Topic: Hydraulic lash adapters are too TIGHT -> leaks air on the intake valves

[slonny]

Greetings,

It is my first time here and I already have a problem... 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.

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

Thanks.

[robvangulik]

A video may be worth a billion, but those who can read clearly have an advantage.
Your bike obviously has a DOHC engine, and as there are no SOHC bikes with hydraulic valve lash adjustment I think there's only a small chance you'll get helpful answers here. Sorry.

[slonny]

I'll take a chance. I hope an SOHC enthusiast has a wider range of view when it comes to bikes. ;-)

[robvangulik]

You never know, you could strike lucky

[scottly]

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?

[slonny]

That is probably it! But that would have been quite a grind. My only qustion is, why did I have the 1st cyclinder leaking at first and after taking camshafts off and now it is the 4th one?

If this is the cause how can I send the cylinder head to a mechanic for a valve job and to do a test on all the valves for leaks without sending the whole bike? Sprokets and cam chain need to be installed to set everything at TDC and test for leaks. Or will just fully assembled head will do with camshaft and sprokets?

[slonny]

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:

Any ideas what it may be?

[05c50]

I may have missed it, but have you done anything to the carbs? When you pulled the plugs, did they all look the same or does the one that's misfiring look heavily fouled or possibly completely dry? It's possible that the idle circuit is restricted and causing the cylinder to misfire. The condition of the plug should give you an indication of what's going on.

...........Paul

[slonny]

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.

[05c50]

I may not have made myself clear. I asked about the plugs to see if the misfiring cylinder showed an overly rich or lean condition. It sounds like you've eliminated an electrical issue, but from your last comment, it sounds like you have a problem with the carb. When you cleaned the carbs, did you remove the jet holder and main nozzle to inspect/clean the cross drilled passages?

......Paul

[Pat S.]

My $0.02 is that it is either a vacuum leak on #1 or a carb problem also. I've used a unlit propane torch to find vacuum leaks and lean misfires with a running engine. You can put a length of hose over the tip to reach hard to get at areas. Turn the valve open approx. 1/2 way but do not light torch and apply gas around intake manifold a carb and lastly at the carb intake throat while it's running. If the cylinder comes back to life you've found your problem.