mercruiser 7.4 big block knocking or popping through carb help needed

[bolo78910]

happy easter everyone. now here i have a mercruiser 7.4L serial number# 00389808
i bought it over the winter as a project, today i started it for the first time, it runs fine but there is a knocking or popping sound coming from the carb area . i made a little video off the sound i just need to figure out how to post it. like i said the knocking is coming through the carb with the engine running it goes knock knock knock . oil pressure is great . any info of some sort would be great .i am planing on taking off the valve cover to see what is going on.

key bottom sound great knock knock is at the top. ur thought guys. thanks

 

[achris]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

Sounds like it's 'lean popping'. Could mean a slight carb or timing adjustment, or fuel filters or a carb strip and clean, or all 3.

Chris.......

 

[Don S]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

Along with the fuel problems, have you done any of the basic tests yet? Like compression test, checking firing order, timing, looking at the cap and rotor for obvious cracks or carbon tracking etc.?

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

thanks much wow love this site. well i have not dun all those things thanks. don i dont kno how to post the video with the engine running, i did manage to post it on youtube this is the search link please hop on youtube and hear the knocking sound thanks. mercruiser 7.4 knocking by bolo

 

[Don S]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

thanks much wow love this site. well i have not dun all those things thanks. don i dont kno how to post the video with the engine running, i did manage to post it on youtube this is the search link please hop on youtube and hear the knocking sound thanks. mercruiser 7.4 knocking by bolo

You forgot the link !!!!!!

 

[achris]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

Here's the link....

mercruiser 7.4 knocking by bolo - YouTube

That is not good. It's regular enough to be a mechanical problem. I'd be checking compression and doing a leakdown test, then check all the spark plugs, spark wires, rotor and cap.... make sure you have the right plug leads on the right plugs!

While it's running, pull a lead off the plugs one at a time to see if you can determine which cylinder is causing the noise.

Chris......

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

thanks guys great help on the link , yea i think am looking at a problem there. the bottom sound good but that knocking sound is right by the carburator when i slightly rev it up it back fire out the carb. ok give it to me i kno its bad news lol

 

[achris]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

Do the tests I suggested. Without the information from those tests there's no way we can make a diagnosis....

Could be anything from a bad spark plug to a blown head gasket to a burnt intake valve.

DO THE TESTS....

Chris......

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

thanks chris very much , now which service manual do i need for this engine guys is it #16

 

[Don S]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

Good grief, just listened to your video. What is it idling at, 200 rpm? Get it up to 600 to 650 where is should be. That "knock" could even be a flapper at that rpm.

serial number# 00389808

Recheck your serial number, that second number should be a letter, and O wasn't used.

 

[achris]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

The more I listen to that video, the more it sounds like a burnt intake valve... But you need to do a compression and leakdown test to confirm that.

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

don thanks , its is 0D389808 DRIVE NUMBER IS 0D403744

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

cris i think ur on the money, thats why when i increase the rpm it popping backfire out the carb . god i hope its only that .

 

[achris]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

don thanks , its is 0D389808 DRIVE NUMBER IS 0D403744

For the engine, Manual #15.... For the drive, manual #11.

(1990 7.4L with a Bravo 1)

cris i think ur on the money, thats why when i increase the rpm it popping backfire out the carb . god i hope its only that .

DO THE COMPRESSION TEST!!! That is the only way you'll know for sure.

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

thanks much let yall know tomorro how i make out,

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

ok guys am back with my finding today , i did not get to do a lot because i found the problem right away.
first i start the engine again , i then start pulling plug wire one at a time as soon as i pull # 5 wire the knocking stop and the engine sound real good oil pressure is good too. i then tale the valve cover off just to see what going on in # 5 what i found was a bent broken push rod . i inch off the the push rod was missing so i know it is somewhere in the engine. i did not have a compression tester at hand to do a test on that cilinder but i crank the motor over while my friend stuck a finger in the plug hole and strong air kick his finger out. ok guys i dont realy want to take the short way out ,but do you all think i can et away with the one inch off broken push rod still in the engine ?? or maybe i should just pull the heads and do a complete head and intake gasket fix.. thanks for any info that would help .

 

[xjdriver]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

The piece is probly in your oil pan, or stuck to the oil pickup screen pull it off and see.

 

[achris]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

Ok... You have one bent push rod and one missing pushrod, yes? Most likely the pushrod that's missing is in the engine valley, below the intake manifold. I would pull the intake and replace the bent and missing pushrod, then do a compression test. If that shows up good, then replace the intake and finish the top end. ok, why did the push rod bend? Check ignition timing and all the plugs and leads, cap and rotor. (I would just replace all the leads, cap and rotor and spark plugs).

At this stage I would also pull the exhaust elbows and check them and their gaskets... and the manifold surfaces too... You may have had a small amount of water in the cylinder which could have caused the pushrod problems. Just a thought

Chris....

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

????? can i get away with one inch of pushrod i do hope it ends up in the oil pan. lol

 

[bolo78910]

Re: mercruiser 7.4 big block knocking or popping through carb help needed

thanks guys , chris only one push rod that is damage , it is missing one inch from the tip it was slightly bent also. so only one.