QJet tuning on SBC 350

[kfa670]

Hi

My first post on the board I just wanted to say what great info there is here.  I also recently bought your book cliff and plan on rebuilding a back up QJet with it soon.  So, I have:

Rochester QJet, can't remember exact number but I know its a 1978 model, I think Federal in application used with an Auto Tranny and the vehicle was a Chevy.  I have 71 mains, Cliffs .44 primary metering rods and DR Secondaries on a G hanger.  High flow needle and seat.  I don't know about the idle tubes and air bleeds yet (still studying the book).  But I did drill out my Idle screw ports to around .088 I think and gained quite a bit of adjustment.

I currently have this carb on a generic freshly rebuilt Goodwrench 350 (Pre 1986 block).  It has a 2602 performer intake, Melling MCT-1 cam, headers and free flowing exhaust, pretty much stock compression.  This combo is in an 1989 Land Cruiser with the stock automatic tranny (A440F) which is in less than spring condition.  I live and drive around 5-7000 feet.  Timing is around 14 base, 34 total mechanical.  I think my vacuum advance can is set to about 12 degrees.

When I bought this rig it didn't run worth crap, but as I've tuned on it a little I seem to be making progress.  I didn't know anything about tuning a couple years ago but have learned a ton.  I have a few issues occasionally though. 

*First, I would just like to consult the gurus, and see if my current carb setup makes sense for my application and elevation.  I can't really complain about my idle, it seems pretty steady at 600 rpm and I can adjust it if need be.  It seems just a tad flat off idle though, and if I flick the throttle sometimes I get black smoke out the exhaust.  I would say I'm too rich still (I went from .042 rods to Cliffs .044s and it seemed to crispen things up a bit), or maybe is it the power piston spring/APT? 

*when I lay my foot into it, it takes right off (for a 6000+ lb rig).  Secondaries seem to transition well and has nice acceleration.  However, if I have my foot into it for say 10-15 seconds, its almost like I start to run out of fuel in my bowl.  It starts to hesitate a little and I pretty much can't gain any more RPM. I further think its a fuel issue because I recently replaced the fuel pump.  And the problem decreased ALOT, but still occurs.  With the older crappier pump.  I could hold my foot down and almost kill the engine entirely, before it seemed to get a drink of fuel, rev back up a little and then fall off again.  Now, it just seems to reach a point and begin to miss a little and no longer build rpm.  Basically, I'm wondering what I can do to fix this?  The new fuel pump helped, but may even put out a little too much pressure, as now sometimes when I come to a stop, I think my bowl is over flowing and I flood (gotta check float level now).  But is there anything I can do so I can keep my dang foot in it without it starting to hesitate a little?

*My MPG sucks.  Maybe 10-11 on highway.  Not really too concerned in this area as its mainly an off-roader (33's, 5inch lift)  I'm wondering if my cruise could be a little leaner though.  I thought I had it too lean at one point because I was getting backfire/sneezing through the carb at cruise, but then discovered the rear two bolts on my air horn had backed all the way out!

Sorry for the long post, If you guys can chip away at some of my issues, I'd be more than appreciative   My main question is if my setup makes sense for the application/altitude.  This is mainly a boondocking woods rig.  It sees steep hills and big boulders.  It works "pretty good" right now, but I feel it could be improved.  Thanks again folks!

[Cliff Ruggles]

I'd need a carburetor number for specific advice.  There were several models made in 1978, with completely different calibrations.....Cliff

[kfa670]

Ok Cliff I was able to get the exact number:

17058204
758  BHT

[Cliff Ruggles]

I'm still slightly confused by the first post.  Have you completely rebuilt the carb with modern components, or going to rebuild the carb at some point?

71 jets are really small for that unit, even at your altitude.  From what I remember, those are large MAB carburetors, and stock jetting is closer to .076", and they are pretty lean with those.....Cliff

[kfa670]

Sorry Cliff I should be a little more clear.  I have another carburetor on the side that I am going to be rebuilding separately using your book. 

The current one in use will also be taken apart again and inspected per your book to gain a little more knowledge of what it has as far as air bleeds etc. As far as modern components, I think everything is up to date and it is in good shape.  I bought it from I-5 Automotive about 2 years ago.  It doesn't see a lot of use, maybe a few thousand miles a year. I have put in the newer nitril float, .044 meters and hi-perf needle and seat, and it came with the .071 jets in it.

Do you think I should try going up on the mains and see what kind of changes I get?  Maybe a .073?

[Cliff Ruggles]

I can't provide any advice on a carb built from that source, could be anyplace for calibration.  Without knowing airbleed sizes, and what else was done to it, I'd just be guessing.....Cliff

[kfa670]

Understood cliff, I'm going to have to take it apart next time I'm home off work, and figure out what they are.

