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

[Scott Danforth]

I would look at timing. if you havent set dwell with a meter, start there

 

[cdavy000]

Find out if you have a valve leaking past seat that would be my inclination with low compression and backfire

Will do

 

[cdavy000]

I would look at timing. if you havent set dwell with a meter, start there

All right I’ll have to pick up a dwell meter. I set the points with a feeler gauge and I have made ignition timing spot on with a timing light numerous times. I read online that you only needed to set points with a feeler gauge but I guess that’s not good enough.

 

[cdavy000]

So I used an ever tough combustion leak tester and I had zero combustion gasses in the coolant, which tells me my gasket isn’t blown on a cylinder-to-coolant portion. I’m not sure if that’s helpful I just thought it might be.

 

[Scott Danforth]

When setting points, going to quote my father and high school autos teacher..... feeler gauge is close, setting with a dwell meter is accurate

 

[Scott06]

So I used an ever tough combustion leak tester and I had zero combustion gasses in the coolant, which tells me my gasket isn’t blown on a cylinder-to-coolant portion. I’m not sure if that’s helpful I just thought it might be.

You said the AF side of the cooling system did not hold pressure, correct? if so there is a leak somewhere could be heat exch.

Would do a poor mans leakdown test or rent a leakdown tester to see if valves are leaking by and verify dwell timing and Scott recommended.

Hard to say from the keyboard but you are loosing compression somewhere, if not head gasket that leaves rings and valves... if that's is the case you are looking at machine work to correct a red head stepchild of an engine.

 

[cdavy000]

You said the AF side of the cooling system did not hold pressure, correct? if so there is a leak somewhere could be heat exch.

Would do a poor mans leakdown test or rent a leakdown tester to see if valves are leaking by and verify dwell timing and Scott recommended.

Hard to say from the keyboard but you are loosing compression somewhere, if not head gasket that leaves rings and valves... if that's is the case you are looking at machine work to correct a red head stepchild of an engine.

No no sorry the cooling system itself holds pressure just fine it’s my coolant cap that wasn’t holding pressure it’s just old and needs to be replaced.

Yeah I just picked up a dwell meter and cylinder leak down tester and I’m going to do tests in the next day or two. I’ll post results when done.

 

[Scott06]

No no sorry the cooling system itself holds pressure just fine it’s my coolant cap that wasn’t holding pressure it’s just old and needs to be replaced.

Yeah I just picked up a dwell meter and cylinder leak down tester and I’m going to do tests in the next day or two. I’ll post results when done.

Ok if cooling system holds pressure head gasket is ok, look elsewhere for low compression rings or valves.

Rule out dwell and timing.

 

[cdavy000]

Ok if cooling system holds pressure head gasket is ok, look elsewhere for low compression rings or valves.

Rule out dwell and timing.

All right sounds good thanks, I will rule out dwell tomorrow and I will do leak down test today to check for worn rings or valves etc.

Is it possible that the head gasket blew between cylinder 1 and 2? Causing high compression in cylinder 1 (126 psi) and low compression in 2 (103psi)

(Based on the equal number of 114 psi in cylinder 3 and 4)

 

[Scott Danforth]

the pressure test says that your head gasket where the cooling passages are is in tact. being an open deck block, it would be unlikely for combustion to transfer from one cylinder to another without entering the combustion side.

you did have the cooling water drained when you did your test, correct?

 

[cdavy000]

the pressure test says that your head gasket where the cooling passages are is in tact. being an open deck block, it would be unlikely for combustion to transfer from one cylinder to another without entering the combustion side.

you did have the cooling water drained when you did your test, correct?

Oh, no I didn’t have the cooling water drained so I assume those results are invalid. But I did get a combustion leak tester, used it on my cooling system while running and no combustion gases were detected in the cooling system.

