Carbon build-up/loss of compression

Pete · 04-23-2004, 11:59 PM

I saw where BillAce is selling a 954 engine with 4,000 miles on it, and he describes carbon build-up on the valves and the resultant loss of compression. So as not to hijack his thread, I thought I'd ask my question here. Is it normal to have that much carbon build-up after only 4,000 miles?

Joel · 04-24-2004, 3:37 AM

I'm going to say it's not entirely abnormal. If the valves have carbon on them, and then the motor does not run for a period of time, the carbon will harden up on the valves that are open. Once the motor is started up, the valves slam hard enough on the head to knock the carbon off the valves. A compression test or leak down test will not get the motor spinning fast enough to get the valves to seat properly.

ratter · 04-24-2004, 5:49 AM

I wouldn't expect carbon build up at the low mileage unless the motor had not been run in properly and was burning a little oil

Zang · 04-24-2004, 11:09 AM

What "fuel" was run? With the right (or rather wrong) type and depending what those 4000 were ridden like (1/4 at a time, all road, stop and go, slow, etc) anything is possible.

BillAce · 04-24-2004, 11:15 AM

The problem was a faulty mass airflow sensor causing a very rich condition. The dealer blamed the exhaust, EMS map and I didn't get the answer until I took it to a private mechanic. The dealer wanted me to sign off on $900 of labor to pull the head and investigate in case. If I can only get my new motor running I'll be all set. I haven't seen my 954 in almost 6 months.

Zang · 04-24-2004, 11:17 AM

That could definately cause it...but I would think you would see an awful lot of smoke if it was that rich!

CBRBob · 04-24-2004, 10:56 PM

Since when do 929/954s have mass air sensors????????????????????????

Zang · 04-24-2004, 11:29 PM

Heck I don't know...I don't have one...

ratter · 04-25-2004, 2:05 AM

Quote:

Originally Posted by CBRBob

Since when do 929/954s have mass air sensors????????????????????????

the whole story sounds suss

CanyonCarver · 04-26-2004, 9:03 PM

I'm guessing he means the MAP (Manifold Air Pressure) sensor. If it was faulty, it could produce the symptoms described.

Holeshot · 04-27-2004, 12:04 PM

Quote:

Originally Posted by CBRBob

Since when do 929/954s have mass air sensors????????????????????????

That's exactly what went though my mind.. I know what a MASS sensor looks like and I sure did not see one (or space for one) on the CBR!

BillAce · 04-27-2004, 12:48 PM

I checked with the mechanic and that's it, my bad, but that's why I'm paying him.

luvtolean · 04-27-2004, 1:05 PM

If the mechanic told you that the motor had excessive carbon build-up causing low compression at 4k, and that it was otherwise OK...it's time to find a new guy.

BillAce · 04-27-2004, 1:38 PM

Quote:

Originally Posted by luvtolean

If the mechanic told you that the motor had excessive carbon build-up causing low compression at 4k, and that it was otherwise OK...it's time to find a new guy.

Here's what I know. I was having trouble starting and fouling the plugs. I killed the battery trying to start it a few times. The dealer told me the valves were carboned up and they would pull the head if I signed off on a $900 labor bill, saying I would pay if it wasn't a defect. I decided to use a motorcycle only shop instead. They perfromed a leak down test and determined the rings were fine, the valves were unable to close completely causing a loss of compression. They measured 80-100 PSI. Does this sound right? They diagnosed and replaced the MAP sensor along the way as well.

luvtolean · 04-27-2004, 1:43 PM

Yikes, 80-100 PSI sounds pretty low. Were all cylinders within that?

BTW for those that don't know, BillAce is a longtime member, and I'm quite sure there's nothing "fishy" about this...other than maybe a mechanic is lost.

There is no way cabon build-up SHOULD be responsible for this, unless you have leaky stem seals or something. Is it smoking?

BillAce · 04-27-2004, 2:29 PM

It did have black smoke @ idle before being torn apart. It ran when I brought it in though. I'll see if I can get pictures of the valves, the pistons looked good, @ least from the top. It actually was running strong, the morning I had trouble starting, I thought the battery was low because it sat for a week. I charged it, went out and at one point did a rol on with a Gix 750, RC51 and 929 and walked them. When it didn't start next time, I figured the dealer should take a look, I rode it there and got the bad news. I just want it back, it's been a while.