Rough running C3 engine when hot

[Forrest Gump]

Latest update/progress to date.
Stripped carb and rebuilt with new gaskets, accelerator pump cup seal and fuel shut off valve.
Carb was actually very clean. I guess the paper filter on carb fuel inlet has done a good job -however I renewed the filter.
Decided to check the fuel supply right back to the fuel tank.
Removed sender unit / filter and found tank and filter to be very clean, no significant debris.
Checked fuel lines from tank to pump all still like new.
Decided to replace fuel pump, fitted new Carter pump.
Started car up and although initially seemed better, still has rough running / poor initial acceleration.
Decided to re-look at ignition system.
Removed all spark plugs - all are whitish/grey/very light brown colour, but perhaps if anything indicating running a little lean.
Checked compression on all cylinders when cold and engine turned 4 or 5 times using starter.
All cylinders range from 160 to 175psi.
Re-fit spark plugs / attach strobe light to no 1 spark plug/lead.
Started engine and ignition timing at idle was way out - perhaps 20 degrees before tdc - I should have checked this right at the start of problems !!!!
Checked distributor and clamp bolt was still tight and distributor body still in original position (I know this because I had put white paint on
distributor body/clamp to reference their relationship).
I now suspect the timing chain may have jumped a tooth !!!!
This would support the sudden change in engine running from superb to rough.
I re-set the ignition timing and engine is running better at idle/low revs, but dies on acceleration, still runs rough.
Hence, still thinking timing chain is the problem.
I dont want to launch straight into removing the radiator, water pump/timing cover etc to get at the timing chain without being absolutely sure.
But how else could the ignition timing suddenly jump so far out?
The only other explanation could be if the drive pin for the distributor has sheared, I will check this.
My next plan is to remove the valve cover to expose the valves for no 1 cylinder and check the timing of the opening of no 1 inlet valve.
If the timing chain has jumped a tooth, I would expect this to be well out also.
If it is out, then I guess I have no option but to replace the timing chain/gears.

All in all this is proving to be challenging/frustrating, but it is teaching me a great deal about owning a 40 year old Corvette !!!
Has anyone else experienced a timining chain jumping a tooth?

Both of these comments your notes that I‘ve high-lighted above still sound carb related to me.
So the engine stumbles when you touch the throttle, but eventually (perhaps with just moderate throttle) it starts to improve as the revs rise?
Are you definitely getting a good squirt into the carb from the accelerator pump?

 

[Nigel]

Well after a frustrating few weeks, I hope I have found the culprit to the intermittent rough running.
Two of the ignition leads for cylinders 1 and 3 run behind the tubular exhaust manifold.
These cannot be seen, and you will see from the attached photo they have overheated/partially disintegrated.
It was only after removing these that the damage could be seen - when idling the spark plugs were still firing.
Following an exhange of information with Scott at Corvette Kingdom I was told that my manifolds are afternmarket tubular mainifolds.
I decided to buy some new ignition leads and run these on new guide rails that fit above the engine manifold, thus keeping the leads well clear of
the exhaust manifolds.
These should arrive on Wednesday, together with some new spings/bob weights for the mechanical advance in the distributor.
(I decided to change these, as the old ones appear weak, and the mechanical advance was advancing the ignition right from idle upwards!!!)
I will let you know how she runs after these are fitted.

 

[johng]

It's very common to fit weaker springs in the distributor to improve performance. Typically people aim to have full mechanical advance of 36 degrees (without the vac advance connected) by around 3000rpm. It would be worth checking what you've currently got before changing the springs, just in case you find stiffer springs affect your performance and you want to swap back.

 

[Corvetteville]

Wow! Those leads really are toast. Glad you found, what surely must be the problem.

 

[antijam]

Well after a frustrating few weeks, I hope I have found the culprit to the intermittent rough running.
Two of the ignition leads for cylinders 1 and 3 run behind the tubular exhaust manifold.
These cannot be seen, and you will see from the attached photo they have overheated/partially disintegrated.
It was only after removing these that the damage could be seen - when idling the spark plugs were still firing.
Following an exhange of information with Scott at Corvette Kingdom I was told that my manifolds are afternmarket tubular mainifolds.
I decided to buy some new ignition leads and run these on new guide rails that fit above the engine manifold, thus keeping the leads well clear of
the exhaust manifolds.
These should arrive on Wednesday, together with some new spings/bob weights for the mechanical advance in the distributor.
(I decided to change these, as the old ones appear weak, and the mechanical advance was advancing the ignition right from idle upwards!!!)
I will let you know how she runs after these are fitted.

I run tubular headers on my 350 and run the leads to the plugs from above. Even with this layout the leads do run very close to the headers (although you can at least see them now) and you may find it useful to fit heat resistant boots that cover the leads and slip over the plugs...


Something like these.

 

[Roscobbc]

Basic electrical fundamentals are that electricity flows via the path of least resistance irrespective of whether a standard HT system or high output MSD type system. An HT lead that provides a seemingly good 'spark' for an engine at idle and perhaps at a constant cruise speed may not flow so well when the the engine is under load (like when accelerating) with the spark plug perhaps struggling to ignite an overly rich mixture - even when the HT leads are 100% if the 'spark' can find an easier path (like 'shorting' to a header flange via a perished or burnt spark plug boot) it will, resulting at best a significantly depleted spark to the plug (and 'sharing' to earth at same time) at worst when under load shorting to header as previously mentioned with zero spark.

Nigel - you have experienced exactly what I previously (on an earlier post) happened on my car. I hope you will be using the very best quality HT leads and connectors to help preventing it happen again. For added protection use the heat resistant cloth sleeves perhaps as suggested for added protection. JohnG has made a valid point about whether (or not) to change the distributor springs (and perhaps bob weights) until you know what your current advance curve comprises. If you have a 'dial back' timing light this is very easy. Simply read-off the the timing amount at specific revs (using the 'dial back') adjustment dial on the timing light) and starting from idle rpm plot-off the advance shown on a sheet of paper against the rpm shown on your rev counter. 'Clock' the idle adjustment screw every 250 or 500 rpm from idle up through 1000rpm, 1500rpm up to 3000 rpm or more and you'll have your advance curve. Obviously this will be a bit noisy and a helper would be useful to do it safely and quickly. Let us know the results. The advance curve should start to operate just off idle, say 700 or so rpm and end before 3500 rpm. If you have a vacuum pipe leading from distributor to carb or inlet manifold disconnect and 'plug' the engine connection to stop a vacuum leak (reconnecting when all done)

 

[Nigel]

Thank's for all the very helpful comments/advice.
Latest update
Fitted New plugs, new support guides for ignition leads which allow leads to run above engine, new leads (made up to shortest direct route) and also heat boots to protect leads/caps at spark plug.
The attached pictures show these, and I am very pleased.
I checked briefly the ignition advance with the old bob weights/springs and fitted a new set (the weakest spring set seemed quite a good match)
Set ignition timing at idle (with vacuum advance disconnected/plugged) at around 12 degrees BTDC.
The car is running very, very well indeed.
Possibly still some fine tuning yet - but the car is running well and ready for The Stars and Stripes weekend at Tatton Park.
I am planning to be there on the Saturday 2nd July.

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