chevy small block and IMPCO 425: timing question

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kikkegek
Posts: 416
Joined: Thu Nov 17, 2011 2:12 pm
Location: Netherlands, The Hague

Re: chevy small block and IMPCO 425: timing question

Post by kikkegek »

thanks guys, yes Steptoe is correct. I use a heated O2 sensor with my Innovate MTX AFR guage.

I havent tried adjusting the lever yet. I only measured if it was at correct height...after the yearlu inspection I might do a little tweaking there, to see if I can get the cruise and light throttle mixtures a little richer...

thanks guys
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

kikkegek
Posts: 416
Joined: Thu Nov 17, 2011 2:12 pm
Location: Netherlands, The Hague

Re: chevy small block and IMPCO 425: timing question

Post by kikkegek »

tried some stuff yesterday with the vaporizer

the lever was exactly at 1/32" below the edge of the housing

then bent it to exactly against the edge of the housing: no noticeable difference

then tried to put the middle of the small bar in the lever at the edge of the housing: no noticeable difference

then bent the lever even further to exactly on top of the housing: idle afr jumped to low 10 for AFR and engine didnt idle anymore...could only keep it running with light throttle (because that always leans the mixture in my case). I tried to lean the idle mixture be turning in the idle mixture screw, but even all the way in, the idle afr stayed at 10...

so moved the lever back to exactly under the edge of the housing. idle afrs were normal again, which in my case is 15 for idle and low 17s for light throttle and cruise....

I'd love to have the idle leaner around 16-17 and the light throttle and cruise a little richer about low 16s.......but I am out of ideas how to do that......

the only option I havent tried it to to something with the idle circuit........maybe put heavier spring in there? of maybe bent that lever? any thoughts on that?
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

Steptoe
Posts: 1504
Joined: Fri Feb 02, 2007 8:32 pm
Location: JAFA , New Zealand

Re: chevy small block and IMPCO 425: timing question

Post by Steptoe »

Maybe Im getting old..from memory 1/32 the heights are different for the L and E converters 1 is 3/32???? the other 5/32" ????
Check in older threads or impco manuals and is the gap between the case height and top of the lever....
get it too close to the diaphragm and u run into issues.
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C3H8
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Location: Winnipeg, Canada

Re: chevy small block and IMPCO 425: timing question

Post by C3H8 »

Model E: factory setting for the secondary lever is to et the pin 1/31" below the top edge of the casing. Measure both sides as the lever can get twisted.

Model L: Never actually measured it. The kit usually included a go-no go gauge but 5/32" above the edge would be about right if memory serves me.

C3H8
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Location: Winnipeg, Canada

Re: chevy small block and IMPCO 425: timing question

Post by C3H8 »

OOPS! set the pin 1/32' below the case.

kikkegek
Posts: 416
Joined: Thu Nov 17, 2011 2:12 pm
Location: Netherlands, The Hague

Re: chevy small block and IMPCO 425: timing question

Post by kikkegek »

C3H8 wrote:OOPS! set the pin 1/32' below the case.
yes, rebuild instructions says 1/32" below casing:

Image

did have a little sucess though. Because I noticed that the difference between idle and light throttle/cruise is so big, I was wondering if I might have some luck leaning the idle mixture just by opening up the the throttle valve a little (idle screw on carb base) to raise the idle rpm. well....it did...idle mixture got a lot leaner and didnt really raise the rpms.

I used the idle-mixture screw on the 425 to richen the lean mixture again a little and what do you know....the idle rpms jumped up...at operating temperature I set the AFR for idle to 15.5-16.0 and let it sit for a night...

something really weird happened. before this my idle AFR would go from 18-19 at cold engine to 14-15 at operating temperature (so it got richer). Now the AFR starts out at cold engine start at 11-12 and leans to about 15-16....light throttle/cruise starts out at about 16.5-17 and leans to low 17's....for now I am fine with this. The big improvement is that the engine cold starts very easy with the rich mixture and wont choke when you go back and forth leaving our parking space.

I'll see what the mileage will do next week...
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

Steptoe
Posts: 1504
Joined: Fri Feb 02, 2007 8:32 pm
Location: JAFA , New Zealand

Re: chevy small block and IMPCO 425: timing question

Post by Steptoe »

I was wondering if I might have some luck leaning the idle mixture just by opening up the the throttle valve a little (idle screw on carb base) to raise the idle rpm. well....it did...idle mixture got a lot leaner and didnt really raise the
I have found with 4 barrel base plates on the 425 set up is the same for when have a 425 bolted on top....
The secondary butterflies have to be set so the idle speed screw is 1/ 12 turns out +/- 1/2 turn.... the difference with the original petrol carb and 425 is on the petrol the mixture screws are also set 1 1/2 turns out +/- 1/2 turn...
If not then one knows there is a vaccuum leak and/ or butterflies havnt been set.
I'll see what the mileage will do next week...
Others have comment and I have also found a mixture around the high 15s low 16 seems to be the most economical ......even adjusting advance to compensate or the extra lean mixture burn time
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