5.7 High RPM Dead Spot

[matysol]

Hello. Volvo I/O, 5.7 liter engine,4bbl carb.,electronic ignition. Engine starts great,idles great,gets up to cruise great, but at higher RPM {3700-3900} engine hesitates and makes a very slight popping sound. Not loud like a backfire. Thought it was starving for fuel since it was only doing this at higher RPM, but after changing filter in carb.,spin on filter and fuel pump(electric) it has not helped. Do not know how long it has been like this because I rarely run the engine above 3400. Any other place to start looking? Thanks. Matt

 

[MooseHootie]

Re: 5.7 High RPM Dead Spot

I have the same symptoms on my 4.3 liter 2 barrel carb. at 3900 or above. I have taken all of your steps too. I am thinking ignition system mabe ?

 

[navigator336]

Re: 5.7 High RPM Dead Spot

You might check your plug gap. I have Indmar 5.7's of the same generation and set-up. The carbed model is supposed to be gapped at .030" while the injected models with the same ignition are spec'd at .045". Your spark might be a fraction late at the higher rpm's.

 

[Bondo]

Re: 5.7 High RPM Dead Spot

The High-Speed Circuit of your Carb is Plugged............<br />Rebuild, or Replace the Carb.........<br />

Your spark might be a fraction late at the higher rpm's.

???????<br />Plug Gap has No effect on Timing........

 

[ahein]

Re: 5.7 High RPM Dead Spot

Check to see if you have the correct spark advance at high rpm. I have the same engine set-up (Volvo 5.7 liter, newly rebuilt Rochester Quadrajet, and Delco Electronic Ignition). I experienced the same symptoms as you did earlier this year. My manual says that spark advance should be 30* at 4200 rpm. When I checked it, I was only getting about 12*. After fidling around for awhile, I found out that the mechanical advance weights were stuck and wouldn't swing out to advance the spark at higher rpm's. After I freed them up I took the boat out again and got up to full rpm's (4400) without any hesitation. Good luck .

 

[navigator336]

Re: 5.7 High RPM Dead Spot

At higher rpm's, a mis-gapped plug can definitely cause a mis-fire, which at least partially fits the syptom listed above. Actually, plug gap does have a very slight effect on timing, but not one you can measure with a timing light. In order for the spark to jump the gap, the high voltage has to ionize the air in the gap. The bigger the gap, the longer it takes to do this. It will also effect the length (in time) of the spark.

 

[Blk-n-Blu]

Re: 5.7 High RPM Dead Spot

Matysol, First I would look for water in the fuel, that is a classic symptom, usually a popping sound in the exhaust that sounds like a backfire. Pull the fuel/ water seperator filter and dump it into a glass container, water will settle on the bottom, Water will also accumulate in the carb bowl and when running the upper RPM range will be pushed into the intake due to the fuel demand. Sparkplugs may have a white crust around the electrode as well. Even after changing the spin on filter water will remain in the carb and symptoms will persist until all water is gone.<br />Just My $0.02