IMPCO start assist

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kikkegek
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IMPCO start assist

Post by kikkegek » Fri Jan 03, 2014 10:02 am

hi guys,

I have been searching and searching, but can find the answer.

Got a Suburban this weekend and it has a IMPCO300A on a Quadrajet and is connected to a IMPCO model-E vaporizer. The vaporizer has a IMPCO Start assist connected.

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Now the manual is pretty clear on how to install it when using a IMPCO 425

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but I dont have a IMPCO 425 (yet) but a 300A on a Quadrajet.

Now it says DONT CONNECT TO MANIFOLD VACUUM.....but you should connect to the "air valve vacuum port".....is that ported vacuum?

what would be the starting procedure? because ported vacuum is zero of nearly zero at when cranking...

please help.
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado


C3H8
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Re: IMPCO start assist

Post by C3H8 » Fri Jan 03, 2014 4:07 pm

viewtopic.php?f=5&t=1531

The start assist on the 300 mixers is attached on the bottom side of the 300 to a port marked EC1. This is venturii vacuum. IMPCO start assist valves are made for low vacuum in inches of water column. Ported vacuum on a gauge that reads in inches of mercury may very well show 0 inches but actually there is 5 to 10 inches of water column above the throttle plates but under the mixer while cranking. You have two choices for your set up. Intsall the fitting that came with the valve into the ECI port under the mixer. It like has a screw in it to block it right now. The link above should take you to another post with pictures showing the location. If it doesn't work go back to Nov 27 in the forum "1995 G20 won't idle" and the pics are in there. Just scroll down.

Another option is to drill and tap a fitting into the side of the adaptor holding the 300 to the throttle body/carb.

C3H8
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Re: IMPCO start assist

Post by C3H8 » Fri Jan 03, 2014 4:15 pm

Sorry missed the starting procedure question. It depends how the valve is activated. Many of the older conversions had a spring loaded switch that had to be pushed during cranking. Others were just wired to the starter solenoid wire (purple) and this activated the valve as soon as cranking began. Many used the push button switch because they did not need the start assist on a hot engine. Most only used it in the morning or if the engine had not been started for many hours. Basically turn the key to the cranking position and at the same time push and hold the button to activate the start assist valve if it is not wired directly. You should verify it is wired properly. I have seen many vehicles wired to run only positions for the switch where the installer did not check for power while cranking.

C3H8
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Re: IMPCO start assist

Post by C3H8 » Fri Jan 03, 2014 4:19 pm

One other thing. I see they have grounded the assist valve to the regulator cover. That's not the best place. The regulator is bolted to the fender which is a painted surface. Frequently these grounds fail due to corrosion. Run the ground to a factory firewall ground or to an existing ground on the radiator support or straight to battery ground.

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kikkegek
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Re: IMPCO start assist

Post by kikkegek » Fri Jan 03, 2014 6:15 pm

he thanks man!

I'll check the ground connection and make sure it gets power while cranking. thanks.

All the ports you mention seem "ported vacuum" connections to me and thus all the same. They are all connected above the throttle plates and thus ported vacuum. So the current ported vacuum connection I use should be fine?

Or are they different from the options you offer. I dont really understand the difference I guess...
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

C3H8
Posts: 995
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: IMPCO start assist

Post by C3H8 » Sat Jan 04, 2014 5:05 am

I'll try to explain.
We're actually dealing with three vacuum sources here. The first is the one everyone that works on vehicles deals with. That is manifold vacuum. Manifold vacuum is measured in inches of mercury (HG). 1" of HG is equal to 13.59 inches water column (WC). An engine at idle is usually around 16 to 18" mercury or about 220 to 250" WC. Manifold vacuum is measured after the throttle plates.

Ported vacuum is taken just above the throttle plates. The minute the throttle plates are opened this vacuum is affected strongly by the manifold vacuum. As the plates are opened further and further manifold vacuum drops and ported vacuum can actually increase as the ports are usually located just above the smallest portion of the carburetor venturii. The air rushing past the venturri increases ported vacuum. Depending on design the ported vacuum may remain several inches higher then manifold vacuum at WOT.

Air valve vacuum is created just under the air gas valve in the IMPCO mixer. IMPCO gas valve assemblies create a vacuum based on the airflow through the mixer. Due to the design the vacuum is very low since it is well above the throttle plates and away from the original carburetor venturii's. The mixers are designed to generate a vacuum of approximately 5" WC (.3" HG) cranking and reach a maximum of 30 to 35" WC (2 to 3" HG) at WOT depending on the engine specifics IE RPM, cam design, air flow characteristics, etc. The air valve vacuum is what signals the regulator to open through the vapour hose. This light vac can take a while to get to the reg on cold starts and pull vapour down the line slowly until the engine actually fires.

