IMPCO 425: new base plate, binds when engine runs
IMPCO 425: new base plate, binds when engine runs
he guys, help me out here.
I had used an old base plate from a vacuum secundairy carb and modified it to mechanical secundaries. I was never satisfied about it. throttle was hard and primairies never closed completely when releasing the throttle slowly. even tougher springs didnt fix it. tapping the throttle would return the engine to normal idle at traffic light. pretty annoying.
so I bought a "allmost" new throttle base plate and already regret it. its a 850 cfm plate and it shows exactly the same issues...but now primairy AND secundairy seem to have it. when slowly opening the throttle they both dont completely return to their closed position, causing high idle.
I have tried everything:
- adjusted the inside of the mixer so the blades dont touch the inside of the mixer.
- checked the gasket to not touch the blades
- wet-polished the bores with 400 sandpaper
- made sure the blades are not completely closed, but just a little lifted over the bores, so they dont bind
- etc...
when the engine is OFF everything is fine.
But as soon as I start the engine, the throttle shafts, both primairy and secundairy, seem to turn with some kind of friction causing them to bind and not close completely when I let go of them.
the return springs are brand new and should be just fine.
anybody else ever had this with a (allmost) new base plate?
here are some pics for info
I had used an old base plate from a vacuum secundairy carb and modified it to mechanical secundaries. I was never satisfied about it. throttle was hard and primairies never closed completely when releasing the throttle slowly. even tougher springs didnt fix it. tapping the throttle would return the engine to normal idle at traffic light. pretty annoying.
so I bought a "allmost" new throttle base plate and already regret it. its a 850 cfm plate and it shows exactly the same issues...but now primairy AND secundairy seem to have it. when slowly opening the throttle they both dont completely return to their closed position, causing high idle.
I have tried everything:
- adjusted the inside of the mixer so the blades dont touch the inside of the mixer.
- checked the gasket to not touch the blades
- wet-polished the bores with 400 sandpaper
- made sure the blades are not completely closed, but just a little lifted over the bores, so they dont bind
- etc...
when the engine is OFF everything is fine.
But as soon as I start the engine, the throttle shafts, both primairy and secundairy, seem to turn with some kind of friction causing them to bind and not close completely when I let go of them.
the return springs are brand new and should be just fine.
anybody else ever had this with a (allmost) new base plate?
here are some pics for info
storage/project: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.10 gears - IMPCO425 mixer
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
Re: IMPCO 425: new base plate, binds when engine runs
I've been looking through you pictures and I can't spot the springs that are commonly used on a holley setup. When you are referring to the return springs are you talking about the springs on the shafts only. Typically Holley carburetors have additional return springs running from an anchor point on the intake to the throttle linkage. The shaft springs alone will not keep the plates closed against the airflow and intake vacuum. You can see many samples of this if you type "Holley throttle linkage return spring images" into your search engine on the internet. Some of these show single or even double return springs as part of the install.
Re: IMPCO 425: new base plate, binds when engine runs
the springs are not the problem.
the problem is that when the engine runs, the throttle shafts feel like they are binding or rubbing.
the problem is that when the engine runs, the throttle shafts feel like they are binding or rubbing.
storage/project: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.10 gears - IMPCO425 mixer
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
Re: IMPCO 425: new base plate, binds when engine runs
When the throttle plate is not screwed onto the mixer what does it feel like?
My suggestion would be cut your losses and get a new 600 throttle plate.
My suggestion would be cut your losses and get a new 600 throttle plate.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Re: IMPCO 425: new base plate, binds when engine runs
It feels smooth
It only feels like it's binding or rubbing with the engine runing. .
It only feels like it's binding or rubbing with the engine runing. .
storage/project: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.10 gears - IMPCO425 mixer
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
Re: IMPCO 425: new base plate, binds when engine runs
The whole time the engine is running or just when it is hot or cold? and it feels smooth when it is on the engine but not when it is running? is that correct?
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Re: IMPCO 425: new base plate, binds when engine runs
OK. Have you looked at the accelerator cable or is it the metal rod style? Also, if it has it, what about the kick down cable for the transmission or is it a standard trans. Could either of these be binding. I have seen old cables that have broken strands and begin to bind on the sheathing.
