Dual Weber carbs on a 1200 maybe - Page 2 - Steve Saunders Goldwing Forums

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post #11 of 18 (permalink) Old 02-27-2019, 08:24 PM
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Year: 1977, et al
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I had 1000 heads on it. I went back to the 1200 heads and I think it’s better, but ran out of warm weather to conclusively say so. A little known fact about the swap: it is often cited that the 1000 cams had the greatest lift and the 1200’s the least lift because of comparing cam lobe lift, the only facts Honda released. The 1000’s also were slightly longer duration cams, (degrees). What’s not been known is that in the 1200 the rocker ratio was improved to give it as much lift or more than the 1000. The 1000 and 1100 still get the nod for possible WOT because they had 38mm intake valves and the 1200 had 36mm intake valves, a 5% increase. Exhaust valves were the same. Piston dome and combustion chamber was changed in the 1200.

The 84-85 year cams had 10 degrees more overlap. The 1200’s had smaller intake ports which gave faster intake charge speeds. Faster speeds equates to more filling in the same amount of open time. They used carbs with larger bell mouthed intakes into a smaller venturi through an exit the matched the ports. They also reduced the turn the charge had to make from carb exit to intake valve.

The change from 75-77 horsepower and torque (78hp 61tq) from Honda was only minimally improved with the 100cc additional of the 1100 (81hp 65tq). The 1200 went smaller on the intake but ended up with a bigger improvement for the same 100 cc addition. (94hp and 77tq ) And did it at lower rpm (5500) Most attributed it to the extra stroke of the 1200 and that must have been some of it. But think about those numbers. There had to be more to it. It’s a 20% jump. There had to be more to it than the longer 5mm stroke.

Here’s a 75 rocker and an 84 rocker on a spindle.

So the 1200 cam’s smaller lobes lift as much with better balance.

I know everyone says it’s the mod to do. I did it and followed a few early ones thinking it would better. I’m surprised know one caught this but the 1200’s bury the rockers from view with the self adjusting valves. Another reason to keep the 1200.

In case you’re wondering if the 75 cams could be put in the 1200 heads? It’s a no-go because the spindles were moved closer together on the 1200. I have bent valves to prove it.
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post #12 of 18 (permalink) Old 03-01-2019, 10:22 AM Thread Starter
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Year: 1980,83,84,85
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The idea I was going to try only because I have the parts and a spare motor was the mod joedrum did on his bike. It was 1200 motor with 1100 heads with older 1000 cams. I spoke to the owner of c5 ignitions and he rode that bike saying it ran great and powerful. The only thing needing to be done so it worked was relieve the piston intake part a bit. From the pictures it was extremely minor amount of material taken away. I am still interested in the dual carb mod though but did want to know a bit more about the Stromberg's carbs as the price is way better than the webers.
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post #13 of 18 (permalink) Old 03-01-2019, 04:18 PM
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Joe and did it at the same time. The 1100 heads are identical to 1000 heads as far as valving and ports. He used what he had with early cams. The 1100 heads are not immediately setup for ignition off the left rear. Read Joe’s build for his he did it.

I have PowerArc ignition off the crank, same as C5 except C5 is off the cam. PowerArc makes C5. What I showed above shows they’re pretty much the same, a little bigger intake valve on the 1000/1100 heads, but I forgot to mention the stock 1200 cams have a little more valve lift on the exhaust side. Joe also had a custom curve for his from Paul. Mine is a cb750 curve. I need to buy the cord and tweak it.

The other difference in our bikes is carburation. Joe’s is a 2bbl Progressive Weber clone. Mine are currently v65 carbs, a little bigger than 1200 carbs but the same design.

It’s possible the single carb is better for the build. The cylinders fire 1-3-2-4 so with four carbs or dual Webers each venturi will get a pulse and then delay three turns. With a single carb and a VW plenum (Progressive or single throat)there is a constant movement of air through the Venturi(s). This may help in filling cylinders.

I have decided to try the dual Progressive carbs, but I really think a properly set up two barrel could be better.
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post #14 of 18 (permalink) Old 03-02-2019, 11:33 AM Thread Starter
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What dual progressive carbs are you thinking of going with?
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post #15 of 18 (permalink) Old 03-03-2019, 02:33 PM
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Hitachi 306 DGV from early Datsuns.
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post #16 of 18 (permalink) Old 03-06-2019, 02:58 AM Thread Starter
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Thanks I'm going to look into that as well.
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post #17 of 18 (permalink) Old 03-07-2019, 07:56 PM
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I stumbled across the Hitachi’s looking for small venturi sizes. The smallest are a 20mm primary and a 26mm secondary, considerably smaller than the Weber two barrel clones people are using. They’re set up quite similar to a Weber. I know of one who used one by itself and he reported it ran great. I’m hoping the 20 is stingey enough for decent economy when cruising but with the ability to give it a good shot when the secondaries open up.

I’ve modded the insides a bit but probably will have some backfiring on decelerating unless the choke flop will keep it rich enough on deceleration.

Sure wish it would warm up to try it out. I still have to weld on some mounts and build a throttle setup.
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post #18 of 18 (permalink) Old 03-14-2019, 09:51 PM
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Garage
1100 with single holley

Cycle innovations made a conversion, cast aluminum intake with a single holley YF like fit a ford 160 cu inline six. It had a special 3 stage venturi and cable spool (visible).

Ran better than OEM, picked up 1mpg, lots of torque, no lag off idle.


plenum would ice a little after startup on damp days, but was fine if you let it warm up.


1st pic is my old wing with conversion
2nd pic is one a friend sent me from daytona last week, appears to be weber twins
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