Carburetor tuning high elevation

[Croz]

Morning RDP, my 210 has a 502 with a Holley 850 double pump carburetor. Our new home and local lakes hover right around 4700’ elevation. My boat was set up for Parker and I know I’ll need to lean it out. It’s currently got a high Idle speed and wanting to die on me up here just running in the driveway. I ordered the jet kit and plan on popping the front bowl off the front and downsizing jets one size at a time while making minimal adjustments to the 4 idle screws. Question is, how do I know if I need to change out the secondaries as well? should the primaries take care of it? Am I getting myself into a shit show?

 

[coolchange]

I’d screw the air bleeds 1/4 turn at a time. Wait for it. 2 down in all four corners is nothing.
Make sure your ignition is good.
Carbs only do 2 things, bog, or stumble.
Find out what power valves you have while in there. A vacuum gauge will help while driving.

 

[Croz]

I’d screw the air bleeds 1/4 turn at a time. Wait for it. 2 down in all four corners is nothing.
Make sure your ignition is good.
Carbs only do 2 things, bog, or stumble.

What do you mean 2 jet sizes down in all 4 corners is nothing? You sound like a carburhater lol. Should I downsize more than one at a time? Say they’re 74’s, should I jump straight to 70’s or adjust idle and try 72s?

 

[Lumpy]

What do you mean 2 jet sizes down in all 4 corners is nothing? You sound like a carburhater lol. Should I downsize more than one at a time? Say they’re 74’s, should I jump straight to 70’s or adjust idle and try 72s?

This isn’t that big of a deal…I’d run it and check the plugs first. Take a picture of a plug in parker then compare. You may be fine, if not, drop a couple sizes and try again. Holley has a ton of info online. One carb N/A should be a snap to dial in. Idle fuel/air mixture screws are for just that “Idle” mixture…not gonna matter when you're on plain but will matter with idle drivability. Once you find out what your vacuum readings are then you can dive into the power valve…you don't want those babies open at idle or you will be fouling plugs big time. My guess is you should be okay with some minor adjustments.

 

[Vamodsquad]

Morning RDP, my 210 has a 502 with a Holley 850 double pump carburetor. Our new home and local lakes hover right around 4700’ elevation. My boat was set up for Parker and I know I’ll need to lean it out. It’s currently got a high Idle speed and wanting to die on me up here just running in the driveway. I ordered the jet kit and plan on popping the front bowl off the front and downsizing jets one size at a time while making minimal adjustments to the 4 idle screws. Question is, how do I know if I need to change out the secondaries as well? should the primaries take care of it? Am I getting myself into a shit show?

Look up the number on that 850 to see what it came with from the factory. It will give you a base line to see where you're at, and which way you need to go.

 

[coolchange]

Carb hater? I’ll come by and dump my box of jets valves, pumps, cams for you to rummage through. 74to 72 is nothing. That’s a morning to afternoon at Parker.

 

[77charger]

I would drop it 2 sizes and go from there.If 78s in there go to 74s take plug reading see how they look etc.

Need to remember a hot day in parker in the summer the density altitude will also be very high as well and way different from a cold winter day in parker.

 

[Croz]

Ok I’ll start by dropping 2 sizes at first, I’m a newb at this.

Carb hater? I’ll come by and dump my box of jets valves, pumps, cams for you to rummage through. 74to 72 is nothing. That’s a morning to afternoon at Parker.

Ha just messing man, trying to figure this out without taking to anyone.

 

[caribbean20]

I was 2 sizes down, primaries and secondaries, on a Holley double pumper, 750 CFM, 502 with Dart aluminum Heads, EMI Thunder exhaust, 21’ Caribbean jet, going from Mohave (maybe 1,000 ft) to the Denver reservoirs (5,300 ft). Still ran just a little rich, but not too bad.

And sadly get used to about 10 - 15% less HP and a few MPH off the top end.

 

[Croz]

I was 2 sizes down, primaries and secondaries, on a Holley double pumper, 750 CFM, 502 with Dart aluminum Heads, EMI Thunder exhaust, 21’ Caribbean jet, going from Mohave (maybe 1,000 ft) to the Denver reservoirs (5,300 ft). Still ran just a little rich, but not too bad.

And sadly get used to about 10 - 15% less HP and a few MPH off the top end.

Not looking forward to the power loss, but my wife is pregnant and due in July, I have a 2.5 year old, so that little bit shouldn’t be a problem cause I will only be cruising for the next few years

 

[caribbean20]

Not looking forward to the power loss, but my wife is pregnant and due in July, I have a 2.5 year old, so that little bit shouldn’t be a problem cause I will only be cruising for the next few years

Can still remember when my wife was pregnant bouncing around in my skiff. Just couldn’t keep my foot out of it.