Re: IMPCO 425: new base plate, binds when engine runs
No Cables attached yet.
And yes only when engine runs. So when there is engine vacuum
And yes only when engine runs. So when there is engine vacuum
storage/project: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.10 gears - IMPCO425 mixer
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
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Re: IMPCO 425: new base plate, binds when engine runs
Maybe you need to very slightly round off any sharp edges on the throttle plates which might bite/bind in the throttle bores when closed.
Just a few wipes with fine (800+ grit) sandpaper will be enough I think if this is the problem here.
Just a few wipes with fine (800+ grit) sandpaper will be enough I think if this is the problem here.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.
Re: IMPCO 425: new base plate, binds when engine runs
SOLVED..
I had the wrong expectation of how smooth the throttle should be and how much spring tension would be needed to close the butterflys on idle.
I ended up adding the pulling spring setup in front of my setup again and added the double spring. now throttle returns to idle base position.
thanks for helping guys.
I thought the spring on the throttle plate would be enough for normal operation. Guess it down work without the extra springs attached.
I had the wrong expectation of how smooth the throttle should be and how much spring tension would be needed to close the butterflys on idle.
I ended up adding the pulling spring setup in front of my setup again and added the double spring. now throttle returns to idle base position.
thanks for helping guys.
I thought the spring on the throttle plate would be enough for normal operation. Guess it down work without the extra springs attached.
storage/project: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.10 gears - IMPCO425 mixer
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
Re: IMPCO 425: new base plate, binds when engine runs
I run holley base plates.. have have rebuilt them.. new bushes , line bored/ reamed etc
It is essential that one doesnt run heavy secondary springs.. doing so causes side wear on the butterfly shafts and bushes... resulting in a small vaccuum leak.. and if running a larger (800 cfm) base u will never get the idle speed down without adjusting the secondary stop screws so the butterflies sit on the bores. Butterflies sitting on the bores ids a BIG No NO...
If the engine is left sitting, the butterflies jamb.. then open the throttle, u can bend the shafts and/ or move the butterfly alignment in the bores. Once this happens, once again there is no way u can get a idle speed due to leaking past the secondary butterflies
The other common issue is torquing down and over torquing...it only takes a couple thou to distort the base plate and how he butterflies sit in the bores... petrol not as critical.. LPG VERY critical
1/ visually check how the butterflies sit in the bores closed , up to the sun or bright light...they should be able to close down closed EVENLY and SYMMETRICALLY with the primary and secondary adjustment screw adjusted right out..
If not re set the butterfly alignments in the shafts / bores.. it may take several attempts to do so.. be annal about.
Re crimp the butterfly /shaft screws.. use new ones is best..and careful not to bend the shafts doing so.
2/ Now set the primary idle screw to 1 1/2 turns off the primary butterflies fully closed.
3/ Set the secondaries to just off sitting on the bores.. and note the position of the screw very closely
4/Check the the holley return springs and butterflies work smooth and gaps symmetrical.
Note: notice how the holly spring doesnt load the return load sideways, but around the movement.....any other springs u add supplement this spring only, they should not 'over ride' the loading
5/Now bolt/ torque up to the mixer.. Again check everything is smooth etc
6/ Now mount to the valley cover... again check all is smooth etc.. even as u torque down
7/Connect the min weight spring required for secure butterfly return
8/Fire up the engine
9/ The idle sshould be either way to low to be unstable or way to low to idle.... DO NOT ADJUST the primary idle screw.. it is already adjusted to correct height
10/ Take a blind rivet, hammer the shaft end flat into a screw driver, file the end flat.. Put a near 90 deg bend in the shaft. U now have a screw drive u can get under the edge of the base plate to adjust the secondary butterfly height.
11/ Now using this tool adjust the secondary butterfly screws to open the secondary butterflies to correct height.
The spec for the primary idle adjustment is 1 1/2 turns +/- 1/2 turn... Once the secondaries are adjusted correct for THAT engine.. so u have some adjustment in the idle speed adjustment screw for FINE TUNING ONLY
If u cant get idle down then u have a vaccuum leak which maybe a port not sealed covered , poor gasket, worn butterfly shaft bushes.. bad PVC valve.. what ever.
