Engine not running right

[johng]

I was out for a drive today and on the way home the engine was not running as normal. It ticks over at 700rpm as usual and goes fine if I floor it, but in between it feels very underpowered. It will accelerate but much more slowly than normal. I do have fuel squirting out the accelerator pump in both barrels, although I'm not sure if it's the correct amount. Also, not sure if it's related but my rev counter is jumping about at mid range revs (2000 to 2500). Any ideas where to start looking?

 

[Oneball]

Accel pumps only affect things as you press the pedal. Not once you’re at full throttle.

Is your rev counter an electronic one? If so you have an electrical fault in the ignition system. We not be HT unless you’ve got some weird fault where the sparks earthing thought the LT side.

Points, condenser, 12v supply. Would be my first port of call. If their fine disconnect the rev counter feed in case it has an internal short.

 

[johng]

Accel pumps only affect things as you press the pedal. Not once you’re at full throttle.

Is your rev counter an electronic one? If so you have an electrical fault in the ignition system. We not be HT unless you’ve got some weird fault where the sparks earthing thought the LT side.

Points, condenser, 12v supply. Would be my first port of call. If their fine disconnect the rev counter feed in case it has an internal short.

That's why I thought about the accel pump, as I don't have a problem at full throttle. If it's the ignition system it seems strange that it runs fine when I floor the throttle. The car seems to accelerate through the rev range fine at full throttle but not at part throttle
I have got an HEI electronic distributor and electronic rev counter, I will try disconnecting the rev counter to see what that does.

 

[Forrest Gump]

Could it be your choke is not opening up properly?

 

[johng]

No the choke is fully open.

 

[Oneball]

The accel pump works when the throttle position changes (increases). It doesn’t matter on the throttle position. If you press the pedal half way down and its more than just a second or so stumble it’s not the accel pump.

Full throttle often hides misfires. They’re still there but you don’t notice as easily.

Could also be a vacuum leak.

 

[johng]

Had a play this afternoon.
Started the car from cold and problem was still there (it only occurred yesterday after I'd driven about 30 miles)
Disconnected the rev counter and it made no difference
Checked the timing which was fine and both the centrifugal and vacuum advance are working normally
Removed the coil and thought I'd found the problem, the contact on the bottom was rusty

Cleaned this up and it made no difference!
The coil primary and secondary resistances are in the ranges that I've found on the internet.
I'm still getting good vacuum at tickover and I can't spot any vacuum leaks
I think it might be gradually getting worse, because now it still ticks over fine, but when you try to accelerate it bogs down worse than before then very slowly picks up speed (the rev counter is still jumping about in the 2000 to 2500 rpm range). When you get to around 3000rpm it all clears and pulls strongly up to the red line and the rev counter becomes smooth again.
I've ordered a coil, ignition control module, distributor cap and rotor from Rockauto which hopefully will be here by Monday.
If that doesn't fix it I guess I'll be stripping down the carb
Oh and forgot to say the engine revs up fine when the car is stationary, it just has a problem when it's under load.

 

[Forrest Gump]

If it was ignition relation I would think it would get worse with more revs, not improve as you're finding?
So it's not miss-firing as such, but more like a bad stumble?
My guess is its not getting enough fuel, the accelerator pump should give really strong squirts and right through the throttle opening range, they put a lot of fuel in. I've got a spare HEI and a spare Quadrajet if you wanted to try either?

 

[johng]

Thanks Andy, it sort of seems as if the idle jets and main jets are fine but something in between isn't, but then again I believe what seems like a carb problem can quite often be ignition. Also it's easier to change ignition parts than strip and rebuild a quadrajet! If the new parts don't fix the problem then I might well pop over and borrow your quadrajet to try and get the car running in time for Helmingham Hall, if that's okay with you.

 

[teamzr1]

Let me wonder a bit on this

Be interesting to see while driving the car

1. attach fuel pressure gauge so can be seen while driving
2. same as to engine vacuum
3. same for voltage output
4. test gas for water and Ethanol level
5. at the HEI, voltage and ground quality

 

[Roscobbc]

Considered making up a mild detergent mix in a squeegy bottle (or perhaps WD 40) and squirt it around the carb when its 'struggling' to see if there's perhaps a vacuum leak somewhere on the carb?

