Engine vibration at start

[Lanceer]

95 7.4l carved motor. About 3 years old. When I start up at the beginning of the day, the motor shudders pretty bad. But when I throttle up to get it to stop shuddering , it works and runs great the rest of the day.(so far) the shudder seems to be coming from the back right as looking at the motor but it's really hard to tell. Does anyone have any thoughts or advice?
thanks

 

[Bondo]

Ayuh,...... Welcome Aboard,...... Is this shudder a knock, or ring, or what,.....

Shudderin' to me, means a rough idle, no pin-point sound though,.....

Do you use the throttle when startin' yer motor,..??

 

[Lanceer]

Doesn't seem like rough idle as I can get it to stop when I throttle up and it doesn't really keep vibrating. It isn't a clacking sound. It's a motor vibration like maybe a bad mount but they are tight. It almost seems like a rotation problem but why can I get it to stop?

 

[Scott Danforth]

if your running lean on one side or other from the carb idle circuits, cylinders 1, 2, 7, and 8 may go lean and cause a rough idle

you could have bad plugs or wires causing a mis-fire which is on one cylinder

you could have bad fuel with water in the fuel

first, pull the contents of your fuel filter in a clear container and check for water (you need a new fuel filter when you do this)

do a balance test (with motor running, pull one cylinder at a time to see which one is not firing). if all the cylinders are fireing, look at your carb.

 

[Lanceer]

I'll check fuel
I do get a small backfire sometimes when I throttle up under load. I think a carb rebuild may be needed. I just never experienced this before and would never think fuel would be a culprit. It's a 4 bbl from 1994. Is there a rebuild kit that is general or should I wait till I can get a number off the carb. On the balance test, as I pull wires what should I look for as I have never done this.

 

[Lanceer]

You know what, as I think about it, it seems like a knock that shakes the whole motor. Crankshaft maybe? I hope not.

 

[Scott Danforth]

a knock that shakes the whole motor is potentially an issue.

on a balance test, every plug you pull (use rubber gloves and a plastic pliers for pulling plug wires) should drop the RPM's about 50 RPM

the cylinder you pull that doesnt change RPM is not firing

 

[Lanceer]

I'll try pulling wires first. For piece of mind, would a problem show up in my oil filter? I wasn't happening for too long
The oil on the dipstick is clean as a whistle

 

[dsburch]

I'd look to the plugs as prime suspects. I've had three sets of brand new bad plugs in the past year and half, two sets in a Merc 60hp and one in my old Mopar. Either weak fire or no fire. Capacitance checks showed resistance numbers all over the map. Same with a 2011 Ford - I noticed the idle shake at about 60k miles, and cured it with new plugs. $0.02 Good luck there.

And yes, be specific on a carb rebuild kit. Go to the numbers on the carb. I think that one is probably a Rochester, if so stick with the rebuild instructions.

 

[Lanceer]

Thanks I will try all. I have to admit I'm kind of a worrywart. I just like a piece of mind