I would take him up on this Donavan then you can read through the whole book
At my wits end with this QuadraJet
- Thread starter Rktpwrd
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Yes, idle circuit first and he isn't fine as is. Neither my Holley 78 reman or my factory 403 Qjet would run worth a dam once I bumped compression 1.5 to 1 with the same 204/214 cam with 1.72 roller rocker arms added. That secondary hanger is perfect, just get CV or CK secondary rods once you sort out the idle circuit. I tried similar rods, CL on a 307, the only time I actually got one to ping.
Try 73 jets and those 44 rods should be great. Doug Roe's book suggests enlarging the idle discharge ports first before doing any other drilling. That is because you can still lean out the mixture screws by turning them in. Other drilling can't be changed back very easily. The pre-emissions size is .095" and you should measure yours first to know a baseline. It also helps to sneak up on that size in increments- a smaller hole may suffice. Then when you are setting the idle forget number of screw turns and get a vacuum gauge. Start with 5 turns out at a high idle and work your way down using the idle screw and mixture screws.When you get down below 1000rpm pay attention to the vacuum gauge. You should be able to adjust down to 700rpm or so and get a good 20 to 22 inches of vacuum as that cam is not radical. Most importantly be patient. Tuning an orphan carb on any engine is not easy!
Alright fellas, I made some progress tonight.
I got my hands on the mixture screw adjusting tool and counted the number of turns they were at before removing them.
2 1/4 on the passenger’s side one, and 1 3/4 on the driver’s side. Not great.
I also obtained a copy of Cliff’s book and studied the entire sections on the idle system. I definitely think this is where the problem lies as the idle circuit wasn’t modified for the improvements and changes I made to the engine when the carb was rebuilt.
Going through step by step following the book, I measured and recorded all the sizes of the components I had in the idle system.
Idle tube: smaller than .040 (smallest drill bit I had, should be able to find out exactly tomorrow)
Idle down channel: .048 - .050
Upper idle bleed: .067
Lower idle bleed: .076 - .077
Idle bypass air: .069
Holes under mixture screws: .067 - .069
Holes in throttle plates: None
As you can see by these measurements, several of the components and ports in the idle system were severely undersized, especially when compared to Cliff’s recommendations for a build with a cam similar to mine.
I need to bring my calipers home from work tomorrow to verify and finish up enlarging the rest of the components, but here’s what the new, modified numbers are at now:
Idle tube: Not enlarged yet
Idle down channel: Not enlarged yet
Upper idle bleed: .070
Lower idle bleed: .070
Idle bypass air: .082
Mixture screw holes: .093
Holes in throttle plates: None
What do you experienced guys think of these numbers with my engine combination?
(See earlier in this thread if you don’t recall the particulars)
Thanks in advance.
D.
I got my hands on the mixture screw adjusting tool and counted the number of turns they were at before removing them.
2 1/4 on the passenger’s side one, and 1 3/4 on the driver’s side. Not great.
I also obtained a copy of Cliff’s book and studied the entire sections on the idle system. I definitely think this is where the problem lies as the idle circuit wasn’t modified for the improvements and changes I made to the engine when the carb was rebuilt.
Going through step by step following the book, I measured and recorded all the sizes of the components I had in the idle system.
Idle tube: smaller than .040 (smallest drill bit I had, should be able to find out exactly tomorrow)
Idle down channel: .048 - .050
Upper idle bleed: .067
Lower idle bleed: .076 - .077
Idle bypass air: .069
Holes under mixture screws: .067 - .069
Holes in throttle plates: None
As you can see by these measurements, several of the components and ports in the idle system were severely undersized, especially when compared to Cliff’s recommendations for a build with a cam similar to mine.
I need to bring my calipers home from work tomorrow to verify and finish up enlarging the rest of the components, but here’s what the new, modified numbers are at now:
Idle tube: Not enlarged yet
Idle down channel: Not enlarged yet
Upper idle bleed: .070
Lower idle bleed: .070
Idle bypass air: .082
Mixture screw holes: .093
Holes in throttle plates: None
What do you experienced guys think of these numbers with my engine combination?
(See earlier in this thread if you don’t recall the particulars)
Thanks in advance.
D.
It will be interesting to see how the modification will improve your carburetor's performance but I do not think the issues you had were caused by the stock characteristics of the carburetor. It would really help if you had a wideband A/F mixture gauge to take out any guesswork.
A wide band is a very good idea but I think the emissions set up carb is the issue. I did as Donovan did and my carbs, yes two, both ran like sh*t, that ran fine beforehand. I was going to be brave like Donovan and follow Cliff's book, I took the easy way out. If you pull this out Donovan, build up the 2004R next, another Pandora's box of the automotive world.
Steve, I’m inclined to agree with Christian on this one. I think the emissions era carb and it’s extremely lean idle circuit is EXACTLY to blame for the issue I’m having.
Everyone proclaims how Cliff’s book is the “bible” to follow when building and modifying these carbs, and in the section on the idle circuit, Cliff notes how modifications MUST be performed to the idle system to have a QuadraJet perform properly on a modified engine.
I do agree, a wideband A/F gauge is an invaluable tuning tool, however literally hundreds of thousands of guys modified and tuned these carbs using old school methods long before A/F gauges became mainstream and commonly available.
The wideband option may still happen in the future and remains an attractive option, however it’s just not in the cards at this time.
Thanks for your input.
Christian, the 200R4 I’m currently using has already been professionally rebuilt, and I’m not a sucker for needless self abuse and punishment, so I ain’t touching it!
If it ain’t broke, don’t fix it...
I love that he is diving in to this and is going to see it through to the end. Bravo! We need more Q-jet gurus in the world. As for myself I have never had to open up anything but the idle discharge ports. The logic is- bigger cam wants more air/fuel at idle and doing that has worked well. I also play with the jets and rods depending on displacement and cam. But then the biggest cam I have ever run was a Crane HMV278- grumpy but not crazy. The beauty of a Q-jet is you can take a six cylinder carb and make it work on a big block.
I'm glad that Donvan's wit's were further down the road than he thought and he's going to dive into this quadrajet.....
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