For starters, this is the first carburated engine I've ever worked on, though it's a simple one, I'm still learning.
My b18 has run solid for the year that I've owned it, up until last thursday... I filled up with premium and pulled out of the station, she died at the first light I came to - started right back up and got me home. The next morning on the way to work, she gave up! backfires galore!

She was due for a tune-up anyway, so I replaced the cap, rotor and plugs, checked for spark - two wires were bad, replaced those... still backfiring...so here comes my carberatto quandry: I've go 4-bolt SU's - when the motor is idling, and I open up the forward carb, she revs just fine, but when I open up the rear, the motor cuts out / backfires... I'd imagine it something to do with the fuel mixture? An thoughts?

Michael H.

Portland OR

I'd check one simple thing first: is there oil in the piston damper on the rear carb? If this has run low, pressing on the gas will cause a short-term lean condition which will result in a severe miss which can be mistaken for a backfire.

Steve

Wow, all this talk about worn cams and wobbly distributors does not make sense to me. Nor does the points closing up, since he re-tuned it after the problem showed up.

As I read it, the car was running fine until he filled up, so it sounds like bad gas...perhaps a load of water got in there and filled the back float chamber, but somehow managed to bypass the front one? Or perhaps the gas he got had a fairly high ethanol content and disloged a chunk of trash and clogged up that carb.

Either way, it sounds like a simple clean-up of the carbs is in order along with a new fuel filter, and perhaps a draining of the fuel tank if it turns out there is water in the carbs.

He omitted any mention of replacing, or even looking at the points, and also said he had no prior experience with older engines, so a points-related problem is not beyond consideration, especially if the distributor cam has some wobble and/or uneven wear to the lobes.

Bill

OK, point made. I read into his post something that was not there.

However I personally have not had an engine that was running strong suddlenly start backfiring and running poorly because of points closing. My experiece is that the car slowly starts running poorer until it really runs bad.

Still, my main point is that it sounds like the car ran well up to teh point he filled up, and that is when it started running poorly...which makes it appear it is more likely fuel related. I certainly would not hurt to check teh dwell or gap and timing, but I'd look at teh fuel system first

Well, exactly what he talks about is what happened to me 5 days ago. I was driving, and all was fine, then a HUGE backfire and car was dead. cleaned the points, got home fine.

then yesterday, it does it again. (having trouble finding points) I was 40 miles from home, so I cleaned them and started heading home. I was doing about 70 when all of a sudden BOOM!. sounded like a cannon going off. I pulled to the side of highway and cleaned he points again to get the last mile home.

My car sounded like a B-52 taking off.

When I got home, I looked under tihe car to see my muffler exploded!!

All because of worn points. It ran perfectly right up to the backfire.




As for finding the points. I have a 67' with a B20 transplant. I wasn't sure of any info on the engine, and every set of points they sold me where wrong. Then I got a smart guy that knew his stuff at NAPA. And he found the set I need, they are for a MARINE application for the volvo.
I ordered a set, should be here today.

Cheers!
Kent

Here is a picture of my exploded muffler due to points gone bad!




Cheers!
Kent

Well, OK then I stand corrected. I guess a round on the house is on me.

Cheers!

I have had bad points cause exactly that. More specifically, a bad condensor and worn points had me fixing the point every 1/2 hrs to get home.
--
Patrick, '68 220, '92 Eurovan (work truck).

No doubt it can, and it's happened to me too...only not immediately in my case.

The clue is when the problem started: almost immediately after filling the car up. I don't think points care if you just filled up or not. :)

Guys;

Thank you Champ6 for making the most sense and voicing what I thought of first also (logic dictates that the FIRST thing to suspect after something has been changed - new fuel added in this case - should be thoroughly checked...it wouldn't be the first time a pump pushed out water, and BTW charged you dearly for it!)...while I don't discount Bill's point, I personally believe the "wobbly distributor shaft" has gotten WAAAAYYYYY too much bad press (and likely caused the recycling of many a distributor before its day), and I further agree with Champ here that it sure as hell wouldn't all of a sudden cause the symptoms experienced, and to that extent...it takes literally thousands of miles and years of wear to wear a distributor to the extent that it would cause the symptoms, and the problems it can cause sure wouldn't come on in a flash as these have...those few degrees which the timing moves because of shaftwear will not even be detectable without a timing light...the points spring keeps any slop in the shaft loaded up anyway...you will feel it when wiggling the shaft by hand, sure, but it has little to do with what happens in use...

Cheers

You're welcome. I do think you ought to check your points and timing too as basic maintenance even though I don't think it is the cause of your symptoms.

What I have observed with a badly worn distributor is that the points gap is inconsistent as the cam rotates which...

A: Makes it impossible to set the points gap correctly.

B: Creates a situation where the points gap is much too small for decent operation
on one or more cylinders, but still works decently on the other cylinders.

The amount of time that elapses between having the engine run (more or less) with such a distributor, and run like crap can be very short when an erratic points gap is concerned; there seems to be a point of wear at which the performance degrades very rapidly, even within a day, in my experience.

Pertronix ignition is very, very good.

And no, I'm not insisting the guy's problem is ignition related, although an ignition problem isolated to one or two cylinders can make it appear as if one of the carbs are not working correctly.

