1979 evinrude 70hp acceleration problems

[bigcatfishing]

Hello everyone. I have a 1979 evinrude 70 hp motor. I couldn't get up on plane and it had a 13 1/4 inch x 17 prop. I changed prop to a 13.5x 15 inch. I can finally get up on plane. My problem is I really have to baby the motor after I put it in gear. Just a little throttle at a time and it is a little rough running at times. When I get to where I can go full throttle it kindof stammers and boggs a little bit then all of a sudden it picks up and runs fast and I get up on plane. At that time it runs pretty good it has just a small miss feel to it. Any suggestions as to why it runs pretty rough getting to WOT then all or a sudden just takes off? I have no ideal. It runs pretty good at idle too. High idle and all works good. Thanks in advance.

[fireman57]

Sounds like you have a clogged jet in one of your carbs. It seems to be starving for fuel. If your old prop worked fine before then there was no need to change it. That points to an engine problem.

[bigcatfishing]

Ok will have to check the carve. They were just rebuilt a month ago. I changed the prop because I never could plane out. Full throttle and the the back would ride real low and never come up. Also the engine rpms never did come up. I drove it approx 1.5 miles and it took about 5 gallons of gas to do it. This time with the new prop it didn't use near the fuel either and I could plane out. It just took a little time.

[grizzleyjoe]

You had the crabs rebult ? . I changed mine out cause it would bog out on a wave or wake . Then I had that problem the when I go full throttle it would brake up . I adjusted th linkage and it works fine now .

[fireman57]

Do you have the engine trimmed properly? The reason I ask is because you said you couldn't plane off before. Did it ever plane with that prop that you know of?

[bigcatfishing]

With the old prop it never did plane out not that I know of. It would at full throttle just ride rather slow with the front up high and the rear down low. I have it trimmed like I was told by the mechanic. I am not real familiar with boats. This is our first. It don't have tilt and trim. It is manual. I have the pin set in the last hole closest to the transom. The prop has a very slight downward angle toward the back. Like I said before it don't run all that bad when it hits plane and the motor winds up. Just a small miss in it. I just had a new power pack, rectifier, starter silenoid, and all three carbs rebuilt. I can't afford another high mechanic bill. So all of your help is greatly appreciated.

[soundchaser]

That engine needs to be able to do 5500 rpms at WOT if it doesn't You need to correct that somehow.

[fireman57]

If the mechanic just did all of that I would take it back to him and tell him that it is not right. I wouldn't pay him anything as he said it was running like it should and it doesn't. Do you have a tach on this engine?

[eavega]

I would also look beyond the carb. Hesitating at speed can be air getting into your fuel lines. Bad fuel pump, cracked lines, etc. Run some clear tubing from your fuel pump to the carbs and see if you get bubbles in the fuel.
As to the speed thing, you have to fool around with the tilt position and the motor height. Either one of those will keep the motor from running at the right speed.

Rgds