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Backfiring through carbs still

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mylesw

Jedi Hopeful
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Hello

I'm still suffering with my car backfiring through the carbs and was wondering if anyone could help me pinpoint the issue... The engine has been rebuilt and this issue has been there from the beginning. It doesn't happen all the time, but generally when the engine is cold or under aceleration. All the ignition system is new or rebuilt. I have checked the valve clearance, ignition timing, dwell and all are ok. I have checked the compression and have low compression on one cylinder (110 psi vs 130 on the rest). I have checked for manifold leaks and retuned the carbs. Last weekend I pulled the spark plugs which were black, which I found slightly odd as backfiring through the carbs seems to be a symptom of a lean mixture....

So the question is, what to do next? I am thinking about getting another coil to see if it is a issue with the strength of the spark - could the coil be faulty? Or change out the distributor cap and rotor arm. Other option is to pull the head and find out what is causing the low compression, but this will cost $$$.

Any ideas on what I should do next? I've spent months trying to solve this issue but seem to be getting nowhere....

Thanks

Myles
 
Ideas:
Swap out coil, condensers (buy 2 or 3) - there are QC issues with both these products.
Check plug wires in the dark to see if any are grounding on the outside of the wires
Fuel pump supply
Plugs - too cold?
Leakage test on suspect cylinder
Test for butterfly shaft leaks
Disconnect distributor wire from remote battery shutoff switch
 
Hi Myles,

Try taking the car out, fully warm, and get it to backfire on acceleration, then repeat with the choke partially out. If it continues to backfire, then it must be something other than mixture. You could also try the colortune to see how your mixture is. I assume your carb needles are correct for the motor?

I have seen a broken valve spring, or a sticky intake valve cause your symptoms. You might also check the distributor advance too. My BJ8 is touchy on cold starts, too little choke and it will backfire, but when warm it doesn't do it.
 
Were the carburetors also rebuilt ?
 
What do you mean by run Marvel Mystery oil through it? I do have Marvel Mystery oil in the carb dashpots.

The carbs were rebuilt by AJ Turner in the UK (and have been apart a few times since to check the bushings, needles etc). The distributor was rebuilt by Jeff at Advance.

I'll take it out this weekend (weather permitting...) and see if I can check the mixture again. Otherwise I have just ordered a new coil, rotor arm and condenser. And I plan to take off the fuel pump to check that also.

We'll see where we go from there....

Thanks for your help.

Myles
 
OK - first test on the fuel pump - disconnected the fuel pipe from the front carb and pumped into an old coffee jar, pumped about 1/8 pint in 30 secs - seems low? Seem to remember it should pump a pint in 30 secs? Thoughts?
 
I once had this problem with my BJ8. The cause turned out to be a failed gasket between the exhaust manifold and the first exhaust pipe. A new gasket fixed the backfire.
 
mylesw said:
OK - first test on the fuel pump - disconnected the fuel pipe from the front carb and pumped into an old coffee jar, pumped about 1/8 pint in 30 secs - seems low? Seem to remember it should pump a pint in 30 secs? Thoughts?
Click to expand...

Specs for SU fuel pumps:
https://homepages.donobi.net/sufuelpumps/Fuel_Pumps/Fuel_pump_specs.pdf
1 pint in 30 sec is in the ballpark.

Maybe your tank pickup and/or filter is clogged.

I was referring to the old technique of squirting Marvel Mystery Oil in the carb opening with the engine running - as a way of blowing out carbon. However you may have identified the problem with insufficient fuel flow.
 
Hi Myles, check the earth connection to distributor/engine. Had the same problem and found the earth connection to engine was missing. Brg Lars
 
Thanks for all your tips.... After blowing air through the fuel pipe I am still suffering with the backfire so I replaced the condensor and the distributor cap but no success. I checked the compression which shows 120 psi on all cylinders except #6 which has 130 psi. So seems ok there. I checked the vacuum advance which is working. Lastly I hooked up a vacuum gauge to the rear carb vacuum port however only read about 5 in Hg at idle which seems low. This seems to indicate a major vacuum leak somewhere? I have sprayed starter fluid on the manifold and carb spindle but these results in no response from the engine. I've already replaced the manifold, and carb spacers and gaskets, and tightened all the manifold nuts and the blanking plugs on the top...where else could a vacuum leak be?
 
If your fuel delivery is truly as you say in Post#8, then surely this is the first thing that needs addressing?

Danny
 
Yep, I cleared the gunk from the fuel line with compressed air and now pumping ~1 pint in 30 secs so that is sorted but does not seem to be the only issue as the car is still backfiring/spitting.
 
Check the floats in the carbs.What about the lead wire from the dist to the coil.I think it has to be electrical maybe dad rotor or condencer
 
If your fuel lines were partially clogged, it's possible that some of the gunk got into the carbs and is partially blocking the small hose that goes from the bottom of the float bowl to the carburetor. Look at your float bowls and see if there is dirt in the bottom. If so, you'll want clean them.
 
So think I have this sorted now - ended up taking the fuel pump off and the carbs. Cleaned the points on the fuel pump, stripped the carbs, cleaned and rebuilt them again. Tightened all the nuts on the manifold and the fuel pipe connection on the fuel pipe. Tuned the carbs and took the car out for a drive today and no backfire. So seems to be cured for now...but still not sure what the issue was. Thanks for all your help on this.

Myles
 
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