tachometer reading for 4 and 6 cyl.

[redlinj]

My 21' Sylvan came with an 88 hp 4 Johnson. I have since replaced that motor with a 1976 200 hp Evinrude. The question is, will I need another tachometer since I now have 6 cylinders as opposed to the original 4 cylinder ob that came with the boat? I ask this because the rpm range at WOT for the 200 hp is supposed to be between 5250 and 5750 and I can only achieve 4600. The prop is 14.5 x 19 which is the standard prop for this engine. I'm thinking of going to a 15 inch pitch, which theoretically, could bring my rpms up by maybe 700-800. Compression is 105 on all cylinders. Thanks

 

[wilde1j]

Re: tachometer reading for 4 and 6 cyl.

You motor should turn 5800@ WOT. Tach is fine.

 

[redlinj]

Re: tachometer reading for 4 and 6 cyl.

Yes, the engine should turn close to 5800, but is 1200 short. If the tach is fine, what could my problem be? Compression, fuel system, prop, or, just a tired engine! Any ideas? Thanks

 

[wilde1j]

Re: tachometer reading for 4 and 6 cyl.

Compression is fine. Look at setup 1st. If AV plate is buried @ WOT, engine needs to come up until it's just dry on top. If engine is too deep, RPM will increase @ WOT somewhat. That's what I would look at first.

 

[redlinj]

Re: tachometer reading for 4 and 6 cyl.

The motor is on a jackplate. We've had it up, down, and all points in between. 4600 rpm's is the max I can get out of it. That's why I was going out on a limb and hoping maybe the tach is off a bit. What's left! carbs, fuel pump/line, timing, plugs? I don't know.

 

[emdsapmgr]

Re: tachometer reading for 4 and 6 cyl.

Depends on which version of 76 200 heads are on the engine. If the heads are the very early ones (very few ever got into the hands of consumers) the compression would be in the 130+ range. If you have the later 76 200 heads (with the factory cut heads) the compression was reduced to 120 lbs per cyl. At 105, your engine is down on compresson. Despite the low compression, your engine should still perform better than it is. Visually check to make sure the carb butterflys are fully opening when the throttle is advanced to full throttle. (you'll have to pull the airbox cover off to make this check.) Make sure the engine is firing on all 6 cyls when it heats up to normal operating temp. You may want to run some Bombardier Engine Tuner throught the engine to see if the compression will come up somewhat.

 

[redlinj]

Re: tachometer reading for 4 and 6 cyl.

How would I know which heads I have? 320643 and 320644 are the numbers that are on the heads. Would you know how those #'s translate? I agree that the engine should preform better.( 4600 rpm and 41 mph with a GPS) Tomorrow I will check the carbs (all 3 of 'em). The engine has 6 new coils and the plugs are ul-77 surface gap, but they are rather old--perhaps I should get new ones. I'll also try and get some of the engine tuner. Just as a point of information, I picked up this engine and it's mate 3 yrs ago. They were both set up on some type of race boat or so the previous owner said. Between the two engines, my brother-in-law and myself made the one we are talking about now. We have overcome hard starting and overheating only to be kinda frustrated by this lack of top end performance. Hope you can help further. Thanks, Jim

 

[emdsapmgr]

Re: tachometer reading for 4 and 6 cyl.

The 643/644 heads are the original heads for that engine. You can't tell which version of them you have unless you pull one off and have a look at the head firing face to check for the factory mill marks. I probably wouldn't pull them just to check. Either way, those heads should make strong compression-more than 105. I'd stay with the ul77 plugs, but change them if you think they may be causing a miss. You should be able to tell when under hard accelleration. Do a decarb and recheck the compression with another gauge to confirm your first compression test reading. Have you gotten into the carbs at all? If the engine sat for some time, it is not unusual for fuel to dry up in the carb passages. The dried gelatin of the fuel can block internal carb passages/jet and result in poor performance.

 

[Haffiman]

Re: tachometer reading for 4 and 6 cyl.

Test a 17" prop (18" Raker), preferably a bow lifter/Raker type.

 

[redlinj]

Re: tachometer reading for 4 and 6 cyl.

OK, so today I removed the airbox and checked the carb butterflies. It appears, after looking real hard and with a bright light, that the bottom butterfly was only partially opening at full throttle. Won't know for sure that this is the problem until Lee (tropical storm)clears out of here and Lake Erie settles down. In the meantime, I will do another compression check with another gauge, and if I can locate one, I'll try out a lower pitch prop. At this point, I THANK all who have offered their 2 cents. Apparently, some things are just too easy for me to overlook. I will post again on the compression recheck. Jim