389 monoblocs on 88SS

t ingermanson

t ingermanson

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Has anyone ever put 1 1/8" 389s on an 88SS?

88SS and early 650SS bikes both had 376s, but there's report of people running up to 932s on 650SSs with good results, so...

I've got sets of 389s and 376s, but wondering if the 389 would just be too big, even with a hot(ter) motor. I'll be making the inlet manifolds and sleeves for the inlet ports, so I can definitely taper everything to speed up the flow, but would rather not spend the time to make it all if there's experience out there telling me otherwise.
 
1inch carbs were spec by Norton for the 88SS, 1 1/16 for the 99SS.
I suspect they will be slighty too big and reduce air flow to the engine, because they are bigger bore.
You will lose the low down power in the power band. I'm building a 88ss and will not consider them, because the smaller carbs will suck more oxygen in , the bigger the carbs you go the less oxygen that gets sucked into the engine.
If you don't beleive me, try sucking water through a quater inch tube , then try it with a one inch tube.
 
Bernhard said:
If you don't beleive me, try sucking water through a quater inch tube , then try it with a one inch tube.
Click to expand...

I tried it with my evening martini. That first one went down fast, I don't remember the second one, and the third one got me arrested.

Now to try with the 1" straw.
 
Bernhard said:
.... because the smaller carbs will suck more oxygen in , the bigger the carbs you go the less oxygen that gets sucked into the engine.
If you don't beleive me, try sucking water through a quater inch tube , then try it with a one inch tube.
Click to expand...
I think you're wrong there - actually the bigger carbs will allow more air (27% oxygen) because of lower friction but...
The jets won't work as well because of the lower air velocity over them - meaning leaner mixture than should be required.

If you don't believe me try sucking air (or oxygen) through a 3mm tube, then do the same with a 12mm tube - you'll get more air, easily, with the larger tube.
Your water analogy won't cut it on this application.
 
t ingermanson said:
Has anyone ever put 1 1/8" 389s on an 88SS?
Click to expand...
I don't know this for sure, but suspect there is a design range of velocities for each type of carb.
eg:
Engine capacity(cubic feet) /2(it's a 4 stroke) x RPM = CFM

CFM / combined carb throat area (in square feet) = ft/min velocity through the throats

So, for a rev range from 1100 - 7000 rpm you can work the range of velocities - are they appropriate for the carbs being used?

If the velocity is much too high you will ice up the carbs, much too low and the engine won't get enough fuel to run.

There's probably a good reason you don't see 389s on 500cc engines - doing the ratios it's like putting a pair of 43mm carbs on a 750 commando - you probably don't see that combo much either.
 
I had a pair of 650 SS carbs on an 88 and they worked OK (seat of pants feel) ,don't think bigger would be helpfull on the road.
 
Ran 1 3/16” monoblocs on a 500 with an SS head that I raced mid 60s to early 70s. Copy of works camshaft, head ported by Harold Hall, Dunstall pipes and megas. Raced in Ireland and Isle of Man with no problems.
 
Dogsbody said:
that I raced
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Dogsbody said:
with no problems.
Click to expand...
There you go.
idle immaterial,
partial throttle/cruise immaterial
7000 rpm and up OH WOW,,,and the cops are chasing me !
With those carbs, of course, you will have no muffler or why bother?

You create the performance characteristic of a chain saw, idle or WOT

robs ss said:
The jets won't work as well because of the lower air velocity over them - meaning leaner mixture than should be required.
Click to expand...
Above is the whole thing (physics) in a nut.
oversized big carbs = bragging rights & crappy response even if rejetted and custom modified and tuned
more correctly sized = smooth throttle transitions from idle/low speed to high speed
1-1/8=28.5mm the normal port size of a S650/136 head and is as big as I would (reluctantly) use.
Driveability will be compromised on a 500. On a 650 it will be somewhat better.

5 speed or close ratio will make the engines torque curve more acceptable when "on the cam".
 
Thank you, everybody.

This will be a hot-ish street motor, but not a race motor, so keeping the power down low is more interesting than coming on the cam at 5k.

I had a feeling they'd be too big. I'm getting the inlet ports flowed due to some generous dremel work to the SS heads I've got. I may still make some inlet manifolds and sleeves for the 389s for a 650 motor. Once the flow work is done, I'll report back with any findings.
 
I'm glad you see sense.
Leave inlet port at manifold as it is at same diameter do not make bigger. You can do work downstream of valve guide to cylinder head valve seat.
I was going to fit a pair of cv carbs off a more modern jap bike, but I'm on my own regarding jetting.
 
For an 88SS, think of an "SS" 22729 cam already as a double strength "combat" cam. LOL
That is all I'm using in MY faux 88SS and the same in my, to follow, 650SS.
28mm concentric carbs-28mm manifolds & stock 1-1/8" head. I'll eventually be riding it to NE dragway for testing.
18T gb sprocket, belt drive, commando clutch. Gona try and chase down a Hayabusa. :cool:
cheers & good luck
DJC
 
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dynodave said:
For an 88SS, think of an "SS" 22729 cam already as a double strength "combat" cam. LOL
That is all I'm using in MY faux 88SS and the same in my, to follow, 650SS.
28mm concentric carbs-28mm manifolds & stock 1-1/8" head. I'll eventually be riding it to NE dragway for testing.
18T gb sprocket, belt drive, commando clutch. Gona try and chase down a Hayabusa. :cool:
cheers & good luck
DJC
Click to expand...
Well, the 389 carbs I've got are 28mm...

How would running the monoblocs be bad, but the concentrics good?
 
Concentric or monobloc should be the same,
I'm going racing ! I tried to buy small enough for the 88 but still big enough for the 650. Once built and tested, everything carries over from one engine to the other. Granted 28 carb is not ideal. I'm trying to only have the 88 in the chassis for one season max...test over. Only change is the 88 gets the small diameter pipes and the 650 will get the bigger atlas pipes.
 
dynodave said:
I'm going racing !
Click to expand...

Fair enough!

Two of the heads I've got have 30 and 31mm inlets, so they'll be sleeved down to 26 up until the guide and the guy who is going to flow the heads is pretty interested in seeing what shape he can come up with downstream of the sleeve by adding to what is currently missing. We'll see. I'm interested to see what he comes up with. He's been involved with winning high performance race car teams and motorbikes for a long time, and was super excited to get to play with an 88ss. Most guys roll their eyes at 60 year old motorbikes...

I'll be taking a stock head with stock carb, manifold, and sleeve as the control, the 30mm and 31mm heads, the 389s with matching manifolds and sleeves. Anything else I should consider? I was thinking of seeing what there is to the larger carb with a stepped down or tapered inlet manifold and sleeve.
 
I have a rather old Weber carb soft back book 1970 or 80's. which I bought while racing my MGB, it has a chart with multiple graph lines for venturi sizes , cc/cylinder and RPM as the arguments. The point was to show the effect on power peak (limit) as the variables changed. The chart RPM's would certainly go well above the mechanical reliability of a NHT. While slide carbs are different from 40DCOE or IDA they can use changeable 30 thru 36mm venturi as desired and you can also buy & use carbs way to big for the engine.
Doing all this R&D is fun and educational. But the measuring and evaluation is often very subjective.unless it goes on a dyno or on the track.
My evaluation was winning over 70 trophies and 2 championships in competition.
 
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