Looking for fuel system contamination is certainly warranted.

Bill

OK, your descrition makes sense, even though I personally have never had it degrade that fast, I don't mean to say it couldn't, but I've just never experienced it. I hadn't thought about how the wobbly shaft really could make it misfire on only one or two cylinders, but sure, it really could.

When my distributor went south as I mentioned, it was all to nothing in about 1/8 mile... admittedly, in perhaps over a half million miles of driving B18/B20 Volvos, it's only happened to me that one time. Not a common thing.

But, he didn't mention having touched the points... so that's where I'd check first personally, as it takes about 15 seconds to remove the cap and rotor, check gap... while in there, another minute to carefully check that no wires (or springs) are (or can become) grounded, and that the shaft is tight...

once that is done, check for spark at the plugs. If there is spark, then move on to fuel issues (which may well be the case... again, my hasty reading didn't pick up on the 'just filled up' part). But I'd still take a quick couple minutes to rule out any ignition issues... it's a lot easier, and a lot less messy.

Oh so many times I've done a complete tune up, and had the car die later in the day due to a fuel pump issue. I *never* rule out coincidence.. they happen a lot.

Not trying to start a war.. just saying how I'd go about the diagnostics... easy clean stuff first, more difficult and dirty last.




--
-Matt I ♥ my ♂

Yes, and to you and spectratone, I'm not looking for a war either. It's just a difference of experiences.

How about we have a friendly wager? If you and spectratone are right about the points, I'll buy you both a virtually beverage of choice ( ore rela one if you're not too far away), and if I'm right you can do the same. Deal?

Now to pdxp220: Please let us know what you find, and we'll all three buy you a virtual beverage of choice!

You're on :-)

I think you'll win too... but I'd still check ignition first :-)

--
-Matt I ♥ my ♂

but I'd still check ignition first :-)

Yeah, and when I want a beer, i go on line and be a smart a** first! ;-P

Thanks you guys for all the info!!

I've got a couple sets of decent HS4 carbs sitting in a bin... I'll check my pionts first off - then check the oil in the rear carb oil damper resivoir - then try to peek into the tank for possible rust/debris.... and I'll be reviewing this thread for sure!! Keep ya'all posted!

- Michael H.

PDX

- '67 P220 rat rod...

It sounds to me like a large plug of crap has entered the jet tube on the rear carb. The quick and dirty solution is to remove the chamber cover on the rear carb, and the piston and chamber, and then blow compressed air through the jet. Normally there is a collection of rust particles in the bottom of the bowl - I think maybe some has gotten into the delivery tube, or lodged in the turn to jet.

At first, my thought was timing gear... but in reading further to find that it got home, and that it idles OK... no. Not that.

I'm with spectratone on this one. Check your points. I bet they closed up.

Or, your distributor could have gone south. Check for wobble in the shaft... sounds similar to a problem I've not thought of in a long, long time. Totally worn out distributor that would wobble about, opening and closing the points at will...

Just pull the cap, wobble the rotor, and check to make sure the points open when they should. I think it'll be obvious.

--
-Matt I ♥ my ♂

I don't think it is carbs, and carb balance is not that big a deal unless it is
ridiculously off. Cams go flat though, not too noticeable at idle but really
when you rev them up. Usually (not sure why) it will be an exhaust lobe so that
after combustion the stuff doesn't have anywhere to go till the intake valve
opens, so «¡¡BANG!!» it comes back out the carb, slamming the carb's piston
down in the process.

I'd pull your valve cover and carefully measure the lift of each valve. It is
easiest to measure from the top head surface (where the valve cover gasket goes)
and the valve spring retainer). ALL of them should lift about 3/8". Since the
backfiring is in the back, start measuring at the back. About 20 years ago I
had the same deal with Li'l Red and after doing a valve job, changing cylinder
heads, changing carbs and a bunch more stuff finally noticed that if I pulled
#4 spark plug wire it stopped backfiring but would make a cloud in front of the
back carb (air cleaner off). Turned out that the exhaust lobe on #4 was pretty
much gone. I had tried to economize on lifters by regrinding them (all the way
through the hardened layer) and they retaliated by eating a cam lobe!
As IPD says, reusing old lifters = false economy.
--
George Downs, Bartlesville, Heart of the USA!

You should try checking your points gap and ignition timing first, adjusting as needed, and then try synchronizing the SU's if the car is still running badly.
Also make sure the chokes are correctly engaging and disengaging.

I live in the Beaverton area, BTW. I've got an air-flow meter for balancing the carb idle, and a timing light.

Bill

if only one carb is working the cam and ign. is good the rear carb is not getting fuel check the bottom bowl for junk a plugged seat or the carb has a bad vuccun diafram if the top of your carb is short take the top off and check the rubber diafram i hope you keep trans oil in both carbs

good luck k.m.beef

last note on points, checking gap is step one, then using a dwell meter is step 2. You also need a timing light to check timing as static timing is pretty innacurate.
--
Patrick, '68 220, '92 Eurovan (work truck).

...so I have to dissagree with your comment...I've checked mine a number of times with a timing light after having set it statically, and it was ALWAYS within +/- 2 Degrees, which would certainly be just fine left like that, and would certainly get you away from any timing which might be causing backfiring...

Cheers