V4 Crossflow Jetting help

[iwombat]

I'm in the process of getting a '80 115 back in condition. I've retrofit a set of 1 & 5/16" carbs and installed boyeson reeds w/ the filler blocks. The boyeson's call for downsizing the idle air jet by 5. I've looked at the jets I have and there's no numbers on 'em anywhere.

So . . . anyone have any suggestions as to what size I should run? Barring that, anyone know what years and what size engines took the 1 & 5/16" carbs? For the idle circuit does the difference in throat size even really matter that much?

Just looking for a reference point and a place to start.

 

[ezeke]

Re: V4 Crossflow Jetting help

Use the dumb end of your sae drill bits to establish the size. The opening and thread should get you to the right orifice. Actually the newer manuals have the drill size conversion charts so you could use any drill.

 

[iwombat]

Re: V4 Crossflow Jetting help

Looks like I've got 31s in now.

 

[72SideWinderSS]

Re: V4 Crossflow Jetting help

For what it's worth, the "78" 140 had 1 3/8" carbs.
The "79" 115 had 1 5/16" carbs.

 

[Benny1963]

Re: V4 Crossflow Jetting help

hi whats up maybe this will help you to the top .
try faztbullet he knows the carbs , but is not on often or hes just watching,
good luck and thanks for the help you have givin me in the past .
im also watching ,thinkin about some mods my self stuffers and boysens and carbs ,
Bennyb

 

[ezeke]

Re: V4 Crossflow Jetting help

31s must have come off a early 1980's V6. The 79 V4 had choked carbs.

 

[iwombat]

Re: V4 Crossflow Jetting help

Hrm. . . interesting theory. They were sold as a pair, but I guess that's possible.

I think I'll start out with 25s and go from there.

 

[Theoutdoorsman]

Re: V4 Crossflow Jetting help

Hrm. . . interesting theory. They were sold as a pair, but I guess that's possible.

I think I'll start out with 25s and go from there.

According to the 1979 OEM Service Manual, the low speed orifice plugs for the 1979 115hp was .027". However, I think the 1979 115 used the smaller throat carb. The low speed orifice plugs for the 1979 140hp used .025". Not absolutely certain as to the orifice plug sizes for the 1978 140hp carbs, but I do know for certain the lower carb used two different size orifice plugs on the port and starboard sides. If you know for certain you have the carbs from the 1978 140hp, I would gladly dig that information up for you. Let me know if you need it, or better yet, I'll post the info here when I get it. Without identifying the correct year the 1 5/16" carbs came off of, you could very well be chasing your tail on this. I'll do some digging for you.............. ALAN

 

[iwombat]

Re: V4 Crossflow Jetting help

The '79 is a different carb entirely. If memory serves, it uses fuel jets. The '80+ uses air jets.
(someone chime in if I'm mistaken)

The carbs I have do not have choke plates, and have the air jets.

 

[Theoutdoorsman]

Re: V4 Crossflow Jetting help

The '79 is a different carb entirely. If memory serves, it uses fuel jets. The '80+ uses air jets.
(someone chime in if I'm mistaken)

The carbs I have do not have choke plates, and have the air jets.

Ok....sorry....... my mistake. You are correct.

 

[iwombat]

Re: V4 Crossflow Jetting help

No worries. It's all a little foggy right there in the late '70s.

On a whole number of fronts.



I am kind of interested in opinions on whether the bore size really matters all that much for the idle and low speed circuits. The late '80 115 came with 1&3/16" carbs. I'm betting I can just baseline off of those in that throttle range.

 

[Theoutdoorsman]

Re: V4 Crossflow Jetting help

Wouldn't the intake need to be changed for this retrofit? It seems the smaller throat carbs would have had a different bolt pattern.......?

 

[ezeke]

Re: V4 Crossflow Jetting help

The crossflow 2 barrel carbs have the same bolt pattern all the way to the 1998 Turbojets and maybe beyond.
This is not rocket science. The only crossflow motors that used the 31 as the stock air jets were the 175HP models as far as I can tell.
That puts the carb at 58.33HP/unit or 116.66/pair before the change in HP rating systems.

 

[iwombat]

Re: V4 Crossflow Jetting help

A good metric. So, looks like 25 is going on the conservative side. 26s might do the trick. Still think I'm starting with 25 and maybe drilling larger - or not.