Hi all, have you had any experience with using a wideband air fuel ratio kit to accurately tune the carbs? New type 4 1911cc engine and fully refurbed solex carbs. Got the van to drive great, plenty of power, but on tickover and deceleration, the exhaust coughs (not bangs) which is annoying. I’ve read everything I can to tune and having stripped and rebuilt the carbs myself, I know them inside out. No vacuum leaks on manifolds, break booster or exhaust but it’s driving me potty. An AFM meter is my last hope but are they worth it seeing as they’re so expensive. Please help! Also installed a vintage speed exhaust - could this be the culprit?
If it “spits” out the top of the carbs, then that is lean at idle. I gather that is what you mean by “cough”? could be a matter of turn out on mixture screw or upgrade of idle jet.
Pops from the exhaust point at lean, I thought.. my progressive usually does it with a blocked or too small idle jet.. So winding out the idle mixture screws or going up a size on the idle jets may help.
Hi @mikedjames @Barry76, this is what I’ve found. Checked for air leaks and no leaks at brake booster, booster elbows, manifolds or interconnecting hoses/pipes, so that’s ruled out to be the cause. Disconnected vacuum pipe from distributor and plugged. Rev’d engine to 3500rpm and no popping from exhaust, advances to a max of 28 degrees. Reconnected vacuum to distributor and rev’d engine to 3500rpm and the engine only pops when I let go of the accelerator cable. Note it advances to around 44 degrees at 3500rpm. Do you think it’s the distributor that’s causing the popping issue? It has previously been taken apart, cleaned and has a new vacuum can fitted. Cheers
Worth remembering you are achieving that popping in circumstances that never occur while driving. From memory, the total (vac) advance is a few more degrees than it should be, technically speaking, but in reality it won't damage anything as it will not advance like that when the engine is under load. Your popping is unburnt fuel igniting in the exhaust as I'm sure you know. When you let the throttle go you cut off air and advance the timing to your 44 degs where it's possible the combination of timing and mixture doesn't always burn completely, builds up until it won't burn at all then pops in the exhaust. Try disconnecting the vacuum advance and go for a drive?
If you shut the throttle fully, you are blocking off the vacuum feed to the distributor,as well as the air to the engine, so it will stay more advanced for a while, also the engine is still spinning fast so it doesnt get enough fuel. So it pops. This may turn out simply to be a characteristic of your tuning and carburettors. You may have to open up the mixture screws a little, trade running overall slightly richer for popping. Or maybe tweak the idle speed up a little with the endstop screws so the airflow cant be shutoff quite so completely with the throttle plate, there may be a point where the carburettor system goes extra lean right at fully closed throttle. Leaks in exhaust joints also contribute to popping. Go round with a hose held to your ear , poking the other end at exhaust joints and listen for crackling sounds from leaks.. VW did fit throttle position dampers to some of the carburettor installations because it would produce more NOx emissions when you released the throttle and the mixture went lean if it closed too fast.
