1974 evinrude 70 hp won't rev in the water ???

[Jd 1978]

Hello all. I have read these forums for some time now but this is my first post ! 1974 evinrude 70 hp will not rev while submerged ?? At wot I'm guessing cause the tach doesn't work that I'm only revving up to maybe 2500 rpm. Maybe less. The warm up lever idles it up higher than wot. I have a tach on order so the rpm guessing will stop soon. But anyway the motor has dead even compression on all three cylinders at 125 apiece. The motor starts easily and idles smooth. On muffs it revs what seems like the full range. It literally screams on the hose. When on the water if you trim the motor up to where it's just barely submerged it revs alot higher. I thought it might ba a prop issue? Can a prop cause you to lose thousands of rpms ?? I tried to see what prop was on mine but I can't find any numbers ? Anybody have a rough idea ? 16 ft renken runabout. Also I took the carbs completely apart and cleaned and blew the heck out of them. I'm lost. At wot and it tops out on rpm it's not hitting a rev limiter or anything cause there is no miss it's steady and smooth just won't rev. You can prbly tell I'm new to boats by reading this but I have been around cars and bikes my whole life. Figured I could relate the two a little easier. Any help would be appreciated. I know I need to get out and get a manual soon. Hoping to do that next weekend. Thanks for your time.

 

[jgriner]

Re: 1974 evinrude 70 hp won't rev in the water ???

im not an expert, but i know u NEVER NEVER NEVER NEVER rev the motor on the hose, its a good way to blow it up, their is no back pressure on the exhaust, and no resistance on the prop, kinda like putting your truck in neutral and flooring it, yeah its going to rev, and rev much higher then if it was in drive.

sounds like for some reason you dont have a cylinder fireing, or something thats y it sturgles to rev. but like i said im not an expert

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Thanks for the advice. Like I said I'm new to boats. Glad I never really set there and revved it for very long.

 

[Daviet]

Re: 1974 evinrude 70 hp won't rev in the water ???

jgriner is correct, NEVER run more than 1200-1500 RPM on the hose.
All it takes is timed spark, compression and fuel to make your engine run. Sounds like your compression is ok. Have you checked the spark, jumps 7/16" and is bright blue? You said you took your carbs completely apart, did you remove the core plug on top of the carb and clean the passages underneith, remove and clean the orifice in the bottom of the bowel? Leave the plug out of the bottom of the bowels and pump the primer bulb, good fuel flow out of the bowels?
On the hose those engines run great on two cylinders, but under load they don't run that well.

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Thanks daviet for the reply. The only way I checked spark is by using my inline tool. The one with a light bulb that lights up. Is there a tool that measures spark distance ? Or should I just hold it 7/16 away from the head ? As far as the the core plug on top of the carb, are you referring to the very top that has the flathead screw with the jet behind it? Or another part all together? I found what seemed to be three jets in each carb. One in very bottom of float bowel. One to the right of float and the one at very top behind the flathead screw. Three jets was strange for me but what was really strange is there was no needle at all. Guess that's why there are three jets ? I havent pumped the primer bulb to see the gas come out of the bowels with the drain removed but I will try that. Thank you for the advice.

 

[Daviet]

Re: 1974 evinrude 70 hp won't rev in the water ???

They make a tool that you can adjust the air gap, most auto parts stores carry them. All the jets in you carbs are fixed, no adjustments. The core plug that I am talking about is #5 in the diagram below.
http://www.ishopmarine.com/ishop/js...CatFilter=BRP&eCatSupFldr=supplier-100-005-EJ

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

I'll pick up one of them tools and measure. And no I did nothing with number five. I was referring to number 26. What size drill bit do I use ? Is there another jet under there ? Thanks for your time.

 

[Daviet]

Re: 1974 evinrude 70 hp won't rev in the water ???

I usually use a 1/8" bit, just barely go through, and use a punch in the hole to pop the plug out. There are not any jets, but there are some very small holes that need to be clean.

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Ok. Picked up a spark tester and at 7/16 the spark is bright blue and jumps the gap easily. Rule out spark then ?? I also did the test with the carbs. Pulled all three bowel drain plugs and pumped the primer bulb. Fuel flowed freely through all three drain holes. Then removed the carbs and those plugs on the top. Under was three little holes that I stuck pieces of long aluminum brush through and filled with carb cleaner and it drained down so I'm pretty sure they are clear. Anymore ideas come to mind ? Thanks.

 

[Daviet]

Re: 1974 evinrude 70 hp won't rev in the water ???

High speed jets in the carbs clean? The ones behind the plugs in the bottom of the bowels. If all are clean, I would suggest trying to borrow a portable tach and see exactly what the RPM is at Wide Open Throttle (WOT).

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Tryin to get my hands on one of them now. Will have to wait until this weekend to get it out on the water though. Will let you know. Thanks.

