Runs rough when hot

[Noah H]

Hello all. 
 

My 230 Flathead daily driver is suffering. In town driving causes rough idle and off idle miss when at operating temp. 

 

Starting the car immediately after I shut it off it fires right up. (After a cruise/drive etc) 
 

Say I were to wait 5-10 min before starting. The engine still hot. it starts very hard. 

there is no heat shield on top of the fuel pump. Which I have read may be the cause? 
 

Highway driving it purrs like a kitten. But I’d like to solve this problem as I will continue daily-ing the car. 
 

Thanks for any input/help. 
 

take care 

 

Noah. 

 

[Doug&Deb]

The heat shield will help. Check the state of tune ( timing, carb settings etc). Is this a sudden problem or has it gradually changed?  Normally my Coronet is hard to start when hot because of heat buildup under the hood. Once I rev the engine a few times to bring the temperature down it runs fine. Also check the routing of the fuel line from the pump to the carb. Make sure it’s not too close to the manifold.

[Noah H]

12 minutes ago, Doug&Deb said:

The heat shield will help. Check the state of tune ( timing, carb settings etc). Is this a sudden problem or has it gradually changed?  Normally my Coronet is hard to start when hot because of heat buildup under the hood. Once I rev the engine a few times to bring the temperature down it runs fine. Also check the routing of the fuel line from the pump to the carb. Make sure it’s not too close to the manifold.

thank you. 
 

I have an inline metal fuel filter coming from the fuel pump to carb which is an inch from the manifold. Rubber lines are also present from pump to carb as well. 

[keithb7]

Running non-ethanol fuel helped for me. Add the fuel pump heat shield. A carb base heat plate helps too. Seen here in the pic. The plate must have reliefs cut in it, in the proper places for carb vacuum passages. If they are absent its easy to cut your own to make it right. 
 

 

[greg g]

This heat shield is available on line from jeep aftermarket  suppliers. Or make your own from sheet metal.  You can get sheet brass from most hardware stores. The phenolic insulator in several thicknesses from Langdons stove bolt web site. You will need extra gaskets for each piece. And longer carb to manifold studs to accommodate the added thickness. Langdon has those too

 

 

Edited September 6, 2021 by greg g

[Sniper]

I really wonder what's going on with these hard hot starts?

 

I have zero problems, I live in a very hot part of the country and I employ none of the suggest fixes for a hot restart issue.

 

I have 13 letters in my last name so I doubt it's luck, lol.

 

Seriously, I'd really like to know the root cause.

 

 

[greg g]

Take a look at the bottom of the carb.  Is it wet looking, is there raw fuel evident?  If so you might want to lower your float level.  Modern gas expands more and quicker than what these carbs were designed for.  Heat soak causes perculation which can cause your engine to become flooded just sitting after a long run.  Seting the float by 1 or 2 32nds can address this.  When this condition occurs, slowly push the gas pedal to the floor and hold it before engaging the starter.  If this works it confirms that perculation is occurring as WOT introduces a lot of air leaning the mixture and clearing the flooded situation.

[Noah H]

55 minutes ago, greg g said:

Take a look at the bottom of the carb.  Is it wet looking, is there raw fuel evident?  If so you might want to lower your float level.  Modern gas expands more and quicker than what these carbs were designed for.  Heat soak causes perculation which can cause your engine to become flooded just sitting after a long run.  Seting the float by 1 or 2 32nds can address this.  When this condition occurs, slowly push the gas pedal to the floor and hold it before engaging the starter.  If this works it confirms that perculation is occurring as WOT introduces a lot of air leaning the mixture and clearing the flooded situation.

no evidence of wetness. I may switch to all metal fuel lines to see if that cures the problem. Thanks for the tips. 

[kencombs]

In addition to all the good suggestions already offered, it wouldn't hurt to check your float level and fuel pressure.  Either of those can contribute to hot start and rough running on restart problems.  And, a lot of replacement pumps seem to provide too much pressure.  That could unseat the inlet valve in the carb and cause seepage into the manifold on a hot shutdown.

[Sam Buchanan]

6 hours ago, Noah H said:

no evidence of wetness. I may switch to all metal fuel lines to see if that cures the problem. Thanks for the tips. 

 

I think metal lines would be more prone to vapor lock than rubber lines. See if you can get your filter farther from the exhaust manifold, having it that close is most likely heating the fuel.

[tom'sB2B]

I also found that lowering the float level helped with hard starts with a hot engine. Might be an easy fix.

[Kilgore47]

 

Had the same problem with the P15.  Hard to start when hot.  So I started looking for the problem.  I installed a plastic fuel filter in place of the metal one so I could see what was going on.  After everything was hot I shut it down and looked under the hood.  I could see gas bubbling up into the filter.  That's bad.  I re routed the fuel line away from the exhaust manifold and re installed the metal filter.  Also split some rubber hose and used it to insulate the fuel line.  Now the car starts with a tap of the button.  Even when hot.

[Noah H]

Tried metal lines and it does run better off idle but still starts hard. I turn the key and get a POP! Almost more of a chuff than a backfire then it starts and runs fine. This has been going on before the fuel line replacement and after as well. 

