Hi guys i pieced together my motor and it is backfiring threw the triples ,the only difference i made to the engine from running was put a 1.6mm head shim in to lower the compression???I have looked at the timing,coil,had the carbs tuned and have no idea whats causing it???Any one have an idea??
TRIPLE CARBS BACK FIRING help
Started by
_troy_
, Oct 24 2007 03:11 PM
64 replies to this topic
#2
_CraigA_
_CraigA_
-
- Guests
Posted 24 October 2007 - 03:13 PM
An air leak possibly? On the intake side.
#3
orangeLJ
-
- Members
-
- 10,259 posts
Yes, yes I do post alot!
- Joined: 02-May 06
Posted 24 October 2007 - 03:23 PM
fouled spark plug, when my car fouls a plug it farts out the carbies like a bitch under load.
#4
_threeblindmice_
_threeblindmice_
-
- Guests
Posted 24 October 2007 - 05:34 PM
Does it backfire out the back going down a hill in top with your foot off the gas ?
#5
_troy_
_troy_
-
- Guests
Posted 24 October 2007 - 05:38 PM
no it doesnt go at all it idles beautifully then when you try to accelerate it backfires severly threw the carbs i checked the plugs and then put new ones in it still backfires and carries on under any acceleration.
There are only 3 things i have done to the motor since it was running
1.Used ARP HEAD STUDS
2.Replaced fuel pump with a universal electronic fuel pump 2-3.5psi 105 litres an hour or 28 gallons an hour.
3.used an ACL head shim to lower compression ,now that may be the problem???
I just had the car down at a performance shop and they cant even work it out,its like its pre igniting or something.
i replaced the distributor and coil and plugs and its still doing it...
if anyone has any suggestions or knows a good mechanic in perth metro area can you let me know,cheers troy
There are only 3 things i have done to the motor since it was running
1.Used ARP HEAD STUDS
2.Replaced fuel pump with a universal electronic fuel pump 2-3.5psi 105 litres an hour or 28 gallons an hour.
3.used an ACL head shim to lower compression ,now that may be the problem???
I just had the car down at a performance shop and they cant even work it out,its like its pre igniting or something.
i replaced the distributor and coil and plugs and its still doing it...
if anyone has any suggestions or knows a good mechanic in perth metro area can you let me know,cheers troy
#6
rodomo
-
- Members
-
- 18,001 posts
To advertise here, call 13TORANA
- Name:R - O - B Dammit!
- Location:Way out west of Melbourne Awstraylya
- Joined: 10-December 05
Posted 24 October 2007 - 08:10 PM
Did you tighten the head down evenly?
Does it have adjustable rockers?
Does it have adjustable rockers?
#7
Dangerous
-
- Members
-
- 948 posts
Forum Fixture
- Location:Adelaide, SA
- Joined: 14-November 05
Posted 24 October 2007 - 08:49 PM
Watch out if you have stromberg CD sidedraughts (ie 150s or 175s, as per LC and LJ XU-1s), as backfiring can rip holes in the diaphragms.
Backfiring off idle under no load usually points to a big timing problem in my experience. Umm, otherwise, what are you using for lifters, valves, and rockers? Maybe you've overadjusted the lifters on reassembly, and some valves aren't completely closing after the lifters have had a chance to pump up.
Backfiring off idle under no load usually points to a big timing problem in my experience. Umm, otherwise, what are you using for lifters, valves, and rockers? Maybe you've overadjusted the lifters on reassembly, and some valves aren't completely closing after the lifters have had a chance to pump up.
#8
wingnut
-
- Members
-
- 101 posts
Forum Member
- Joined: 09-December 05
Posted 24 October 2007 - 08:50 PM
It sounds like lean fuel mixture to me.
Presumably they are SU or Stromberg CD carbs.
You say the carbs were tuned ?? How ? on the running engine or the work bench ?