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Ok. Was finally able to get the boat out. At idle I was running at 1000 rpm. Tried to lower it while at the dock but when I did the motor would die off ?? And at wot the rpm got around 2100 to 2200. That number is prbly off because the only portable tach I could get ahold of was a photo sensor tach. I had to hold it pointed at the tape strip so I'm sure it's not accurate but prbly close enough to see the motor isn't revving high in the water. One thing I did notice this time was the motor jumped and tried to rev higher and the boat tried to take off. These higher revs were short lived and I wasn't able to get a reading on the increase. So maybe I'm back to a carb problem ?? Does anybody know what jets should be in my engine ? Also I found a wasp nest in the lower unit that I picked out. If I didn't get all that out of the vents could that cause a problem ?? I'm not sure how to proceed. Thanks.

 

[Daviet]

Re: 1974 evinrude 70 hp won't rev in the water ???

OK, all it takes to make your engine run is timed spark, compression and spark. Your compression numbers look good. Here is a couple of things to try. With the front cover off of the carbs, have someone run the boat for you at WOT, use a spray bottle with fuel mixture in it, squirt fuel into each carb and see if you have a cylinder that picks up. If nothing happens with the fuel, use an inductive timing light and check each plug wire and see if you have a dead cylinder under load.
Is there any chance that the flywheel key could be defective?

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

I was wondering how to proceed and those two tests make sense. Unfortunately I will have to wait til the weekend to try it out. I know nothing about the flywheel key ? How do I check it out ? Also was wondering what the timing was supposed to be set at ? Appreciate the help !

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

ok, finally got the boat back out on the water...... while at WOT running across the lake we shot the fuel mixture into the carbs there was no change unless you squirted several shots in there and then the motor bogged down.... also hooked a timimg light to the spark plug wires and it only came on with the middle cylinder.... that really didnt make sense to me, but i kept changing it around and the timing light only worked on the middle ?? any ideas with that ?

 

[patrick4266]

Re: 1974 evinrude 70 hp won't rev in the water ???

sounds like your running on 1 cyld. Move your coils around and see if the spark go's to a different cyld.

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Thanks Patrick. I didn't think to do that because when I sprayed fuel in each cylinder the motor bogged down. If the cylinder wasn't firing would the motor bog down with extra gas ??I'm going to a local boat yard to pick up some coils cause mine looks old and to rule them out. But I gotta wait to take the boat out this weekend to test them out. Anything else anyone can think of? Thanks.

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

My coils did jump a 7/16 gap during cranking. Is it possible for them to not work properly under load ? Wouldn't make sense to me but like I said I am new to boats. Thanks.

 

[jtexas]

Re: 1974 evinrude 70 hp won't rev in the water ???

The spark plug gap is only 3/1000 of an inch, so if it's strong enough to jump 7/16" in the open air, that pretty much confirms you're getting spark in the hole under heat & compression.

That's assuming the spark plugs are in good condition. If the plugs are too old, or carbon-fouled, the electricity will find an easier path than jumping an air gap. Your motor will run best on champion QL77JC4 at .030.

I can't explain your result with the timing light, but I can tell you that my motor will run on 2 cylinders (very smoothly, just lacks power under load), but I've never gotten it to run with only one working cylinder -- not saying it can't happen, but experience tells me it's pretty unlikely.

I think you are right, if it bogs when you spray the gas, that cylinder must have been firing.

You don't mention carb kits; order three kits from iboats (I been using the sierra kits with good results), replace all the gaskets and o-rings, and especially the float valves (needles *and* seats). You'll need the core plug replacements anyway. Just set 'em in place and flatten with a hammer and punch, or a small socket works, or really any flat tool. Doesn't take much. Don't worry drilling 'em out, there's some space under there, just try not to poke through too hard.

You really can't tell from visual inspection whether a gasket or oring is letting air in or gas out or whatever, and worn float valves will affect performance before they wear down enough to tell with the naked eye. Ok, maybe with enough experience you can learn to eyeball it & tell, but I sure can't.

Click on the "Top Secret File" at the top of this forum and follow the link to Carb Cleaning and Rebuilding for some more good tips.

Also, make sure the carb throttles are all closed at idle, parallel throughout their range of motion, and perfectly horizontal (not beyond) at WOT, and that the choke plates stay open (except when you engage the choke).

 

[Jd 1978]

Re: 1974 evinrude 70 hp won't rev in the water ???

Thanks for the reply jtexas. I double checked my throttle plates and choke plates and all is fine and move as they should. I went ahead and ordered those carb kits you spoke of part number 18-7222. And as far as the plugs they are new but I'm running champion ql78c. Would that effect anything ? Autozone cross referenced the plugs that were in the motor when I got it. So my plan is to play around with these used coils I picked up swap them around a bit to see if anything changes. Unfortunately I will have to wait til the weekend to test anything. Thanks.