View Full Version : Excessive backfiring

01-05-04, 07:33 PM
Can anyone help/give any advice. Whenever i take my foot off the accelerator the engine almost continuosly seems to backfire.
What can i do to overcome this problem???


02-05-04, 12:11 AM
Check timing and check carb, make sure it`s not running too lean.


02-05-04, 12:12 AM
Check for vacuum leaks. Disconnected pcv hose, Broken vac advance diaphragm in distributor etc.

Check also for exhaust leaks near the engine too.

Also it could be an ignition advance issue. If you are running a 4 barrel carb, and have your vac advance connected to the "full" port, try with the vac advance connected to the "timed" port (above throttle plate, ie no vacuum) when the throttle is against it's idle stop.
If you had been running on the "full" vacuum port, you may need to re-set your idle speed.


03-05-04, 07:18 AM
Hi Blue, I had a similar problem with my car when I first put it on the road as when ever I took my foot off the throttle. Turned out in the end that the metering rods in the carb were wrong when I took it to a rolling road it was fine on acceleration but when cruising it was running really rich. New rod fitted plus several tweaks on the timing and never done it since at all.


Martin :tu :tu :tu :tu

10-05-04, 07:51 PM
Hi, I had a similar problem and found your suggestions very helpful. I have sealed the leaks on my exhausts, sorted my inlet manifold leaks, refitted my PCV pipes and this has reduced my backfire to a crackle. I have checked the static timing and had a look at a couple of pots with a colour tune and cant see anything radically wrong, there again I cant see very much at all! The problem may be running too lean as there is no problem if there is a bit of choke.

Trouble is Im not really sure what my starting point is, I bought a 3.9 but the inlet manifold/carbs and dis where separate. I was told that they had all once been together but Im not sure so I need to start from scratch. I have set the static timing to 4 degrees before as per the RR manual but is this right, particularly as Im using unleaded!

Can anyone give me a base level starting point that I can work from, and or suggest a way forward.



10-05-04, 08:39 PM
PS. Should have said the dis is a 35DLM8 and I think the SU's are HIF6.


11-05-04, 05:44 PM
Mixture too lean, air leak on exhaust or carb, ignition too far retarded or all four!! Eric ;-)

12-05-04, 07:02 PM
Yup, It was all 4 and I still have the problem, what I day I have had. Having sorted out all the little leaks etc I looked at the timing again and found that the dis was a piece of c**p so I stripped it down and cannibalised another dis to make one fully functioning one.

But still got the crackle, played around with the timing and found that by advancing the dynamic to 10 or 12 BTDC I just about get rid of the problem. What worries me is that the manual says, for a 3.9 with 9.35:1, it should be set to 4 BTDC dynamic. However, I note from various posts that 10 BTDC is recommended as a base point for 3.5s.

Any views of what the timing should be set to or where I can get further info would be greatly appreciated.


12-05-04, 08:26 PM
Mac, normal 3:5 is 6 deg BTDC static, but with timing light is 10deg BTDC @ 1200 RPM as a rule of thumb, but you seem to have sorted the problem anyway which is good eric :tu

Purple AK
12-05-04, 08:34 PM
Have you checked your timing marks are true against TDC on No1 Piston?
They are known to slip.

12-05-04, 09:06 PM
Thanks Eric, I hope I am near to sorting it, trouble is I am still at the chassis stage and am concerned that if I have set it too far advanced I will get a load of pinking when it is driven.

12-05-04, 09:09 PM
Chris, that is very interesting and could well be the problem (or answer), I had assumed that the front pully would be keyed on somehow, I will check tomorrow, Many thanks


Purple AK
12-05-04, 11:20 PM
Mac the front pulley is keyed on but the damper ( Which the timing marks are on) is bonded to the centre, and prone to slipping!. I think you'll find several posts relating to this problem if you do a search.

13-05-04, 05:32 PM

Still not totally sure whether the timing marks have moved or not as its so difficult to tell whether there is any movement between 3 & +3 degrees, I want to believe that it is 4 degrees out but suspect it is spot on. Cant explain why it runs so well if its set to 12 degrees plus though.

I think I will have to put it back to 4 and set up the carbs.


13-05-04, 08:21 PM
Chris sorry mate but I have never heard of a RV8 damper coming apart!If you know your business there is no need for a timing light either. ;-) :tu

Happy Jim
13-05-04, 10:06 PM
!If you know your business there is no need for a
>timing light either. ;-) :tu

Hence why I use one :tu

Purple AK
13-05-04, 10:19 PM
I stand corrected by your vast knowledge :tu I am sure that there have been several posts relating to the subject though :tu

14-05-04, 08:31 AM
Come on James you know you have vast info on the RV8 :)

Funny thing is I have just bought a timing light, having made that comment how sad is that! x( As a point of interest you can actually set the timing up with a vacuum gauge as well Eric :tu Hope to see you Sunday??

14-05-04, 08:41 AM
Morning Chris, not that vast mate, have been messing round with a colleagues Dax off and on for about three weeks now, changed more bits than you can shake a stick at and still it has a misfire when hot!!!!

You are probably right mate re the damper but I have never come across it before myself, but I think there only bonded onto the rubber? Its also possible to get the wrong timing marks when using some of the Range Rover pulleys, key ways in a different position cheers Eric. :tu

Happy Jim
14-05-04, 12:51 PM

Might be tight for this Sunday, I've still got a list of things to fit (heater matrix, battery, hydraulics (and bleed), seats, rewire fan/horn/reverse light/speedo sensor (lousy job 1st time round, decided to do it proper-like this time). All of course only happening when I get the complete double drive free, ban the kids coming within half a mile, lock the dog up and wrap my self in bubble wrap (to avoid scratching the paint of course :tu - won't last x( )