what a difference a spring makes

Keith V

Keith V

Well-known member
Mar 26, 2008
3,599
395
83
Ive been tuning my Qjet for a while now. I have a wide band O2 so I know what it's doing (even if I don't know what I'm doing).

I had it running at 15/16:1 pretty nice but it would go lean from about 15-8" of vacuum, then fine with the secondaries.

So I put in the 8" power piston spring, I really didn't want to as it seemed way too high.

But wow, that woke it up!

She now runs a nice 15/16:1 till you give her a little gas then it drops nicely into the high 13's as you put the throttle down.

At about 5 inches the secondaries open and if you can stop looking at the fuel gauge dropping like a rock, it really goes!

So I'm running 68 jets and the 8-12" blue spring.

Keith Vasilakes

Mounds View, MN

1975 ex-Royale.

Microlevel Ride height controller
 
Keith,

It will be interesting to see if this change has any effect on your fuel economy.

I don't have an A/F gauge, so I don't have access to the info you have, but my spark plugs indicate a slightly lean mixture even with #72 main jets.
I have no idea which power piston spring is in my carb, but it sounds like it may be worth my while to change it to the one you recommend just to be
sure.

I have always read (on this forum) that the proper spring is red, but I think the color code may have been changed and the important thing is the
vacuum spec on the spring, not the color.
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
I'm really hoping for a bit of improvement MPG wise, the vacuum looks to be a bit higher, and it struggles less on uphill grades.

I could see that it was going lean before the power piston started working, so the heavier spring made sense.

WBO2's are cheap these days, I cost justified mine just by fuel usage. Hopefully I will be right about that ha ha ha.

I would not use my coach as a tuning example, except that you need time and a WBO2 to do it right ?

my engine is over boared, re-camed, has C heads, filled crossover, headers and 3" exhaust. It's not stock in the least.

________________________________
From: Gmclist on behalf of Carl Stouffer
Sent: Monday, October 23, 2017 9:19:29 PM
To: gmclist
Subject: Re: [GMCnet] what a difference a spring makes

Keith,

It will be interesting to see if this change has any effect on your fuel economy.

I don't have an A/F gauge, so I don't have access to the info you have, but my spark plugs indicate a slightly lean mixture even with #72 main jets.
I have no idea which power piston spring is in my carb, but it sounds like it may be worth my while to change it to the one you recommend just to be
sure.

I have always read (on this forum) that the proper spring is red, but I think the color code may have been changed and the important thing is the
vacuum spec on the spring, not the color.
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member

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> I'm really hoping for a bit of improvement MPG wise, the vacuum looks to be a bit higher, and it struggles less on uphill grades.
>
> I could see that it was going lean before the power piston started working, so the heavier spring made sense.
>
> WBO2's are cheap these days, I cost justified mine just by fuel usage. Hopefully I will be right about that ha ha ha.
>
>
> I would not use my coach as a tuning example, except that you need time and a WBO2 to do it right 😊
>
> my engine is over bored, re-camed, has C heads, filled crossover, headers and 3" exhaust. It's not stock in the least

I'm sure you're right Keith. Trying to tune the Q-jet without a WBO2 gauge would be a flying you-know-what in the dark at best. My engine is all
stock except for a double roller timing chain (set straight up) a new pair of K-heads, a Gary Rockwell intake manifold, and a Paterson distributor
with MSD dash adjustable ignition. I am running a 3" exhaust though and all that through a 3.70 final drive.

The fuel requirements would be different for a fresh engine with a different cam, for sure.

--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
Last night I was reading about carb tuning and one thing I kept seeing is people saying you cannot read spark plugs anymore with reformulated gas as they are always white.

So I guess I'd be careful about that.

I was also looking at power vs economy curves for AFR and it looks like I might still be a bit lean. I just know that the next fatter jets will put my cruise AFR into the 13:1 range and I'd rather stay out of there during cruise.

I wonder how accurate Jets are, should I try another set of 69's?

________________________________
From: Gmclist on behalf of Carl Stouffer
Sent: Tuesday, October 24, 2017 1:24:18 PM
To: gmclist
Subject: Re: [GMCnet] what a difference a spring makes

> I'm really hoping for a bit of improvement MPG wise, the vacuum looks to be a bit higher, and it struggles less on uphill grades.
>
> I could see that it was going lean before the power piston started working, so the heavier spring made sense.
>
> WBO2's are cheap these days, I cost justified mine just by fuel usage. Hopefully I will be right about that ha ha ha.
>
>
> I would not use my coach as a tuning example, except that you need time and a WBO2 to do it right 😊
>
> my engine is over bored, re-camed, has C heads, filled crossover, headers and 3" exhaust. It's not stock in the least

I'm sure you're right Keith. Trying to tune the Q-jet without a WBO2 gauge would be a flying you-know-what in the dark at best. My engine is all
stock except for a double roller timing chain (set straight up) a new pair of K-heads, a Gary Rockwell intake manifold, and a Paterson distributor
with MSD dash adjustable ignition. I am running a 3" exhaust though and all that through a 3.70 final drive.

The fuel requirements would be different for a fresh engine with a different cam, for sure.

