Topic: Sticky valve?

[kaysystems]

Can some of the experts listen to the sound on this page

http://homepage.mac.com/d.kay/FileSharing7.html

and give an oppinion please. - works best connected to a stereo system or with headphones. I moved the mic from the back of the bike to beside the engine half way through.

The bike runs a bit rough. The valve clearances are correct (fed up checking them) and the cam chain is tensioned.

I think that I  have a sticking or bent valve.

Thanks.

David 

[TwoTired]

Well, I'm not an experts.  But, I gave it a listen.

Wouldn't a sticking or bent valve show up on a compression test?

Have you done a carb sync? 

Do you get any clues from reading plugs?

Are all your pilot screws set the same?

Do all the head pipes feel the same temp?

[kaysystems]

Compression is 167 on 1 & 4 and 154 on 3 with 157 on 2.

Plugs show a bit rich except #2. All the needles are at the TOP except on 2 which is in the middle and is the only one the correct colour. New pistons & ring with a rebore last year, but I don't beleive the valves were touched.

David

[TwoTired]

Was the compression test done hot or cold?  And, did you try a little oil in the cylinders to see if that makes a compression difference?

The numbers given are not alarming.
167 +167 + 154 + 157 =645,  645/4=161.25 Avg compression, 10%=16Lbs  161+/-16 =145 low limit 177 hi limit.
All your numbers are well withing 10%  Cylinders and valves are unlikely suspects.

The number 2 spark plug points to and issue with operation of that cylinder different from the rest.  But, the compression is good enough that it shouldn't be an issue.  I'd guess at this point that there is an issue with carburetion.

Why aren't all the needles in the same position?  If all the carbs have the same components, but are adjusted differently, how could you expect all cylinders to fire evenly?  I think you need to make them all the same.

Carb sync, afterwards.  I'd check the idle screw adjustments for same settings.  If those show even vacuum from the cylinders, and you still have issues, then I would suspect a partial blockage in a fuel circuit of #2 carb. 

Have you replaced any of the internals of the carbs?  Carb kit pieces not stock, that might have different metering characteristics between carbs?

Have you checked for fuel contamination?

[kaysystems]

Was the compression test done hot or cold?  And, did you try a little oil in the cylinders to see if that makes a compression difference?

The numbers given are not alarming.
167 +167 + 154 + 157 =645,  645/4=161.25 Avg compression, 10%=16Lbs  161+/-16 =145 low limit 177 hi limit.
All your numbers are well withing 10%  Cylinders and valves are unlikely suspects.

The number 2 spark plug points to and issue with operation of that cylinder different from the rest.  But, the compression is good enough that it shouldn't be an issue.  I'd guess at this point that there is an issue with carburetion.

Why aren't all the needles in the same position?  If all the carbs have the same components, but are adjusted differently, how could you expect all cylinders to fire evenly?  I think you need to make them all the same.

Carb sync, afterwards.  I'd check the idle screw adjustments for same settings.  If those show even vacuum from the cylinders, and you still have issues, then I would suspect a partial blockage in a fuel circuit of #2 carb. 

Have you replaced any of the internals of the carbs?  Carb kit pieces not stock, that might have different metering characteristics between carbs?

Have you checked for fuel contamination?

Carbs synced

the odd carb is set as it is the only way to get it to run. if #2 is in the lowest position, the plug is snow white, suggesting maybe a leak?

Compression check done hot. Oil made no difference.

All carbs rebuilt after Berryman soak.

I still have to replace the needles and needle jets with honda ones.

Float levels identical at 26mm.

I even swapped the carb set, coils & plugs. no difference.

David

[TwoTired]

Yes, check carefully for a vacuum leak in the number 2 runner between head and carb.

A different set of carbs does the exact same thing?

Is this a recent development or did it coincide with the top end job?

Is the cam getting oiled properly. Is it all wet under the valve tappet covers.  Any reason to suspect the cam lobes are worn unevenly?

Um, did you sync the carbs at idle?  And, did it maintain the sync when checked at higher RPMs?

