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Downsides to too large carbs on street?

  • Thread starter Thread starter Zweihammer
  • Start date Start date
Z

Zweihammer

Guest
Hello All!
I have an 82 GS1100E, largely stock engine wise as far as I know. It has a Supertrapp 4 into 1 but that seems to be it. I am monoshocking the rear and won't have room for the airbox and I already know the stock CV's run like crap without it, and I would like something with accel pumps. The carbs as they are now have a serious lag. Pulls like a train a second later.

I have K&N's, I also have 39mm FCR's, though they are not for an 1100. I don't know what they would fit. I was going to sell them to raise some capitol for some Mikuni RS's. I see used sets go thorough e*ay all the time in the 300 range in the larger bores. I would like 36's, but they seem to command higher prices.

My question is can larger carbs be jetted down for the street on a stock bike without huge downsides like bog or flooding?

Thanks, Erick
 
I had 38's on my 1166 and it would bog if I got on the gas too fast. The 36's were much more responsive.
 
Would you use a Hummer to tow a 12 ft dingy/boat? It can be done but wont be suitable. Stock motor to a GOOD street motor would only need 33mm smoothbores.
 
Stock carbs work fine with 4-1 and pods if jetted correctly...
Lots of us have that set up...
 
a set of correctly jetted bandit 36mm carbs with filters will liven the old warhorse up
 
The old trade off, do you want the Absolute Maximum Blast or a nice easy to ride engine with good gas mileage?
 
Thanks for all the responses!

I guess my problem is I don't have a good point of reference. I have no well tuned stocker to compare to. As of now, I haven't changed anything. It starts nicely with the choke when cold, idles at around 1200 to 1400 after warming up. When riding though, it always lags behind twisting the throttle a second or so, isn't crisp at all under load. But it does pull strongly (until the clutch slips a bit at 4500, why just 4500? not above or below?) It cruises nicely, but you really have to think 5 seconds down the road. Not so great in the twisties or traffic.

It could be any number of things. I haven't done a comp or leak down test yet. But the plugs look nice. Doesn't smoke or seem to burn oil. Perhaps the engine us just tired and this the best I am going to get at this point.

I just look at $120+ for a dynajet(sp?) kit and I think I am halfway to a used set of something else. I just know I won't have room for the airbox and I am trying to come up with a game plan now.

Cheers, Erick
 
carburetors of large diameter are a great boasting subject.

really!! smaller mech-slides will give you the use able "street" rpm power

34's are a good street choice up to 1300cc.

36s' will out flow a 2 liter engine with ease!

2 theories in carb dia. intake valve size or cylinder volume.

I love (32 or 34) cv's for the off idle to 3-4K shifting , it makes in town stop and go traffic riding so much easier.

my big race engine 1428 has 44mm trons on a 126c.f.m. ported head BUT the bike is slower than a lot of smaller bikes off the line. There is a trade off for a over carbbed bike, it is slower out of the hole. ! However ! it gets scary fast in 4 gear at 10K and then shift into 5 gear -- hang on!! O WOW fast 151 in a quarter mile. I wish they were a little longer tracks to strech her legs out a little more.

try smaller carbs you might be surprised how little you need W.O.T. power compared to off idle zip.
 
I have jet kits for a LOT less than that & I also have a set of RS 36s for sale. Call me at 714-356-7845 if interested. Ray.
 
trippivot said:
carburetors of large diameter are a great boasting subject.



34's are a good street choice up to 1300cc.
If you want to restrict power to 8000 RPM!!!


36s' will out flow a 2 liter engine with ease!
BS!!! BS!!! BS!!! Put a set of 36 RSs on your 1428 & see how MUCH it slows down!!!

try smaller carbs you might be surprised how little you need W.O.T. power compared to off idle zip.
Click to expand...
THIS last line is true. Ray.
 
