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Rough Engine, High Idle *calling all carb geniuses*

Discussion in 'Shop Talk' started by TexasDoc, Nov 19, 2011.

  1. TexasDoc

    TexasDoc Member

    I am having two problems or two symptoms of the same problem. I just rebuilt my first engine - 351W. Comp Cam XE274H (specs here), AFR 185cc heads, Holley 650cfm Street Avenger (vac secondary).

    The engine starts easily, but runs rough. It idles at 1700 when choked. However, I cannot lower the curb idle once the engine warms up. The curb idle screw is backed completely off the throttle linkage but the car still idles at 1200rpm. The throttle seems to have full travel without binding. Fuel level is set just below the sight windows. Fuel pressure is 5. I have tried to adjust the fuel mix screws, but it doesn't seem to make much difference so I set them at 1.5 turns out. I have tried adjusting the secondaries to have a "square transfer circuit" (basically just a pinhole size). It still idles fast, even when warm.

    Secondly, the engine runs a little rough at idle. The vacuum is reading approx 15 but rapidly "flutters" between 14-16. If I increase the throttle, the engine is smooth, but at idle, it is rough.

    I got this carb new from Summit, but I think it is a "returned item". The literature that came with the carb says that it is set from the factory, but when I got it, one idle mix screw was out 1/2 turn and the other was out 2 turns. The transfer circuit was open about 1/4 inch. I've studied various places on the net looking for answers, but haven't found any yet.

    My next step is to confirm no air leaks. I don't have anything connected to the carb except the vacuum gauge (no brake booster, vac advance from dist is plugged, no PCV). Gonna do the carb cleaner spray trick. Any other ideas from you carb guru's? I am hoping both of these symptoms are from the same problem rather than two separate issues. If I can't find an air leak, I'll pull the plugs and check them, but they are new with less than 30 minutes total run time.
  2. I had the same problem with a carb from Summit, it had obviously been used, was in the wrong box and out of tune. The problem I had with the idle sounds like yours, would not go lower than about 1200 rpm. The problem was that the choke idle cam was hitting on the carb body and not allowing the butterfly to close completely. I had to take the carb off and look fron the bottom to see this. Just had to bend a tab a little bit to allow it to close all the way.
    Hope this helps
  3. TexasDoc

    TexasDoc Member

    Thanks. Any ideas about the rough idle?

  4. Roush #1

    Roush #1 Member

    Sounds like a vacuum leak to me. Carb spacer involved? If so, make sure the carb gasket fits the spacer fully. Some spacers are hollow on the bottom side and the carb gasket may not be sealing adequately.
    Also check the base of the carb and the spacer to insure neither is warped.
    Worse case scenario is an intake manifold gasket. Putting a small strip of paper over the oil fill hole with the engine running will give you an idea if it's a intake gasket. The paper drawing in consistently over the hole is an indication the intake gasket is leaking.
    A PCV valve tester would be the tool to use to check for an intake gasket leak, but the paper will work as well.
    Hope this helps.
  5. TexasDoc

    TexasDoc Member

    Couldn't find a vacuum leak. No carb spacer. Carb (Holley avenger 670cfm) is directly attached to the intake (Edelbrock performer RPM airgap). With the engine running and warmed, I set the timing at 12 deg on the dizzy. I have the transfer slot squared. All the vacuum ports on the carb are plugged except the small one where the vacuum is attached.

    I ran the carb cleaner spray all around the carb without noticing any change in RPM. The only time I noticed any change was when I sprayed the carb cleaner directly down the air horns and it bogged and almost died. I took off the carb and checked - no binding of any of the arms or connectors.

    With the transfer slots squared, I couldn't get it to idle below 1200RPM. Even with the curb idle (hot) screw backed all the way out, the idle was at 1200.

    Roush: I'm not sure exactly what you are describing. You state to put a piece of paper over the "oil fill hole". D you mean the fill hole on the valve covers where I pit the motor oil in? I don't think that this is it because I can see smoke coming out of those holes on both sides. I sprayed carb cleaner all around the carb and along the top of where the intake sits on the heads on both sides - no change in rpms. I didn't spray along the front or back of the engine where the intake sits on the block. I thought I put down a good bead of silicone before attaching the intake.

    FWIW, I don't seem to notice much affect on the vacuum when adjusting the idle mixture screws until one or the other is mostly screwed in and then the engine wants to die. Being a carb noob, I can't figure this one out.
  6. Prof

    Prof Member

    What rockers did you use? How did you set lash?
    Sounds like what happened on my AFR heads when I had rockers too tight. You gotta adjust them with the engine idling (oil goes everywhere).
    What procedure did you use?
  7. TexasDoc

    TexasDoc Member

    I posted a video on Youtube of the engine running. http://www.youtube.com/watch?v=IVFFvMIy6d4&sns=em

    Rollers from Summit. Proform rollers similar to these: http://www.summitracing.com/parts/PRO-66911/

    Hydraulic lifters. For lash, I tightened them while rotating the pushrod until the pushrod started having tension. Then I tightened 1/2 turn tighter.

