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Hard Starts When Engine is Hot TPI to Carb Swap

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Old 06-20-2018, 10:52 AM   #1  
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Hard Starts When Engine is Hot TPI to Carb Swap

I have a 87 iroc, initially was TPI fuel injected. I swapped to a carbed 355 and am having trouble starting the engine when it is warm. It turns over for maybe 20 secs before it catches. After this, it will start immediately after I shut it off, but if I let it sit for maybe 15 mins after being warm, it will start hard again. I have a 750 double pumper Holley

What Have I tried?
Temp Control Carb Spacer
Replaced 60 psi fuel pump with 14 psi pump with pressure regulator (with return line)
Set fuel pressure to 3-5 psi
Wrapped Fuel Lines
Adjusted Floats to decrease fuel in bowls
Replaced starter with racing started (wrapped also to reduce temp)

Someone help me PLEASE!!!! Have had this issue since the engine swap two years ago, and have taken it to several shops and no one knows what to do.
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Old 06-20-2018, 11:45 AM   #2  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Can you see any fuel in the bowls (if you have a sight glass) prior to trying to do a hot start? Kind of sounds like your bowls are empty.
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Old 06-20-2018, 12:04 PM   #3  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Can you see any fuel in the bowls (if you have a sight glass) prior to trying to do a hot start? Kind of sounds like your bowls are empty.
Right now the bowls are 1/4 full but i have tried it with them totally full, 3/4, 1/2 to almost empty. In either case the problem still exists. The only other piece of information I have is when its trying to start while hot, the fumes smell is very noticeable which is expected I guess since its trying to start. It almost seems like its flooded but fuel pressure is low, and regulated below what holly requires. I am supposed to be running 8psi. I definetely think its in the delivery somewhere between the electric fuel pump and the carb. I have also switched the diameter of the return line ...up and down and no luck. I am trying to figure this out before i go through the hassle of using a mechanical fuel pump amd changing all my lines.
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Old 06-20-2018, 01:29 PM   #4  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Run the car to warm it up. Turn it off. Let it sit for 15-20 minutes. Pull the carb and look down in the manifold. It sounds like gas is leaking from the carb into the engine while it is sitting and not running. (I think that is what Jorlain was saying in his post above)
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Old 06-20-2018, 01:37 PM   #5  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Run the car to warm it up. Turn it off. Let it sit for 15-20 minutes. Pull the carb and look down in the manifold. It sounds like gas is leaking from the carb into the engine while it is sitting and not running. (I think that is what Jorlain was saying in his post above)
Been down this road before to. Thanks for suggesting. Sad thing is i have put 3 brand new carbs on to check this issue before. Same issue with a edelbrock 750 and 2 different holley 750 carbs
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Old 06-20-2018, 02:37 PM   #6  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Run the car to warm it up. Turn it off. Let it sit for 15-20 minutes. Pull the carb and look down in the manifold. It sounds like gas is leaking from the carb into the engine while it is sitting and not running. (I think that is what Jorlain was saying in his post above)
That or the fuel is evaporating.

With it hot, if you leave the key on for say 6-10 seconds prior to cranking (allowing the fuel pump to build pressure and\or fill the bowls) does it still take an additional 10 seconds before it fires?

Another thing to try would be to try pumping the throttle while looking down the carb during a time it normally wouldn't start right away. Do you get a full pump shot? If you give it a couple of pumps does it start normally or faster?
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Old 06-20-2018, 02:54 PM   #7  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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That or the fuel is evaporating.

With it hot, if you leave the key on for say 6-10 seconds prior to cranking (allowing the fuel pump to build pressure and\or fill the bowls) does it still take an additional 10 seconds before it fires?

Another thing to try would be to try pumping the throttle while looking down the carb during a time it normally wouldn't start right away. Do you get a full pump shot? If you give it a couple of pumps does it start normally or faster?
i have looked down the carb before and it does give a full shot. Now as far as pumping priming the pump (for lack of better words) with the key, that doesnt change the hard start. Giving it gas doesnt alter the issue either.

