Hi everyone,

I have a problem with a 1966 122 which I can't seem to make sense of. For about 6 months it has been developing an increasingly serious backfire problem through the carbs.
I had the carbs professionally rebuilt, replaced the coil and all ignition components electrical components, installed a pertronix ignitor, and have gone through long and repeated setting and re-setting of the timing, valves, and carbs. Everything appears to be working properly but the backfire won't go away. At this point it is virtually un-driveable.
No vacuum leaks, no timing gear noise, 165 psi compression in all cylinders. HELP!

Thanks to everyone for the input- I had a chance to take a closer look today, removed the valve cover and cranked the engine. Sure enough- two of the valves are not moving as much as the others. By a lot, like 1/4"! I'm planning to replace the camshaft, but before I begin, just wondered if any of you had tips for that job?

Should I remove the engine first? What else should be replaced while I'm in there?

Thanks again for all the help!

Jacob

You don't have to remove the engine, but replacing the cam involves removing the cylinder head to change the lifters, so it is a big job.

Besides getting quality parts, the most important step is the proper cam break in procedure and the continuing use of engine oil with ZDDP or using a ZDDP additive.

If you don't have one yet, get a repair manual. My 1st manual was a Haynes and it is easily available.

However, the Haynes got all its information from the original Volvo manuals and Volvos torque specification for the camshaft nut is too high. IIRC, there was a TSB revising the torque spec because the cams were breaking with the original spec.

Volvos original cam nut torque specification is 94-105 ftlbs. I use 75 ftlbs, some red Loctite and stake the nut.

My tips are to use an Elring head gasket set, update the front crankshaft seal to neoprene and to use a quality steel timing gear set if you have a fiber timing gear. I don't consider the Cloyes set as a quality steel timing gear set although it is a lot cheaper.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

I've done (sadly) a few cam swaps on my various pushrod Volvo's over the years. It's the weak spot, really.

The radiator and grille needs to come off, then there's room for the cam to come out the front end of the engine. The head needs to come off to, as mentioned. It's a good time to consider having it rebuilt with hardened exhaust seats, if it doesn't have them already.
--
'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

NorthNH and B20Paul are the ones in this thread suggesting making sure u have good ignition and I am with them. Too many times I have suspected exotic problems when it was just a bad cap or rotor etc....use your timing gun on all plug lines as welll as the main line from coil to look for regular spark...if you are getting intermittent spark find out why....Petronix uses the original cap and rotor and in my experience many "new" C and Rs are garbage entire runs can fail so you need to pick up replacements from different store/manufacturer. Guess it could be valves but sort the ignition first...
re overheating while sitting in traffic.....120s do that ....the WP does not push enough water idle and on a hot days the temp will climb while in traffic. give it a little gas so it runs 200 or more rpm faster should cool it down. BTW if the "choke" mech. is set up right, pulling it out a bit will give you 200+ rpm without making the mix richer.
--
Patrick, '68 220 , '53 GMC 4104, '97 VW Transporter.

Try clipping the timing light to each plug wire. Check for consistent firing of all cylinders.

Step one is to swap your electronic ignition for a stock setup, that is by far the easiest and quickest way to see if it is fuel, ignition, exhaust or mechanical. My guess is failing ignition.

I've knocked the lobes off of plenty of cams and it doesn't give those symptoms. Compression test will rarely find a bad lobe because you don't need much of a cam to fill the cylinders at cranking speed. If you could crank the motor at 4000rpm and do a compression test, that would tell you alot!
Intake lobe loss just gives a reduction in power without any other symptoms, Exhaust lobe loss gives less of a power loss, but your car will sound like a Subaru.

If it's not the ignition, I would check the exhaust system, mufflers can collapse, a simple rev to 4000 and feel what's coming out your tail pipe.
--
69 142S Overdrive + 69 164S Manual

I had a Jensen-built P1800 in a few years ago that still had its original B cam. It would idle smoothly and run okay at low revs, but anything over 3000 would make it pop out the carbs like crazy. I kept the cam I took out of that one as an example of just how flat lobes can get and still run.

My old 1800E did the same thing with a flat exhaust lobe. It would start just fine, had great even compression, and would run normally at low throttle/rpm. But as soon as the pedal went down, or the revs got a little higher, lots of backfires.

This was a headscratcher for me for a while until I noticed one of the rockers not moving up and down much at all while I set the valve clearances for the Nth time.

I'm assuming that at higher RPM's the cylinder simply couldn't exhaust very well, and when the fresh intake charge came in there was still a fair amount of hot exhaust in there, enough to light off the intake charge immediately.

