Misfiring when hot

Been chasing a ghost for over a year now, still no success.

Symptoms:
- Coolant temp gauge reading very hot after only a few minutes of running, but no other signs of overheating
- After crawling along in low range a short while or flogging it up hills in high range, the engine misfires badly. It will stall if I push the throttle hard, but it'll still idle roughly. Adjusting the mixture (to make it more rich) seemed to make it slightly more tolerant

Parts replaced:
- engine oil
- thermostat
- fan clutch
- water pump
- belts
- hoses
- expansion tank cap
- radiator
- temp sender
- temp gauge
- fuel filter
- air filter

Tinkering thus far:
- rad & cooling jacket flush, forward & reverse
- attempted to rule out air lock; ran without rad cap while on incline to bleed
- attempted to rule out HGF; no bubbles/residue visible
- adjust ignition timing
- balance carbs

I'm not really convinced it's overheating. IR thermometer says the top hose is 82-85C with an 88C thermostat. Bottom hose reads about 75C. All seems spot-on, even tho the gauge reads past the red.

Last week I discovered the timing marks on the craft shaft pulley are off, so finding TDC and getting the timing just right is still a problem.

It's idling too fast. Just fitted a rev counter and it's idling at 1000-1500RPM; the timing is over-advanced (also getting pinging under load), so I'll retard the timing slightly and that should help.

Any ideas greatly appreciated!

Non gen cap leads and rotor?
Incorrectly set up fuel setting on carbs
Are the carbs strombergs or su?

vougese39 said:

Non gen cap leads and rotor?

Click to expand...

I have no idea what you just said. I think it has something to do with the distributor cap leads & rotor. The previous owner replaced the cap leads and I replaced the cap when I first got it (carbon element was all but gone). The rotor is original, as far as I know. What should I be looking for?

Also, there's allow manor of crap deep inside the distributor. The protective shroud below the cap is cracked and it looks like a bird nested there many years ago.

Carbs are SUs - definitely could be the carbs. Actually, I suspect them to be causing some portion of the problem. The engine isn not idling right, either, but I want to retard the timing slightly and then see whether that helps.

when replacing ingnitionparts you need to do all genuine plugs,leads,cap,arm,ht lead to coil and coil, a simple check on carbs is to remove pipe work to them and watch mixture is equal in both and looks right throughout rev range

Thanks James - how do I watch the mixture? If I take off the pipes I can see the carb's pistons moving and compare movement. Is that what you mean?

I just roughly re-balanced the mixture and backed on the timing advance. It idles more normally now, but still a little uneven. I think my pulley markings are FUBAR, so I need to do some mad science to find TDC.

Get the ignition timing sorted first; then check the vac advance is working correctly. You say you get a hot running problem, could you have a fuel vapourisation problem here? How did you balance the carbs? This is not something that can really be done by eye - you'll need a pair of vacuum gauges or a carb balancer to do this properly. A compression test might also be useful - run wet and dry tests, if compression rises on wet test then possibly worn piston rings, if no change then possible head gasket or valves. HTH.

classic kev said:

check the vac advance is working correctly. You say you get a hot running problem, could you have a fuel vapourisation problem here? How did you balance the carbs? .... A compression test might also be useful - run wet and dry tests, if compression rises on wet test then possibly worn piston rings, if no change then possible head gasket or valves. HTH.

Click to expand...

Thanks, I'll have to look into getting a compression tester and a vacuum tester.

Definitely could be fuel vaporization, but I have no idea how to test for that. Any ideas?

Today I just balanced the mixture screws using the first part of the method described in the workshop manual: align both nozzles with carb bridges and then turn clockwise 3.5 turns. I have no gas analyser so I think that's the best I can do there. I took it to a mechanic who set it way too rich - plugs are black now - so I wanted to get it back to something closer to normal.

Ok starting at the top
Replace the cap leads and rotor arm with dealer only gen items
Carbs out of balance you need a carb balance done to ensure the linkage is free and not binding
Guessing the timing is out without a proper timing light is a folly!
When starting to tune carbs start by ensuring you have a base setting which ,WANs tune the fuel screws totally in then out 3 turns but until you have had the emissions set you are guessing