might be fixed with a good carb sync?

Yes. Handling the carburetors WILL make the sync get out. You must set sync after each carb removal and before tuning for all speeds.
All it takes is a touch of the adjusters and it's out of synch again.

The instructions for jet kits only give you a starting point for the fuel mixture screws. Called the "bench setting". The same is true for jet needle clip positions and even main jets. Although the instructions are usually pretty close for main jets.
It is not at all uncommon for fuel mixture screws to end up being as different as up to 3/4 of a turn different from one to the other. Sync FIRST, or you'll never get them right.
If you do not have an Exhaust Gas Analyzer the only way to get the fuel screws set correctly is by running the bike mostly on the idle / slow speed circuit then 'read' the spark plugs. Even with an EGA reading spark plugs will be more accurate.
For all other speeds an on-board EGA is useful but again, reading spark plugs is the way to get it Spot On... AND, You need to use new, unused spark plugs pretty much after most every adjustment made.

S F

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WintrSol

· #3 · 3 mo ago

Another test for idle mixture is the drop test. Raise the rpm to about 3000rpm, then abruptly release the throttle, and judge how quickly it settles to idle. Does it drop below idle, then come back (rich), or hang a second (lean). These are crude ways to assess how far off you are. Then use fresh plugs and asses the final settings.
Also, sync can be two separate settings, one at idle, and the other with the throttle held open to about 3000rpm. Depends on how your throttles are connected. For the open throttle adjustment, I will raise the rpm to about 3500, then ease it down, watching for the vacuum to shift (seems more sensitive, to me).

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jkwon21

Discussion Starter · #4 · 3 mo ago

Thank you, everyone for the suggestions and advice. I will do some more troubleshooting tonight, including a carb sync. One question I have is regarding the possibility of this occurring due to enlarged bypass holes.

The kit requires enlarging the carb bypass holes using a small drill bit and while I believe I did it properly, it's a bit difficult to tell. To my limited knowledge, these bypass holes control the fueling and if too large, would manifest in more fuel being delivered leading to a high idle. Is this correct?

My idle is steady, so does that effectively rule out the possibility the bypass holes are too large?

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WintrSol

· #5 · 3 mo ago

What 'bypass' holes? The ones in the brass tube above the jets, or some other? Got an image, or a link to the instructions?

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jkwon21

Discussion Starter · #6 · 3 mo ago

WintrSol said:
What 'bypass' holes? The ones in the brass tube above the jets, or some other? Got an image, or a link to the instructions?

Here is Pat's website detailing how to install Ivan's Jet Kit.

Loading…

www.cartestsoftware.com

If you scroll down to the section titled "Enlarge Pilot Bypass Holes," there are lots of handy photos to look at.

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SemiFast

· #7 · 3 mo ago (Edited)

jkwon21 said:
If you scroll down to the section titled "Enlarge Pilot Bypass Holes," there are lots of handy photos to look at.

This is exactly why I use and recommend Factory Pro Jet Kits. None of that

is needed, should be done, or should be recommended, IMO.

I didn't look further but what fuel mixture screw setting is recommended by Ivan after the drilling?

Turns out I do not agree Ivan.

Yes. Set carb sync and check back in with us...

S F
EDIT: I kinda jumped out of my skin when I saw what was recommended by Ivan, so I backed up a little and edited....

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WintrSol

· #8 · 3 mo ago

The bypass holes indicated provide pilot mixture as the throttles are opened, until the jet needle/needle jet and main jet take over. In theory, the bypass holes shouldn't contribute to the mixture at idle, although they will provide a little. That is assuming the holes are clean and accurate.

I agree with S F, drilling carb bodies sets my teeth on edge.