Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: 55 Tony on August 10, 2017, 02:23:04 PM
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Hi I just rebuilt a 17084266 and sorry to say I did not use Cliff's kit. I just found this website and it looks like I hit the jackpot! I did buy his book but I'm ADD and skipped through it a few times not sure what all I've grasped.
I had to turn up the idle to keep it running, I forget how many turns first on the cold idle then also 2.25 turns for the warm/hot idle. I have a wideband A/F gauge so that helps tremendously in tuning. Anyway I've opened up the two idle mixture screws just about as far as they go and still it's very lean at idle and doesn't want to idle, (making me turn up the other 2 screws).
The main idle speed screw I had adjusted while it was apart and set the butterflies half way into the Oh can't think of the name right now, the idle *slots*? To keep it running I had to turn the screw to open the butterflies 2.25 turns in. Still at idle the A/F gauge reads 17 to pegged past 18 with the idle mixture screws probably close to falling out!
I took it out for a test drive using the same 72 jets and 51 rods it came with and I forget the secondary rod letters, that it came with but the mixture is GREAT throughout rpm ranges and WOT. It's only the idle that is so lean.
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It would really help knowing what type of engine it's sitting on and also, what was done to the carb (modifications, if so) beside "rebuilding" it.
Frank.
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I made no modifications to the carb, just rebuilt it. It is an 800cfm.
It is on a modified 454 10:1+ compression, a healthy roller cam, Edlebrock rpm manifold, 1.75" headers, timing 7
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I don't know what is going on with my post but I had writen this?
Timing 7
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OK this newsgroup must not like the degree symbol, it cuts off everything after it, I'll try a third time.
Timing is 7* static, 37* all out, plus vacuum advance ported to manifold. Let's see if the third time is a charm.
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What's the cam?
You really need more initial timing.
How much vacuum do you have at idle?
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No to more initial timing advance. At 8-9 degrees it's hard to start, kicks back and breaks starters.
Idle vac is 15 but the idle is a bit fast at 800.
Cam:
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=436&sb=0
Previous q-jet idled as low as 500 in gear when good and hot. (17058212 I think?)
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Your vac. adv. is hooked to manifold vac.?
Probably needs to have the idle circuit enlarged to provide more fuel.
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Yes, lots more idle fuel with a big roller cam. Most every other circuit will need attention as well, in my opinion.
Also, if that is a healthy roller cam, 800 rpm at idle isn't that fast.
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Yes vacuum advance is to the manifold. I tested every port on the carb and none gave more vacuum than the manifold. The one ported one was about equal but with more of a lag when accelerating. Didn't all cars have vacuum advance to the manifold before smog laws came around?
As in my original post, all other circuits are working perfectly throughout their ranges. No need for changes there, I am working on the idle only.
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You will need to modify the idle system for that much camshaft.
I would get into the book deeper to familiarize yourself with what will need to be done to the idle system.
Since you didn't use our kit, I'd also recommend installing the high flow N/S assembly, small float, HP accl pump and secondary cam/spring, at a minimum.....Cliff
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"No to more initial timing advance. At 8-9 degrees it's hard to start, kicks back and breaks starters."
I'd also check cranking pressure. With that much cam in a 10 to 1 CR 454 it should not be "bucking" the starter on hot re-starts with only 8-9 degrees of initial timing.......
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What is cranking pressure and how is it measured?
I do have high compression, 195 to 205 psi.
Also how does this effect idle mixture? That's my only problem.
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Edit:
Sorry, never heard of it called cranking pressure before, so the 195 to 205 is my cranking pressure.
The cam is advanced 2 degrees which is partially responsible for the high compression.
As for "bucking" the starter, that even happens on COLD starts if I advance over 7 degrees. Could be my compression ratio is higher than I thought? Don't ask why but deck height was never measured. Only made sure the pistons didn't hit the valves. My first rebuild.
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Was the cam ICL set according to the cam card or did you just advance it 2 degrees from the "dot to dot" position?
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Could be your timing marks are off and you're getting a false reading.
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Was the cam ICL set according to the cam card or did you just advance it 2 degrees from the "dot to dot" position?
First degreed the cam then advanced it 2* from dot to dot.
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Could be your timing marks are off and you're getting a false reading.
Nope, timing mark is lined up with the key on the crankshaft.
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I'm not sure why you would correctly degree a cam on a tight LSA that already uses an advanced ICL then advance it some more? I would move the cam back to the correct position since you are seeing high cranking pressure and difficult hot restarts with relatively low initial timing numbers.....Cliff
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Like I said, my first build/rebuild. Someone with a lot of engine building experience was giving me help online. Comp Cams said all it would do is decrease the power band of the motor by about 500 rpm making more low end torque. Since then I've found I have all the torque I could possibly use plus more and if it ever comes apart I will install it straight up. It's not that big a deal to make me want to take it apart unless I have to because I don't rev it over 5500 anyway.
How does this factor in with the lean idle anyway?
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You got poor advice from the experienced engine builder and from Comp Cams.
The intake closing point is a critical event in an engine, and advancing camshafts closes the intake valve much earlier so everything above the piston starts to get compressed sooner, plus you change scavenging by the exhaust moving things ahead some.
