Engine, Carb or Prop? need suggestions or ideas please.

[Zyen]

hello fellow iBoaters.
this is long post, as i want to convey as much info as possible on problems, what i've tried, and specifics on this engine.
TL-DR - my sh~t aint working right and i dont know why.

so, i've considered myself a fairly decent mechanic for the last 40 or so years, but this is driving me nuts.
cant get over 3800/3900 rpm and cant figure out why.

chevy 4.3l v6. OMC 800 stringer. upper gearcase is 21/20. overall ratio is 1.76:1 iirc.

quick background: rebuilt boat, broke (newish) engine, bought new short block.
reused heads, manifold, dizzy, carb, coil and all drive parts.

on previous engine, i dont recall trying WOT, but seem to remember running about 3600 rpm on a afternoon river cruise using a 23p 3blade aluminum prop with 4 on board.
this engine came with boat when i got it almost 14 years ago. (for those that don't know THAT saga, see sig for posts).

while breaking in new engine, can not get rpm above ~3500 or so with same 23p prop.
on first 4-5 hours, did not try over 3/4 throttle or go above 3k rpm, per instructions. this was first try to get rpms up.
put on 19p ss 3 blade and tried again. engine seemed even more sluggish and hard to plane.
bog bad and loses power once secondaries open. does not catch up.

remembering black coated pistons in previous engine and current running problems, i looked into jets/rods with more detail as fuel mileage was also horrible (about 8 gallons over 1.5-2hours).
had 70/43b in carb. all info i've found suggested 66/36b for this carb/engine combo (per merc manual). found and installed 67/36b to err on rich side, if possible.
also found 2nd rods were DG with an N hangar. should be DH. i had DP handy and they're very close, so installed them with an L hangar to get a bit more fuel when opening.

acquired and installed 21p aluminum 3 blade.
back to lake to continue break-in, test and ride around a bit.
holeshot is around 4s or so, which is better than previous run.
can open throttle fully without killing engine now, but it sound like it's still bogging.
partially closing throttle (to close secondaries) does not yield increase in rpm/power.
adjusting trim up/down does not yield much change in either rpm or speed.

details/specifics...
(edit: added boat data)
boat is Baretta 191ri. 18.7' centerline, 91" beam, 2735# with me, all gear and full tank (24gal)

dizzy is for 4.3 even-fire chevy v6. prestolite ibm7021 marine with points. (part # 14095800)
(had previous problem with wrong dizzy years ago...boat/engine came with dizzy from 3.8l)

carb is Rochester quadrajet 17082515

plugs are tan/khaki but may not be accurate from limited run time of 7-8 hours.
plug gap is 0.035", point gap is 0.018" and dwell is 40°.
base timing is 2°, total is 17° all in at 2800rpm.
idle is smooth as silk at 575 rpm.
both upper and lower gearcases were rebuilt years ago by me. torn down, cleaned, new seals/orings/gaskets and fresh oil before install on previous engine. this included preload testing on all shafts. checked for binding and turned drive by hand before installing on new engine. drive turns free, in/out of both fwd/rev without problems.

have not gotten out the borescope to view piston tops for combustion environment.
with limited run time, not sure how well i could see lean/rich conditions.

im out of ideas...

any suggestions on what else to look for or test would be much appreciated.

allan

 

[kd4pbs]

Plug color would indicate you're probably mixing fuel and air at the proper rates. 2° idle and 17° seems like not much. 4.3 is much like a small block V8 without a couple of cylinders, and they usually want much more. Might want to early it up until you start to hear some pinging, then back off a bit to see what that engine wants. Running higher octane fuel of course would allow more.

