Won't rev over 3000 - out of ideas

[warrenz]

Hi

My 73 240 has been off the road for 2 years undergoing restoration, now painted and back together I cannot get it to run properly.

 

It now idles smoothly and will rev to 4K sitting in the garage in neutral. If you trundle along without exceeding 2500 revs it drives OK but when you press the accelerator to the floor there is only a gradual acceleration and some hesitation and when it get to just over 3000 revs it just does not go much higher and the exhaust note becomes very flat. To me it seems like it is not running on all 6 cylinders.

 

Before taking it off the road it ran strong all the way to the 6000+ revs

 

Since putting the body back together I have:-

Filled it with fresh 98 octane premium unleaded fuel

Changed the fuel filter and checked inside the fuel pump

Cleaned and reset the gap on the spark plugs (NGK BP 5ES)

Changed the points and reset the gap and ignition timing. Distributor is a 65A so set timing to 10 BTDC, also tried 17 BTDC before I read label on dist.

Checked drop on piston inside SU carbs, found one would not fully lift due to broken spacer floating around inside, now OK

Reset mixture, wound mixture screw up fully and then backed it off half a turn. Front 3 cylinders the plugs are a nice brown colour, the back 3 the plugs are black. Also tried 2.5 turns as per ZTherapy and is smoother and more responsive low down but all plugs are now black.

 

Since it will rev freely in neutral or with no load I don't suspect a spark problem I am thinking a fuel problem. Before it came off the road the back 3 plugs were always black and the front 3 brown but it revved out fine.

 

I have run out of ideas. Has anyone got any suggestions.

 

Warren

[aegean]

Have you checked fuel pump pressure ?

[warrenz]

No, is there a simple way to do that?

 

Warren

[ZD44TT]

maybe drive it till you reach that cut out point then imediately turn the engine off pull over and check your floats to see if theres any fuel left..........if no fuel left then you'll know it's fuel pressure related.

 

I had a similar problem only mine happend abit high up the rev band........it was a dying fuel pump I found out later on.

[Zeddophile]

Since it will rev freely in neutral or with no load I don't suspect a spark problem I am thinking a fuel problem.

 

In general that usually indicates ignition.  One of the common characteristics of an ignition fault is that its only present under load.  As an example, when I started controlling spark on my range rover, at between 4,000 and 4,500 it would act like it was hitting a rev limiter.  The solution was to increase the minimum coil discharge time by 0.1 milliseconds.  Of course, no matter what the problem seems to present itself as, it can always be something completely different.

 

You've changed the points, and cleaned the plugs, but have you changed the condenser?  This annoying little can has been known to cause all kinds of issues - its about a $5 part, so its cheap to try that.

 

Its odd that the back 3 always run black, there must be a difference in the carbies?  I'd start off with checking the float level in the carbies, and check for any crud floating around in them, make sure the damping fluid in the dashpots is at the correct level, then set the carbies up like so:  Since the front one seems to be burning correctly at half a turn out, try it at one full turn out, then set the back one up so that it has the same amount of vacuum through it as the front one.  Don't get stressed for the moment about the difference in them, just see if having them balanced makes an improvement.

 

Since you say it runs better with the screws set to 2.5 turns, that suggests to me that it is too lean with the set screw at half a turn, but 2.5 may be too far in the other direction.  Hence the suggestion to start with a single turn at the front, then balance the second one off the front.  Even when rebuilt, you may well find that two carbies have different requirements for the mixture screws to turn out the same end result.  I know you say it always used to burn half the plugs differently, but its quite possible that the adjustment has changed far enough out to make it unbalanced.

 

For an example to show you what I mean, my alfa EFI setup ran very well the other week, to the point of extreme traction issues on the wet bitumen.  After a little while though, it fouled number 1 spark plug with fuel.  What I worked out, was though it felt smooth, the intake box I'd made up to feed the throttle bodies was actually starving number 1 of air.  You had to really listen to the note under load to realise that in fact it was actually only running on 3 cylinders, and free revving it you couldn't tell at all.  Removing the box and putting pod filters on each throttle gained a large amount of power, and much better running.

[aegean]

You will need a gauge to test it, but a simple test is the disconnect the fuel line where it enters the first carb and attach a plastic tube into a graduated jar. Turn the key on and measure the amount of fuel that flows over a 30 second period. It should push about 1 Lt.

 

No, is there a simple way to do that?

 

Warren

[pauly_adams]

Hey I had the same problem what type of carbies are the su are they flat tops or round also I set my timing to 7 degrees and presto it now revs to over 6000rpm I also scraped my su s and put on a 350 holley my su s where rooted and it could only rev to 4700rpm and used heaps of fuel so set your timeing to 7degrees before top dead center fixed my prob

 

Cheers pauly

[warrenz]

Hi Pauly, the carbs are the original round top SU's. I can only get the timing back to about 10 BTDC as I run out of adjustment on the distributor. I would have thought I should be able to set it back to 0.

 

Warren

[Zeddophile]

If you can't move the timing back past 0 degrees, or even back past 10, you need to lift your distributor out, and rotate the shaft so that when you put it back in, the drive gear is engaged one tooth back (in reference to direction of rotation) from where it is now.  Can be confusing, but when you do it, think about which way the rotor spins, how rotating the dizzy advances and retards the timing, and from there you should be able to logically follow which way to go to retard the timing.

[warrenz]

Was thinking that the distributor needed to be rotated slightly myself.

 

I checked the vacuum advance on the distributor and it is working. I decided to recheck the gap on the points whilst I was doing this and somehow they had slipped back to about 10 thou from the 20 thou that I set them at. Obviously not tight enough with the securing screws on the points, but screw heads are  soft and I was trying not to destroy them.

 

Reset and then checked timing and was > 20BTDC so reset to lowest setting possible. Test drive was better below 3000 but still this rev limiter effect at 3000. I cannot get the timing  less than about 20 BTDC as I have run out of distributor movement, so need to rotate it.

 

I will have to put this on hold for a few days as I am off on a business trip.

 

Warren

[modular9]

My car gets doughy after 3000 rpm. I tracked it down to the dizzy not advancing (or retarding i never can remember which ) as it should. Electronic dizzy on the way ! Perhaps a re look at the dizzy ? Or check your fuel hookup lines. i had one line that was badly perished and some air was getting into the system after replacing i got a little better performance.

[warrenz]

Problem almost solved.

 

Whilst I was away my Pertronix Electronic Ignition and coil, that I had ordered previously, arrived from the US. So fitted them along with a new distributor cap as the original one was very worn, reset timing and found that the lack of distributor movement was no longer an issue. I also re-adjusted mixture on SU's and car was much better. I then cleaned up another set of less used plugs and now it pulls strongly until about 5500 revs before it starts to mildly pop.

 

So almost there, but at least is drivable in normal traffic. A new set of plugs and leads on the way to complete the ignition update and when I get my carby balance tool back I will set up carby's properly.

 

Thanks for your advice.

 

Warren

[warrenz]

Problem fully solved - case closed.

 

I have now fitted a new rotor in the distributor, new plug leads and coil lead and a brand new set of spark plugs. Revs strongly all the way to 6000 revs before the tacho drops to 0. (need to move one of the wires from the Pertronix ignition to the other side of the ballast resistor.

 

I am pretty sure it was all points related, but since everything else was old and tired I think they all just added to the problem. The plan was always to replace all the ignition components as part of putting it back in the road anyway.

 

So ready for January cruise, hope it will not be 40 degrees that day.

 

Thanks for all the advice.

 

Warren