Rough running C3 engine when hot

[Nigel]

Thanks for super fast replies.
Mechanical advance and vacuum advance appear to be ok and working.
It sounds as though I should carefully pull the distributor and check the drive gear teeth.
I know the engine was refurbished back in 2013 by a genuine car mechanic, this included a re-bore, new pistons, crank re-grind etc and also included an Eldelbrock 2102 Performer Plus cam set.
I don't know if the distributor or the timing chain/gears were re-placed.
The car has done less than 2,000 miles since and been running superb up till now.

 

[Oneball]

When you checked the timing at idle with the strobe did you have the vacuum advance disconnected?

 

[Corvetteville]

Hi Nigel, I recall seeing an episode of Wheeler Dealers, where the distributor drive cog teeth on the end of the shaft were partially worn away, causing rough running. Admittedly it was a C2 but basically the same.

 

[Nigel]

Thanks, I think the vacuum advance was still connected at the time, as otherwise I couldn't get it to idle.
It was running slow at about 500rpm, but I will have another go before launching on anything major.

 

[Forrest Gump]

It’s possible that the harmonic balancer has moved, its not unheard of if old, and would put your tdc mark out. Worth checking before you get too far into timing settings.

Edit….I re-read your comment on how recent the engine build was so unlikely balancer has moved really but perhaps still worth eliminating given everything else you’ve checked.
With all the major parts and work put into your engine I wouldn’t think it skimped on a new timing gear set which are low cost no-brainer. I think it highly unlikely to jump a tooth unless there was major wear.

 

[Oneball]

Thanks, I think the vacuum advance was still connected at the time, as otherwise I couldn't get it to idle.
It was running slow at about 500rpm, but I will have another go before launching on anything major.

If the vac advance makes a difference at idle it’ll be connected below the throttle plate. So your timing idle setting should be done without the vacuum advance connected. For example 5 deg BTDC. It should run at this. Reconnecting the vac advance will then give you more advance at idle so your initial measurement of 20 BTDC probably isn’t too far out from what it should be.

 

[Forrest Gump]

Thanks, I think the vacuum advance was still connected at the time, as otherwise I couldn't get it to idle.
It was running slow at about 500rpm, but I will have another go before launching on anything major.

Oh, that will give you a bucket load of extra advance at idle.

 

[Nigel]

Many Thanks for your further posts.
The harmonic balancer is not something I had considered at all. I will check it out as it could have major implications
on timing settings.
Then, I will re-check the idle without the vacuum advance connected.
Regards
Nigel

 

[johng]

If your timing has been set for max performance then it's likely to be in the range 14 to 18 degrees without vacuum advance. Do you know what it was before you had this problem?

 

[Nigel]

I believe it was set around 6 degrees btc.

 

[Forrest Gump]

Many Thanks for your further posts.
The harmonic balancer is not something I had considered at all. I will check it out as it could have major implications
on timing settings.
Then, I will re-check the idle without the vacuum advance connected.
Regards
Nigel

I don’t think your balancer has moved tbh, it was a thought when you said your timing looked a long way off, but don’t think your timing is necessarily the problem either reading the subsequent posts.
A burnt HT lead will give a miss-fire under acceleration, but may still run fine at idle - have you carefully inspected each lead.

 

[johng]

Just had a thought, doesn't the 81 Quadrajet have an ECM controller? Could it be that something in the electronics is breaking down with heat.

 

[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.

 

[johng]

Did a bit of googling and it seems an 81 did have an ECM controlled carb and distributor. This thread on the US forum suggests the sort of issues if you don't have a functioning ECM and also how to identify if you have an ECM controlled carb and distributor. If that's what you've got then you may have either a faulty ECM or a faulty sensor.
1981 runs without computer / ecm? - CorvetteForum - Chevrolet Corvette Forum Discussion

 

[Nigel]

Hello,
Although the car initially would have had an ECM fitted it no longer has.
The ECM has been removed and now has an earlier carb (from 1979), and distributor fitted as this has the vacuum advance with it.
This set up has been performing brilliantly for the past 18 months from when I bought the car, until now.

 

[Forrest Gump]

I’ve got a spare working HEI distributor you can try if you are anywhere near Essex?
I did have a spare Quadrajet carb too but lent that to someone who was having problems…I must retrieve that.

 

[Nigel]

Thanks for your kind offer, sadly I am not near you, but up in North Wales.

 

[Nigel]

I have carried out a resistance check on all the ht leads.
They are Accel High Temp Super Stock Radio Suppression Core 8mm wire leads.
six were around 12,000 ohms, and two were 6,000 ohms.
I changed the two 6,000 ohm leads for others but made no difference to the running - even when engine was accelerating.

One of the two 6,000 ohm leads had been in contact with an edge of the exhaust manifold and showed some burn mark to the outside surface however
re-routing the lead or even replacing it made no difference to the running.

 

[Oneball]

If I’ve read it right things that are left to check:

Spark plugs

Vacuum leak - manifold gasket or vac pipes, spray some easy start around then see if engine note changes.

Stuck PCV valve

Engine earth.

Dissy cap

Blocked exhaust - does it still have the cat?

 

[Roscobbc]

Just to add even more on the check list (which you have probably done anyway).
Coolant level.
Vacuum test.
Spark plug operation.
Float bowl levels.
A continuously rich mixture on idle snd through transition to main jet can in time foul spark plugs - even though the electrodes may come up a good colour after a run.
Vacuum test itself won't always point directly to a fault but could show some clues.