Running on 1 cylinder. Fuel ok, spark ok, compression ok..

[thibay]

Hi there,

I rescued my '76 447 XS from a 20 years sleep.

Went thru the carbs (cleaned, new kit, new floats).
Cleaned the tank, new oil and cleaned filters.
New breakers as well.

The problem is that she runs on the left cylinder. I think I got the right cylinder running at a time (right exhaust became hot) but didn't happened since.

The right cylinder gets fuel: spark plug is wet
The right cylinder gets spark too
Swapped the spark plugs, wires and coils. Doesn't change anything.
Swapped the wires that come from the condenser, same problem.
Breaker points aren't throwing sparks or flames while it's running so I guess the condenser is still good.

Carbs are set to the factory setting, 1 turn 3/4 if I remember well.

Any idea ? I'm stuck

Regards,
Thib

 

[Grinder]

No expert here but I would think if the plug is wet, either too much fuel (float setting?) or no spark if compression is good.

 

[thibay]

Compression is good, I get 120psi on both cylinders. And I get spark too.

Both floats are leveled with the same setting, highest edge is 25mm from carb base.

But I remember I got fuel leaking from one of outlet pipes on the right carb during the first fill. I cut the petcock it stopped, reopened it never leaked again.

Too much fuel could prevent mixture to burn ?

 

[retiredgentleman]

thibay;

If you ordered the carb kit from Mikesxs, you have the wrong jets in your carbs. 5twins has a chart in the "Tech" section that shows the correct jets for 76 and 77. The Haynes manual also lists the correct jet sizes.

The kit from Mikesxs has jets that are for 78 and 79

76 and 77 MJ #122.5...............PJ #25

If you used a #135 MJ, its much too large.

120 psi is not good compression................140 to 145 is normal.

Factory mixture screw setting is only a starting point (for the correct Pilot jet), use the "Dead Cylinder" method to adjust the mixture screws.

Getting spark is good, but if its a weak yellow spark, then you get poor combustion. What colour is your spark? If you are still using the old original ignition coils/leads/caps, they are pretty much worn out.................replace with a new dual output coil for a good strong spark.

 

[thibay]

Thanks for your time retiredgentleman.

I was aware of the jet problems in the mikexs kit. So I cleaned and replaced the original correct size jets.

I made a mistake reading compression.
Maybe it's an interesting value: 130 PSI left, 150 PSI right.

The firing cylinder is the one with the weakest compression. I believe I'm out of the 10% compression tolerance between the two cylinders.

I'll try that "dead cylinder" method. Does it work even if it doesn't fire ?

My sparks, both sides are blue and intense. It's not a yellowish spark as I have in mind.
I bet a complete tunep kit would help to improve a bad working engine but since I swapped the plugs/cables/coils shouldn't I get something out the right cylinder if it works well on the left?

What do you think about the compression difference. Is this a problem ?

Thanks

 

[xjwmx]

"Too much fuel could prevent mixture to burn ? "

Yes. More than once I wasn't firing on one cylinder and I traced the problem to a leaky float valve or seat o-ring. It gets so rich it won't fire.

But - I have bs34 carbs, and you have bs38 and I don't know about those.

 

[thibay]

Update: well it looks like it's a fuel delivery problem.
I vaporized some start pilot on the right air filter while the engine was running and the dead cylinder fired.

I guess that the right plug was slighlty wet because it's not getting enough fuel to detonate.

Will keep that post updated.

 

[retiredgentleman]

You could try putting a new spark plug in the side that's not firing.

Everyone says they cleaned their carbs. Quite often it turns out later, they did not know much about the interior of the carbs. Are you 100% confident that the pilot jet you re-used is completely clear? Did you blow carb cleaner and air through the pilot circuit passageways?

 

[Jerm]

I had this exact problem a few days ago and it was a spark plug that had crapped it's pants.

 

[thibay]

You could try putting a new spark plug in the side that's not firing.

Everyone says they cleaned their carbs. Quite often it turns out later, they did not know much about the interior of the carbs. Are you 100% confident that the pilot jet you re-used is completely clear? Did you blow carb cleaner and air through the pilot circuit passageways?

