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

[nola mike]

Did I miss the part where those compression #'s were addressed? 14mm is still poor vacuum. I can't tell if my video is laggy or if that's the motor, but it looks like your vacuum is low, fluctuating, AND having a sharp drop (HG, rings, burnt/sticky valves?). And your leakdown test numbers are probably wrong; I'd expect much higher leakage given those compression numbers, although more important where the leak is occurring.

 

[Scott06]

What brand of carburetor did you buy as your new one?

Chinesium... to the OP if you shut the idle screws all the way down and it still runs gas may be overflowing the float bowl would look down the bores during idle. This assume you are at a normal idle and not into the transition slots like idling at 1000 rpm.

Did I miss the part where those compression #'s were addressed? 14mm is still poor vacuum. I can't tell if my video is laggy or if that's the motor, but it looks like your vacuum is low, fluctuating, AND having a sharp drop (HG, rings, burnt/sticky valves?). And your leakdown test numbers are probably wrong; I'd expect much higher leakage given those compression numbers, although more important where the leak is occurring.

No he did the leakdown which showed leakage by the rings but 10% leak down is on the high end of OK. Not sure where to go with that other than the leakdown is higher or compression is a gauge issue and is actually higher.

What made me mention the leakdown to him as I have a 65 GTO that I did a backyard hone/ring/new bearings/heads had a valve job years ago has never idled quite right and vaccuum has always been low 15-16" and not 100% steady... but compression is 175-180 lbs (10.75:1) . I get a ton of blow by now that I finally got it street legal ... It has 20% leak down. Likely I should have bored it, clearly rings never seated... rear main seal leaks like mad ... and it would have been so much cheaper to rebuild back in the day

Not sure where to go on this one but carb is suspect given its origin.

 

[ROY WILLIAMS]

Yeah I can hear start stumble at about 3/4 turn but I’m able to turn it all the way in without it stalling, although it will stumble hard. It’s possible my idle rpm is too high, which is in turn creating enough vacuum to continue to suck fuel through the Venturi’s. Although I’ve adjusted it to idle at 750 in gear like it’s supposed to be.

THE 1-1/4 OF THE CARB MIXTURE SCREW,,, TIGHTED CW IT AND CCW 1-1/4 TURNS ... THE IGNITION IS THE BTDC 4-6 DEG [650-800 RPM ]... THE DWELL METER IS THE AROUND 28 DEG ..THE RPM IS 650-700 RPMS IDLE ENGINE ..
THE CARB IS THE FLOAT ADJUSTED IT .. DOWN AND THE UPSIDE CARB DID THE TOP CARB .

 

[Scott Danforth]

Chinesium... to the OP if you shut the idle screws all the way down and it still runs gas may be overflowing the float bowl would look down the bores during idle. This assume you are at a normal idle and not into the transition slots like idling at 1000 rpm.

Missed that

yes I went with the Amazon special

@cdavy000 , I would do a good cleaning of your original carb

Little anecdotal information. Briggs and Stratton bought carbs from Guangzhou china for their snow engines back in the late 2000's early 2010's. The reason is so that they could be competitive (or so they claimed)

Because of that, every other snow blower would either run super rich, or super lean right out of the box. So much for the theory that B&S tested their products. Turns out the manufacturing chips were never cleaned out of the carbs and the castings were porous.... pull the lid on the carb and you would see aluminum chips and in some instances porosity so bad the carb would weep.

It was a crap shoot if the head was going to melt or the thing not run. Warranty was extremely high on all the units with B&S motors. so high in fact that we helped LCT develop the winter motors to replace the Briggs

The lesson I learned from that.........Never ever ever buy a chinese carb.....ever.

every one on iboats that tried the cheap Amazon carbs has reported issues.

 

[Scott Danforth]

...I have a 65 GTO that I did a backyard hone/ring/new bearings/heads had a valve job years ago has never idled quite right and vaccuum has always been low 15-16" and not 100% steady... but compression is 175-180 lbs (10.75:1) . I get a ton of blow by now that I finally got it street legal ... It has 20% leak down. Likely I should have bored it, clearly rings never seated... rear main seal leaks like mad ... and it would have been so much cheaper to rebuild back in the day

did you try the bon-ami trick?

 

[nola mike]

No he did the leakdown which showed leakage by the rings but 10% leak down is on the high end of OK. Not sure where to go with that other than the leakdown is higher or compression is a gauge issue and is actually higher.

Yeah, that's why I suspect his numbers are wrong. A 104 psi is leaking a lot from somewhere, right?...shouldn't have normal leakdown numbers. If you can't build good compression, you're by definition not sealing the chamber well.

 

[Scott Danforth]

Or the cam is washed out

 

[Scott06]

Or the cam is washed out

I thought his valve train video showed relatively even lift on all 8 . Of course I look at more threads than centerfolds in my younger days could be mixing them up…

 

[Scott Danforth]

Miss January......

 

[Scott06]

did you try the bon-ami trick?

not sure what it is? I assume carbon removal from rings. Did that with mercruiser power tune and seafoam soak. Had less carbon coming out than when I have used it previously

I think root cause was it was assembled by an 18 yr old idiot that I see in the Mirror

 

[cdavy000]

What brand of carburetor did you buy as your new one?

Here is the link to the new carb.

Amazon.com

 

[cdavy000]

Did I miss the part where those compression #'s were addressed? 14mm is still poor vacuum. I can't tell if my video is laggy or if that's the motor, but it looks like your vacuum is low, fluctuating, AND having a sharp drop (HG, rings, burnt/sticky valves?). And your leakdown test numbers are probably wrong; I'd expect much higher leakage given those compression numbers, although more important where the leak is occurring.

