1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

[Scott06]

I hooked a vacuum pump up to my power valve to check for leaking and it seems to hold about 20 max and then quickly starts leaking. At 7” Hg it leaks very slowly and stops leaking at 3.6” Hg. If anyone knows if this is normal it would be much appreciated thank you.

I also noticed that the Venturi holes on the new carb are slightly larger than the ones on my old carb.

that is not how a power valve on a rochester 2 jet or Mercarb works.

There is an piston/actuator that comes down from the airhorn and pushes the power valve open. At high vacuum/low throttle opening the piston gets pulled up into the airhorn and doesnt push the plunger on power valve open, when vacuum drops the piston comes down and open the valve to richen fuel mixture.

you are just pulling the power valve open with the vacuum. here is a vid on how it works

 

[cdavy000]

that is not how a power valve on a rochester 2 jet or Mercarb works.

There is an piston/actuator that comes down from the airhorn and pushes the power valve open. At high vacuum/low throttle opening the piston gets pulled up into the airhorn and doesnt push the plunger on power valve open, when vacuum drops the piston comes down and open the valve to richen fuel mixture.

you are just pulling the power valve open with the vacuum. here is a vid on how it works

I was trying to check for a blown power valve since I’ve had so many carb backfires. I wanted to see if maybe the diaphragm ruptured and it was leaking while I had the carb open

 

[cdavy000]

After bathing the intake manifold and carb in carb cleaner I found a vacuum leak where the throttle valve is controlled by the throttle linkage (pictures below, I have red straw pointing to where leak is)

Any ideas on how I might seal this considering the rod needs to be able to rotate around In there? Really really thick grease? thanks guys.

 

[nola mike]

After bathing the intake manifold and carb in carb cleaner I found a vacuum leak where the throttle valve is controlled by the throttle linkage (pictures below, I have red straw pointing to where leak is)

Any ideas on how I might seal this considering the rod needs to be able to rotate around In there? Really really thick grease? thanks guys.

Imma channel my inner @Scott Danforth here and say... Rebuild your original

 

[Scott06]

After bathing the intake manifold and carb in carb cleaner I found a vacuum leak where the throttle valve is controlled by the throttle linkage (pictures below, I have red straw pointing to where leak is)

Any ideas on how I might seal this considering the rod needs to be able to rotate around In there? Really really thick grease? thanks guys.

normally those are a tight fit between baseplate and throttle shaft. When restoring carbs some better rebuilders will bush the throttle shaft to seal. On a new carb this should not be send it back and rebuild the oem carb or have it rebuilt by someone competent. This is the rub with these offshore carbs, some work some have horrible manufacturing defects

 

[cdavy000]

Imma channel my inner @Scott Danforth here and say... Rebuild your original

I have tried rebuilding the original and it didn’t work I think the 37 year old carb is just too worn.

 

[cdavy000]

normally those are a tight fit between baseplate and throttle shaft. When restoring carbs some better rebuilders will bush the throttle shaft to seal. On a new carb this should not be send it back and rebuild the oem carb or have it rebuilt by someone competent. This is the rub with these offshore carbs, some work some have horrible manufacturing defects

Rebuilding OEM carb didn’t work so I suppose I’ll try to find a new OEM one to purchase

 

[Scott06]

I was trying to check for a blown power valve since I’ve had so many carb backfires. I wanted to see if maybe the diaphragm ruptured and it was leaking while I had the carb open

That is on old holley power valves not Rochester derivatives. the newer holley power valves are not susceptible to backfire damage anyways .

 

[Scott06]

Rebuilding OEM carb didn’t work so I suppose I’ll try to find a new OEM one to purchase

what about it is not rebuildable? Willing to bet that with some guidance you can properly rebuild it. anyone here who says they haven’t re-rebuilt a carb is FOS… my success rate improved by getting a ultrasonic cleaner from harbor freight used it with both carb cleaner and also simple green in it. All 4 carbs i rebuilt using it have worked great afterwards.

 

[cdavy000]

what about it is not rebuildable? Willing to bet that with some guidance you can properly rebuild it. anyone here who says they haven’t re-rebuilt a carb is FOS… my success rate improved by getting a ultrasonic cleaner from harbor freight used it with both carb cleaner and also simple green in it. All 4 carbs i rebuilt using it have worked great afterwards.

I bought a kit from mikes carburetor parts and followed his videos. The carb worked better but it still took a few seconds of cranking to start and I still had the carb backfire issue. With the new one it starts instantly but I still have this carb backfire and apparently a leak.

 

[Scott Danforth]

only one carb out of 200+ have I ever deemed beyond rebuildable. there was too much corrosion. the issue most people make on rebuilding a carb is not putting the effort in on cleaning the gunk from the passages.

 

[Dubed]

Forgive me if Im wrong, but doesnt this thread go back to extremely low compression which would suggest it needs a rebuild??
i would probably start there and not wory about feeding a dead motor fuel. Also, is this the original stator ignition and water cooled regulator?, or have you upgraded to an alternator? These motors are pretty particular when everything is perfect. I would get back to basics, then try your original rebuilt carb, and toss the Amazon one. Seems your having the same issues with both carbs

 

[dubs283]

is this the original stator ignition and water cooled regulator?

The ignition system for this model(s) mcm engines has always been battery supplied voltage conventional/points from the factory. The only task the stator/flywheel/balancer/regulator system has is to provide ample charging/maintainence for the battery

That said, this thread is long but the focus has been primarily on fuel supply, namely the carburetor. OP has rebuilt the original carb once with same symptoms. Installed a new/reman/aftermarket carb with same results. Also the ignition system has been suspect, mainly dwell and idle timing which according to OP is correct

If I were diagnosing first hand I would first verify correct/quality ignition/tune up components, correct firing order, correct dwell and idle timing. I would then verify the timing is advancing properly with rpm and load. If issue still persisted I would verify correct fuel pressure/vacuum at idle and under a load

If issue still persisted I would have a helper drive the boat while adding fuel directly to the carburetor while the engine is experiencing the issue

But thats just my method of troubleshooting

 

[ROY WILLIAMS]

After bathing the intake manifold and carb in carb cleaner I found a vacuum leak where the throttle valve is controlled by the throttle linkage (pictures below, I have red straw pointing to where leak is)

Any ideas on how I might seal this considering the rod needs to be able to rotate around In there? Really really thick grease? thanks guys.

the alum corroded of that carb !!!!!!!!!!!!!

 

[cdavy000]

only one carb out of 200+ have I ever deemed beyond rebuildable. there was too much corrosion. the issue most people make on rebuilding a carb is not putting the effort in on cleaning the gunk from the passages.

I only soaked the parts in carb cleaner for a day and then boiled them in vinegar and water, what might you suggest to clean the passageways?

 

[cdavy000]

Forgive me if Im wrong, but doesnt this thread go back to extremely low compression which would suggest it needs a rebuild??
i would probably start there and not wory about feeding a dead motor fuel. Also, is this the original stator ignition and water cooled regulator?, or have you upgraded to an alternator? These motors are pretty particular when everything is perfect. I would get back to basics, then try your original rebuilt carb, and toss the Amazon one. Seems your having the same issues with both carbs

Yes it is upgraded to the alternator set up

 

[cdavy000]

the alum corroded of that carb !!!!!!!!!!!!!

The aluminum seems pretty clean to me what makes you think it might be corroded?

 

[Scott Danforth]

I only soaked the parts in carb cleaner for a day and then boiled them in vinegar and water, what might you suggest to clean the passageways?

see post #119

 

[cdavy000]

see post #119

Gotcha thanks