--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member

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> Last night I was reading about carb tuning and one thing I kept seeing is people saying you cannot read spark plugs anymore with reformulated gas
> as they are always white.
>
> So I guess I'd be careful about that.
>
>
> I was also looking at power vs economy curves for AFR and it looks like I might still be a bit lean. I just know that the next fatter jets will
> put my cruise AFR into the 13:1 range and I'd rather stay out of there during cruise.
>
> I wonder how accurate Jets are, should I try another set of 69's?

It IS kind of a PITA to swap out jets, but it wouldn't hurt to try the next size up if you're up for it.

I went to 72s on the recommendations I got from here and even with those, I ended up with a cracked exhaust manifold and, eventually, a burned exhaust
valve. Now whether those were related to my AF mixture is a matter of conjecture, but maybe.
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
Lean mixtures make more heat. Gasoline rich mixtures carry off a bunch of
it. Your choice. Probably safest to keep that ratio on the fat side of 15:0
to 1.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> > Last night I was reading about carb tuning and one thing I kept seeing
> is people saying you cannot read spark plugs anymore with reformulated gas
> > as they are always white.
> >
> > So I guess I'd be careful about that.
> >
> >
> > I was also looking at power vs economy curves for AFR and it looks like
> I might still be a bit lean. I just know that the next fatter jets will
> > put my cruise AFR into the 13:1 range and I'd rather stay out of there
> during cruise.
> >
> > I wonder how accurate Jets are, should I try another set of 69's?
>
>
> It IS kind of a PITA to swap out jets, but it wouldn't hurt to try the
> next size up if you're up for it.
>
> I went to 72s on the recommendations I got from here and even with those,
> I ended up with a cracked exhaust manifold and, eventually, a burned exhaust
> valve. Now whether those were related to my AF mixture is a matter of
> conjecture, but maybe.
> --
> Carl Stouffer
> '75 ex Palm Beach
> Tucson, AZ.
> Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive,
> Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American
> Eagles,
> Solar battery charging. GMCSJ and GMCMI member
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> Last night I was reading about carb tuning and one thing I kept seeing is people saying you cannot read spark plugs anymore with reformulated gas
> as they are always white.
>
> So I guess I'd be careful about that.
>
> I was also looking at power vs economy curves for AFR and it looks like I might still be a bit lean. I just know that the next fatter jets will
> put my cruise AFR into the 13:1 range and I'd rather stay out of there during cruise.
>
> I wonder how accurate Jets are, should I try another set of 69's?

Kieth and Carl,

I fear I have to jump in here and still not try to muddy things up.

Kieth, the reason you read that you cannot read spark plugs anymore is two-fold and sort of correct.
There are two issues here: One is that crapahol makes an engine look like it is running lean, and the other is that part of the color we used to see
was the lead in the fuel.

The jets used in old carburetors were very precise. The chance of you finding jets of the same nimber that make different flows for the same
condition is very remote. (About a hundred years ago - honestly over 40 - I did have a problem with a carburetor that turned out to be a big Fing
burr in the jet as manufactured.)

BUT Rochesters do have other parts that can make a difference and any part can be damaged or worn. I have seen main jets and needles that were worn,
but this was at the end of a very long durability run. If it can happen there, it can happen in the real world - it is just less likely. There is a
spring on the power piston that can make a difference too.

Without very expensive collection of test stands, there isn't much more you can do.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
Thanks Matt,

I'm being very overly precise about AFR and I'm probably beyond my measurement accuracy anyway.

Thats the problem with having tools, you want to trust them implicitly.

________________________________
From: Gmclist on behalf of Matt Colie
Sent: Tuesday, October 24, 2017 5:15:12 PM
To: gmclist
Subject: Re: [GMCnet] what a difference a spring makes

> Last night I was reading about carb tuning and one thing I kept seeing is people saying you cannot read spark plugs anymore with reformulated gas
> as they are always white.
>
> So I guess I'd be careful about that.
>
> I was also looking at power vs economy curves for AFR and it looks like I might still be a bit lean. I just know that the next fatter jets will
> put my cruise AFR into the 13:1 range and I'd rather stay out of there during cruise.
>
> I wonder how accurate Jets are, should I try another set of 69's?

Kieth and Carl,

I fear I have to jump in here and still not try to muddy things up.

Kieth, the reason you read that you cannot read spark plugs anymore is two-fold and sort of correct.
There are two issues here: One is that crapahol makes an engine look like it is running lean, and the other is that part of the color we used to see
was the lead in the fuel.

The jets used in old carburetors were very precise. The chance of you finding jets of the same nimber that make different flows for the same
condition is very remote. (About a hundred years ago - honestly over 40 - I did have a problem with a carburetor that turned out to be a big Fing
burr in the jet as manufactured.)

BUT Rochesters do have other parts that can make a difference and any part can be damaged or worn. I have seen main jets and needles that were worn,
but this was at the end of a very long durability run. If it can happen there, it can happen in the real world - it is just less likely. There is a
spring on the power piston that can make a difference too.

Without very expensive collection of test stands, there isn't much more you can do.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

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