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Old 06-17-2012, 12:44 AM
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stumbling in high gear, low RPM , sounds like metal and rocks

So Ive been tuning everything on my engine to get it right, and trying to listen to advice Ive received here.

I noticed when I upshift into 3rd or 4th , and slow down( back off the gas only, not when I apply brakes) to low RPM - say 1,000 - 1,500ish

My car starts making a sorta clunking sound.. like bolts and rocks being thrown around inside the trans or crankcase. I mean what my mind pictures is if my crank shaft were broken and slapping inside of my engine or something) At first I thought it was my built 700R4, so took that out and brought it to Bowtie overdrives. they took it apart, checked it out, rebuilt it, and tested it in their trucks, their respoinse, nothing wrong with it..

So I started thinking its my engine.

Each time this has happened ( the noise) and the car feels sluggish, I just hit the brakes and slow down, or rev it up and it goes away..

But last time I was thinkin.. wtf! im just gonna let this cruise and see what happens for a sec. so I did

The car started to sort of lurch back and forth like it was stumbling, then I stepped on it, and it kinda choked, then revved up fine..

This is what it seems to me, the car being under load ( low RPM, high gear) is where the problem is.. when I hit the brakes, Im guessing it slows down the engines work, since its not trying to push the car anymore.. or rev it up, the load is gone...


I read somewhere this could be an ignition issue..?

Car is a carbed 350 chevy. racing heads, mild cam 236 @ .50 duration, headers, 3,000 RPM stall, 700r4 trans, edelbrock performer air gap intake, 24 degrees base timing non vacuum advance dizzy
any thoughts??

thanks guys
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Old 06-17-2012, 07:22 AM
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A combination of carb, cam, rear gear. The cam is not designed for low rpm cruise. You are "lugging" the engine. This sets up harmonics in the drive train.
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Old 06-17-2012, 07:39 AM
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Your engine is most likely detonating from too much advance at the lower rpm. If you keep running it like that you will destroy your engine!
I read one of your earlier posts about timing. Did you install the 10 degree advance limit bushings(2ea)? That Fbird recommended? 24+18=42 Way to much. Should be around 34-38 max. You said something about 40ish timing being ok with vacuum advance. Thats only with a very light load as the vacuum drops when you open the throttle, and the advance is then dropped to the initial+mechanical setting. Since you don't have vacuum advance on your distributor, you should never get over 38 tops.
Just to verify that its a timing issue, drop your initial down to say 16-18 and see if it still acts up at low rpm in 4th gear. If it doesn't act as you described then you know it had too much timing earlier.
As far as the timing jumping around, it could be loose springs in the mechanical advance mechanism or just sticking advance weights. Could be a sloppy timing chain. Could be worn out gear on the end of the distributor, worn out bushings in the distributor, balancer outer ring shifting on the hub,
bad timing light, or a combination of any one of these.
A decent test for the timing chain or distributor gear/bushings, or both is to remove the distributor cap, use a wrench or some other means of rotating the balancer back and forth a small amount, and see if the distributor rotor moves instantly as the balancer. Any delay in movement between the two means there is slop somewhere(gears/chain/bushings, or combination of all).
Not to make it too confusing, but you may also have the wrong heat range plugs. If they have too high of a heat range they can cause detonation or pre-ignition. After doing the previous test for slop in the chain/distributor, and it looks ok, then try using a lower heat range plug. I bet if you inspect them now, you will see little black specs on the insulator from detonation. Don't go to a lower heat range plug until you verify that the timing is correct.
Hope it helps,
ssmonty

PS Whats your compression ratio?

Last edited by ssmonty; 06-17-2012 at 08:07 AM.
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Old 06-17-2012, 08:19 AM
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I see Bobcrman beat me to it. Thats what I get for typing so slow.
Anyway I thought that I'd mention that a decent way to set timing is to advance it until you hear detonation in 4th gear accelerating from a low rpm with WOT(high load). Back off throttle as soon as you hear detonation. Reduce initial timing and try the acceleration test again. Keep adjusting initial timing and the advance springs to get the quickest advance curve until you hear detonating while accelerating from low rpm in 4th gear.
FWIW
ssmonty
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Old 06-17-2012, 08:57 AM
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Agree with ssmonty.

