Triumph Rat Motorcycle Forums banner

1 - 11 of 11 Posts

·
Registered
Joined
·
2,472 Posts
Discussion Starter #1
YOU WANT TECH, I'LL GIVE YA A TECH TOPIC!
'kay, here goes:

1) Does anybody know what size & just what kind of bung I should buy to utilize those fittings on the down-tubes of our headers? It would afford us a more reliable reading of what our air/fuel ratio would be via each carburetor (our cross over pipe prevents such a reading).

2) If I wanted to drill out my 2.5mm air hole to 3mm, should I AVOID from doing it by hand..... should it be done with a drill press instead?

3) I see that the steps suggested for variations of main jet sizes seem to be in 5 point increments.
(*e.g.. 105 for free breathing pipes, 110 with an added free flowing air filter, 115 with addition of a bellmouth, 120 when eliminating an air box baffle, 130 if the air box is replaced with pods) per Thruxton needles. My question is this:
I can go to a motorcycle shop and buy main jet sizes in all sorts of sizes (and I have lotsa jets now) In the world of Bonneville's Keihin's CVK, just what is "one full step" in jet sizes???

Now this is really weird.....

4) I'm currently running a 790 with the divider plate still in my air box (I ran w/o the plate in my BonnieBlack). The only other differences are a slight variation in my modified D&D cans, and my NH bellmouth having been powder coated a smoooooth and sexy gloss Black. Now, I just got the thing "sniffed" & Dyno'ed with some 118's in the Thruxton needle equipped carbs, and it was running waaaaay lean. It was running upwards to 17 (Air/Fuel), between 3500rpm & 4200rpm. 14 to 15.5 (Air/Fuel) from 5000rpm to 7800.
This baffles me because I may have actually been running smaller main jets in my BonnieBlack (sans it's divider plate)! How is it that my current bike seems to want a bigger main jet?!?!?!

5) So...... I had two size jets readily available with which to replace the 118's, some 122.5's or 130's. THUS my question, "what is a full step" in main jet size? I thought that because I was running soooo lean, that simply going from 118 to 122 wasn't enough to address my lean mixture, so I stuck my 130's in there (it's been raining, haven't taken her for a spin yet). In the meantime, I also concluded that my cold start problem was as a result of having the 40 Pilots still in there, so I bought some 42's yesterday. I'm NOT gonna take those carb's apart again (my hands are still chapped from all that gas from last night's switch out), until I have the bike sniffed with these 130's (that's assuming the danm thing'll start this afternoon).

So, to sum up.......
1) What size bung and what do I need to make myself a pair of sniffer sticks?
2) Can I just drill by hand, or find somebody with a drill press?
3) What's a quarter step, half step, or full step in jet sizes?
4) How many 'steps' would YOU increase from my lean running 118's, given my lean reading a couple days ago?

Speak among yourselves, share your thoughts, help me with some wisdom here guru's of things carburated.....
:)



[ This message was edited by: FattRat on 2006-12-09 11:37 ]
 

·
Registered
Joined
·
1,598 Posts
Hi pat ,I'm sitting here chilling out with a broken right hand in a cast instead of riding ,maybe I can shed some disinformation on this topic.
1. I don't know!
2. I asked one of our famous on line vendors about this and was advised to NOT drill out the slide hole,very easy to mess up he said.He advised using a different spring on top to quicken the throttle response;non invasive and easily un do able if you don't like it.
3.Jets come in divisions of 2.5...I would think that would be a step.
4.I was also running 117.5s in a similar setup...as you had previously posted ,they run pretty good set up like that!I thought I was lean up at the very top end,but acceleration was great everywhere else.So I went up to 122.5 mains ,already had 42 pilots,Thruxneedles no shim.Pulls even stronger up top ,but seems maybe a tad rich in the mids.I was gonna go down to 120 mains befoe I read this from you ,now I will stay where I'm at with the mains and get it on a dyno after I have the valve check done (soon).When I went up to 122.5s .my milage took a definite slide ,15/20%.I adjusted the pilot mix and it seems to be better now,but not as before.
SO you were only a little lean up top and way lean in the mids;put a shim on the needles and bring the mains down to 122.5 or 125...you should be about there.
I have a theory about Thrux needles in 790 motors,could be wrong.They seem to work magic in setups jetted in the teens,but are harder to get a even A/F mix across the rev range as you go up in main size;they are very rich needles.Your reported midrange leaness is indeed a mystery! Good luck I'm sure you will get it sorted.
I also think the Hinckley motors like to run on the lean side,just consider how they deliver them from the showroom.They would never do that if they thought they would burn up.
 

