Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: 5stareventsdj on October 05, 2012, 03:13:55 PM
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I've been trying to diagnose a WOT problem. Problem is I'm not getting the rpms that I should. Under load I'm getting 3900 rpm max when I should be seeing 4200-4600 at my configuration. I think I may have tracked it down to the secondary air plates not openening. Is it possible that the choke pulloff is holding these closed? I see the linkage that travels between these plates and the choke pulloff. For me to open the choke plates the pulloff needs to be extended. It currently is retracted all the time. Or at least it seems to be. Is this choke pulloff supposed to weaken at low vacuum? At what rpm should the choke pulloff start to release and when should the air flaps open? Do I have this all backwards in my head?
Greg
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I got this on another quadrajet post: "When the engine fully warmed up, the choke flap should be straight up and down. This is the full open position. This is important because there is a secondary lock-out that prevents the secondary throttle blades from opening if the choke is still applied." My choke is opening fully (straight up and down). Is it possible the my flaps are still being locked. Trouble shooting further, with the engine running a cannot push the air flaps open easily as the pullof if retracted. With the engine off, the pulloff extends and the flaps open. I got brave (or possibly crazy) and hit the throttle a couple of times and didnt see the plate open. I didn't hold the throttle open though it was just a quick acceleration and backed off immediately.
Thanks,
Greg
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The pull off will lose vacuum when the engine goes into high load (aggressive throttle/WOT, triggering the secondaries will likely cause this to happen).
You should measure the retraction time of the pull off and get it to 3 sec or faster (cliff's book has guidelines depending on the build you're going for.)
To do this, retract the pull off arm manually and cap the vacuum port on it with your finger. Let go and time the time it takes to fully retract.
The next step is to get the air valve tension set up correctly. I believe 3/4-7/8 turn past the point the spring engages the rod is a decent starting point.
It's not so much RPM as it is load, CFM demand. A 454 would likely open the secondaries up sooner than a 350, given equal "temperament".
Good luck, I'm a newb so my advice probably isn't great! ;)
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Thank you for the reply. Its as good as any place to start although I think I may have a problem with the mechanism that locks the secondary air valves. I will make those adjustments though and report back.
Greg
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I would remove the lock-out on the Marine carb if present. It is only there to prevent the secondaries from coming in when the engine is cold and choke not fully open. They can hang-up and cause issues.
Also check the link going from the pull-off to the air flap shaft. It must hold the flaps tightly shut at idle (high vacuum), and allow them to fully open at full throttle (low vacuum). Bend the rod as needed so it allows this to happen......Cliff
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Will do. Thank you Cliff. I am going to also check the release time on the choke pulloff, but I think the stop is my big problem. :-)
Greg
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Typically doesn't have to be really quick on a Marine unit, 2 seconds or so should be fine......Cliff
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It appears that my set up doesn't have a secondary lockup. Does this appear to be correct?
Not sure how pics work in this but I attached one.
Thanks!!
Greg
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greg this might sound stupid. but have you (with the engine off ) put the throttle in wide open and looked past the sec. air flap to see if the lower plates are opening up to full position? it sounds like you have a linkage problem ( ether the throttle cable or the mechanical linkage between the primaries and secondaries needs to adjusted ) and your only running on the primaries. ive never worked on a marine carb but from the picture you dont have any lockout on the secondaries. jmtc. dean
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Thanks for the reply. I've been reading alot about the secondary lockout looking at pics, videos and Cliff's book. I was ready to disable mine until I looked at it to see that I don't have the lockout that I have heard so much about.
I have looked inside to see that the secondary throttle is opening. with the air cleaner (spark arrester) off, I haven't seen the air plates open on the secondary side. I have only checked out of the water though and am not able to give full throttle with cooling from my garden hose.
I hope to get on the water to confirm. MY WOT problem is driving me crazy. I can't tell you how much time & effort I've spent on this.
Greg
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greg .it was posted earlier you need cfm demand to get them to open .sitting in the drive way snapping the throttle isnt enough demand the only way they will open is if you remove the pulloff link and set the spring tension very light .that was the great design of the quad it only gives the engine the fuel and air it needs. dean
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Thanks again. I was out working on it again. I started cold. Although the choke is opening fullly, it's taking a long time to do it (10 minutes). It's a divorced choke and the linkage is nearly straight. I may look for a new bitmattalic spring. In the meantime, I am going to adjust the spring in the housing.
I shut the engine off after it was warmed up and was playing with the fast idle cam, trying to figure out how it works. While moving the throttle back and forth, I noticed drops of gas coming from the throttle shaft, near the fast idle adjustment. Does this mean I need to do the throttle bushings? When I had the carb off to rebuild it I didn't feel any play. Is there a gasket that may need to be replaced? My instincts tell me that if gas can come out there that air can get it. This was not happening while the engine was running just when it was off.