[wagoneer]

Valves need to be backed off a bit? Maybe getting too hot and not enough space to cool off.

[Noah H]

3 hours ago, wagoneer said:

Valves need to be backed off a bit? Maybe getting too hot and not enough space to cool off.

I thought the same thing but I’ve adjusted them a couple months ago and never had this problem? 

[Noah H]

1 minute ago, Noah H said:

I thought the same thing but I’ve adjusted them a couple months ago and never had this problem? 

 

3 hours ago, wagoneer said:

Valves need to be backed off a bit? Maybe getting too hot and not enough space to cool off.

I also hear distinctive clicking like a sewing machine and I’ve heard this is better than not hearing any sound at all. 

[wagoneer]

One sure way to overcome vaper lock is to use a backup electric fuel pump to help push through the bubbles. Mine only starts hard after a few weeks when the lines are empty and evaporated. I turn it on for some seconds, and then it fires right up using the mechanical (though using both together floods the carb with too much pressure -- need a regulator for that). 

 

Also, if you've gone through the effort of replacing to metal, you can always exaggerate the fuel line routing toward the fender to ensure there is enough "cooler" air.  Definitely get a heat shield for the fuel ( $20 from DCM - https://dcmclassics.com/fuel-system/423-e-743-heat-shield-for-fuel-pump-ss.html) or probably cheaper elsewhere.

 

10 hours ago, Noah H said:

turn the key and get a POP! Almost more of a chuff than a backfire then it starts and runs fine. This has been going on before the fuel line replacement and after as well. 

 

Here is a clue to focus on , and maybe points to the carb and engine, rather than the fuel, per se.

 

Pops and chuffs and backfires are timing problems or possibly stuck valves. The clicking noise may also indicate as much; your engine should purr. . Worth going back and verifying the valve clearance, and doing a hot clearance check (There have been a threads on this recently and in the technical section); easier to search out the many threads using google.  Check your plugs, if any are black/sooty, consider adjusting those first. You are using it as a daily driver, but maybe hotter plugs to burn away the carbon.

 

When hard starts occur, I think of the common garden blower. There are those that start instantly (usually when new), and then years later it takes a few pulls. Usually it's some carb adjustment, or flooding the engine, but occasionally it's deeper. 

 

Be methodical and rule out portions at a time, and eventually you run into the issue.  You started with fuel input, then move down toward the target and check carb, float, and then into the delivery of the fuel.

 

1. timing problem - investigate the backfire - and check your engine timing. This may be a separate issue but contributing to the problem. 

2. Clicking noise - The clicking noise could be a sticking valve allowing for spark to ignite fuel not in the chamber, Once started, the valve gets going. Try running with the valve covers off. 

3. Carb too rich / vapor lock

4. spark plugs too sooty due to carbon buildup (use hotter plugs)

5. Engine overheating - cooler thermostat (160) and / or electric fans

6. Valves too hot and sticking 

 

[Noah H]

12 minutes ago, wagoneer said:

One sure way to overcome vaper lock is to use a backup electric fuel pump to help push through the bubbles. Mine only starts hard after a few weeks when the lines are empty and evaporated. I turn it on for some seconds, and then it fires right up using the mechanical (though using both together floods the carb with too much pressure -- need a regulator for that). 

 

Also, if you've gone through the effort of replacing to metal, you can always exaggerate the fuel line routing toward the fender to ensure there is enough "cooler" air.  Definitely get a heat shield for the fuel ( $20 from DCM - https://dcmclassics.com/fuel-system/423-e-743-heat-shield-for-fuel-pump-ss.html) or probably cheaper elsewhere.

 

 

Here is a clue to focus on , and maybe points to the carb and engine, rather than the fuel, per se.

 

Pops and chuffs and backfires are timing problems or possibly stuck valves. The clicking noise may also indicate as much; your engine should purr. . Worth going back and verifying the valve clearance, and doing a hot clearance check (There have been a threads on this recently and in the technical section); easier to search out the many threads using google.  Check your plugs, if any are black/sooty, consider adjusting those first. You are using it as a daily driver, but maybe hotter plugs to burn away the carbon.

 

When hard starts occur, I think of the common garden blower. There are those that start instantly (usually when new), and then years later it takes a few pulls. Usually it's some carb adjustment, or flooding the engine, but occasionally it's deeper. 

 

Be methodical and rule out portions at a time, and eventually you run into the issue.  You started with fuel input, then move down toward the target and check carb, float, and then into the delivery of the fuel.

 

1. timing problem - investigate the backfire - and check your engine timing. This may be a separate issue but contributing to the problem. 

2. Clicking noise - The clicking noise could be a sticking valve allowing for spark to ignite fuel not in the chamber, Once started, the valve gets going. Try running with the valve covers off. 

3. Carb too rich / vapor lock

4. spark plugs too sooty due to carbon buildup (use hotter plugs)

5. Engine overheating - cooler thermostat (160) and / or electric fans

6. Valves too hot and sticking 

 

Thank you, I really appreciate your detailed response. My timing/dwell is correct and it still occurs. I am going to looking into valves tomorrow. I do run Autolite 308s as a hotter plug as I daily drive this car. I checked them today (after about 3000 miles and they are very clean and crisp.