crossflow v4 carb throat vs jet questions

[ryguy1101]

Please don't groan....I'm trying to rebuild my carbs and adjust my jet sizes using primarily my pile of spare parts, but I have a few questions:

Background: I have been running a pair of 1 5/16 throat carbs (1982 generation) w/ #36 (throat jets), and #29 and #67 (bowl jets) for a couple years on my rebuilt bubbleback V4 with a '85-'87 era ignition system and approx 100PSI/cylinder. With those jets it seems to run great at cruising/planing speed, but rich (very smokey with LOTS of unburned fuel in the exhuast) at idle, and the spark plugs seem to agree. A day out cruising and the plug insulators are somewhat chocolatey like they're supposed to be, while a day of idling or trolling and they're pretty wet and black. wanted to try trading out some of the jets to see if I could lean out the idle mixture just a bit and found out the hard way that I over-tightened the #67 jets in the float bowl last time I serviced the carbs. Needless to say, those jets are stripped and the bowls are ruined. Herein lie my questions.

1) I have two sets of spare carbs w/ 1 3/16 throats from the same generation crossflow. The float bowls have different stamped part numbers but look identical visually. Can anyone tell me if they are interchangeable with the bowls from the 1 5/16 carbs or if the air/fuel ports are different somehow, irrespective of jets?

2) My original goal was to replace the jets I had been running with the standard 115 HP jets from that era (27, 34, 60). Will the 1 5/16 throat be too large for the reduced jet sizes/fuel input and therefore make the engine run too lean?

3) Ultimately, with the stated compression, two different carb sizes, and multiple jet sizes I have on hand, does anyone have a specific recommendation for plugs, jets, and carb throat combination? I'd like to make sure this engine is tuned right and running efficiently.

4) For my own own reference, what throat size carb historically went with what HP V4 engine?

Thanks for any input,

Ryan

 

[Haffiman]

Re: crossflow v4 carb throat vs jet questions

The key to correct combustion is the mix between fuel and air.
Reduce (smaller throat) the amount of air, and you need to reduce the amount of fuel to get a clean combustion.
In short, if going down on ventury diameter, in most cases you need to go down in jets as well and loose power!!
As for mixing float bowls between different carbs, use the parts cataloge (evinrude.com or marineengine) as guide and reference.

 

[ryguy1101]

Re: crossflow v4 carb throat vs jet questions

Haff-

Good suggestion regarding using parts lists to compare bowl numbers across all HP options. The bowl sets are apparently the same for all 88-140 crossflows from that generation which answers question number 1 above. However, they list the upper and lower bowls with different part numbers. Any ideas what the difference could be between upper and lower float bowls?

Also, still would appreciate some thoughts on questions 2, 3, and 4.

Thanks,

Ryan

 

[ryguy1101]

Re: crossflow v4 carb throat vs jet questions

:redface:bump??

 

[emdsapmgr]

Re: crossflow v4 carb throat vs jet questions

Hard to tell why the bowl part numbers are different. The part numbers have been discontinued, so there is no supercession history on them. You are running a set of 140 carbs on your 115. I am sure that they are overfueling your powerhead. Not only at idle, but probably slightly on top end. I'd be careful when you rejet. If you switch your140 carbs to the jets from your old 115, you will wind up with a problem. You'll be ingesting a lot more air than fuel and a lean condition will start to destroy the powerhead. Better to slightly lean out (go down one jets size) the 140 jets one size at a time (a long-drawn out testing process.) Keep reading your plugs each time and take lots of notes about rpm's at WOT, time to plane, etc. so you have a baseline on each set of jets. Spend most of you time getting the high speed jets dialed in first. Your test runs should be short, brief, followed by a high rpm throttle chop and then read the plugs. You can replace the carb bowls, but you should look for bowls that are identical to the ones you have. Your current bowls have two jets on each side of the bowls. Later bowls have the mid-jet inside the bowl. I run a similar set of 1 5/16" 135 carbs on my 79 85 hp block. Have had to fiddle a lot with the carb jetting. Huge performance difference from the original tiny stock carbs. Took a while to dial them in, but was worth the performance gains. You need to get your hands on the OMC master jet chart which is found in many of the mid 80's factory service manuals.

 

[ryguy1101]

Re: crossflow v4 carb throat vs jet questions

Thanks emd, you've given me good advice in the past as well. So, basically what I'm hearing is that stepping down fully from std 140 jets to std 115 jets using the same wide-throat carb will lean out my mixture excessively and that I should instead reduce them incrementally, one size at a time. Can I safely assume that the 67 jets regulate the high speed delivery while the 29's feed her at low speed? What do the #36 jets in the carb throats do?

What size jets are you running on your 135 carbs? Is that 85 block standard bore?

Also, is it possible my relatively low compression is responsible for the excessive smoke and black plugs (i.e. incomplete combustion)?

Thanks again,

Ryan

 

[emdsapmgr]

Re: crossflow v4 carb throat vs jet questions

Yes, I'd start with the jets that are in the carbs and go down. I am running 74 135 carbs. They had huge main fuel jets originally (.069) I have rejetted downward to .068 and probably still need to go down further. (ran out of jets from my surplus jet box.) The engine is a terror out of the hole, but overfuels somewhat still at WOT. If I want to keep playing with it, I will probably find that .066 will be the best for overall performance. I have not changed the idles from stock. (this carb does not have mid jets.) You may also find that once you get the high speed jets dialed in, that you may not need to fiddle with the idles/mids. Mine idles perfectly at 650 in gear. That year 79 85 block is 99.6 cubes. It had a bad hole when I got it, so it's now all .30 oversize. I did port the block prior to reassembly and am running the 140 rubber intake stuffers. It is still running the stock 85 heads, which are actually fairly high compression- 125 each cyl. If your heads are the newer bathtub heads (lower compession) that's another reason to step down in main jet size.

 

[ryguy1101]

Re: crossflow v4 carb throat vs jet questions

update on results. Ultimately found out I was running mid-jets two sizes too large (29's instead of the factory spec 25's for 140HP engines). Finally took a ride yesterday and it ran great. Purrred like a kitten (albeit a 25 year old one) at startup and at speed. Only thing left to do is check the plugs to see how they look after a day of mixed idle and cruise. Thanks again for input. Will post again if anything changes, and may post photos of plugs if significant.

 

[dave2517]

I know this is an old thread but I just ordered a set of carbs from a 135 for my 85hp....my 85 is a 1977 so I think I have the small bore....other than the carbs I am going to put a set of boysen power reeds in and the filler blocks.....any jet suggestions to get me close?

 

[GA_Boater]

I know this is an old thread but I just ordered a set of carbs from a 135 for my 85hp....my 85 is a 1977 so I think I have the small bore....other than the carbs I am going to put a set of boysen power reeds in and the filler blocks.....any jet suggestions to get me close?

You have a carb thread! That is where you ask these questions, not in dead 4 year old threads.

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