[kfa670]

Well, its been a while... but I was finally able to get a few measurements on my carb.  These are approximate but pretty damn close I think.  My Numbered drill bit set is in a little rough shape and I'm missing a few.

17054208

Idle Tubes: .042
Idle Down Channel: .046
Upper Idle Air Bleed (Main Cast): 0.064-0.067
Lower Idle Air Bleed: 0.072
Idle Bypass Air: 0.065
Mixture Screw holes: 0.088

MAB (body): 0.112
MAB (Air horn): 0.112

Main Jets Currently 0.072
Primary Rods Currently: 0.046K

So I'm assuming I have a big MAB carb, but I know for a fact they aren't .125" as that size bit wouldn't fit.  This carb came from an aftermarket source but the only strange thing I found with it is that it had two different main well bleed tubes.  One was much smaller than the other, and wasn't at quite the same depth.  I drilled out the smaller one to 0.042 which matched the other one and put it at the same height. 

Since Cliff mentioned the original specs have much bigger main jets, I put in some 0.074s and gave it a try.  It started up SO much easier and the Mixture screws were way more effective.  I took it for a drive and while the idle and everything is great, off Idle (with .046 rods) has got to be WAY to rich.  Off idle it is sluggish and smokes like crazy.  If I'm in neutral, and I blip the throttle, It isn't very snappy and I get a puff of black smoke out the exhaust.  If I disconnect the accel pump, it is much more responsive and basically doesn't smoke at all.  However, With the .074s in, the top end is MUCH improved.  Actually feels like it has horsepower!

So my question is, do my measurements make sense, and what should I do to clean up the richness off-idle?  I have some .048, .050, and .052 rods coming, but they are all the M style, hopefully that will do the trick?

[Cliff Ruggles]

WAY too much idle tube for that carburetor, and most likely why it is rich right off idle.

I'm still confused some, aren't we working with carb number 17058204?

I don't know what 17054208 would be from?.......Cliff

[kfa670]

Yep sorry, its a

17058204

I get a little dyslexic with the keyboard sometimes...

I'll order up some idle tubes, what size would you suggest?  And in your book you say to use the .026 inch tip metering rods with these large MAB carbs... I only have 3 sizes in hand, a .046K a .041K and your .044Ks.  Stock for this carb were .052K?  Will M Rods work if I adjust jetting accordingly? 

Thanks Cliff

[Cliff Ruggles]

I don't like to use "K" series rods with those units, as they only allow .005" of tuning ability on the upper section.  I'd set it up with a full tapered rod instead.

"M" series don't work well in large MAB carburetors.

I can supply idle tubes, tuning parts, and anything else that you need for it when you are ready......Cliff

[kfa670]

Well, this thing has about frustrated me enough haha.

I can tune for an awesome mid-range or an awesome top end, but can't seem to get both.  And always rich off idle with puffs of black smoke.

So, what kind of recipe would you suggest for this carb? 

Idle tubes?
Rods?

I understand I may have to play with the mains a little to get it just right, but would you suggest altering any other parameters of the idle circuit?  Let me know and I will place an order

Thanks

[kfa670]

I forgot to mention I tried a few different combinations:

.071 Mains
44K rods (from Cliffs HP)

-This has given me the best mid-range and a pretty decent idle so far.  A small hesitation off idle and some black smoke.  Pretty flat feeling on the top-end.  This is the best I have gotten the truck to run.

.076 mains
.052Ms

-Attempting to keep the mid-range the same and add just a little more fuel on top-end.  Mid-range sucked, Top-end a little better, idle and off idle not great. 

The best vacuum I can ever get at idle is about 14g.  I'm at about 6000k elevation. 

You mention using a full taper rod... would that be the older style?  I didn't think they would work that well in a newer unit?

And again, if you can put me a list together for a good kit for this MAB, I'm more than happy to get out the credit card   Had enough of taking this thing off and putting it back on...

Thanks again for all the help!

[kfa670]

Or should I just quite messing around and put some smaller MABs in this dang thing too....

[omaha]

yes, high altitude can present some problems. I think airbleeds would be a good move. Maybe somewhere in the .070 to .080. range. Slower pump shot (outside hole) and less stroke would help. You got less air  out (up) there so less fuel to go with the less air.  full taper rods would be more adjustable. maybe a lower tension spring in the APT so you don't over fuel it. Most of us "flatlanders" carbs will smoke if we take them up to those altitudes. That is why you need the slower fuel curve. Also the heavy weight of the vehicle . maybe .034 - .036 in the idle tubes. The idle transitions into the off idle so it is important to have it set up right. This is my opinion only and I know others may have more, better advice on the set up but I think the general direction would be this. good luck and don't give up now, youll get it figured out.