Get them to do an exhaust analysis or borrow a coloutune plug and see whats
going on in the combustion chambers. It will get hot quickly if its too lean so
whats the temp guage telling you ?
Good luck.
JR.
Presumably they are SU or Stromberg CD carbs.
You say the carbs were tuned ?? How ? on the running engine or the work bench ?
Get them to do an exhaust analysis or borrow a coloutune plug and see whats
going on in the combustion chambers. It will get hot quickly if its too lean so
whats the temp guage telling you ?
Good luck.
JR.
#9
flygtr028
-
- Members
-
- 51 posts
Forum Member
- Location:Sydney
- Car:1970 LC GTR Daytona Bronze
- Joined: 03-June 07
Posted 24 October 2007 - 08:52 PM
If you have used a 1.6mm shim you must reset valve clearance.
#10
_Bomber Watson_
_Bomber Watson_
-
- Guests
Posted 24 October 2007 - 09:07 PM
Perhaps even pushrod lengh??
#11
_troy_
_troy_
-
- Guests
Posted 25 October 2007 - 11:29 AM
yea the rockers have been reset a few times ,i lost the cam card but its a 35/75 cam set to 20 thou on the adjustable roller rockers i set them 3 times then now set them to 15 thou still backfiring,im using double valve springs.It sort of does sound like a timing problem but there may be a point about the push rod length ,u wouldnt think 1.6 mm would be a major issue??Im running 1 3/4 triple SU carbs which had been set on a bench and then fine tuned when it was idling ,the temp is fine i richened the carbs up and leaned them off to no avail and advanced the timin and backed it off turning the distributor and still it back fired!The lifters are small block chev solids,the roller rockers are yella terra with double valve springs on a 186 H head,but when you set the clearance on the roller rockers wouldnt that take up the 1.6mm on the pushrod length because i used the head shim?? Thanks for the input guys 
#12
rodomo
-
- Members
-
- 18,001 posts
To advertise here, call 13TORANA
- Name:R - O - B Dammit!
- Location:Way out west of Melbourne Awstraylya
- Joined: 10-December 05
Posted 25 October 2007 - 12:14 PM
Is it back-firing through 1 carby in particular?
Or
Any of the 3 randomly?
Or
Any of the 3 randomly?
#13
_rorym_
_rorym_
-
- Guests
Posted 25 October 2007 - 04:25 PM
I am with Dangerous its electrical/timing. Condenser?..vacuum advance ?..out a tooth?..harmonic hasnt slipped and you have the mark in the wrong place?...by the way..2-3 psi seems low to me..we had 7 psi on Yellow.
R
R
Edited by rorym, 25 October 2007 - 04:27 PM.
#14
orangeLJ
-
- Members
-
- 10,259 posts
Yes, yes I do post alot!
- Joined: 02-May 06
Posted 25 October 2007 - 04:47 PM
rory i run 2.5 PSI on my adjustable fuel regulator, the tripple SUs only need about that much, any more and they piss fuel out the overflow. im sure some hungrier carbies like webbers would need more pressure but the little SUs work well on that sort of pressure.
#15
_troy_
_troy_
-
- Guests
Posted 25 October 2007 - 06:21 PM
it was mainly the middle carbie backfiring now its all 3 doin it,i got an electric dizzie module but i swapped it for a points one and its still back firing,the bottom end hasnt been apart but whats the chances of the harmonic slipping or the adjustable timing gears slipping a tooth??i noticed the manifold is letting a tiny bit of air in not much though,?
Edited by troy, 25 October 2007 - 06:22 PM.
#17
rodomo
-
- Members
-
- 18,001 posts
To advertise here, call 13TORANA
- Name:R - O - B Dammit!
- Location:Way out west of Melbourne Awstraylya
- Joined: 10-December 05
Posted 25 October 2007 - 07:55 PM
Is it still idleing O.K?it was mainly the middle carbie backfiring now its all 3 doin it,
How do these plates seal? Do you need 2 head gaskets?
