Crossflow venturi size.....Effects?

[mnw001]

I'm replace carbs on a 79 140 with 1 3/8 venturi's. Can anyone tell me if I can boost low end performnance with a smaller venturi or would that greatly effect top end on this motor. I've heard this but am unclear about its effect. <br /><br />Question 2 anyone know if carbs from a 100hp cross with a 1 3/8 ven would work on my 79. I'm not sure of the year for the 100hp.<br /><br />Question 3 anyone know the size of the ven on the 79 150 the part number on the carb is 389992 but don't know the size and I don't have it here to look at it.

 

[DHPMARINE]

Re: Crossflow venturi size.....Effects?

A smalller venturi would mean a richer mixture.It might get up and going quicker.But on top end,a motor is an air pump.What you put in venturi wise also has to get out.That means exhaust.<br /><br />your 140 already has the bubble,so you'd have to pull the powerhead,and look at the exhaust there.<br />Maybe it can be opened up,but I don't know how.<br /><br />Another idea,your motor is 26 years old.And you want more power out of it ? Even if you just rebuilt it,many parts are still 26 years old.<br /><br />I don't think the V6 carbs will bolt on.Try to buy a gasket for it and see.<br /><br />DHP

 

[ob]

Re: Crossflow venturi size.....Effects?

On the other side of the coin,most two strokes will readily come out of the hole without any modifications or back yard engineering.What is your engine propped to turn at wide open throttle?If overpropped,it will definately be sluggish coming out of the hole.If you don't have a tach ,just post the diameter and pitch of the prop you are running.Also if your rig is not eqipped with a tach ,I recommend having one installed.Running without one is flying blind so to speak.

 

[rodbolt]

Re: Crossflow venturi size.....Effects?

hello<br /> the crossflow v-6 carbs will bolt right up. doesnt mean they are jetted correctly. look upsome information on the venturi principle. however the 140 crossflow was almost at the edge of reliability in 79. the 140 used a tuned exhaust. hence the bubble back. it also used filler blocks on the intake reeds. while there is some performance potential to gain it starts at the cost of low speed drivability and engine longevity. <br />speed costs money. how fast ya wanna go ?

 

[mnw001]

Re: Crossflow venturi size.....Effects?

Ok I guess I didn't explain quite enough<br />I'm only changing carbs because one of mine is aparntly worn out I get a small pop or backfire at idle because of the plate not closing completely due to wear in the throttle shaft bushings. I have tried all kinds of things. My boat is a 18' aluminum walleye boat. I'm not really interested in top end but I do load the boat heavily going boat camping once in a while. So I just thought if I'm changing carb's anyway if it ran a little stronger on the bottom that would be fine. but I'm also wanting the best cruise economy I can get which probably isn't going to change much on this motor. I understand that the 79 140 was geared higher than ealier engines and I know that I can adjust the prop to hit my rpm range. <br /> I did think that the v6 carbs would work and I was planing on using my stock sea level jets or 2 or 3 thousands smaller due to my 5200 foot altitude..<br />Thanks everyone I love having this forum to bounce ideas off of you guys. Your really helpful

 

[rodbolt]

Re: Crossflow venturi size.....Effects?

hello<br /> its much easier to rejet the idle air jets on your original carb to compensate for wear.<br /> remove the airbox. with the engine idling and the popping going on, use a tooth pick and one at a time stick it in the idle air jet. the one that smooths the motor out as you close off the air is the one to make a few sizes smaller. sometimes you can make all of them smaller. the jets can be changed with just the air box cover removed.

 

[R.Johnson]

Re: Crossflow venturi size.....Effects?

If you are set on going to smaller carbs, I have a set of carbs with the 1-5/16 venturi's off of a 135 I will exchange with you. I want a set of carbs with the 1-3/8 venturi's for a 135 I am playing with. I can fix that throttle shaft.

 

[mnw001]

Re: Crossflow venturi size.....Effects?

Rodbolt<br />that confused me<br />first of all the throttle plate is staying open just a few thousandths allowing extra air <br />The idle jets control fuel as I understand not air. If I have too much air creating a lean pop I need more fuel to offset that DON'T I? They are accessed thru a cover screw on the side of the carb. I don't know of any changed air jets for the idle circuit unless it is the two small brass orfices that are pressed into the body of the carb. I have size 30 idle jets which are stock sea level and I'm at 5000 feet so I allready should have a rich idle mixture which is fine. Help me understand please if I'm wrong

 

[mnw001]

Re: Crossflow venturi size.....Effects?