When you say it’s unlikely for combustion to transfer between cylinders without it entering the combustion side, I don’t think I understand what that means. Are you saying that it’s unlikely for combustion to leak between cylinders through a hole in the head gasket? And when you say “combustion side” were you referring to the combustion chamber? I was under the impression that combustion was only in the cylinders, although I could be dead wrong. Sorry I just didn’t understand that part

 

[Scott Danforth]

When you say it’s unlikely for combustion to transfer between cylinders without it entering the combustion side, I don’t think I understand what that means.

Here is a picture of the open deck.

see how any combustion gasses seeping past the cylinders except that little bit of landing in the middle would have to go into the cooling water jacket.


here is a picture of the open deck being closed for a high-performance hot-rod build.

 

[cdavy000]

Here is a picture of the open deck.

see how any combustion gasses seeping past the cylinders except that little bit of landing in the middle would have to go into the cooling water jacket.
View attachment 390956

here is a picture of the open deck being closed for a high-performance hot-rod build. View attachment 390955

Ah yes now I understand that’s a great visual, yes I would agree that would be extremely unlikely for it to only blow in that tiny little landing, thank you. I’m about to perform a leak down test so hopefully that will shed some light on this

 

[Scott Danforth]

I would drain the coolant prior to the leakdown test. I would also re-test the water jacket

 

[flashback]

Here is a picture of the open deck.

see how any combustion gasses seeping past the cylinders except that little bit of landing in the middle would have to go into the cooling water jacket.
View attachment 390956

here is a picture of the open deck being closed for a high-performance hot-rod build. View attachment 390955

The hotrod build is interesting, how is it cooled if you fill it with weld. I guess they do it for extra support on the cylinders?

 

[cdavy000]

Okay so leakdown test finished - finding TDC on all cylinders is a B***H btw- and it seems that the results are good, but I’m not sure

I left in compression numbers for reference

#1 126psi, 9% leak (most of the air out of oil fill cap but a little out of intake)

#2 103psi, 10% leak (all air out of oil fill cap)

#3 114, 10% leak (almost all air out of oil fill cap but ever so slightly out of intake)

#4 114, 10% leak (can hear air half out of oil fill cap and half out of intake)

 

[cdavy000]

I would drain the coolant prior to the leakdown test. I would also re-test the water jacket

I seemed to have seen your suggestion to drain coolant before leak down test too late. I will test it when I do coolant change.

 

[cdavy000]

When setting points, going to quote my father and high school autos teacher..... feeler gauge is close, setting with a dwell meter is accurate

Okay so I am trying to set dwell with a meter but my dwell is bouncing sporadically. It hits 27 degrees and 52 degrees at the extremes as well as everything in between. I’m not really able to accurately set it as it’s bouncing so much. I replaced the condenser and points just last year.

 

[kenny nunez]

With the spark plugs out and the coil wire grounded spin the engine and you will be able to get an accurate setting. If the dwell still moves and does not hold steady the distributor bushings or the centrifugal advance parts under the breaker plate could be worn out.

 

[cdavy000]

With the spark plugs out and the coil wire grounded spin the engine and you will be able to get an accurate setting. If the dwell still moves and does not hold steady the distributor bushings or the centrifugal advance parts under the breaker plate could be worn out.

I had the spark plugs out but I only pulled the ignition coil wire out of the coil, I didn’t ground it, do you think that could be the issue? Also when i tried to set the dwell, whenever I cranked the engine sometimes the spark would jump from the points and I could see it hit engine parts like 6 inches away, I’m not sure if that’s normal. I replaced advance springs last year so I’m hoping that isn’t the issue, but it’s possible bushing is bad. I basically just took what I could get and watched the highest and lowest readings on the dwell meter while cranking and adjusted it where the reading in between those two was the dwell number I used. After that I started the engine and the vacuum was MUCH more stable. The needle only slowly bounced between 13-14 psi when warm (11.5-13.5 when cold) and I could audibly hear the engine revving up a tiny bit each time it bounced.