Thoughts on Misfire Please.

[Gremlin]

1986 Bentley running with a R(?)90 unit and spuds into manifolds just before the carbs.

Recently it has developed an intermitant misfire.

Just before I went on holiday half way thro last month I made a slight adjustment to the tickover screw on the unit and then the next day went away for 10 days. I mention this because these cars dont like being stood up.
When I next used it I drove approx 1.5 miles all ok, but when I started to drive home it developed a misfire. Did some fiddling and gave it a run and it sorted itself out... except for the fact that it does much the same thing every day when I use it. If I stamp on the peddle it will clear itself but will start to misfire again when I slow down. It doesnt cut out at any point. Problem seems to go away when engine is hotter. Doesnt misfire on petrol.
a:- Could it be the cold weather which coincides with the start of the problem.
b:- Could it be not enough anti- freeze? (although there is enough gas when I give it some welly.)
c:- Gunge in the vapouriser.
d:- Too rich or weak?

[Gilbertd]

None of the above. Most likely it's asking for a new set of spark plugs. LPG needs a better spark than petrol so even if it runs without misfire on petrol, it may well misfire on gas. Leave the LPG system alone until it has new plugs in it.

[Gremlin]

Agreed it might well do with some.. I bought a set the other day and only had time to put 2 of them in. The old ones looked as if they had only been in for a few days, but the have been in there since I bought the car nearly 2 years ago.
Oddly my Mercedes has slight misfire on petrol but perfect on gas.

[Gilbertd]

In cold weather, particularly cold, damp weather (and we've certainly had plenty of that) it isn't uncommon for HT leads to start to suffer. Give them, and the top of the coil(s) and dizzy cap a good clean, that may be all it needs. Years ago someone I knew thought his car had a moral conscience. No matter what the weather the car would start first time every time if he was at home but if he stayed overnight at his girlfriends it wouldn't. That was down to a buildup of dust and oily crud on top of the ignition coil allowing the sparks to track down to the LT terminals. When he was at home the car was garaged in the dry but out on the street exposed to all the damp air when at his girlfriends......

[Gremlin]

Years ago someone I knew thought his car had a moral conscience. No matter what the weather the car would start first time every time if he was at home but if he stayed overnight at his girlfriends it wouldn't.

I know exactly what you mean. Park on grass or gravel does the same to some cars. My mother kept complaining about her car not starting very well...I'd bring it home and it would be perfect.

[Gremlin]

Managed to get around to changing the rest of the plugs and things went ok....for 2 days, then it started to misfire again. Then stopped. Then started.

Clip on distibutor cap didnt really appear to be doing its job as well as it should, so attacked it with a pair of pliers.. Sprayed all the leads and cap with sealer and touch wood its now been firing on all 8 cylinders for 3 days, and, fires up on first turn of the key instead of having to have 2 goes.