Backfire!

[CraigBarnard]

Ok new problem.
We are dealing with a 120hp mecruiser from a 1965 Starcraft Cheiftain.

Idles just fine.

Try and rev it up over idle and it backfires badly.

All new ignition parts. Plugs, cap, rotor and wires.
New fuel filter. (2)
Carb cleaned and new kit installed. (3) times.
Changed the points to pointless and then back again.
Changed the coil.
Changed the fuel pump.
All new fuel lines.
Ran on a independant fresh and clean fuel source even bypassed the filter at one point, straight into the pump.
Checked the neutral switch.
Checked and rechecked the timing then checked it again.
Checked for vacuum leaks.
Compression test good, close to 150 on all four.
Leakdown test good.

If you pump the throttle to spray additional fuel into the carb it will improve (a little) but normal increase in throttle will cause a backfire
I am at a loss and so are two seasoned marine mechanics.

Any ideas? Help?

Thanks

 

[reddogg]

Re: Backfire!

How did the engine run before you did all that to it? My gut is saying it's something to do with the points, do you still have the original points?

Red

 

[CraigBarnard]

Re: Backfire!

How did the engine run before you did all that to it? My gut is saying it's something to do with the points, do you still have the original points?

Red

New boat to me. Original owner said it ran ok but the real history will never really be known.

"all that" was done to try and stop the backfire problem.

I changed to pointless ignition then back to points again. Same issue.

Still at a loss, lol

 

[gchaney]

Re: Backfire!

Sounds like you have an extreme lean condition check your intake for a crack or faulty manifold gasket. When you rebuilt the Carb did you put new jets in it or just clean?

 

[CraigBarnard]

Re: Backfire!

Sounds like you have an extreme lean condition check your intake for a crack or faulty manifold gasket. When you rebuilt the Carb did you put new jets in it or just clean?

The carb was dealt with by a marine mechanic. I believe he changed the jets or serviced them at the least.
Checked for vacuum leaks already and none seem to be present.

 

[Mahoney]

Re: Backfire!

Check and recheck your spark plug wires and sequence. Whenever you change ignition components it is extremely easy to get the rotor in 180 degrees off or switch a plug wire.

 

[Peaceful Pirate]

Re: Backfire!

Just a couple of thoughts. Verify that the valves are properly adjusted. Instructions are in your service manual. Second, trouble shoot your ignition from the ballast resister to your coil. Check for proper resistance through the ballast resister and +/- 7 to 9 volts at the coil with the engine off and ignition switch in the ?Run? position. Best of luck to you.

 

[1977 SILVERLINE]

Re: Backfire!

Pull your distributer cap and check if the distributer has a mechanical advance. If it does the rotor should turn opposite of normal rotation and spring back normally.

 

[CraigBarnard]

Re: Backfire!

Just a couple of thoughts. Verify that the valves are properly adjusted. Instructions are in your service manual. Second, trouble shoot your ignition from the ballast resister to your coil. Check for proper resistance through the ballast resister and +/- 7 to 9 volts at the coil with the engine off and ignition switch in the ?Run? position. Best of luck to you.

No ballast resistor in circuit.

 

[CraigBarnard]

Re: Backfire!

Pull your distributer cap and check if the distributer has a mechanical advance. If it does the rotor should turn opposite of normal rotation and spring back normally.

Mechanical advance working properly.

 

[Peaceful Pirate]

Re: Backfire!

No ballast resister? Interesting. Would think in a ?65 there should be a resister unless rewired sometime in the past. ?65/?66 specs call for a ballast resister and a coil without an internal resister. Without a ballast resister, you need to have either a resister wire in line between the battery, usually off of the starter, to the coil, or a coil with an internal resister. Would recommend you look over your appropriate wiring diagram and verify what you have.

 

[CraigBarnard]

Re: Backfire!

Okay looks like the problem has to be internal.

I was an automotive mechanic for the last 20 some odd years and i could not figure it out.
I know marine stuff is similar but differant enough to throw me a few curves so i sent it to a marine mechanic customer of mine.
The marine mechanic i took it to has thrown his hands in the air and given up also.

Me thinks some idiot has pieced this motor together with oddds and ends parts. This has happened to me before.

Now i am on the lookout for a replacement motor.

