fuel leaking from Weber front and rear trumpets

PostPost by: sprintsoft » Mon Sep 28, 2020 5:48 pm

Hi John,

From what you say I don't think you have a leak problem as such, it seems to be an induction issue.

Do you have results of a compression test, looking to see if cylinders 1 and 4 are dramatically different?

Airbox off, have you tried placing your palm briefly over the 4 trumpets while it's running - looking for different suction - or, if you have something like the Synchrometer airflow meter - check to see if there is any flow inwards on 1 and 4.

Iain
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PostPost by: NWElan » Mon Sep 28, 2020 5:53 pm

I'll need to get an assistant to check the accelerator pump jets though the engine ran fine before I took it out to replace the water pump and the jets have not been touched.

I looked down the carburetor throats and confirmed the throttle valves are opening with the throttle. Plus, the throttle valves for cylinder 2 & 3 are tied to those of 1 & 4.

Thank you for the suggestions.
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PostPost by: NWElan » Mon Sep 28, 2020 6:21 pm

You made the right call Iain. Zero compression in cylinders 1 & 4. 120 and 122 for 2 & 3.

Time to take off the camshaft cover that I so carefully sealed and installed.

Thank you all for the many ideas and suggestions.
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PostPost by: sprintsoft » Mon Sep 28, 2020 7:39 pm

Hi John,

That's an interesting result you have there!

1. Check the cam timing is correct.

2. Check valve clearances, to have no compression its possible there maybe some at reduced or zero clearance.

Good luck
Iain
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PostPost by: NWElan » Mon Sep 28, 2020 7:59 pm

Ken at Dave Bean suggests I may have bent the intake valves on cylinders 1 and 4.

That would explain the lack of compression and fuel being blown back out of the carburetors.
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PostPost by: nmauduit » Mon Sep 28, 2020 8:15 pm

NWElan wrote:Ken at Dave Bean suggests I may have bent the intake valves on cylinders 1 and 4.

That would explain the lack of compression and fuel being blown back out of the carburetors.


that usually does not happen by itself : did you remove the distribution chain or the camshafts? or fully undo the tensioner bolt (which may lead to slip several teeth on the sprockets)? the initial post did not suggest that valve timing had been altered as only carbs were mentionned... yet bending valves is easy to do when dealing with valve timing without sufficient attention to the procedure.
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PostPost by: sprintsoft » Mon Sep 28, 2020 8:31 pm

Bent valves is a possibility but I wouldn't jump to that conclusion just yet, take your time and check the cam timing is spot on.
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PostPost by: MarkDa » Mon Sep 28, 2020 8:44 pm

Glad to hear that you're on the right track.
Compression on 2-3 isn't that high either if you had the throttles open when testing.
Might also be an indication of mis-timing?
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PostPost by: NWElan » Mon Sep 28, 2020 10:02 pm

Yes, I did take out all the valves to decarbon them when I had the head off to change the water pump. I used a bench wire wheel for the job. Are the valve stems soft enough to be bent in the process? The valves went back smoothly into the guides with little play so I believed I didn't need new guides. I also lapped the valves.

I noted the warning not to set the head on extended valves and was careful to support the head only at the ends to leave plenty of clearance below the head when on the bench though I can't rule out accidental hitting the valves 100%.

I will check valve clearances and timing before pulling the head. Can cylinders 2 & 3 still fire if timing is off?
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PostPost by: pharriso » Mon Sep 28, 2020 11:04 pm

Why are you pulling the head? Are you assuming that valves are bent already?

You suspect your cam timing is wrong, so If this were me, I would:
1. Remove cam cover
2. Remove cam sprockets
3. Undo cam caps, so cams lift
4.Set crank at TDC
5. Set Cams for #1 cylinder at TDC on firing stroke
6. Replace cam sprokets, make sure cam timing is good
7. replace cam cover
8. perform compression test (cold, but will give an indication of bent valves if any...)
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PostPost by: MarkDa » Tue Sep 29, 2020 6:59 am

Definitely worth checking clearances again.
Lapping them in may have reduced clearance marginally.
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PostPost by: alan.barker » Tue Sep 29, 2020 7:23 am

Hi John,
i hope the Valve clearances are ok because if n°1 and n°4 are very large it would mean bent which is stopping them to close.
Good luck hoping the Clearances are ok for you.
cheers
Alan
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PostPost by: NWElan » Tue Sep 29, 2020 1:54 pm

I'm pretty sure I've figured out why I have bent valves for cylinders 1 & 4.

I was looking at the photos I took as I was lowering the head onto the engine block for re-assembly. I had the crankshaft at TDC so pistons 1 & 4 were even with the top of the block.

I remember after I had the head installed, I was rotating the camshafts to align them and noticed how hard it was to rotate them at certain points. I reasoned it was due to the cams compressing the valve springs.

Of course it was hard because the valves for cylinders 1 & 4 were hitting the top of the pistons.
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PostPost by: 512BB » Tue Sep 29, 2020 2:20 pm

You might want to consider investing in 4 wooden dowels when you have sorted your valves. You still have to think about what the camshafts are doing, ie what valves they are opening, but it makes it so easy to see where the pistons are at any given time in the cycle.

Leslie
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PostPost by: alan.barker » Tue Sep 29, 2020 3:03 pm

Hi Leslie,
i've never done that but i very much like the idea.
Before i replace Head i always put at TDC.
Before i lift into position i always fit Sprockets tempory and set to TDC marks.
Your idea with the wood dowels certainly avoids silly mistakes.
Cheers
Alan
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