Author Topic: M4ME 17085580 primary circuit  (Read 650 times)

Offline theraymondguy

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M4ME 17085580 primary circuit
« on: October 03, 2021, 01:47:42 PM »
1985 Pontiac Trans Am, Canadian non ccc on lightly modded LG4 305 (9.5:1, 416 heads, headers, Accel distributor and the wimpiest cam gm ever put in a small block).

My first automobile carb rebuild, Ive rebuilt dozens of motorcycle carbs and thrice more small engine carbs.  I rebuilt this carb and installed primary throttle bushings. I have approximately 14 months of run time on the rebuild.  I believe the carb was all original, however the caps were removed from the primary adjustment screws, likely had been rebuilt at some point.  The accelerator pump has the blue seal, the float assembly is new.

Engine comes off choke perfect, idles at 550rpm in gear (as per the under hood decal) pulls a smooth 19 inHg and has all emissions installed and plumbed as per the hood decal (there *might* be a ‘bb’ in the egr vacumm line…).  The engine pulls clean off the line and all the way to 5000. 8 degrees of initial timing and 36 degrees all in.  1.5” primary headers and factory air cleaner with a wix filter.

However. 

It takes 5 (five) full turns out on the adjustment screws to achieve this, which seems out of line.  I’ve sprayed enough carb cleaner around it the carb to have removed most of the coating, and still can’t find a vacuum leak at the intake or anywhere else.

The primary here are marked “72”, I cannot find any identification on the primary needles, but they clearly have three diameters or the main body and two smaller diameters if you prefer.

In a motorcycle I’d have already swapped in larger primary jets, but I’m in unfamiliar territory - almost need permission to move forward.  Some messing with a spare accelerator arm has proven that there’s more power to be had with more fuel (probably could use some tweaking on the apt valve).

I have questions about tuning for power as well, with plans to install an intake (old school performer) and mild cam (204/214 112 LSA) this winter.

I did chat with Cliff briefly on a Facebook Quadrajet page.  Cliff recommended the “M” style primary needle iirc and a phone call about jets.

I’d Rather work through issues here, where a guy doesn’t have to worry about what I’m taking someone else away from or misinterpreted dialogue.

Thanks for your thoughts and input,
Rob.
« Last Edit: October 03, 2021, 02:03:04 PM by theraymondguy »

Offline Kenth

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Re: M4ME 17085580 primary circuit
« Reply #1 on: October 04, 2021, 12:04:04 AM »
Replacing main jetting rods and jets will do nothing for the idle mixture at the mixture needles/screws.
The idle circuit is a stand alone circuit in the carb.
It is very common this circuit is too lean set up for todays fuels as these contain less BTU´s than yesteryears fuels.
In the idle circuit you have idle tubes (jets), upper and lower idle airbleeds, idle downchannel restrictions, off-idel slots and last idle needle holes.
To get control of the idle mixture needles one has to open the idle tubes .002"-.005", idle downchannel restrictions the same amount and the idle mixture needle holes to .085"-.095".
Start small until you are satisfied.

If you don´t already have Cliff´s book, i´ll recommend to buy it.

Offline theraymondguy

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Re: M4ME 17085580 primary circuit
« Reply #2 on: October 05, 2021, 02:55:19 PM »
Thank you, that sounds like the right direction.

Ive been through Cliffs online store a number of times and not seen the tools to complete these adjustments, where do you purchase them? Mcmaster Carr?

Offline old cars

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Re: M4ME 17085580 primary circuit
« Reply #3 on: October 06, 2021, 03:12:51 AM »
"It takes 5 (five) full turns out on the adjustment screws to achieve this, which seems out of line."

I assume you are talking about the idle mixture screws. Remove screws and drill passage to .090"

With 19" vacuum at idle you could try this first (before moving on to idle tubes ,if necessary)
Also quality of fuel depends where you live. Are you in Canada?

Offline old cars

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Re: M4ME 17085580 primary circuit
« Reply #4 on: October 06, 2021, 03:31:33 AM »
Ontario, Canada gasoline is 10% ethanol in regular fuel (approximate) Premium is ethanol free.

Offline tayto

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Re: M4ME 17085580 primary circuit
« Reply #5 on: October 06, 2021, 06:26:15 AM »
The only fuel in western canada that doesn't have ethanol in it is Shell 91 and Chevron 94, FWIW.

Offline theraymondguy

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Re: M4ME 17085580 primary circuit
« Reply #6 on: October 06, 2021, 03:57:07 PM »

Ontario, Canada gasoline is 10% ethanol in regular fuel (approximate) Premium is ethanol free.

I am in Ontario Canada.  I run Costco premium which I'm told by the attendants is ethanol free (I try to ask a different attendant every time I go, always the same answer).