Compression numbers were
#1 126psi
#2 103psi
#3 114psi
#4 114psi
Yes, vacuum is low, fluctuating and about a 1 to 2” hg rise and drop. I mean I’m not sure how the leakdown numbers would be wrong I put the cylinders at TDC, plugged it up to shop air at 100 psi and read the leakdown %.

 

[cdavy000]

Chinesium... to the OP if you shut the idle screws all the way down and it still runs gas may be overflowing the float bowl would look down the bores during idle. This assume you are at a normal idle and not into the transition slots like idling at 1000 rpm.

No he did the leakdown which showed leakage by the rings but 10% leak down is on the high end of OK. Not sure where to go with that other than the leakdown is higher or compression is a gauge issue and is actually higher.

What made me mention the leakdown to him as I have a 65 GTO that I did a backyard hone/ring/new bearings/heads had a valve job years ago has never idled quite right and vaccuum has always been low 15-16" and not 100% steady... but compression is 175-180 lbs (10.75:1) . I get a ton of blow by now that I finally got it street legal ... It has 20% leak down. Likely I should have bored it, clearly rings never seated... rear main seal leaks like mad ... and it would have been so much cheaper to rebuild back in the day

Not sure where to go on this one but carb is suspect given its origin.

In neutral it’s about 850 rpm, but in gear it’s at 750 rpm which is what I’ve read the in gear specs should be.

I have only one idle mixture screw. I don’t recall seeing any gas coming out of the bores during idle. But I can give it a closer look tomorrow.

 

[cdavy000]

THE 1-1/4 OF THE CARB MIXTURE SCREW,,, TIGHTED CW IT AND CCW 1-1/4 TURNS ... THE IGNITION IS THE BTDC 4-6 DEG [650-800 RPM ]... THE DWELL METER IS THE AROUND 28 DEG ..THE RPM IS 650-700 RPMS IDLE ENGINE ..
THE CARB IS THE FLOAT ADJUSTED IT .. DOWN AND THE UPSIDE CARB DID THE TOP CARB .

When you say “650-700 rpm’s idle engine” are you saying idle in gear or in neutral.

Also if I turn down the idle like 100-150 rpm’s the vacuum will drop like 1.5 to 2” Hg.

I haven’t personally adjusted the float as it is a new carb but I can do that tomorrow.

 

[Scott Danforth]

Idle is 650 in gear in the water

Your compression numbers mean the motor needs a rebuild.

The carb is highly suspect

 

[cdavy000]

Missed that

@cdavy000 , I would do a good cleaning of your original carb

Little anecdotal information. Briggs and Stratton bought carbs from Guangzhou china for their snow engines back in the late 2000's early 2010's. The reason is so that they could be competitive (or so they claimed)

Because of that, every other snow blower would either run super rich, or super lean right out of the box. So much for the theory that B&S tested their products. Turns out the manufacturing chips were never cleaned out of the carbs and the castings were porous.... pull the lid on the carb and you would see aluminum chips and in some instances porosity so bad the carb would weep.

It was a crap shoot if the head was going to melt or the thing not run. Warranty was extremely high on all the units with B&S motors. so high in fact that we helped LCT develop the winter motors to replace the Briggs

The lesson I learned from that.........Never ever ever buy a chinese carb.....ever.

every one on iboats that tried the cheap Amazon carbs has reported issues.

I spent about two weeks last year thouroughly cleaning the old carb and I put a rebuild kit on it and it didn’t seem to get much better at all. It would take 2.5 seconds of cranking to start the engine no matter if it was cold or hot or whatever and it had the same backfiring problem. I threw on the new carb and it starts in a half second or less every time, so I guess I’m just reluctant to go back. The backfire seemed to stop for a couple weeks after I put on this new carb but the carb backfire came back, although it’s possible I adjusted timing or points or something in that time.

 

[cdavy000]

Idle is 650 in gear in the water

Your compression numbers mean the motor needs a rebuild.

The carb is highly suspect

All right I will turn the idle rpm down to 650 in gear in the water tommorow and report back. It’s possible that the engine will stall if I completely lean out the fuel mixture at that rpm so I will post results.

I’m not sure if I already said it but the carb will only backfire under heavy acceleration if I am under like 1200 rpm’s.

Yes I fear the motor will need a rebuild I just was hoping there might be something I can do before spending a month or two dismantling it.

 

[cdavy000]

Or the cam is washed out

Unfortunately very possible.

 

[cdavy000]

Oh I just realized that it might be important to state that if I floor it at idle and it backfires through the carb I have to pull back on the throttle or the engine will sputter for 2 seconds and die. But if it’s at like 1000 ish rpm’s when I do it, it will hesitate and then accelerate normally.

 

[ROY WILLIAMS]

When you say “650-700 rpm’s idle engine” are you saying idle in gear or in neutral.

Also if I turn down the idle like 100-150 rpm’s the vacuum will drop like 1.5 to 2” Hg.

I haven’t personally adjusted the float as it is a new carb but I can do that tomorrow.

my manual mercruiser 470 engine is the 650-700 rpm idle !!!!
my MCM470 is the manual all those engines and the outdrives !!!
the vacuum are 17-21 inches of the manifold intake ,,,..
the ignition of the BTDC 4-6 degrees idle 650-700 RPMS ..
the gasoline is the alcohol new days ... back in the decades gasoline is no alcohol ..