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Wes Harden

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So after converting my 77 sub, sbc, q jet carb, np205, 4.10, 33, to use a 700r4, the truck ran horrible up any grade in 3 and 4th. I pulled the carb off and ran spray carb cleaner and compressed air through all passages, new gaskets, etc. Recheck/set adjustments. Truck ran pretty good for @ 200 miles. Last Monday evening I was south bound on I15 in Victorville, I could not get over 45 50 mph w/o the engine cutting in and out. 4th gear @1500 rpms, manual shift into 3rd no change. So after cresting the hill. 65 70 no problem. Down hill , level runs great, around town no issues.
I have considered the pick up coil in distributor, but haven't investigated.
Suspect a lean miss fire at under load, basically because the q jet isn't designed to meter fuel for power at 1500 rpms.
Question is how to get the q jet to meter more fuel under those vacuum conditions. 4.56 would probably help, and I am leaning that way, but $$ aren't available atm.
I would like to hear if others, with a carbed 700r4 are running well or what remedies they came up with.
My current plan is to either up the main jets, or reduce the main rod diameter, and if possible adjust the main rod vacuum piston to lift sooner.
 
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Bent77

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The fuel lines on the frame are notorious for causing this kind of issue. Sucks enough air to not work under a load. Make sure you are maintaining a consistent fuel pressure

You may have a vacuum loss someplace as well after the conversion
 

Wes Harden

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IvI' gone over the fuel lines 2x now. The sender unit is new, all steel lines are whole and clear, all hose has been replaced.
I have checked for vacuum leak, nothing external. I will be resealing my intake in near future, oil leak. So I will inspect for a vac leak from bottom of intake.
 

Bent77

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I suspect something you changed/touched (or close to and got disturbed) since the swap if the issue wasn’t apparent before
 

K5wrench

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You said it ran good for 200 miles after carb cleaning.

Then you suddenly have this problem.

Check the torque of all the screws holding the top of the carb together and all four bolts holding the carb onto the intake.

This happened to my 76.
Every pass i climbed in Nevada.
Finally I found the carb top was just loose and really screwing with the air bleeds, main jet wells.. everything.
Give them a check.
 

Wes Harden

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You said it ran good for 200 miles after carb cleaning.

Then you suddenly have this problem.

Check the torque of all the screws holding the top of the carb together and all four bolts holding the carb onto the intake.

This happened to my 76.
Every pass i climbed in Nevada.
Finally I found the carb top was just loose and really screwing with the air bleeds, main jet wells.. everything.
Give them a check.

I'll check this after dinner :waytogo:
 

Wes Harden

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I've also seen the tubes, that are pressed into the top of the carb, fall out leading to crappy low rpm fueling.

Yes I have seen this too. I expected to see it when i had it apart in January, but they are in tight.

So I checked the tightness of air horn cover screws, the screws in the air horn i got almost a full turn. The front to bolts were almost finger tight. Didn't get a chance to test on a hill, but re-torquing those couldn't hurt any.
 

blazer74

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Do not over tighten the 2 large front mounting bolts, that causes warping of the airhorn.
Snug plus a little is all you need.
 

Wes Harden

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Well finally took the burb out drove 40 miles freeway speeds 65ish ran good. Drive through town 5, 6 miles great, drive 25 miles freeway speeds 70ish. 3/4 up a 4-5% grade runs low on power slows down starts bucking and surging. Crested the hill @ 40 mph. Popped the gear shift to neutral, snap the throttle, motor revs good.
Still not sure the vacuum signal is right to raise the main metering rods out of the main jets. At cruise I am taching 2k, manifold vacuum is going to be over 15inch, the main lettering rods are controlled by manifold vacuum.
I was thinking the gm use a Qjet with the 700r4, but then I remembered those where CCC carbs.
Frustrated
 

500$k5

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Sounds like it is draining the float bowl of fuel. Q -jets have a small float bowl, and must have a good volume of fuel pulling grades. Put a temporary fuel pressure gauge on it, so you can see it while driving, and see if the pressure drops off while climbing the hill.
 

dyeager535

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I was also going to say something about the float/bowl. Back when my truck was running a Q-jet, I had no issues driving at ~60MPH for long periods of time. UNTIL I got to a hill. A bit after starting up the hill, power would start falling off. Flat ground, power was back. Since it was repeatable, was fairly easy to tell when I got it fixed.

