Rough running C3 engine when hot

N

Nigel

CCCUK Member
Hello,
Suddenly my 1981 C3 has developed a rough running engine when it gets warm.
It happened as I set off one morning, all fine for first mile, then slowed down to go around roundabout and when I accelerated away the engine seemed to be running on only about four cylinders instead of eight.
It ran better at higher revs but same problem after slowed down to near standstill and accelerating away.
Initial thought was fuel problem, having occured on roundabout, or ignition coil?
I have changed the fuel filter in the Rochester quadrajet carb inlet, changed ignition coil and ignition terminal block/condenser/module in distributor, but no change.
I have removed all spark plugs and they are a good colour.
I have carried out a resistance check on ignition leads, six were around 11K ohms, two were around 6K.
I have just changed the two leads which had lower readings, but still no improvement.
I am not sure what to look at next?
Has anyone else had similar problems? or can suggest what to try next?
 
antijam

antijam

CCCUK Member
Maybe an accelerator pump problem - possibly if the rubber is original the cup may have deteriorated due to ethanol attack?
 
Oneball

Oneball

CCCUK Member
I’d start by pulling each HT lead off in turn. That way you can determine if you have a cylinder specific misfire.
 
62 C1

62 C1

CCCUK Member
When it cools down again does it go back to smooth running? Trying to isolate whether it is heat coincident or something has failed in hot and cold situations. You said you replaced the coil - was it with a new one and have you checked the low and high resistance across the coil with a multi-meter? The definitely can suffer heat problems.
 
Forrest Gump

Forrest Gump

CCCUK regional rep
Did you notice any smoke from the exhausts at all when accelerating from slower speeds? If it begins to clear and run better with more revs I would be thinking it is overfuelling at lower revs so maybe debris in the float needle valve. That would come on suddenly. Coil and ignition problems usually get worse with more revs as the demand on them increases.
 
N

Nigel

CCCUK Member
Hello,
Many thanks for the various replies.
It is a Rochester Quadrajet Carb.
The coil was replaced with new from Corvette Kingdom, I haven't tested the resistance accross the coil as yet.
When the engine cools down and is started again from cold it starts, runs, accelerates perfectly, then as stated after a mile or so
after slowing right down and then accelerating the engine is very rough to begin with, improves once it has gained some revs, but not running right.
I haven't noticed any smoke from exhaust on acceleration.
I am not running with any choke at all.
I have removed all plugs and they are all appear consistent and good grey/light brown colour.
I have checked resistance on plug leads, they are 8mm wire and six were around 11k ohms, two showed 6k ohms. I have changed the two lower value leads for others and it made no difference.
I have noticed once on starting engine again after a run some smoke from exhaust on initial start up, cleared within a few seconds.
My suspicion from when the problem started is fuel related, especially as it came on after going quite quickly around a tight roundabout, as though
debrit may have moved/lodged somewhere. But I am puzzled as to why it runs so well initially from cold.
 
Oneball

Oneball

CCCUK Member
When you say you’re not running any choke at all does it work when it’s cold? If not I’d say stuck float, so carb is running rich which it likes when cold but not when warm. If the choke does work cold I’d say blocked jet, so it’s running lean but the choke compensates a bit when cold.
 
Roscobbc

Roscobbc

Moderator
Current ethanol based fuel can potentially create both carb, fuel pump and fuel line issues. Bottom line is that older carbs, fuel pumps and fuel lines that, perhaps have been on the car many years will be adversely affected by it. Rubber parts (fuel tube, fuel pump diaphragms) will break down and become porous. Current, known quality, branded replacement parts should be fine with Ethanol based fuels. I've had carb gaskets (even Holly ones) break-up with tiny particles jamming float needles open (causing flooding) also had fuel pumps braking down and working fine when on a light load (like cruising, give the car some serious beans and the engine dies due to fuel starvation due to failing fuel pump diaphragm) - have you checked your fuel pump?
Is your '81 Cross Fire? - it might be a Quadrajet but is it an 'electronic' version.........either will have their own specific issues other than above.
 
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N

Nigel

CCCUK Member
Hello, No its not a crossfire, the carb is not electronic, there is no electronic engine management system.
The car was rebuilt from chassis up in 2013 and all metal fuel lines, rubber hoses etc replaced with new.
But I am not sure if the carb was refurbished, I will need to check the history file.
 
Roscobbc

Roscobbc

Moderator
Nigel said:
Hello, No its not a crossfire, the carb is not electronic, there is no electronic engine management system.
The car was rebuilt from chassis up in 2013 and all metal fuel lines, rubber hoses etc replaced with new.
But I am not sure if the carb was refurbished, I will need to check the history file.
Click to expand...
It was about that time when there was a lot of cheap sub-standard rubber hosing on the market. For example braided stainless fuel piping looke really cool but perished within 6 months - problems was that with the stainless braiding you coundn't see the degrading of the rubber - if you were lucky you might see the petrol seeping out of the braiding.......unlucky? perhaps a fire?
 
62 C1

62 C1

CCCUK Member
Does definitely sound carb/fuel related if it is a new coil. If you do test the resistance at the coil if you remove it a put the bottom section in very hot water for a few mins you can check for heat related changes. Sounds like cleaning and setting up the carb is where to start. Does the car smell heavily of fuel before you start. When I had bad fuel lines it stank especially with 99 Octane in it. Are you running a mechanical or electric fuel pump? On a non-Corvette I have had an electrical fuel pump start to fail when it heated up due to a cracked magnet that was created friction with the armature and then ran very unevenly.
 
N

Nigel

CCCUK Member
Hello,
No detectable smell of fuel before starting or afterwards.
It is a mechanical fuel pump, I have disconnected the fuel output line at the carb and turned engine over with starter motor.
It seems to be pushing a good volume of fuel but I have not checked the pressure as yet.
There is no sign of petrol leaking from drain holes in pump body.
 
N

Nigel

CCCUK Member
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?
 
Corvetteville

Corvetteville

CCCUK Member
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.
 
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