misfire under heavy load

M

Majicmark

New Member
I have got a problem with my 2200sc (P6) 1976 & could do with some input as i am running out of ideas.
The car starts & runs fine most of the time & is usually fairly economical. Always runs on all cylinders & idles fine does not pink or lack power until it is under heavy load going up Cornish hills (ie steep long drags). The engine then seems to miss but not an obvious 1 cylinder, it seems to be across all the range & power starts to drop off & fuel economy decreases drastically (nearly to the point of watching the gauge move). Once you clear the hill & the car is running on the flat it clears & runs smooth again. I have checked valve clearances & i have a couple of tight ones I.E 6 thou but good compression all around 200 psi. I have changed the points, condenser, rotor arm, coil & HT leads all to no avail. I checked the feed to coil 12v cranking & 9v when running (ballast?). I re-changed all above from a good known vehicle to be sure & still fault happens. The ignition timing is set to 4 degrees before TDC (as per book) but do i need to alter this more to compensate for unleaded fuel?. It feels more like a timing fault than a fuel fault but i am open to any ideas. The distributor is the same as my minor one with adjustable micrometer on end of vac unit & i wonder if this is correct ?. The timing chain is good & no noise when running. Any input would be appreciated.
Thanks Mark
 
Majicmark said:
The ignition timing is set to 4 degrees before TDC (as per book) but do i need to alter this more to compensate for unleaded fuel?. The distributor is the same as my minor one with adjustable micrometer on end of vac unit & i wonder if this is correct ?.
Click to expand...

Book timing is actually 8°BTDC for all the 2200's but it may have been retarded to eliminate pinking. The distributor should have the vernier adjustment. As for the problem itself, you don't mention having replaced the distributor cap, and if it's a new condenser it could be faulty even though its new. If fuel consumption goes up when you get the problem then it's unlikely to be fuel starvation, and it's unlikely to flood just because it's put under load, so it does sound like something electrical breaking down. I'd go back to the beginning and start again as you're probably missing something along the way, it's just a bit difficult to think what it could be given the list of things you've changed. If it's running a ballasted system, are you using a 9v or a 12v coil?
It should be a 9v, if you're using a 12v then that could be the problem.
 
I had a similar problem with mine. I believe it was down to a poor quality modern rotor arm. The black type with the riveted contact are the problem, the rivet works loose and the spark brakes down under load. I swapped to the red type when I upgraded to electronic ignition and haven't had the problem since.
 
Thanks for replies . Should have said I changed cap aswell & am running new ballast ( old stock) coil (tried two different ones)& also rivet type & bonded Lucas rotor arms transferred known good one from my minor & still does it . I had timing set at 8 degrees yesterday which made running worse which also suggested timing to me . I read via Internet that low octane fuels may need timing set to TDC have any of you heard of this? . Also export Market cars being set to 4 degrees ATDC. Although mine is home Market so can't see this would be right . All rotor arms tried have had leading edge to them & I wondered if later straight type may help. Maybe clutching at straws!. I have fitted 5 different condensers including one from the minor which was good on that engine and also checked dist earth wire which is good. I took base plate off & checked fly weights too all look ok.
 
I have two offers here. First is, I wonder if you clocked the significance of Harvey's comment about the coil? The P6 has a ballasted feed to the coil which, as you observe, results in a 9v feed once the engine is running. You need to have the correct coil to go with this, ie a lower internal resistance to the primary. Essentially you are looking at a 9v coil that is over-volted during starting. A normal 12v coil will produce the symptoms you describe because it is producing a very weak spark which can't take the richer mixture when you are under load.

The other offer is the the flyweight springs in the distributor. I recently worked on a 2200 auto which we took to be rolling roaded, and the distributor advance curve was completely wrong due to weak springs. The symptoms of this were not dissimilar to those you report. New springs are available from, amongst others, the Distributor Doctor.

Chris
 
Thanks Chris . I had a new old stock 1.5ohm resisted coil fitted but bought another coil listed for car today to be sure. I tested the supply by earthing neg terminal of coil & when ign on had 6.8 volts on pos terminal. Also I thought about fly weight springs & as I had a spare dist with good springs I tried them too but fault still there. Car goes well & will pull up hill for a while. I have noticed today though that car seems to have an uneven idle when hot so am beginning to wonder if I have a valve or gasket fault. The other thing I havent checked is valve timing so will do tomorrow. Also stripped carb today ( float was set wrong but all clean). I have two tight valve clearances as mentioned before but when I got car had no clearance on one cylinder , this caused misfire from cold & at low revs but cleared higher up the range. Hc s were also sky high then but ok now. As you all say I must be missing something but can't for the life of me see what.
 
Try getting a new set of distributor springs rather than 2nd hand - when installed under tension in the distributor they'll relax with age even if not used. I agree about cam timing, they are known to be sensitive to this. And I think you ought to sort out those two tight clearances, again they are known to be sensitive to these being correct.

After that, I might be persuaded to invest in a rolling road session. Partly this is because getting the ignition timing correct seems to be particularely inportant and, as you've observed, the Rover marks leave rather a lot to be desired! Also that you'd finish up with hard information about what the engine is doing at particular engine speeds - and that can only help future diagnosis.

Once set up correctly, the 4 cylinder is a flyer, so it's worth persevering!

Chris
 
GOT the BUGGER.
I spent the last two days (on & off) stripping out & re setting valve clearances & put it all back together today.The car ran fine for a short while then missed & would not rev clean at all (worse than ever). I re checked timing & showed ok. As i knew all valve clearances etc were ok I thought,go back to basics & found the spot welds on my latest new condensor were breaking away from body. I had the old dist from our minor (which had a good condensor) so thought i would change base plate complete (to include earth wire etc). Anyway once fitted i rechecked the timing & it was 6 degrees ATDC without altering anything from before other than swapping base plate. I then re set timing to 8 degrees BTDC & took it for a run. Went like s**t of a shovel compared to how it had ever run since i got it. After investigating i found the old base plate had worn a trough between the two layers & therefore the advance/retard mech was not getting full travel & more importantly not returning all the way. Anyway im waffling now but suffice to say problem well & truly solved & thanks for all your inputs, sometimes you just need to bounce ideas about to make sure you are not loosing it (my old college tutor used to say disappearing up our own exhaust pipes). Thanks all Mark
 
You must log in or register to reply here.
Back
Top