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

[cdavy000]

Opened up the distributor again today and I noticed a bunch of rotational play of the whole spring/weight mechanism itself. (First video)

I also showed that there is some distance the springs have to travel before the weights are contacted. (Second video)

If any of this is abnormal please let me know thank you!

 

[cdavy000]

So the engine is audibly increasing and decreasing in rpms while sitting at idle, and the vacuum gauge is increasing and decreasing vacuum along side it. So I went with a timing light to check if timing was steady and it seems that timing is constantly advancing and retarding by itself moderately slowly (about 3-4 degrees) at the same time as the engine is increasing and decreasing in rpm’s. Which leads me to believe that the unsteady timing is what’s causing the unsteady idle and therefore the vacuum gauge bounce. Any ideas on what might be causing an unsteady timing?

 

[Scott Danforth]

It's the other way around.

Your increasing and decreasing idle is called a lean surge caused by your carb.

The mechanical advance is just following the RPMs

 

[Scott06]

It's the other way around.

Your increasing and decreasing idle is called a lean surge caused by your carb.

The mechanical advance is just following the RPMs

Agreed wonder if there is a vacuum leak at carb base.
would try shooting starting fluid at base of carb to see if rpm changes

 

[Scott Danforth]

Or float level is fluctuating covering and hiding either a jet, or passage or casting flaw

 

[cdavy000]

Agreed wonder if there is a vacuum leak at carb base.
would try shooting starting fluid at base of carb to see if rpm changes

I Will try that tomorrow, although I tried to smoke the system to check for leaks and did not find any

 

[cdavy000]

It's the other way around.

Your increasing and decreasing idle is called a lean surge caused by your carb.

The mechanical advance is just following the RPMs

Ah I see

 

[cdavy000]

Or float level is fluctuating covering and hiding either a jet, or passage or casting flaw

It is very possible that my jet size is wrong, the carb came with extra jets of different sizes but I just went with the one already in the carb. Could incorrect jet size cause this?

 

[ROY WILLIAMS]

Opened up the distributor again today and I noticed a bunch of rotational play of the whole spring/weight mechanism itself. (First video)

I also showed that there is some distance the springs have to travel before the weights are contacted. (Second video)

If any of this is abnormal please let me know thank you!

the DOW CORNING DC-4 ELECTRICIAL GREASE IS THE WEIGHTS OF THE DISTIBUTOR .. I was the the corroded weights did the good ,,, no corroded ..
also the DC-4 GREASE distributor shaft of the points ..

 

[flashback]

Yes I think your carb needs some attention.

 

[cdavy000]

Yes I think your carb needs some attention.

Alright I’ll try to size up and size down jets and see what happens.

 

[flashback]

Alright I’ll try to size up and size down jets and see what happens.

Think you may want to size up, but be sure it's clean and float is set.

 

[cdavy000]

Think you may want to size up, but be sure it's clean and float is set.

Will do thank you

 

[Scott06]

It is very possible that my jet size is wrong, the carb came with extra jets of different sizes but I just went with the one already in the carb. Could incorrect jet size cause this?

Jets do not affect idle and transition circuit . If you look down the bore and see when fuel starts dropping off the boosters you will be close to 2000 rpm, below that you are on the idle circuit

 

[Scott Danforth]

Jets do not affect idle and transition circuit . If you look down the bore and see when fuel starts dropping off the boosters you will be close to 2000 rpm, below that you are on the idle circuit

Agreed in the original Rochester.

If the venturi bleeds are not sized properly on the Amazon special the OP has, it could be transitioning sooner

 

[cdavy000]

Agreed in the original Rochester.

If the venturi bleeds are not sized properly on the Amazon special the OP has, it could be transitioning sooner

I could try to put the venturis from the old carb on the new one. I wouldn’t be too surprised if they fit. I guess I’ll see

 

[Scott Danforth]

I would just rebuild the old one. Could be any number of things wrong with the new one

 

[cdavy000]

I would just rebuild the old one. Could be any number of things wrong with the new one

I have tried to rebuild the old one and it didn’t seem to work. Soaked every tiny peace in a pail of cleaner overnight and replaced everything with a kit.

 

[Scott Danforth]

I have tried to rebuild the old one and it didn’t seem to work. Soaked every tiny peace in a pail of cleaner overnight and replaced everything with a kit.

run a piece of fishing line thru the passages to make sure they are clear.

 

[cdavy000]

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.