Rejetting Qjet-How?

M

marcus mcgee

New member
Sep 29, 1997
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I am going to rejet my Qjet. Since the engine rebuild the engine has
been running somewhat warmer at high speed than before. At idle it
drops real quick back to 160 or so. Running at 70-75 it was around 190
and up to 215 on the hills. It has a good hi volume water pump, good
fan clutch and new balanced flow thermostat. Checked plugs after an
extended 70 mph run and they read lean. I am assumeing the lean
condition is the reason for the higher temps.

The new engine has the RV cam, and the heads were redone and it has
headers. How hard or easy is it to change jets. I assume I will go up
two sizes and if it is running rich then back it down one size. The
carb has 73's, IIRC, in now.

Any suggestions from the Qjet wizards.

Marcus
 
Patrick;

If you could find the diagram that would be great. The carb is rebuilt only
about 4,000 miles ago. You say it is possible to do it without removing the
carb. Is this advisable as I would like to keep it as simple as possible.
Any place on the web with a how-to page?
I may wait till I get to Atlanta and let you help. (not really, but if I
have to go back a notch I may do it in Atlanta)

Marcus

> Marcus,
>
> The jets can be changed without removing the carb, but it's easier with
> it out - especially if you're not familiar with the insides of a Qjet.
> Remove the screw that holds the secondary metering rod hanger and lift
> the rods out. There's about seven screws that hold the carb top plate
> in place and you'll need to disconnect the links for the accelerator
> pump, the choke/secondary interlock and the choke. With all that off,
> the top should just lift off. When the top comes off, this allows the
> power piston with the primary metering rods to pop up. Carefully remove
> the piston, primary rods(watch out for the little spring that locks the
> primary rods onto the piston) and the top gasket and you should see a
> float bowl insert(piece of plastic). Lift out the insert and you can
> see the jets underneath. Be sure to use a screwdriver with a wide
> enough blade so that you don't bugger up the top of the jets.
>
> When reinstalling the top plate, take care to insert the power piston so
> that the primary rods go into the jets where they belong and don't tear
> the skirt on the accelerator pump piston. There's a specific tightening
> sequence to the top plate screws and you need to be careful not to
> overtighten or you'll warp the top plate(bummer). You probably should
> replace the top plate gasket instead of reusing the old one. If you
> want, I'll try to find the tightening sequence diagram and scan it for
> you.
>
> It's harder to describe than it is to do, but it can be scary if you
> haven't been in there before.
>
> Patrick
>

> >
> > How hard or easy is it to change jets. I assume I will go up
> > two sizes and if it is running rich then back it down one size. The
> > carb has 73's, IIRC, in now.
> --
> Patrick Flowers
> Mailto:patrick
>
> The GMC Motorhome Page
> http://www.gmcmotorhome.com
 
don't you have to change the rods also ??

gene

>Marcus,
>
>The jets can be changed without removing the carb, but it's easier with
>it out - especially if you're not familiar with the insides of a Qjet.
>Remove the screw that holds the secondary metering rod hanger and lift
>the rods out. There's about seven screws that hold the carb top plate
>in place and you'll need to disconnect the links for the accelerator
>pump, the choke/secondary interlock and the choke. With all that off,
>the top should just lift off. When the top comes off, this allows the
>power piston with the primary metering rods to pop up. Carefully remove
>the piston, primary rods(watch out for the little spring that locks the
>primary rods onto the piston) and the top gasket and you should see a
>float bowl insert(piece of plastic). Lift out the insert and you can
>see the jets underneath. Be sure to use a screwdriver with a wide
>enough blade so that you don't bugger up the top of the jets.
>
>When reinstalling the top plate, take care to insert the power piston so
>that the primary rods go into the jets where they belong and don't tear
>the skirt on the accelerator pump piston. There's a specific tightening
>sequence to the top plate screws and you need to be careful not to
>overtighten or you'll warp the top plate(bummer). You probably should
>replace the top plate gasket instead of reusing the old one. If you
>want, I'll try to find the tightening sequence diagram and scan it for
>you.
>
>It's harder to describe than it is to do, but it can be scary if you
>haven't been in there before.
>
>Patrick
>

>>
>> How hard or easy is it to change jets. I assume I will go up
>> two sizes and if it is running rich then back it down one size. The
>> carb has 73's, IIRC, in now.
>--
>Patrick Flowers
>Mailto:patrick
>
>The GMC Motorhome Page
>http://www.gmcmotorhome.com
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/
 
Marcus,

The jets can be changed without removing the carb, but it's easier with
it out - especially if you're not familiar with the insides of a Qjet.
Remove the screw that holds the secondary metering rod hanger and lift
the rods out. There's about seven screws that hold the carb top plate
in place and you'll need to disconnect the links for the accelerator
pump, the choke/secondary interlock and the choke. With all that off,
the top should just lift off. When the top comes off, this allows the
power piston with the primary metering rods to pop up. Carefully remove
the piston, primary rods(watch out for the little spring that locks the
primary rods onto the piston) and the top gasket and you should see a
float bowl insert(piece of plastic). Lift out the insert and you can
see the jets underneath. Be sure to use a screwdriver with a wide
enough blade so that you don't bugger up the top of the jets.

When reinstalling the top plate, take care to insert the power piston so
that the primary rods go into the jets where they belong and don't tear
the skirt on the accelerator pump piston. There's a specific tightening
sequence to the top plate screws and you need to be careful not to
overtighten or you'll warp the top plate(bummer). You probably should
replace the top plate gasket instead of reusing the old one. If you
want, I'll try to find the tightening sequence diagram and scan it for
you.

