Engine Speed variation

howard · Jul 14, 2009

So I was chugging away downtown today, everything was peachy. But eventually I got out on a straight stretch and was going a steady 40 mph or so, and started getting an alarming engine speed fluctuation. It was as if I was stomping the gas, then letting off...continuously. No smoke, overheating, etc., just a weird pulsating engine speed. I could let up on the gas, and it would even out, but as soon as I put the pedal back down, the pulsations would start again. I limped it back home and parked it.

What could be causing this? Vacuum leak? Clogged gas filter? Stupid driver? Any suggestions would be appreciated.

New plugs/wires, fresh gas, in-line filter, tanks have been cleaned/dipped/sealed.

SIVAllan · Jul 14, 2009

howard said:
So I was chugging away downtown today, everything was peachy. But eventually I got out on a straight stretch and was going a steady 40 mph or so, and started getting an alarming engine speed fluctuation. It was as if I was stomping the gas, then letting off...continuously. No smoke, overheating, etc., just a weird pulsating engine speed. I could let up on the gas, and it would even out, but as soon as I put the pedal back down, the pulsations would start again. I limped it back home and parked it.

What could be causing this? Vacuum leak? Clogged gas filter? Stupid driver? Any suggestions would be appreciated.

New plugs/wires, fresh gas, in-line filter, tanks have been cleaned/dipped/sealed.

Howard,

I didn't realize you were up and running, sorting out a few last glitches...

Congratulations!

To try and help with the current issue though - you use a stock mechanical fuel pump feeding a stock Solex carb ??

Allan

64beam · Jul 14, 2009

Hi Von,

It sounds like you may have a fuel delivery problem which could be the fuel pump or the carburettor causing the fuel starvation at speed. Check that you are getting fuel to the carburettor, check your float level and make sure that there are no blockages in jets, etc.

Regards, Robin.

RootesRooter · Jul 14, 2009

Sounds like paint flakes from the inside of the gas tanks are blocking the fuel line. Get the front end of the car up the so you can pull the float bowl without gas going everywhere. Odds are good you'll find a bunch of black flakes sitting in the recesses of the pump.

howard · Jul 15, 2009

I'm still running the stock fuel pump... with a new 32/36 Weber carb. The fuel tanks have been removed and reconditioned so there isn't any more paint in them to come loose (they've been coated internally).

I have been driving the car for a few days, trying to mentally list what needs to be "ironed out" so I can be happy with it. Now this. Bummer.

I may have to let it sit for a few days before I can get time to get back to it. Many irons in the fire. It does act like a fuel delivery issue, though. I just wonder why, if it is, it doesn't manifest itself at lower speeds.

alpine_64 · Jul 15, 2009

howard said:
So I was chugging away downtown today, everything was peachy. But eventually I got out on a straight stretch and was going a steady 40 mph or so, and started getting an alarming engine speed fluctuation. It was as if I was stomping the gas, then letting off...continuously. No smoke, overheating, etc., just a weird pulsating engine speed. I could let up on the gas, and it would even out, but as soon as I put the pedal back down, the pulsations would start again. I limped it back home and parked it.

What could be causing this? Vacuum leak? Clogged gas filter? Stupid driver? Any suggestions would be appreciated.

New plugs/wires, fresh gas, in-line filter, tanks have been cleaned/dipped/sealed.

Howard,

Did the engine make a deep drowning out sound and revs drop off? That can be a VAc leak, it would idle ok, but once under more load start to pull in to much air and starve the motor.

The other thing could be fuel pump pressure if you are running a electric pump, blocked filter that is allowing enough petrol to feed light throttle but not full.. or as mentioned, paint flake floating and under full suction blocking the flow.

64beam · Jul 15, 2009

howard said:
It does act like a fuel delivery issue, though. I just wonder why, if it is, it doesn't manifest itself at lower speeds.

At lower speeds your fuel bowl in the carburettor has more time to refill, where as at higher speeds the bowl will empty quicker. Did you flush out your fuel lines prior to running your car? Scale can build up in the fuel lines as well.

Good luck,

Regards, Robin.

SIVAllan · Jul 15, 2009

howard said:
What could be causing this? Vacuum leak? Clogged gas filter? Stupid driver? Any suggestions would be appreciated.

You might consider inspecting the fuel line from tanks/crossover tube to carb, for air leaks...

Rule out the line, the pump, etc one step at a time, until it works Ok again, or only the carb remains...

husky drvr · Jul 17, 2009

Don't get fixated on the idea of a fuel problem, might be ignition.

howard · Jul 18, 2009

Still not 100% sure I solved the problem, but I noticed a cracked rubber gas line going to the Weber, so I replaced it. The problem has not resurfaced, but the jury's still out. I need to get the car out for a good 45 minute drive and see if it happens when she's hot and at higher speeds.

It would be a good thing to check into getting some new fuel lines. What is a good source/what's everybody else doing to replace theirs?

SIVAllan · Jul 19, 2009

Von,

I used standard rubber line available at any parts store, and followed the same route as the factory metal line.

Allan

howard · Jul 20, 2009

Replace the old metal lines with rubber? Isn't that dangerous?

SIVAllan · Jul 20, 2009

howard said:
Replace the old metal lines with rubber? Isn't that dangerous?

Not as good as factory metal lines; possibly as safe as the SV factory plastic line...

Next time your car is on stands, take a look at the factory route. If that's safe enough to prevent ruptures from road debris, then it should be ok. The SIV line at the most "close contact to the road" area - beneath the boot - is protected by a metal shield made to the chassis.

puff4 · Jul 20, 2009

I dunno. I have yet to hear of anyone ever having one of the original plastic lines actually fail. Ends being damaged, screwing them up when disconnecting and reconnecting, yes... but outright failure, never. I think the original lines are actually pretty safe.

SIVAllan · Jul 20, 2009

I did not mean to say or imply the factory plastic lines are unsafe. I'm sure they must be good.

The question whether rubber lines is safe is a good one; I've not heard of a failure but then I don't hear these type of things

.

The advantages of a rubber line are availability and low cost.

Another consideration is that the plastic lines fitting to the crossover tube does not fit the SIV so, buyer beware...

Allan

howard · Jul 22, 2009

Well obviously the cracked fuel line wasn't the problem: the car is still running erratically. Conked out on me on the road home yesterday

. I shall proceed with the easy stuff first- like replacing the in-line fuel filter, for starters. Then move on to the pump and lines.

howard · Jul 25, 2009

I believe the situation is resolved: Initially, I think the rough running was a clogged fuel filter. I replaced it, and adjusted the carb to run smoothly. Next day, though, when I tried to get it to run smooth, it was awful! I left it parked.

Today I finally got around to viewing the problem, and LO AND BEHOLD if the choke cable's outer sheath hadn't broken loose under the dash. You could pull the choke out fine, but when it came time to push it back in it stayed out (but looked as if it were pushed in). I soldered it all back together and now it works great.

Took the car for a spin on the old highway (where you don't have to go 70mph) and she ran like she ought to. I believe, as the old timers here say, "Ah'm back in bidness!"

kenny_ii · Jul 26, 2009

If you're pretty sure you've had a rich fuel issue for a while, you might want to change the oil. A lot of that extra fuel has ened up up in the oil, where is shouldn't be.

Tim