NHGuy

I have a forged piston 383 with 10:1 compression running Mercruiser Thunderbolt V ignition. I put on a Quickfuel M750 this year thinking it would get me the last few hundred rpm at the top of the range. Previous carburetor was a Weber that came with the original 350 Mag. I had worked the Weber with different jets and kept coming up lean.

Unbeknownst to me, at the end of last year my knock sensor wire came off which allowed too much timing and I blew my stainless shim head gaskets. So this July I discovered the head gasket issue, checked the heads and deck. And replaced them with MLS gaskets.

Now with the current setup I am fuel fouling the #5 spark at anything below 1000, 1100 rpm. I had though it was from the timing so I ran it up to 14 degrees and changed out my plugs with some that are one step cold. Dart (my heads) recommends that with each point of compression.

I'm thinking the colder plugs will foul easier, but I had them so I put them in. And the engine ran bad after a couple of minutes below 1200. So I will probably grab a set of standard plugs today. I'm on vacation and it's an easy step.

Only #5 has the problem with the plugs I removed yesterday.

Here's the cam spec
Comp Cams OE roller style CS 268XFI HR13 Cam
Valve Lift .570” Intake .565” Exhaust
Duration at .050 lift, 218 Intake 224 Exhaust
113 Degree Lobe Separation Angle
Intake Centerline 109 Degrees


Any thoughts? I am not coming up with a solution except to maybe add more timing.

There's a chance the engine is getting idle fuel through the primary flap. I have the four corner idle set very lean which shows the most vacuum.
If I add more timing and it's good I might get a Daytona sensors box so I can control the timing completely. After this experience I am shy to continue theThunderbolt V.

I really need to get the idle speed down for no wake zones.

SB

iTrader: (1)

What plugs and gap ?

Have you done a compression test on that cylinder ?

It is next to cyl #7 in the firing order - wondering if ignition wire's causing misfire, or water from exhaust reversion ? Both more likely with 5+7 than other cylinders because of this firing order and being right next to each other physically. Also common for even knowledgeable people to screw up and switch plug ends on #5 and #7 by mistake.

Is this #5 issue new with this carb ?

As you know, I have an engine with same cam, but 350cid with less compression, BG 650 Sea Demon, Brodix heads.

It has 12* Initial, 34* Total with TBolt IV, MSD HVC canister coil (no real reason why other than using these coils forever), and I'll have to check which heat range plugs. I do know they are NGK V Power series. Top of my head -6 heat range. Again, just because I have used this type plug forever in performance things and like the HVC coil, have have never given me a reason to change to something else.Only changed the plugs twice in 10 yrs just because. I always run .035" gap because yet again, all ways works for me on all performance engines.

It can idle all day. Yesterday in fact, idled in gear for 1hr and change watching / following a young Bald Eagle learning to fly.

If you are on Winni during the week you can borrow the carb if you want. Not week-end because the boat needs it.
As a note if this is a possibility - BG uses a different bowl inlet thread for fuel line. The set up on it has a -6 inlet to the 'Y' set up.

NHGuy

Well, my story changes. When I went out to the boat this morning I took a good look at things and realized my #3 and #5 plugs were wet with water not gas, fuel floats were too high, and my fuel pressure was too.
I snugged the intake bolts a bit and reset the fuel levels, It's better but it still loads up. I did not pull any of the even side plugs due to time.
I wonder if I f'd up the intake gasket. I remember a poor "drop" when putting the intake on. I moved the gaskets with the intake and then had to reposition them.
My risers are new last I year. I have EMI Thunder manifolds and the plus 3" extra height stainless risers.
I put in some exhaust baffles, suppose I could take them out to see if they are causing reversion. But I do like the sound reduction.
On the plus side I put some good hours on the boat the past couple of days. It runs great at higher rpm.
I will recheck the wires, I always go off the marks on the Mercruiser caps.

Was the bald eagle in that big tree behind Black Cat Island? We have seen them there before. Awesome birds they are for sure.

