Tricky problem from 1000 to 2000 rpm. Need help driving me insane.

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ianpala

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Hey guys, 

Always like to start by thanking the community for its invaluable help and expertise, so thanks :) 

So... I haven't been able to figure out this issue since I put in my new cam and went from a 2bbl to a 4bbl carb.  My engine struggles between around 1000 and 2000 RPM. Seems to clear up right at 2000 with an increase in power.  Rough, chugging, low power, subtle misses. Pronounced when engine is cold or going uphill.  Also a hesitation, if not a backfire out the carb upon initial acceleration, (the acceleration problem has been cleared up mostly with a red accelerator pump cam and jump from .33 to .35 pump nozzle).  At first I thought this was all due to the primaries simply running lean (checked plugs after driving and shutting off engine and were white etc...) went from 68, to 70, to 72 primary jets... runs a little better but it is still there, so I think its something else.  I've played with my timing everywhere between 10-13 initial and 33-35 total... seems to run its smoothest when its less advanced, but problem is always there, have it at 10 initial 33 total right now. My vacuum at idle is lower than I think it should be... its pulling like 9 or 10 inches of mercury.  Also, the carb likes the 2 idle mixture screws at different depths. Checked everywhere for vacuum leaks, none found. Fuel pressure good, vacuum advance good and going to carb, problem still there if I run off manifold vacuum. 

AT THIS POINT I'm thinking I either have a vacuum leak on the underside of my intake manifold or a defective carb out the box. Is there anything else you guys can think of, or do you know of a way I can rule out the intake leak or isolate the problem etc?  

351 2v.  

  • Quickfuel slayer 600, Edlebrock performer 2750 intake
  • Comp Cam 32-242-4  .513/.520  262/270, Roller rockers
  • AOD, posi, 3.70 or 3.72 rear gears (can't remember), 
  • Dual Exhaust

Thanks again, this has been escaping me for about a year and its driving me CRAZY.

 
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Don C

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Have you pulled the bowls to see if there is gunk in them? Sounds like it's too lean, either plugged passages in the carburetor or vacuum leak. You didn't  say what cam you have. Your throttle plates may be open too far at idle, messing with the transfer slots. Others with this problem have solved it by drilling a small hole in each primary throttle plate to increase the air and reduce the throttle plate opening. Have you checked the idle setting on the secondaries? They can mess up the low speed performance, too, if not set right. Every new carburetor has to be checked over before installing it. Especially now days, seems like less and less stuff is good right out of the box.

 

ianpala

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I've taken apart and cleaned the carb a ton. I've shot brake cleaner through all the passages and none seem obstructed. It's definitely been running lean, I'll cruise for 5 minutes through out the trouble rpm range and shut off the engine, stop and then read the plugs and they either look new or barely white. Throttle plate is barely open, t-slot is good.  I have not messed with the idle setting on the secondaries, I figured in this rpm range it would be all my primary circuit. 

 

71coop

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Is 262/270 advertised duration?

 

Kilgon

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Several things you can try.  To check for vacuum leaks spray some carb cleaner or starting fluid around the intake at the heads and carb base and see what the engine does.  If rpm changes then you have a leak.  Unless you damaged the gasket  I would seriously  doubt you have a leak underneath the intake.   Did you try discounting all hoses and lines and plugging the connector to see it your vacuum goes up?  If it does then reconnect one at a time until you find out which one is causing your vacuum to drop.   On the stumble or hesitation I would try a larger acceleration pump and go to a 50cc.  This will give you more fuel to burn until the primaries kick in.  Also, there is a happy median for your idle screws where they will both be at the same depth.  Does the engine stall if you run either of them in - if so they are working ok.  I would start with them 1 1/2 turns out and work from there. It takes time and patience and you need to either use your vacuum gauge or watch rpm's as you do it.  Once you get it in the ballpark start adjusting them an 1/8 of a turn at a time.  You have to remember that this takes time and you want to allow a few seconds for change to take effect after each turn.  I also like to rev the engine after I make a few adjustments to allow for the primaries to kick in and then return to idle.   Don't know if you a running a standard dizzy or what.  If you know at what rpm your full curve is in then I would set the timing at least to  34 at that rpm and try see what that does for you at idle.  

 
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ianpala

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The cam specs listed in the original post are the advertised specs. I've checked extensively for vacuum leaks with carb cleaner and plugging, so I'm confident there are none unless it's under the intake and figured it would have been done to damage during installation. I'm using an electronic distributor one of summit's.  I'm also confident the idle screws are good they are around 1 1/2 turn adjusted with vacuum gauge in gear. I've had it set at 34 and 35 total timing, it made no difference to the problem described, was able to rule out timing as the issue. I can be coasting at 1700 rpm for 15 minutes and the problem will be consistent the entire time, then the second I hit around 2000 it smooths out (imagine this is when the secondaries kick in or potential vacuum leak plateaus?).  honestly I could keep going up in primary jet sizes, but I've already jumped 4 sizes up, and I feel like I shouldn't have to do this and that it points towards an underlying problem. The only thing I haven't tried is what Don mentioned, adjusting the secondaries (maybe adjust them to open a little faster?).  I also took it to a great mechanic here in LA and he couldn't figure it out either and double checked everything as well.  He seems to think its the same thing, intake leak or defective carb.  I just got of the phone with him and he to plug the PCV hole on the right valve cover and then feel over the breather hole on the back of the left valve cover, if I feel vacuum its an intake leak, there should be no vacuum or it should be pushing air out, so I'll try that.  Unfortunately my breather grommet is pretty shot, so I'll have to wait until my new one comes in to really get a good read. 

