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

[cdavy000]

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 ..

I’ll turn down my idle today and see what happens. My timing is 4 btdc as per the Mercruiser service bulletin. Not sure what you’re saying about the gasoline but I run non-ethanol 89 octane.

 

[cdavy000]

Okay so it seems that the left Venturi is dripping fuel. About one drop ever 1.5 seconds. The right Venturi is not dripping.

After I adjust in gear idle to 650 rpm the left Venturi is dripping one drop every 4 seconds. Right Venturi is not dripping.

It stops dripping after engine is shut off.

 

[Scott06]

Okay so it seems that the left Venturi is dripping fuel. About one drop ever 1.5 seconds. The right Venturi is not dripping.

After I adjust in gear idle to 650 rpm the left Venturi is dripping one drop every 4 seconds. Right Venturi is not dripping.

It stops dripping after engine is shut off.

At idle nothing should be dripping out of the venturis. The only fuel flow is below the throttle plates in idle circuit holes. Either bad needle and seat /float, incorrectly set float level, or too much fuel pressure.

Since you had same issue with both carbs might lean towards fuel pressure. But not sure over filled bowl would cause backfire... might be hung up and not quite closing

Not sure what you’re saying about the gasoline but I run non-ethanol 89 octane.

he was talking about 10% ethanol fuel now vs the old days. If your run non ethanol should be no issue

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 %.

What isn't making sense to several of us is the low compression numbers-

cyl2 is essentially dead , 3& 4 are not far behind

And yet you have low leakdown % - 10 is very acceptable

No one is saying this to be difficult those two just don't make sense together. Would suspect either the compression gauge is not accurate or leak down is not accurate based off experience.

Most obvious thought with low compression and backfire is intake valve not seating hence the leakdown test, but leakdown shows healthy engine...

Have you hooked up a timing light when you blip the throttle and get the backfire? Does the ignition timing go way advanced or something odd? Since you have mechanical advance and everything on a boat corrodes wonder if advance plate is wonky.

 

[cdavy000]

I took the top off the carb to check float level and it seems there is a spring at the bottom of the accelerator pump. That was not on my old carb but I suppose it could be normal I just thought I should point it out.

 

[cdavy000]

At idle nothing should be dripping out of the venturis. The only fuel flow is below the throttle plates in idle circuit holes. Either bad needle and seat /float, incorrectly set float level, or too much fuel pressure.

Since you had same issue with both carbs might lean towards fuel pressure. But not sure over filled bowl would cause backfire... might be hung up and not quite closing

he was talking about 10% ethanol fuel now vs the old days. If your run non ethanol should be no issue

What isn't making sense to several of us is the low compression numbers-

cyl2 is essentially dead , 3& 4 are not far behind

And yet you have low leakdown % - 10 is very acceptable

No one is saying this to be difficult those two just don't make sense together. Would suspect either the compression gauge is not accurate or leak down is not accurate based off experience.

Most obvious thought with low compression and backfire is intake valve not seating hence the leakdown test, but leakdown shows healthy engine...

Have you hooked up a timing light when you blip the throttle and get the backfire? Does the ignition timing go way advanced or something odd? Since you have mechanical advance and everything on a boat corrodes wonder if advance plate is wonky.

I have not watched timing while carb backfires with a timing light that’s a good idea I will try that next

 

[cdavy000]

After adjusting float it seemed that the spring loaded needle was set at 8/16 instead of 9/16 so I fixed that. And then the float drop was at 1 and 8/32 instead of 1 and 3/32 so I fixed that.

 

[kenny nunez]

On your # 48 post you stated that the dwell is 30. The correct dwell for your engine is 40. You need to get that corrected, as far as the timing 4* is not enough. I like to set the timing with the spark plugs out with the spark plugs on the wires and grounded, spin the engine and then set it. That way there is no way the centrifugal advance will be starting to advance the spark. This is a good way if the carburetor has a problem with the idle circuit. Try setting the timing at 6* with this method. If you can get pure gasoline try 91 octane and bump the timing to 8. The vacuum will pick up and you will be able to close the idle stop screw even more, as you know 1.5 turns from seated should be the sweet spot for the idle mixture.
While this will not correct a worn engine it may get it running better.

 

[cdavy000]

On your # 48 post you stated that the dwell is 30. The correct dwell for your engine is 40. You need to get that corrected, as far as the timing 4* is not enough. I like to set the timing with the spark plugs out with the spark plugs on the wires and grounded, spin the engine and then set it. That way there is no way the centrifugal advance will be starting to advance the spark. This is a good way if the carburetor has a problem with the idle circuit. Try setting the timing at 6* with this method. If you can get pure gasoline try 91 octane and bump the timing to 8. The vacuum will pick up and you will be able to close the idle stop screw even more, as you know 1.5 turns from seated should be the sweet spot for the idle mixture.
While this will not correct a worn engine it may get it running better.

