Anglefire Posted February 2, 2019 Report Share Posted February 2, 2019 I had a nice run out in the cold this morning (See topic "Snow"!) and I drove it reasonably hard - upto 70 on the dual carriage way and came down a couple of small hills (Bear in mind we are talking around Birmingham and not Wales!) and on the overrun was popping a little bit. I've done a bit of a search - and the answer varies from its normal to a rich mixture to a lean mixture and to a leak in either the exhaust or manifold. Interestingly enough the idle speed has increased as well and is a little too high (Again!) So my initial thought was timing - but I'm now inclined to think a leak? I've not had another play as its too flippin cold! Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 2, 2019 Report Share Posted February 2, 2019 make sure the jets return after using the choke as said in snow, a little weak at idle Pete Link to comment Share on other sites More sharing options...
Anglefire Posted February 2, 2019 Author Report Share Posted February 2, 2019 Actually I did check the chokes when I got to my friends and all good - nicely shut. I'll have a look at the mixture again - it was running at about 2'C ambient and its not been out properly in that temperature in my ownership before. Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 2, 2019 Report Share Posted February 2, 2019 so its suffering from SHOCK ha! 2 Link to comment Share on other sites More sharing options...
daverclasper Posted February 3, 2019 Report Share Posted February 3, 2019 I know your joking/chocking!, Pete, but could it may be the case, slightly different tolerances on carbs/cables etc, making slight difference, as well as ambient temp?. Link to comment Share on other sites More sharing options...
Anglefire Posted February 3, 2019 Author Report Share Posted February 3, 2019 I am aware that weather can affect the tune of these olduns - particularly as it was dry and cold - so the air is denser so would lean out the mixture a little. Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 3, 2019 Report Share Posted February 3, 2019 Dave as carbs are made up from all manner of bits and pieces , yes variations and tolerances all play a part in making nothing standard or hard and fast , variables thats why all the bits are adjustable, Everything from the fuel put in to the owner are all varyables Pete Link to comment Share on other sites More sharing options...
SpitFire6 Posted February 4, 2019 Report Share Posted February 4, 2019 Hi, is popping the sound you get with an overly lean mixture? Excessively lean mixture fails to ignite & fresh air with unburnt fuel exits the exhaust valve. Over several cycles, the AFR in the exhaust is good for ignition? I know when I decrease the fuel pressure, my car pops too much on overrun. Gets attention for those that can't see the car. Cheers, Iain. Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 5, 2019 Report Share Posted February 5, 2019 it was normal to shut of ignition allow some overrun, switch back on and kaboom, best done under bridge , or near guys working done a manhole, the commer pb with a side exhuast was perfect. has a reputation for splitting open silencers then you get old and modern fueling wont work Is there no fun left Pete 2 1 Link to comment Share on other sites More sharing options...
dave.vitesse Posted February 5, 2019 Report Share Posted February 5, 2019 2 hours ago, Pete Lewis said: it was normal to shut of ignition allow some overrun, switch back on and kaboom, best done under bridge , or near guys working done a manhole, the commer pb with a side exhuast was perfect. has a reputation for splitting open silencers then you get old and modern fueling wont work Is there no fun left Pete Pete, if you did that on a modern car you would end up with a dead cat in the road! Back then if you remember it was soot and bits of baffle that came out. One way of achieving a free flow exhaust system. You will get thut thut (Did I spell that right) noises, but as we have said should not get any bangs on over-run. An 88C thermostat helps the engine (and heater!) in the winter, but remember to fit the 82C in the spring. Been there done in, forgot that is. Dave Link to comment Share on other sites More sharing options...
poppyman Posted February 5, 2019 Report Share Posted February 5, 2019 2 hours ago, Pete Lewis said: it was normal to shut of ignition allow some overrun, switch back on and kaboom, best done under bridge , or near guys working done a manhole, the commer pb with a side exhuast was perfect. has a reputation for splitting open silencers then you get old and modern fueling wont work Is there no fun left Pete I used to find the old petrol transits good for that, they often set off car alarms with the boom....... A jubilee clip around the silencer stopped them splitting. The best fun was people pumping their tyres up at petrol stations Tony. Link to comment Share on other sites More sharing options...
Anglefire Posted February 9, 2019 Author Report Share Posted February 9, 2019 I've bought another manifold gasket (They are only £6.80 ish from Ebay for a genuine Payen one) and if I get chance I'm going to whip off the manifolds, inspect the faces and refit with some high temperature silicone sealant (Yes I know you shouldn't need sealant) and see if that cures my backfiring and seemingly high idle again, I shall be satisfied. Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 9, 2019 Report Share Posted February 9, 2019 dont forget the earlier thread on seized nuts on the studs, or bottomed out on crud so it feels tight but not clamping the gasket Pete Link to comment Share on other sites More sharing options...