So far we have tried all of these suggestions over a two year span. The only other thing I can think of is there is some over pressure when the engine is shut off? I took my carb off and put it on my friends 86 which has a mechanical pump and he has zero issues. I swapped him for his carb, and i get the same hard start. We swapped back and his runs fine with either carb. I think its related to either the plumbing set up or soemething in the fuel delivery. The electric pump is currently set to turn on when the ignition is activated, then shuts off when the ignition is turned off.

i did some research on another thread and there are a few other people with the same issue with a variety of different carbs.
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Old 06-20-2018, 09:20 PM   #8  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Just adjust the floats to 1/4 and ran it for an hour and let it sit for 35 mins and now there is nothing in the bowls. Is this a sign of evaporation? If so besides the thermal gasket and wrapping the fuel line, what can be done?
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Old 06-20-2018, 10:12 PM   #9  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

That much fuel disappearing in 30 minutes makes it sound like a leak rather than a fuel boil\evaporation issue. But you've changed carbs, so that seems unlikely. But, you might want to take a peek down into the intake anyways to see.

What type of fuel line are you using? And what are you using to insulate it? What are your engine temps like? How are the lines routed? Obviously, the hotter the fuel is the faster it'll evaporate. I think modern fuels might even evaporate more quickly due to the ethanol in them.
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Old 06-21-2018, 03:51 AM   #10  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

It is 1 of 3 things: boiling/evaporating, leaking, or draining back towards the tank through the fuel lines.
That might mean that there is a vacuum created in your tank while running and the fuel is getting sucked out of the carb when you stop the engine. Is your tank vented properly?
How hot is it under your friends hood compared to yours?

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Old 06-21-2018, 07:01 AM   #11  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Carbs don't like ethanol. Major contributor to hard starts while hot...
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Old 06-21-2018, 09:19 AM   #12  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Something ez to try is leave the gas cap loose so no pressure can build See if the problem persists. You may get some odors but you can deal with that for now.
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Old 07-08-2018, 06:37 PM   #13  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Something ez to try is leave the gas cap loose so no pressure can build See if the problem persists. You may get some odors but you can deal with that for now.
i am going to try this. If this renders the problem, whats tje permanent fix?
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Old 07-08-2018, 06:41 PM   #14  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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It is 1 of 3 things: boiling/evaporating, leaking, or draining back towards the tank through the fuel lines.
That might mean that there is a vacuum created in your tank while running and the fuel is getting sucked out of the carb when you stop the engine. Is your tank vented properly?
How hot is it under your friends hood compared to yours?
Tank may not be vented properly. How should this be done? Also my engine runs at 165 degrees. Using the IR gun the carb gets to 90 degrees max on a hot day, no feul bowling. My regulator has a return line. I also have another fuel line that is not being used and it has a rubber cap on it.
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Old 07-08-2018, 06:42 PM   #15  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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It is 1 of 3 things: boiling/evaporating, leaking, or draining back towards the tank through the fuel lines.
That might mean that there is a vacuum created in your tank while running and the fuel is getting sucked out of the carb when you stop the engine. Is your tank vented properly?
How hot is it under your friends hood compared to yours?
also i may add my friends engine runs slightly hotter around 190 degrees.
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Old 07-08-2018, 06:46 PM   #16  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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also i may add my friends engine runs slightly hotter around 190 degrees.
I do notice that after sitting, the fuel pressure drops on my fuel line pressure gauge but maybe this is expected?
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Old 07-08-2018, 08:00 PM   #17  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Tank may not be vented properly. How should this be done? I also have another fuel line that is not being used and it has a rubber cap on it.
That unused line most likely went to your charcoal canister, which I'm betting is not there either. There is also supposed to be a "UFO" valve under the car by the driver's rear wheel. Together, these take care of your tank venting. Cap them off and you now have problems.
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Old 07-08-2018, 08:31 PM   #18  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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That unused line most likely went to your charcoal canister, which I'm betting is not there either. There is also supposed to be a "UFO" valve under the car by the driver's rear wheel. Together, these take care of your tank venting. Cap them off and you now have problems.
If you dont mind can you post pics of both the ufo valve and canister? The gas line that is not used is under the hood on the drivers side. I see something by the tire but i want to make sure I am looking at both right parts. So are you saying both parts need to be there in order to eliminate this issue? Also I have a vented gas cap as well. Not sure if this will cause an issue.
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Old 07-08-2018, 08:40 PM   #19  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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also i may add my friends engine runs slightly hotter around 190 degrees.
also i do notice after sitting there is no gas in the bowls. Could this possible vacuum be the cause?
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Old 07-08-2018, 10:10 PM   #20  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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That unused line most likely went to your charcoal canister, which I'm betting is not there either. There is also supposed to be a "UFO" valve under the car by the driver's rear wheel. Together, these take care of your tank venting. Cap them off and you now have problems.
my canister is gone. They removed it when i did the tpi swap. Do i just get a canister and run that line to it? Do i plug the red plug back into it?
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Old 07-08-2018, 10:17 PM   #21  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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also i do notice after sitting there is no gas in the bowls. Could this possible vacuum be the cause?
If a vacuum is being created in the tank as the fuel is consumed, then it is entirely possible. But if your cap is vented, a vacuum should not be created. Do what midias said above - Something ez to try is leave the gas cap loose so no pressure can build. See if the problem persists. You may get some odors but you can deal with that for now.
Leaving the cap loose would also prevent a vacuum from being created.
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Old 07-08-2018, 10:27 PM   #22  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap



