ChrisR-4A Posted January 7, 2017 Report Share Posted January 7, 2017 lain, try connecting a strobe light to a plug lead to determine if knock is every revolution or every second revolution. This could indicate if its source is valve train or crankshaft. Crankshaft could be timing chain/tensioner otherwise little ends. You say it goes if you blip the throttle, if you just increase the tickover on the screws at what revs does it disappear, 900, 1000 or? Chris Quote Link to post Share on other sites
Hamish Posted January 7, 2017 Report Share Posted January 7, 2017 My first thought, as I think the tap tap seems less frequent than the engine speed would suggest. Could it be the fuel pump. Doesn't that lever operate less frequently ?? Â But then if the noise is the dizzy end? My idea doesn't add up ?!?!? A weird one for those more expert than I. Quote Link to post Share on other sites
Paul Down Posted January 7, 2017 Report Share Posted January 7, 2017 Have you checked the timing chain? Tensioner perhaps? Quote Link to post Share on other sites
Peter Cobbold Posted January 8, 2017 Report Share Posted January 8, 2017 (edited) Iain, Is it spark knock on all cylinders? 800rpm should deliver 400 sparks in 60 seconds, or 6 per second. Thast roughly the knock frequency I can hear in second video ca 00:24 Try richening the mixture or/and retarding the static timing. I reckon this worked much the same way: "Slightest touch on the throttle via linkage and the noise goes.." Â Peter Edited January 8, 2017 by Peter Cobbold Quote Link to post Share on other sites
peejay4A Posted January 8, 2017 Report Share Posted January 8, 2017 Following up on Marcus' post #59 have you checked the lift in each cam lobe. I had a similar noise that I couldn't eliminate by closing the rocker gaps. When I measured the cam lift with a dial gauge I found that cam number one had significantly lower lift than the others. On removing the cam sure enough it was rounded off and had taken the follower with it. Other lobes were also on the way out. This was the cam that was in the engine when I received it back in 2011 which turned out to have been badly rebuilt. Quote Link to post Share on other sites
Rob Salisbury Posted January 8, 2017 Report Share Posted January 8, 2017 I had a noise very similar to that and was convinced it was valve/tappet/cam related, started to strip the engine and found a manky inlet manifold gasket (small hole burnt through underneath) ... got one of the uprated ones from Revingbone and stuck it back together again, ... quiet as a mouse!!, ... well as quiet as a 4A can be on Webers!!. Cheers rob Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 Pete, I haven't measure cam lift directly, but given that all valves have a similar clearance when looking/measuring for binding I don't think its that...  Peter, tried resetting the ignition.......oddly retarding the engine makes it worse! Mixture looks OK. But see postulated problem below and it might make sense.  Neil, I just wonder, we have a new cam and an old dizzy drive gear........so I took it all apart again and there is quit a bit of backlash in the gears, more than spec? I don't know, but it certainly knocks.  See what you think.    An explanation could be,​ when cold the oil is thick the pump therefore drags on the drive gear and helps keep the gears in mesh. As the oil warms up the pump drag is less, the gears chatter, until put under load ( increase revs ) and then they mesh. It would also explain why the tang on the bottom of the shaft that engages with the pump has witness marks that appear quiet shinny on each pickup point..........  Many thanks  Iain Quote Link to post Share on other sites
peejay4A Posted January 8, 2017 Report Share Posted January 8, 2017 Iain, You can't judge whether a lobe has failed just because the rocker gaps are even. Quote Link to post Share on other sites
Guest ntc Posted January 8, 2017 Report Share Posted January 8, 2017 Pete, I haven't measure cam lift directly, but given that all valves have a similar clearance when looking/measuring for binding I don't think its that...  Peter, tried resetting the ignition.......oddly retarding the engine makes it worse! Mixture looks OK. But see postulated problem below and it might make sense.  Neil, I just wonder, we have a new cam and an old dizzy drive gear........so I took it all apart again and there is quit a bit of backlash in the gears, more than spec? I don't know, but it certainly knocks.  See what you think.    An explanation could be,​ when cold the oil is thick the pump therefore drags on the drive gear and helps keep the gears in mesh. As the oil warms up the pump drag is less, the gears chatter, until put under load ( increase revs ) and then they mesh. It would also explain why the tang on the bottom of the shaft that engages with the pump has witness marks that appear quiet shinny on each pickup point..........  Many thanks  Iain Iain Got it in one same sound on the second vid. Quote Link to post Share on other sites
