Hard to start when hot

[bobs1916]

I  would   welcome  ideas  as  to   why  it  has  been  difficult  to   start  my  1918 DB  when  it  is  hot (engine).  When  running  it  is  smooth  as  silk.  No  backfire or  misses. recently   put  in   new  pistons   and  valves.  It  will   easily  start  when  cold and   easily  starts   when   hot  if   you  pop  the  clutch,  This  is a  magneto   car  with  crank  only  start , NO BATTERY that   has  recently  had  a  magneto rebuild.  I  am  thinking  about  vaporized   fuel in  the  carb ??? 

[22touring]

Hot start problems like yours are typically caused by vapor lock due to vaporized fuel in the fuel lines.  Modern fuels containing ethanol tend to vaporize worse than old fuels did.  I would suggest insulating your fuel lines in the engine compartment with a product such as ThermoTec "Thermosleeve" or Heatshield Products "Thermoflect" sleeve.  It worked for me.

[Grimy]

22touring is right, of course, but there are other measures. I claim no familiarity with the routing of Dodge fuel lines. Here are two other measures if insulating the fuel lines does not solve the problem:

 

1. Especially *IF* the fuel lines are routed away from heat sources, fuel may be vaporizing in the carb bowl upon shutdown. When these cars were built,, fuel was difficult to vaporize completely (just the opposite of our issue today), so exhaust downpipes were usually at the front of the engine, where their heat could be blown by the fan at the carb bowl.  Even on shutdown, there's a lot of heat from the downpipe at the carb. So consider insulating the first foot or so of the downpipe.

 

2. The vacuum tank itself will get a lot of heat from the exhaust manifold.  For my cars with updraft carbs and vacuum tanks, I've fabricated blankets from Thermotec materials and use Velcro to secure them.  About 30 seconds to remove for show, and 2 minutes to re-install for go...

[Spinneyhill]

3 hours ago, bobs1916 said:

 It  will   easily  start  when  cold and   easily  starts   when   hot  if   you  pop  the  clutch, 

 

What do you mean by "pop the clutch"? How could the clutch affect evaporation of fuel from the carb.?

 

Does it have a vacuum tank? If so, does the fuel run into the carb. by gravity? If so, there is quite a lot of fuel to evaporate from the carb. if one includes the contents of the vacuum tank? If you stop the engine hot, it is hard to start 2 mins later, thus it cannot be fuel evaporation because there is not enough time. If it were hard to start cold, it is conceivable it could have all evaporated as the engine cooled?

 

Where is the coil mounted? In a hot place?

 

 

Edited November 16, 2016 by Spinneyhill (see edit history)

[zeke01]

After examining every component of my fuel system at least twice,I found that my "vapor lock" problem turned out to be a bad coil breaking down when hot. It would work well enough if the car was running but wouldn't start it,. It was behaving exactly as the OP car in question. Remember, 80% of all fuel problems are electrical in nature. Zeke

[nearchoclatetown]

bob, have you done a compression test while it is hot? It sounds like tight valves. You have covered most everything else, mag, etc. 

[bobs1916]

15 hours ago, Spinneyhill said:

 

What do you mean by "pop the clutch"? How could the clutch affect evaporation of fuel from the carb.?

 

Does it have a vacuum tank? If so, does the fuel run into the carb. by gravity? If so, there is quite a lot of fuel to evaporate from the carb. if one includes the contents of the vacuum tank? If you stop the engine hot, it is hard to start 2 mins later, thus it cannot be fuel evaporation because there is not enough time. If it were hard to start cold, it is conceivable it could have all evaporated as the engine cooled?

 

Where is the coil mounted? In a hot place?

 

 

Pop  the  clutch   is  to  roll   the   car  in  gear  while   depressing  clutch  pedal  then  while  it  is  rolling   with  ignition  on  let  up  on  the  clutch to  start  engine.   NO  vacuum  tank yes  gravity   feed , no  coil   it  is  a  magneto. 

[bobs1916]

10 hours ago, nearchoclatetown said:

bob, have you done a compression test while it is hot? It sounds like tight valves. You have covered most everything else, mag, etc. 

 

10 hours ago, nearchoclatetown said:

Doug  I  have  not   checked   compression   when  hot .  What  would  I  be  looking  for  and   what  is  the   solution?

 

 

[bobs1916]

19 hours ago, 22touring said:

Hot start problems like yours are typically caused by vapor lock due to vaporized fuel in the fuel lines.  Modern fuels containing ethanol tend to vaporize worse than old fuels did.  I would suggest insulating your fuel lines in the engine compartment with a product such as ThermoTec "Thermosleeve" or Heatshield Products "Thermoflect" sleeve.  It worked for me.

Thanks

[nearchoclatetown]

Bob, it just a guess. But if valves are tight compression would be lower when engine gets hot making it hard to start. Have you ever tried to choke it when hot and hard to start. Also are you positive timing is dead on?  I forgot to retard my truck last week with Mark and it kicked back pretty hard.