UFO Valve - no longer available
You have to take yours apart and clean, or get really lucky finding one used.

Below - charcoal canister.


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Old 07-08-2018, 10:31 PM   #23  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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my canister is gone. They removed it when i did the tpi swap. Do i just get a canister and run that line to it? Do i plug the red plug back into it?
That red plug is the solenoid that was controlled by your ECM.
The ECM is gone also, I assume.
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Old 07-08-2018, 10:36 PM   #24  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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I do notice that after sitting, the fuel pressure drops on my fuel line pressure gauge but maybe this is expected?
Yes, you are feeding a carb now, not fuel rails and injectors.
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Old 07-09-2018, 12:23 PM   #25  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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If a vacuum is being created in the tank as the fuel is consumed, then it is entirely possible. But if your cap is vented, a vacuum should not be created. Do what midias said above - Something ez to try is leave the gas cap loose so no pressure can build. See if the problem persists. You may get some odors but you can deal with that for now.
Leaving the cap loose would also prevent a vacuum from being created.

Took the gas cap off and drove it, still has the same issue. I still have the UFO valve in place and in line. The canister is gone. UFO valve. Car has 70,000 miles on it.
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Old 07-09-2018, 09:55 PM   #26  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Took the gas cap off and drove it, still has the same issue. I still have the UFO valve in place and in line.
This is very strange. It has to be a problem with your car, because you said that it does not happen when you swap your carb to your buddies car, and it still happens if you put his carb on your engine.
I'm starting to run out of ideas.


