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Discussion Starter · #1 ·
I am running a 70 350/350hp Qjet on my 70 396. I sent the carb off to custom carbs for a restoration and street tune. Got the motor fired up and working through some adjustments. The carb has some "flat" spots. Want to do some early trouble shooting but I dont know QJets very well.

Im way up here in Denver (5K ft), I told the guys as custom carbs my cam specs and altitude but not sure if they are used to building carbs for this elevation.

I was told to check my metering rod springs (power piston springs?) to see if they are working as they should at my elevation. I'm only running about 10" of vacuum at idle with my cam setup.

I see there are different springs you can buy but not sure what spring does what. I'm assuming with less vacuum than lower elevation I may not have enough to pull the springs and close the meter rods.

Whats a good way to test if they are working properly?

Thanks,

Dave
 

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I am running a 70 350/350hp Qjet on my 70 396. I sent the carb off to custom carbs for a restoration and street tune. Got the motor fired up and working through some adjustments. The carb has some "flat" spots. Want to do some early trouble shooting but I dont know QJets very well.

Im way up here in Denver (5K ft), I told the guys as custom carbs my cam specs and altitude but not sure if they are used to building carbs for this elevation.

I was told to check my metering rod springs (power piston springs?) to see if they are working as they should at my elevation. I'm only running about 10" of vacuum at idle with my cam setup.

I see there are different springs you can buy but not sure what spring does what. I'm assuming with less vacuum than lower elevation I may not have enough to pull the springs and close the meter rods.

Whats a good way to test if they are working properly?

Thanks,

Dave
You can test the primary power valve spring by using a straw or other small, light rod (I use a cut off piece of plastic artist brush stem) and inserting it in the vent tube opening. Angle the bottom of it forward and you should be on the top of the primary power piston. Push down and it should go down about 1/4" against light spring pressure. If the bottom isn't angled forward enough it will be on the float and pushing the float down will flood the carb. Start the engine and if the spring is light enough, idle vacuum will pull the power valve down and the item inserted in the vent will go down about 1/4". Blipping the throttle will lower vacuum and cause it to go back up temporarily. You can mark the item at both positions with a sharpie to make it easier.
Springs are available in 4 pack assortments. With 10" of idle vacuum you would need the lightest spring IMO. That's getting close to NO spring being light enough. The stock '70 carbs with stock setup are very lean on the primary side for emissions and your cam makes it worse, BUT at your altitude that richens things somewhat but still probably too lean.
 

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I know there is no one size fits all formula for Qjets, but I am sure if you told a reputable rebuilder your cam specs and altitude he would be able to come up with some sort of an in the ball park setup that would allow your car to run well when he worked your carb over. There are "formulas" for altitude and jetting that can make it easier to get it where it needs to be by actually starting you off in the right place. That being said, as with any carb at any altitude, on the car testing is where you nail it for YOUR car.

You need to know what jets and rods you have in your carb before making changes. Did you get a sheet with your carb stating this? Changes should be done in an orderly fashion in %'s that won't be dangerous for your engine. If "flat spot" = surging at cruise, you probably are too lean. What do the plugs look like? If it's running pig rich due to the power piston spring being too TIGHT (working against the vacuum), well, one would assume the plugs would be sooty. That's an easy check to verify what you find with von's test.

Super nice weather lately getting you antsy, huh ;) ?
 

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Discussion Starter · #5 ·
Thanks for the replies. I did use www.customrebuiltcarbs.com so I will have to call and talk with them. Unfortunately I didn't get any paperwork back with the carb that talk about what springs, jets, etc were used. I heard great things about their work so I know they built it correctly. Agree, now that the car is finally running with the fresh motor I can start tweaking. I plan to get a Qjet book to read up on it, I know there are alot of adjustments but a buddy told me to check the spring pressure since we have such low vacuum up here.

I'll tinker with it and call the guys who built it for some info on whats in it.

Dave
 

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Thanks for the replies. I did use www.customrebuiltcarbs.com so I will have to call and talk with them. Unfortunately I didn't get any paperwork back with the carb that talk about what springs, jets, etc were used. I heard great things about their work so I know they built it correctly. Agree, now that the car is finally running with the fresh motor I can start tweaking. I plan to get a Qjet book to read up on it, I know there are alot of adjustments but a buddy told me to check the spring pressure since we have such low vacuum up here.

I'll tinker with it and call the guys who built it for some info on whats in it.

Dave
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Even though your newly resto'd q-jet may need some tweaking to run better ytoum need to 1st ensure you have proper timing(espcially enoiugh base timing ) if you running any kind
of non stock aftermarket gm cam like from comp/crane/Lunati/etc.

So what cam and specs are you running,compression too?

