Severe Misfire

[archigator]

My Sprint began to backfire upon decelleration a few weeks ago, but otherwise seemed to run fine. (A couple of months ago I had my original mechanical fuel pump rebuilt, and before I reinstalled it I totally drained my stainless steel gas tank and replaced the original gas line from the tank to the fuel pump.) At that time I also took off the fuel sender and visually inspected the inside of the tank with a flashlight. It was clean as a whistle, so I don't think I had any issues with crud in the fuel.

Last weekend when I went to crank up the Sprint for its weekly Sunday drive, the car began to backfire upon starting and barely made it around the block. This weekend I changed the plugs, ignition wires, coil, rotor, and converted to a Pertronix electronic ignition. I took the Webers off to make sure that the o-rings were properly seated and fired her up. I did not fiddle with the settings on the Webers at all, as they had been running fine for several years without being touched. When I pull the throtle, they all "squirt" as they always have. I pulled out the jets, cleaned and reinstalled them. The distributor shaft doesn't seem to have any loose play in it.

I can only think that it's either ignition timing or bad gas. I've read the previous posts regarding similar misfire issues, and I can't think of what else it might be. Any suggestions?

Thanks!


Gary
'71 Sprint FHC
Miami, Florida

[ardee_selby]

Gary,

Hmmmmm...I take it you have read Randys' epic saga!

However...you haven't mentioned the distributor cap as such (when you say "ignition wires" I assume that's the HT leads) No tell-tale signs of tracking/cracks in the cap when leads were replaced?

Good Luck - Richard

[bilcoh]

If Richard nails this one on the first try, I'm ready to send him money to support his "faith healing" skills.

Gary, I doubt I've got much to add. I don't know anything about EI. Does it still use the distributor to manage the timing? You mentioned rebuilding the original fuel pump. Any chance the distributor is loose and the timing is off slightly? Sounds like you've got fuel, and something more dastardly, like a head gasket, would be giving trouble on acceleration at least as much as deceleration, right?

Good luck. I'm sure I'd be more help if I'd had a running car any of the last 25 years.

Dave

[archigator]

I forgot to mention that I replaced the distributor cap as well.

I hope it's not a head gasket! I had a head gasket go on a Fiat years ago, and it blew white smoke when it went. I'll check the oil and water tomorrow morning.

I'm still hoping it's bad gas...

Gary
'71 Sprint FHC
Miami, Florida

[bilcoh]

I'm still hoping it's bad gas...

Whether a Lotus or our health, it's the same statement, right? "Please, God, don't let it be serious!"

Hope it's gas.

[rgh0]

My golden rule is " The last maintenance work you have done is the most likely source of your current problem"

I would start by looking at the fuel pump pressure of you rebuilt pump and whether the carb needle valves are sealing ok if the new pump is putting up the correct pressure but possible more than the old pump before the rebuild.

cheers
Rohan

[Sea Ranch]

My thoughts are with you, Gary. You will triumph.

But I do generally agree with Rohan on backtracking to latest repairs/mods.

Regards, from the other corner of the continent,

Randy

[elansprint71]

Rohan's rule is a good maxim.
Do you have a Color-tune available? Do they even sell these in the US? It is a glass spark-plug which allows you to see what the situation is when the mixture explodes. A very useful piece of kit.

[archigator]

I think I may have a blown head gasket... water not visible in the top of the radiator, after running the engine I pull the dipstick and the "oil" is all the way up the stick. That ain't right!

No white smoke, no white frothing on the dipstick, no overheating issues, but I got a bad feeling...

Gary
'71 Sprint FHC
Miami, Florida

[PaulW]

I forgot to mention that I replaced the distributor cap as well.

In line with other comments about revisiting work just done - try re-fitting the old distributor cap. I had exactly the same problem and the culprit was the new cap which looked the business, but wouldn't do the job at all.
Paul W.

[ardee_selby]

I think I may have a blown head gasket... water not visible in the top of the radiator, after running the engine I pull the dipstick and the "oil" is all the way up the stick. That ain't right!

No white smoke, no white frothing on the dipstick, no overheating issues, but I got a bad feeling...

Gary
'71 Sprint FHC
Miami, Florida

Gary,

When you pulled the old plugs...were any tell-tale signs noticed? Odd coloured deposits? (If they're in the trash...maybe worth another look-see for a clue)

Re: Timescale. Problem "a few weeks ago" & fuel pump "a couple of months ago" Could be the same thing...but if not, was it OK in the intervening period?

Cheers - Richard

[bilcoh]

Hope I'm wrong, Gary. I feel bad for even thinking of that as a possibility. But having once suffered from a blown head gasket, what you described doesn't sound like one. You get bogging down, not backfiring. Your issue sounds much more like fuel/fire mixture.

Actually, when it turns out NOT to be a head gasket, and only something slightly less expensive and troublesome, you'll think of me and be grateful, right?

Best of luck,

Dave

[StressCraxx]

Possible easy and cheap fix. Look for a vacuum leak from the #1 cyl vacuum fitting for headlights, carb bodies to intake.

Vac line disconnected from fitting, cracked plastic tee at crossmember, leaky headlight lift valve....

Been there...

Regards,
Dan Wise

[oldelanman]

I pull the dipstick and the "oil" is all the way up the stick. That ain't right!

Hi Gary,
That could be fuel in the sump and not water - maybe your rebuilt fuel pump is faulty as Rohan suggested. A leaking diaphragm will result in fuel leaking into the sump and possibly reduced fuel pressure which might affect the mixture and lead to the backfire. Does the dipstick smell of fuel ?

Hope it's not the head gasket - good luck.
Regards,

[elansprint71]

As the water is "disappearing" and the "oil" is rising, I now suspect the head gasket. If the oil is rising, it has to be a liquid which is topping it up.

Don't forget that if there is no water around the sensor, you will get no overheating reading on the gauge. The gauge can only show true(ish) temperature when water is present.

I recently had a head gasket leak where the water was being pressurised (by cylinder pressure) and being pushed out of the cooling system, I had no indication of overheating.

There are many different sets of symptoms of a leaking head gasket, it just depends which bit of the gasket has failed.

Next step, for me, would be to drain the liquid out of the sump and see what you have got, oil and water will still separate even when some emulsification has taken place. There may be mayo but maybe not.