You may also want to look at changing prop/impeller size for the elevation change. I was running a Berk B mag bronze impeller in my jet.

 

[River Dirt 2]

On my sons 240, relatively stock 454 with 750 we went down 75 to 70 on the primary’s and 1/4 turn in on the idle/fuel screws for Lake Arrowhead. No changes to the secondary’s since it’s a 35 mph speed limit so it does not see the secondary’s much if ever.

We also go from a 21 Rev 4 to a 19 Rev 4 prop.

 

[Toolman]

I worked for several years with Byron Hines of Vance and Hines racing. When they would go to Mile High dragstrip in Colorado, they would decrease the CFM‘s of the carburetors. Changing the jets isn’t gonna help that much. You have to keep velocity up which means you have to go down in the size of the throat of the carburetor to keep that velocity moving. You will lose performance and there’s no way around that, but it will give you the best performance possible at that altitude.

 

[rivermobster]

No body wants to see what his plugs look like now, before he makes Any changes?

@Croz

Post em up, and let's take a look.

 

[caribbean20]

I worked for several years with Byron Hines of and Hines racing. When they would go to Mile High dragstrip in Colorado, they would decrease the CFM‘s of the carburetors. Changing the jets isn’t gonna help that much. You have to keep velocity up which means you have to go down in the size of the throat of the carburetor to keep that velocity moving. You will lose performance and there’s no way around that, but it will give you the best performance possible at that altitude.

Good point, that 850 may be a bit much for 4,700 ft.

 

[boatnam2]

Lake Tahoe sits at an elevation of about 6,200 feet, and carburetors—like your Holley 850 on a MerCruiser 502—run richer at higher altitudes due to the thinner air. This affects air-fuel ratios and can lead to poor performance and fouled plugs if not adjusted.

General Jetting Rule for High Altitude​

For every 2,000 feet of elevation, it's common to reduce the main jet size by 2% to 4%. At Lake Tahoe’s ~6,200 ft elevation:

• You're looking at roughly 6% to 12% reduction in jet size.

Holley Jet Sizes and Example​

Holley jets are numbered (e.g., 80, 82, 84, etc.). If you're currently running:

• Primary jets: 84
• Secondary jets: 86

A 6–12% reduction would suggest dropping 5 to 10 jet numbers total (e.g., from 84 down to 76–79 on primaries).

Safe Recommendation:​

• Drop 4–6 jet sizes to start.

 

[Todd Mohr]

2 sizes down front and back (-4 on the jet #) lean out the idle screws a 1/4 turn at a time. Power valve should be good maybe 1 size smaller (6.5 to a 5.5). A 2 inch smaller prop and you should be good. Or put a small supercharger and bring sea level performance to 5k elevation. A 177 would do the trick.

 

[rivermobster]

@Croz

Also post up what your initial, and total timing, are currently set to, when you post up the spark plug pics.

 

[JUSTWANNARACE]

My first question would be, how long has the boat been sitting?

Not wanting to idle in the driveway or stumble/bog when you whack the throttle could be as simple as fuel sludge plugging the jet. Simple cleaning would be your first step. Pop the bowl and take a look.

That is where I would start!

 

[River Dirt 2]

After a full season at Arrowhead, 35 mph and shit gas, “good enough for the girls I go with”

 

[rivermobster]

My first question would be, how long has the boat been sitting?

Not wanting to idle in the driveway or stumble/bog when you whack the throttle could be as simple as fuel sludge plugging the jet. Simple cleaning would be your first step. Pop the bowl and take a look.

That is where I would start!

Here is SoCal, all the alcohol in the fuel turns the accel pump diaphragm, into a rock, in about 6 months!

You guys have that same issue up there?

 

[Croz]

Great responses and a lot of good info here. Thank you guys! I’ve already ran the boat up here only in the driveway. It was too rich for sure, so a plug pic may be deceiving since I’ve ran it since moving up here. When im home this evening I’ll check back in, get some plug pics. I have a 26p 4 blade, I’ll switch back to my 23p 3 blade if I have to. John Cogan is who built the motor, he recommended downsizing prop as well. Will be interesting to what unfolds. I’ll get a vid or it running after I swap jets and can compare to a vid I took last weekend.

 

[Croz]

My first question would be, how long has the boat been sitting?

Not wanting to idle in the driveway or stumble/bog when you whack the throttle could be as simple as fuel sludge plugging the jet. Simple cleaning would be your first step. Pop the bowl and take a look.

That is where I would start!

Boat sat since thanksgiving and fired up immediately up here, like I had a choke on it. Started strong and sounded great till it didn’t. Rev’d it up a little and didn’t sound bad, but the idle was high, then low, then rough. Adjusted the needle and seats which cleaned it up a bit, starting to understand how the primaries and secondaries work at different times.

