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

Zyen

Zyen

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i was wrong about the secondary rods i had installed. they were DR, not DP.
since i was getting pig rich at wot on secondarys, i changed them to DP and tested again.
wot now goes ~10.4 then down to 11.2
video of this run, for those interested.
since it's in a truck, the aft does jump around a lot depending on throttle/load, and shifting gears. of course, on a boat it wont do this.

most all afr >13.8 is high vacuum/light load/coasting.
 
jimmbo

jimmbo

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Marine Carbs are generally Calibrated richer than a Street Legal Carb, as the Engines they are on, run under a completely different Operating Parameter.
 
Zyen

Zyen

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jimmbo said:
Marine Carbs are generally Calibrated richer than a Street Legal Carb, as the Engines they are on, run under a completely different Operating Parameter.
Click to expand...
right, and that is what i was looking for....a bit rich on the truck.

just plugged everything back in the boat. verified timing marks on balancer, hooked up shop tach and timing light to check total advance.
its currently running ~13° total, all in by 3000 rpm. as it's in between my marks for 10 and 15, im guessing here, but not close to my 18° max mark (about 1/4" from it).

will reset timing and take it to lake for testing again...eventually.
gotta fix a bad toe-out on the trailer's front axle first.
 
Zyen

Zyen

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hello fellow iboaters...quick update
over last few weeks, i found the timing to be set at 1° btdc per omc manual for 87 pump gas, for a total advance of 13° above 2800
talking with my engine builder, they recommend 8-10° base, so i set it to 8° which put total to 20°.
we were finally able to get out today and rode around a couple hours. there was no major difference in how it ran after adjusting timing.
it was 89°f and ~54% humidity.
i had the 21p prop to start, and got wot of 4100 @ 41mph
pulled cowl off engine to listen to it on the off chance i'd be able to hear pinging, but no out-of-place noises were heard.
changed to the 17p 4 blade and went again.
it got 4700 @ 39mph then started backfiring thru carb.
i didnt bring the 19p, and didnt want to put on the 23p after the 21 was low at wot.
still something weird going on here i havent figured out yet...maybe it's too lean.

im gonna get this figured out.

cheers all
 
Lou C

Lou C

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Did you check the tension on the secondary air door spring? That’s a major source of Quadrajet bog, those secondary air doors opening too soon… May need to tighten it up….remember on these carbs the actual secondary blades are mechanically operated but no air gets sucked in till the air doors open & that’s controlled by the spring
 
Zyen

Zyen

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Lou C said:
Did you check the tension on the secondary air door spring? That’s a major source of Quadrajet bog, those secondary air doors opening too soon… May need to tighten it up….remember on these carbs the actual secondary blades are mechanically operated but no air gets sucked in till the air doors open & that’s controlled by the spring
Click to expand...
i have a qjet on my truck, so i do have experience with them. :)

yes, i have it set according to Cliff Ruggles paper that was sent with the rebuild kit. it states 1/2 turn for this model, so that's what i have it set to.

when going from lowest-plane-speed (about 2600rpm on the 17p) to wot, there is no bog or hesitation, so i think the secondary tension is right.
when rpms get >4k, it sounds like it's bogging. it sounds like low power. hard to tell on the water.

however, and i know this is apples-to-oranges comparison, when i put this carb on the truck (ford 300 i6), the air flap did very well set at 1/2 turn. afr was showing around 11.5 at wot and >3200rpm
 
Zyen

Zyen

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Pmt133 said:
Timing looks good. Agree they're a little lean.
Click to expand...
thanks for confirming
with this being a 4.3 and all the horror stories i've heard about detonation, i've been concerned about total timing, but i agree with ya...the plugs are showing timing is looking good. im leaving it there.

with them looking lean, that'll get me back to carb.
the jets/rods are from mercury specs, but i have no idea on compression or timing (i couldnt find it in the book i was looking at), so it could be way different than what im running.

at this point, i think my next option is to richen it up and try again.
i have a set of 69/39b that will go in next. this is ~6% increase. but i may just put in the 70 jets which is ~10% increase.
 
P

Pmt133

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Zyen said:
thanks for confirming
with this being a 4.3 and all the horror stories i've heard about detonation, i've been concerned about total timing, but i agree with ya...the plugs are showing timing is looking good. im leaving it there.

with them looking lean, that'll get me back to carb.
the jets/rods are from mercury specs, but i have no idea on compression or timing (i couldnt find it in the book i was looking at), so it could be way different than what im running.

at this point, i think my next option is to richen it up and try again.
i have a set of 69/39b that will go in next. this is ~6% increase. but i may just put in the 70 jets which is ~10% increase.
Click to expand...
Just to confirm. This reman engine has a q-jet on it. Is it a vortec (8 bolt) head/intake or the old style. I don't believe merc ran the q-jet with the better heads is why I ask. Then there are a few different cams that could be in there. Even though its by the book, there's no saying what you have matches what the books supposed to be either.
 
Zyen

Zyen

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Oct 4, 2011
Messages
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Pmt133 said:
Just to confirm. This reman engine has a q-jet on it. Is it a vortec (8 bolt) head/intake or the old style. I don't believe merc ran the q-jet with the better heads is why I ask. Then there are a few different cams that could be in there. Even though its by the book, there's no saying what you have matches what the books supposed to be either.
Click to expand...
this is a reman short block. it is non-vortec, 181 cast heads (supposedly the best non-vortec for marine). it has a qjet, 12 bolt cast intake with exhaust crossover for divorced choke. the qjet came with the 4.3 that was in the boat when i got it. it has been cleaned and rebuilt before putting on this engine.

EDIT: i found cam specs, so i am adding them here...
Camshaft - Hydraulic Roller, Advertised Duration 272 intake/294 exhaust, Valve Lift .430 intake/.436 exhaust, Camshaft Lift .269 intake/.273 exhaust. Replaces Mercruiser #431-803186
END EDIT

i agree with you...there are no other specs detailing what that jet/rod configuration is for, so i could be way off.
i've also posted about the carb on Cliff's forums, but no replies as of yet.
 
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