1974 70 HP 70ESL74M Carb Orifices Different than Book??

[Bay Boater]

It's a little more than an orifice question but not much.

This motor is at an Adirondack Scout camp - only gets used occasionally over two months of the year.

I came in this year to get it going and had to futz over it a while to get it to do that. Not too bad now, but it's still quirky and I found some deviations from the book in terms of the orifices used in the carburetor. Can these explain my symptoms? I suspect in the past someone may have put carbs on from another motor thinking they were the same since externally they are the same.

Throttle and choke synchro, pickup timing, and related things are set up by the shop manual.

The motor is easy to start but needs a lot of choke. Even when warm it likes a tap of choke and it spins right up.

But when I put it in gear at idle it goes dead. If I put it in gear and aggressively advance the throttle it will pick up and run steady. But when I throttle back it comes down very smoothly to a bit above idle and dies.

Have also had some carbon tracking on the no. 3 spark plug (flat plugs) but that may have been started by a weak spark I had for a while on that one. But the sparks are all strong now and that one still builds a carbon track after I wipe it off. The motor will start skipping after 40 minutes or so and re-cleaning the plug brings it back for a while. It may just be following the path of the original track so I will be looking at a new plug there.

The low speed or idle orifice (#27 in the parts diagram) is called out to be a no.30 (0.030" dia) but the carbs have no.27's installed. The orifice plug (#28 in the parts diagram) is called out to be a no.33 but the carbs have no.35's installed.

Could these mis-matched orifii account for the bad behavior?

The other orifice plug in the bottom of the carburetor bowl is called out to be a D61 and is correct.

Other things going on include very old gas - some a year old and some a year old mixed with last years old gas. Will be testing fresh soon. Also, that old gas may have been mixed with too much oil.

 

[Rustywrench]

First you might ditch the surface gap plugs. Go with L77JC4 Champions or equivalent NGK. The surface gap plugs are too cold for idling applications. Someone has richen the engine somewhat. That may not be a bad thing. They have only gone a little bit. An acceptable amount if a problem was encountered. Your idle speed may be set a little low? Do you have a tach on the boat? About 750 in gear is good. Might be as much as 1000/1100 in neutral. Fresher fuel will also help today's fuel doesn't last long.

 

[jimmbo]

It might have carbs from a 73 65hp, or from 1975 or newer 70 or 75hp. Since you didn't provide a link tot he parts diagram it isn't clear exactly which orifices are which, airbleeds or fuel jets. One of the joys of buying a used engine, you really don't know what you are getting. It is hard to say that the carb isn't actually metering correctly for its calibrations.
Agree about the idle speed being low, but those engines usually run real nice at 600 - 650. You need to unhook the throttle cable when setting the idle speed, then when it is correct, you have to adjust the cable length so there is a slight preload on the cable(ensures consistent idle speeds but making sure the linkage is against the idle stop).
Regarding the surface gaps, I like them myself, the plugs that came from the factory in my Merc, were still in it when I traded 14 yrs later. That engine idled a lot and it didn't have a thermostat to keep it warm at low speed, the plugs never fowled

 

[dsweet2010]

I have the same year and the same exact motor .I experienced similar weird issues due to the auto choke mechanism which I did not even know existed . Apparently this was an idea they tried then that was very unsuccessful and discontinued . If you have not already bypass that, there is reference on this website of how to do so. I would be willing to bet that that is failing intermittently causing choke issues whilewhile starting and while running. Ever since I disabled mine, it has done perfect.

 

[racerone]

Post compression numbers.

 

[Bay Boater]

Responding to all at once...

The flat plugs are not the root cause of this problem so I'll save them for possible future work. A fresh plug to replace the one contaminated with the carbon track is my only plan there for now.

I don't have a tach on the boat. I can tell by ear if it's low enough to shift safely and that's about it for now.

Parts diagram attached. I think they are all the same (from the Johnson book?) but who knows for sure? This is the diagram off the iboats parts pages.

Part #27 is the "idle orifice" and is supposed to be no.30. I have a no. 27 installed.

Part #28 is the "idle restr. orifice" and is supposed to be no.33. I have a no.35 installed.

Part #45 is the "orifice plug 61D" and the correct one is installed.

Auto-choke is disconnected with both solenoid coil leads tied together and the temperature sensor removed and lost somewhere. Overtemperature sensor still in place and operating.

Compressions top to botton are 125, 125, and 115. BUT, sometime in the past this motor lost a cylinder and whoever rebuilt it replaced the domed piston with a flat one in no.3, so that compression should be a little lower even if the rings are equal to the others.

The motor has an entirely new ignition, installed two years ago. Coils, power pack, sensor ring, and charge coil. Nothing in its behavior has suggested an ignition problem.

I have some loose 30's and 33's from another set of junk carbs. My present plan is to replace them and run on fresh gas. The fact that both of the orifii needing changing are related to low speed / idle gives me some grounds for hope.

 

[Bay Boater]

Changed the orifii this weekend to the correct ones. Night and day difference. I wonder how many misbehaving motors out there have had their carbs replaced with the wrong ones - the person replacing them thinking they are the same because they also came off a 70 hp or something close and they look identical from the outside? But they have different orifii inside intended for a different motor.

 

[jimmbo]

'Close' only really works with Horseshoes, Grenades, and Nuclear Weapons. Carbs that look the same, No.