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You should be able to swap the Keihin FCRs for a set of Mikuni RS 36mm, or just sell them and pick up another set of carbs. If you don't want to spend upwards of $300 on decent RS carbs, do what some have suggested and look for some Gixxer 36mm Mikunis, if improved response is what you're after. These are way better than the stock 1100 carbs, if properly jetted, and the best bang for the buck power-wise for the 1100s, IMHO, without the invasive surgery needed for big bore kits etc . I'm running Mikuni BDST 36 (semi-flat downdraft carbs) and the bike was able to walk away from an 83 1100E sporting Dyno-jet kitted stock carbs with K&N pods, at will, anywhere in the rev range, so I don't think 36mm should be considered 'over carbed'.
I have a pristine set of RS 36's that I've yet to try....the bike is running very well now & I'm reluctant to mess with it.
Tony.
 
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I agree that generally, the smaller workable size is your best option for the street. But..

I run a set of FCR's on a mostly stock CB900F and Keihin's recommended size for that combo is FCR37's. I was a little concerned that these may be overkill size wise, however they replaced a set of CR31's that I had on the bike and both the bottom and and top end improved greatly. My point is, I believe you can go a little larger than you would normally suspect with the FCR's as they deliver fuel very precisely and they have an accelerator pump (unlike CR's).

If you do plan to try them, make sure they are sidedraft and not downdraft and that the spacing and spigots are correct. FCR's are not easy to re-space and spigots aren't cheap.
 
I am leaning toward the RS 36's versus another set of FCR's as the RS's have provisions for a choke. Though I live in sunny California and have owned and driven many cars without working chokes. I am able to pump them a couple of times to prime them, though. Are FCR's difficult to live with in a mild climate? I do know what you mean about re-spacing the FCR's. I got mine in a trade and thought I could re-space them without too much hassle, nein.

I know the CV's are user friendly, easy to run, good mileage, etc. I just want something that snaps. This isn't my daily, so mileage isn't much of a concern. I will be unhappy if it is temperamental to start, I am getting too old for that.

Cheers, Erick
 
Zweihammer said:
I am able to pump them a couple of times to prime them, though. Are FCR's difficult to live with in a mild climate?
Click to expand...

They start up fine - you just have to twist the throttle a few times to prime them up. Most have an external idle adjustment, so you can simply turn that up until the bike warms up a bit. A proper choke would be nice though.

The only other downside is the noise - like dry clutches you either love it or hate it;

http://s29.photobucket.com/albums/c...0F Restore/?action=view&current=FCR37s010.flv
 
Hello Everybody!
Sorry for the cross post, I am asking this same question in a thread in appearance mods I started on my build, but since I asked the original question here and this is more a performance question I will repeat myself here;

Well, I scored a set of Mikuni RS38's off the E*ay last night with a buy it now of $225. I really wanted some 36s but I could not ponder much as I thought this opportunity wouldn't last. I have had deals bought out from under me while I stared at the screen. I just confirmed they weren't old slabby flatslides and hit the button.

So I guess I will find out first hand what too-big carbs will do. I already have a set of fcr 39's with spacing that didn't fit the GS (or I would have tried them last summer) and I have grand plans for a big bore some day, but that is a year or two away and as far as I can tell I have a stock 1100 with a header.

Any advice on jet sizes I should start with? I plan to go through these before I try to use them, any advice? I also have an odd thought of machining some 1mm thick, tapered, hard plastic reducer bushings to bring the venturis down to 36mm. I would press them in for the time being and take them out later once I can utilize more flow. I have heard of many instances of boring a carb body larger, or using filler in a manifold to alter the flow characteristics, why wouldn't it work in reverse? Or am I worrying for nothing?

Cheers, Erick
 
actually cv's run friggin sweet without the airbox. They are vacuum based carbs. whatcha needa do is is pull the tops off and bore out the vacuum holds just a bit. slap some pods on her. I'm a firm believer that airboxes were invented by the government to make life boring for us citizens.
 
Erick, give me a call at 714-356-7845 & I'll help you out. I wanted to talk to you anyway. Ray.
 
IU've been in the same boat Zweihammer, been trying to snag some 36mm slingshots for months now, ALMOST bought the 38's a couple times, but didn't.
 
Gonzo, I have a set & will put whatever main jets in them you want. I want $400.00 + shipping. Let me know if you are interested, Ray.
 
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