Won't rev above 3K - running short 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

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

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 just about run out of ideas. Has anyone got any suggestions.

Cheers

Warren

[xray]

Dude, can I feel your pain! I'm having similar problems--actually almost identical-- except mine stumbles at around 2500-negligible difference I guess.

The folks at ZTherapy say baseline fuel mixture should be 2 1/1 turns down from full lock (lean). Maybe changing that will help?

What oil is in your piston damper? I currently have ATF but will be going to straight 20-weight (if I can find any) thinking that under load the piston raises too quickly and leans out the mixture too much....

I was running BP6ES plugs and fouling them quite quickly, despite COlortune analysis showing a neutral to lean condition in the combustion chamber...Lean mixture confirmed by raising the piston on the carbs causing the engine to stumble and almost die. This makes me think it's timing or weak spark causing the combustion not to be hot enough to burn off the carbon--I think?!?!

I've also read that significant carbon buildup in the combustion chamber and valves can be a cause of something like this, so I'm weighing the Seafoam treatment. Clearly, I'm at the bottom of the barrel of ideas here too!

I've tried advancing my timing 5-10 degrees thinking it will make a better burn but it just makes my engine run much rougher...have you tried it?

My other thought is that the projected tip of the BP6 plug may get fouled by the incoming fuel mixture...I am planning on locating some B6ES or equivalent plugs to see if this helps, and may go hotter with B5ES

Other than that, I'm considering problems with the coil, theoptical distributor and even cam-to-crankshaft timing problems...good luck on yours and PLEASE let me (us) know what fixes your issues...

Steve

[WarrenZ]

Hi Xray,

I tried both 5ES and 6ES plugs with no difference. The difference I found before taking it off the road was the the 6ES plugs fouled quickly so went for the hotter 5ES plug. This happened when they stopped selling leaded fuel in Australia and you could only but 91 octane unleaded.

I had the timing at 17 BDTC as I did not check the number stamped on the distributor I just read the book and went with 240 without pollution gear, either setting does not fix the problem.

Damper oil is Castrol Everyday Oil which I have been using since 2001 and was OK before even though may not be 20SAE

I will try 2.5 turns and let you know.

Thanks

Warren

[WarrenZ]

Hi Xray,

2.5 turns makes a difference low down as it is more responsive and smoother but the same problem at 3000+ revs. The plugs are now all black, so it is getting too much fuel.

The Haynes manual say 0.5 to 1 turn, so why does ZTherapy say 2.5 unless they are setting the refurbished carbs up with different jets.

Cheers

Warren

[dogma420]

To me, it really sounds like something timing based...

when you were restoring it, was anything done to the motor during this time? I can't tell from the description...

did it run fine, then 2 years later after sitting, and nothing touched on the motor, did it then start running bad?

[WarrenZ]

Nothing was done to motor during the restoration and it was started a moved around during the time and driven to the body shop a couple of times for quotes etc where it ran OK.

When it came back from the paint shop it was running on 5 cylinders and I found 1 plug had died. I took for a short drive after replacing the plug and I think it was OK. Then once it was legally driveable I changed all the fluids and filters before driving on the road.

The only thing I can line up with is that it was after that it started playing up, I did change the fuel filter, coincidence ? The filter is a Valvoline filter with a clear bowl and when revving it in the garage it has ample fuel in it. If it was a starvation problem I would have thought the plugs would not be black and sooty.

Warren

[xray]

Carbon fouling is not always from too much fuel. Retarded timing, cold spark/ bad ignition are other things to consider, especially since your nozzle was only lowered 1/2 turn and it fouled the plugs. BTW, did you also check your float level? If it's too high, it will run rich even with "lean" mixture nut settings....

[EScanlon]

You mention that it will accelerate OK in the garage, but not once it's in gear and under load. One item that can escape notice is the Vacuum Advance Plate. This is the sliding disks underneath the Point Plate inside the Distributor that allow the points to be advanced by the vacuum.

Simple enough to check if it's working properly. Disconnect the Vacuum Hose that is connected to the Vacuum Pot, remove the Distributor Cap and suck on the hose. You should see the Point Plate rotate and once the vacuum is released, rotate back. First movement should be noticeable and smooth, return movement should spring back easily. The other test to do with the Vacuum Pot is to suck on the hose, and then hold it either with your tongue or a finger. If it is leaking or won't hold a vacuum you'll see the point plate return (make sure you're making a good seal on the hose).

FWIW

E

[xray]

I hope I'm not hijacking this thread too badly, seeing as Warren and I both are having similar issues...

Enrique, I have seen your posts regarding the vac advance in prior threads, and taking your advice I inspected, cleaned and regreased the bearings that hold the advance plate. My mechanism has full action at vacuum around 15-20 in (IIRC), holds vacuum without leaking but that doesn't seem to fix my situation....maybe it will fix Warren's though.

Thinking along electrical causes, would a bad ground have a similar effect in this scenario? Or a short in the tachometer loop?

Thanks,

Steve

[3RD STRIKE]

Hey Warren, I have read all the posts on your prob. ann the one thing i havent seen a comment on is the fuel pickup screen in the tank. I had a prob. simular to yours. First off I have a fuel injected l28. So this may not help you. But mine started starving for fuel @2500-3000 rpm,s. When i found the prob. it turned out to be the the fuel pickup strainer in the tank. I took it out & replaced and it worked fine. That is 1prob I had w/fuel sys. The other prob. I had was the exact same situation, I rebuilt the engine and it took me about a year to get it back together. When i got it running I had the same prob. again . I never really figured out what the prob. was that time ,but this is what i did. It had a fuel cell in it but i had took it out an put the factory tank back in while it was tore apart ,so i blew all of the fuel lines out,installed a new fuel cell, and a glass fuel filter in. Ichanged all of this at one time so im not sure what fixed it. I think it was the venting line off of the tank.

[WarrenZ]

Seems the vacuum advance on the distributor 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 I could get. 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 I guess i need to pull the distributor out and rotate one notch and reset timing to correct setting !

Seems to be this is the likely area of the problem but I have to go on a business trip for a few days so will get back to it later in the week.

Warren

[WarrenZ]

Problem almost solved - Whilst I was away my Pertronix ignition and coil were delivered from the US. So I fitted them along with a new distributor cap (as the other one was badly worn) and reset ignition timing (lack of distributor movement no longer an issue) and re-adjusted mixture on SU's to about 1.5 turns and it was better but not totally fixed. I then cleaned up another set of used plugs and it now revs strongly to about 5500 before it starts to pop a little, so almost there. But at least it is now OK to drive in normal traffic.

One thing I did not mention before was that I inadvertently left the ignition switched on for 24 hours, which of course flattened the battery but maybe did not do the coil any good either.

I was pretty sure it was spark related of some sort, so maybe a bit of all of the above. New plugs and leads to come to complete the total make over.

Anyway thanks for all the advice.

Warren