ALSO: I remember there being a passage on the heads and intake in-between the 4 ports.  My intake gaskets did not have a hole for this passage, so I cut one out in order for them to connect. Maybe this was supposed to stay blocked? or maybe it caused a tare upon installation. Just a thought 

 

Don C

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No, you don't adjust the secondaries to open faster, if it worked it would just be a bandaid covering up the real issue, just make sure they are set to the correct idle specification.

It's easy to get the intake installed at an angle, so it doesn't contact the gasket correctly. The best way is to install long studs in the four center holes to act as a guide when lowering the manifold into place. Finger tighten the rest of the bolts, remove the studs install the four center bolts. Tighten all in three steps:

1. Torque all bolts in sequence to 8-10 ft. lbs.

2. Torque all bolts in sequence to 15-20 ft. lbs.

3. Torque all bolts in sequence to specifications:

     5/16" bolts should be torqued to 23-25 ft. lbs.

     3/8" bolts should be torqued to 28-32 ft. lbs.

Don't using the shop manual final torque, that's for iron intakes. Retorque after a few times running it, with the engine cool.

The center holes you uncovered are for the exhaust gas crossover, helps for cold weather running and needed for the original automatic choke with the choke stove in the manifold. Most everyone keeps the crossover holes covered when using newer carburetors with electric chokes.

 
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TommyK

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At 1000-2000 rpm you are probably still on the t slot. I would bet the engine smooths out above 2000 rpm because the primary main jet is beginning to flow.

If you set your idle mixture screws using the highest vacuum method your idle circuit is likely too lean especially with an aftermarket cam. As an experiment try backing out the idle mixture screws another half or full turn and see if that mitigates the problem. Alternatively, you can stick some small gauge wire (like copper telephone wire) in the idle air bleeds and temporarily secure it. This will simulate a smaller idle air bleed which will richen the idle/transition circuit.

Generally speaking, an increase in initial timing will usually yield some amount of additional idle vacuum up to a point. The first thing I would do is increase the initial timing until you achieve or are close to the highest idle vacuum reading possible and then reset the idle mixture screws to achieve the best idle quality (not highest vacuum) and begin carb tuning from there. When all is said and done you may have to recurve the distributor to limit the total timing to a safe limit which generally speaking is 34-38 degrees.

 

ianpala

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Going to order a fresh intake gasket and torque to your specs Don.  Thanks Tommy, I'll try backing out my idle mix screws a half turn and see how that goes. the only rebuttal I may have to the idle circuit being too lean due to high vacuum is my idle vacuum is only 9-10 inches/mercury   My throttle plates are almost fully closed, haven't needed to raise my idle speed at all. Do not have access to an extra 600cfm carb to test out. Honestly thinking of just buying another Quickfuel 600cfm, but want to turn over every stone before I do. 

 

ianpala

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Taking off the heads today. The intake manifold gasket looked cracked! Might have happen during removal though. Had some over heating and coolant loss so re-doing the head gaskets and taking the heads to a engine shop to make sure there are no cracks etc... BUT...
 

Now I can’t get my screw in studs out to save my life. The guide plates block the head bolts. Have been trying the 2 bolt method and can’t get out a single stud. Any tips? Thinking of taking a torch to it to try to break it lose. Really don’t want to have to weld a nut to the studs. 
 
-Ian 

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Don C

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You're not getting the nuts tight enough, just use the wrench on the bottom one when trying to loosen them after getting the nuts tight. You may have to resort to threadlocker on the nuts. The studs may have red threadlocker on them, in which case you may have to use some heat. 

 

TommyK

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That looks like the Crane stud conversion kit. As Don suggested some judicious use of heat should help. The lower portion of the stud is only 5/16" so be careful not to snap them.

The intake manifold gasket looks like a Fel-Pro print-o-seal. They are known to fail like that.

 
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They do also make stud extractors like an ez out for studs. You will not break like an ez out. I do not have a set never needed them. It cams in and locks on the diameter of the stud. Yes heat breaks the bond of loctite. 

 

ianpala

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Got them out with some torch to the pedestal!   Does anyone have an intake manifold gasket they swear by? I got the same type (Fel-pro 1240) and am hoping with Don's torque specs and absence of Silicone they will do better this time.  Also, looks like my head Gaskets were leaking between my cylinders which is kinda bananas bc they are only about a year and a half old.  Hoping it was due to erroneously using copper coat on newer blue Fel-pro Permatorque head gaskets that ask for no additional sealers.  Having the heads checked out by Burbank Speed and Machine to check for warping etc...   Definitely looks like the intake gaskets were leaking due to the coloring of the corresponding combustion chambers.   And looks like the rest were running rich due to that 4 size jump in the primaries! What a mess. 

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c9zx

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Printo-leak gaskets are worthless. I've been using SCE or Cometic the last few years with no problems. Chuck

 
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+1 on what Chuck said. I have also been using Mr. Gasket intake gaskets with no problems. I think the part number is #214. 
Make sure you get the head gaskets on correct, they are easy to put on backwards.

 

Don C

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Felpro has definitely gone downhill. The last time I used Mr. Gasket they were still good.

I would put a straight edge across the block, make sure you don't have a problem there.

 
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