96 Yam 250 cylinder not firing?

[Sealuvrrr]

I changed the spark plugs mid season and the engine seemed to run fine ever since other than an occasional cough which I attributed to a dirty carb. In the process of winterizing, I pulled the plugs to fog the cylinders. When I pulled the middle plug on the left side as you look at the motor from the boat, I noticed that the plug was totally clean, just like the day I put it in. How did the engine run well? What do I do to diagnose the problem? Thanks!

 

[MikDee]

Re: 96 Yam 250 cylinder not firing?

IMO, It could be a bad plug wire, coil or powerpack, or a washed plug from water intrusion.

 

[JUSTINTIME]

Re: 96 Yam 250 cylinder not firing?

IMO, It could be a bad plug wire, coil or powerpack, or a washed plug from water intrusion.

check compression first

 

[MikDee]

Re: 96 Yam 250 cylinder not firing?

Thanks Justin, That's another thing I meant to tell you to check, low compression, or a bad head gasket could be letting some water in causing this.

 

[Sealuvrrr]

Re: 96 Yam 250 cylinder not firing?

Thanks guys...The plug looked brand new! Like there was no gas, water or anything ever on it.

 

[rodbolt]

Re: 96 Yam 250 cylinder not firing?

whats the compression on each cylinder?
for future reference please use the industry stndard.
port bank and stbd bank
port bank from top to bottom is cylinders 2,4 and 6 stbd bank is cylinders 1,3 and 5.
keeps confusion to a minimum.
give me the numbers and maybe I can assist from there.

 

[Sealuvrrr]

Re: 96 Yam 250 cylinder not firing?

Industry standard..it is cylinder number 3. Can this engine run strong on 5 cylinders? I have not done any diagnostics yet. I pickled the cylinders and put it away for the winter. If the cylinder was not firing, then why did the engine run ok? What could cause the plug to be spotlessly clean? What steps should I take in the spring?

 

[MikDee]

Re: 96 Yam 250 cylinder not firing?

It may still fire on that cylinder, if the water intrusion is not that bad, but that spark plug is telling you the story, it is getting washed at the same time IMO.
Wait, let me say industry standard is basically are they all the same? and an industry standard figure is 100lbs, what are they compared to this figure? any discrepancy on that cylinder is reason enough to be suspect, I would pull the head & check it out in the spring.

 

[Sealuvrrr]

Re: 96 Yam 250 cylinder not firing?

I did a compression check and came up with:
1-2 108
3-4 108
5-6 105

I also switched the spark plug wires between 3 and 5 and 3 is still dead. I put my hand over the throat of the #3 carb with the engine running and felt suction. Since the plug is squeaky clean, I think the cylinder is not getting any gas at all. What next? Carb rebuild?

 

[rodbolt]

Re: 96 Yam 250 cylinder not firing?

compression numbers look good, check the low pressure pumps, if they leak it can fuel wash a cylinder.
next I would test spark, can be done with an inline spark tester or a Kv tester.
also check the spark plug cap resistance.

 

[Sealuvrrr]

Re: 96 Yam 250 cylinder not firing?

You don't think its carburation? The plug is totally dry. I disconnected the plug wires totally between 3 and 5 and swapped them. It made no difference. How do I check the low pressure pump?

 

[MikDee]

Re: 96 Yam 250 cylinder not firing?

If the plug really is dry not being washed clean, it sounds like an intake valve, or reed? is not opening? no fuel is getting to that cylinder

 

[rodbolt]

Re: 96 Yam 250 cylinder not firing?

now you MUST check the spark, its the next step. it MUST be able to jump a 7/16ths gap.
you can buy or make a spark tester.
the pumps can be tested with a hand held pressure vacum tester as per the service manual.
the plug cap resistance can be checked with a digital volt meter.
use a logical approach to your trouble shooting, try not to splatter gun it and read then re read the service manual until you understand what each component does and what will happen if it does not.
thats why I always say, test spark, test compression then go fix the fuel system.
its really rare to see a reed fail on a V yamaha, I have seen the occasional gasket blow out but I cant recall ever seing a reed fail on a V motor.

 

[Sealuvrrr]

Re: 96 Yam 250 cylinder not firing?

I tested the spark with an inline tester and all cylinders are fine.

In trying to decide what to do next I noticed that on the #3 carb, there is a small bolt missing on the left side as you look down the throat. The other carbs have this bolt. Where the threads are there is a hole and a hose that connects down to the carb float housing. There is a valve in the hole and on the other side there is a rod that extends into the carb mouth.

Sorry to describe it this way but I am not a mechanic. Could this be why the cylinder is not getting gas?