Author Topic: 396 L34 hard to start when hot  (Read 1275 times)

Henry Cloutier

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Re: 396 L34 hard to start when hot
« Reply #15 on: October 11, 2020, 12:29:28 AM »

  Just curious, What is your timing at now?

Henry
Hank

PHAT69AMX

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Re: 396 L34 hard to start when hot
« Reply #16 on: October 27, 2020, 04:50:14 AM »
What might be the numbers stamped on the Distributor, or really what's needed, is what are the numbers stamped on the bottom of the Pole Piece in your Distributor and is the Sleeve still in place on the mechanical advance travel limiter pin?  The Initial Timing Advance should not be changed without first knowing how much potential total mechanical advance is in the distributor.  This must be considered to avoid over advance.  Agree, more advance at idle helps many things, but also agree too much initial advance can cause difficult hot starting.  So one must first know how much Total Mechanical Advance the Distributor is capable of in order to determine how much advance at idle is enough and not too much.  But this is complicated by if the mechanical Advance may already be working some even at idle, which it sometimes does, depending on weights and springs and center cam.  So the 'right way' to set the timing advance is at 'high' rpm using a dial back timing light and adjusting the distributor to set Total Advance with the Mechanical Advance all-in and NO Vacuum advance at all, vacuum advance disconnected and plugged.  Then whatever the timing advance is at idle is whatever it is.  What matters is total advance and all-in by when.

  Also, was the top distributor bushing grease well re-packed with grease?  Has the distributor been checked for sideplay?  Sideplay can reap intermittent havoc with distributor ignitions.  That top bushing is not motor oil lubricated, it is only lubricated by the grease well in the distributor housing, and if original 50 years is a long time.  Distributor grease well pictured is filled with wheel bearing grease.  The correct original GM Grease that should be used in the Distributor top bushing grease well is red and a jar of it is pictured.

RAS68

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Re: 396 L34 hard to start when hot
« Reply #17 on: November 13, 2020, 05:56:03 PM »
Thanks again to everyone for the ideas and suggestions. The last post about mechanical advance is very interesting - Im going to look into that as well. I backed off the timing some and it does start better when hot. I also found that it will start pretty easy when hot if I floor the gas pedal during start or just before I turn I key. What does that mean?
Allen
'68 SS L34 07C NOR

Leon in Mn.

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Re: 396 L34 hard to start when hot
« Reply #18 on: November 13, 2020, 10:37:15 PM »
I thought I would chime in here as well as back in the day I  went to school on these vehicles (old school but not THAT old) and worked on them as well.  All of these comments are great, but another issue is fuel percolating/and evaporating in the float bowl at high temps.  This can cause vapor lock and like one poster stated to check the float level.  It can "blubber over" into the manifold and cause a temporary flooding.  Do you know for sure that you have the heat shield plate under your Quadrajet? I'm assuming you have the Quadrajet? If that is not in place and in the right position along with the gasket in the right position, it can cause many problems.  It can also burn out the plugs in the bottom of the carb which can leak fuel out of the bowl and into the hot manifold below, not only draining the fuel bowl, but causing flooding.  Sometimes hooking up another vehicle just gives the car that extra spin and voltage to overcome the hard start which normally it wouldn't need.  Try searching for "manifold hot slot problem" on these cars and you will find a lot of info, and what you can do if you didn't yet find the remedy.   Something else that would cause a flooding like scenario is to make sure your heat riser isn't stuck in the closed position .......Just something to think about if you are still looking for the problem.   :)
 Leon

Henry Cloutier

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Re: 396 L34 hard to start when hot
« Reply #19 on: November 18, 2020, 01:57:49 AM »

Hi there,
I'm certainly no engineer, and there is most likely a better explanation for easier starting with the gas pedal held to the floor. I always equated it to unloading the engine since the throttle blades are open allowing the engine the best opportunity to spin freely when hot. Near as I can figure the engine when hot is at its optimum with the piston rings fully seated and friction at it's highest. When the timing is to far advanced the engine is trying to fire with no inertia to sustain the motion. The engine kicks back because the starter has limited ability to overcome this kick back and struggles to turn over. Lessening the initial timing allows the engine to fire closer to TDC which helps the engine sustain the inertia generated buy the starter during the start sequence. Once running the engine can handle more timing which is better for overall performance.
Fuel boiling and vapor lock are common with these old cars. Back in the day us back yard guys clipped close pins to the fuel lines to act as heat sinks; not very attractive but worked really well.

Best regards,

Henry 
Hank

Kelley W King

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Re: 396 L34 hard to start when hot
« Reply #20 on: November 18, 2020, 12:39:59 PM »
Since you are a "back in the day" guy Henry, try aluminum foil around the lines. Adds a little shine to the engine also. On a serious note then heat sink plate kits with the 1/2 spacer is only about $35 bucks if you have room. Worth a try maybe.
69 Z28 RS Scuncio Hi Performance
69 SS L78
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Henry Cloutier

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Re: 396 L34 hard to start when hot
« Reply #21 on: November 18, 2020, 10:03:00 PM »

Dang,
 I wish I had thought of the foil-"back in the day," perhaps it would have helped with the radio reception as well!
Henry
Hank

Stingr69

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Re: 396 L34 hard to start when hot
« Reply #22 on: November 20, 2020, 02:05:12 PM »
I agree with the heat boiling the current crappy gas in the carb is a concern.  The hot slot has been an issue forever.  If you have the hot slot intake it needs to be addressed as they are carb cookers.  A heat insulator under the carb is an easy fix.

169INDY

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Re: 396 L34 hard to start when hot
« Reply #23 on: November 20, 2020, 05:12:59 PM »
I think there are a few threads about Drill & Tap and "set" pipe plugs in the hot slot end passage hole and block it off for good, esp nice since what 99% do NOT daily drive these old buckets of bolts.
Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS
68 Z28 M21 LOS

z28z11

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Re: 396 L34 hard to start when hot
« Reply #24 on: November 20, 2020, 10:48:15 PM »
I think there are a few threads about Drill & Tap and "set" pipe plugs in the hot slot end passage hole and block it off for good, esp nice since what 99% do NOT daily drive these old buckets of bolts.

You dont have to drill and tap it to block it - takes a 7/16 freeze plug, just drive it in securely (it won't back out), gasket it with the stainless shim under the gasket, and forget it. It's actually a safety precaution as well - you don't need hot exhaust crossover temps under the Quad bowls -

Regards,
Steve
1968 Z28 M21/U17 BRG/W
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
1969 Z11 L48/M35/C60/C06  1949 3100 5wd 235/6

 

anything