    On the video, you can hear a clicking. That may be a lifter or rocker. That wouldn't make the idle fast tho, right. Maybe the roughy idle, but not the fast idle, right? I will check the rockers...
  8. Lakeway Cobra

    Lakeway Cobra Member

    It is tough to tell sound on videos but to me, it sounds like your valves are way out of adjustment which would cause erratic running if way out. When you said you adjusted them, did you do so individually while the lifter was at the bottom of the cam lobe? See the following instructions. I do mine this way and have no problems.

    "To adjust preload on hydraulic lifter camshafts with
    adjustable rocker arms, turn the engine in the normal direction
    of rotation until the exhaust lifter starts to move up, then
    adjust the intake valve to zero lash with no preload, then 1/2
    turn more. Turn the engine over again until the intake
    opens completely and then is almost all the way back down.
    Now, set the exhaust valve to zero lash, then 1/2 turn
    more. Continue the above procedure for each cylinder until
    all valves are adjusted to the same amount of preload. This
    procedure will give you the correct lifter preload for any
    hydraulic lifter cam with adjustable rocker arms."

    As for the high idle, the squared method doesn't always work on the secondaries. Try closing it down a little and see if that helps. Make sure your valves are adjusted properly first though. My guess is that could affect vacuum which will throw a lot of things out.
  9. TexasDoc

    TexasDoc Member

    I did adjust each rocker with the cam on the base of each lobe. I will check them again...
  10. Lakeway Cobra

    Lakeway Cobra Member

    I seem to recall the XE cams are known for making some clatter. Something to do with the very fast ramp rates it has. At least that is what I remember from when I did research on cam selection. If you find your valves are adjusted right and still make noise, that could be what it is.

    This is a flat tappet cam and not roller correct? Any chance the break in procedure did not work out correctly and you have a flat lobe? Don't want to alarm you but this happens often with flat tappet cams unless the break in procedure is followed exactly.

    Where are you located? I bet there are some guy's that would come by to take a look/listen if close by.
  11. TexasDoc

    TexasDoc Member

    Yes, it is a flat cam. My block is a 1982 so rollers weren't an option. I followed the break-in procedure exactly and don't think I flattened a lobe.

    I took off the valve covers and some of the rockers were visibly loose. I tightened them back up using the correct procedure, ensuring they were on the base lobe for each one. I double checked them again before putting the covers back on.

    I ran out of daylight but just got to start it. It didn't idle very well. Time to go back and check the timing, mixture screws, and idle screws... Family tomorrow, work Friday. Hopefully I will get to work on it some more Saturday.

    (I am in Keller, TX)
  12. Lakeway Cobra

    Lakeway Cobra Member

    Sounds like you are doing everything right. I had some problems with idle when I first built my motor and it drove me nuts. Finally returned the carb for a brand new one (the one I had was a factory rebuild) and that solved my problems. Hopefully yours is something simple like that.

    I'd go ahead and close the secondarys till you just barely see the slot and see if that helps the high idle at all. It won't fix the rough idle though.

    Good luck. I'd come by but Austin is a long way from Keller!
  13. TexasDoc

    TexasDoc Member

    Something more is going on... Remember... flat tappet cam with hydraulic lifters.

    Ever since I tightened the rockers, the engine will not run. Pulled the plugs and checked the compression - #2 is at 195, the rest at 170-180. However, with the spark plug in #2, there is a hissing sound that is internal to the engine/intake when #2 is on the compression stroke. I don't get the hissing with the spark plug out. I double-checked the rocker lash yet it still leaks. Next thing I'm going to try is to set the lash at zero without the extra 1/2 turn. I'm also going to try the soapy bubbles around the head and the spark plugs to see where it is leaking.

    If that doesn't work, I guess I'm going to have to pull the head. I don't think it is a ring - compression is good. I don't think it is a head gasket either. I don't think it is a wiped cam lobe or it wouldn't open at all, right? Don't think it is a bent rod - that would prevent it from opening, not prevent it from closing all the way, right? Seems like it is a valve.

    Any other thoughts or suggestions?

  14. Lakeway Cobra

    Lakeway Cobra Member

    Hopefully some others will chime in with more engine experience than me. I can give you some opinions, but take it for what its worth as I only speak from experience working on my own cars and certainly am not an expert.

    I feel that you can wipe a lobe and it will still open and close the valves. Just not accurately.

    I have also seen (heard) new motors make a very slight hissing sound after shut down. I've heard this coming out of the headers and assume it is just the compression leaking down before all the internals have properly seated. The car I've heard do this stopped doing it after about 500 miles.

    I would also guess that your compression check may not be that far off for a new motor. Perhaps you need some more time on it before everything is fully seated for an accurate compression test/comparison. I could be wrong on this as it is just an amateurs hunch.

    There is an engine builder on Club Cobra who I understand is very helpful and willing to talk through things with owners. Look up B2 Motorsport. I think it is Brent Lykins or something like that. I'd find someone of his caliber and start asking questions.

    My gut tells me that your cam did not break in properly. Do some searches on the XE series cams and break in failures on flat tappets. I seem to recall they can be prone to being very hard on the cam with those fast ramp rates. I may be way off on the guess though.

