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I thought I changed this?
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1,213 Posts
Discussion Starter #1
Needing fairly urgent help on this !!

Just changed out the cam on my stock 302 for a Thumpr.
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=891&sb=2

Changed the valve springs too, and the timing gear.
I had it running fine, but the pushrods were too short, so I ordered oversize ones.
Since I put them in, it would run ok, but wouldn't idle.
I never run it long, because even the oversized rods were too short, so it barely got off choke. Ever since I got the car, it's been fine when cold, and when warm, but at that spot in the middle, it's always been a bit of a pig. I could have adjusted the choke, but I heard many horror stories about Edelbrock, so decided that what wasn't broke didn't need fixing !!
That may have been why it wouldn't idle then, because it never got hot.

I was offered a set of quench heads, so I took them, and had them milled, and converted to 7/16" screw in studs, guideplates, and hardened pushrods.
I'm running 1.6 roller rockers
Set up the valves properly, set it to zero lash, then 1 turn, and tighten it up.
I finished that job this weekend, but when I tried to start the car, it just won't run right.

It won't idle, it can be kept at idle with throttle, but as soon as it's given gas, it starts "hunting", and then backfires through the exhaust.
Strangely tho, when I applied load to it, to move it into the garage, it didn't backfire, or stall out, but I certainly wouldn't describe it as "responsive"

I've checked the cam timing, and checked it again, and it was all fine.

I tried to stab the distributor in at tdc on #1, but then if I turn the distributor clockwise, it cranks irregulary, like the timing's off.
I also tried setting it to 6 degrees before tdc, then stabbing the dist, and while it's a little better, it's still not enough to make the difference.

Got me scratching my head, and I'm starting to run out of time, I need to get the car running this week so I can get it off for inspection.

It's got an MSD blaster coil, and converted to breakerless ignition.
Edelbrock 4 bbl, with performer manifold.
Since I haven't built my exhausts yet, it's still running open headers, I don't know if that's a factor ?

I keep coming back to timing, but, as I say, I've checked, and checked again.
The only thing I can think of is that on the timing gear http://www.compperformancegroupstores.com/store/merchant.mvc?Store_Code=CC&Screen=PROD&Product_Code=2135
There are 3 possible keyways to locate the timing.
IIRC, o degrees, and 4 degrees advanced, or retarded.
I didn't degree the cam, but I'm wondering if I should maybe have had it set to 4 degrees advanced, rather than setting it to 0
Could it be that 1 turn of the adjusters is too much, and the valves aren't closing properly ?


Any help gratefully appreciated.
Bob
:beer: :beer:
 

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Premium Member
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13,577 Posts
Thump'r is a strange animal, so it may act differently and "normal" observations/advice not apply. Which Thump'r did you buy? The mildest one, the mid-one or the most radical one? They all are stated to have low vacuum at idle, but this is not your problem (yet).

I will go out on a limb and say exhaust (or lack thereof) may be a big contributor to your issues with a cam like the Thump'r with a lot of overlap. I would slap a pair of mufflers on the collectors (temporarily) and see if this changes things.

On the valve adjustment, I have always run 3/4 turn from zero lash, my buddy who owns the machine shop likes 1/2 turn. 1 turn should be fine too, as long as you are not bottoming out the plunger travel in the lifter (and keeping the valves open thereby).

Regards,

Bob
 

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I thought I changed this?
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1,213 Posts
Discussion Starter #3
Thanks Bob. :wave:

I had wondered if the lack of exhaust could be a factor. I'm trying to remember if I took the exhausts off at the same time as doing the first pushrod swap, but I can't recall !!
I know for definite when I fired it up after the cam change, I had exhausts on, because I have a video.

The Thumpr is only a fairly mild one, it's got 107 overlap, but it's designed to work with a stock convertor, so it should be fine.

I might whip the valve covers off tomorrow and try it with 1/2 to 3/4 turn, and see if it makes any difference too.

I guess I could learn to live with the odd backfire if I could only get it running properly !!
 

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I thought I changed this?
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1,213 Posts
Discussion Starter #4
Ok, gonna go down to the car today and stick the exhausts on, and see what happens.
Also, thinking about the vacuum, it's got a pcv which is worked by vacuum, so I think I'll disconnect that and blank it, see if that makes any difference. It's a pretty big hose, so it's probably sapping much needed vacuum from everywhere else.

Will post up the results later, meantime, if anyone else has any other advice, it'd be much appreciated in case my work today doesn't make a difference.
 

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This maybe a dumb question....Are you running 351 firing order? I see the cam is for 1969-85 W.
 

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I thought I changed this?
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1,213 Posts
Discussion Starter #6
I solved it !!!

Turns out it was a doofus problem !
Seems when I reconnected the distributor, I managed to swap two of the wires !! I had marked them, but the marks had smudged.

Swapped them over, and hey presto, off it goes.
Got it running no problem now, and timed it by ear, so I'll stick the strobe on it tomorrow. Building the exhausts took a little longer than planned !!

Thanks for all the advice guys, good to know that there's experts on hand when you need them.
You can all treat yourselves to an extra chocolate biscuit tonite !!! :wave:
 

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Glad to hear it!! :beer:
 

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This same thing happened to me. We were changing the headers, three of us. One guy did a side on removing the plugs and putting back. Somone got distracted and there you have it. Plus when you get in your 50's you don't see as well. Stuff happens.
 

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Looks and sounds mean. Probably not too many Cougars like yours in Scotland. Congrats.
 
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