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.

  • At idle do the RPMs fluctuate at all?
    – cinelli
    Apr 1, 2013 at 3:56
  • No it idles fairly smoothly
    – Ian Oakes
    Apr 6, 2013 at 10:07
  • Did you ever discover what the root cause of the issue was? I predicted it would be that the bike required a valve adjustment and thinner shims on some of the cylinders. Feb 18, 2016 at 1:08

3 Answers 3


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.


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.

  • 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, 2013 at 10:06
  • Glad I could help.
    – cinelli
    Apr 7, 2013 at 3:18


Based upon the information you have provided I see no indication that the rings are bad.

  • You aren't burning any oil

  • You do not have oily carbon buildup on your spark plugs

Bad rings will almost always give you those two symptoms, if they aren't sealing well against the cylinder walls you burn oil. That's it.

Another point of interest is that you have a shim and bucket valve train. More on that later.

Here is an image of a shim and bucket valve train. Notice that there are no rocker arms or push rods. The cam acts directly on the bucket and shim which acts directly on the valve stem. You adjust the valves by changing out different shims to get the appropriate valve clearence. In this image, the shim is below the bucket. This is what the CBR has. It's called an Under the Bucket Shim.

enter image description here


Other posters have told you to buy new pistons and rings.

If you buy new rings, you will need to have a new nikasil coating put on all of your cylinders. No shops that I have ever encountered offer this as a service. You have to have them either ship it out their provider or you find one and send it off to them.

If you bore your cylinders and put the 1st step overbore into the bike you may as well send it out to a place that does the nikasil. If you skip the nikasil, the rings will quickly wear out and you will have an issue.

Here is a link explaining the virtues of nikasil.

BUT, since none of this really applies, let's move on.


Even though your compression went up with the oil in the cylinder that is not proof that the rings are bad. It just seals them up a bit better from normal compression loss and helps to increase the compression.

The issue you are experiencing is out of adjustment valves. The symptoms are these depending on the severity.

  • Hard starting

  • Rough idle

  • Poor low RPM response

  • Power loss

What you are looking for is counter-intuitive. You are looking for no valve clearance as opposed to too much.

A shim and bucket setup will lead to zero clearance situations where the valve can actually hang open and there is a tiny gap between the valve seat and valve face in the head. When this condition arises the vehicle is hard starting, poor idling and general runs like a bag of hammers. One valve hanging open can lead to a very poor running/idling condition that improves with higher RPM. Running a compression test can give you breadcrumbs and hints but a leak down test will truly determine if this condition is occurring. It's a very common issue and is considered general maintenance to adjust your valves.

This happens as the valves get super hot and get beat in and then the engine cools, the valve shortens. So you get into a situation where the condition can get worse when the engine heats up and the valve doesn't make contact with face. It will actually hang open just a bit.

Along with the mechanical beating of the valve face against the valve seat, you get a gap.

All you need to do is adjust the valves. You remove the cams after measuring everything out with a feeler gauge and documenting it and then subtract the value you need to get valve clearance and goto the Honda shop and order up some shims. Be sure you put the shims in a cupcake backing pan or ice tray, something where you can keep track of where they go, exhaust cylinder 3 etc...

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