R J<br />I'm not really wanting to use smaller unless they would be better on the motor, If I can determine that perhaps we could trade except that I disassembled the throttle shaft on one carb to inspect it and one of the damn plate screws broke off in the shaft. might be a trick to get it out of there. The shaft did come out ok and it does show wear on one side..

 

[rodbolt]

Re: Crossflow venturi size.....Effects?

fishslayer<br /> the carb throttle plate scres are "bucked" or peened over after installation to prevent then from vibrating out during operation. DO NOT reuse the screws. and when you install new ones make sure to peen them. a screw through the reeds is a nasty thing. but overall I am thinking you may need to study your carbs more. the idle air jets are in the top of the carb throat just in front of the venturi restriction. the small one in the fuel bowl is intermetiate fuel and the large one is main fuel. the idle pick up tubes are the small brass ones with the neck, pressed inside the main emulsion tube. but idle air fuel is controled by the air jets in the carb throats. so by decreasing the size of the idle air jet it will cause more fuel to be picked up in the idle fuel tube thus compensating for some throttle shaft wear.<br /> like I say a toothpick in the jet will tell you which cyl is lean. sometimes on extreme cases you can close them off with your finger while running. try to do all tests with the boat in the water,<br /> good luck and keep posting

 

[Dhadley]

Re: Crossflow venturi size.....Effects?

Just a bit of info on crossflow carbs -- any crossflow carb will bolt up to any crossflow intake manifold. The the throttle pick up is further in on a V6 carb than a V4 carb (shaft length). You can use the V6 carb on a V4 by modifying the cam follower. No big deal.<br /><br />As for jetting -- the carbs are jetted according to the calibration of each carb. Not by what motor its on. Jets are only a minor part of the final calibration.<br /><br />Venturi size -- smaller venturi's = more accleration but tend to limit rpm's compared to large venturi carbs. Larger venturi's = more top rpm but take longer to get there.

 

[jimmbo]

Re: Crossflow venturi size.....Effects?

Dhadley, Don't you mean any crossflow carb after 1968?

 

[Dhadley]

Re: Crossflow venturi size.....Effects?

Youre correct Jimmbo. Good catch. Any V4 cf carb 1969 & newer and any V6 cf.

 

[mnw001]

Re: Crossflow venturi size.....Effects?

Dhadley<br />Are there changable idle air jets on the 79 140 carb <br />Thx

 

[Dhadley]

Re: Crossflow venturi size.....Effects?

The idle jets in a 79 140 are fuel jets and can be changed.

 

[rodbolt]

Re: Crossflow venturi size.....Effects?

hello<br /> I have 1 1979 and an 83 140. Dhadley is correct I was looking at the wrong ones. your 79 has a choke and uses Idle fuel jets. the jet is located under a plug just below the throttle shafts. on the carbs that used a primer they were in the venturis. I would try rejetting your carbs before replacing them. there were some tech bullitens someplace about the incorrect venturi sizes on some v4-v6 carbs I just cannot remember them. it fairly common to have to rejet idle and intermediate jets to compensate for wear and rebuilds and sometimes we had to do it to new motors that liked to cough at idle with no other failures found.

 

[mnw001]

Re: Crossflow venturi size.....Effects?

Well Rodbolt that all makes sinse but I know for a fact that the one throttle plate is staying open just a little which puts air in the crankcase instead of using the idle circuit. so it seems like because this was caused by wear in would just continue to get worse even if I added more fuel to it. If it were running poorly and nothing was wrong I would then want to change jets for sure. for the price of used carbs on ebay I'd just as soon try to get a good one and see how it runs then. What do you think about that approach? I still really appreciate all the help. I thought that 85 or newer used primers so you say your 83 has them. Are they stock?

 

[rodbolt]

Re: Crossflow venturi size.....Effects?

yes<br /> its all stock. just sank.<br /> both were sunk in saltwater so not many powerhead parts are salvagable. both have carbs with locked up throttle shafts. I also have a couple of cross flow v6's that I am scrapping out.

 

[mnw001]

Re: Crossflow venturi size.....Effects?

one more question if I'm changing from choke to primers can I use the same fuel pump and is there a primer pump or solennoid that takes the place of the choke solenoid<br />thx again<br /><br />Rod Could you email your number I'd like to talk with you about the carbs<br />mnw001@hotmail.com

 

[jy118lfd]

Re: Crossflow venturi size.....Effects?

Fish sorry I can not find the 140 carbs