Mono jet won't start without priming - The 1947 - Present Chevrolet & GMC Truck Message Board Network

retro4now · 02-24-2016, 07:45 AM

Hi
I'm new to this forum and am finding it very useful.
I just bought a 1971 GMC step side with a later model 250 six.
It had been sitting quite awhile, so I had the tank professionally flushed and cleaned, changed the fuel lines, new sending unit, new fuel filters, and rebuilt the carb.
It's a one barrel MonoJet. It runs fine except for one thing.... When it sits over night it won't start without priming with some gas, no matter how much I crank it or pump it. Then it starts right up and every time after that until the next day.

Any ideas where I should start looking. I haven't had a chance to pull the top off yet first thing and check the float bowl. Seems like it might be leaking down from somewhere, but I don't know where that might be.

Thanks
Gerry
Albuquerque, NM
'71 GMC C1500

**special-K** · 02-24-2016, 09:42 AM

It does sound like leak down, but it should strat once fuel is pumped. I wonder if your pump is weak or possibly partially gunked up. I didn't see you mention dealing with the pump at all. I'd buy a new one anyway to complete the fuel system refresh, they're pretty cheap. If you end up needing to rebuilt the carb, you'll have a completely new system

57taskforce · 02-24-2016, 10:01 AM

Pump. That's where I'd start. My k/20 does the same thing and I know for a fact it's the pump.

geezer#99 · 02-24-2016, 10:15 AM

Choke!!
Is your choke working?

retro4now · 02-24-2016, 05:00 PM

Thanks for the replies.
I did forget to mention that I did install a new pump as well. I rebuilt the carb and I used to rebuild carbs in my sleep, but I got to admit I haven't touched on in over a decade, so I very well could have missed something.
The choke seems to be working, I step on the pedal when it's cold and the butterfly closes.
I definitely think it's a fuel leak down problem, but where. No leaks externally, and I have a clear fuel filter installed above and in front of the valve cover and it's always full of fuel.
So I'm thinking the fuel must be leaking down into the intake from the carb. somewhere.
thanks again
Gerry

Wrenchbender Ret · 02-24-2016, 05:14 PM

How about the accelerator pump. If you look down the throat & work the throttle you should see a squirt of gas going in the throat.

retro4now · 02-29-2016, 03:13 PM

Update: First start of the day, I pulled the air cleaner cover and looked down inside and gave the throttle a couple of blips. Sure enough, it's squirting fuel. However, the choke plate didn't close. When I manually closed it, the truck started right up without priming. So, looks like I'll be checking the choke adjustment and/or replacing the bi-metallic spring.

thanks for all the suggestions.

Gerry
Albuquerque, NM
1971 GMC, 250 six

**special-K** · 02-29-2016, 09:41 PM

Glad you figured it out. I hate gremlins

retro4now · 03-03-2016, 07:42 AM

Put a manual choke cable on it yesterday. Works great and starts right up.
Thanks to everyone for the advice.
Gerry

02-24-2016, 07:45 AM	#1
retro4now Registered User Join Date: Feb 2016 Location: Albuquerque Posts: 8	Mono jet won't start without priming Hi I'm new to this forum and am finding it very useful. I just bought a 1971 GMC step side with a later model 250 six. It had been sitting quite awhile, so I had the tank professionally flushed and cleaned, changed the fuel lines, new sending unit, new fuel filters, and rebuilt the carb. It's a one barrel MonoJet. It runs fine except for one thing.... When it sits over night it won't start without priming with some gas, no matter how much I crank it or pump it. Then it starts right up and every time after that until the next day. Any ideas where I should start looking. I haven't had a chance to pull the top off yet first thing and check the float bowl. Seems like it might be leaking down from somewhere, but I don't know where that might be. Thanks Gerry Albuquerque, NM '71 GMC C1500

02-24-2016, 09:42 AM	#2
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,861	Re: Mono jet won't start without priming It does sound like leak down, but it should strat once fuel is pumped. I wonder if your pump is weak or possibly partially gunked up. I didn't see you mention dealing with the pump at all. I'd buy a new one anyway to complete the fuel system refresh, they're pretty cheap. If you end up needing to rebuilt the carb, you'll have a completely new system __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

02-24-2016, 10:01 AM	#3
57taskforce All about them K’s Join Date: Jun 2014 Location: Farmington, New Mexico Posts: 6,336	Re: Mono jet won't start without priming Pump. That's where I'd start. My k/20 does the same thing and I know for a fact it's the pump. __________________ Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 315/75/16's ‘87 IROC-Z all original 50K mile survivor TPI 305 IROC Blue ‘10 Camaro 2SS/RS Aqua Blue Metallic #93 -version 2.0

02-24-2016, 10:15 AM	#4
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,570	Re: Mono jet won't start without priming Choke!! Is your choke working?

02-24-2016, 05:00 PM	#5
retro4now Registered User Join Date: Feb 2016 Location: Albuquerque Posts: 8	Re: Mono jet won't start without priming Thanks for the replies. I did forget to mention that I did install a new pump as well. I rebuilt the carb and I used to rebuild carbs in my sleep, but I got to admit I haven't touched on in over a decade, so I very well could have missed something. The choke seems to be working, I step on the pedal when it's cold and the butterfly closes. I definitely think it's a fuel leak down problem, but where. No leaks externally, and I have a clear fuel filter installed above and in front of the valve cover and it's always full of fuel. So I'm thinking the fuel must be leaking down into the intake from the carb. somewhere. thanks again Gerry

02-24-2016, 05:14 PM	#6
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,207	Re: Mono jet won't start without priming How about the accelerator pump. If you look down the throat & work the throttle you should see a squirt of gas going in the throat.

02-29-2016, 03:13 PM	#7
retro4now Registered User Join Date: Feb 2016 Location: Albuquerque Posts: 8	Re: Mono jet won't start without priming Update: First start of the day, I pulled the air cleaner cover and looked down inside and gave the throttle a couple of blips. Sure enough, it's squirting fuel. However, the choke plate didn't close. When I manually closed it, the truck started right up without priming. So, looks like I'll be checking the choke adjustment and/or replacing the bi-metallic spring. thanks for all the suggestions. Gerry Albuquerque, NM 1971 GMC, 250 six

02-29-2016, 09:41 PM	#8
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,861	Re: Mono jet won't start without priming Glad you figured it out. I hate gremlins __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

03-03-2016, 07:42 AM	#9
retro4now Registered User Join Date: Feb 2016 Location: Albuquerque Posts: 8	Re: Mono jet won't start without priming Put a manual choke cable on it yesterday. Works great and starts right up. Thanks to everyone for the advice. Gerry



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