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Force backfire works but then comes back
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Author:  zookz [ Tue Mar 05, 2013 1:51 pm ]
Post subject:  Force backfire works but then comes back
Gday all, I have a mate with a 87 Sierra that has been extremely temperamental and bloody frustrating. When it plays up it runs rough, kangaroo hops, bogs down, no power, backfires etc etc
But when I do a mexican tune up (2-3) it runs perfect! Until it comes back to the same problem after another small drive.
Have checked fuel pressure (4-5psi)
Changed Coil
Leads
Checked Plugs
Adjusted mixture and idle
Adjusted timing

What can this be? The air filter looks new...?


I also swapped carbys! But could the second carby be faulty 2? 8O

Maybe need a full carby kit?

Cheers
Author:  JrZook [ Tue Mar 05, 2013 4:10 pm ]
Post subject:  Re: Force backfire works but then comes back
zookz wrote:

Maybe need a full carby kit



Sounds like it
Author:  Mike57 [ Tue Mar 05, 2013 6:03 pm ]
Post subject:  Re: Force backfire works but then comes back
The backfiring indicates a timing issue or a sticking valve. I am not an expert but does a Siera have a timing belt or chain. Is the belt slipping. Could there be a sticking valve which could happen if a stem was bent or there was some overheating at some stage.
Author:  droverdave [ Wed Mar 06, 2013 7:51 am ]
Post subject:  Re: Force backfire works but then comes back
what is a mexican tune up.
Author:  Bruce [ Wed Mar 06, 2013 8:08 am ]
Post subject:  Re: Force backfire works but then comes back
Swapping leads around. EG: putting number 4 lead onto number 1 on the dizzy. Good backyard stuff for fixing a blocked carby in the bush.
Author:  atari4x4 [ Wed Mar 06, 2013 8:57 am ]
Post subject:  Re: Force backfire works but then comes back
very similar to the french tune up, where you rev it up & smother the carby to try & suck the crap through the jets.
Author:  alien [ Wed Mar 06, 2013 9:11 am ]
Post subject:  Re: Force backfire works but then comes back
maybe check the auto-choke isnt going... mine did all sorts of weird shit when that happened... i replaced the auto choke twice before dtiching it for a weber with manual choke so it wouldnt happen again, cos its a pain in the ass.
Author:  zookz [ Wed Mar 06, 2013 10:44 am ]
Post subject:  Re: Force backfire works but then comes back
The auto choke has been disconnected by me so it's not that...
Maybe contaminated fuel?
It's just really weird because when I backfire it, it runs so sweet.
Author:  Matthew [ Wed Mar 06, 2013 10:50 am ]
Post subject:  Re: Force backfire works but then comes back
Have you got a fuel filter between the fuel pump and carby?
Author:  atari4x4 [ Wed Mar 06, 2013 10:50 am ]
Post subject:  Re: Force backfire works but then comes back
could be dirty fuel tank/lines, heaps of people run a 2nd fuel filter just before the carby

EDIT: beaten
Author:  Built4thrashing [ Wed Mar 06, 2013 3:28 pm ]
Post subject:  Re: Force backfire works but then comes back
Check the fuel pump is not stuffed and sucking oil into the fuel.

Blocked PCV Valve.

Crap leads or rotor button
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