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Carby backfiring......
https://www.ausmini.com/forums/viewtopic.php?f=2&t=84465		 Page 1 of 2
Author:  carter [ Wed Feb 05, 2014 7:50 pm ]
Post subject:  Carby backfiring......
I'm feeling pretty down about my car at the moment. I can't drive it cause it overheats. I have had the motor rebuilt but glazed the bore so my brother and I chucked a new set of rings in. I changed fuel pump as I believe this was the reason for the glazed bore. I can't drive it now it overheats and when first started it backfires out the carb..... I've tried timing as in static getting it on tdc then turn the dizzy till it sparks but it still over heats. Also it wS running fine before I pulled it apart to throw a new set of rings in it just burnt heaps of oil. Pleae help :( thank you to all the wisdom that's hopefully coming my way......
Author:  peterb [ Wed Feb 05, 2014 8:07 pm ]
Post subject:  Re: Carby backfiring......
If it's running lean, (partially blocked jet) it can run hot and back fire. Check for air leaks at intake manifold, as this can cause lean mixture too.
Author:  carter [ Wed Feb 05, 2014 8:13 pm ]
Post subject:  Re: Carby backfiring......
Lean is not enough petrol right? I initially thought this so I adjusted the carby screw to give it more fuel. How would I tell for maifold leaks on the intake I had a look but I can't really say for defiant eyes it's leaking.....
Author:  peterb [ Wed Feb 05, 2014 8:20 pm ]
Post subject:  Re: Carby backfiring......
You can usually tell it's 'sucking air'/ lean, by the fact it goes a bit better when you pull the choke out slightly. Or take it for 'normal' drive up road then stop and sniff exhaust fumes and if it's lean, it smells funny. By the way, just sniff it to check, no getting high on the fumes. :lol:
Author:  1018cc [ Wed Feb 05, 2014 8:24 pm ]
Post subject:  Re: Carby backfiring......
Spray carby cleaner around the joints of the inlet manifold. If there is a leak the engine will rev up.
Author:  carter [ Wed Feb 05, 2014 8:27 pm ]
Post subject:  Re: Carby backfiring......
Could I just pull the inlet off and put some gasket silicone on it to ensure a seal or is this a bad idea?
Author:  carter [ Wed Feb 05, 2014 8:28 pm ]
Post subject:  Re: Carby backfiring......
And with sniffing the exhaust. My cars smells like petrol/ smells all the time.....
Author:  1018cc [ Wed Feb 05, 2014 9:05 pm ]
Post subject:  Re: Carby backfiring......
I've never used silicone on fuel parts before, generally I use petrol to clean silicone off things so I don't think it would last well. Are you sure it is a vacuum leak? It is a lot of effort to do what you are saying if it isn't. When adjusting the mixtures you are looking for the highest rpm (smoothest) idle you can achieve. Try not to work on hunches otherwise you might only be making it harder on yourself. Engines are simple and I've been caught out before by the blindingly obvious, it just takes time to solve. Have you also looked for fuel leaks?
Author:  carter [ Wed Feb 05, 2014 9:28 pm ]
Post subject:  Re: Carby backfiring......
Fuel leaks ? No I haven't I know my fuel tank has a small leak this would do anything though would it? Also I smell petrol in my car but that may be from my boot..... I have a facet fuel pump which is 1.5 to 3.5 psi electric on my car as well.....how would you recommend me to tune it? I've tried adjusting the mixture screw and it doesn't seem to change anything. Not sure if it's a vacuum leak. I don't believe it's a head gasket as I'm not loosing water, no water in oil no bubbles in radiator and it takes a while to warm up compared to blown head gaskets etc I have had before on previous motors. Has good compression also.
Author:  1018cc [ Thu Feb 06, 2014 6:10 am ]
Post subject:  Re: Carby backfiring......
Watch this video. It is for an edlebrock carb so it has nothing to do with SU's but listen to the description of how he achieves the highest smooth idle. You may also have a problem with your carbs if adjusting the mixture doesn't change anything. If you lean them right out the engine will want to stall. If your carbs are not doing this there could be a problem with them. I would do tests in the following order:
1) spray carby cleaner around on the joints of the manifold, connection to the head, top, bottom and sideways etc to check for vacuum leaks. The engine will rev up if there is one. Doing it this way will also help determine where the leak is.
2) adjust the carbs to ensure they are sucking in the same amount of air as each other. There is a screw on top of the carb that will adjust this and you can tell if they are sucking in the same amount of air by getting a hose (fuel, vac etc) and putting one end in the throat of the carb and the other end to your each. Switch between the two carbs and adjust until the amount of air rushing in sounds the same.
3) if you don't find any vacuum leaks, check the carbs over to ensure that when you turn the nut, the idle mixture is changing (just make sure the carbs are working correctly even if it isn't tuned well)
4) if you can't find any issues with the above two items then try and tune the carbs listening for what the engine does (listen to what the guy is saying in the youtube video). You should be able to get it to run pretty good using what the guy describes in the video.