I read in the other post that you had trouble getting the ARP's to seal.
What exactly was that issue?
#18
_sunburst_metalic_
_sunburst_metalic_
-
- Guests
Posted 25 October 2007 - 08:35 PM
Have you got a carb balancer, they don't need to be out much to backfire, follow the post for the tripple adjustment ie remove throttle linkages, eye set the idle screws, set the main jet adjustments all even at 2.5 turns. set the idle with balancer, refit the linkages and tighten the lock nuts, from memory the middle just starts to open the 1 and 3 follow. Pretty easy sorted mine big time. I now just muck around with the mixture when i tune it depending on the colur of the plugs.
Sunburst
Sunburst
#19
_troy_
_troy_
-
- Guests
Posted 26 October 2007 - 12:00 AM
It idles beautifully no problems there,the head studs had a slight water leak coming up the stud shaft so i used master pipe sealant loctite that fixed that problem,the shim gsits on the block then you use hylomar sealant to glue the head gasket down and then the head goes on ACL make them,im really stuck and have run out of ideas ,it must be somethin unusua 
#20
rodomo
-
- Members
-
- 18,001 posts
To advertise here, call 13TORANA
- Name:R - O - B Dammit!
- Location:Way out west of Melbourne Awstraylya
- Joined: 10-December 05
Posted 26 October 2007 - 09:12 AM
Try re-tensioning the head, cold, in the order of sequence.
#21
Dr Terry
-
- Moderators
-
- 3,279 posts
Technical + Numbers Guru + Moderator
- Location:Eastwood (Sydney) NSW
- Joined: 13-November 05
Posted 26 October 2007 - 01:29 PM
Hi Guys.
A common cause of side-draught carbies backfiring is that the pistons are rising too quickly, try a slightly heavier grade of oil in the dampers.
Retarded ignition timing is also a cause, but I guess you've already tried advancing the dissy a bit.
Dr Terry
A common cause of side-draught carbies backfiring is that the pistons are rising too quickly, try a slightly heavier grade of oil in the dampers.
Retarded ignition timing is also a cause, but I guess you've already tried advancing the dissy a bit.
Dr Terry
#22
_gtrtorana_
_gtrtorana_
-
- Guests
Posted 26 October 2007 - 02:37 PM
I don't run any oil in the dampers on my 6 but mine are strombergs, maybe SU's you have to. Is your fuel OK?
#23
orangeLJ
-
- Members
-
- 10,259 posts
Yes, yes I do post alot!
- Joined: 02-May 06
Posted 26 October 2007 - 04:40 PM
pete (why PSI) doesnt run any in his SUs, well he didnt used to when he had them, and never had a drama, i run sewing machine oil in my carbs without too many hassles.
#24
_troy_
_troy_
-
- Guests
Posted 27 October 2007 - 04:14 PM
hmm sounds like i am goin to have to throw the motor in the bin after spending thousands on it,Its such a shame that there isnt any reliable decent mechanics left that can sort this shit out,not far off chuckin a match into it 
#25
_willo_
_willo_
-
- Guests
Posted 27 October 2007 - 04:45 PM
mate i fully understand your frustration.. i had the same problem heaps of times and there were different causes on several occasions .. the main reasons were 1, adjustable roller rockers vibrating loose after being set correctly 2, air leaks on intake manifolds....sorted the rocker problem with some super strength locktite and a shitload of patience...sorted the air leaks by using some brand new extractors and having the exhaust shop match the thickness of the extractors to the inlet manifolds prior to fitting and then meticuously following tightening sequence and torque settigs...DO NOT give up ...just keep trying and trying ...you will sort it eventually and will be surprised by the diff when you do...btw i use auto trans fluid in mine and run 3.5 psi but my setup is on a stockcar so does not have much relevance to something used on the road...good luck with it and keep at it cheers shane
1 user(s) are reading this topic
0 members, 1 guests, 0 anonymous users