Detonation. Put some 93 octane gas in it and see if the problem clears up a bit.
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Old 06-18-2012, 12:53 PM
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Quote:
Originally Posted by ssmonty
Your engine is most likely detonating from too much advance at the lower rpm. If you keep running it like that you will destroy your engine!
I read one of your earlier posts about timing. Did you install the 10 degree advance limit bushings(2ea)? That Fbird recommended? 24+18=42 Way to much. Should be around 34-38 max. You said something about 40ish timing being ok with vacuum advance. Thats only with a very light load as the vacuum drops when you open the throttle, and the advance is then dropped to the initial+mechanical setting. Since you don't have vacuum advance on your distributor, you should never get over 38 tops.
Just to verify that its a timing issue, drop your initial down to say 16-18 and see if it still acts up at low rpm in 4th gear. If it doesn't act as you described then you know it had too much timing earlier.
As far as the timing jumping around, it could be loose springs in the mechanical advance mechanism or just sticking advance weights. Could be a sloppy timing chain. Could be worn out gear on the end of the distributor, worn out bushings in the distributor, balancer outer ring shifting on the hub,
bad timing light, or a combination of any one of these.
A decent test for the timing chain or distributor gear/bushings, or both is to remove the distributor cap, use a wrench or some other means of rotating the balancer back and forth a small amount, and see if the distributor rotor moves instantly as the balancer. Any delay in movement between the two means there is slop somewhere(gears/chain/bushings, or combination of all).
Not to make it too confusing, but you may also have the wrong heat range plugs. If they have too high of a heat range they can cause detonation or pre-ignition. After doing the previous test for slop in the chain/distributor, and it looks ok, then try using a lower heat range plug. I bet if you inspect them now, you will see little black specs on the insulator from detonation. Don't go to a lower heat range plug until you verify that the timing is correct.
Hope it helps,
ssmonty

PS Whats your compression ratio?
thanks for the help

After seeing the post about timing from F-bird, I looked at the link for the advance bushings. Ive spent so much money I didnt want to spend $40 on 2 metal bushings, so I made my own.
I tried your advice by dropping my initial timing to around 18, and detonation is gone.
THANKS!!!!
so Im also assuming after reading the last post, if I want my timing a little higher, gonna have to run higher octane gas

As for the timing bounce, the distributor is brand new. probably has about 500 miles on it. I tried your method, which I had also just recently read about turning the crank, and watching the rotor. I see instant movement when I rotate the balancer/crank. so it cant be a loose chain, or worn timing gear etc...

I tried separating plug wire 5,7 a little more, and It didnt really change anything.


For the plugs, When I pulled them, before doing all the affore mentioned stuff, they were like a bright orange color ( which I thought was odd)

Compression ratio im guessing around 9.5:1 or so

So now, Ive resolved the detonation issue, and still have the timing bounce issue

however.. AHHHH a new problem...
now after all this and running my car a little bit hard for 2 quick runs,
In idle, and when I go to reverse or drive( just backing up or pulling forward a little in my driveway).. almost feels like a faint vibration or metally noise coming from inside.. I cant hear it outside my car, only inside because the engine and trans are so close to the floor. ALso I can hear it through my exhaust.. to me that means internal...

Do you think my base timing, with no load, is too low causing this, and when I bump it up, its fine, but under load its causing detonation..??

This new problem has me stumped even more now. The timing bounce is my next problem to solve - is it possible it could be caused from my crane cams ignition box?
thanks aagin

Last edited by Ocean-Angler; 06-18-2012 at 01:00 PM.
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Old 06-18-2012, 03:46 PM
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Just a thought,
It sounds like a new build? When you initially installed the distributor, did you do it without the gasket first to make sure that it wasn't bottoming out on the oil pump drive shaft? The distributor should be test fit by installing it without the gasket at its base, and it should sit flush with the manifold. That verifies that you have enough clearance after adding the gasket to keep from pushing the oil pump gear down onto the oil pump cover plate(where the oil pick-up is mounted), and cause the gear to bind/jump around, and can cause the timing to jump around and possibly make the noise you here.
I know it can be a pain to remove/install the dist. in some cars, but if you didn't test fit it without the gasket its worth trying IMO. If it does sit up off the manifold, you can install an extra gasket to raise it up some.
As far as the plugs color, I'm colorblind and never was any good at reading plugs. Sorry.
FWIW
ssmonty
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Old 06-18-2012, 04:16 PM
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Something I should have mentioned earlier is that sometimes the distributor won't bind the pump until you tighten down the distributor clamp. You can buy a distributor shim kit from Summit PN MOR-26150 for about $5. It has 3 sizes of shims, .030,.060, & .100 inch. just for the purpose of raising the distributor.
Its not an uncommon occurrence when installing new heads/intake manifold, or the block has been decked.
Just don't get carried away and use too many or you might raise it too high, and not have enough engagement with the pump driveshaft.
ssmonty
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Old 06-19-2012, 01:45 AM
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thanks for the tip
Ill try that next.
yea the whole top end is new -
new intake, new heads, new dzzy, new cam and lifters. I know when i dropped the distributor down in, it was a total pain in the *** to line up and grab the notch in the oil pump. But when it did, it dropped right down ( although that was with the gasket in there)
so even though it seemed to drop right in, that gasket mighta been hiding that it ws bottomed out?
ill try removing the gasket, and reseating it and see how it fits.. it shooouuuld fit.. but ill check and let u know how that goes..

yea the new noise is terrible and annoying..
to rule out horrible scenerios -
if the crank was warped, or rods were.. or I had some kind of busted bearings.. I mean had serious engine damage that would require me to remove the pan and inspect the block, wouldnt the sound be more noticable at high rpms, or at diff rpms, and not just low rpms.. right?