This brings us to the SV. For whatever reason IMPCO chose to design it with a relatively weak coil and a seat that cannot withstand high vacuum. Attaching the valve to full manifold vacuum could hamper it opening. More importantly manifold vacuum is hard on the regulator diaphragm. Also the valve will leak fuel at manifold vacuum once the engine has started as it was not designed to withstand manifold vacuum. Also, manifold vacuum could affect the distance the secondary diaphragm is pulled open during engine operation if the seat does leak affecting the mixtures. This valve is a great start assist but connected incorrectly it can cause minor operational concerns. Also if it is attached to manifold vacuum it is quite easy to flood an engine on start up.

The best location is the EC1 port on the bottom side of your mixer. I see you have an electric solenoid for a lock off so the EC1 port should be free. This is the port usually used with a vacuum lock off. Like the 425 mixer this port connects directly with air valve vacuum. In a crunch the adaptor is the next best as the area is large and the vacuum in this area is closest to air valve vacuum. Ported vacuum works but there is a possibility of this having a higher vacuum then desired as engine RPM increases. The solution is to test the port and see what type of vacuum is generated while driving the vehicle. If it stays under 3 inches it would be OK but most ported vacuum sources can generate 10 to 12 inches of HG at various times during engine operation.

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kikkegek
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Re: IMPCO start assist

Post by kikkegek » Sat Jan 04, 2014 7:24 am

thank you for the very clear answer. I'll make sure I move the Start Assist from the ported vacuum its on now to the EC1 port you pointed out. really appreciate that.
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

C3H8
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Location: Winnipeg, Canada

Re: IMPCO start assist

Post by C3H8 » Sun Jan 05, 2014 12:39 pm

One quick reminder. Note in the instructions there is a jet(s) (J1-20) for various engine sizes. There is a recommended jet for 4, 6, and 8 cylinder engines. Non are cast in stone. You may want to experiment with these to get the best starting. In many cases we did not even insert a jet on the larger V8's.

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kikkegek
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Re: IMPCO start assist

Post by kikkegek » Sun Jan 05, 2014 1:00 pm

C3H8 wrote:One quick reminder. Note in the instructions there is a jet(s) (J1-20) for various engine sizes. There is a recommended jet for 4, 6, and 8 cylinder engines. Non are cast in stone. You may want to experiment with these to get the best starting. In many cases we did not even insert a jet on the larger V8's.
cool! I'll check...thanks man!
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

kuchen
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Re: IMPCO start assist

Post by kuchen » Mon Jan 06, 2014 3:52 pm

C3H8 wrote:.... There is a recommended jet for 4, 6, and 8 cylinder engines.
My SV was only supplied with one jet for smaller engines (the instructions are pretty unprecise about what a small engine is)
What would the recommended orfrice diameter for a 307ci Oldsmobile V8 be ?

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kikkegek
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Re: IMPCO start assist

Post by kikkegek » Mon Jan 06, 2014 4:37 pm

our engine backfire usually on cold morning start with the Start Assist. I start it with the throttle blades opened up just a little and currently the start assist is connected to ported vacuum.

would the backfire be casued by the wrong connection to ported vacuum?

or could it be that there is none or the wrong orifice in it?

if I dont open up the blades a little the car wont start at all.
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

C3H8
Posts: 995
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: IMPCO start assist

Post by C3H8 » Mon Jan 06, 2014 5:34 pm

To kuchen: smaller displacement would be V6 4.0l and under. All others I would try without the orifice to start with. Maybe my memory is getting foggy or mixer up but I thought there was originally a couple of orifices but we may have custom drilled the existing orifice for mid range (4 to 5 L).

To kikkegek: Backfires into the intake usually means a lean condition, ignition issue or a combination of both. On a carbureted engine the starting method preferred is cracking the throttle a little (1/4 is fine). Check your SV and see if it has the orifice in it. If it does remove it. I don't believe your current ported connection would cause the backfire. One other consideration is a potential valve sealing issue depending on the mileage. You may be getting some sealing issues on a cold engine allowing ignited gas to cause a backfire. Done a compression or leak down test recently?

kuchen
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Re: IMPCO start assist

Post by kuchen » Wed Jan 15, 2014 1:52 pm

the rear vacuum break of this Q-Jet is rendered useless, as a plasic strap holds the choke-flap open at all times.
Is this a bug or a feature ?

C3H8
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Re: IMPCO start assist

Post by C3H8 » Wed Jan 15, 2014 2:51 pm

A little better explanation please. Are you referring to the vacuum operated plate above the secondary butterflies? If so normally this is left functioning the same as on gasoline.

kuchen
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Re: IMPCO start assist

Post by kuchen » Wed Jan 15, 2014 3:17 pm

No, not the secondary air valve, the choke flap on the primary side that is pulled open by a dashpot called rear vacuum break.
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