Rough place to start for timing with street perf bbc/sbc wityh mild to moderate perf cam is 18 deg base timing (set with idle low like 550-600rpm & vac adv unhooked & plugged) ) + dist setup to give approx 18 deg at crank for approx 36 deg total all in by 2500-2600rpm.

Post setting timing try vac adv on full int vacuum & also reset idle mixturte and idsle speed too.

Ok after doing that if the motor still has the stumble issues etc could maybe be cause by the accel pump cup slipping/flipping over on accell pump rod causing it to not give a full /proper shot casing a stumble comming off idle and when you hit the throttle @# cruise too.

If thats what happened go to link i patsed below for q-jet prts to get a new pupm cup thats resistant to toadys ethenol'd pump fuel. The std rubber pump arm cup that comes in some rbld kits gets very slippery,distorted & bloated allowing it to litterally slip off or flip over upsidedown on the accell pump mrod in short order post being exposed to todays different blend pumpp fuel esp with ethenol thr std rubber accell pump cup was never deisnged to handle in the 1st place.

Also,position of rod from carb linkage in the accell pump arm (either hole 1 or 2) can also make for a stumble so could try changing that to see if that helps,if not oput back in original pisition/hole)

I have als found the stock idle circut on the q-jet is a bit lean enough that causes a lean idle to main transition .

Hads that happen on my mild perf 396 and having the idle down tubes drilled just a little larger fattened uop the off idle to main-cruise mix enough to get rid of the stumble.

But its also possible your carb is setup slightly lean when at cruise too making things even worse srtock setup for your 70 chevelle carb is 78 mains/49b rod on prim side with AX
sec rods which could be a little lean for a perf setup esp with aftermarket cam/good breathing exhaust,etc.

Hard to find larger mains then 78 for q-jets but to fatten up cruise mixture you could try going down 1 or maybe 2 steps max in prim rods from stock 49b to 48b or 47b.

Could fatten up secondaries a smidge if need be by going to sec rods with slightly smaller tips.

The stumble issue is normally a lean issues but being too fat at idle and or when comming off idle can also cause a stumble esp the accell pump adj etc.

To chk that is easy,push in the roll pin that holds the accell pump actuating arm to the carbs airhorn so it pops out.

Then try giving the motor some throttle and if the stumble goes away the carbs idle & or idle to main cruise is setup too rich,but if the stumble is still there then its too lean for likely 1 of the reasons i already stated above.

And thers another thing that can make the motor stumble when you give it throttle @ cruise and thats if the power piston spring is too light not allonging the prim rods to be partially puished up/out of main jets to slightly richen the cruise mixture.

But get the timing right 1st before going after the carb of you can end up chasing your tail with carb issues that may be mostly timing issues that your covering up by incorrectly adj mult carb circuts to overcome improper timing.

PS,id also install a lerger hi flow needle & seat with q-jet on street perf motor too.

Heres the links to the hi flow needle & seat along with any any other q-jet parts you may need too. Also included a link toi Sean Murphy Induction thats a good source for q-jet resto/rbld/perf mod's etc. Sean usually get's q-jet tuning right the 1st time out of the box even for modified street perf motors.

http://gesslerheadporting.com/

http://www.quadrajetparts.com/

http://quadrajetparts.com/needle-seats-c-30.html

http://www.smicarburetor.com/

Sorry for the length of this post but i felt you needed to see what some of the avg/main causes were for the runability issues your having with your newly resto'd q-jet all in one post since there are mult timing & carb tuning issues that can cause the problems your having.

Scott
 

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I'm still curious what is meant by "flat spots".
 

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Discussion Starter · #8 ·
Well I gave up on the timing/carb issue for the moment. A different issue has popped up since my previous posts. Problem I'm having is I have so many untested parts on the new motor. I'm so far in the weeds on trouble shooting I can't see the track.

Quick background: Fresh motor .. burning oil ... motor ran "decent" but lack of performance and on initial throttle it I wouldn't gain any power until around 1500 rpm .. found out bad valve seals .. fixed them. Went to pickup the car and could barely get it moving in first gear, throttle was very mushy "flat spot" and it would pop in the carb and almost stall ... At Idle the motor ran choppy and would "smooth" out after about 1500 rpm or so. Was a similiar issue that I had earlier but more intense to the point it would die on me.

When I was checking the spark plugs I got a nice zap .. got me thinking about my wires. I am running stock reproduction spark plug wires with the HEI Module and MSD Blaster 2 coil. Right now I HOPING I need better wires and I'm getting crossfire and have burned out my plug wires. Waiting to get a set to test it out.

Once that's check I need to go into my timing but I think I'm pretty close, can't recall where its at right now (car is with the builder till I can tow it) but I will use the info posted above. Before I ran into the car stalling I had the timing and initial carb adjustments set okay, but was hard to tune it in with the burning oil.

Let me add I'm at 5000Ft elevation.
 

