Starting problems

[sunger]

I am stumped by this one. I have a 1968 427/400 (tri-power) that is perfect in every way but one, and that is starting. I usually drive it about every two weeks, and when I first get in I pump the gas twice and it starts immediately. If I drive for a while and make a short stop, like for gas, it restarts immediately with no pumping. If I stop for a longer period of time, 20 minutes or so, it is very slow to start and frequently backfires through the carb when it does. I have tried pumping a lot, and not pumping, and everywhere in the middle; it just takes about 10 seconds of cranking and then it fires, runs a little rough for another 5 to 10 seconds, and then smooth and perfect. I am thinking carb, but then I can't figure why it starts quickly after sitting for 2 weeks, and quickly after sitting for 5 minutes. I can smell gas after I park it in the garage, but it can't be draining out or it woudn't start after the 2 weeks. I appreciate any thoughts or experience you have had.

 

[raybies]

Sunger
It sounds to me as if whats happening is that un atomized fuel is pooling up in the bottom of the plenum, and when you shut the car off for a while the fuel runs down as liquid into the cylinders causing a fat condition, or wetting the plugs.
Your float levels may need to be checked or floats replaced. These plastic floats tend to suck up fuel and become heavy and less boyant after time.
You could also have an accelerator pump dripping fuel.
Also I would take a quick look at throttle blade openings and make sure that there is not excessive opening at the closed throttle position. Any way you slice it sound like a carb issue and the tri-power setup's have always been a little touchy. I had a 1962 Impala coupe that had a 348 with a tri-power setup and I was forever needing to give my carbs attention.
Oh and just a quick P.S. In 2 weeks time any gas that has dripped down in the manifold or cyilinders would dry up or drain past the rings thats why you dont have the problem starting it after extended sits.

 

[sunger]

I can't say that I completely understand everything you said, but I do understand some. Would the condition as you state cause a fuel smell in the garage when I park the car? I know enough about carbs to adjust them a little, but thats all. Would you think it is rebuild time for that center carb?
I really appreciate your help; this has been driving me nuts.

 

[jims427400]

sunger,
I too feel your pain. I've had the same problem from time to time. And I had tripowers totally rebuilt last winter.
I think raybies may be right, my 67 runs like a top, but the 68 is very touchy. There are many service manuals that explain float adjustment and correct linkage adjustment. When I reinstalled my 67s tripowers I had the seconderies opening when they should have been completely closed. Once I figured it out, havent had a problem.
Know I'm trying to figure out how to adjust the choke on the 68. I wonder if that may be where your problem is coming from.
Great question ... I'll be watching for other opinions, plenty of tripower owners on this forum. You've come to the right place.
Any pics of your car..

 

[raybies]

Ok let me try to simplify this.
Like I said before, if your floats are of the plastic type and not the brass floats they can take on fuel after time it soaks into the plastic making it heavier and thus forth making the float impossible to lift to the seat closed position before the level of fuel reaches an overflow state. (reduced boyancy due to saturation)
If the throttle blades (meaning any of the 3) are too far open when the engine is hot and at a closed throttle position this can cause accelerator pump drip. Either one of the 2 reasons I've given you would leave the fuel smell your talking about due to the fact that after you shut the engine off your still discharging fuel via leakage in the carb system onto the hot plenum of the manifold, and down into the cylinders which by the way is a real problem because it washes the cyilinder wall and rings. meaning the wall and rings are dry (oil washed away) on start, henceforth excessive ring and wall wear over time.
Quick check is this. Next time you drive it, do your thing with it and after you get home leave it sit for about 10 minutes then with the air cleaners off and a flashlight in hand engine off, open the throttle blades to wide open and hold them there, look down the throat of the carbs and if there's a puddle of fuel on the floor of the plenum or if its wet then its time to do some carb work and adjustment.
Hope any of this helped you.
Ray.

 

[sunger]

I will do that this weekend and let you know. What you say makes sense, and I never thought about the oil being washed away.
Between your and Jim's answers I have some work to do. I thank both of you,and I'll post my answer this weekend.
Thanks again.

 

[JohnZ]

Quick check is this. Next time you drive it, do your thing with it and after you get home leave it sit for about 10 minutes then with the air cleaners off and a flashlight in hand engine off, open the throttle blades to wide open and hold them there, look down the throat of the carbs and if there's a puddle of fuel on the floor of the plenum or if its wet then its time to do some carb work and adjustment.
Hope any of this helped you.
Ray.

Before you open the throttle blades, see if any raw fuel is dribbling from the boosters onto the throttle blades - it will be obvious. If so, it's percolation (fuel boiling in the float bowl during hot-soak after shutdown); lower the float about one hex flat and see if that helps.