 

[Croz]

@Croz

Also post up what your initial, and total timing, are currently set to, when you post up the spark plug pics.

I have zero clue about finding timing. This is all a new learning curve from me. I bought the boat from John Cogan (concept marine) and have had many trouble free years, which has been great, but I haven’t had the chance to learn through trial and error, when there’s hardly any errors haha.

 

[Bigbore500r]

No body wants to see what his plugs look like now, before he makes Any changes?

@Croz

Post em up, and let's take a look.

Exactly!

Don't change jets yet....
The jets have hardly any effect on idle.

Idle should be able to be cleaned up with mixture screws, if those end up being less than 1/2 turn from "closed", it means the idle feed restrictors in the metering blocks are too large (common problem with all aftermarket holleys).

Once you get the idle dialed in, toss fresh plugs in it and run it, then look at a few plugs and see if jetting needs anything.
Rich wastes fuel and hurts power . . . but too lean KILLS

 

[Nordie]

For the power valve I always divided the total vacuum in half and ran that number of power valve. It always seemed to work well for me.

Plugs will tell you everything you need to know on properly tuning the engine. I wouldn't be afraid to drop 2 jet sizes on all four corners, and if the carb is a 4 corner idle adjust accordingly.

Do a lot of research, Holley carbs are actually very easy to dial in.

 

[coolchange]

Boat sat since thanksgiving and fired up immediately up here, like I had a choke on it. Started strong and sounded great till it didn’t. Rev’d it up a little and didn’t sound bad, but the idle was high, then low, then rough. Adjusted the needle and seats which cleaned it up a bit, starting to understand how the primaries and secondaries work at different times.

I’d look down the throat at the boosters for fuel. If it started easier than expected and went to shit may have a failed power valve

 

[Bigbore500r]

Boat sat since thanksgiving and fired up immediately up here, like I had a choke on it. Started strong and sounded great till it didn’t. Rev’d it up a little and didn’t sound bad, but the idle was high, then low, then rough. Adjusted the needle and seats which cleaned it up a bit, starting to understand how the primaries and secondaries work at different times.

If adjusting the needle and seat cleaned it up....you possibly had a leaking needle/seat, Or...you've adjusted the float level so low that you've effectively leaned out your idle circuit .....which won't be good for the boat under load, it will starve for fuel and be very inconsistent. Make sure the float level is just below the inspection hole when running, if fuel dribbles out when you pull the plug, drop the level a hair until it doesn't then lock it down. If you have fuel dripping from boosters when it's sitting there idling, or after you shut the motor off, it means your float level is too high or you have a leaking needle and seat. Check that out.

 

[JUSTWANNARACE]

Here is SoCal, all the alcohol in the fuel turns the accel pump diaphragm, into a rock, in about 6 months!

You guys have that same issue up there?

It is usually 10% here. But the little country store by my house sells non-ethanol regular and premium out of the pump so that is what the boat gets. That is what the wife runs in the blazer as well. Anything that is not a sealed fuel system gets non-ethanol.

But I have ran into the OP is dealing with. Boat fired right up then went to shit. Ended up sludge/gel sucked into the jets. Been non-ethanol since.

 

[PRIMO]

Buy a second Carburetor / Drop the CFM / Switch out when you go down the hill / GENIUS....:

 

[rivermobster]

I have zero clue about finding timing. This is all a new learning curve from me. I bought the boat from John Cogan (concept marine) and have had many trouble free years, which has been great, but I haven’t had the chance to learn through trial and error, when there’s hardly any errors haha.

With the above being the case...

Take the boat to someone in the area you'll be using it in. They will be familiar with All the tuning requirements at that elevation.

There is far more to tuning than swapping out some jets. Let someone that has lots of experience dial your baby in for you.

Like Jerry said...

Mistakes can be an expensive learning experience!

 

[coolchange]

I think that comes with about 80s out of the box. You didn’t say if it’s black smoke at idle. Just pull the carb and rebuild it. Probably got spiders in the bowls

 

[lbhsbz]

My first question would be, how long has the boat been sitting?

Not wanting to idle in the driveway or stumble/bog when you whack the throttle could be as simple as fuel sludge plugging the jet. Simple cleaning would be your first step. Pop the bowl and take a look.

That is where I would start!

Thats a great point....especially with ethanol laced fuel.

After I blew my knee, I parked my DRZ400 (daily commuter) for about a month and then decided to sell it. Went to fire it up and take it for a quick lap around the neighborhood before the buyer showed up and it would not idle off the choke. Gas was exactly 1 month old, vented tank. I took a chance before diving into the carb and drained the fuel and added a few gallons of fresh fuel....fired up and ran great. Could be just shit gas....it degrades pretty quick in a vented system.