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Popping in the carb can be a timing issue and/or lean issue.
For fastest diagnosis and excellent results, look at SWHEATONs' post and follow to a "Tee.":yes:
 

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Discussion Starter · #10 ·
I will for sure .. but need to fix the wires that are sparking
 

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Went to pickup the car...car is with the builder till I can tow it
Any reason why the "builder" can't fix your problems? I'm assuming he actually was paid to do the work? When you take your car to someone else and you pay them to fix it, the least they can do is actually FIX IT....?
 

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Discussion Starter · #13 ·
To clarify, the "builder" is the machine shop who did the block and heads ... he didn't do the first fire up or messed with the carb, distr, etc.
 

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To clarify, the "builder" is the machine shop who did the block and heads ... he didn't do the first fire up or messed with the carb, distr, etc.
I think they're talking about Custom Rebuilt Carbs if that's who did the restoration and tune on your carb.
 

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Discussion Starter · #15 ·
Oh yeah .. the carb was from them. So many issues lol. When I get the car back to the house I will give them a call and see what they think. I already sent them an email, hopefully they can tell me what was put in it. I'm not sold its the carb yet, but def something I need to check.
 

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Oh yeah .. the carb was from them. So many issues lol. When I get the car back to the house I will give them a call and see what they think. I already sent them an email, hopefully they can tell me what was put in it. I'm not sold its the carb yet, but def something I need to check.
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YUP,since your questioning the cond of plugs & wires get the basic ign parts squared away 1st,then go after timing & lastly go after the the carb "only if " getting ign parts & timing squared away doesnt fix it.

Let us know how you make out.

Good luck.

Scott
 

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Discussion Starter · #18 ·
My engine builder just gave me a call and said he had some time to look at the. He did a compression check and all was well. He said all my plugs were fouled that he just put in. He grabbed a pair of performance spark plug wires and test out the car and doubled checked/reset the timing and added a vacuum advance. He said a WORLD of difference. Flat spot is 99% gone, the car is running good and now has the power that has been missing. He said the wires were definitely shot, just couldn't handle the spark.

Still have some tweaking to do with the carb, he said it is not dialed in yet.

But great news! I have so many new/untested parts on this motor it gets difficult to trouble shoot.

Appreciate all your comments, I'll def be looking deeper into the tweaking of the carb and all but happy that the car is running again. 1 problem down, ?? many more lol

Thanks guys.

Dave
 

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My engine builder just gave me a call and said he had some time to look at the. He did a compression check and all was well. He said all my plugs were fouled that he just put in. He grabbed a pair of performance spark plug wires and test out the car and doubled checked/reset the timing and added a vacuum advance. He said a WORLD of difference. Flat spot is 99% gone, the car is running good and now has the power that has been missing. He said the wires were definitely shot, just couldn't handle the spark.

Still have some tweaking to do with the carb, he said it is not dialed in yet.

But great news! I have so many new/untested parts on this motor it gets difficult to trouble shoot.

Appreciate all your comments, I'll def be looking deeper into the tweaking of the carb and all but happy that the car is running again. 1 problem down, ?? many more lol

Thanks guys.

Dave
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Ah,setting timing (maybe adv it some) and then adding vac adv which add a fair amount of timing too & replacing the fuel fouled plugs is likely what fixed it running poorly.

When the timning esp base timing is overly retarded that can lead to carb issues /low vac signal making carb run richer at idle/low speeds which can quickly fuel foul the plugs.

The new plug wires couldn't hurt either but thats why we suggested going after the timing 1st before the carb. That's because 8 or 10 times there are runability issues with a motor that seem to be carb related it's not & turns out to be retarded timing issues and no vac adv esp for for street motor.

Bet the motor has a lot more umph from the additional timing when adv timing at dist and also adding more timing from vac adv too that aftermarket perf cams (even miild ones) need plenty of to run properly on the street.

18 base/36-38 total all in by 2500-2600rpm & 10-12 deg max from vac adv for roughly 50 deg timing at cruise is a great place to start for a street perf timing curve.

Scott
 

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Discussion Starter · #20 · (Edited)
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To chk that is easy,push in the roll pin that holds the accell pump actuating arm to the carbs airhorn so it pops out.

Then try giving the motor some throttle and if the stumble goes away the carbs idle & or idle to main cruise is setup too rich,but if the stumble is still there then its too lean for likely 1 of the reasons i already stated above.

Scott

I did the test for the Power piston spring and the vacuum is pulling the spring down and I can feel it rising when giving throttle so I think Im good there.

Scott not sure what you meant about the accel pump and how to check that. What I did do is at Idle I manually depressed the pump down and the car stalled. Not sure if that's normal or not.

btw I am getting a shot of fuel when I slowly depress the pump, but push too much or too quick and it stalls
 
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