Lookin for clues...

[kaysystems]

Yes, check carefully for a vacuum leak in the number 2 runner between head and carb.

A different set of carbs does the exact same thing?

Is this a recent development or did it coincide with the top end job?

Is the cam getting oiled properly. Is it all wet under the valve tappet covers.  Any reason to suspect the cam lobes are worn unevenly?

Um, did you sync the carbs at idle?  And, did it maintain the sync when checked at higher RPMs?

Lookin for clues...

I sprayed the rubber iintake manifold with WD40. nothing

Yes a different set of carbs gives the same results.

I bought the bike 2 years ago. The problem only showed up after the new pistons, because the cam timing was off before!!

I'll check the oil on the cam this week. From what I saw last time I checked the clearances, everythng was wet.

Carbs synced at idle & fast running.

David

[TwoTired]

I gave the MP3 another listen.

Sometimes you can pinpoint engine noise with a stethescope.  I've used aquarium tubing as a substitute.  One end goes in your ear.  The other end is your probe.  Use a piece of tubing no longer than you need.   You can hear vacuum leaks this way sometimes.

Is the spiting it's doing going back through the carbs?   Does it only occur on #2 or the others, too?
With the air cleaner side the the carbs open to free air, put your "stethescope" near the intake of each carb.  The spitters should be audible.

Are you thinking the cam timing was off far enough for the pistons to contact the valves?

If a valve is binding, you should be able to feel for it though the open tappet covers.  Put a piston slightly after TDC during the power stroke.  You can depress the valve manually with a large screwdriver or wooden dowel and get a feel for how hard it is to depress the valve and see if it returns smoothly. Then compare the pressure with those on other cylinders.  I've done this with the CB550.  I assume this is possible with the 750.  If you can figure a way to do this, it might help confirm or alay fears about the valves.

The last part of the sound file sounds pretty raspy, like the cam chain is rubbing on something.  Is the tensioner set correctly?  Are the wheels still in good condition?  Maybe it's a trick of the microphone?

I heard a CBX that idled similar to yours.  Even though the carbs were cleaned twice one of them was still spitting/popping.  We drained the carb bowl of the offending cylinder and refilled it with aerosol carb cleaner for an over night soak.  Next morning the problem was solved.  These were CV carbs, though.

Were I you, if I could push on the valves and felt they were free and equal, I'd focus in the Carbs or intake runners.  But, then I don't relish taking engine out of frames, either.

[kaysystems]

With the old carbs, there was spitting through one carb, not sure whether #2 or #1.

The mic didn't really pick up a loud tappet style noise that is only heard when running tbove 2000rpm. The chain is tensioned.

The cam chain was installed one tooth too retarded so I really think that the exhaust valves must have hit the pistons. I need to get the old pistone from the guy who took it apart. I think it was about 30 degrees retarded at the cam.

I have a new set of needles & Jets (Honda ones) that I want to put in, so when the carbs are off I'll clean them out again.

I'll try the Stethoscope, but I'll run it a bit until the winter, then I'll haul the engine out & do the head with new valves & check the chain & wheels plus anything else that I can find.

Thanks for your help

David

[kaysystems]

Got a picture of what apparently is inside the engine!!

http://homepage.mac.com/d.kay/FileSharing7.html

Looks like I'll do it myself next time & change the valvess

David

[TwoTired]

I really can't tell from the picture if those are valve reliefs cut into the piston dome.  Or, marks from a valve hit.

If that was a valve hit on your current valves, I am mightily impressed at your compression numbers.

[kaysystems]

I managed to get other pictures too. Definitely 4 exhaust valve hits

The compression is pretty good - new pistons & rings with a rebore. They also skimmed the head as it was warped beyond spec.

I'm now looking forward to the compression with new valves, but I'll do it all myself. I thought the engine would be more complex to work on, but looks fine.

The only other bike I worked on is my '66 Ducati 250 OHC single. EVERYTHING in that engine is shimmed, including the bevel gears to drive the cam.

David