Intake valve oddity

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TheCounty

TheCounty

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Issues arose with 750 commando.
Compression fine, carbs fine, ignition A-Ok, timing spot on, no "noises" etc.
On idle, everything nice.
At about 1500 rpm on accelleration spitting/missing occured.
With intake filter off, found that at 1500 and up the right carb was blowing fuel out in a light spray. No intake pressure at all with hand over carb.
Figured intake valve spring(s) toast, or valve sticking.
Pulled head & all looks fine ?????
Only oddity is that the hardened end of the intake pushrod at the upper (rocker) end falls/slides off pushrod easily.
Other ends are all tight on the aluminum pushrods.
Not sure how, but could this be the issue.
All comments welcome, but please accept that the carb/ignition/timing is fine.

Cheers
TheCounty
 
Wild guess... Your exhaust valve is bent or sticking open during the intake stroke, so when your piston is descending, the exhaust valve is partially open. That makes the vacuum that your piston would exhert on the intake side weak because the cylinder's exhaust valve isn't sealed...

check the piston top and see if there's any sign of a collision with the exhaust valve. I wouldn't imagine a weak exhaust spring alone would have that effect at 1500 rpm, but a bent valve stem probably could.
 
Check the cam. Jim


TheCounty said:
Issues arose with 750 commando.
Compression fine, carbs fine, ignition A-Ok, timing spot on, no "noises" etc.
On idle, everything nice.
At about 1500 rpm on accelleration spitting/missing occured.
With intake filter off, found that at 1500 and up the right carb was blowing fuel out in a light spray. No intake pressure at all with hand over carb.
Figured intake valve spring(s) toast, or valve sticking.
Pulled head & all looks fine ?????
Only oddity is that the hardened end of the intake pushrod at the upper (rocker) end falls/slides off pushrod easily.
Other ends are all tight on the aluminum pushrods.
Not sure how, but could this be the issue.
All comments welcome, but please accept that the carb/ignition/timing is fine.

Cheers
TheCounty
Click to expand...
 
How long has this been going on? Did it just happen at once, or was there a progressive drop in performance?

If compression is fine, that should rule out a sticking valve.

Blowing back seems to be a clear case of the intake valve open on compression stroke. My guess is a weak intake spring. Most likely, your engine would benefit from all new springs.

Another possibility is a worn cam lobe. (Oops! I see Jim beat me to the post on that)

Slick.
 
Happened all of a sudden.
No sign of piston collision at all.
Carbon build-up minor.
Cam lift ok.
 
TheCounty said:
Only oddity is that the hardened end of the intake pushrod at the upper (rocker) end falls/slides off pushrod easily.
Click to expand...

End should not be that loose so I would replace the pushrod.

This is a stretch but maybe the loose end on the pushrod is acting like a hydraulic lifter and finds it's happy spot at 1,500 rpm where it's getting jacked up enough on oil and preventing the valve from seating. Lower rpm than that and there's not enough "pumping up".

Broken valve spring is a good candidate but would expect some witness marks on piston at higher rpm.

Replace the pushrod and see what does not happen.
 
TheCounty said:
Happened all of a sudden.
No sign of piston collision at all.
Carbon build-up minor.
Cam lift ok.
Click to expand...

Did you check the cam lift on the exhaust valve?

Are you using a fiberglass or coated tank. Rosin from the glass will coat the intake valve stem and cause it to hang open. Jim
 
TheCounty wrote:
Only oddity is that the hardened end of the intake pushrod at the upper (rocker) end falls/slides off pushrod easily.


End should not be that loose so I would replace the pushrod.

This is a stretch but maybe the loose end on the pushrod is acting like a hydraulic lifter and finds it's happy spot at 1,500 rpm where it's getting jacked up enough on oil and preventing the valve from seating. Lower rpm than that and there's not enough "pumping up".

Broken valve spring is a good candidate but would expect some witness marks on piston at higher rpm.

