monty Posted October 3, 2006 Report Share Posted October 3, 2006 Having tried to alleviate my rear oilseal leakage by checking & cleaning the breather system etc. I now, since the cleaning (not much needed & no blockages), seem to have a engine 'running on' problem. It could just be a coincidence but on disconnecting the breather pipe from the inlet manifold the running on stops! Could it also possibly be the fuel I am using? 95 octane unleaded. Should I up the rating to say BP Ultimate or Shell Optimax (97/98)? (Engine has unleaded head). Anty suggestions please. Many thanks, Monty. Quote Link to post Share on other sites
littlejim Posted October 4, 2006 Report Share Posted October 4, 2006 Monty, no one has replied to you yet, so I'll put in my (limited) experience which may not be very helpful. I had to drive mine 500 miles from Melbourne to Canberra, and tried to use 98 Octane (AUS different to US octane rating system) all the way, plus lead replacement fluid. It still ran on rather noticeably after switching off. I subsequently replaced the dud condenser in the distributor which improved the performance under load an incredible amount, and did a 'by the book' carburettor tune and balance. After the carburettor tune the 'running on' reduced to almost zero. In fact it really dropped off to zero. I noticed it once afterwards for about 2 revs. In other words I found the tune up made the difference, I was already using the higher octane fuel. Quote Link to post Share on other sites
67_gt6 Posted October 4, 2006 Report Share Posted October 4, 2006 (edited) Monty - possibly the engine is running so lean that it is causing pre-ignition. Removing the breather pipe may lean it out to the point where the mixture can't ignite? I would certainly go with Little Jim's advice and tune it. If the mixture/ignition timing/cooling are all correct it shouldn't run on. I would also use higher octane fuel. Andy Edited October 4, 2006 by 67_gt6 Quote Link to post Share on other sites
jmathias Posted October 4, 2006 Report Share Posted October 4, 2006 (edited) running on (the ignition of the fuel/air mixture without the aid of a spark) is usually caused by one of two factors - either the mixture is detonating under compression alone, or the combustion is being triggered by something in the combustion chamber. The latter is usually a build-up of carbon which retains heat and acts as a sort of glowplug to ignite the mixture. If your plugs have a heavy coating of carbon, that might be the cause. Premature detonation under compression means that the mixture is wrong - the combination of the carburettor setting and the octane of the fuel you're using means that what's going into the cylinder is igniting all by itself when it's squeezed. The breather system has an effect , assuming you have the type where the breather pipe from the rocker cover feeds back into the air inlet to the carbs. If your breather was partially blocked previously, then maybe your carbs were set to compensate for that, and now that the breather is working properly, the setting is wrong? Intuitively, I'd have thought that premature detonation would be a result of a too rich mixture, so I guess I'd try leaning off the carbs a little to see if it cures the problem. <just read 67_gt6's contribution:> I stand corrected ! I should have paid more attention in my thermo lectures at college - never really understood combustion properly. Try enriching the mixture a bit instead... Edited October 4, 2006 by jmathias Quote Link to post Share on other sites
D Murton Posted October 4, 2006 Report Share Posted October 4, 2006 Monty When I bought my 4a 2 years ago I had the same problem even though it is fitted with an anti run on valve. It has an unleaded head so I ran it on normal unleaded and from the plug colour the mixture looked OK. If I fill up with Optimax the running on disapears but not immediatley. I can only assume that Optimax burns hotter or cleaner and removes some of the carbon deposits that are glowing and causing the engine to run on, If I change back to normal unleaded it can take some time for the running on to return. I cant say I notice any real improvement in performance with Optimax althouth it does seem to run more smoothly DaveM Quote Link to post Share on other sites
stuart Posted October 5, 2006 Report Share Posted October 5, 2006 Monty I had same problem on my unleaded converted 4a when it was running on its original strombergs with the diaphragm type breather and never cured it until i fitted a pair of twin`40,s hasnt run on since!! Bit extreme i grant you but i am convinced its the poor octane value and dubious make up of standard unleaded. I never had this problem with my previous 4a but that was in the days of good old four star with the occasional friday night treat of a tank full of five star. Ah those were the days! Have you tried an octane booster? As other threads seem to say that the likes of optimax seem to help. Stuart. Quote Link to post Share on other sites
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