Knocking and a Flat Spot...holley prob?
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 1
From: SA
Knocking and a Flat Spot...holley prob?
Hi guys,
Had this small problem start with my 350. Under load and only between 1800 and 2500 rpm i get a rattling noise from my engine. Its sounds like marbles and i presume its Knocking. If it was something else loose it should rattle up past 2500, right? Also the more throttle open the louder it is. My engine is a rebuilt stock 72 with 8.5:1 comp ratio. I have used 98oct premium unleaded but it still knocks. Gonna try Avgas 100 tomorrow.
It may tie in with another prob i have...when cruising on light throttle at 35 mph (around 1500rpm) the engine will stumble as if its running out of fuel or being flooded. This condition takes 20 to 30 seconds of steady light throttle to appear. Once it stumbles i open the throttle more, it flatspots then revs ok. Im thinking this problem and the one outlined above may be carb related. I have a holley 650 vac sec.
I have checked timing and its fine. One of my primary exhaust tubes was crushed by previous owner to around a 1/3 of its original diameter. Would this cause the cylinder to get hot and detonate? It only happens once engine is warmed up.
Any advice on which order to start pulling things for testing?
:crazy:
Had this small problem start with my 350. Under load and only between 1800 and 2500 rpm i get a rattling noise from my engine. Its sounds like marbles and i presume its Knocking. If it was something else loose it should rattle up past 2500, right? Also the more throttle open the louder it is. My engine is a rebuilt stock 72 with 8.5:1 comp ratio. I have used 98oct premium unleaded but it still knocks. Gonna try Avgas 100 tomorrow.
It may tie in with another prob i have...when cruising on light throttle at 35 mph (around 1500rpm) the engine will stumble as if its running out of fuel or being flooded. This condition takes 20 to 30 seconds of steady light throttle to appear. Once it stumbles i open the throttle more, it flatspots then revs ok. Im thinking this problem and the one outlined above may be carb related. I have a holley 650 vac sec.
I have checked timing and its fine. One of my primary exhaust tubes was crushed by previous owner to around a 1/3 of its original diameter. Would this cause the cylinder to get hot and detonate? It only happens once engine is warmed up.
Any advice on which order to start pulling things for testing?
:crazy:
Moderator
Joined: Jun 1999
Posts: 7,117
Likes: 51
From: Dove Mountain Arizona
Re: Knocking and a Flat Spot...holley prob? (xrated)
I'm going to suggest installing a new power valve. More info is needed really though, things like: Did it always stumble? Did the carb just get rebuilt? 98 octane should stop an 8.1 cr engine from detonating easily. Float level needs to be right as do jets. If it just started, we should be able to narrow it down for you. What vacuum reading do you have at idle?
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 1
From: SA
Re: Knocking and a Flat Spot...holley prob? (MasterDave)
its been stumbling since i bought the car about 3 months ago. I have no idea what vacuum i have at idle. This knocking was not there when i bought the car but started about 6 weeks after (This is when i started givin it stick) and seems to get slowly worse. Carb looks like it has been rebuilt by previous owner. It is running rich at idle. I need to get it dyno tuned. I might pull the carb off and get it overhauled.
What jets should be in the primaries?
[Modified by xrated, 12:49 PM 10/6/2001]
What jets should be in the primaries?
I'm going to suggest installing a new power valve. More info is needed really though, things like: Did it always stumble? Did the carb just get rebuilt? 98 octane should stop an 8.1 cr engine from detonating easily. Float level needs to be right as do jets. If it just started, we should be able to narrow it down for you. What vacuum reading do you have at idle?
[Modified by xrated, 12:49 PM 10/6/2001]
Moderator
Joined: Jun 1999
Posts: 7,117
Likes: 51
From: Dove Mountain Arizona
Re: Knocking and a Flat Spot...holley prob? (xrated)
Thanks for the info. You do not need to 'Dyno' it to tune it though. A thorough tune up (including a carb check) at a reputable shop could take care of it for you. If the engine is in 'stock' trim it should be easy to do. Not so sure of the knock though, you state it happened after you rode it kinda hard, once again a good mechanics ear can tell you if it's mechanical damage or pinging. Let me know how it goes.... :cheers:
[Modified by MasterDave, 12:22 PM 10/6/2001]
[Modified by MasterDave, 12:22 PM 10/6/2001]