OK, describe your fuel supply system (type of pump, regulator, pressures, etc).
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Old 07-10-2018, 07:56 AM   #27  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Not sure if this has anything to do with it, everyone is looking at fuel, but How advanced is your Timing? When my Bird was *HOT* from climbing up hills or on a hot day, the timing was so advanced that you could barely start the car over 210 degrees. Spin the distributor back a bit and off I went, I lived in a High Elevation Area as well.
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Old 07-10-2018, 10:14 AM   #28  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Not sure if this has anything to do with it, everyone is looking at fuel, but How advanced is your Timing? When my Bird was *HOT* from climbing up hills or on a hot day, the timing was so advanced that you could barely start the car over 210 degrees. Spin the distributor back a bit and off I went, I lived in a High Elevation Area as well.
My timing has been adjusted several times but we left it alone thinking it was a heating issue. Maybe we will revisit this because it occurs when the car is not hot at all.
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Old 07-10-2018, 10:20 AM   #29  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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Took the gas cap off and drove it, still has the same issue. I still have the UFO valve in place and in line.
This is very strange. It has to be a problem with your car, because you said that it does not happen when you swap your carb to your buddies car, and it still happens if you put his carb on your engine.
I'm starting to run out of ideas.


OK, describe your fuel supply system (type of pump, regulator, pressures, etc).
Just realized we left out a major component..my friend has a mechanical fuel pump so we are not in the same playing field. I am trying not to go that route due to plumbing of has lines and such.

i started with the stock 60 psi fuel pump but now have a 14 psi tbi pump. I have tried mallory and holley regulators. My current is a mallory with return line. Holley recommends 8 psi for carb but i run 4-6 psi.

Floats are down to where you can barely see fuel in sight plug.
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Old 07-11-2018, 01:30 AM   #30  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Float level should be just at the bottom of the plug's threads, so you're good.
6 psi would work. Where is your regulator installed, before or after the carb? I'm guessing before, as in FP, Regulator, Carb.
Here is the better way, if you can set it up like this. In this way the regulator cannot act as a restriction to the carb.

If you had to add a mechanical FP, the TBI pump could give the fuel a push from the tank to the block mounted mechanical.
GM also did that to try to solve the "vapor lock" problem. L69 cars came set up this way from the factory.
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Old 07-11-2018, 08:02 AM   #31  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Yes mine setup is currently similar to this with the exception of my electrical fuel pump is inside the tank. Everything else is the same. I still think the issue is somewhere with the fuel delivery after the engine is shut off. I think we are getting close.
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Old 07-11-2018, 10:43 AM   #32  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

Here is my current setup


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Old 07-11-2018, 11:22 PM   #33  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

1) You MUST be using a bypass regulator.
2) Change the line routing to this. The only thing you need to change to this is a log type of fuel line at the carb.
But for a test, you can get away with a tee.
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Old 07-14-2018, 01:47 PM   #34  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

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1) You MUST be using a bypass regulator.
2) Change the line routing to this. The only thing you need to change to this is a log type of fuel line at the carb.
But for a test, you can get away with a tee.


Found out the issue. The problem is when the car shuts off, the pump shuts off, but I dont think the fuel goes back thru the return line fast enough and I still have pressure in the line going to the carb. I installed a switch to the fuel pump, and if i hit the switch 6 seconds before i shut the car off, the pressure goes to <1psi and when i go to start, it does not hesitate, it starts right up, no issues.

This is a temporary fix but what can I do to force the fuel back to the tank faster, and relieve the pressure in the line?
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Old 07-14-2018, 10:31 PM   #35  
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Re: Hard Starts When Engine is Hot TPI to Carb Swap

OK great, you're onto something.
3 things come to my mind.
1) your needles & seats in the carb are not completely sealing. So the pressure left in the line after you shut the engine off is forced past - "flooding" your engine. A not-so-soon-after restart takes a lot of cranking to get that fuel out of there.
2) if it only takes 6 seconds for the line pressure to drop, then the return line is working fine.
3) or is it? What is the part number of your Mallory regulator? If it is anything like the Holley 12-803, then that regulator is a non-return style and will not work properly in this situation. https://www.summitracing.com/parts/h...SABEgLByPD_BwE
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Hot!, Hot!, Hot! Headers bigjeepinyj TPI 7 10-14-2002 02:51 PM
Horn Problem, hard to explain, please try to help!!HARD HARD HARD! CANT FIGURE IT OUT nesluopetan Electronics 5 03-04-2002 01:10 AM


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