Topic: troubleshooting, cold compression......HELP

[Jersey550F]

This is for a CB550F:

I cannot get my bike to start. Here are the facts:

I have spark

The plugs smell of fuel when I take them out

It back fires once and a while

My timing and point gaps are perfect

My cold compression readings are 60-40-50-60 (I replaced the rings and gaskets on the top end this winter, I did not bore the cylinders as they were within wear limits)

Is my compression shot to hell? I am not sure how big of a difference there is in compression readings cold verses warm.

I have not re-checked my valve clearances yet but I should still get a better response than just a few backfires shouldn't I?


If anybody has some logical troubleshooting steps from this point on please let me know. I am having a meltdown.

[TwoTired]

Did you remove the glaze from the cylinder walls with a cross-hatch patern so the new rings will quickly seat in?
 Your compression number are pretty low.   Did you check it with the throttle AND choke open, using the kick start lever for the highest reading possible?

Squirt some oil in the cylinders.  This will help seal up the rings and give you better compression.  It might also help get the bike started.

If the plugs are obviously wet with gas, and you have verified that all the spark plugs arc across their gaps, then maybe you are over choking the engine (flooded).  Kick it a few times with the throttle wide open and the choke off to clear it out.  If it roars, back off on the throttle.

Lastly the spark advancer can be assembled 180 degrees out.  The assembly is keyed, but not the cam collar. You might verify that spark occurs near TDC and not BDC.

[Jersey550F]

I did take the glaze off the cyclinder walls, I'll try some oil and the kickstart instead....

I will also check to see if my spark is happening at TDC.

I don't think that I am flooding the engine as my plugs aren't wet w/ gas, just smell of it.

I am going to re-check my t-chain tension and then maybe tackle my valves tomorrow. As for right now I am going to button up the bike and head for a cold one.  Thanks for the response Lloyd.

[Robert]

Once upon a time there was a little tinkerer that didn't bother to check the valve clearence before doing the comp test. he had low compression on one of the cylinders and took it all apart again to find..nothing. after that little tinkerer had it all together again, he did check the clearence and it turned out, one of the valves had no clearance at all and was not able to close, thus low compression. the tinkerer was me.

Since you had the head off, are you positive that the camshaft is mounted exactly right? I once had the head off and mounted the camshaft several degrees off (due to a broken dowel pin of the spark advancer ) . same symptoms as you have (backfire every now and then, will never run).

...and on another occasion I reassambled the spark advancer 180° off (I don't know if that's possible with the 550, with the 650 it is), which also gives backfire (albeit only one every two minutes...a really big one)

Sorry no logical steps, just sharing some experiences.

[cben750f0]

this may sound silly, but ar the spark plugs on the right cylinders, i found when i got the spark plugs messed up, it did the same thing... just my two cents... peace

[bryanj]

Those compressions are WAY to low are you checking with the choke off and the throttle WIDE open? If so you have big problems!

[cb750_chris]

Go with the wet test.  It will help you get the bike started.  The worst case is that you can get the bike running and see if the compression improves.  NOTE the compression will get better while wet but you want to check it later after the affects of the oil are gone and the rings might have started doing thier job on thier own.  You want to be up in the 115 or better range and a difference of more than 5 between cylinders isn't good either. 

Throtle wide open is important. 

[MRieck]

Those compressions are WAY to low are you checking with the choke off and the throttle WIDE open? If so you have big problems!

I agree.

[Jersey550F]

Ok, sure enough my cam was 180 degrees off.  I haven't re-checked my compression as I was anxious to see if it would start up. I added a teaspoon of oil to the top of the pistons and now I am getting pretty regular backfires. what is this a symptom of, a fuel issue or timing issue?

also, this bike was running like a top before I tackled the top end, so I am assuming that carb adjustments shouldn't be in the mix right?

[TwoTired]

Point cam or Camshaft?

With the valve camshaft off 180 degrees there is risk that valves were open when the pistons came up.  These strikes can easily bend valves so they are unable to seat.  That would give you low compression, indeed.  Any struck valves will likely need replacing, now.