Mishako129

Hi guys. My cb7 has a slight but persistent misfire when it is cold and the revs are above 1000 rpms. It does not do this when it is warm and idling normally, only when cold and trying to warm up. The car has good compression, 185 in all 4 cylinders, it just was rebuilt, has only 7,000 miles on it.


Just changed the spark plugs, no fix.

Mishako129

Does anyone know what I can do to figure out what's wrong or how to fix it? Its a small problem but it could get worse. It does it only when its warming up. The miss is very slight, like a small vibration. I can feel it in the steering wheel.

brakedrum

I'm going to assume "slight" means barely noticeable if not paying close attention? If so, maybe that's just how your engine warms up. Engines have a lot going on when first started. Electrical things kicking in demand power and make the IACV compensate which causes little fluctuations in idle and the way the idle sounds. Mechanically, there's timing tensioners and belts that have to get settled in after sitting for hours and getting cold. Many other things involving fuel, etc. If she runs good and idles properly after everything is in full swing, then I wouldn't worry about it beyond maybe throwing the timing light on it to make sure it's TDC.

Mishako129

What I mean by slight is that its not anything that big. Its definitely there though, and it sounds quite unnatural.

jdowen2

Try an ascertain where the vibration is coming from. Get a stethescope or long screw driver and with your ear on the handle end listen to various parts of the engine. Without something definite only guesses are possible.

brakedrum

You could mess with your Fast Idle Thermo Valve (FITV) a bit. I used to give the wax ring a couple of counter twists for winter versus summer, but stopped a few years ago and just leave it just slightly looser than full tight. There are some days where it will idle slow when it starts and then rise up as everything gets going, and then slow as the coolant warms and shuts the FITV. If it gets brutally cold this winter, I might adjust the FITV, but I made it through a few winters without having to touch it.

You could also try removing and cleaning that FITV and the Idle Air Control Valve (IACV). Keep in mind that the FITV and IACV do what the engine needs in order to keep it running. Sometimes they might do some odd sounding stuff but are only responding to the atmospheric conditions and what the computer is telling the IACV air requirements are to keep things running. The FITV sort of works strictly based on coolant temp, though. Maybe try adjusting it in very small increments to let in a little more air at start up and see if it changes what you are noticing.

You could also dig up some info on valve lash adjusting, which these cars do need from time to time. You'll also hear it called valve clearance adjustment. But I think that is overkill for what yours is doing.

I still think if you turn that key and she ignites and begins to give you the signature Honda hum, then be glad. Especially if its running good under load, getting decent mileage and idles properly. Let all the systems do their little tradeoff at start up and then just drive off happy.