I tried both new plugs yesterday with no luck. Plugs had already been swapped earlier.

Good point about the carb cleaning.
I guess I didn't cleaned enough some passageways althought I blowed air/acetone/brakecleaner in every hole. Went with thin wire inside too.

When I found the XS it was almost lying on its right side so it got all sediments, dry fuel and so on over the years. There was some white solid powder which I may haven't removed completely.

Will definitely go through the right carb, again

 

[thibay]

Hey everyone.
The problem came indeed from the right carb. Especially the float level was too high so fuel coudln't fill the carb enough.

The bike now runs like a champ on both cylinders

I noticed a light "rattling" sound coming from the engine. Like if something wasn't tightened. Guess it may come from the camshaft chain tension. Any good procedure to recommend ?

Cheers,
Thib

 

[gggGary]

Wow glad to hear you stuck it out and found the carb problem. Do a search on cam cahin or timing chain adjustment there are multiple threads with good procedures. Have you gone through the "I bought a new bike now what" guide yet? have you removed and cleaned BOTH oil screens. How many miles on the old gal? You may just need an adjustment but the rubber cam chain guides get old and break up too. Set the valves again, maybe after an hour or two of running. Anytime a bike has been akakened after a long nap a second oil change after an hour or two of running with another look at the filters is a good idea. Helps you to "know" if there are issues. lurking. Welcome to the flock of true believers!

 

[thibay]

Hi Gary, yes I went through both oil filters. They weren't too dirty at all and I didn't saw plastic chips into oil.

I removed the tensioner cover yesterday: the pin is coming in and out of the body +-3mm but I don't know if it's good or not. Will check the forum for that. Damn Haynes manual doesn't talk about that..

The old lady has 49.000Km. I will fix the front brake and fork then ride it for 1 hour like you said. Then valves and oil change.
Can't wait to ride it

 

[gggGary]

That's a fair amount of hours on it. If it hasn't been opened before, a "top" may be in it's future but if well maintained when it was "in service" it may go a while yet. Would be interesting to recheck compression at the "couple of hours service" too. It is normal for the adjuster to go in and out a fair bit. I think more than one newby has ruined his guides by tightening till the movement stopped.

 

[retiredgentleman]

Hi Gary, yes I went through both oil filters. They weren't too dirty at all and I didn't saw plastic chips into oil.

I removed the tensioner cover yesterday: the pin is coming in and out of the body +-3mm but I don't know if it's good or not. Will check the forum for that. Damn Haynes manual doesn't talk about that..

The old lady has 49.000Km. I will fix the front brake and fork then ride it for 1 hour like you said. Then valves and oil change.
Can't wait to ride it

3 mm is too much. I don't do any measurement at all. I adjust mine after riding when the engine is hot. I remove the large nut. I adjust until the in/out motion stops, and then back off 3 flats with my 10 mm wrench for a small amount of motion. Put the large nut back on when the motor cools down.

 

[thibay]

Hi retiredgentleman,
By 3 flats you mean 3 times the height of the 10mm wrench flat section ?

 

[retiredgentleman]

No, I mean you rotate the nut with the wrench on one flat, then move the wrench to the next flat and rotate again, move to the third flat and rotate once more. Using the open end of a 10 mm wrench on the adjuster nut.

 

[thibay]

Ok thanks !
I'm sorting the fork and brake stuff then I'll look into that, keeping this post alive.

 

[gggGary]

I am usually with RG's advice but am not so sure on this one. Guess I would have to try it both ways and see how much different the settings are and which way they differ between RG's, the factory method, and adjust while idling but not hot method.

 

[xjwmx]

^I think there's a large range that's ok for the adjustment. I think that's why people have issues with the factory method of adjusting it. People don't like making it flush, then being able to turn the crank a rotation and have it not be flush any more. But I think using the factory method puts it within the large good range. Also, it does seem to tighten when it's hot. At least if it's loose a certain amount it will stop rattling on it's own as the engine sits there running and heating up.