constant back fire

[lil_mickey]

My VH 253 4Brl Quadrajet has a constant backfire above 3000 revs,

The kind of sound it is made me think it was a bottom end bearing if that helps anyone figure out what i mean by constant.

I only realised what was actualy happening when i cracked the s#!ts and pulled the throttle wide open while my head was under the bonet with the air cleaner off...

I checked the timing and then fixed as per the manual(8deg BTDC) no success, i have tried disconnecting the vac-advance but still the same.

Carb had a kit through it recently,
All valves working properly,

Please help...

Thanks

[DANNY8]

Maybe check that ur rockers are all torqued up nicely.

[Nackerz]

check your leads and plugs and retune the carby as that will be out of whack mate

[SL/ENUT]

try timing the motor more, my 308 sle is one 12 deg btdc, runs very smoothly.

[ari666]

yeah mines at 11 too. you sure you got all your leads on the right plugs? two leads on the wrong way can mean it runs ok-ish but then misfires like crazy under load.

also, check the plastic cam on your secondary flaps (top ones) your secondary meter rods are attached to the hanger which is then lifted by that cam when the flaps open up. that cam often suffers from a bad case of rot and falls to bits, so that when your secondary flaps open the rods dont rise. it lets a bunch of air in but no fuel and could possible cause popping (lean misfire)

[VKCOMMO]

my guess would be leads, get out your gregory's and let that help you with that

[lil_mickey]

Maybe check that ur rockers are all torqued up nicely.

Had the Rocker covers off last night, all are fine

check your leads and plugs and retune the carby as that will be out of whack mate

Replaced the leads and plugs tonight better, but still not good.
Just after I did the carb, 3 and a half weeks ago, I took the car on a 2000Km "test drive" and it ran perfect.

try timing the motor more, my 308 sle is one 12 deg btdc, runs very smoothly.

Tried this, but it was just as bad(after changing leads and plugs)
Found I had the least trouble @4 degree BTDC (where it is now)

yeah mines at 11 too. you sure you got all your leads on the right plugs? two leads on the wrong way can mean it runs ok-ish but then misfires like crazy under load.

also, check the plastic cam on your secondary flaps (top ones) your secondary meter rods are attached to the hanger which is then lifted by that cam when the flaps open up. that cam often suffers from a bad case of rot and falls to bits, so that when your secondary flaps open the rods dont rise. it lets a bunch of air in but no fuel and could possible cause popping (lean misfire)

Leads all in order, checked while I was replacing them tonight.

Also checked that the cam lifted the metering rods when i manually opened the sec flaps, I didn't know that was ever a problem, so thanks

my guess would be leads, get out your gregory's and let that help you with that

Used the max Ellery's, it told me the same as the manifold... Leads were/are in correct order and they are now new, Thanks


Will see if i can post a video of whats going on after the leads, plugs and timing were done

Thanks for all the responses so far

[SL/ENUT]

I have a feeling someone has pull the dizzy out and turned it a bit. No way should a 253 be running on 4deg btdc. Have a look on you strut tower at the sticker, has a tune bit on there.

[delcowizzid]

more like the balancer has moved timing could be anywhere may need to remark TDC on the balancer and check timing

[lil_mickey]

I have a feeling someone has pull the dizzy out and turned it a bit. No way should a 253 be running on 4deg btdc. Have a look on you strut tower at the sticker, has a tune bit on there.

This started all of a sudden, and nothing was moved,
My dad and I the only ones that ever lift the bonnet on my cars (after a very bad experience)
so thats what has me so puzzled i will have to post the vid tomorrow arvo as i have work a 3:30am.

Btw, the sticker has been removed...lol

[ari666]

more like the balancer has moved timing could be anywhere may need to remark TDC on the balancer and check timing

yeah thats a very valid point you make. easily could have spun the balancer and the popping is just pinging.

with your eyes closed (just bear with me on this one, lol) turn the dizzy clockwise to retard it till it runs really low rpm, then turn it counter clock wise and stop when the revs stop going up. whip out your light and check your mark, then retard 5 more degrees.

if your balancer is out, that process will get you in the ballpark, then its all about road tuning from there.

[lil_mickey]

yeah thats a very valid point you make. easily could have spun the balancer and the popping is just pinging.

with your eyes closed (just bear with me on this one, lol) turn the dizzy clockwise to retard it till it runs really low rpm, then turn it counter clock wise and stop when the revs stop going up. whip out your light and check your mark, then retard 5 more degrees.

if your balancer is out, that process will get you in the ballpark, then its all about road tuning from there.

Thanks,

I will give it a go when I get home

[strgas]

sounds like a weak valve spring at revs and the valve maybe be floating .

[lil_mickey]

sounds like a weak valve spring at revs and the valve maybe be floating .

I'm thinking that,

i have messed around with the timing for hours and still no cigar...

Moving house in two weeks and working 12 hours atm so i will have to look at it after xmas...

Sorry bout the vid, it is still on its way... lol

[ari666]

whats your vacuum doing? you taken a vac reading?

[lil_mickey]

whats your vacuum doing? you taken a vac reading?

will check tonight wit a vac guage

[ari666]

found this, i have not read it but the vac chart down the page will be what you wanna look for from your readings.
Tips on Reading Gauges: Vacuum Gauges

[lil_mickey]

This is not under load (clearly)

Also i couldn't hold the revs because of neighbors..

Haven't checked vac yet...