Thank you all for your help.   I'll be trying the idle mixture mod first and report back with results.

Offline theraymondguy

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Re: M4ME 17085580 primary circuit
« Reply #7 on: January 09, 2022, 09:16:27 AM »
Okay, it's been a while but here are the measurements of my m4me 17085580 carb.
I have now installed the 204/214 cam.

I have purchased Cliffs book, still somewhat unclear about certain component locations

Idle tubes 0.033"
Idle down channel 0.047" - restricted port beside idle tube port
Upper idle air bleed 0.070" - on 45 degree angle, above idle down channel and into primary bore
Lower idle air bleed 0.070" - into bottom of primary bore
Idle bypass air 0.070" into throttle plate
Mixture screw holes 0.070" I have opened these u to 0.093" the closest I could get to 0.090".
Holes in primary plates - none

I have purchased a micro drill kit and numbered drill set. I have a new rebuild kit (using for gaskets only),

Some of you may recognize the camshaft specs there is an example for this cam in Cliffs book.  However that example carb has no Idle bypass air ports.
« Last Edit: January 09, 2022, 09:24:44 AM by theraymondguy »

Offline Cliff Ruggles

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Re: M4ME 17085580 primary circuit
« Reply #8 on: January 11, 2022, 04:38:07 AM »
It takes 5 (five) full turns out on the adjustment screws to achieve this, which seems out of line.

No, that's actually pretty "normal" for a later model carburetor with the metric pitch idle mixture screws and small holes typically found under them.  Most come in around 5-7 turns vs 2.5-3.5 turns with the early units that have #10-32 pitch threads.

Your "blue" pump seal will fail in ethanol unless you bought one of mine.  They are dubbed as "ethanol resistant" but are offshore production and swell up in this new fuel.

Idle tuning should be based on CID, compression ratio and cam being used.  When you install an aftermarket cam with more seat timing, tighter LSA or both this reduces the "signal" to the carburetor and may require larger idle tubes, DCR's, smaller idle bleeds and idle bypass air to make things happy. 

The "recipes" in my book work very well, but one still has to custom tune at times as it's pretty rare for any two engines to behave in exactly the same fashion.  The little 305 9.5 to 1 compression with a 204/214 cam would behave about like a 350 with 8.5 to 1 compression (same cam) as far as idle fuel requirements, for example.

Initial timing is a big player with these things.  When you install a larger cam and make the engine less efficient at idle they will want more timing as well.  I do NOT like to run more than about 10-12 degrees initial timing in one of these engines.  When you find yourself having to increase initial timing pretty far and open up idle system passages quite a bit you just missed the cam choice some.......Cliff

Offline tayto

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Re: M4ME 17085580 primary circuit
« Reply #9 on: January 11, 2022, 08:30:42 AM »
Cliff, by "miss the cam choice some" do you mean too big of a cam was installed? I found my 355 with TBI heads liked more timing at idle around 14-16*. This is under what the factory TBIs ran (usually high zzzzz, low 20s @ idle). It woild run fine at 10 or less, but made 1 to 2 in-hg less @ idle.

Offline theraymondguy

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Re: M4ME 17085580 primary circuit
« Reply #10 on: January 11, 2022, 06:22:33 PM »
While the 204/214 112 LSA is not a big cam, it's a fair bit more cam than the stock 178/194 109 LSA I removed.

I'm going through the process of altering the carb now.  I'm focusing on the details, slowing myself down.

I opened up the idle tubes and down channel restrictions to the recipe (were 0.033" and 0.040").  I also opened up the secondary wells holes as mentioned in the rebuilding section.  FWIW not all drywall screws are created equal.

Thankfully it's a hobby car, I can piddle with it all winter long.  Unfortunately I can't tune it much till we have a warm day but I should be a fair bit closer than I was.  Next winter it'll be cylinder heads (305 Vortec or rebuild and port the 416s) and I'll no doubt be open in up the carb again.

I believe I had timing at 12* @ 800 RPM prior to pulling the carb.


Offline Cliff Ruggles

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Re: M4ME 17085580 primary circuit
« Reply #11 on: January 12, 2022, 03:38:33 AM »
Unless the Vortec heads have a working exhaust crossover I'd stick with the 416's.  No need to port them for what you are doing.  The little 305 with a small cam isn't moving enough air to require larger ports.  Nothing wrong with a nice clean-up, remove casting flash and some bowl blending, but resist the urge to hog them out it will do nothing but slow the air down and hurt low end and mid-range power.........

Offline Cliff Ruggles

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Re: M4ME 17085580 primary circuit
« Reply #12 on: January 12, 2022, 03:57:44 AM »
"Cliff, by "miss the cam choice some" do you mean too big of a cam was installed?"

