Slight misfire while cruising

[kcsun]

I have a 1973 CR with fuel injection and a Lumenition ignition system and K&N air filter. I have noticed that when I am "cruising" at about 50mph or 60 in overdrive i get a slight hesitation/misfire. This is with just a slight pressure on the throttle to hold the speed. If I put my foot down the car pulls like a train with no issues and also accelerates from standstill without any issues. I only run 98 fuel and have filled up quite a few times during the summer but still have the same issue. It has a Bosch pump with the separate paper fuel filter which I have changed. I then changed the spark plugs but still have the same problem. A friend lent me his Magnecor leads, dizzy cap and rotor arm and still the same problem. When I start the car it needs the choke out but I can fully return it after only a mile (it does go fully back). When I came home after a show last weekend (about 60 miles) my wife was in the drive as I turned the car round and she said that it stunk of fuel (running rich?)

I have attached photos of the new spark plugs which have now done about 500 miles

Any ideas??

kc

 

kc

Edited August 31, 2024 by kcsun

[John L]

Which plugs are you using?  They do look a bit like they are running rich.

It looks like the throttle bodies could do with a balance perhaps, and it may also be injectors leaking or not atomising correctly.

Do a suck test on the vacuum pipe to the MU, it should hold a vacuum, if this is leaking the MU will run rich.

May need a bit more advance perhaps, tell us what you have now.

John

[kcsun]

Hi John they are Champion N9YC, bought and fitted in March

I was thinking of the injectors, may get them out out and test in a bottle. I would have thought it would show up with one spark plug looking totally different, although No3 does look different

Bit late tonight to go out sucking pipes , but will have a look/suck tomorrow

kc

Edited August 31, 2024 by kcsun

[Mike C]

I agree, you're running really rich. I personally use NGK BUR6ET spark plugs because  the 3 electrode design will fire even when wet with fuel.

[Mk2 Chopper]

The petrol injection fuelling amount is controlled by vacuum, so the first things to do are check/ adjust the valves (tappets), sync the throttle butterflies, check all the rubber pipes for leaks on the induction side, on a CR there are twice the balance pipes between the throttle bodies than a CP. Then check and adjust the ignition, spark plug gap, timing and so on. 

If after you still run rich, you can do checks on the fuel pressure, compression check, vacuum reading going to the MU etc. If everything checks out, you might need to get your MU recalibrated.

Gareth 

 

[Mike C]

If it helps my engine must have 10 inches Hg vacuum at idle to run correctly. I measure vacuum with a gauge on the brake booster connection. 

[astontr6]

8 hours ago, Mk2 Chopper said:

The petrol injection fuelling amount is controlled by vacuum, so the first things to do are check/ adjust the valves (tappets), sync the throttle butterflies, check all the rubber pipes for leaks on the induction side, on a CR there are twice the balance pipes between the throttle bodies than a CP. Then check and adjust the ignition, spark plug gap, timing and so on. 

If after you still run rich, you can do checks on the fuel pressure, compression check, vacuum reading going to the MU etc. If everything checks out, you might need to get your MU recalibrated.

Gareth 

 

+1 Bruce. I only use grade 5 NGK and have done for years, without any ill affects.