Need some advice on a hot start problem.

I have a 454 in my 68 and recently have had problems restarting it after running it and getting up to normal temp. If I let it sit until cold again, it starts fine. If I try to start it immediately after shutting it off hot, it will restart ok, but if I let it sit say 10 or 15 min and then try to restart, it acts like it is maybe flooded. You can sometimes hold the pedal to the floor and it will cough and sometimes start after much cranking (my new mini starter will turn over the motor just fine during this episode).

I just rebuilt the q jet, installed oem spec stock jets, primary and secondaries and new float and inlet needle/seat, and running non ethanol gas. I did set the new float height at 1/4" so maybe a float height of 3/8" would be better?
Also installed a new mini starter and new battery cables and engine to frame ground.
So I am thinking this is a fuel problem, fuel boiling in the carb bowl or vapor lock??? But this is a new problem and it is a lot cooler with fall weather.

Suggestions??? Or I did a search and maybe I need to swap out the coil? It would be easy to try this next.

Any ideas are welcome.

Much thanks!

 

I had this problem a few years ago with a 383. It would only do it after driving - never a problem if the car was "cold". Turns out my fuel was boiling in the lines feeding the carb, and when really hot (i.e., middle of July/august) it could boil in the bowl.

I also noticed that 10-15 minutes after shutting down was often a problem, and found that the heat was soaking into the fuel bowls, causing gas to boil. I could see it on my holley if I looked down into the carb.

I added a return line from the carb back to the tank (I used the stock return, but had to lose my stock fuel pump to do so). No more hot start problems. The return line gave the boiling fuel in the feed line an easy path away from the carb.

I also added a heat shield under the carb to protect the fuel bowls and looked for an insulating gasket to protect fuel in the bowl.

I'm not positive that is the same problem with your car, but maybe it's worth checking out. I believe some of our modern blends of gas boil very easily, and this is one way to manage that problem.

 

The only way I would replace the coil is VERIFYING that you have no spark when cranking it under these conditions of it not starting and running.

Do you have a fuel return line connected????

DO you still have the clutch fan for cooling or do you have electric fans???

Do you have an infrared thermometer??? That way you can see how hot your fuel lines are getting.

Are you sure that the rubber fuel hose that supplies fuel to your fuel pump is not getting sucked shut...because it has been heated up and bit more flexible due to running for a while. I know this is a long shot idea...but it is something I always check. I have seen so many where someone has used section of hose off a spool of hose instead of the correct molded 'S' hose and I can see that it is already somewhat crimped closed....and that is when it is cold and not been run.

Also..have you checked to make sure your carb is not dumping fuel into the intake when you shut it off???

DUB

 

I remember there is something in the bottom of q-jet fuel bowls that is a slow leaker. shut it off, it starts right up. give it 10 min or so, dumps the fuel into the intake and causes a flood issue. after cool-off, it has had time to evaporate out. i gotta go search offshore only for this... they are a bunch of true big block gearheads.

 

I would recommend buying a generic GM coil and taking it with you. Next time it refuses to start, swap it out promptly, see if that works. It's cheap and easy to check.
On mine it took me forever to find because when it did fail, as soon as I opened the hood, the coil would start cooling down (it was on the firewall), and then the car would soon just start. Of course, I didn't really know "why" and I was guessing all these things about a hot carb and/or manifold (except I hadn't changed anything????).....anyway....

Only reason I found it is because one night, after a lot of no-success troubleshooting, I was just sitting there spacing, with the engine idling kinda-sorta smoothly, and the timing light flashing on the garage wall........ and then I happened to notice a "skip" in the light flashing sequence. So...... then I started paying attention, and sure enough .... the timing light flash was quite steady, except for an occasional skip. I swapped out the coil with another one of out my old Datsun 210 sport sedan, and it was immediately noticeable that it was a skosh smoother and no more timing light skipping.
After I replaced that coil, I never had another case of the engine just dying after driving around on hot days like that.

It's a cheap, easy, and portable option that you can take with you to test/check/fix.

 

The main metering well plugs? It can happen.

 

Much thanks for all of the helpful advice!

yes, I hooked up the return line.

Have the clutch fan and no electric fan, I got the biggest 69+ DeWitts big block radiator with 1-1/4 tubes and looks like engine does not overheat, but the engine compartment would likely be cooler with an electric fan. Might have to rethink this. I know some think a carb heat shield is a good thing and might have been OEM?

Will check the fuel lines temp and see if I got spark during the no start condition, good ideas all. Also, I think I have the "hose off the spool" and not the S formed hose, so will check that too. The last S formed hose I bought cracked and failed soon after I installed it, but no doubt the S hose is better.

