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Discussion Starter #1
I am having difficult time figuring out the problem with my recently acquired '91 Honda Nighthawk 750 that has 34k miles on it. 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. Wouldn't that result in symmetrical leaks on cylinders #1 and #4? Besides ignition timing is right on the mark. This was tested several times with ignition timing gun.

Does anyone have any idea what may be the problem? Taking the engine apart again without arriving at logical conclusion of what the problem may be is not the way I want to go.

Thanks.
 

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Ace Tuner
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Sounds like you have found the problem.
With 45~50% leakage thru the intake valves and 34k miles on them possibly the valve seats are too wide and not sealing.
The valve seat should only be about 1 mm wide.
You can test for a good seal by poring solvent in the port and watch for leakage at the valve. (Cyl head off of course).

I don't guess the hydraulic lash adjuster could keep the valve from closing completely.
If it fails it makes a lot of noise and you would expect the valve not to open enough.
 

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Discussion Starter #3
I have already checked the valves for leakage and did a valve job on the burned valve. The rest if the valves were lapped and sealed very well. Tested before tge head ws put back.
 

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Discussion Starter #5
I posted in my initial post that I have done leak down test and the air was leaking out of carburator on cylinder 1 & 2.
 

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Ace Tuner
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I have already checked the valves for leakage and did a valve job on the burned valve. The rest if the valves were lapped and sealed very well. Tested before tge head ws put back.
I posted in my initial post that I have done leak down test and the air was leaking out of carburator on cylinder 1 & 2.
While doing the leak down test and getting 50% leakage coming out of the carburetor it has to be going past the intake valve for some reason.

If the valves passed the solvent test.......Hmmm.

Could some trash have gotten on the valve face or seats?
Could the rocker arm be holding the valves open a little?
Try wiggling the rocker arms when the valve is closed. If it's TIGHT, maybe something is going on with the hydraulic lash adjusters.
At this point I'm out of ideas unless those valves somehow got bent during reassembly.
 

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Discussion Starter #7
I have taken cams off 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 check the cam timing prior to that. Everything was spot on. I will reasamble the everything and do the leak dowm test again. If it still leaks I will follow your advice and check all the things you mentioned.

Thanks.
 

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Discussion Starter #8 (Edited)
hydraulic lash adapters?

For some strange reason not 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 not adjusting correctly. 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!

Any idea why not? I followed bleeding procedure and installed them afterwards. Still the same issue. All the parts were working fine before.

Just like you mentioned, SemiFast. They are too TIGHT... But they were fine before and I didn't do anything harmful to them.

Here are some videos. Video is worth a billion words.

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

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How are you adjusting your hydraulic valves?

The systems I'm familiar with it's not uncommon for a valve to be held open a bit after adjustment if the lifters are well bled. Eventually the lifter bleeds down and the valve closes. It can making starting an engine after valve work a real pain.
 

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Discussion Starter #10
It makes sense. I haven't closed the valve cover yet. All the leak down tests were done after installation of the camshafts and cam timing adjustment just to see if the valves sealed all the way.

Are you suggesting to put everything back together and let tge bike run for some time to readjust the hydraulic lash adapters?

On Honda Nighthawk 750 they are automatically adjusted. Plus it seems that there are no vulnerable parts in the adjusters that could have been damaged during the process of bleeding.
 

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I really don't know enough about your era of Honda to make suggestions. If what I said above is true, the valve spring pressure will eventually bleed the lifter down and the valve will close. Maybe try again in the morning?
 

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Discussion Starter #12
Got a reply from the other forum:

"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."

forums.y-blocksforever.com/Topic118337.aspx

Can't argue with that.
 

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Ace Tuner
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Didn't think of the valve installed height. Probably because the leaking valves were lapped. That shouldn't change the measurable height.
Too high of an installed height could come into play if the hydraulic lash adjusters and rockerarms are in place while doing the leakdown test.
(Or trying to make the thing run for that matter).
Also, I agree with the leakdown and compression info for the most part.
The standard is, anything more than 10% leakage is considered to be a fail. I've seen 11~12% leakage with no problems.
So 10% is not a hard and fast rule, more like a guideline. I do agree that compression should be lower with 50% leakage. :confused:

They were thinking of push rods actuating the valves with the cam in the block, not an overhead cam engine when it was said.........
... 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.
This part does not apply to an overhead cam engine like yours.
 

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Discussion Starter #14
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.

youtu.be/TwdCvOng7HU

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

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Discussion Starter #16
There are no leaks now, but I am getting misfire on one of the cylinders. I have done several tests after the reassembly. As far as I understood, to diagnose misfire you have to test for compression, spark, and fuel.

Timing is spot on.
Compression is good.
There is spark on all the cylinders, but by following a method that I found on youtube I have narrowed down which cylinder is misfiring. The idle changes when I disconnect spark plug boot from all the cylinders but the first one. Backfire also goes away when the spark plug wire is removed from the first cylinder. When it is reconnected, the engine is misfiring again.

There is definitely spark on cylinder one, judging by the sound and spark plug tester. The spark is blue/purple, which means the spark is strong. The gap seems to be fine too. The spark is shooting right in the center. All the carburetors are synchronized as well.

Here is a video of all the tests: youtu.be/h_qiEDMHHqc

Any ideas what it may be?
 

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A bad plug cap could dampen the spark under compression but still spark outside the engine.
Even testing the plug cap with an ohm meter does not always show it to be bad.
You can get a cheap aftermarket (NGK) plug cap to use as a way to test.
 

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Discussion Starter #18
I have tried switching spark plugs from cylinders 1 and 2. The problem is still there.

Coil A: cylinders 1 & 4 - long wires
Coil B: cylinders 2 & 3 - short wires

Switched wires 1 & 4 on Coil A. The problem is still there.
Switched Coils A and B. The problem is still there.

Dissassembled carburators 1 and 4 and switched floats and needle valves. Still misfires.
While I was at it I also checked if any of the jets are clogged. The jets are not clogged. About a month ago I cleaned out the carbs by soaking them in pinesol for a few days. All the jets were cleaned out as well.

I have not checked any of these: vacuum leak (intake manifold cracking, throttle shaft seals failing, intake manifold gasket not sealing)

All I can say is that all the intake manifolds are in good condition. Carburetor assembly was not taken apart when it was cleaned, so I doubt it is throttle shaft seals. Intake manifolds were never taken off the head. I don't think it is related to a vacuum leak, otherwise I would have had that leak before servicing the head.

When I try to mess with pilot screw and screw it all the way in, the misfire stops. As soon as I undo it even one turn out, the misfire reappears. With more turn of the screw misfire appears at lower RPM range. With one turn it appears somewhere at 3,500 - 4000 RPM. With 2 1/2 turns it appears from 1000 and up.
 
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