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MG Midget and Sprite Technical - backfire

The motor backfires when it is cold in the morning. Especially when I press the throttle Choking helps. Setting the ignition more early cures it but than there is no acceleration and no top speed.
I see the ignition timing wander at idle although I have a programable 123 ignition. I can hear the duplex chain especially with a warm motor. I could not hear it before. It is a 948 cc with H1 carburetors, maniflow exaust, 100.000 km after a total rebuild. Will I replace the duplex?

Flip
Flip Brühl

Sounds like it's running lean, I'd check for a vacuum leak such as a bad vacuum hose / connection or a leak around an intake gasket.
SK Kowznofski

With a 100,000 miles on the clock its a good guess that you may need a new timing chain and sprockets amoung other things

im assuming back fire thur the carbs and not the exhauste

id start by looking for an air leak... probably around the carb butterfly spindle

next change fuel make sure your using a current fuel mix and not one for a summer blend...here the seasonal change is just going into effect and its noticeable

also try using a higher octan and see if that helps

make sure ykur filters are clean

run a pipe cleaner thur the carb jet tubes as they can devolop crud on the enterior walls causing a lean constriction

Id put a timing gun on it and get that sorted

also make sure the springs and dawgs inside the dissy are funtioning properly and not gotten weak

then redial in the carbs and keep rechecking your dissy timing

i deally you want 1000 rpm at 10 degrees idel and have the carbs adjust to almost the lowest idle speed setting but not the bottom ...

and actually before dojng that check your valve lash

lots. Of good genral insighton google ... engine backfiring

usually backfiring is a lean fuel condition or a ignition timming issue

prop
Prop and the Blackhole Midget

Btw... have you noticed the engine running hotter then normal durjng this backfiring period of time

thats a good indication of a lean fuel problem as a lean fuel set up makes a hotter running engine

prop
Prop and the Blackhole Midget

Prop,

Flip said he had 100k kilometres on the car not miles, that's 62k miles so I wouldn't expect significant wear in a duplex chain at that mileage due to normal use so maybe something has let go and damaged it. It'll likely need investigating.
David Billington

Since the problem seems to go away when the engine is warm, it leads me to believe carb to intake or intake to head leak which goes away as engine heat makes the metals expand. First place I'd check would be to run a wrench over all those fasteners in case some have loosened. Least amount of work & no cost involved. I'm cheap & lazy so I always start with the potentially free and easy solutions ;>)
SK Kowznofski

Metrics... there a sovit communist conspiracy....haha

Good points above....i need to read the details a little closer i missed some meaningful admissions

prop
Prop and the Blackhole Midget

Yes , Thank you for the advices.
I fastened all the nuts of the manifolds and yes, no backfiring anymore! Prop, I made the butterfly spindle from Tufnol, so no leaks since then. Metrics and communists?

What to do with the scatter. There is a lot of play between the crankshaft and the ignition (Duplex chain, camshaft, distributor drive, distibutor). So much that the 123 computer can not get rid of it...

Flip
Flip Brühl

Flip What you are describing I think is called points bounce, have you checked that the 123 may have settings for "debounce" ??
I use this kit with points in my 1500 and it cures slap and play including debounce.

http://archive.siliconchip.com.au/cms/A_112976/article.html

You can buy one from Jaycar here in OZ for about $60AUD
Takes about 2 hours to assemble if you are moderately experienced.
Cheers
Rod
R W Bowers

Wandering and scatter I see as different issues - unless you have both of course. Scatter at idle as seen when using the strobe is often down to electrical noise or interference when using non insulated ignition leads. So if you've kept the early plug caps and copper wires it's almost certain you'll get it.Switching to double insulated silicon leads will usually fix it.
If you have mechanical scatter at idle you'll get more of a snatch and grab pattern on the strobe that steadies as revs are increased.In this situation renewing components,including cam bearings and even adding the later nylon tensioner should help remove any slack from crank to ignition,quieten things down and give you a steady idle. On my Frog I've done both of these things and together with sintered rockers the valve train all but inaudible.
Fergus

I use silicon leads and yes the scatter is away when the revs are increased. So I have to renew things. At the other hand, I can live with the uneven idle.

Flip
Flip Brühl

This thread was discussed between 02/10/2016 and 06/10/2016

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