 

[Terminal Velocity]

Where are you located. There may be a member that could help you out. I'm in Salt Lake City.
Edit: Just did a search and see you are now in Carson City.

 

[coolchange]

Thats a great point....especially with ethanol laced fuel.

After I blew my knee, I parked my DRZ400 (daily commuter) for about a month and then decided to sell it. Went to fire it up and take it for a quick lap around the neighborhood before the buyer showed up and it would not idle off the choke. Gas was exactly 1 month old, vented tank. I took a chance before diving into the carb and drained the fuel and added a few gallons of fresh fuel....fired up and ran great. Could be just shit gas....it degrades pretty quick in a vented system.

Didn’t think about that! Old gas always my first go to. Customers would drop their vintage stuff off and I’d call them 2 hours later to pick it up. Couldn’t get them to not leave gas in their driven 4 times a year cars!

 

[lbhsbz]

Didn’t think about that! Old gas always my first go to. Customers would drop their vintage stuff off and I’d call them 2 hours later to pick it up. Couldn’t get them to not leave gas in their driven 4 times a year cars!

That said....guy dropped off an MGB at my house after it hadn't run in 15 years. After I went through the basics, gave it a shot of ether and hit the key....fired right up and ran fine on whatever was in the tank lol....mighta still been the good stuff with MTBE in it. dunno.

Bought a Scirocco that had been sitting for 10+ years and the same thing....brought a battery with me, aired up the tires, hit the key and it started like it was shut off yesterday....drove it home and it even passed smog the next day. That was most definitely pre-ethanol gas though.

 

[Croz]

Where are you located. There may be a member that could help you out. I'm in Salt Lake City.
Edit: Just did a search and see you are now in Carson City.

Yes about 15 mins south of Carson in Minden, topaz and lohanton will be the 2 main lakes I go to. Tahoe is closest but also most intimidating

 

[Croz]

I think that comes with about 80s out of the box. You didn’t say if it’s black smoke at idle. Just pull the carb and rebuild it. Probably got spiders in the bowls

Black smoke was happening when the needle and seat was stuck over thanksgiving. I typically run the non ethanol from Parker oil. There’s non ethanol options for 87 up here at the Maverik I plan to run

 

[Todd Mohr]

Call Bob at Competion carbs in Reno, 775-331-5609. He will rebuild you carb and dial it in with the proper jetting and all adjustments. He has built 4-5 carbs for me when I lived up there and has incredible knowledge. Your carb will be perfect and it's most likely needing a rebuild. He will ask what motor it's going on and where and how you use your boat. Only downside is he's usually 30 days out from when you drop it off, but well worth the time if you are unsure about tuning it yourself.

 

[Croz]

Call Bob at Competion carbs in Reno, 775-331-5609. He will rebuild you carb and dial it in with the proper jetting and all adjustments. He has built 4-5 carbs for me when I lived up there and has incredible knowledge. Your carb will be perfect and it's most likely needing a rebuild. He will ask what motor it's going on and where and how you use your boat. Only downside is he's usually 30 days out from when you drop it off, but well worth the time if you are unsure about tuning it yourself.

Great thank you for the recommendation!

 

[lbhsbz]

Yes about 15 mins south of Carson in Minden, topaz and lohanton will be the 2 main lakes I go to. Tahoe is closest but also most intimidating

Beautiful area up there....if my in-laws didn't live there, I'd probably move there. Trying to get further away though...not closer.

 

[77charger]

Here is SoCal, all the alcohol in the fuel turns the accel pump diaphragm, into a rock, in about 6 months!

You guys have that same issue up there

But they can drive from the mtns to the beach get the parts and be back the same day.LOL

 

[Mototrig]

Subscribing

 

[Lumpy]

Jesus!

Carb needs a rebuild.

Don't confuse the poor guy…doesn’t anyone read?

Post #4 Ya… I said that for fuck sake.

Don’t change a thing…run it to temp and pull a God Damn plug for Christ sake…didn't any of you go to auto shop?

Wish gn7 was still alive…he wouldn’t be as nice.

Hell, send me the carb I’ll do it.

 

[Croz]

 

[wallnutz]

She is just a tad fat

 

[Croz]

She is just a tad fat

Could be from before with the stuck needle and seat?? These plugs have a season on them. Boat is running TITS, idle is a tad low but fires up like it’s EFI

 

[River Dirt 2]

View attachment 1504541

“Good enough for the girls I go with”

 

[DarkHorseRacing]

Could be from before with the stuck needle and seat?? These plugs have a season on them. Boat is running TITS, idle is a tad low but fires up like it’s EFI

Replace the plugs and run it hard, shut it down and then read them. Don't let it idle much after the hard run.

 

[wzuber]

View attachment 1504541

He'll, that thing is piglosi rich....