 

[Scott Danforth]

cant get over 3800/3900 rpm and cant figure out why.

chevy 4.3l v6. OMC 800 stringer.

while breaking in new engine, can not get rpm above ~3500 or so with same 23p prop.

boat is Baretta 191ri. 18.7' centerline, 91" beam, 2735# with me, all gear and full tank (24gal)

dizzy is for 4.3 even-fire chevy v6. prestolite ibm7021 marine with points. (part # 14095800)
(had previous problem with wrong dizzy years ago...boat/engine came with dizzy from 3.8l)

how did you mate up a 4.3 to the stringer drive? only the 1985 4.3 has the right crank (2-piece RMS) to mate up to the OMC button coupling unless you had a button coupling custom made

are you sure the motor is not the 3.8 liter?

with your boat being a 1984, the 4.3 didnt exist yet. the 1985 4.3 was only out for about 6 months before they changed from a 2-piece RMS to a 1-piece RMS.

did you verify the boat weight on a scale?

are you sure on the dwell?
3.8 is 0.17 / 31 degrees
4.3 is 0.19 / 39 +/-2 degrees

did you verify the bearing torque in the upper and lower after shimming? if you are too tight, that adds a significant amount of drag

I think you are over-propped

 

[Zyen]

Plug color would indicate you're probably mixing fuel and air at the proper rates. 2° idle and 17° seems like not much. 4.3 is much like a small block V8 without a couple of cylinders, and they usually want much more. Might want to early it up until you start to hear some pinging, then back off a bit to see what that engine wants. Running higher octane fuel of course would allow more.

i had thought that too, but everything i've read (on forums with well-respected experts) say that the 4.3 is already on the edge of detonation, and to never set total timing more than 18°. i have a chart somewhere comparing the 4.3 with the 5.0/5.7 and the v6 advance stops at 18°.
OMC specs call for base to be 1° btdc. this is why i stopped there.
thanks for the reply.

 

[kenny nunez]

I just did a search and the 1986 4.3 base timing is 6*. All your other specs are correct. Are you sure that you have a genuine Marine carburetor and the fuel pressure is 5-6 lbs? Maybe you should contact Cliff’s Quadrajets to make sure you are going in the right direction with the changes. This guy knows everything about them.
Like SD said you also may be over propped. Also did you have the original drive coupler machined to match the 4.3 and does your engine have a balance shaft?
Are you sure that there is no trapped water in the hull?

 

[Zyen]

how did you mate up a 4.3 to the stringer drive? only the 1985 4.3 has the right crank (2-piece RMS) to mate up to the OMC button coupling unless you had a button coupling custom made

are you sure the motor is not the 3.8 liter?

with your boat being a 1984, the 4.3 didnt exist yet. the 1985 4.3 was only out for about 6 months before they changed from a 2-piece RMS to a 1-piece RMS.

did you verify the boat weight on a scale?

are you sure on the dwell?
3.8 is 0.17 / 31 degrees
4.3 is 0.19 / 39 +/-2 degrees

did you verify the bearing torque in the upper and lower after shimming? if you are too tight, that adds a significant amount of drag

I think you are over-propped

hey scott. i hoped you'd see this.
the boat originally came with a mercruiser 470 and 23p prop.
that was gone long before i got it.
so my frankendrive is just that. the 800 stringer is from a 3.8 and when i originally got it (in 2011), all the parts were from a 3.8 and would barely run. my rebuild thread covers the hull work, but does not cover all the engine work i had done to get things "right"...what i could.
the first block was a 357 casting with 181 heads, which was a 1995 GM 4.3
the "GM 4.3" part was cast in the block.
the coupling fits the flywheel/crank as is. i dont know what it was from, but the fitup is perfect. i spent a couple days lining everything up and verifying before torquing it all down. i've heard it may have been from a 5.0 but i didn't see any numbers on it when i was rebuilding.
i do know this crank has same flywheel bolt pattern as the 5.0
new short block is 090m. definitely GM 4.3 and has the 1pc rms.

boat weight certified on CAT scale by my house....if im remembering it right. havent thought about it since last year.

i am certain on the dwell. the gap was just "to start with", but the dwell i wanted exact. and it is exactly 40°.

rolling torque is within nominal specs...i'd have to dig out my paper to tell you exact numbers, but it was a couple oz on the heavy side of middle for all shimmed bearings. this was taken with lip seals in place, which added 6in/lbs iirc.

i had to reshim the engine driveshaft while i had things apart due to changing burnt bearings. this was the only bearing set i changed, but all shafts had to be reshimmed to get rolling torque within specs.

i'm thinking it's overpropped also, but almost 1k rpm?
even so, i found a 17p and have it on order. i'll try that when it gets in.

this weekend, im gonna put the 19p on and get some hard numbers with it.


thanks for the insights.

