Mik allen bolt in head.

Hey Guys,

I recently had my '74 commando head rebuilt by a local "Norton Guy". Everything looked great when complete and the new valves, seals, rings and bore really improved compression. I could really feel a difference at both take off and through the power range.

Two days ago while riding the bike died. It was apparent that it was starved for gasoline (so i thought) so yesterday i pulled apart the carb and everything looked great. Kept digging and sure enough when looking at the inlet port, one of the two allen bolts that secure the manifold to the head had come loose and fallen and wedged into the port against the valve guide. He didn't use Locktight when putting the manifold back on the head!!

I was able to get the bolt out, no damage to the bolt, and no visible damage to the valve stem or guide.

She starts right up again but the left side (where the bolt had fallen in) is firing much weaker than the right side. It still fires and there is compression. But the difference between the left side and the right side is night and day. Very weak coming out of the exhaust. To the point that if the right side misses the bike almost dies.

What possible damage should i be looking for into. I'm tempted to just leave it alone and try to get some riding in this summer but would like your opinions on what damage could have occurred that may be worth addressing immediately.

thanks,

Any backfiring? Do a quick compression test, report back

You running a single carb?

concours said:

Any backfiring? Do a quick compression test, report back

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Yes. Backfiring, especially with the throttle 1/2 open or more. I tried to do a compression test with the Harbor Freight compression tester and it's not giving me an accurate read. 60 psi (on both sides) warm.

Single Mikuni VM34.

This has happened to the best of us with or without lock-tight assist. The bolt probably scored the valve seating area. Your pressure numbers are low. Ask your tech to pull and fix the head. new valves or regrind at the minimum.
I now have a modified allen the I use to get those bolts tight as possible.
When this happened to me and my Fastback, I was @ 10 miles from home. Made it back with lots of choke-somehow!
Michael

do a compression test while holding the throttle open - those numbers aren't real

mikegray660 said:

do a compression test while holding the throttle open - those numbers aren't real

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That is with the throttle open. I think the tool itself is not sealing in the spark plug hole well enough to get a proper read.

<I now have a modified allen the I use to get those bolts tight as possible.>

Fill us in here, with a pix if possible. Cannot say ever had a problem here.

Although you you're not going to like it, your best case scenario is replacing that valve that got bumped by the piston.

Worst case will be valve, guide and then of course the seat either re cut or replaced.

The fact that you are attempting to run it is causing damage to the seat so STOP.

I would dare say that checking your valve clearance will show no gap at that valve.

you are correct in he will most likely be removing the head. I would suspect a bent valve and when you check the valve clearance it might be bigger as the bent valve is not going all the way back to the seat.

pete.v said:

Although you you're not going to like it, your best case scenario is replacing that valve that got bumped by the piston.

Worst case will be valve, guide and then of course the seat either re cut or replaced.

The fact that you are attempting to run it is causing damage to the seat so STOP.

I would dare say that checking your valve clearance will show no gap at that valve.

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bill said:

you are correct in he will most likely be removing the head. I would suspect a bent valve and when you check the valve clearance it might be bigger as the bent valve is not going all the way back to the seat.

pete.v said:

Although you you're not going to like it, your best case scenario is replacing that valve that got bumped by the piston.

Worst case will be valve, guide and then of course the seat either re cut or replaced.

The fact that you are attempting to run it is causing damage to the seat so STOP.

I would dare say that checking your valve clearance will show no gap at that valve.

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Yes, that's what I meant.

Compression - 135 psi on the left side (allen bolt side) and 140 psi on the right side. Cold.

Thoughts?

jsouthard said:

Compression - 135 psi on the left side (allen bolt side) and 140 psi on the right side. Cold.

Thoughts?

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I think those are respectable "cold" numbers. They are respectable "hot" numbers as well.

I do not know what damage, if any. that the seat has taken, but I do believe it will improve over time.

I'm lost here. How can the allen bolt that secures the manifold to the head possibly get into the inlet track?
Then, if it is the bolts I am thinking of, Loctite would make them really difficult to remove, they are fiddly as it is.

Hi,
Compression tests are useful, but a cylinder leak-down tester will pin-point the trouble.

GB

Flo said:

I'm lost here. How can the allen bolt that secures the manifold to the head possibly get into the inlet track?
Then, if it is the bolts I am thinking of, Loctite would make them really difficult to remove, they are fiddly as it is.

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The "Mik" the OP mentioned meant it's a single Mikuni, running a 2-1 manifold, it uses two bolts IN the intake passages.

Have you contacted the guy that did the work? If you try to continue riding the bike I think you can forget about him taking any responsibility for what happened and any kind of damage caused by your running the bike. It seems it would be reasonable to expect him to take the head off and check out what's going on, at his expense. Do you have some kind of history with this guy?

Update;

I pulled the head and found a broken pushrod. The left intake rod tip had broken. I did a leak down test and valves are seated well. Installed the new pushrods and she's running like new. Thanks for your help guys!

jsouthard said:

Update;

I pulled the head and found a broken pushrod. The left intake rod tip had broken. I did a leak down test and valves are seated well. Installed the new pushrods and she's running like new. Thanks for your help guys!

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Thanks for closing the loop, good info. Glad it's back up.

Hi Flo

I was thinking the same thing. The allen heads are outside the manifolds.

But then I thought of my single carb Sunbeam/Norvil etc one & that has the two inner fixings tightened through the carb end of the manifold.

Chris