Float Levels

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Spub
Mach 3
Posts: 94
Joined: Mon Feb 27, 2006 4:01 pm
Location: Pasadena, CA

Float Levels

Post by Spub »

I have a 77 900SS with PHM Dellortos. The floats were set at the stock 18mm. There is a long twisty mountain road behind my house (Angeles Crest Highway) and at certain points on this road, the one of the carbs seemed to cut out. After much observation, I determined it was the rear carb, and it appeared to be the fuel level, since the problem sent away once I slowed down and the bike stood up. I have experimented with increasing the fuel level in this carb by bending the float arms, but I was wondering if anyone else had experienced this problem. Funny thing is, I would have suspected the front carb to be more suceptible to this, since it did not sit completely vertical, whereas the rear sits vertical. I also note now that when I try to flood the rear carb, fuel does not spill from the overflow when it is flooded, as it did when it had a lower float level. This does not bother me, other than the fact I used the leaking fuel as a guide to let me know when the carb was sufficiently flooded prior to start up. Just wondered why this might be so...

I have also read in several tuning articles the suggestion to run the rear carb with the needle one notch down compared to the front, making it run slightly richer. Have not tried this, but wondered why the rear carb needed to run slightly richer......
1982 Laverda 180 JOTA, 1975 Moto Morini 3 1/2 Sport, 1993 Moto Guzzi Daytona, 2010 KTM Adventure 990, 1974 Laverda SFC #17188

Vince
750 Sport [BEVEL]
Posts: 207
Joined: Tue Nov 09, 2004 12:28 am

Post by Vince »

Richer helps keep it cooler,they get a bit hotter than the front,I have heard the same for middle cylinders of Laverda tripals,if your running them that hard every little bit helps,the float level has the most effect tuning wise on high speed cruising with small throtal openings,keep a look out for surging,thats a sigh of it running lean,if I remember right there is a couple of sizes of float bowl nut that gives more capasity to the bowl,might be worth looking at the bigger one.

Spub
Mach 3
Posts: 94
Joined: Mon Feb 27, 2006 4:01 pm
Location: Pasadena, CA

Post by Spub »

Vince:

I guess that makes sense, thanks. After a little more investigation, I guess I was a bit hasty adjusting the float level. Turns out, the little rubber sealing washer inside the left side petcock had turned with the lever, blocking the hole that allowed fuel to flow. This was hard to ascertain, since fuel does not flow unless the carb floats are depressed or otherwise are lowered to open the float jet. I took apart a few "sugarcube"petocks I have lying around (having 3 brands of Italian bikes you tend to collect things like this) I noted that while they may look the same and are marked the same, there are differences inside. One type has small steel tube like inserts that protrude from the three holes, and the rubber washer fits over this, the steel inserts keeping it in place when the lever applies a torque force against it. The second type seems to bond the washer to the back wall of the inside of the petcock, and this type seems the least prone to the problem I was experiencing. The last type has a rubber washer with three protrusions molded into the back side (i.e. it is not flat on one side) and these protrusions insert into the three holes on the inside. The faulting petcock had this type of washer, and they had sheared off over time. I think this type would be the most prone to the problem I was having. Thing is, this type of problem is hard to diagnose if you use the X over hose connecting tthe petcocks.
1982 Laverda 180 JOTA, 1975 Moto Morini 3 1/2 Sport, 1993 Moto Guzzi Daytona, 2010 KTM Adventure 990, 1974 Laverda SFC #17188

Vince
750 Sport [BEVEL]
Posts: 207
Joined: Tue Nov 09, 2004 12:28 am

Post by Vince »

Good get,there is a lot of blokes putting flat slides on Laverdas at the moment weth some fuel starvation issues at high speed ,they have timed the flow from the petcocks to check.

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