Also..have you checked to make sure your carb is not dumping fuel into the intake when you shut it off???
I am thinking you could hold all of the plates open and see if fuel is dribbling in? sources could be a leaking inlet needle valve or leaking main jet plugs? I think one of the items in Lars papers said a porous carb casting occasionally occurs too?

And I looked the main metering well plugs and they "looked" fine, but I did not test them per Chris Ruggles' book suggestion, hmmm! some saying about "assume"? making an aXX out of you and me???

And I got a couple of extra coils and would be an easy check, might check fine cold and change when hot, I think coils do funny things.

Anyway all of this is very helpful and gets me jump started with new ideas to find the fix.

Thanks again! you guys on this forum are great!

 

2 things to think about:

- Is your return off of the pump or from the carb? My fuel boiling issue was between pump and carb - so where I started the return line was important.

- Even if the heat shield wasn't OEM, I am convinced modern blends of fuel boil much easier than back in the day - so adding heat shield or an insulating gasket may be good insurance anyway.

Hope you find the root cause quickly, I know this can be frustrating.

 

My return is off the fuel pump. Question: I have the std design Qjet with a single metal line from the fuel pump to the Q jet, so the return line comes off of the fuel pump. I am not sure how you would plumb up the std design Q jet with a fuel return off the carb??? Or did you simply cut a tee into the pump to carb fuel line and tie it back into the return line?? Interesting!

Agree, heat shield is directionally correct problem or no problem as cooler fuel is good.

Thanks for the response! (Yep although I have decided this is not a problem as long as you are not in a hurry to come back from anywhere!)

 

Now you can take a coil with you and the tools or simply have one of these to plug into your spark plug wire and check for a spark. There are several different designs and they are really inexpensive.

Heck you can make your own out of a spark plug and spark plug wire and then attach a wire to the threaded portion of the spark plug and ground the wire on your engine and it will spark just like if it was in your cylinder head when you crank it.



DUB

 

You're taking me back 35+ yrs but I remember a friend or 3, all with quadrajets, that had this problem. Something is not sealing (internally) and quadrajet experts know what to do. I believe the fix involved epoxy.

 

I built my own fuel line and tee'd off just a couple inches before the line reached the carb. Then I changed the stock fuel pump for one w/out a return, and ran the new return line down to the return line on the frame.

I had to do it this way this as the fuel was boiling between pump and carb, and was especially bad if I tried to restart after driving the car - I believe heat was soaking into the lines and if hot enough, the bowl after shutting off the engine.

I have run this type of setup with both a QJet and a Holley, and it does require you to build your own lines. Since going this route, I have never had this problem repeat.

 

After fiddling around quite a bit, I think I solved my problem:

I did swap out the coil, old coil had 5.3 K between tower and -, new autozone coil had 9.8K between the tower and -, so probably a good thing.

However a post by cromwell74 got me thinking as I have removed the choke linkage, no problem in hot Louisiana, except:

quote:
The choke was disconnected by previous owner because it would sometimes go on/get stuck when hot and flooded the carb

So, I checked during my hot "no start" episode and sure enough the choke flapper was closed. I suppose I could have removed the choke flapper but I wired it up for the time being and have not had the problem since. (I hope).

Much thanks for all of the help and advice by everyone and you might want to add the hot choke issue to your list of things to check next this problem pops up.

Thanks again!

 

Glad you got it fixed. I had a similar problem with the 454 installed in my 68 by the PO. When the engine was hot and turned off, the residual heat boiled the fuel out of my QFT carb. The carb has glass sight windows on each barrel, so I could see the fuel level going down. Hopefully your Q-jet does not do that, but keep the info in mind just in case. I had to add an electric fuel pump to re-fill the carb faster than the mechanical one, which required cranking for 20-30 seconds to get fuel back in the carb.

 

Sounds like vapor lock to me. I have owned my 1970 since 1972. It is mostly original. I think it has always had this problem.
The only difference is I do not have the carb heat shield. The car came with a Holly spread bore on it.The heat shield was not there.
After 1975 , I drove it to work about every day. It always started at night.

But I have tried lot of changes to prevent the vapor lock.
I wrapped my headers, put on an electric fuel pump, put heat insulation over the fuel line, thick insulator gasket, and other things to try to improve it.
But the problem still persists.

Heat is also a killer on the magnetism of the starter magnets. I replaced numerous starters because they would not start that car after it heated up. I put in a high torque starter to fix that problem. But now, the vapor lock is more prevalent.