·
Registered
Joined
·
2,561 Posts
OK-main jets kick in at 5,000 rpm- needles control most everything else. 42 Pilots are a good move IMHO. I run 135 mains in my 865 with the NARK and K&N's. D&D pipes or TORS(early short ones) which seem to make little difference in
performance just less noise. Needles are the answer I think. My needles are stock and work great, although I have not sniffed or dynoed the bike. My Bonnie is set up like a lot of folks who run 140 plus mains?? Are they too rich? When I go to 4.000 feet altitude (NC/Tenn Mountain rides) the 135 jet are too rich-bike won't hardly run above 5,000rpm. Guess I need to get it dynoed and snifffed! Only it runs too good to be too far off. (FAST TOO!) maybe u should try some later T100 865 needles??? Goood luck.
PS -no drills please. I ruined a Harley carb one time drilling it for more gas flow-got it too rich! be careful.
Bigger main jet need could be due to increased flow maybe due to a different air filter
characteristics(mo flow) ?
 

·
Registered
Joined
·
1,576 Posts
Pat, here's my tuppence worth:

In answer to question 2, I used a battery drill and ran the 3mm drill straight through the slide holes, no sweat. It won't f*ck up your carburettion, mine's ok still. I supported the slide with a block of wood beneath it before drilling, but you have to be reasonably careful to run straight through without 'picking up'. Judging by your handywork on your bike, you have a good pair of hands, you'll be ok.

Jetting, I can only speak for my 270' 865 motor. With the pipes I have on at the moment (de-baffled and perforated stockers) and an opened out airbox, minus the baffle plate, I'm currently on 42 pilots with 1 1/2 turns out on older style adjustable Bella Corse thumb screws (quite coarse adjustment) Thruxton needles with 2 shims and 130 mains.

I hasten to add these are unscientific butt-dyno figures, but I can tell when my bike runs right and when it runs sh!te. I tried 122 mains first after I opened out the airbox, then 130 and it ran better on 130. As a rough experiment I later removed the filter completely, it ran much worse, so I take it I'm not drastically rich. I'm probably the rich side of right though.

Anyhow, I'm changing pipes and fitting pods soon so I'll be back to square one myself.

ps. If my future experiments go as hoped, the entire Scrambler exhaust system will be going cheap. don't ask why, just let me know if you're interested.
 

·
Registered
Joined
·
1,995 Posts
I'd start with a 125 main, this should be enough of a increase to get your a/f in the ballpark between 5000 and 7800. To address the lean condition between 3500 and 4200 I would shim the needles up a bit...maybe 2 shims, and increase the pilot's to 42..maybe 45 but not until I got it back on a dyno with the 42's and the needle's shimmed.

I don't know that I would drill the slide's Pat, once you drill them out...you cant go back. I think I'd swap out the spring's in the slide's. Easily reversable if you do not get the desired result.

Not sure about the weather out there, but consider that cooler air is denser, you may need to jet a little fatter this time of year compared to the heat of the summer.

As for the fitting's in the down tubes, I havent seen a dyno operator use them to this point, if you do drill them out so they can be used on the next trip to the dyno odd's are good your a/f will read differently due to different positioning of the 02 sensor.


Nice pic of the bike man, looking at her, with her black fork's and black cup's for the instruments...you could surely paint or powdercoat that headlight bucket black :wink: :upthumb:
 

·
Registered
Joined
·
2,472 Posts
Discussion Starter #6
THANKS GUYS..... you're the greatest!

I just got back again & again from a couple 30 mile rides before it rains again (mirrors to tighten, crooked handlebars, etc.). Lemme tell ya first what led up to today:

Prior to sticking the 118's in, I was running 122.5's. When I switched out my Black D&D's with my home-made "FattRat" pipes and took the 122's out..... the bike began to run like shite. So after I had been working on the bike earlier this week, I find that a zip tie I used to secure my in-line fuel filter was pinched under the cap of one of my carb's, i.e. a compromised seal on my right carburetor. But the pipes were now changed, and I put the 118 main jet in the other carb' already, so I went ahead and finished up with the task at hand. Now I just GOTTA KNOW where I stand with my jetting! Thus the Dyno run when I was able to slip away from work early Thursday (a very lean 59 SAE hp, & 46 torque).