Replace the pushrod and see what does not happen.
Click to expand...

So far this is the only factual fault you have got and clearly something is holding the inlet valve open. I would have thought broken and weak springs would be easy to check now the head is off. If you can find nothing else stick it back together with a new push-rod and give it a go.
Dave
 
cash said:
So far this is the only factual fault you have got and clearly something is holding the inlet valve open. I would have thought broken and weak springs would be easy to check now the head is off. If you can find nothing else stick it back together with a new push-rod and give it a go.
Dave
Click to expand...

Just be sure that before you do you have lifted the barrels to be 100% that cam and lifters are as you expect them to be! Or you are just going to be back here again!
 
Thank you "Dances with Shrapnel".
I left the idea of the push rod end acting like a hydraulic lifter out of the post purposely, as it does seem a bit of a stretch.
Now that someone else has tossed this into the ring, I feel a bit better discussing it.
The valves, seats, cam lift, springs etc all seem OK. I was hoping that someone had slayed this particular dragon before though.
Anyhow, the head is getting new guides/seals/ & proper valve job before going back together with a new pushrod of course. May be a few days though due to
12 shifts, wife's to-do list, etc.
Thanks to all & if anyone has run across this issue, please post.
I'll follow up down the road.
Cheers
Mark aka jaguar/TheCounty

ps. For those interested I put this bike together back in 1991 and have put over 50,000 enthusiastic miles on it since. (... and lots of venoms/roadriders)
Not much to complain about for sure.
 
I have seen a lot of pushrods ends loose. Never seen it cause a problem other than slowly increasing valve clearance as the aluminum end wears down. Jim
 
So if you have no intake pressure above 1500 rpm then that would tell me the cylinder was not able to exhaust it's charge.
If the exhaust valve is working then you might want to make sure the exhaust pipe and muffler on that cylinder is not blocked. I have seen it happen, both with a broken baffle in the muffler and with a plugged catalytic converter. Jim
 
Exhaust clear. One of my earlier checks & good thought.
Reminds me of the story of the pet rat in the bike exhaust, owner lights match to see, residual fumes ignite & cannonball rat into owner's face breaking nose & killing rat...
I used a flashlight just to be safe (chuckle).
Cheers
 
TheCounty said:
ps. For those interested I put this bike together back in 1991 and have put over 50,000 enthusiastic miles on it since. (... and lots of venoms/roadriders)
Not much to complain about for sure.
Click to expand...

Well you must have done a decent job back then, and it is clear you intend to do a decent job now, at least on the head.

Maybe I misunderstand, but are you resisting lifting the barrels for a particular reason?

After 50,000 miles it might seem reasonable even if just to make sure the rings and grooves are in good order!

Your choice, but if you do the head as you say, refit, you may soon need to lift it all again.
 
Hi SteveA
The compression was good & the bores look great and the old girl still pulled real strong before this setback. Otherwise I would strip her right down again.
Should be good for quite a while & she always gets premium care.
Cheers
 
Most engines will spray fuel back through the carbs while the revs are below the 'cam spot'. One thing which will really stuff performance is a poorly seating valve. Years ago when I was racing my short stroke Triumph 500, it suddenly lost it's edge. I searched high and low for the problem and raced the bike for about 4 meetings before I found it. I had fitted new valve guides, but the only ones available at the time were cast iron. A small triangular chip had come away from one of the inlet guides and gone through the motor, dinging the seat of the exhaust valve on the way through. When I looked carefully, I could also see a mark on the piston.
 
TheCounty said:
Hi SteveA
The compression was good & the bores look great and the old girl still pulled real strong before this setback. Otherwise I would strip her right down again.
Should be good for quite a while & she always gets premium care.
Cheers
Click to expand...

Understood, but that does not give any indication of the state of the cam and followers. I was unpleasantly surprised when I last took my barrels off last!

Not suggesting you need to strip the bottom end, just lift the barrels to confirm how everything is that you cannot see.
 
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