Now connect throttle linkages...again there should be no binding.
If u require 2 return springs.. safety , track regulations whatever... have them working in OPPOSITE directions.. ie one at the bottom to the rear of the engine.. the other at top to the front of the engine...
The above applies to petrol, LPG methanol .. all engines
With a sb on lpg a 800 cfm baseplate is ball park , secondaries just off the bores+ about 1/4 to 1/2 turn....ANY wear in the shaft plates and u will not get the idle down... on LPG.. petrol is slightly more tolerant due to different mixture ratios.
It is essential that one doesnt run heavy secondary springs.. doing so causes side wear on the butterfly shafts and bushes... resulting in a small vaccuum leak.. and if running a larger (800 cfm) base u will never get the idle speed down without adjusting the secondary stop screws so the butterflies sit on the bores. Butterflies sitting on the bores ids a BIG No NO...
If the engine is left sitting, the butterflies jamb.. then open the throttle, u can bend the shafts and/ or move the butterfly alignment in the bores. Once this happens, once again there is no way u can get a idle speed due to leaking past the secondary butterflies
The other common issue is torquing down and over torquing...it only takes a couple thou to distort the base plate and how he butterflies sit in the bores... petrol not as critical.. LPG VERY critical
1/ visually check how the butterflies sit in the bores closed , up to the sun or bright light...they should be able to close down closed EVENLY and SYMMETRICALLY with the primary and secondary adjustment screw adjusted right out..
If not re set the butterfly alignments in the shafts / bores.. it may take several attempts to do so.. be annal about.
Re crimp the butterfly /shaft screws.. use new ones is best..and careful not to bend the shafts doing so.
2/ Now set the primary idle screw to 1 1/2 turns off the primary butterflies fully closed.
3/ Set the secondaries to just off sitting on the bores.. and note the position of the screw very closely
4/Check the the holley return springs and butterflies work smooth and gaps symmetrical.
Note: notice how the holly spring doesnt load the return load sideways, but around the movement.....any other springs u add supplement this spring only, they should not 'over ride' the loading
5/Now bolt/ torque up to the mixer.. Again check everything is smooth etc
6/ Now mount to the valley cover... again check all is smooth etc.. even as u torque down
7/Connect the min weight spring required for secure butterfly return
8/Fire up the engine
9/ The idle sshould be either way to low to be unstable or way to low to idle.... DO NOT ADJUST the primary idle screw.. it is already adjusted to correct height
10/ Take a blind rivet, hammer the shaft end flat into a screw driver, file the end flat.. Put a near 90 deg bend in the shaft. U now have a screw drive u can get under the edge of the base plate to adjust the secondary butterfly height.
11/ Now using this tool adjust the secondary butterfly screws to open the secondary butterflies to correct height.
The spec for the primary idle adjustment is 1 1/2 turns +/- 1/2 turn... Once the secondaries are adjusted correct for THAT engine.. so u have some adjustment in the idle speed adjustment screw for FINE TUNING ONLY
If u cant get idle down then u have a vaccuum leak which maybe a port not sealed covered , poor gasket, worn butterfly shaft bushes.. bad PVC valve.. what ever.
Now connect throttle linkages...again there should be no binding.
If u require 2 return springs.. safety , track regulations whatever... have them working in OPPOSITE directions.. ie one at the bottom to the rear of the engine.. the other at top to the front of the engine...
The above applies to petrol, LPG methanol .. all engines
With a sb on lpg a 800 cfm baseplate is ball park , secondaries just off the bores+ about 1/4 to 1/2 turn....ANY wear in the shaft plates and u will not get the idle down... on LPG.. petrol is slightly more tolerant due to different mixture ratios.
My Spelling is Not Incorrect...It's 'Creative'
Re: IMPCO 425: new base plate, binds when engine runs
another thing. also found out the PCV valve was bad, causing a vacuum leak and thus higher idle. I replaced it and engine didnt even idle anymore. So I could open up te idle a lilttle more and binding is now completely gone
storage/project: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.10 gears - IMPCO425 mixer
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado
driver/project: 1977 GMC C15 Suburban - 454 big block - 4L80E- LPG VSI - 3.08 gears
sold: 1986 Chevrolet Suburban K20 Silverado