That's fair. Like I said earlier.. im torn with this being a numbers car trying to keep things accurate. The idea from the start was basically original equipment, but with a cam swap.

I think it's safe to say we're past that now.

 

Do you have a build sheet for the carb? Does it feel flat and then suddenly come alive after enough throttle without opening the secondaries?

 

I do not, and that's part of the reason I need to try to get the carb close before going to dyno..I dont know what size of anything is in it.
it pulls strong, and I don't notice any dead spots.. but then again I've barely driven it. After going for a while, it starts feeling like it's gonna stall as if it's running out of fuel.

 

Call your carb guy, describe your running out of fuel feeling, I bet he recommends a stronger spring in the power piston. Thats a pretty common tuning change with a cam swap. Thats an easy one to change. You'll have all sorts of part throttle/cruise issues if its too strong.

 

I think the weaker power piston spring allows the weaker vacuum to draw the tapered power piston rods to be pulled down, which leans it out. A stronger power piston spring pops the power piston in the full up postion which makes the the mixture richer.

 

You are correct, the spring should be swapped for a stronger one, my train of thought was backwards last night!

 

Yeah, we talked about it when it went down the first time. I think though when it got opened up and he saw one of the rods had fallen out, we both just kinda forgot about that other issue..lol.

He did say he also made changes to the idle circuit, so maybe that issue was hoping to have been addressed that way..but I'm not sure.

 

Nope, I believe you’d need a lighter one due to the reduced vacuum of a larger cam. Otherwise it’ll allow the power piston to rise, lifting the metering rods out of the jet, and potentially making it run richer at idle.
These things are a pita to tune, most of you have no idea.
And the notion that Qjets “give better mileage” is a farce as well. They can be made to run as rich as anything else, and often do.

 

I understand this, I have built several q-jets including one thats deep into the 11s and street driven.

I'd say his carb builder has certainly swapped the spring from stock, and at this point it's probably too weak. Sounds to me like he is describing the piston not overcoming the vacuum soon enough based on the what he said.

Carb tuning through a forum is impossible, and the OP isn't interested in trying to solve the problem on his own, so its a moot point anyway.

 

The builder is open to fixing it, and I don't have experience with QJets. I have a bench full of them to learn on, but my goal is to have (was to have) it in the ballpark by the time the engine gets pulled and sent off. But at this point Im wondering whether its worth trying to keep messing with getting the QJet right, or just stick a Holley back on and call it a day. As much of a headache as it was getting the linkage lined up for the previous holley, I don't really want that headache again..but it is what it is at this point.

 

your WOT looks pretty good at mid 12’s so your full throttle afr isn’t your problem. You need to do a full throttle run through all the gears to see if it’s staying between 12.5 and 13

the lean part throttle/cruise can be richened up with a rod change on the primary while keeping the same jets by going to a more aggressive tapered rod while keeping the same tip diameter. If you know what rods are in it now, I can give you a number that will richen up part throttle/cruise.

you can also check to see if your power piston is staying fully seated at idle and not fluttering up and down by inserting a plastic straw down the fuel bowl vent and resting it on top of the power piston. You can check it yourself with engine idling..but needs to be checked in gear idling, which needs two people..have the other person watch the marks on the straw as you drop it in gear…it should stay down.

when you tap the throttle like doing a small brake torque, the straw should rise instantly and drop back down after its back to idle speed.

first make sure your power piston moves up and down freely,, mark the straw at max up and max down positions before starting

 

That's inspired. I like that.

 

Interesting, ill have to see if I can get a helper to test that with the straw. I'm hesitant to do a full throttle pull through the gears due to the detonation, but I do understand the point.

I do not know what's in it unfortunately, but my basic knowledge had me wondering if it just needed to go up a bit on the rods and/or jets.

Based on the fact the idle doesn't return consistently, if I understand the power piston issue correctly I'd think that is a point of contention as well.

 

It could be that, or throttle plates that aren’t centered causing them to bind and not fully close.

 

Make sure the secondary air valve is closing consistently too. I had a fit with the one on my WIfe's car to the point that I found another used carb with the same part number and used the top plate from it. Hers was warped and as much as I tried to get it straightened out I could not get it to close right.

 

Sticky advance weights will cause an inconsistent idle also.

 

i listened to your video a couple times..I couldn’t hear any detonation on hard acceleration…is it that bad?

 

it's not as bad as it was before, but its there.

 

you’ll have to back off on the total a touch then. Keep trying until it’s gone

maybe trying limiting the distributor centrifugal with a bushing to keep the initial the same. do you know what your total advance is at 3,000+ rpm?

 

Total at 32°, it put my initial at 18°. I'm currently backed off to 16°, but it only detonated with this carburetor on it.

 

it shouldn’t be detonating with only 32 and 12.5 afr under WOT. are you using a dial back timing light?

you aren’t making nearly enough cyl pressure with that type of cranking psi. what octane are you using?

 

That's the thing though, the detonation didn't start until this carb was put on. I even retarded the timing from where it was previously just out of curiosity and it's still detonating with this carb. I'm running 93 octane

 

Swapped the Jet carb back on after dinner this evening. Only had time to pull it out of the garage, let it warm up and get a video of it idling/a couple revs in the driveway.


I'm hoping the weather is decent this weekend and I can run the same route I did for the other carb.

 

That first part of the video was ridiculously lean. I like the later Qjets, the easily adjustable APT makes tuning them simple. Just tap a plug in the air horn and notch the adjuster for a small screwdriver. I built a very aggressive Qjet, I offered to sell to you. I realized I didn't want a big cam motor. I just followed one of Cliff's recipes and used his high performance kit and bushed the base plate. The guy that bought it off me on here, said it was close on his 403. It started out as a 83 non CCC 307 Canadian spec 800 cfm Qjet. It was too lean at idle for a stock 73 Olds 350! You have the same luck as me, none, hopefully it isn't big dollars to fix this motor or somehow a proper carb fixes it.

 

Yeah, I do like the APT adjustment on the Jet carb (2nd in video).

The carb issue is a secondary issue (no pun intended) to the primary engine issues..im just hoping to get the carb closer before the engine goes out to be fixed.

 

I'm sorry, I hate to hammer on this but I cannot get my head around the logic of zeroing in on the carburetor before the engine goes out to solve multiple issues, because, despite your best efforts, by the time the engine gets back the carb will most likely no longer be "right".

 

Yeah, I've flip flopped back and forth on that thought, but my hope is that it'll get within range to where it can be adjusted at that be it...rather than put it together, go to test it, have a carb thats too lean, have to wait on someone else, etc.

 

Yeah, the first carb is worse than my Terminator X Max on throttle tip in, cold. What does the Jet carb read for part throttle?

 

Not sure yet, I've not had a chance to run it up the street for a cruise and for a WOT/hard throttle blast.