Lastly, I was able to confirm that the secondary throttle is working properly. If I push down the air flaps, I can see them opening when I move the throttle.
Would a leak in the throttle shaft be enough to cause the engine to run lean at WOT and cause me not reach WOT RPMs?
Sorry for so many questions. It's my first carb and I don't want to give up on it.
Greg
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1. Does this mean I need to do the throttle bushings? When I had the carb off to rebuild it I didn't feel any play. Is there a gasket that may need to be replaced? My instincts tell me that if gas can come out there that air can get it. This was not happening while the engine was running just when it was off.
2. Would a leak in the throttle shaft be enough to cause the engine to run lean at WOT and cause me not reach WOT RPMs?
1. The only gasket to worry about there is the throttle body-main body gasket. Did it seal well? Is there any chance your main body was missing the alignment dowels to the throttle body? (there should be two). If they are missing, Cliff has a procedure in his book that describes how to align the throttle body with the main body.
2. Not likely. The secondaries are massive and the amount of air that could get through the shaft area is minimal.
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1. The only gasket to worry about there is the throttle body-main body gasket. Did it seal well? Is there any chance your main body was missing the alignment dowels to the throttle body? (there should be two). If they are missing, Cliff has a procedure in his book that describes how to align the throttle body with the main body.
Yes that gasket sealed well. The only time gas comes out is when I pump the throttle while the engine is not running. The dowels were there. Alignment went nice.
2. Not likely. The secondaries are massive and the amount of air that could get through the shaft area is minimal.
Good to know. Should I be concerned that gas is coming out of the throttle shaft? It only happens when I pump the throttle and the engine isn't running.
Going back out now to mess with the choke. Hopefully it's cold enough.
Greg
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I would be concerned about the fuel coming out. Especially on the primary side.
I was responding to the vac leak @ WOT. Something that minor shouldn't have a huge, if any, impact on WOT. It could have a massive effect on idle, though.
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Idle runs nice and smooth at 550 rpm. I may do the bushings anyway.
Greg
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greg couple of questions? when did this problem start? did you pull the carb and overhaul because of this problem. this could be a timing advance problem in the dist. or is your cam broke on the secondaries not raising the sec. rods out of the holes to give it enough fuel.when it doesn't run as fast as you think it should. does it act like its running out of fuel.give us a run down on how this all started and someone will have the answer to help you fix the problem. dean
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greg sorry for the questions i didnt see your post on the sept.25. how did the carb work on the old engine? dean
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Hi Dean,
I can't say for sure. I bought my boat in August 2012. Engine failed in September after a couple of miserable outings. Manifolds were bad. I wish I knew then what I know now. It's quite possible the carb was not right then. I did do a rebuild using Cliffs Marine kit. It's running real nice. Since I replaced the motor I've been trying to troubleshoot my WOT problem. Stopped counting the $$$ spent quite some time ago. :-(
It's been better with every step though. I'm just trying to get it right.
Thanks for following up. This is my new favorite forum.
Greg
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greg i hate to ask dumb questions but sometimes you get to the bottom of the problem .did you check the cam timing?maybe the cam is timed wrong.1or 2 degrees .how about comp.?did you get a slug low comp. engine.dean
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greg i hate to ask dumb questions but sometimes you get to the bottom of the problem .did you check the cam timing?maybe the cam is timed wrong.1or 2 degrees .how about comp.?did you get a slug low comp. engine.dean
Not dumb questions at all. Thank you for asking them. I've never checked cam timing. Not quite sure how to do it. I did check compression 150psi on all 8 cylinders. I will look into checking cam timing. I also haven't used a stop to verify tdc. I will put that on my list of things to learn/do.
I bought the engine remanufactured from a reputable engine builder. I didn't expect I would have to check these things... :-(
Greg
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greg i built a 1970 429 about 11.5 comp ratio.when i did the comp test it tested about 250 .i could burn diesel.to make a long story short i had a (reputable) shop do the the heads .and guess what? 10,000 miles later i had it back apart because he never put hardened seats in like he said he did. and the valves where half way sucked up in the head.just a suggestion go the the shop and find out what heads, pistons ,and cam are in it and the chevy guys on this site will tell you if it 150 hp or 300. with 150 hp it may only rev up to 3800.jmtc. dean
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Did the boat run 4600 rpm before the motor broke? Maybe your boat has too tall a prop for the motor to turn at the correct rpm.
What is your boat's weight, length and hull deadrise? These factors all affect the amount of power it takes to propel it. What is your drive ratio and what is your prop pitch? If the prop is too tall the motor won't be able to reach correct wide open throttle RPM.
I guess what I am saying is, are you sure the motor is the problem?