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This is a follow up question to What are the causes of fluctuating intake manifold vacuum

I have a 2005 Honda CBR600 F4i which suffers from occasional hesitation at low rpm when accelerating.

After a compression test on a hot engine I got the following results in psi before and after squirting some oil down each of the cylinders. The manual indicates that I should be seeing 178 psi.

Cylinder      1    2    3    4
Before       95   90   95   90
After       105  135  135  135

The spark plugs seem to indicate that the engine is running reasonably well, they are darkish grey. The engine doesn't blow smoke and always starts easily.

Given that I've already identified that the intake manifold vacuum fluctuates between 5 and 20 cm Hg at idle, what is your interpretation of the above figures and what work would you recommend getting done.

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How much mileage has the bike done? – timbo Dec 16 '14 at 0:15

2 Answers 2

Measure each cylinder and buy over sized pistons / rings to accommodate the sizes.. this will be your cheapest option because installing new liners and buying stock pistons is very expensive / allot of machining work. If you don't have the time or money, keep driving it the way it is - if its not smoking bad you should get another 35k or more off of it.

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You cannot just buy oversize pistons and slap them in. It doesn't work that way. You can buy 1/4mm over stock increments 3 sizes above. Wiseco and other manufacturers will require machining as will stock 1/4 over. Additionally, there is a nicad coating on the cylinder that will be required, you'll have to send the cylinders out (top half of case) to have that component done along with the boring/honing. This isn't a Ford. – DucatiKiller Feb 12 at 12:27

Wow! Textbook case of worn rings. The oil helps seal the ringland area. That's a large increase on cylinders 2, 3, and 4. Your fuel efficiency should be pretty poor with those compression numbers. Is it?

If your fuel efficiency is miserable, it's time for a rebuild. Depending on the car it might be cheaper just to replace the engine with one from a engine supplier.

Alternatively, you can drive it until it dies, which may be years down the line as long as you don't have to worry about emissions or you don't care about fuel efficiency.

Another option is to hand it to any new driver coming up in your family. They tend to wear things out anyway so it can't hurt to let them do it in a car with less to lose. Added benefit: less HP available.

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Sorry forgot to mention that it's a sports bike in this question, I've now updated it. So the replacement cost is quite high. Do you think it will be enough to replace the rings or should I consider going the next oversize? – Ian Oakes Apr 7 '13 at 21:20
It's something you'd have to check when rebuilding. It's going to be at least the rings. – Nick Apr 8 '13 at 0:46

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