I had rebuilt the carb, so I was certain float level was "spec" for that carb (since all the rebuild kits, back then anyway, showed float height based on carb ID number, which of course simply means what it was installed on, and is no help when installing on a different engine), however that carb was for a heavily emissions laden 305, not the nearly emissions free 350 I was running, which I believe also had headers at that point. Took the air horn off, changed the float level a smidge, and drove it. Problem solved.
 

blazer74

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I’ll share my experience.
Ethanol in the fuel doesn’t help, could be a bit of vapor lock. Fought similar but not exact with my 74 K5 and a Qjet.
Added a filter up near the carb with a return using the canister vent line to the tank with a vented gas cap.
Seems to have corrected the issue. Never had these problems years ago untill the ethanol fuels showed up.
I’ve even tried the so called non E fuel with no help.
Like 500 said Tap in a fuel pressure gage and see what’s happening when the problem occurs.
Been there done it. Pressure drops off and surging bucking begins and that’s with an Edelbrock street fuel pump which I swapped in for this issue.
The hotter outside the worse it is.
Just my thoughts on this.
 

Joel Wilson

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What kind of condition was the fuel filter in when you cleaned the carb? What I have done on every SBC with a Qjet is to put an inline fuel filter between fuel pump and carb. Much easier to change and diagnose problems with and it keeps you from stripping the threads on the carb inlet itself. Are you sure the fuel pump is up to snuff? With enough fuel pressure it should refill that bowl right now. Odd that it came up after the carb cleaning and the 700r4 swap. The gearing isn't that much different between TH350 and a 700r4 in 1,2,3 and you downshifted to 3rd and it was still there. So, I'm thinking fuel pump, fuel filter unless something got tweaked during the carb cleaning process.
 

Wes Harden

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Filter was new when carb cleaned, another new filter when carb reinstalled. I also bumped the float level 1/16th up. Pump was new account Delco for small block, in December, when I replaced all the rubber hoses. Tank was removed and cleaned in March with new pick up and sock.
The pump I used, is listed as 40GPH 8psi, just ordered a Jegs 15650 pump 80GPH and 6.5 to 8 psi. I stilled need to reseal intake for oil leak, I am going to get on that. May be an internal vac leak.
 

Wes Harden

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Found this post on a Corvette forum.
Anyone else know about this?

"I have to assume this is a Qjet knockoff.....IF so, I remember some years ago being told by a Qjet man that the troubles with most of them lied in the 'off idle transferr' circuit.....
has to do with the little rectangular slots in the leading edge of the primary throttle plates....what happens is it's set too lean, causing engines to bog off light throttle...then the vacuum drops raising the metering rods, then it goes too rich...

It was explained to me to be one set of the brass tubes extending from the top of the carb down into the middle casting...these tubes need be enlarged a size or two, with a numbered bit....
been a few years, so that's all I remember.....I think Lars is a better source of information on this, and maybe you can PM him or someone...."

My motor is not over modified. Mild cam single 3" exhuast. I am going to put a high volume fuel pump. My expectations that the pump will fix this issue are not high. Thou I vaguely remember using a HV pump on another K20 burb, with mild sbc and 465, so....
 

dyeager535

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The Qjets are incredibly complex in terms of adjustments available, and what adjustment affects what in terms of performance.

I'd get one of the Rochester Q-jet books if you really want to dig into it. No single change is going to work to fix every possible issue for a random Q-jet thrown on a random motor. And the later the carb, the more adjustments to consider. A late 60's Q-jet isn't going to have nearly the components and adjustments as one from the mid-80's.

A pressure gauge between the carb and pump would tell you if the pump isn't keeping up under load, or even initially is on the low end of spec. I want to say the later

Honestly, if you are going to spend the time to learn to set up a Q-jet "perfectly", learning to tune fuel injection is just as difficult. Not saying convert, but I feel it's every bit as difficult, especially if you start throwing timing into the mix. Anyone that says carbs (at least Q-jets) are simpler than injection have not read one of the Q-jet books. The Q-jet is a good/great carb, and can be fairly easily tuned to get adequate service, but to really dial them in takes a LOT of work. Heck, I've got a check-ball type needle/seat replacement that is supposed to flow more fuel, which I'm guessing is not even sold anymore.
 

dyeager535

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Yep, that one, although there is another that is written that is pretty thorough too, although I haven't read it. I've seen it recommended more than Roe's.

Also, IIRC, the Roe book doesn't cover much of the later variants and their tuning? What year is the carb you have?
 

Wes Harden

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It is an edlebrock 1902, I bought new probably 2004ish. I think I still have the original maybe 2 original 1977 4mv's.
 
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