720 hard to start after sitting

[MaxChlan]

On 1/11/2020 at 4:42 PM, Charlie69 said:

When my truck sits for a while and I suspect the fuel bowl is empty I turn my switch barely into the start position and I can here the fuel pump come on but this does not engage the starter, I keep the key in this position for a few seconds to fill the fuel bowl, I release the key and hit the throttle pedal 1 time to set the choke and I start the truck.  This prevents the dry bowl cranking.

I do this too. I live in MN, and drive my 720 maybe once a week. My normal procedure is two pumps, and it generally it either starts and stays running, or will starvefor fuel and die. In that case I'll let it sit for a couple seconds, then start it again. The only time I've had to break out the starter fluid is when it was -27 ambient. Below is the fuel pump wiring/operation from the 1986 manual:

 

 

I can definitely see how you'd have a hard-start issue if the bowl is emptying for whatever reason, combined with a weak fuel pump or semi-clogged filter. I think a good solution would be to wire it to accessory with a switch. 

Edited January 13, 2020 by MaxChlan

[Charlie69]

13 hours ago, Madkaw said:

 Not sure a bigger carb is he answer. Not sure the 38 would have a larger bowl. The 32/36 is more than adequate on a stock motor if jetted correctly. 

 

The 32/36 and the 38 both have the same bowl vents that allow evaporation of fuel.

[Madkaw]

I just checked on my truck and the fuel pump comes on when the ignition is on. I have replaced the fuel pump relay previously . So maybe this is why I have no issue with the truck starting . Not sure if this faulty or not. I guess worst case scenario I can install an inertia switch in line with the fuel pump in case of accident .

[TimmyG]

On 1/12/2020 at 6:45 PM, datzenmike said:

 

 The 38/38 is a six cylinder or small V8 carburetor, so too big for a 4 cylinder engine. Sure it will work, if you learn how to 'step into it', (the 32/36 is the same way) but engine will display all the symptoms of being over carbureted above idle and at low speeds. It would work better on a larger displacement Z24 than an L20B. The 32/36 would work better on an L20B than an L16 and so on.  

 

Ran a 38 on my first 720 z24. Loved it. Never bogged like this 32/36. Never had to slowly step into it. I was 16 for fuck's sake. Lol. 

Edited January 17, 2020 by TimmyG
Missed a word. Brain fast. Fingers slow.

[Charlie69]

3 hours ago, Madkaw said:

I just checked on my truck and the fuel pump comes on when the ignition is on. I have replaced the fuel pump relay previously . So maybe this is why I have no issue with the truck starting . Not sure if this faulty or not. I guess worst case scenario I can install an inertia switch in line with the fuel pump in case of accident .

If you have the aftermarket solid state fuel pump relay this might explain the fuel pump coming on with the ignition switch or the replacement relay is wired to bypass the safety features to cut power to the fuel in case of accident.

 

Please post manufacturer and part # of replacement part as I would like to buy one for testing purposes.

[Charlie69]

3 hours ago, TimmyG said:

 

Ran a 38 on my first 720 z24. Loved it. Never bogged like this 32/36. Never had to slowly step into. I was 16 for fucks sake lol. 

When I was 16 fuel did not have the additives and actually had color to it. Regular was about $0.35 a gallon.

[Madkaw]

4 hours ago, TimmyG said:

 

Ran a 38 on my first 720 z24. Loved it. Never bogged like this 32/36. Never had to slowly step into. I was 16 for fucks sake lol. 

I think the 32/36 has to be tuned - it’s not a plug and play as much as they may say it fits that model. With my wide band O2 sensor I could see how terribly lean it was . My truck scoots pretty good and I have 3.88 gears. Still need to add some more fuel at WOT .

[Madkaw]

1 hour ago, Charlie69 said:

If you have the aftermarket solid state fuel pump relay this might explain the fuel pump coming on with the ignition switch or the replacement relay is wired to bypass the safety features to cut power to the fuel in case of accident.

 

Please post manufacturer and part # of replacement part as I would like to buy one for testing purposes.

Pretty sure I just ordered it from Rock Auto . I’ll try to remember to look

[TimmyG]

7 hours ago, Madkaw said:

I think the 32/36 has to be tuned - it’s not a plug and play as much as they may say it fits that model. With my wide band O2 sensor I could see how terribly lean it was . My truck scoots pretty good and I have 3.88 gears. Still need to add some more fuel at WOT .

 

My plugs look very lean. I haven't put hardly any miles on the truck since I bought it 5 months ago, and who knows how old they are. 

[Madkaw]

37 minutes ago, TimmyG said:

 

My plugs look very lean. I haven't put hardly any miles on the truck since I bought it 5 months ago, and who knows how old they are. 

I went up two jet sizes on the main jets already and I could probably go up to 160 on the main jet - stock is 140. I have NOT changed the idle jet yet- in fact I have been able to turn the mixture screw in a good half a turn or more . 
The MAIN AIR jet needs to be enlarged and the main fuel jet at the same time . 
I went from 16-18 AFR during moderate loading to 14’s . WOT is still in the 14’s - which I want to see in the 13’s . 
The REDLINE kit I got only had up to 155 main jet , so I ordered a 160. I haven’t had a chance to install it yet . The performance difference is very noticeable and my MPG has gotten a bit better I believe , but need more miles to verify 

[TimmyG]

On 1/16/2020 at 5:32 PM, Madkaw said:

I went up two jet sizes on the main jets already and I could probably go up to 160 on the main jet - stock is 140. I have NOT changed the idle jet yet- in fact I have been able to turn the mixture screw in a good half a turn or more . 
The MAIN AIR jet needs to be enlarged and the main fuel jet at the same time . 
I went from 16-18 AFR during moderate loading to 14’s . WOT is still in the 14’s - which I want to see in the 13’s . 
The REDLINE kit I got only had up to 155 main jet , so I ordered a 160. I haven’t had a chance to install it yet . The performance difference is very noticeable and my MPG has gotten a bit better I believe , but need more miles to verify 

 

Thanks for the info! I definitely think the stock jetting isn't quite right...even though I have little experience with carburetors.

[Charlie69]

All new carbs need to be tuned to the vehicle they are installed in. 

[TimmyG]

2 minutes ago, Charlie69 said:

All new carbs need to be tuned to the vehicle they are installed in. 

 

Yeah. Carb was on the truck when I bought it. 

[Madkaw]

4 hours ago, Charlie69 said:

All new carbs need to be tuned to the vehicle they are installed in. 

Very few actually tune the carb completely . That would require jet changes for optimum performance - and these trucks need all the help they can get . The idle jet adjustment can cover a lot , but getting performance and mpg takes jet sizing . 

[TimmyG]

Just now, Madkaw said:

Very few actually tune the carb completely . That would require jet changes for optimum performance - and these trucks need all the help they can get . The idle jet adjustment can cover a lot , but getting performance and mpg takes jet sizing . 

 

It's on my list of things to do. I don't drive the truck much lately so it's not first on my list.