If you have any problems with any of the steps do not proceed to the next until you sort out the problem. You'll never be able to tune it correctly if you have a vacuum leak.

<edit, added a step in>
Author:  JimL [ Thu Feb 06, 2014 8:19 am ]
Post subject:  Re: Carby backfiring......
aren't these common symptoms of too far advanced timing? Have you tried turning the dizzy anticlockwise a couple of degrees while the car is running and listen if the idle speed increases a bit?
Author:  1018cc [ Thu Feb 06, 2014 9:02 am ]
Post subject:  Re: Carby backfiring......
JimL wrote:
aren't these common symptoms of too far advanced timing? Have you tried turning the dizzy anticlockwise a couple of degrees while the car is running and listen if the idle speed increases a bit?


The odd carby backfire I have experienced has been from a bit too lean. Having said that I know the timing has been correct in the cars that it has happened to me in (a recently recurved mini and another car with a MegaJolt). The easiest way to check the timing (if you don't have a timing light) is to statically time the car with a 12V test light. Should get you in the ballpark.
Author:  FNQ [ Thu Feb 06, 2014 9:04 am ]
Post subject:  Re: Carby backfiring......
Hi sorry to hear about your woes. As 1018 says, be methodical about your diagnosing. Follow 1018 steps. This back tracks a little though..... usually you make absolutely sure timing, ignition, firing order, spark etc is spot on first. Backfiring through carby check timing first,Check timing with vac blocked off - are you sure the dizzy/ coil combo is ok etc are you sure you havent got a misaligned timing chain?
then check for lean / partial blockage/filter/aircleaner/leaks/ mixture/ fuel flow at carbs etc.
What carb/carbs are you running? Again as 1018 said take out and check they move etc, also to reinstall at recommended needle/bridge height ( eg 12 flats out)
If you think high pressure pushed past the float valves, they may have changed the float levels/ or damaged the valves,
Author:  carter [ Fri Feb 07, 2014 8:11 pm ]
Post subject:  Re: Carby backfiring......
G'day fellas had another look at my car and it's still got problems. Although it's not backfiring out the carby any more as it appears a couple of the nuts may have been a bit loose (silly me) but it's still over heating. So today I changed the thermostat to a 84 instead of the 72 so we will see if that does anything after I take it for a drive. I sprayed carb cleaner around the inlet and nothing so I don't think it's leaking here. Just running a single 1 1/2 carby and static times it again just to get it in the ball park see if I was right. And it's basically where it was so the timings right.....
Author:  Thorlek [ Fri Feb 07, 2014 8:22 pm ]
Post subject:  Re: Carby backfiring......
carter wrote:
. So today I changed the thermostat to a 84 instead of the 72 so we will see if that does anything after I take it for a drive.


Wouldn't that have the opposite affect? the thermostat will stay closed longer, so the engine will get hotter quicker.

Did you clean out the radiator recently? (when the engine was rebuilt) and made sure the cooling system is holding pressure?

if you've upgraded from a 998 to a worked 1100 or 1275 but kept the old radiator, it could just need to be upgraded
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