Popping through carb, vibration, loss of power 1.9 DG

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Savara
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Popping through carb, vibration, loss of power 1.9 DG

Post by Savara »

Hi all. After replacing 4 of the lifters in the van they now seem to have quietened down but - another issue. The engine starts well but coughs/pops every 2 seconds or so. Sometimes the noise goes away for 20 seconds then returns. With every cough/pop the engine hesitates. I've tried adjusting the timing, no difference. Swapped carbs, no difference. Any ideas? ..

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Last edited by Savara on 21 Jul 2018, 15:07, edited 1 time in total.
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CJH
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by CJH »

Blow-back through one of the valves, due to an over-tightened tappet?

One thing I found when trying to get my tappet adjustment right is that the tappets need time to expand/relax to their starting point. When you stop the engine, two valves will always be open and under compression from the cam. To adjust the pre-load you have to rotate the crank so that those valves are fully closed, but I believe that it takes time for the tappet to recover from being compressed - especially a new tappet with nice tight tolerances - since the only thing acting to decompress the tappet is a rather puny little spring underneath the plunger.

I found that I was able to get 6 out of the 8 valves done in one go, but the last two required the engine to be rotated so that they weren't under compression, and then left for a few hours to make sure they were fully expanded before adjusting the pre-load.

So, if you've done yours all in one go you may have added the standard 2 turns of pre-load to a nice tight new tappet that was already a little bit compressed, which might mean that valve isn't closing properly.
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by kevtherev »

A simple compression check will confirm a valve thats opening too early
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by Savara »

Thanks for the input. I thought we were definitely on the right track with valves but now I'm baffled. I've adjusted and adjusted these things, they are now super loose, like 1 mm, no preload at all. I adjusted them yesterday and then again this morning. Run engine, plugs out, compression check - all around the 150-160 mark, seemingly perfect. Still the same, no power, shuddering and phut, phut, phut...
I've had the rockers off again, checked push rods. One of the lifters is like super squishy, really easy to push with a screwdriver but this at least expands easily so hard to put the rockers on and leave any pressure on there. What next? Will check the timing again..
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Savara
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by Savara »

What's doing my head in with this issue is that nothing seems to make any difference at all. Whatever I adjust timing, valves, carbs, always the same. Grrr...
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Savara
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by Savara »

Apologies for the double post - I've just had both rockers off again and rebuilt very carefully to ensure push rods are seated in the followers properly. minute adjustments only required if anything to the adjusters as you would expect. Another compression test - all 150 psi or so. Spark plugs in, starts like a dream, no valve train noise just the hesitation and the phut phut phut. If a valve was bent that would mean zero compression surely and at least some awful noises?
What next?? Go to the pub perhaps? Hmmm..

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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by HansT »

Just a shot in the dark, really, but how are your HT leads and spark plugs? Nice light brown on the plug insulators?
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by Savara »

Leads tested ok with multimeter, plugs a beautiful golden brown/grey
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by daysofthunder »

I have exactly the same issue with mine.

Compression test fine , hydraulic followers replaced and rockers set as per manual.

Still Popping back through the carb lack of power and dies under load.

Im in the process of removing the engine now due to lack of confidence and finding oil sheen in the coolant.

Spare engines going in when i have chance.

My engine has a fair amount of end float so thats also a reason for removing it so please do not think you have run out of options ...... keep at it
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Re: Popping through carb, vibration, loss of power 1.9 DG

Post by kevtherev »

How is the brake servo hose?
If the hose is leaking air into the manifold it will cause a major misfire
AGG 2.0L 8V. (Golf GTi MkIII)

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