Not at all.  If you find that you have to run a LOT of timing at idle to make one of these engines happy it certainly doesn't like the cam you chose.

Even with that said the "trend" has always been to install a cam that puts some "lope" in the exhaust note which is associated with making more power.  99 times out of 100 it's not really helping much and the "menacing" idle quality is simply telling us that the cam used has more overlap than the engine is happy with, lower vacuum at idle speed and will require more timing and idle fuel to be happy.

Even with that said customers tend to go that direction and a few years ago I got myself in all sorts of trouble helping one out.  He had built his 350 following guidelines on the Forums and in magazine articles and it sounded bad ass but was a complete "turd" in actual use. 

He lowered the compression for pump gas (not sure who came up with that deal but you aren't going to make MORE power with LESS compression), then installed a tight LSA cam with advanced intake closing to bring the lost power back.  For sure it sounded great standing behind it but with a stock converter and 4000lb vehicle it was a complete turd in actual use.  You'd have to drop the car off of a floor jack to do a decent burnout!  The engine was lackluster for power as well until you got it spun up some. 

I was enlisted to help so we did another 350 with 10.6 to 1 compression, super tight quench and had Lunati grind a custom cam for it based on the 327/350hp cam.  The new engine idled really nice, but relatively smooth, STRONG power right off idle and still pulling really hard well past 5500rpms.  It would ROAST the tires right off the car and when it did hook up had you planted hard in the seat till you decided to move the shifter.  I thought it was a nice improvement and I was very happy with it.  The owner was NOT.

Despite nailing down mid 20's for fuel economy and tons of power over a broad RPM range he HATED the smooth idle quality.  He bitched that no one paid it any mind when he idled thru the Dairy Queen car cruises. 

First time that happened to me but I completely understand now that some customers goals for the project are to have a "menacing" idle so everyone that hears the car thinks it's bad-ass and having an engine that idles smooth isn't going to cut it........
« Last Edit: January 12, 2022, 02:13:27 PM by Cliff Ruggles »

Offline theraymondguy

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Re: M4ME 17085580 primary circuit
« Reply #13 on: January 12, 2022, 01:47:46 PM »
"Cliff, by "miss the cam choice some" do you mean too big of a cam was installed?"

Not at all.  If you find that you have to run a LOT of timing at idle to make one of these engines happy it certainly doesn't like the cam you chose.

Even with that said the "trend" has always been to install a cam that puts some "lope" in the exhaust note which is associated with making more power.  99 times out of 100 it's not really helping much and the "menacing" idle quality is simply telling us that the cam used has more overlap than the engine is happy with, lower vacuum at idle speed and will require more timing and idle fuel to happy.

I was enlisted to help so we did another 350 with 10.6 to 1 compression, super tight quench and had Lunati grind a custom cam for it based on the 327/350hp cam.  The new engine idled really nice, but relatively smooth, STRONG power right off idle and still pulling really hard well past 5500rpms.  It would ROAST the tires right off the car and when it did hook up had you planted hard in the seat till you decided to move the shifter.  I thought it was a nice improvement and I was very happy with it.  The owner was NOT.

Despite nailing down mid 20's for fuel economy and tons of power over a broad RPM range he HATED the smooth idle quality.  He bitched that no one paid it any mind when he idled thru the Dairy Queen car cruises. 

First time that happened to me but I completely understand now that some customers goals for the project are to have a "menacing" idle so everyone that hears the car thinks it's bad-ass and having an engine that idles smooth isn't going to cut it........

I was informed early in my career as a heavy equipment technician, "no good deed goes unpunished".

If I were to ever get a tattoo, this and a few other pearls would be likely choices as I seem to forget them so frequently.

Cliff, would you mind expanding on your thoughts for the exhaust cross over?

I literally made the decision to not block it moments before installing the intake - thinking that

An aluminum intake will shed the heat easily,
I have no heat riser or stove pipe with the headers, a bit of heat on the bottom of the carb would help the carb while the engine was coming up  to temp.



Offline Cliff Ruggles

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Re: M4ME 17085580 primary circuit
« Reply #14 on: January 12, 2022, 02:18:59 PM »
Since we are talking about a wet-flow system a working crossover is going to be beneficial.  It will heat the intake and improve efficiency allowing us to run a leaner mixture and greatly improve engine performance thru warm-up. 

Not having one is NEVER a good idea if you plan on using the vehicle in cold weather, so keep them open on your plow truck engine or you will absolutely hate it in the Winter months.

For part time, warm weather and casual use you can get away with a blocked crossover but it does not improve power or performance since the intake eventually heat soaks anyhow. 

You also loose your factory choke if it is divorced or "hot air" so plan accordingly. 

So basically I'm recommending to keep them open......Cliff