55hp Chrylser Outboard backfiring through leg:

[th69judge]

Hi:

I have been trying to get a 55hp Chrylser o/b running with no luck.

Motor sounds like it wants to fire up and then backfires down through the leg. The diaphram in the fuel pump has been replaced, the carburator cleaned & checked. Also the compression, points, condensers, coils and flywheel also checked. the flywheel key and keyway good.

This motor has had very little use, 20 tanks of gas or less but has been lying up for the last few years.

The motor is currently at the reapir shop and has the techs. baffeled. They have been in business for decades servicing all types of marinne eng. especially o/b motors.

My question: Is the crank shaft in 2 pieces and if so is it possible for the top and bottom to shift out of alignment thus causing a timing problem. If so should the engine start on one cylinder?

Any input or suggestions will be much appreciated.

Thanks for your time.

Regards,
Dave

 

[wickware]

Re: 55hp Chrylser Outboard backfiring through leg:

IMO! This will be a Frank?s question. You might get a quicker response if PM him or add ?Frank? at the start of your post (Frank! 55hp Chrylser Outboard backfiring through leg). This is an interesting situation and question. My limited expience is with a 1970 35 HP Chrysler.

Good Luck!

 

[th69judge]

Re: 55hp Chrylser Outboard backfiring through leg:

IMO! This will be a Frank?s question. You might get a quicker response if PM him or add ?Frank? at the start of your post (Frank! 55hp Chrylser Outboard backfiring through leg). This is an interesting situation and question. My limited expience is with a 1970 35 HP Chrysler.

Good Luck!

IMO ???????

PM ???????


Thanks for the response will follow up on this advise.

 

[tater76]

Re: 55hp Chrylser Outboard backfiring through leg:

Sounds like timing is wrong. I have twin 55hp's, and yes they will run on one cylinder, but no they don't have a split crank.
Timing should be set static at 28 degrees BTDC, I'm sure the shop knows this? Remember, your motor need 3 things to run.... compression, fuel, and spark. Rule out each, one at a time until you find the issue.

 

[rickryder]

Re: 55hp Chrylser Outboard backfiring through leg:

IMO ??????? In My Opinion

PM ??????? Private Message


Thanks for the response will follow up on this advise.

Here is the link to my 4shared account to download the OEM service manual for your motor http://www.4shared.com/zip/QWWFr2YC/ob2803.html

 

[Frank Acampora]

Re: 55hp Chrylser Outboard backfiring through leg:

The engine has two sets of points and each must be set accurately. Gap is .020 at the high point of the lobe. Points must be clean and springs must not be weak. The crankshaft key also holds the cam in place. If the lower extension is broken, the cam will not stay correctly placed. The cam itself has a right side and wrong side. The arrow must be on top. Check or replace the condensers. It is unusual but one may be bad causing misfiring and ignition in the exhaust chest. Look for a frayed wire in the stop circuit--the two brown wires that look like lamp cord. Check all wires at the points--especially the condensers--they tend to rub on the underside of the flywheel and short.

 

[wickware]

Re: 55hp Chrylser Outboard backfiring through leg:

I hope I did not offend other Techs and myself asking for Frank?s opinion due to the question of possibly ?a split crank? throwing me while napping out of the TX Heat (after 50 yrs and not one split shaft). From what I?m seeing my 1970 35 HP Chrysler might be questioned as a split shaft. Why was the term mentioned/used (2 sets of points)? Hello! I was qualified if this is the case. LOL! It?s time to put that ohmmeter, etc. and witts together in the old mag system (A Bug! In Computer Language).

 

[Frank Acampora]

Re: 55hp Chrylser Outboard backfiring through leg:

Wick: Some of the foreign engines do use a multiple part crankshaft with the crankpins pressed into the crank cheeks-- Thus: A split crankshaft.

 

[tater76]

Re: 55hp Chrylser Outboard backfiring through leg:

Whats the reasoning behind the split Frank? I have not run into this yet, and would love to see a pic if anyone has one?

 

[th69judge]

Re: 55hp Chrylser Outboard backfiring through leg:

Hi All

Thanks for all the feedback and suggestions. I will definitely try them all, I see no reason why this o/b won't start. Glad to hear that the crank is 1 piece that rules out 1 potential serious cause.

Will post an update of o/b when all suggestions have been ruled out.