Thanks for the offer on the carb, That's a lot to go through though. I suppose I can reinstall my Weber to rule out the carb too.

SB

iTrader: (1)

No, but we did go past BLACK Cat that day to show some the Kona Boat House. Bald Eagles are all over the place now, a whole bunch of them, but the Bald Eagles we follow are near Camp Tecumseh.

Anyway, for carb you would just have to hook to one #6 male.

I'm not sure what intake gaskets you use but NAPA can usually get you 1204's in a day.

SB

iTrader: (1)

On carbs with glass sight bowls I set no lower than 1/4 up window and no higher than 1/2 way up window. I shoot closer to the 1/4 because all the bouncing boats do.

How much fuel psi did you have ? Was it causing fuel to come out of boosters or or vent at idle ?

Some other member here last year, or this year ?, had a low speed issue with his Quickfuel from having it bolted to tight to the intake. Never heard of that, but he swore he loosened it and then tightened it less and issues went away.

Mr Maine

SB, that was probably me. 850 quick fuels. I have the carb, a gasket, the throttle plate, gasket, spacer, then gasket. So a lot of squish and room for uneven tightening. After i read qf instructions to carefully tighten evenly to only 8 ft lbs no issue.

NHGuy

I'm home today for the doctor. Conveniently picked a rain day.

I think I will do a carb cleaner check around the carb base and intake to see if I can detect a vacuum leak.

And yes I did a compression check, all were 150 ish +/- 7 psi.
.
I'm going to bring a set of factory plugs gapped down to your .036". Mercruiser calls for .045" which I have been following. So if there is any deficiency in ignition that could be involved.

I'll redo my intake gaskets too. I didn't seal the intake gaskets on both sides last time. I (foolishly) depended on the gasket without a sealer.

My boat lives on a rack in West Alton this year. Suppose I will bring up the trailer and do it there so I'm not sitting at a dock somewhere with the intake off.

As to the eagle watching. I never learned my way around Melvin Bay and Moultonboro Bay. I should explore over there. I bet it's a little less crazy on weekends.

NHGuy

The fuel pressure is another conversation. I was working with it yesterday. I bought the Quickfuel regulator for the Quickfuel carb.
It was supposed to come out of the box set to their required 6.5 psi. Wasn't. So I adjusted it.

Then when I looked yesterday morning it was too high, over 7. So I adjusted it.

Couldn't get it below the 7 plus unless it went down to 5! No matter what I do it's always 5 or 7.3 psi. I just couldn't get the sweet spot on the regulator. Frustrating. I left it at the 5 for yesterday afternoon.

I think I will call their customer service line to see what they suggest.

BTW after doing some reading on 4 corner idle carbs I turned the secondary idle screws way down. That may have helped me too.

If any of you have feed back on that issue I'd like to have it.

Over on the car boards it is commonly held that if you don't run a wild cam the secondary idle isn't needed. I'm debating just closing them altogether.

SB do you think that my risers are adequate against reversion, I'd sure think so.

SB

iTrader: (1)

My rear idle mixture screws are barely open. I can check for you, but I believe around 1/2 turn open or there abouts. Since you will have carb cleaner with you, put red nozzle right at prim and sec idle bleed holes and spray a few shots thru..yeh, I know, new carb..but you never know and I do this a few times a year since it is a common issue. Might as well put a few shots thru the high speed bleeds too.

Even though many Holley type carbs do fine with over 7psi, when one has a regulator we all usually try to bet at 7psi or just under.
Again, just one of those things.

It is not less crazy at all. It used to be up to about 15yrs ago - we have so many rocks people where afraid of this area...including Police - but now the cops know the area and with all the new houses built, people have flooded are area too. That's why we don't go out Saturdays. We boat Sunday mid afternoon until the night. Much more peaceful.

ezstriper

if you can hook up a wideband you would be way ahead...even seeing what the idle AFR is