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View Full Version : Stumble at 3,000 - 3,300 RPM - Need Help - Driving Me Crazy!



bigboi
11-11-2007, 11:27 AM
I have a stumble when I am criusing in 3rd gear starting at 3,000 RPM. When I am in 3rd gear at around 2,500-2,600RPM it's smooth and consistent.

I have a Holley 650 Double Pumper w/ Mechanical Secondaries. It was just rebuilt by The Carb Shop so it should be working good.

One thing I have to deal with is the timing. The previous owner didn't have a white mark on the Harmonic Balancer, but there was a black permanent marker line on the Harmonic Balancer where there is an engraved line by the manufacturer that says TDC. I put a white line on it and put a white line on the timing tab.

When I fire up the engine and hookup the timing light I have to dial the timing light dial to about 40* before the two white lines come together. This seems way high for timing at idle. When I turn the distributor clockwise to retard I can't get it down to around 10* because the engine dies. This must be because the engine timing is being retarded too much. So long story short I don't think the TDC mark on the Harmonic Balancer is correct.

I played with the timing and each time I test drove the car. I got it where "I think" the timing is Ooookay - this is my feeling by listening to it and the throttle response crispness. The exhaust pops when I let off the gas, but I think this is just cold air coming back into the exhaust (I'm running 3" pipes w/ 1 Chamber Flowmasters and they dump at the axle). Acceleration is hard and crisp which is good.

But... I am still wondering why the hell I am stumbling cruising at 3,000 RPM. Could it be that my mechanical secondaries are kicking in at 3,000 RPM? Is this causing it...

Dude... I am at a loss here... Any help is appreciated.

Project69
11-11-2007, 11:34 AM
Do you have the right timing tag for that harmonic balancer? Just checking.

bigboi
11-11-2007, 11:38 AM
I thought all timing tabs were pretty much standard. 2 bolts hold the tab on all chevy big blocks. Right?


Do you have the right timing tag for that harmonic balancer? Just checking.

Project69
11-11-2007, 11:53 AM
Yes, but the TDC mark was moved 8-9 degress on the damper so you would need the tab for that damper. They moved it in 1970, so if you have a 1970 model damper then you need the newer tab. Get my drift.

bigboi
11-11-2007, 01:45 PM
Gotcha. Even if I add the 8-9 degrees that my tab is off. It still looks like I am at 30* initial - too much huh?

Here is a question... If I retard the timing until it dies. At what degree does the engine usually die? Maybe I can get an idea of where my timing is at when I can get that data point figured out.


Yes, but the TDC mark was moved 8-9 degress on the damper so you would need the tab for that damper. They moved it in 1970, so if you have a 1970 model damper then you need the newer tab. Get my drift.

Project69
11-11-2007, 01:57 PM
It usually should be around 4-6 degrees before TDC. Thats what my 327 is set at. Im not sure when the engine might die though. You did unplug the vaccum from the distributor when you checked timing right?

Just trying to get you a steady base with your timing issue. But your stumble sounds like a carb. issue to me. Your getting more fuel then needed at that RPM range then normal.

Shaker455
11-11-2007, 02:41 PM
Before you start anything it's a "MUST" that you know where your timing's at.

Pop off the drivers valve cover and take note of the line on the balancer as it closes in on the tab.

Pop your dist cap and mark the spot where #1 lines up on the cap.

When you come up on zero the intake and exhaust valves will be closed and there will be almost no pressure on the rockers.

Striker on rotor will be in line with #1 on the cap.

bigboi
11-11-2007, 05:41 PM
Welp... I took the car out and romped on it tonight and now I got a clicking, knocking, slightly screechy sound coming from the motor... Sheeeeeeeet. Something is messsssed up.

Gonna pop the valve covers and hope its a bent pushrod. If it's topend I can fix easily. If its bottomend I'm in trouble.

Project69
11-11-2007, 06:04 PM
d@mn that sucks. If its not one problem its another. Dont you love this hobby haha.

bigboi
11-11-2007, 09:35 PM
yeh it was a bummer man...

i dropped it into second to get on the freeway, hammered it, slapped it to 3rd, hammered it, and caaa clink clink, pulled off at the next exit and sure enough it was broken.

i guess i know what i will be doing after work and next weekend...


d@mn that sucks. If its not one problem its another. Dont you love this hobby haha.