Thursday night I put the 130 mains in, and today I was finally able to take her for a spin. I gotta tell ya, after those 60 miles with the 130's, the bike feels really light on it's feet.... all the way up to an indicated 115 mph. 'Jenks' suggested that if the bike has a strong top-end when COLD and slows upon warming..... the main jet is too big. Well, she just seems to have gotten BETTER upon warming, not worst. Quick to accelerate, and quick to respond to the twist of the wrist at 70 mph! WooHOO!

After a couple quick 30 mile jaunts, those danm handlebars all crooked & c0ckeyed since I put her together JUS' GOTTA BE ADDRESSED!!! My first attempt today didn't work, they simply sprung right back into their c0ckeyed position after I went a mile down the road! :mad: :mad: :mad: This time I took it all apart and attacked it again.... I'm reluctant to take it for a spin for fear it'll pop right back where it was.

The clouds are getting really dark, the streets are still dry, and I gotta find out if the bike really DOES run as good as it was an hour ago. My luck would have it though that as soon as I get on the freeway, it'll start to storm'n.

Thanks again,
I may not bother drilling out the air hole, the 42's are gonna go in shortly, and SCC.... you're right about the denser air, I've been feeling asthmatic the last 24 hours.... yawning, gasping for air, feeling sluggish from the heavy air & humidity (I couldn't do tropic weather several years ago when I went to a Club Med in Martinique....). And you really think I could get away with painting that headlamp housing Black??? I'm kinda thinking it needs a little chrome to keep the Red & Black broken up, but I wanna be as cool as YOU guys with your Black housings!

:cool:
 

·
Registered
Joined
·
505 Posts
I used a 7/64 drill in a drill press which gives a 2.8mm hole, with no problems at all, but I can't honestly say I notriced any difference in pickup.
Biggest main jet I can use with THruxton needles in a 790 is a 140 without getting sooty plugs.
I'm still pulling hard at 8000RPM
 

·
Registered
Joined
·
1,576 Posts
Pat,

just to add to what I already said, I did the slide drilling at the same time as other mod's, so I don't have a before/after on the drilling alone. I took Peter's advice on it based on his results, all I'm saying is it didn't do any harm and my bike goes like the clappers :-D
 

·
Registered
Joined
·
79 Posts
I also drilled my slides,but drilled them by hand.The best result was opening them up flush with the outside wall.The difference you'll see with a 42 pilot is that you'll get the same or close to it fuel air mixture simply by using fewer turns out.On a CV carb the needle to me is the most noticeable change that I normally make.What I mean by that is,with a rich needle you might have to select a leaner main jet which is what I go with.I''ve noticed alot of guys using 155 mains.With my slides drilled out I can't do this,I'm stuck using a152.5 or a 150,but to me a 150 pulled harder than a 155 because the needle position is higher,so thats just fine with me.
 

·
Registered
Joined
·
2,472 Posts
Discussion Starter #10
The bike runs good, the day nonproductive, and the ride home..... WET!

I couldn't help myself, I rode and rided, and ridden some mo'. I could see the storm clouds looming to the west, and though initially skirting along them, I found myself heading straight for 'em on my way to Locke.

The roads were wet, and my once pristine bike (waxed many times over) was now awash in the muck kicked up by the cars ahead of me. But I soldiered on, I wanted a Jack & water at Al's......

When I arrived, the couple sitting next to me at the bar suggested that I "...wasn't gonna let the rain slow ME down" I suggested that it did 'slow me down', but it didn't stop me. I smiled and said, 'but the bike was running so good and I, well..... couldn't stop!"

I hadn't been seated longer than seven minutes, then threw my jacket on and headed for the door. I filmed the below linked clip, put my helmet on, and the heavens OPENED.... dumping a maelstrom of water. Well, the bellmouth did quite well in forty minutes of rain on the freeway. My leathers were soaked, but now they're clean.

Life (and motorcycles) is good!
:wink:
The Bonnie in Chinatown

[ This message was edited by: FattRat on 2006-12-09 20:34 ]
 

·
Registered
Joined
·
1,251 Posts
Hivel, I don't mean to call you out but our carbs operate on the position of the throttle, not by RPM. Take a look at the various charts floating around and you will find the correct operation.

Ratt, for what it's worth, I drilled my slides by hand and had no problems. It's your option to drill or not.

Try taking a piece of masking tape, tape it on the switch housing beside the throttle and make reference marks in 1/4 increments to WOT. Take a trip to the dyno and reference the readings of the sniffer with the marks on the tape. This will give you an accurate reading of where you are lean at. Make sure you ask the dyno operater to perform pulls in the 1/4 increments for you.

Most operators will just do WOT pulls unless you ask!

Greg
 
1 - 11 of 11 Posts
Top