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I have a 2005 Honda CBR600 F4i which suffers from occasional hesitation at low rpm when accelerating. I also get occasional fault codes from the ECU indicating a vacuum leak.

Today I connected a vacuum gauge between the MAP sensor and the intake manifold, the manual indicates that I should be seeing between 150 - 250 mm Hg.

While I was running the bike at idle the gauge was jumping rapidly between 5 and 20 cm Hg. As far as I can tell this indicates a leaking valve.

When I increase the engine speed up to about 3000 rpm the vacuum stops fluctuating and sits on about 18 cm Hg.

Generally the bike performs extremely well and apart from the occasional low rpm hesitation, there are no obvious issues above 3,000 rpm

But before I start tearing the engine down is there any else I should be looking at first?

I've just done a compression test if your interested you can see the results here.

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At idle do the RPMs fluctuate at all? – cinelli Apr 1 '13 at 3:56
No it idles fairly smoothly – Ian Oakes Apr 6 '13 at 10:07
Had you removed your carbs or disassembled them in way prior to these symptoms occurring? – DucatiKiller Feb 16 at 23:23

2 Answers 2

Perform the obvious checks first. Check the air filter and pull the spark plugs and inspect and replace if needed. Take note of their condition. (smell like fuel? / Showing signs of a rich/lean condition? / broken? / etc)

Now, lets have a look at the fuel pressure regulator.

enter image description here

The fuel pressure regulator is operated by the vacuum line that's attached to it. Since vacuum readings are directly related to throttle position the more throttle, the more fuel (quick/short explanation).

When there is no vacuum it supplies maximum pressure. Also may cause extra undesired fuel to leak into one of the throttle.

There are a few different ways these regulator fail. - The vacuum line going to breaks is disconnected. - The diaphragm that is inside of it fails.

When this happens it can cause a ton of different symptoms. One of them being your exact issue.

To check it you'll have to lift the tank up. Then locate the fuel pressure regulator (FPR) (use the photo above for reference) and disconnect the vacuum line attached to it.

Inspect the vacuum line. Are there any signs of fuel?

Keeping the vacuum line disconnected turn the key to the ON postion ###(DO NOT START THE BIKE)###. Does this causes fuel to drip?

If so, then we've found our culprit. The vacuum line should be completely dry and, the spark plugs should not have excess fuel on them what so ever.

Make sure that while you have everything opened up to take a look at any other vacuum lines you can see and replace them as needed. Since you're not reporting that the RPMs are fluctuating at idle and it's simply the hesitation & vacuum readings you're getting. Then I'm almost certain it's going to be the FPR and you most likely wont find any other vacuum leaks.

NOTE: If you're using any fuel additives. Now is the time to stop. It seems to show up much more in bikes that use them.

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Thanks for the great answer, I too have suspected the FPR in the past due rich running and fuel smells after stopping. So I replaced it with a new one about 2 months ago and these symptoms have gone away. – Ian Oakes Apr 6 '13 at 10:06
Glad I could help. – cinelli Apr 7 '13 at 3:18

It sure sounds like a valve is sticking or leaking. There's a remote possibility that your troubles are caused by a clogged catalytic converter or a leaky exhaust. If you can't find any other clues it might be worth it to ask someone at your local bike shop for an opinion before tearing the whole darn thing apart.

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