Hot air choke help

[dustinsapple]

I recently rebuilt a 1988 OMC 4.3, I used a 2002 Vortec block to replace my cracked block. I switched every thing from the Marine motor over to the newer Vortec block.

I did not bother removing the balance shaft and the hot air choke tube (heats the choke via exhaust gases) was in my way so I just eliminated it all together. Anyways long story short, I no longer have the hot air tube that runs though my intake manifold. I just have the hot air choke vacuumed off on a Tee with the vacuum nipple on the back of motor.

Problem is now obviously with a regular vacuum line not running though the intake manifold I highly doubt my hot air choke is getting heated up enough to fully open once the motor is hot, and frankly looks original and is probably trash anyways.

Should I somehow switch to something electric? OR could I just remove the thing all together? If I only ever plan to start to warm weather could I just rig it so that the upper choke butterfly is always open?

Thanks

 

[Lou C]

The Vortec design dies not have an exhaust cross over the intake as the pre Vortec does so there is no way a hot air choke can be made to work. There are electric conversion kits available and that’s the way I’d go.

 

[Lou C]

Ahh I'm a little confused...did you replace the whole long block (block and heads from the Vortec) or did you put the old pre-vortec heads on the vortec/balance shaft block? Because the intake in your pic looks like the old pre-vortec intake...
good thing you did not try to remove the balance shaft!

Look on carburation.com for electric choke conversions.

 

[dustinsapple]

I only used the block from the Vortec engine. I am using my original marine heads and intake manifold.

 

[GA_Boater]

Using the original heads and intake worked until the block cracked, so how can it be in the way? Is there something you haven't told us?

The choke helps in both cold and warm weather starting

Your talking about vacuum confuses me. I think trying to run a heated choke actuator with vacuum sucks.

 

[dustinsapple]

Using the original heads and intake worked until the block cracked, so how can it be in the way? Is there something you haven't told us?

The choke helps in both cold and warm weather starting

Your talking about vacuum confuses me. I think trying to run a heated choke actuator with vacuum sucks.

Sorry,

I bought a boat with a 4.3 OMC cheap because previous owner did not winterize it properly and the block had cracked in a couple spots.

I bought a 2002 Vortec complete motor from a GMC 1500. I stripped it down to the block and rebuilt the motor with all my marine parts. Flywheel, Balancer, Bell housing, Heads, Intake manifold, Carb etc everything from the original OMC. The Vortec block has a balance shaft, so when I went to install the intake manifold (from OMC, 12 bolt manifold that matches the original Corba heads) it had the metal tray for the steam ports and inside that tray also ran the heat tube for the choke. I hammered down that tray to make it clear the balance shaft, and put a couple new holes in it so it could breath as I didn't weld the steam ports shut (cross over ports whatever) on the intake manifold to plug them off permanently. So with this being done I lost the ability to fit the original heat tube into the intake manifold. Currently I just have the choke twisted so its always open all the way. (Just have to pump the throttle a couple times and it starts right up).

I ordered an electric choke now anyways so that I can have the choke work as it should.

Pictures attached are;

-the cracks I found in the original block
-new Vortec Frankenstein motor in the boat awaiting the intake manifold
-original intake manifold before modification
-hammer out underside to clear the balance shaft

 

[dustinsapple]

The motor actually runs good, other than the choke issue.

I am however only getting 3900-4100 RPM WOT. Tried 2 different props, 15x17 and 14.25x21. (I say 39-41 as it’s depending on which prop and water conditions) but that’s my WOT range I’ve seen so far through testing and tinkering with the boat.

I’ve been working my way through Dons checklist, (think that was the name of the mod).

Timing is good. 6* idle and 18* total advance. Checked points distributor, .019 gap and 36.5* dwell.

I recently re adjusted the valves as I might have accidentally adjusted them on the exhaust stroke. Also removed the water fuel separator and the anti siphon valve, to make sure it’s not a fueling issue as I obviously had to switch to electric fuel pump since Vortec block doesn’t have spot for mechanical fuel pump.

I haven’t had a chance to water test since re adjusting valves, removing anti siphon and water separator. I will report back though.

 

[Scott Danforth]

why not use the vortec heads (about 20-25hp increase over the non-vortec heads) and simply get a different intake?

electric pump doesnt mean you dont have a fuel system issue.