It's harder to describe than it is to do, but it can be scary if you
haven't been in there before.

Patrick

>
> How hard or easy is it to change jets. I assume I will go up
> two sizes and if it is running rich then back it down one size. The
> carb has 73's, IIRC, in now.
- --
Patrick Flowers
Mailto:patrick

The GMC Motorhome Page
http://www.gmcmotorhome.com
 
Radiator was removed prior to rebuild and boiled, cleaned and given a
clean bill of health. Unless something happened during the pull and
insert, but there is no evidence of damage to the cores.

It also runs fine and 185 cool at 55-60. I know - just drive at 55, but
my foot is heavy and I love to pass SOB's.

Marcus

> Marcus wrote :
>
> ".........Since the engine rebuild the engine has been
> running somewhat warmer at high speed than before. At
> idle it drops real quick back to 160 or so. Running
> at 70-75 it was around 190 and up to 215 on the hills"
>
> I know you said that the plug looked like they were
> running lean -- BUT -- Your symptoms are also somewhat
> charactoristic of a blocked radiator:
> .Temp fine at idle
> .Runs more that 10 degrees above the thermostat
> rating at high speed or under load.
>
> Plus "running up hills" will almost always lower the
> vacuum enough that the needles will be raise from
> there normal (high vac position) and thereby
> enriching the mixture.
>
> Just my 2 cents worth ?????
>
>
>
> =====
> Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
> ___________ (MY TOYS) (904)672-0571
> /_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
> *O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
> Photos--> http://albums.photopoint.com/j/AlbumList?u=192176
> __________________________________________________
> Do You Yahoo!?
> Talk to your friends online with Yahoo! Messenger.
> http://im.yahoo.com
 
Marcus wrote :

".........Since the engine rebuild the engine has been
running somewhat warmer at high speed than before. At
idle it drops real quick back to 160 or so. Running
at 70-75 it was around 190 and up to 215 on the hills"

I know you said that the plug looked like they were
running lean -- BUT -- Your symptoms are also somewhat
charactoristic of a blocked radiator:
.Temp fine at idle
.Runs more that 10 degrees above the thermostat
rating at high speed or under load.

Plus "running up hills" will almost always lower the
vacuum enough that the needles will be raise from
there normal (high vac position) and thereby
enriching the mixture.

Just my 2 cents worth ?????



=====
Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
. ___________ (MY TOYS) (904)672-0571
./_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
.*O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
Photos--> http://albums.photopoint.com/j/AlbumList?u=192176
__________________________________________________
Do You Yahoo!?
Talk to your friends online with Yahoo! Messenger.
http://im.yahoo.com
 
>
> don't you have to change the rods also ??

Nope - you can change the jets by themselves. This should richen it
across the rpm range. You change rods to change the mixture "profile" -
at least that's my understanding.

Patrick
- --
Patrick Flowers
Mailto:patrick

The GMC Motorhome Page
http://www.gmcmotorhome.com
 
After looking at the diagrams, I noticed that I forgot to say that you
need to remove the front two carb mounting bolts also to get the top
plate off the carburetor. Also, there are 9 screws that hold the top
plate down, not seven(in addition to the two front mounting bolts.

Patrick
- --
Patrick Flowers
Mailto:patrick

The GMC Motorhome Page
http://www.gmcmotorhome.com
 
>It also runs fine and 185 cool at 55-60. I know - just drive at 55, but my
foot is heavy and I love to pass SOB's.

Reminds me of last trip in the Pumpkin-Rose, The Boss picked up on my
driving habits, Told her I'd keep it at 65 and was doing pretty good, She
was making things to eat, And I guess she felt the speed increase, She said
I bet your coming up on another RV huh!! You just have to pass-em all don't
you!!!! 8-)>

Ron

- ----- Original Message -----
From: "Marcus McGee"
To:
Sent: Saturday, February 05, 2000 1:18 PM
Subject: Re: GMC: Rejetting Qjet-How?

> Radiator was removed prior to rebuild and boiled, cleaned and given a
> clean bill of health. Unless something happened during the pull and
> insert, but there is no evidence of damage to the cores.
>
> It also runs fine and 185 cool at 55-60. I know - just drive at 55, but
> my foot is heavy and I love to pass SOB's.
>
> Marcus
>

>
> > Marcus wrote :
> >
> > ".........Since the engine rebuild the engine has been
> > running somewhat warmer at high speed than before. At
> > idle it drops real quick back to 160 or so. Running
> > at 70-75 it was around 190 and up to 215 on the hills"
> >
> > I know you said that the plug looked like they were
> > running lean -- BUT -- Your symptoms are also somewhat
> > charactoristic of a blocked radiator:
> > .Temp fine at idle
> > .Runs more that 10 degrees above the thermostat
> > rating at high speed or under load.
> >
> > Plus "running up hills" will almost always lower the
> > vacuum enough that the needles will be raise from
> > there normal (high vac position) and thereby
> > enriching the mixture.
> >
> > Just my 2 cents worth ?????
> >
> >
> >
> > =====
> > Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
> > ___________ (MY TOYS) (904)672-0571
> > /_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
> > *O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
> > Photos--> http://albums.photopoint.com/j/AlbumList?u=192176
> > __________________________________________________
> > Do You Yahoo!?
> > Talk to your friends online with Yahoo! Messenger.
> > http://im.yahoo.com
>
 
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