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Hole in gas tank should have read hole in petrol tank CAP.lots of people have problems with these old beasts going onto one cylinder .for some reason it's very often the left that drops out.
 

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Discussion Starter #62
Just a shot in the dark I always open both Petcocks do you? have you made sure hole in gas tank isn't blocked
My mind went to that earlier. In fact, one of the petcocks was partially blocked due to a swelled seal. I replaced both petcocks as a matter of course with the better UK ones a few weeks back. Yup always open both.

stay-up floats must sit in a different position to give a similar fuel level in the bowl
Thanks Stuart. I think I found a good direction for doing this: GME - Setting AMAL Fuel LevelChains
 

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Discussion Starter #64
Hi Jonner - Someone is loaning me their (working) premier AMAL carb to try out. This should definitively tell me whether carb-related or not. Should arrive in the mail tomorrow, and will try it out this week. Will report back soon.
 

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Hello Neal well that's a brilliant way of eliminating the carbs as source of problem .or confirmation that they are .make sure you set the carbs up re synchronizing throttle slides . can't wait to hear result good luck.
 

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Discussion Starter #66
Tried out the premiers today. Sadly did not make any difference. Exact same issue: cylinder only fires when carb flooded or throttle cracked >50% open. This is consistent with the original carbs, so in my mind this finally rules out carb as culprit. I suppose I have to start tearing down the engine since must be mechanical. Just not sure what else it could be.
 

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Hi Neal, A few thoughts before you do tear down. Sounds like an air leak on intake.

Remove intake valve cover. Measure the valve spring movement comparing right & left intake valves. A ruler is close enough. Don't need to split hairs. Next put right rocker set to where you would check valve clearance. Take a screw driver & gently pry the valve spring cap forward. The idea is feel the valve stem play. This does no damage if done gently. You'll readily see/feel the play. Memorize it. Play can happen sideways also, but not so much. So check sideways too.

Now rotate motor & check the left intake stem play. Are they similar or even just close. Worn intake guide will leak more air than good side. It will be very worn if this is problem. They have a fair amount of play on cold motor. The idea is compare left to right sides.

Also look at valve guide through spring to see if it wiggles in head. It should not wiggle at all.

I've seen shocking amount of wear on guides with no oil consumption or smoke.

If that seems ok, remove intake manifold & look inside manifold for any odd things like a hole to the atmosphere or inside head. Often if leak is inside head you'll see oil streak coming from it.

Since you add extra fuel & it makes cyl fire you have lean mixture. Carb has proven good, so unwanted air is getting in. You need to find the source of leak. Obviously easier said than done.

On tear down, be very careful on inspection. Seen many tear downs as a guess & root problem was NOT FOUND & corrected. Motor had same problem after overhaul.

We ran into leaky intake gaskets, cracked intake manifold often at work. We had blanking plate for air intake. Had smoke generator machine & injected smoke through fitting in blanking plate. This smoke test is common in industry for finding leaks in evap systems also.

You could fashion a blanking plate even out of wood. Turn motor to close valve. Blow air into hole in wood, even with your mouth & see if it holds or not. Epoxy 1/4" tube into blanking plate.

You may need to compare the bad side to the good side. If you hear/feel air coming out spark plug hole, the intake valve is leaking. From your compression test it should be good though. You should not be able to blow air with you mouth through intake valve seat if good.

Don't expect workshop mechanic to have magic. They may not be as knowledgeable as you at this point. You've been through a lot. On odd problems, you sometimes arrive at what it is by what it isn't. Sounds to me like you are down to finding source(s) of air leak.

Cylinder leak down test would be good before tear down also. But you need compressor & test set. Run motor if possible, just before leak down test. Best to warm motor before compression test also. Several kicks to get gauge as high as it will go is normal Count kicks left to right. Hold full throttle during kicking.

You've been through the wringer on this. Good luck!!

Don
 

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Hey Neal now you know it's not carbs and you have reasonable compression .when I've had leak on inlet engine just raced a bit on side sucking air.why not switch your attention to spark try replacing plugs then leads and then coils .I think you can get good spark plugs out but they can break down under compression. Still don't think it's mechanical.
 

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Hey Neal now you know it's not carbs and you have reasonable compression .when I've had leak on inlet engine just raced a bit on side sucking air.why not switch your attention to spark try re
Hi first post for me. I would be investigating the ignition, I ran into exactly this issue, albeit on a Honda a few years ago, and only turned to the ignition as a last resort. The cylinder had good compression and would fire if I injected fuel into the inlet but would not fire without additional fuel. Turned out to be faulty HT Coil,it seems the spark was too weak to fire the normal mixture under compression and would only fire a mixture I when I enrichened it. Changed HT coil for that cylinder and it fixed it. Not saying this is your fault but it doesn't sound mechanical so its either an air leak or ignition fault
 

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Hi first post for me. I would be investigating the ignition, I ran into exactly this issue, albeit on a Honda a few years ago, and only turned to the ignition as a last resort. The cylinder had good compression and would fire if I injected fuel into the inlet but would not fire without additional fuel. Turned out to be faulty HT Coil,it seems the spark was too weak to fire the normal mixture under compression and would only fire a mixture I when I enrichened it. Changed HT coil for that cylinder and it fixed it. Not saying this is your fault but it doesn't sound mechanical so its either an air leak or ignition fault
Sensible comment Mark.we all wait with baited breath to hear what has caused all this headache for Neal. RR
 
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