 

[Zyen]

I just did a search and the 1986 4.3 base timing is 6*. All your other specs are correct. Are you sure that you have a genuine Marine carburetor and the fuel pressure is 5-6 lbs? Maybe you should contact Cliff’s Quadrajets to make sure you are going in the right direction with the changes. This guy knows everything about them.
Like SD said you also may be over propped. Also did you have the original drive coupler machined to match the 4.3 and does your engine have a balance shaft?
Are you sure that there is no trapped water in the hull?

hey kenny.
the base timing of 6° is for 93 octane and 1° for 87. im running 87 non-ethanol (it's cheap and close).
pressure is ~7# at 29gph according to my testing.
when i get home im gonna post on cliff's site and get some opinions there on this carb.
i know it wasnt built right when i got it.

the coupler matched the crank/flywheel bolts when i got it. it *may* have an omc part #, but i dont recall seeing one, but no, *I* didnt have it machined.
yes, it is a balance shaft engine.
i am sure there is no trapped water. as soon as i finished glassing the floor, i loaded it on the trailer and weighed it.

thanks for the reply.

 

[Scott Danforth]

There is no 2-piece RMS button coupler for a stringer 800 that I have found , and the 1-piecs RMS pattern is different.

The bolt circle on the 2 piece rear main seal crank is 3.58" while the 1 piece is 3.00" bolt circle

The Ford pattern is close to the 1-piece.... however with one hole rotated about 4 degrees

If there were, it would open up a bunch of avenues for people. I know the Ford coupler can be modified by drilling another batch of holes, or welding the one up and redrilling

 

[Zyen]

i researched the sh!t outta the coupling when i got the new shortblock.
it required the staggered-bolt starter, which requires the 14" flywheel, which i didn't have either of...so before i started engine assembly, i had to make sure a 14" flywheel would fit. then i had to make sure the coupling would fit my crank, so researched that for a while, too.
i found and bought a 14" flywheel for a 4.3 (mercury 12557590) and proceeded to trim my housing to fit. here's a pic (edit: sorry for the huge picture...dunno how to resize it.)


this is the 14" flywheel in the housing, with the coupling centered under it.

the coupling for all 3.8/5.0/5.7 that i found is OMC# 0980994. i do remember verifying this number on the coupling i have, but no pic of it.

after verifying everything fits and torqueing it down, i checked runout to make damn sure it was right. runout was ~0.001" at the coupler edge.
i did this because it came with the 12.75" flywheel.

 

[jimmbo]

Was that a Marine 4.3 or some Auto-Truck engine you put in?

 

[Zyen]

Was that a Marine 4.3 or some Auto-Truck engine you put in?

marine shortblock from a marine engine builder in Florida.
everything i put in is marine rated.
(edit: except for the sla battery for my stereo)

 

[jimmbo]

Okay
I was curious, cause Time and Time Again, I have come across those who drop Auto-Truck Engines into a Boat, and the Performance is worse than an Engine of half the Hp. It might eventually get to the correct Rpm Range, but might take 10 minutes, of Poor Acceleration

 

[Scott Danforth]

The coupler for a stringer does not care about the flywheel, only the crank pattern



The only Stringer coupler for GM motors only fits the 1985 and older 2-piece RMS cranks

 

[Zyen]

Scott,
yes, the coupler is for a pre-1986 stringer drive.
the couplers changed in 1986.

 

[Pmt133]

I just wanted to add, our boats are similar in size and having the same 090m block... while I can't verify what exactly is in your mill I can say with a 1.84 merc drive I can sling a 19p up to the rev limiter. The internals should be similar for a marine engine. Obviously not apples to apples but moving in the right direction. I'd think a 21p would be closer than what you are seeing though...