 

[Lou C]

Ah ok got it that's what I thought you meant, but wasn't sure how you made it work. It's interesting that those pre-vortec intakes came in 2 slightly different styles for the tin cover underneath, the ones that came on the non-balance shafted motors were curved like the one you modified, then there was another style that had a much flatter tin cover, presumably so it would clear the balance shaft, I have a couple of spare intakes one of each style. And the cast iron part is the same only difference is the tin cover which I believe was to shield the oil in the cam valley from the heat of the exhaust cross over.
It is odd to say the least that the original block could crack like that and the heads and intake did not. Surprising, were the heads magnafluxed? Are you sure they are not cracked?
I would have used the Vortec heads and just gotten a Vortec marine intake. That would also allow you to upgrade to a 4bbl for better top end performance. Vortec heads+4bbl would add approx 40-50 hp. Worth it in my opinion. Esp for the chance to upgrade to a new carb for less problems.

You need to get the rpms up higher, I have used a 15x17 on mine (4600) and a 15.5x15 (4900) I think its better to prop at the high end of the range as long as its not over 5000, esp if you are testing it lightly loaded.
The OMC spec for the 4bbl 4.3 was 4600-4800.

 

[dustinsapple]

I only visually inspected the heads, after cleaning them up and I did not see any cracks or anything. I was going to use the vortec heads at first but when I looked into buying the marine 4bbl intake plus carb it was pretty expensive. Probably more than I’m into this boat in total if I had went Edelbrock new. I kept the vortec heads and everything needed, if I ever come across a used intake for the vortec heads and 4bbl for a good price I’ll probably pick them up and maybe rebuild again year. At this point in the season I just wanna be out on the water.

I read the 2bbl 88 omc should be around 4200-4600rpm, and I’m very close to the lower end of that spectrum. I’m going out this weekend so with all my latest tweaks I’ll see what kind of WOT rpm I get.

If I do end up rebuilding again and using vortec heads and 4bbl I’ll probably end up switching out the points distributor for the newer style ones.

 

[dustinsapple]

Oh also, in neutral when I burp the throttle I’ve shot it up past 5000rpm. I’m only limited under load. The 2 props I have only seems to have a difference in rpm by 1-200. But the 14.25x21 is like 5mph faster than the 15x17.

15x17 at 4000 rpm I get 36mph
14.25 at 3900ish rpm I get 41mph

 

[Lou C]

I only visually inspected the heads, after cleaning them up and I did not see any cracks or anything. I was going to use the vortec heads at first but when I looked into buying the marine 4bbl intake plus carb it was pretty expensive. Probably more than I’m into this boat in total if I had went Edelbrock new. I kept the vortec heads and everything needed, if I ever come across a used intake for the vortec heads and 4bbl for a good price I’ll probably pick them up and maybe rebuild again year. At this point in the season I just wanna be out on the water.

I read the 2bbl 88 omc should be around 4200-4600rpm, and I’m very close to the lower end of that spectrum. I’m going out this weekend so with all my latest tweaks I’ll see what kind of WOT rpm I get.

If I do end up rebuilding again and using vortec heads and 4bbl I’ll probably end up switching out the points distributor for the newer style ones.

keep the points you will have less trouble with making the ESA work for the Cobra outdrive. I still have them in mine.
my old heads were cracked from a previous overheat and you could not see the cracks until the machine shop pointed them out to me. While they did not leak water into the cyls, the cooling passages were getting eroded from years of raw water cooling in salt water so I replaced them with a set of reman pre-vortec heads. $550 for the pair, I used my old intake and rebuilt the Quadrajet.

 

[Scott Danforth]

in neutral, without load, that motor would turn 5000 RPM with only 1 cylinder firing.

 

[Lou C]

Oh also, in neutral when I burp the throttle I’ve shot it up past 5000rpm. I’m only limited under load. The 2 props I have only seems to have a difference in rpm by 1-200. But the 14.25x21 is like 5mph faster than the 15x17.

15x17 at 4000 rpm I get 36mph
14.25 at 3900ish rpm I get 41mph

there is 4" difference pitch that should be like 200 rpm per 1" something else is going on. make sure you are getting full mechanical advance on the Prestolite (18* total at 3200 rpm if I recall, 6* base + 12 centrifugal), might need some fuel system work...

 

[dustinsapple]

Well as for the props I have and RPM, I read that 1” drop in diameter increases rpm by 500,
so call it 375, since it’s only 3/4”
then drop 100-150 rpm per increase in pitch 4” so call it decrease by 500rpm.

15x17 4000-4100rpm
14.25x21 3900-4000rpm

That all seems on point, but again still not in the proper WOT rpm range.

I haven’t be able to retest in the water since removing the anti siphon and water separator, and the readjusted valves. So hopefully this will fix my WOT rpm issue.

Ill report back after the weekend with results.

My timing is pretty much bang on at 6* idle, 12 advance for 18*. Verified with timing lights and marked harmonic balancer.

I used a whiteout pen to mark my timing degrees by transferring from a piece of tape. Used circumference, divided by 360 and layed out onto a tape with a mic so it’s pretty accurate.

Thanks fellas