Anglefire Posted February 9, 2019 Author Report Share Posted February 9, 2019 They are studs and nuts so not bottoming out. Could be dirty threads but don’t recall them being bad. It could be the manifold clamps fouling on the exhaust manifold flange though. Link to comment Share on other sites More sharing options...
daverclasper Posted February 9, 2019 Report Share Posted February 9, 2019 Hi. I thought that sealant maybe not a good idea (unless pretty flaxable) as the joint will move about a bit?. Cheers, Dave Link to comment Share on other sites More sharing options...
Anglefire Posted February 9, 2019 Author Report Share Posted February 9, 2019 It's flexible in that it is silicone - and TBH it shouldn't move much - a few thou with heat expansion possibly Link to comment Share on other sites More sharing options...
SpitFire6 Posted February 9, 2019 Report Share Posted February 9, 2019 Yep, no more than 4 mil max. Link to comment Share on other sites More sharing options...
Anglefire Posted February 10, 2019 Author Report Share Posted February 10, 2019 This afternoon I made a bit of time to whip off the carbs and replace the gasket - with little high temperature sealant both sides of the inlet holes. Ran it up and it ran as rough as a rough thing - needed to apply some throttle - but choke didn't help after a few minutes. Idled at about 400rpm until it stopped! So turned the idle screws 1/4 turn down and then idled nicely at about 7-800rpm. So does suggest there was a leak. The only this I did notice was there was some oil lying in the inlet manifold - and I am using more oil than it should, so was wondering if that was normal - or whether it suggests the valve guides are worn? Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 10, 2019 Report Share Posted February 10, 2019 Residue in the inlet is to me quite normal Does sound like you had a leaker Pete Link to comment Share on other sites More sharing options...
Anglefire Posted September 8, 2019 Author Report Share Posted September 8, 2019 Went out today for a 50mile round trip. I have backed off the needles (Su carbs) so it is running a little leaner at idle at least (exhaust is still black) and coming down a couple of hills on the overrun I was getting a slight pop from the exhaust. The idle speed that I did reduce after my last run out is running a little too high again (about 1000rpm) - I’ve not adjusted it again yet. When I got back home I sprayed the inlet manifold with some brake cleaner and no change to the idle speed (which is a relief!) but I sprayed it around the carb spindles and it did change slightly on both sides. Not much but a definite change. I have looked at the inside of the carbs and the top of the one (front) needle is a bit damp from fuel. So I think I will check the fuel pressure (the pump is a new one (12months or so) and I wonder having read other things on here) that the new pumps tend to over pressure the lines. I already have a fuel pressure gauge that I can fit. The float is obviously another possibility as well as the valve in the float chamber. But they have been replaced previously too Another other thoughts? Link to comment Share on other sites More sharing options...
ShaunW Posted September 9, 2019 Report Share Posted September 9, 2019 I don't know about cars but on bikes, backfiring is usually a leaky exhaust, or most often not enough baffles on aftermarket exhausts. You only get a backfire in the presence of oxygen, it's getting in somehow. Can someone explain to me why a lean mixture is often suggested? I'd have thought unburnt fuel was the issue and too rich would be the cause...unless being too lean means it doesn't combust properly. Link to comment Share on other sites More sharing options...
Pete Lewis Posted September 9, 2019 Report Share Posted September 9, 2019 Lean gives you unburnt fuel from incomplete burns youre on the right track Builds up till it gets ignited when air fuel ratio lets it go off nicely with a hot exhaust Pete Link to comment Share on other sites More sharing options...
ShaunW Posted September 10, 2019 Report Share Posted September 10, 2019 Thanks Pete. Everyday's a school day. Link to comment Share on other sites More sharing options...
Pete Lewis Posted September 10, 2019 Report Share Posted September 10, 2019 You can induce one by throttle back , ignition off , wait a moment, switch back on , and see if you have split the silencer Our vans had a side exhaust it was a common trick on test runs to let one go if a guy was working down a manhole . fool about laughing, but you do grow up eventually Pete 1 Link to comment Share on other sites More sharing options...
poppyman Posted September 10, 2019 Report Share Posted September 10, 2019 1 hour ago, Pete Lewis said: but you do grow up eventually Pete Never..........If you lose your sense of humour and stop laughing you end up a miserable t**t.... Tony. 1 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now