    Good luck.
  15. TexasDoc

    TexasDoc Member

    Yes, 3 wiped lobes. Grrrrrrr.
    Gonna install a roller conversion.
  16. Lakeway Cobra

    Lakeway Cobra Member

    Sorry to hear that. At least you have the problem isolated.

    I have a comp cam magnum 280h flat tappet cam and very happy with it. It is a good choice in the event you don't want to spend the $ on a roller conversion. I used Gibbs break in oil and ran the motor at 2,500 RPM for 30 minutes for the break in. It is an easier cam to break in and less strenous on the valvetrain.
  17. TexasDoc

    TexasDoc Member

    I think I'm going to go the roller route. I did all the break-in stuff too. I would rather spend the $ and not have to deal with it again.

    Anyone with suggestions on which cam? I'm looking for a daily driver with no track plans. A little bit of a rough idle is fine. Comp has the XR276 which is similar to the one I had, but I'm now a little scared of the XE series. Do I need to be worried about the cam with a roller setup?

    I haven't researched other brands yet either.

  18. Prof

    Prof Member

    I watched your video. Good documentation on everything.

    Check your ROCKER adjustments and inspect your DISTRIBUTOR gear for compatibility with roller (steel vs. bronze).

    Good luck.
  19. TexasDoc

    TexasDoc Member

    Thanks, Prof, Lakeway, and to all who have helped.

    I have a Proform distributor that has a steel gear. Researching now what the cam will need. Looking at Comp's website, they are really pushing the composite gears (>$100) but most of the forum posts state I need a mealonized iron gear. Yet when I look, they don't have any iron gears for sale. I also don't know the shaft diameter of my dizzy, so not sure which size to order if I need to change out the steel gear that is on it.

    Hopefully I can swap the cam without removing the heads. Anybody know if I have to removed the oil pan, pump, and oil pump shaft before I take the cam out. I don't think so. Since the pump shaft doesn't directly engage the cam, I shouldn't. The pump shaft connects to the bottom of the dizzy gear.

    Still researching...
  20. Lakeway Cobra

    Lakeway Cobra Member

    Here are some cams I would be looking at....these are just links to the cams. You'll have to cross reference for lifter kits and make sure they are compatible with your upgrade to roller style with your application....






    Maybe even the Ford E303.

    I personally like the comp cams magnum series and I know of a few cobra guys running the Trick Flow cam and like it.

    You won't need to remove the pan or the heads. Just the intake manifold, water pump, etc. Good point on selecting the right distributor gear. Very important.

    Additional edit.... Prof's suggestion of checking your rockers is important. I'm assuming he means the pushrod length to make sure the contact point stays within the specs. Do you have an adjustable pushrod to double check?

    Also, I always thought that steel gears were for roller cams. Iron gears for flat tappets. ???
  21. Prof

    Prof Member

    Cam choices: Lobe separation---

    Looking back, I wish someone had told me that I might do stack injection someday. That day is here. Now I have to break open finished engines (Keith Craft 331 stroker and 351 race motor) to change one small thing on the cam...

    If you can go with a 112 degree or 114 degree cam, you will have compatibility to ANY induction system you go with (carb, EFI, forced induction, etc.). Going from a 109 degree to a 114 degree cam takes away some of the rumpity-rump and shakity-shake, but--like I said--if you ever go another direction--you will need the 112/114 for fuel injection.

    I won't go into the many months I've investigated this and the hours I've spent on the phone with pros.

    Wish someone had told me that back when the motors were being built...

  22. Barnsnake

    Barnsnake Member

    You might want to drop the pan anyway to clean it. Checking a rod bearing and a main bearing might be a good idea as well. All the junk from those lobes went somewhere. Hopefully the filter caught it all and the oil pump didn't get any damage.

  23. TexasDoc

    TexasDoc Member


    Pulled the heads. Head gaskets were mush. Literally. Not sure what caused the composite sandwich layers to turn to mush, but I had a significant leak/hiss in all cylinders. One guy (self-proclaimed race car engine builder) told me that this happened due to my timing being off - unburned hydrocarbons in the cylinders ate away the composite material. Glad I took them off. Got some Cometic gaskets (MLS) to replace them with.

    Went with the roller setup. Got the Comp Cams XR276 roller with retrofit roller lifters (with dog-bones and spider). Degree'd the cam to 106 (had to put it at 4 deg retard on the timing sprocket to get it right). Reinstalled the timing cover, water pump, and harmonic balancer.

    Oil pan is off and cleaned - no significant debris. Rotating assembly looks good. Cylinders look good. Pushrods, pistons, and bushings look good (didn't disassemble them, just wiggled and they feel snug). Oil pump and pickup appear intact - (I will test with a priming tool before restart).

    Now I need to reinstall the heads, intake, carb and fuel lines, alternator, radiator, fan and hoses, headers, exhaust, distributor/wires/plugs, and rewire everything, prime, then try to restart. Easy. :lol:

    Currently I'm investigating whether or not I can use my current distributor gear with the new cam. It is a Proform HEI distributor with a steel gear. But that's a topic for another thread...

    Thanks everyone for their help. Tom

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