 

[johng]

Got to say Ross I haven't considered that, as it only struggles when I'm driving the car and trying to accelerate! Now if you'd like to stand up in the passenger seat and lean over into the engine compartment whilst I'm driving Having said that I think I've heard you can check for leaks by spraying brake cleaner or something flammable around potential leak areas to see if that increases rpm.

 

[Roscobbc]

Got to say Ross I haven't considered that, as it only struggles when I'm driving the car and trying to accelerate! Now if you'd like to stand up in the passenger seat and lean over into the engine compartment whilst I'm driving Having said that I think I've heard you can check for leaks by spraying brake cleaner or something flammable around potential leak areas to see if that increases rpm.

This isn't likely to be the same issue but I recall some years ago when I first had my Vette a slight missfire developed under acceleration.......not enough to actually stop a cylinder firing but noticeable in slightly reduced acceleration (and of course you could hear it). One specific plug would be sooty and wet following inspection. It wasn't enough to stop use of the car......just irritating. I decided to upgrade the ignition system and installed a MSD 'box' and coil. The car certainly 'liked' that and would run much smoother at lower rpm........but the issue still returned.......so give it some 'beans' and a slight missfire. I took the car out one evening less the hood. The 'issue' became very apparent, so.......under light acceleration......no problems. Under heavy acceleration the increased combustion demand of a rich mixture was causing a spark plug boot to arc against the cylinder head and effectively 'share' the spark between the plug and earth. At night without the hood it was quite obvious. Normal light load running didn't prove a problem but the increased HT load with the rich mixture was causing the HT spark to try and take an 'easier' path......in my case 'share' the load with earth. Moral of the story was that electricity whether 12v or HT 'likes' to take the easiest path.......installation of the MSD system didn't change that.......in fact it made it worse as the increased power meant that the 'spark' would now jump a far larger gap than before. Supsequent inspection of the HT lead and plug boot showed a very small burn mark where the lead fits in to the boot......removal of the boot showed much more burning and damage within the cap and lead where the 'spark' had been 'escaping'.
Not for one minute suggesting this is your issue John......simply that you perhaps look for the 'illogical'.

 

[johng]

Some progress. The bits I ordered from Rockauto late on Tuesday evening arrived at lunchtime today. Thought I'd fit just the new coil and distributor cap first, to see if I could narrow down what the problem is. The new coil came with an earth strap that provides an earth connection to the 3 pin plug for the coil (2 pins for the coil and a third for the earth), which it said must be fitted to avoid damaging the coil and ignition module. My old coil didn't have an earth connection at all! Coil fitted with the earth and the problem has changed but not gone away entirely. Now the rev counter no longer jumps about and the car accelerates quicker, but there is still a hesitation when you first hit the throttle. I'll try the new rotor and ignition control module tomorrow.

 

[johng]

Decided not to wait until tomorrow, so fitted the new rotor and ignition control module. Didn't make any difference, but driving about I thought it sounded a bit off beat so thought maybe it's only running on 7. Went home and checked the exhaust manifold temperatures by each cylinder. Numbers 2 and 8 are about half the temperature of all the rest, so maybe i'm running on 6! Think some new plugs and leads might be called for.

 

[johng]

Further update, connected my timing light to each cylinder in turn and they are all firing. Measured the exhaust temps again and realised that the drivers side is running much hotter than the passenger side. I'm beginning to think I have got a carb problem after all, maybe just a jet or passage way blocked in the passenger side of the carb. I've stripped it down and blown through all the passage ways I can see and rebuilt it, but ran out of time to refit to the car. Hopefully tomorrow.

 

[Oneball]

There isn’t really a passenger side of the carb, even with a single plane manifold. If you’ve got a dual plane then there isn’t one at all.

Manifold leak would explain one bank running differently to the other.

 

[johng]

Looking into my manifold it's definitely split in two

 

[Oneball]

You’ve got a dual plane then but the split isn’t left and right bank.

The left side of the split feeds 2 cylinders in the left bank and 2 cylinders in the right bank. The right side of the split does the same.

 

[Oneball]