need help, erratic engine

[vercingetorix]

Must be my turn for car trouble!

Homeboy was running great until about two weeks ago, Then started acting bad at cold start up, backfiring through the carbs and dying and really difficult to start, but would straighten out once it warmed up.

Then refused to idle and began missing at cruise speed. Now all of the above are worse and idle is at best erratic(up to 1200 and drops to under 500, though its set about 800) and at worst it stalls when I take my foot off the gas at stop lights.

I changed the fuel filter even though it appeared OK and checked the timing which was right on the mark. I checked the compression two months ago and all cylinders were great.

I'm stumped. valve lash? vacuum leak? E-1280 dying? Sticking valve?

Now I'm paranoid to leave the driveway with it.

Any input greatly appreciated.

Best,

H Houghton

[Arne]

I'm wondering about insufficient fuel pressure, not keeping the float bowls full. How old and what type is the fuel pump?

[vercingetorix]

Pump is a stock mechanical and a couple of years old.

Would the dampner have any effect?

[xray]

Hard to say....usually, if you think it's fuel-related the problem is electrical. If you think it's electrical then the problem is fuel! Since Arne has taken the fuel side I'll go electrical....

Wandering idle could be vacuum-leak related, or could be from an inconsistent misfire. Are you getting spark out of all plugs? Have you checked them recently for carbon buildup? Carbon deposits on the distributor cap? Coil specs within normal limits? I'm no E12-80 expert since I have Pertronix, so someone else will have to troubleshoot that side fo things.

Backfiring (as opposed to afterfiring through the exhaust) could be from a lean condition, possibly from vacuum leaks or poor fuel supply. Double check floats/nozzles, choke mechanism for proper function as suggested earlier. Mech fuel pumps are cheap, so replace it if you haven't already or if it's old.

Best way to proceed is methodically down one path (fuel), eliminating problems as you go. If that fails, pursue ignition issues and see if that fixes it.

You said the timing is correct, could you tell us what your exact numbers are (crank angle/static and with vacuum)?

Edited October 18, 2008 by xray

[vercingetorix]

The timing is about 14deg w/o vacuum hose and 20 exactly with it on. I dont know what the spread is supposed to be so maybe the vacuum advance is leaking?

I'll pull plugs tomorrow and order a new fuel pump as well. As the whole situation kind of crept up on me, I think Arne may be right. But I'll let you know as soon as I know.

Thanks,

H Houghton

[MikeW]

In my limited experience a fuel supply problem tends to present itself under heavy load such as acceleration or going up a hill. At idle very little fuel is needed so it may run fine with a weak pump or clogged filter. The fact that you're even having issue at idle make me think electrical as well. I think looking at the plugs will be informative. You might have one or two fouled plugs which would cause a bad idle but given enough extra fuel the other cylinders have enough power to allow you to still drive. The backfire could actually be an afterfire in the exhaust system as a result of a spark that only works periodically - igniting fuel that didn't ignite on previous cycles.

[justinr54321]

Aso check your valve clearances, for they too can cause problems if not set right. With mine, when I first got it, was running fine, and then I started having similar problems. It ended up being that I had several rocker arms off their "rockers," (pun), but anyhow, once I put those on and got the clearances set, problem went away, except for the ones one would usually get with the original mechanical dizzy...

Almost forgot, check all the rubber fuel hoses for any crakcs or leaks as well, especially the ones that go from the float bowls to the nozzles.

Anyhow, keep us posted and let us know what's up.

Justin

Edited October 18, 2008 by justinr54321

[vercingetorix]

Checked plugs this PM and everybody was burning fine, no fouling, all a nice tan.

I'll mess with the fuel next unless anyone has another thought on this.

Also checked the distibutor rotor and cap, probably could use a new one but looked fine.

Best,

H Houghton

Edited October 19, 2008 by vercingetorix

[vercingetorix]

Finally got to the bottom of this maddness! After checking the obvious,fuel,spark etc. I thought again about XRAY's comment about vacuum leaks and began to go over hoses etc. and lo and behold a gaping leak. The vacuum fitting right in front of the one for the vacuum asist for the brakes was completely open I reackon it about 1/4 of an inch, some how the end of the rubber cap had come clean off leaving a rubber sleeve with no end so it was kind of diffcult to see. I suspect that the end was old and rotted and began to leaking causing my beginng problem and then perhaps a backfire through the carbs just blew the end right off, I can't think of any other explanation, stinking bizzare!

Anyway I replaced the rubber cap and homeboy fired right up, idled right down and is now running perfectly. Who'd of thunk it!

Thanks for all the input,without all the differing takes and detail I'd still be chasing myself!

Thanks again to all, esp. XRAY

This site is the best on the net.

[MikeW]

I'm glad you got it figured out and that it ended up being simple and cheap. I recall that when I drove my car home from the paint shop the brakes weren't working well. I did a lot of downshifting. When I got home the problem was obvious: the big vacuum line was completely disconnected from the brake booster. I don't recall, however, that there were any running issues. I only understand enough of this to be dangerous but can someone explain it to me? I could see where a vacuum leak would cause low vacuum throughout and affect the brakes and the vacuum advance on the distributor but why would it cause the engine to want to knock off at idle?

[xray]

Woo hoo! Excellent work! Glad to see the fix identified and easily repaired!

Congrats

[MikeW]

I could see where a vacuum leak would cause low vacuum throughout and affect the brakes and the vacuum advance on the distributor but why would it cause the engine to want to knock off at idle?

Ok, I've done some reading on the web and understand it a little better now. It seems that a vacuum leak is most apparent at idle. The leak can effectively cause the engine to run too lean.