Stumble/flatspot

[Graeme_McKie]

My mum's FC Special has a stumble/flatspot that I am having a bit of trouble rectifying. The engine was rebuilt in 1987 but has not done many miles in that time. It is bored to 138, balanced, runs a mild Watson cam which I dialled in, a set of headers and a sports exhaust. It also has a PCV kit fitted. I recently pulled the head and had it worked for unleaded. The dwell is at 38, timing on the datum, vacuum diaphragm ok, throttle pump working fine, no play in the throttle shaft. Stumble goes if I run a 54 main jet but this makes her run to fat. Standard 138 main is 51. Currently running a 53. Float is spot on. Have tried lowering/raising float. Next is I am going to fit a new condensor. I noticed late last night that the points faces are not lining up. The moving point face is contacting the fixed point face halfway down. It is though the points pivot on thre breaker plate is not true. I am a motor mechanic and have plenty of patience. Would like to make the old grey run smooth. Any ideas from any old grey motor heads. Thankyou.

[craiga]

Is the accelerator pump link in the correct position? I can check 100% when I get in front of the workshop manual but from memory it should be on the outer most hole. Your point that the problem goes away when you add a bigger main jet does seem to point to a fuel issue.

If the link is in the right spot you should remove the accelorator pump mechanism and check the plunger condition. Sometimes they can flip back or not seal correctly, leading to a lack of fuel under acceleration/load. Lube them up with a very light smear of vaso when you install.

cheers,

Craig.

[Graeme McKie]

Throttle pump link must be in the middle hole of the lever on the throttle shaft. This is where I have it.

[fastjbav6]

The centre hole is correct on the acc lever from memory.
Has it got stale fuel?
Do you get a good squirt of fuel down the carby if you pump the accelerator with the engine off?
If not check the acc pump circuit.
If it runs better with a larger main jet it may indicate you may have a vacuum leak or faulty PCV valve.
If it has a PCV kit fitted, i assume it's just like the Red motor PCV system.
PCV valve in the rocker cover and hose at the base of the carby and a non vented oil filler cap.
If this is the case disconnect and plug it, however make sure the crankcase is vented  with the original breather cap and side pipe and test drive it.
If you have an improvement then you may have a faulty PCV valve.
With the engine running, disconnect the PCV valve and the rev's should drop by approx 50 rpm as you place thumb over valve, and you should hear the valve "click".
The engine should run roughly with your thumb off the valve.
When running with the new PCV set up, are you using a non vented oil filler cap?
Hope this helps.

Seb

[Graeme McKie]

The PCV kit fitted is an after market set up made by Dufor. You remove the old draught pipe and fit the formed rubber adaptor on to side plate and fit in your PCV valve. The intake manifold already had a union fitted for the vacuum supply. I am still using the original filler cap. Will this affect anything? I also mentioned that the point  faces did not meet correctly. I went through my spare parts section ( plenty of FC, HR and Commer 'knocker' parts) and found a distributor with points fitted. I noticed that shims were fitted to the pivot to align the point faces and a small Jesus Christ clip fitted to keep it all nice and snug. I qiuckly at work fitted the shims to the top of the points and fitted the clip. Drove her home and she seemed much better. Tomorrow I will pull the distributor out, fit new points and condensor and shim the points to align correctly. Every day is a school day. P.S. Anybody have any contacts re my Commer 'knocker' resto.

[Dr_Terry]

Hi Guys.

You mentioned that you had a mild Watson cam & you are running the ignition timing at the stock setting.

Generally speaking, the longer the cam duration, the more initial timing advance you will need. For a 'mild' cam you will probably need 3 to 5 degrees more initial timing than stock.

Problem is, then you will have too much advance at the top end. This is what is meant by getting your distributor regraphed or recurved.

The initial timing depends on compression ratio & cam size, but the total timing depends on head design. That means for any given engine, like a Holden 6 cylinder for example, the total timing required is always the same (around 36 to 38 degrees) but the timing at idle will depend on cam etc & the curve (or advance rate) is then tailored to suit.

In short you need to have less centrifugal advance in the dissy & more initial timing to give the same final advance as a stocker. The biggger the cam, the more initial required. Try experimenting with a few more degrees timing & listen for pinging, but if your accelerator pump is working properly then it all depends on the timing.

Regards.

Dr Terry.

[andykotzur]

Graeme.  When you pull the dizzy , with the points out , see if the pivit (swivel point on the points plate) is buggered.  Mine was, and it took me a year and lots of frustration to find it.  If the swivel is worn,  its impossible to time correctly because the gap keeps changing
good luck