Water skiing season is upon us.



Once again thanks for your input.

 

[Frank Acampora]

Re: 55hp Chrylser Outboard backfiring through leg:

When using the crankpin as a bearing surface, split cranks are way easier and cheaper to manufacture. -- read cheap, old school Japanese engines. I doubt that modern ones would have this type crank.

 

[tater76]

Re: 55hp Chrylser Outboard backfiring through leg:

Ahh... cheap makes sense, thanks.

 

[orbanp]

Re: 55hp Chrylser Outboard backfiring through leg:

When using the crankpin as a bearing surface, split cranks are way easier and cheaper to manufacture. -- read cheap, old school Japanese engines. I doubt that modern ones would have this type crank.

Actually, probably the cheapest way to manufacture a crankshaft is the one piece type. The whole automotive industry is set up for manufacturing crankshafts like that, starting from cast, forged, or billet machined blanks.
The built up crankshaft have the advantage of not having a split race for a roller bearing. While a journal bearing works with a split bearing cap just fine, any misalignment at the parting line for a split roller bearing cap reduces bearing life significantly!
Built up crankshafts can be made by pressing together the components, or by bolting them together with special couplings like the Hirth coupling.
The pressed together assembly is cheaper of the two methods, used to be used widely in Europe and Japan.
A bolted together crankshaft also allows for in-situ assembly, in this case the crankcase design could be simplified/optimized. The bolted together crankshafts are used for some big diesel engines, for aero engines, and for racing engines. The Hirth coupling itself is very expensive!
Built up crankshaft have another point in their favor. The offset part of the crankshaft is usually made like a big disk. It is like the counterweight on a one-piece crankshaft. In two-stroke motors those disks "fill out" the crankcase volume more than a crankshaft with a simple offset. There is a number of merit for crankcase efficiency for two stroke motors, the ratio of crankcase volume when the piston is at TDC and at BDC, like the compression ratio for the cylinder. A higher number provides for better scavenging, resulting in more power.
Built up crankshafts also tend to be more rigid.
So it is not just cost...

Peter

 

[Frank Acampora]

Re: 55hp Chrylser Outboard backfiring through leg:

But Peter: You are comparing apples and oranges. Automotive cranks have pressure oiling and babbit bearings. Outboards use oil misting and the crankpins (along with the rods) and center bearings are fully hardened. This takes special tooling to harden them. A loose crankpin can be hardened in batches then later pressed in. Easier and cheaper.

Remember that even on the early Japanese outboards with multi part crankshafts, for ease of assembly they still used split cap technology rods.

 

[orbanp]

Re: 55hp Chrylser Outboard backfiring through leg:

Hi Frank,

I have no numbers to support the cost of manufacture of a single vs. built up crankshaft, so I guess we could argue endlessly ;-)

I know that manufacturing gravitates towards the lowest cost because of its very competitive nature. I also know that assembly is expensive.
If you look at the number of components of a built up crankshaft with its additional manufacturing steps just for the mating surfaces, I would have a hard time to believe that a built up crankshaft is cheaper to make than the single piece one.
Heat treating single pins might be cheaper than a whole crankshaft, but that is only one part of the cost of manufacturing.
If you look at present day Japanese (e.g. Yamaha) two-stroke outboards, they also have single piece crankshafts.

BTW, automotive crankshafts with journal bearings also require hardening of the bearing surfaces, though they use different heat treatment processes, as a journal bearing creates different stresses on the bearing surface than a roller bearing.

Peter

 

[th69judge]

Re: 55hp Chrylser Outboard backfiring through leg:

Hi All:

Good News & Bad News.

Ran out of options as suggested still no go so I spoke with the tech and told him I had a spare flywheel, coils and points plate. Dropped them off and he installed the spare flywheel only. Turned the key and it fired up immediately, ran very smoothly in the water barrel, pumping lots of water but stopped after about 5 - 10 minutes. A quick check of compression is showing 150 lbs lower cylinder and 20 lbs upper cylinder. It appears that the rings broken but won't know until head is removed.

Thanks for all your advise. I guess the upside is the engine did run but we are still at a loss as to what is wrong with the original flywheel. Side by side they look the same in all aspects.

Now all I have to do is find a way to remove one bolt that is seized in the head but free in the block.

Once again many thanks.

Regards,
Dave