Hard to start when hot

[dave73]

So I've been driving my car more often recently and working through the bugs. Engine is a 512, holley sd intake, holley 850 double pumper and mechanical fuel pump - all parts brand new with about 200 miles on break in so far.

If I start it up when it's cold, 2 pumps of the gas pedal and it fires right up. If I drive it and park for a few minutes, it starts right back up no problem. I'm having issues when it gets up to operating temperature, then sits for awhile and doesn't fully cool down. Yesterday, I drove it to a local car show, car got up to operating temperature then was parked and sat for about 3 hours. It was hot out, about 90 degrees and real humid. After 3 hours, I went to start it and it would crank and acted like it was flooded. I had to hold the pedal to the floor and it fired up after about 10 seconds.

Is this heat soak? Should I be looking at timing or carb adjustment? I read quite a few forum posts of guys having the same issue and recommended using a 1" phenolic spacer to insulate the carb from the intake. Open to options here...

[Burdar]

The fuel is evaporating out of your carb as it sits in the heat.  If you have room for a spacer, that might help some.  I think the best solution would be to add an electric fuel pump.  I think you can put them in parallel and use it only to fill the carb during startup.  I'm thinking about doing this.  It doesn't solve the fuel evaporation problem but it allows to you fill the carb before trying to start the engine.

[303 Mopar]

Sounds like the fuel in the bowls of your carb and possibly in your lines are vaporizing.  Do you have a return line? 

Couple of options.  I installed a Cool Carb spacer www.coolcarb.com and it significantly reduced the temp of the bowls on my carb.  You can also install the Fel-Pro 1215 intake gasket set with the heat-cross over block in the valley pan https://www.summitracing.com/parts/fel-1215?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-fel-pro&gclid=CjwKEAjw7J3KBRCxv93Q3KSukXQSJADzFzVScmixeZ8so22F-26x_Hyfe5eKHjMDp3Fx0f9EQgRsvBoC_2zw_wcB.  Finally, you can install a thermo wrap on your fuel lines that will help reduce the heat too https://www.summitracing.com/search/product-line/dei-fire-sleeves?SortBy=Default&SortOrder=Ascending&tw=dei%20fire&sw=DEI%20Fire%20Sleeves.

[Cuda Cody]

If it's a loss of fuel in the bowl issue, it should do it when it's cold too as the fuel will not fill back up until you crack it a bunch.  If your engine starts good after sitting all night but not when warm, then it most likely is your compression.  I had this happen before on a engine that had a little higher compression then I normally like.  The solution was to back the timing off a bit so it would not have to fight the compression on the way up.  When the engine is warm the pistons seal a bit better so it's harder to fight the compression when it warm.  Thus if you back the timing off a few degrees it might be able to fire off a little easier when it's warm.  Just an easy thing to try.   

What compression is your engine?  @dave73chally

[dave73]

No fuel return line at the moment. What is the advantage?

What's the difference between the cool carb spacer and a generic phenolic spacer? Is one more effective than the other or is it subjective?

Compression is 10:1 - I'll try backing off the timing a few and see how it likes it

Also, I still have the 5/16" fuel line setup on the car. Haven't gotten around to switching it out for 3/8" yet.  Is that aiding in this problem at all?

[303 Mopar]

The return line will allow fuel to return to the tank to give it a chance to cool before being pumped back to the engine, thus cooling the entire fuel system.

I like the cool carb spacer because of the construction material, that it is super thin for hood clearance, and that extends out to protect the fuel bowls more than just a basic spacer. 

I installed this on my '68 Charger with a 505 ci stroker. The Charger had fuel starving issues and vapor locked when hot. It only had a 5/16" fuel line with a stock mechanical pump.  I added a new sender and a 3/8" line as the main and converted the 5/16" line to the return all run through a 3 nipple filter. The new Fel-Pro valley pan will block the heat crossover, the Cool Carb will provide heat protection for the carb, and the DEI wrap added a layer of protection to keep the fuel cool. 

In my non-scientific tests with measurements from a heat laser gun, the engine temp was 195 after 30-45 min of driving in traffic around town.  Before the Cool Carb shield the carb bowls temp was 148.  After the Cool Carb the carb temp was 127.  So about a 20 degree drop, or 15%. 

No more hot starting issues.

[Cuda Cody]

You might try running some race fuel.... have you tried that yet?

I'm betting this is a compression issue and timing issue.  It's one of the reasons I don't run high over about 9.5 to 1 anymore.  You're only going to be able to back it off so far until it's not going to like it so lets hope that a few degrees helps.   

Wouldn't it start slow when cold if it was a lack of fuel in the carb bowl?  I really would lean towards this being a compression & timing deal.    

[dave73]

Did you have any issues with the cool carb? I did a quick google search and found a thread where a guy with a BB mope had issues with his melting apart...

Cody - my engine builder built this to 10:1 with aluminum heads so I could run 93 pump. I'll mess with the timing and hope for the best. I don't want to have to run race gas, that'd suck for accessibility

[Burdar]

What are you using for an ignition system? I think MSD makes a starter retard box. It reduces timing for easier starting.

[GrandpaKevin]

What distributor are you running?

I had the exact same issue that I spent hours trying to figure out thinking it was fuel/hot temp related.

It started fine when cold but after driving and getting it up to temperature then turning it off and letting it sit for a short while, no start until it cooled down.

After talking to an old Mopar mechanic he quickly diagnosed it was a bad magnetic pickup in the Mopar style electronic distributor.

Replaced the magnetic pickup and problem was solved.

Good luck.

[dave73]

I'm running a brand new msd pro billet distributor

[GrandpaKevin]

I'm running a brand new msd pro billet distributor

Have you checked for spark when the car won't start?

How about gas dripping out of a booster when the car is shut off for possible stuck needle and seat

[Husk Challenger]

I have exactly the same problem on my 383 and i am also running MSD distributor from short time.
I am still in tuning process and next try is to reduce timing and see if it fix the problem.

First i was thinking of vapor lock because when the engine is warm, if i stop it and trying to start, it starts right up.

What timing are you running?
Stock bushing and springs?

[Shoooter]

A friend of mine had the same type of problem.  It turned out to be the starter. When the motor got hot the starter didn't have enough power to turn it over fast enough.

[Cuda Cody]

Yes, that's another issue that can happen.  If the starter is turning slow it most likely is the bushings that have gotten hot.  Good thinking @Shoooter

A friend of mine had the same type of problem.  It turned out to be the starter. When the motor got hot the starter didn't have enough power to turn it over fast enough.