Im just hoping I dont have to take this apart... I have to remove my engine to remove the pan.. and I cant do that here in this pick anal HOA plkace i live in
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Old 06-19-2012, 07:12 AM
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Quote:
Originally Posted by Ocean-Angler
thanks for the tip
Ill try that next.
yea the whole top end is new -
new intake, new heads, new dzzy, new cam and lifters. I know when i dropped the distributor down in, it was a total pain in the *** to line up and grab the notch in the oil pump. But when it did, it dropped right down ( although that was with the gasket in there)
so even though it seemed to drop right in, that gasket mighta been hiding that it ws bottomed out?
ill try removing the gasket, and reseating it and see how it fits.. it shooouuuld fit.. but ill check and let u know how that goes..

yea the new noise is terrible and annoying..
to rule out horrible scenerios -
if the crank was warped, or rods were.. or I had some kind of busted bearings.. I mean had serious engine damage that would require me to remove the pan and inspect the block, wouldnt the sound be more noticable at high rpms, or at diff rpms, and not just low rpms.. right?

Im just hoping I dont have to take this apart... I have to remove my engine to remove the pan.. and I cant do that here in this pick anal HOA plkace i live in
No need to remove the distributor to see if there's some clearance between the pump and distributor!

Remove the cap and rotor, then lift up on the plate that has the weights and springs attached to it. If there is up-and-down movement w/the clamp tight you're OK. This is also covered here.
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Old 06-19-2012, 06:11 PM
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Thanks Cobalt!
My bad. I see after reading the wiki that what worked for me wasn't the right way to check it.
To the OP, you never said what your oil pressure was at the rpm you heard the noises. I have to point out that if you ran it long with detonation you may have pounded the bearings so much that now they have more clearance than they are supposed to and that will lower your oil pressure and possibly cause some noise or worse. Hope thats not the case.
Also you need to try a different timing light to see if it has the same bouncing around. Some lights don't like ignition boxes.
Sorry if I caused you any inconvenience!
ssmonty
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Old 06-19-2012, 07:04 PM
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Quote:
Originally Posted by ssmonty
Thanks Cobalt!
My bad. I see after reading the wiki that what worked for me wasn't the right way to check it.
I think the way you described checking the distributor height is fine for an engine at mock up or assembly time, or if the distributor is out for some reason. It's just not the easiest way if the distributor is already installed. But it's not like it was bad info or anything.
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Old 06-20-2012, 05:23 PM
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thanks for the info guys
I tried checking the play by pulling up on the whole weight, advance plate assembly, and there is a very minimal amount of up and down movement.. I can hear it sorta click when I slide it up and down.. theres some play in there.. Im guessing like.. idk ten to 20 thousandths or somethign.. just minimal amount, but nonetheless there is play
Oil pressure.. ahh crap.. not sure, I havent put my gauges back in yet, the whole interior is gutted.

Ill post a couple pics to get an idea of whats going on so we can figure this out
thanks a ton guys
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Old 06-20-2012, 06:30 PM
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also, whats weird about the timing/detonation issue -
After listening to u before, and lowering my initial timing, the detonation went away..
But I lost throttle response
Also, should it detonate - I have a 3,000RPM stall, 3.55 gears and only 26" tall tires... wouldnt that have helped to begin with?
almost wondering If I should go witha vacuum advance, to get my low initial timing, then have the right timing in 3rd or 4th when I back off the gas..
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Old 06-20-2012, 09:36 PM
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Quote:
Originally Posted by Ocean-Angler
thanks for the info guys
I tried checking the play by pulling up on the whole weight, advance plate assembly, and there is a very minimal amount of up and down movement.. I can hear it sorta click when I slide it up and down.. theres some play in there.. Im guessing like.. idk ten to 20 thousandths or somethign.. just minimal amount, but nonetheless there is play
Oil pressure.. ahh crap.. not sure, I havent put my gauges back in yet, the whole interior is gutted.

Ill post a couple pics to get an idea of whats going on so we can figure this out
thanks a ton guys
I may not have been clear enough on this: If you are lifting up on the rotor or the "ears" that the rotor is attached to, you may well be feeling the clearance between the part of the distributor that has the reluctor wheel and the main shaft- these two parts are separate and have clearance between them that can be easily mistaken for the distributor shaft-to-oil pump play. You need to lift up on the very topmost plate under the rotor that has the weights and springs attached to it.

In any event, 0.010"-0.020" clearance is not enough, I suggest you don't drive it until you get a shim under the distributor as described in the link in my first post, above. You can use an extra distributor gasket (as many as it takes to have some clearance w/the clamp tight, 0.030"-0.060" is fine and you can cut your own from paper gasket stock or even make your own plastic shims from a coffee can lid, etc.) until you get the nylon shims.
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