Greetings,

My project bike is like an untamed horse, just doesn't want to give in. The bike is '91 Honda Nighthawk 750 with 34k miles. After running compression test on the bike I found out that the second cylinder had lower numbers than the rest. It happened to be a burned exhaust valve. New valve was ordered and the seat had been recut. Since the rest of the valves were in good condition, they were lapped.

After reassembly I did ignition timing test and synchronized the carbs. Most of the problems that were due to unsynchronized carbs went away. However, the bike continues to backfire on acceleration through the left exhaust pipe and through the carburetor on the first cylinder.

Leak down test revealed that cylinders 1 and 2 have leaking intake valves. There is no leak into adjacent cylinder, so the head gasket is good. The other two cylinders are in very good condition.
#1 - 50%
#2 - 45%
#3 - 12%
#4 - 10%

All the cylinders have uniform compression at WOT (130/130/135/140). Compression test was done on cold engine. Strangely enough numbers tend to decrease slightly when the engine is warm.

At first I thought that I may have messed on timing when I was putting on the cam chain. I have taken cams off again and did a leak down test on cold engine. All the valves are sealed and the percentage is within the green zone. 20% on the ones that were 12% when the engine was warm.

I have checked the cam timing prior to that. Everything was spot on. I reassembled everything and did the leak down test again. For some strange reason cylinder 2 and 4 are leaking out of intake valves. I have analyzed everything farther and it seems to me that hydraulic lash adapters are too tight. The lobes on the camshaft are pressing down on the lifters at TDC very slightly. I think lash adapters have to compensate for that, but they don't!

Hydraulic lash adapters are automatically adjusted. Any idea what the problem may be? All the parts were working fine before any engine work was done. Even with the burned out valve the bike was running smoothly.

Someone suggested that "grinding the seats may have allowed the valve stems to sit deeper into the head, beyond the range of the lash adjusters? You may be able to grind off the ends of the stems to compensate?"

Seems that this could have happened, but before I took the camshafts off for the second time cylinder 1 and 2 were the ones leaking. Now it is #4 and #2. Doesn't make any sense!

Here are some videos. If a picture is worth a million words, then a video is worth a billion!

http://youtu.be/YWZnvXt4Puw
http://youtu.be/zu5MBRIlBig
http://youtu.be/KRgEW-aUpFE

Thanks.

Welcome to the Ford Y-Block V8 engine site.  While the Ford Y engines are fitted with solid lifter camshafts, many of us here are fluent with hydraulic lifter designs.  Here are my thoughts on what is happening with your setup.
 
If you didn’t reset the stem heights of the valves after grinding on the valves and seats, then you likely have too much preload on the adjusters.  There should be a spec height for that height adjustment in the service manual.  Another player here is the replacement head gasket thickness as if it’s less than the original, then the lifter preload will also be increased.  As an addition, your listed leak-down numbers do not jive with your compression test numbers.  At a 50% leak-down reading, the compression reading should also be considerably lower.  As a general rule, I tear engines down with as much as 12% leakage.  Engines in good condition should not exhibit any more than 5% leakage cold and should read 2% or less warmed up.

Lorena, Texas (South of Waco)

Welcome to the Ford 'Y-Blocks Forever' website.  Ted has offered a very comprehensive answer to your question.

Back-in-the-day, when doing valve jobs, usual practice in grinding 45 degree valve faces was to note the depth of grinding necessary to resurface the face and then remove half of that amount from the valve stem length.  While this was the 'rule-of-thumb', Ted's assembled-in-place valve stem height measurement recommendation will yield a more accurate needed adjustment to stem length because how much the seat is cut affects how much the valve stem moves toward the rocker arm.  I think...  

Let us know how you make out with this. 

Best Wishes,


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Thanks for this valuable information!

I looked through the service manual and found this:

"Continue priming the hydraulic tappet until the air bubbles stop and the tappet no longer collapses.

Quickly try to compress the tappet by hand. Measure the
compression stroke with the dial gauge.

Compression stroke : 0 - 0.2 mm(0 - 0.008 in)"

These are the only specs. Do you think machine shop be able to test the head for leakage and readjust the heights to specs if I bring the entire cylinder head assembly?

Most manufacturers that  have a non adjustable valve train will  sell  push rods that  are  0.060 shorter and 0.060 longer to  makeup for  this when  machine work has  been performed
The  push rods  will be a lot cheaper than  having the  valves redone!

Unless the head(S) were milled too much or the valve sunk during the Valve refacing. Or both.

Frank/Rebop

Bristol, In ( by Elkhart) 

I looked through the manual and could not find any information on installed valve stem height. If I was to go to machine shop to get valve stem tips ground how much would I ask them to take off?

To me it seems that I could measure all the valve stem hights on the valves that are not leaking and take the smallest measurement as the hight to which all the leaking valves need to be adjusted.

I hope I am not over complicating this. :-)

Japaneses motorcycles have always been and will always be throw away motorcycles.  Ride them till they stop, throw them away and buy a new one. Wash and repeat.  Japaneses bikes are so common that they are not worth anything and will cost more to maintain and repair then what they are worth.  I would suggest selling your honda 750 and purchacing a used low mileage garage kept Harley sportster.  A honda 750 and a harley sportster have a similar look and similar performace.  The difference is that a harley sportster is a 2 cylinder push rod engine with 2 valves per cylinder and 1 carburetor, where a honda 750 is a 4 cylinder over head cam engine, with 4 valves per cylinder and 4 carburetors.  Do the math

2 vs 4 cylinders
4 vs 16 valves
1 vs 4 carbs
Belt vs chain

Its easy to see what bike is going to be cheaper and easier to live with.  Just my 2 cents.

You might consider making your own "solid" valve lifters and using shims to get the right clearance.  Thats what they did before hydraulic lifters.

Grumpy

Took the head off today and attempted to measure installed valve stem height with a caliper. After several attempts I thought it would be nicer to see what is going when the whole thing is assembled as it would be on the bike. That way I would know for sure which ones are leaking and which ones are looking good. Things didn't look good... Here is the video of what I found. BTW the camshafts are exactly at TDC.

https://youtu.be/TwdCvOng7HU

I will take it tomorrow to a machine shop and ask their opinion.