Chilliman Posted January 8, 2017 Report Share Posted January 8, 2017 Well done Iain I think you've got it, the one I've just built was a new cam & original drive gear but when the gear was engaged and seated as you have in the video I didn't have any perceivable movement at all without raising the drive upwards. Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 Iain, You can't judge whether a lobe has failed just because the rocker gaps are even. Pete, agreed but a new cam and new followers and no readjustment required in the first 750 miles makes me think the cam and followers are ok. Well done Iain I think you've got it, the one I've just built was a new cam & original drive gear but when the gear was engaged and seated as you have in the video I didn't have any perceivable movement at all without raising the drive upwards. John I am hoping you and Neil are right.........Now where to get a new dizzy drive? Moss don't list.  Cheers  Iain Quote Link to post Share on other sites
peejay4A Posted January 8, 2017 Report Share Posted January 8, 2017 I should have read back to your first post. Sorry about that. Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 (edited) No probs Pete.....I should have checked the blasted gear meshing way back! The cam and followers was my first thought.......But seems to have been a red herring:-) Iain Edited January 8, 2017 by iain Quote Link to post Share on other sites
Chilliman Posted January 8, 2017 Report Share Posted January 8, 2017 Â Â Now where to get a new dizzy drive? If you get stuck Iain give me a shout as I've got a good quality used unit you can have as a loaner to make sure that it is the problem before you commit the hard earned pennies. My only slight concern is that it's the cam not the drive gear that has the slop in it? Quote Link to post Share on other sites
Guest ntc Posted January 8, 2017 Report Share Posted January 8, 2017 If you get stuck Iain give me a shout as I've got a good quality used unit you can have as a loaner to make sure that it is the problem before you commit the hard earned pennies. My only slight concern is that it's the cam not the drive gear that has the slop in it? +1 and the damage may have already been done ? I hope not. Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 John that's a kind offer, even if it's just offered up to see if it's the drive gear or the cam gear that has gone AWOL. Lest hope it's just the drive gear that has the wear in it.....but it was ok with the old cam :-( Quote Link to post Share on other sites
Drewmotty Posted January 8, 2017 Report Share Posted January 8, 2017 I've got a used spare too if you want to compare a couple. Quote Link to post Share on other sites
Chilliman Posted January 8, 2017 Report Share Posted January 8, 2017 No probs, I know roughly where you are but send me a pm with your address and I'll drop it off, I'm tucked up Monday & Tuesday but will be passing J27 on Wednesday. It might be as well if I drop off a spare good cam at the same time and you could always rig something on the bench if need be to compare all the tolerance before you do any serious spannering! Quote Link to post Share on other sites
Guest ntc Posted January 8, 2017 Report Share Posted January 8, 2017 No probs, I know roughly where you are but send me a pm with your address and I'll drop it off, I'm tucked up Monday & Tuesday but will be passing J27 on Wednesday. It might be as well if I drop off a spare good cam at the same time and you could always rig something on the bench if need be to compare all the tolerance before you do any serious spannering! That is what the club and forum is all about I hope nice one John. Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 Certainly is Neil. I hope we have found the problem. Thanks to all for the input so far.........:-) Â Iain Quote Link to post Share on other sites
Peter Cobbold Posted January 8, 2017 Report Share Posted January 8, 2017 Iain , woudn't that amount of slop have shown up on the strobe? Peter Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 John and Andrew you have Pm's ta  Iain Quote Link to post Share on other sites
iain Posted January 8, 2017 Author Report Share Posted January 8, 2017 Iain , woudn't that amount of slop have shown up on the strobe? Peter Peter  you would have thought , but no it didn't. No variance at all.  Iain Quote Link to post Share on other sites
GT6M Posted January 9, 2017 Report Share Posted January 9, 2017 I cannae see that slak as being the problemo, when its running, the slak will be took up.  slack like that generally shows up wid wide variations in timing, when a lamp is used,t,see the figures ont crank  this did,nt, hence cant see it  M Quote Link to post Share on other sites
Guest ntc Posted January 9, 2017 Report Share Posted January 9, 2017 The drive trys to climb up and will not affect timing light. Quote Link to post Share on other sites
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