It states on the top of the engine that dwell needs to be 28-34.

Mercruiser sent out a service bulletin for the MCM 470 that the timing needs to be decreased to 4 deg from 8 deg due to the different fuel used today.

I thought the engine had to be idling in neutral to set the timing? (I could very well be wrong)

unfortunately I can’t get non-ethanol 91, I can get 91 octane with ethanol but I was under the presumption running ethanol will eventually cause wear or damage. I am willing to try it but I just want to make sure it won’t cause problems.

 

[cdavy000]

Okay so I just re-did the compression numbers and now, after the MMO soak, I’m getting these numbers.

#1- 140
#2- 135
#3- 135
#4- 135

But I also noticed that if I crank it until the reading is steady and then wait for about 2 minutes it will slowly climb to 140 on cylinders 2,3,4 and 145 on cylinder 1.

Sometimes it will increase to 140 on cylinder 2,3,4 and 145 on cylinder 1 if I cranked it until it was steady, stopped for 2 seconds, and then cranked it again. At one point after doing that I managed to get cylinder 1 to 155 psi.

Not sure what this means but I felt I should add.

 

[Scott Danforth]

youre supposed to spin the motor about 5 times for each cylinder when taking readings

I would go back and redo the leak down test as well.

you are still down from spec, however better. I suggest testing your gauge.

additionally, the carb is still suspect. especially with your comment in post 82

 

[cdavy000]

youre supposed to spin the motor about 5 times for each cylinder when taking readings

I would go back and redo the leak down test as well.

you are still down from spec, however better. I suggest testing your gauge.

additionally, the carb is still suspect. especially with your comment in post 82

5 times was about what I did when waiting for reading to cap out.

I will redo the leakdown test now.

I will look into the dripping left Venturi on the carb. If anyone has any suggestions on what that dripping might be it would be much appreciated

 

[cdavy000]

Re- did leakdown numbers
#1 10%
#2 16%
#3 12%
#4 10%
I did this with engine cold, engine isn’t sitting-over-night cold but the coolant and engine felt cool to the touch.

 

[kenny nunez]

Just for kicks try setting the timing a 6. It cannot hurt, try to get some good gas at a marina. Sometimes you have to experiment. Lucky for my area I still can get 92 pure gas.

 

[cdavy000]

Just for kicks try setting the timing a 6. It cannot hurt, try to get some good gas at a marina. Sometimes you have to experiment. Lucky for my area I still can get 92 pure gas.

Yeah I wish I could get 92 lol. I’ve tried advancing and retarding timing within 10 degrees both ways it doesn’t seem to change anything unfortunately. Maybe I’ll throw some octane booster in my 87 gas and see what happens

 

[ROY WILLIAMS]

I took the top off the carb to check float level and it seems there is a spring at the bottom of the accelerator pump. That was not on my old carb but I suppose it could be normal I just thought I should point it out.

I was that carb picture the alum carb is corroded !!! the carb pump metal corroded

 

[ROY WILLIAMS]

Yeah I wish I could get 92 lol. I’ve tried advancing and retarding timing within 10 degrees both ways it doesn’t seem to change anything unfortunately. Maybe I’ll throw some octane booster in my 87 gas and see what happens

my 470 engine is the 93 octane gasoline .. timing 4-6 deg ....700 rpms

 

[nola mike]

Ok at this point I'd check the vacuum again... That bouncing would make me think a valve isn't seating. Confusing at this point... Your compression and leak down look good now

 

[cdavy000]

Ok at this point I'd check the vacuum again... That bouncing would make me think a valve isn't seating. Confusing at this point... Your compression and leak down look good now

I haven’t unplugged the vacuum gauge for a while now. Once I get the dwell perfect, the needle still vibrates quickly and bounces between 13-14” of hg. Only way I have found to increase vacuum at idle is to increase idle speed. But then my idle would be too high and out of spec.

It also might be worthy to note that the needle doesn’t vibrate or bounce when I increase the idle to 1000+ rpm, which is also when the vacuum reaches 20-ish inches of Hg.

 

[cdavy000]

my 470 engine is the 93 octane gasoline .. timing 4-6 deg ....700 rpms

I will try my 89 non-ethanol gas with octane booster… and then try to find 93 octane somewhere. Do you run non-ethanol?

 

[Scott Danforth]

It's not fuel

Your carb is still suspect

Set idle to spec and run the boat. If you still have popping, look at the chineseum carb