Any suggestions on what vehicles I can find a donor? Will an automotive 2.5 (iron duke) work?
How about a straight six? If i have to pull it i would love to put in something cool in its place.

 

[Peaceful Pirate]

Re: Backfire!

Starcraft Marine Chieftains are nice old boats, but like most of the older boats, they can be a pain in the butt! We do understand. Looking out the window here, we see a ?68 Golden Triangle deck-boat, two ?59 Glastrons and a ?64 Sears & Roebuck (build by Arkansas Traveler).

Ok, back to your Chieftain, if you don?t have one yet, get hold of a Mercruiser Service Manual #1, C-90-68648. It come in two books, Vol 1 & Vol 2, and can be downloaded here on Iboats, or you can pick up a printed copy on eBay for little or nothing. Forget the other options and stick with the Mercruiser manuals. Also, you can occasionally find the supplement manual to Manual #1 around with most of the relevant Service Bulletins.

You would have no problem pulling out a 120 and replacing it with a 140 HP marine engine. Forget trying to put an automotive engine; there are too many differences an automotive and marine engine to make that option viable. Especially since there are so damn many 140?s available. A great deal of boats from the 70?s, 80?s and even 90?s sitting around in boats that have rotten floors and transoms. When we need one, we look on Craigslist and pickup a rig someone wants hauled off for little or nothing. The only caution here is watch for ones where the engine well has been left open. Most have water intrusion down the carb and thus water in at least one cylinder. Even here; however, they can sometimes be punched 20/30 over and rebuilt for $400 or less, especially with you background.

If you are pulling a 120, like noted above, the 140 is a direct replacement. To go to the straight six your footprint in the well is two longer and in most cases will require a great deal of reworking. Depending on you well, you might get a V-6 4.3 in there, but it will require the replacement of everything else, where the 140 is a direct drop-in. Our guess is you will be happy with the performance of a 140 since if you really had wanted speed, you wouldn?t be in a ?65 Chieftain in the first place!

Hope this helps. If we can be of any assistance, drop us a note and we will see what we can do to help.

 

[CraigBarnard]

Re: Backfire!

Is the 140hp a 4 cyl. as well?

 

[Peaceful Pirate]

Re: Backfire!

Yes sir. Basically identical to the 120; the 120 is 2.5 and the 140 3.0.

 

[fishrdan]

Re: Backfire!

You mentioned that the engine might be pieced together, have you pulled the manifold to see if the gasket is intact and the head and manifold match? There are several different styles over the years and maybe someone put together a bad combination.

If you are pulling a 120, like noted above, the 140 is a direct replacement.

The 140HP (3.0L) is just a stroked 120 (2.5L) 4cyl GM marine engine, but I wouldn't say it's a direct replacement,,, UNLESS you get a complete 140 of the same vintage. I wouldn't try to piece-meal one together. Also, there are differences through out the years, especially since yours is a 65'. If I were to do the engine swap, I'd replace everything from the waterpump to the prop. Get a rotted glass boat like Pirate suggested. That old 65 drive is going to be hard to get parts for, upgrading to an Alpha drive would be great.

 

[CraigBarnard]

Re: Backfire!

Went looking for a donar boat or motor and I found a 140 hp mercruiser complete.
It was only a 20 minute drive away too.
Picked it up this morning.
Everything appears to be there but with my luck concerning boats I figure I will find something bad.
Intake/exhaust manifold is definatley differant looking but as others have mentioned things change over the years.
Its a newer motor so hopefully parts etc will be more readily available. Perhaps even a better designed motor.

One question.
Are the blocks the same? I have a worry I may find a crack from improper storage and if so i was hoping to reuse the block from my 120 if necessary.

Thanks to everyone for all their assistance so far.

 

[Peaceful Pirate]

Re: Backfire!

No can do!! The bore dia and head are different.

 

[CraigBarnard]

Re: Backfire!

Okay. Lets just hope the assembly is a good one.
Thanks

 

[Peaceful Pirate]

Re: Backfire!

Just a quick thought for you to keep in mind. As you now know, 140?s are available, so you might want to keep your eyes open for another one, and possibly an outdrive, in decent shape. That way if something goes out, you have parts available, or even a back-up that you can drop in to keep you going while you repair the other unit. On a good day, you can do an engine change out in three to four hours, even less if you push it; and an outdrive change out is easily a 30 min job.