mercruiser 7.4L popping through carb

[7228sedan]

I have a 1991 mercruiser 7.4 with a recently rebuilt q-jet carb. The motor starts & idles great. When she revs up to around 2200 RPM's; the motor starts loosing power & pops back through the carb. I thought it was a timing issue; I set the timing to 8 btdc "I think" I'm pretty sure that the machined groove in the balancer is the timing mark. It revved up to about 3,000 that time & then started popping again. At least she planed off. I rebuilt the carb myself, it was my 1st time rebuilding a carb. I've done plenty of motors before but this is my 1st carbed engine. I'm an EFI guy by trade. Any ideas?

 

[searay3]

Re: mercruiser 7.4L popping through carb

It sounds like it's going lean under load. Fuel filters etc new and clean? Float adjustment maybe. Did yu follow all the adjustment specs? Match the gaskets exactly? Might have to go thru the carb again. Make sure the gasket under the carb is correct, tight and sealed.

 

[zbnutcase]

Re: mercruiser 7.4L popping through carb

Yup sounds like a carb issue still.

 

[Fireman431]

Re: mercruiser 7.4L popping through carb

Just a thought...why not switch it over to EFI?

 

[xltier]

Re: mercruiser 7.4L popping through carb

you should do a compression test.there r other things that r more likely to be causing it.bent pushrod,bad valve,worn camshaft lobe.etc. try holding the choke closed a little to draw more fuel.if it still does it ,its not fuel related.you could have a exhaust valve not opening right.then when the intake valve opens again the compression from the previous stroke pops out.

 

[bomar76]

Re: mercruiser 7.4L popping through carb

Not all carb rebuilds are sucessful.
I really doubt you have any of the issues mentioned above.
Why did you rebuild the carb?
Same issues?
Your situation is classic for fuel delivery problems.

Plugged pickup in tank
Bad vent in tank
Bad filter
Filter sucking air (has happened to me)
Poor fuel pump output
Bad carb
Really bad fuel
.....are some of the areas of concern.

 

[littlebookworm]

Re: mercruiser 7.4L popping through carb

The q-jet is a relatively simple four-barrel, but it is tempermental when it comes to the float and accelerator pump settings. The repair kit comes with a list of settings. Follow them carefully. When setting the float level check if your particular carb calls for setting it with the gasket or without the gasket on the body. Some do, some don't. Bottom line - disassemble and redo the rebuild. Keep in mind some carbs, even after a perfect rebuild, still do not function well. Then it's time for a replacement. Good luck. Hy

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

Thanks for the input all, I am going to go down to the boat today & pull the carb off & go over it again. I have a feeling that the float is not set correctly. I also want to recheck the primary metering rods. I have a feeling that I incorrectly installed the retaining spring. The fuel pump, filter & lines are all new. I could not get her to start this spring without pouring some fuel down the throat. I noticed that the accelerator pump was not squirting & took the carb apart to clean it out. I'm typically a Ford guy & very familiar with the inner workings of multi port EFI, this is new to me. Can anyone confirm if the machined line on the harmonic ballancer is the timing mark? I assume that it is as there are no other marks or lines on the ballancer. My concern is that they were very shallow & are now gone with the majority of the paint off of the balancer. I set the timing at 8 BTDC according to that line.

Thanks

 

[PitchFork]

Re: mercruiser 7.4L popping through carb

Yes only one line on the HB and if you set the timing to 8? off that line that is correct as I'm assuming that is the spec for your engine. I have the same situation but mine is popping at 4100RPM. I am going to change the float level as I changed last season due to hard warm starts and I may have changed it too much. It just may be the float level is set wrong and the fuel in the bowl is too low and sucking air through the jets.

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

One other question I have is, do the idle mixture screws adjust fuel flow, or air flow? I've heard different things from different people.

 

[system-f]

Re: mercruiser 7.4L popping through carb

fuel flow

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

fuel flow

I thought so, thank you for clarifying. I have them set 3 turns out as a base line.

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

Just a thought...why not switch it over to EFI?

A 7.4 MPI motor would be great however, the cost associated with converting my motor to the MPI motor would be about the same as a new long block. Not to mention the 7.4L MPI was rated at 310 HP while my 4bbl was rated at 340. I only have 1 motor so I need to make every pony count with a 12,000 lb displacement.

 

[fossill]

Re: mercruiser 7.4L popping through carb

Dont forget the basics starting with the ignition system including plugs and wires when troubleshooting.

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

Dont forget the basics starting with the ignition system including plugs and wires when troubleshooting.

They're all new. Accell 9mm wires, plugs gapped to spec, cap & rotor new.

 

[PitchFork]

Re: mercruiser 7.4L popping through carb

One other question I have is, do the idle mixture screws adjust fuel flow, or air flow? I've heard different things from different people.

I do not think the idle mixture screws are your issue popping through the carb at higher RPMs. The idle mixture screws I believe only effect the carb at idle speeds and have no effect at WOT or mid range RPMs. I may be wrong hopefully someone else can chime in.

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

I do not think the idle mixture screws are your issue popping through the carb at higher RPMs. The idle mixture screws I believe only effect the carb at idle speeds and have no effect at WOT or mid range RPMs. I may be wrong hopefully someone else can chime in.

No, I didn't think that their setting would affect the popping problem. I just want to ensure that this carb is tuned correctly. At the same time, I wanted to educate myself.

 

[fossill]

Re: mercruiser 7.4L popping through carb

you should do a compression test.there r other things that r more likely to be causing it.bent pushrod,bad valve,worn camshaft lobe.etc. try holding the choke closed a little to draw more fuel.if it still does it ,its not fuel related.you could have a exhaust valve not opening right.then when the intake valve opens again the compression from the previous stroke pops out.

I seen the same problem in two Merc 5.7L's which ended up being 2 worn cam lobes on one engine and the other engine had the head gasket give way between two cylinders. I agree with the compression test as well as a leakdown test.

 

[6meter]

Re: mercruiser 7.4L popping through carb

COIL. Had the same problem. Redid the carb twice and I've done a ton of them. New wires and plugs. Not the problem. The coil has broken down. Put a new one in. they're only $20.

 

[7228sedan]

Re: mercruiser 7.4L popping through carb

COIL. Had the same problem. Redid the carb twice and I've done a ton of them. New wires and plugs. Not the problem. The coil has broken down. Put a new one in. they're only $20.

I thought about that, but I get the pop when I rev the motor quickly in neutral as well. Would the coil show a problem in free rev situation not under load?