popping/backfire help

[wil]

Hi All,

I have a 1600cc early bay with twin dellorto 36 carbs, filter king regulator & stainless exhaust, everything else is stock, running a stock vac dizzy. been running this setup for 6 yrs

During a drive the other day it started to backfire/pop - nothing has been changed, messed with, modified for years so I thought it must be 'muck' in the carbs. Here's what I've done so far but to no avail;

Clean all jets one by one
Clean inlet needles & checked floats are working
Cleaned spark plugs (they were quite black)
Cleaned the leads at the plug ends as some of them had started to rust
Checked rotor arm & cap
Changed fuel pump with new replacement

There doesn't seem to be a pattern to the backfiring/popping but it does seem fine until it's warmed up - then it does it on acceleration/idle & deceleration quite randomly. The only other thing i've noticed is the tinware seems much hotter than usual but i could be imagining this!

Anything I've missed should check next? Spoke to a local 'specialist' & he thought it sounded like a burnt valve...really would like to sort this myself before taking it to a garage!

Oh - & it drives fine, no loss of power or hunting on acceleration...

Thanks in advance

 

[meowdmucker]

Could you have a small hole/joint leak somewhere in the exhaust?

 

[Walt]

Cleaned spark plugs (they were quite black)

I'd be tempted to try some new plugs.

Might not be the actual cause of your issue but I once witnessed a new set of plugs sort out a misfire.

 

[Oslo_mark69]

Check valve clearance and some decoke juice

 

[Trikky2]

Do you have servo brakes?

 

[Mr Brightside]

Do you have servo brakes?

What are you thinking Tricky ?
air leak in servo, pipe or faulty non return valve?

 

[Tristan71]

It's running lean bigger idles

 

[Trikky2]

Do you have servo brakes?

What are you thinking Tricky ?
air leak in servo, pipe or faulty non return valve?

Well its one of the possibilities.

Assuming the valves and their clearances are ok (cheap compression tester would check this) then popping back can only be caused by a mixture or ignition fault. If the ign timing is ok then the dissy cap needs checking for signs of tracking.

If all that checks out then its down to weak mixture, in which case your looking for too little fuel (blocked jets) or too much air (leaking gaskets/faults letting air in).

Hence the thought of servo - easy to check - just block off the pipe temporarily.

 

[wil]

Do you have servo brakes?

Hi - no servo brakes

 

[wil]

Small progress - drove round to local 'specialist' we pulled each lead from the dizzy & when we pulled cylinder 1 lead it made no difference (now why didn't i think about doing that! Just shows two heads are better than one!) - leaving it to cool down & will check valve clearences. Will keep you posted...

 

[wil]

...valves adjusted & no improvement - compression test = 40 so he's sugesting a bent valve...

In need of a spare engine quick time!!!

 

[Tristan71]

What did the other cylinders test out at

 

[wil]

What did the other cylinders test out at

not sure - i doubt he tested the others... i'll ask him tomorrow

 

[beef]

Are your manifold boots still air tight? Sounds similar to how mine was when I imported it.

 

[69 panel porn]

I've been having the same problem for a few months now and still haven't got to the bottom of it I'm checkin the valve clearnecs to but I've changed
The fuel pump
Fuel lines
Fuel filter
Regulator
Rebuilt the carbs
New plug
New leads
New dizzy
New cap and roto arm
New coil
But I'll try pulling a lead of wilst it's running to see if anything changes after I've checked my valve clearnecs any thing else any 1 could think of

 

[Jimbly]

I've had similar and getting worse for a couple of weeks and eventually, it ran like a sack of proverbial on the way home from Dub Vacation...

Comp tested the cylinders I could get to and saw 10bar on each, give or take (150psi-ish).

Started working my way through it and noticed the carb/manifold nuts had worked themselves almost finger loose, so tightened them, as well as the manifold/head nuts (again, those I could get to).

Re-set valves (couple slightly tight), new points and condenser (old ones pitted), re-set timing and last night, much, much better. Still very slight popping on over-run (that'll be the inaccessible nuts, I guess) but no more spitting back through the carbs.

Rest of nuts will have to wait until winter - just can't face pulling the motor again this summer. Only a couple more shows to go...

Hope that helps.

Jim

 

[wil]

Hi guys - hope you get yours sorted, do keep us posted on your progress.

No progress here - although i checked the valve clearences & they were both closed up on number one so will adjust these when I get the chance. Will also do a compression test myself to see whats happening & make sure the manifold nuts are tight. Got a spare cylinder head on it's way to me as we speak & new plugs & other bits to try. Not trusting the garages diagnostics :roll:

 

[robo]

hi ya, mine had simular fault and turned out to be inlet manifold rubbers,cheap to replace but a bit fiddley..

 

[ZedBed]

Hi guys - hope you get yours sorted, do keep us posted on your progress.

No progress here - although i checked the valve clearences & they were both closed up on number one so will adjust these when I get the chance. Will also do a compression test myself to see whats happening & make sure the manifold nuts are tight. Got a spare cylinder head on it's way to me as we speak & new plugs & other bits to try. Not trusting the garages diagnostics :roll:

You checked the clearence, found the gaps closed up, but popped the cover back on anyway and carried on? Good luck with that.

 

[mi2itsdl]

Just had similar issue on mine fixed. Thought it was either going to be a blocked idle jet or carbs out of balance.

Turned out to be coil was knackered and was cooking itself.

The guys at Autotechniks put an ignition spark tester on one of the spark plugs, and you could see that there was not a constant pulse. It was fluctuating quite bad, therefore causing the unburnt fuel to ignite, when it got a spark. Only happened at low revs, but was banging and popping quite bad.

replacement coil and all seems ok.

Have you tried this yet?