By the time I figured that out, the cam lobe was *really* flat.
--
'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

I have heard of a Pertronix failing in the way you describe, causing misfires.

99% of the time when Pertronix fails, they just stop working entirely, but it could be the culprit here.

If you haven't already, you might try throwing a set of points on the car to see if that clears it up.

Aside from that, I would use a remote starter and engage the starter with the valve cover off to make sure all valves are opening and closing the way they should.

I doubt this is a carb issue from what you've described.
--
http://dylans544.blogspot.com

Backfire through the carbs is typically a timing or valve issue. If the intake valve is shut when the spark occurs...

Does it happen as soon as the car is started or only when the engine is hot. If only when hot, do you have a valve that has a thin spot on the edge ?


Greg

Hey, thanks for your reply! It is worst when I step on the gas hard or
accelerate up a hill, and there is no power.

Doesn't matter if the engine is hot or not.

Timing isn't the problem, I'm sure.

How do you identify a flat spot or a sticking valve? I do to hear any loud
noise, but have wondered if it's a worn timing gear or distributor problem.

could it lean back fire?

It was too late during the night to post a reply (tap,tap tapping would have disturbed "she who must be obeyed") but my first thought was as 63pv wrote---a lean misfire--caused by a number of possibilities--lean adjustment from the get go - dropped needle - low float chamber level - low oil damper level - uneven throttle opening - etc. -- Dave

"... she who must be obeyed ..."

Thank you for the John Mortimer / Leo McKern "Rumpole of the Bailey" reference!
--

"... she who must be obeyed ..."

Thank you for the John Mortimer / Leo McKern "Rupole of the Bailey" reference!
--

Thanks, Dave.

I have been looking for issues that could cause a lean backfire, but just had the carbs (SU HS6) rebuilt by sucarbs.com, and have inspected for all of the things you suggested. I recently put a heavier oil (SAE 20, as recommended by the original manual) into the dashpots. Have tried enriching the mixture all the way and moving the timing in both directions. Nothing makes any noticieable difference.

Have you checked for a flat camshaft lobes?
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

Also remember a flat lobe will still seal the combustion chamber and indicate good compression....I say Eric is on to the real gremlin.

Good idea! Not yet, but I'll have a look this weekend! Any advice on doing the inspection?

Turn the engine by hand with the valve cover off, and watch the exhaust valves. I'll bet you have one or more that barely move.

Thanks Phil,

I'll give it a look this weekend- but wondering, wouldn't a worn exhaust lobe be apparent in the compression test?

Also, it started overheating when stopped in traffic around the same time that the backfire problem began, and I've noticed that the timing mark jumps around a bit when I'm looking with the light.

Took the distributor apart and checked / cleaned the centrifugal advance mechanism, it all seems fine.

Were it me, first thing I'd do is replace distributor with a spare. Not so easy though, if you don't happen to have a spare kicking around.

I will be curious to hear what you find when you check movement of valves. If Eric is right, well... picture your compression test. Intake opens, piston goes down sucking in air. Then compresses it on it's up stroke. Both valves closed, so compression test is unaffected.

Speed things up to running speed, and picture exhaust gasses being unable to escape because the business end of a cam lobe isn't there. Yep, that would wreak some havoc for sure. I have a feeling that Eric is onto something. And it's sure easy to check.



--
-Matt I ♥ my ♂

Will repeat here what I wrote elsewhere...

"...and have gone through long and repeated setting and re-setting of the timing, valves, and carbs. Everything appears to be working properly..."

Did the timing mark jump around for six months while you were wandering around your engine's pieces/parts?
If so, was it wise to ignore the clear indication that your distributor and whatever that petronix ignitor thing is was not working as well as your visual inspection indicated to you?

If good uniform compression, it's a waste of time to touch carbs until ignition system is functioning correctly, and yours isn't...

Assuming everything else distributor wear-related has been eliminated eg obvious shaft wear/play, distrib gear, etc, your problem is most likely with the weights/springs controlling advance, that's why it idles fine (weights not in play), and that's why it overheats (jumping-around timing mark=erratic advance=overheating) and backfires.

But I don't have any way of factoring in the electronic add-on, which I've had no use for in 40 years of Amazons and Bosch disributors.

Timing lights and dwell meters are wonderful things, and they don't lie...
Good luck,

Michael

.............or you've got one or two clearances over-tight and those valves aren't seating at all.

How old is your fuel? Did the problem come on suddenly?

Thanks for your response! Problem has been going on for 6 months, not old
fuel.