• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Engine Backfire

Share this page

G

gsable

Member
Country flag
Offline
Hi Guys,
I have been diligently working on my BJ8 and have the following problem. I am very frustrated so any help would be appreciated.

The car only has 38,000 miles (??). The SU's have been totally rebuilt. The PO had fitted a Petronix ignition.
The car stars great and runs smooth until about 2,500 rpms's. It then exhibits a "crack" sound like a backfire. If you increase the throttle slowly, it doesn't occur, but if you give it power quickly, it stumbles and I get that sound at about 2,500 rpm's.

Here's what I've tried so far:
- Checked timing and it seems fine.
- Pressurized each cylinder and check valve seating - perfect.
- Check rocker adjustment with only minor changes.
- Check compression in each cylinder - fine
- Changed from Champion plugs gapped at .025 to NGK gapped at .030 to see if there was a difference - no change.

HELP!!!!!!!!!!!!!, before I blow my brains out.
 
Check the fuel pump for correct pressure and output. Sounds like you're going very "lean" when you give it a lot of throttle. The carbs can backfire through the aircleaners and they make pretty lound sounds.

Blow the fuel lines out with an airhose,

Check for proper vacuum advance too!

How do the plugs look? Good color or weak color?
 
The most likely thing would be the SU "dampers". The little round cap/hydraulic pistons on the top of the carbs.

Remove the cap assemblies, don't interchange the two, & check for fluid in the inner chambers. They should be filled to about 3/8" from the very top of the carbs.

You can suck any old fluid out & refill with 20 or 30 wt. oil. When you replace the dampers/caps you should feel some hydraulic resistance as the caps are lowered. No resistance when the caps are raised. There is a one way hydraulic valve on the damper piston.

The best fluid viscosity will depend on your particular engine. Some use ATF, a few use much heavier. 20 was the original recommended.

The dampers take the place of the accelerator pumps on more conventional carbs.
D
 
Thanks, Dave
I will check the dampers again. What is the likelihood that is is either the Petronix or the coil?
 
It's easy enough to check. You can pull off a spark plug wire and hold it next to ground and see if you get a good hot spark. Backfiring when giving it a quick turn of the throttle turned out to be a bad original coil on my car a couple of years ago.
Check the primary side of the coil with your ohm meter for 3 ohms. If you find it substantially more or less than 3 ohms then the coil is bad. I doubt it is the petronix. If that was not working I don't think it will fire at all. Of course there are many other things that can cause a backfire when giving it a quick throttle but since you asked about the coil, you should know that it is a definite possibility to cause your symptom.
Ed
 
Have you checked the timing advance? If you have a vacuum distributor check the advance to see if it is free. If no vacuum advance, check and see if the movement is good. Might have moisture getting into the advance and not working properly. Moisture comes from the heat and cool of the engine. Low miles will then let it work on the inevitable rust.
 
Is this a new or recent problem?
Have you changed/repaired/rebuilt anything recently?
Always best to check out anything that's been changed first!
 
Larry,
I checked the advance today and it seem to be working. I suspect that this might be the problem, so I will check again.
Thanks for the input.
Gsable
 
I’m with the guys that think its something to do with the dampers. When you whack the throttle open, the vacuum change causes the dampers to rise. If they rise too fast, you get a lean mixture which can cause a backfire through the carbs. As Dave said too little or too light of an oil will let the dampers rise faster.
 
what type of spark plugs are you using and when did you change them last...I was getting crazy backfires on my 100/6...then I changed from the bosch plugs to ngks..BR5ES...almost all of the backfire is now gone...except on startup...all from changing the plugs...unbelievable...just a thought
 
Back
Top