Weber selection and initial jet tuning

[240260280]

You can pull each plug one-at-a-time while idling to see which cylinder is at issue.

[blodi]

Pulled each plug wire...no change. Which I understand to be good news as it would maybe indicate rod knock? You really hear it mostly on decel after reving. Sound like it's coming from the top end. Pulled the valve cover and nothing obvious...I'll check valve lash. No signs of exhaust leak around the header that I could find either. 

[240260280]

I did a search and one possible cause could be timing chain tensioner not taking up slack and the noise would be chain slap. You may be able to replicate by revving a warmed engine by hand.
 

For valve noise: You can pull plugs and use a remote starter to turn over engine. While it turns, a mechanics stethoscope can be used to hear valve noises.

 

At high rpm deceleration, there is considerable vacuum pull in the cylinders but it should not rattle anything.

[240260280]

 

Just a thought:

 

I think the characteristic shape we see with all E Tubes where the richness over-shoots on launch may be due to the squirt from the accelerator system.

The area in the dashed box below is what I refer too. I think the liquid fuel squirt is too much and it over enriches the A/F ratio. 

 

 

I also think it continues for a while as the liquid will hit the manifold walls and will take time to evaporate and become consumed. (See the MP4 video below)

 

Weber 40 DCOE acceleration jet.mp4

 

To test my hypothesis, once your valve train is sorted and you do some F2 runs. I would be grateful if your could  do a  run with the spill jet (item 12 in drawing below) blocked by metric bolts. This will turn off the accelerator circuit and hopefully remove the rich dip on start.

 

Thanks!

 

 

 

 

 

Edited October 10, 2017 by 240260280

[blodi]

Will do! Bare with me here over the next few weeks. Going to be out of town and then am moving over the next two week after that. 

I checked a few valves last night and they are all tight .008 and .010 respectively as I set them. The chain did have a good amount of slack in it on the drivers side, but then I realized that was because I had moved the cam gear by hand. I turned it over a few times with the starter and the slack seemed tight/good. I got some new plugs too that I'll throw in for good measure. Might be next week til I can go through every valve thoroughly. Really hoping it's not a rod knock. Seemed to start randomly as I was driving into work last week out of nowhere, hadn't done any pulls or anything. 

 

 

Edited October 10, 2017 by blodi

[240260280]

lNo need to rush.  Life is more important.

 

I read the Tom Monroe Engine Rebuild book's section on engine noise and it mentions rod knock can occur on deceleration.  If the noise is still there, it may be best to drop the pan and see what is going on.  A first step would be to  do an oil change and filter the oil  to see if there are bearing fragments.

[blodi]

I re adjusted the valves to .010 and .012 and put in new plugs.....no change.  Noise also now seems to happen at idle sometimes and during constant RPM when I hold the throttle open.  But, the noise does seem to be a bit intermittent.  I'm suspecting maybe exhaust leak (I can't see signs of one) or maybe an oil spray bar issue? The bar is intact and no obvious blockage just from a visual inspection. I also hooked up my timing light and the noise is certainly not a fast as the timing light flashes. 

Probably won't have time to dive much more into it for a couple weeks here. I'm afraid I may not then get much more done on the tuning either before winter sets in. We will see. 

 

[240260280]

Good luck.  Sometimes you can feel for the blast from an exhaust leak or see  a build up of soot.

[blodi]

Just an update...kinda...the car has pretty much been parked for the last month and half as I've been dealing with moving and new house projects. I did take it for a short drive a few days ago..the engine was doing the tapping intermittently for a little while then it went away completely after a while.  So, not really sure what it was/is.  Car was running great though. Not sure if I'll have chance to drive it again this season to do anymore tuning/tapping investigation...snow will be upon us soon   Just wanted to make sure everyone knew i hadn't forgotten the thread...but it may go dormant for a while here. 

[240260280]

Thanks for the update!  I'm glad the tapping has gone. We can pick it up in the spring.

I have been reading a great deal about  DCOE's in the last month and it is possible the rich at high rpms could be due to the acceleration jet leaking. We can look into it in the spring.

I should have my SK's running then and can work in parallel.

Merry Christmas!

 

[Patcon]

Be sure all the throttle plate screws are still in place...

[240260280]

FYI what Passini says about Pump Jet leaking at WOT (btw Keith Franke claimed this did not happen). It is why I tune main circuit with bleed/spill jet's blocked.

• A very large pump jet can enrichen the top-end mixture to such an extent that a larger air corrector may be needed in order to prevent over-richness. This may, of course, in certain contexts achieve the desired mixture slope but, on the whole, large pump jets making up for the main system weakening off too much in the top range cannot be recommended.
   
• The pump jet is also in operation during full-throttle running. There being sufficient depression at high revs and a wide throttle opening to lift the weight holding down the delivery valve ball. Once the flow has been initiated, only the weight returning the ball to its seat will stop it entirely because the pump jet orifice lies below the fuel level in the float chamber.  This is one important reason for not mounting DCOE carburettors with the trumpets tilted upwards more than about five degrees, or the siphon effect will overcome the weight and the flow will continue in the form of a dribble which will enrichen the mixture detrimentally.
   
• In addition to the depression in the venturi, high gas speeds past the pump jet are in themselves capable of drawing fuel because of the streaming effect of the low-pressure area which forms downstream of an obstruction (e.g. pump jet protrusion into the throat). The very mixed blessing conferred by the fact that there are, in most cases, several combinations of main jet, air jet, emulsion tube, nozzle, and pump jet (leaking) that will all produce the same flat-out (WOT) maximum power.
   
• None of these three items (rod length, discharge bleed, spring) have any effect on the delivery from the pump jet at full chat (WOT) when it is acting as a high-speed fuel bleed (leaking). 
   
• Figure 5 shows that the pump jet is also in operation, there being sufficient depression at high r.p.m. and a wide throttle opening to lift the weight holding down the delivery valve ball. Once flowing, only the weight returning the ball to its seat will stop the flow entirely, because the pump jet orifice lies below the fuel level in the float chamber. This is one important reason for not mounting the DCOE carburettors with the trumpets tilted upwards more than about five degrees (absolute max: 7degrees), or the siphon will overcome the weight and the flow will continue in the form of a dribble and enrichen the mixture detrimentally.
   
• The pump jet is the only one that delivers fuel in liquid form and not as an emulsion. So, unless the gas velocity is sufficiently high there will be little break-up of the stream. High gas speeds past the pump jet in themselves are capable of drawing fuel due to a 'streaming' effect of a low pressure area downstream of an obstruction.