Been hearing all sorts of remarks over the years about "advancing the cam to make more low end power" and to account for timing chain stretch, among other things. Those comments are mostly regurgitated information from very poorly informed folks and from doing this for many years I know that they have very limited experience with actually moving camshafts around and looking at the results....FWIW.
The cam you used was already on a tight LSA with an advanced intake lobe position so moving it further ahead obviously was not a good move since you have high cranking pressure and the engine doesn't like much initial timing. It shows 292 degrees duration @ .006" tappet lift and 232 @ .050", so it's PLENTY of cam for what you are doing. It shouldn't be making a lot of cranking pressure or finicky to a lot of initial timing either being on a tight LSA and sporting a lot of seat timing.
Not a big secret here or anywhere else that I'm not a big fan of tight LSA camshafts in "street" engines. They are done that way for the "bling" factor more than anything else. Folks love to hear the miss-fire in the exhaust or "lope", but at the same time they narrow the power curve, pull power down in the rpm range, increase octane requirements, and put a lot of unburned fuel out the exhaust (stinky).
Cam position also effects idle quality to some degree, as well as efficiency, power output, and vehicle performance.
Difficult cranking when hot, "kicking back", and breaking starters with as little at 8-9 degrees initial timing is a HUGE indicator the camshaft is too far advanced, or your actual compression ratio is much higher than predicted, or a little of both.
From what I've seen tuning engine with similar characteristics, it will also be finicky with the timing curve, total timing, and how much timing you can add by the VA at cruise as well.
For idle tuning you simply have to get enough fuel to the mixture screws to provide full control from rich to lean, and enough bypass air to keep it on the idle system (no nozzle drip).
Many late model carburetors, especially truck units will also have a LOT of idle transfer slot in the baseplate. This can provide a good amount of fuel to the idle system even if the mixture screws are seated, so it can make idle tuning a little more challenging.......Cliff
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It runs pretty darn well, maybe someday I'll have an excuse to take it apart and back off the 2 degree advance and see if there is a noticeable difference. For now it's staying the way it is.
I would like to get a pair of "normal" idle mixture screws, the ones it has are rectangle shaped heads. Do you know what size they are and do you sell what I need? Also what version fix do you recommend for the leaky bowl? I just got a set of numbered drill bits today so I may work on the idle circuit soon.
Oh, and no I'm not one to choose a cam for how it sounds. But I will also admit I don't know a lot about choosing a cam. I saw they listed some good for "street machines" but this one said for "hot street machines". And as it is it's very streetable. No problems at all cruising through town in stop and go traffic or slow on back country roads. My torque converter is only about 500rpm higher stall than a stock factory one. No problem going from idle to WOT, well except for keeping rubber on the tires.
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Any seat of the pants assessments of engine power and vehicle performance are next to useless, especially with big block engines with decent compression.
A well thought out low compression 454 will pull the end out a ditch, and completely IMPOSSIBLE to hold it with any sort of street tire, even with a stock converter and stock gearing. The last 454 I helped a customer out with here cranked out just over 500hp/600tq with a smaller custom ground HR cam that you are using, less compression, factory "flat" intake, factory Q-jet, and original points distributor.
Matters not in the big scheme of things, what matters is that you don't knock the rod bearings out of it with too much timing. If it's finicky to initial timing and high cranking pressure, it will not do well with much total timing either, so be conservative in that area when tuning.
We sell EVERYTHING for the Quadrajet carburetor, and many of our parts are custom made for us and far superior to anything else out there. I push parts to keep the doors open, no big secret there, but even more important is to have folks get good results and dead solid reliable in long term service.
This is why our accl pump have custom wound springs on them, and a custom made seal that's at least 1000 times better than the "soft" blue seals showing up on accl pumps these days, and no one else out there has a lifetime Warranty on them......Cliff
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:-[ :-[
If it seems impossible maybe it is ... I found out my timing light is shot. My timing for this ordeal has been right around 17-18* not 7 or 8. Still got to dig into that book again, haven't yet found what I'm looking for.
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17-18 initial is too much for that engine combination, imho. It can and most probably will "buck" the starter on hot restarts especially when it's well heat soaked.
I don't like to see about 10-12 degrees initial timing, with 10-12 degrees in the mechanical curve (20-24 at the crank) and another 10-15 degrees from the vacuum unit.
As you improve fuel delivery from the carburetors idle system it will idle better with less initial timing........Cliff
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Finally got what I needed out of the book! Enlarged the idle tubes just a little and the holes for the idle adjusting screws and now it idles strong and the A/F gauge hovers from 14.4 to 15.2. I had to back off the idle rpm adjustment and I was able to loosen up the electric choke so it cuts out sooner. Also opened up the secondary supply holes because it was "running out of fuel" when getting on it good. I had done that trick on my previous q-jet also so I knew that would work.
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Also opened up the secondary supply holes because it was "running out of fuel" when getting on it good. I had done that trick on my previous q-jet also so I knew that would work.
can you elaborate on this ?
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can you elaborate on this ?
Sorry for the delay, I've been a busy guy lately. If you have Cliffs book, it's on page 81. I had done mine long before reading about it and generally go larger. I've had no negative affects that I know of from drilling them too large. Maybe Cliff will fill me in on that.
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thanks for the answer. And what size did you drill them ?
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thanks for the answer. And what size did you drill them ?
I believe 1/8".