450 MK3 lags at partial throttle

[DucRedux]

Throttle response on my 450 MK3 has been off for a while. Riding around, here in the Catskills-elevation 2300 ft., the response is good at half to full throttle, but at low speeds and partial throttle the bike retards and surges. You can feel it more in the saddle than in the engine sound. The carburetor has had its share of problems from bad gas to the clogging, gummy leachate caused by ethanol (it has a now ruined fiberglass resin fuel tank.) The VHB 29 carburetor has been fully rebuilt, is clean and has the correct jets and needle. Last week I removed the jets and needle, cleaned them again and moved the needle position up. Idle improved immediately, but the partial throttle running did not seem to improve much. Might I have an air leak in the gasket or manifold, even though it looks tight and sound? Could it be related to fuel level in the bowl or a clog in the internal passageways? Consider that I could not find a correct float level measurement for the VHB. Might it even be ignition related since this model has electronic ignition? Diagnosis and troubleshooting thoughts are appreciated.

[double diamond]

Changing the needle position shouldn't affect idle at all if the carb is working properly. Idle should be affected only by the idle mixture screw. When you "rebuilt" the carb, did you replace the needle jet and needle? These part will wear imperceptibly but have a noticeable effect on part throttle performance.

[DucRedux]

Needle and all jets are new, less than 2000 miles of wear. When the needle was re-positioned all I did was softly polish the brass of light lacquer and corrosion. The Dellorto tuning guide talks about a "progression or transition" flow in the carb when opening and closing the throttle, with apertures forward of the needle allowing differing fuel-air mixtures. The idle jet passageway in the body is just forward of the needle and I'm wondering whether the body aperture is completely clear, or whether the fuel level in the bowl is too low and airspace above it affecting aerosolization? (When the bike was new in 1974, I always had to fiddle with the adjustments, but then I didn't know what I was doing -things haven't changed.)

[graeme]

Hello, you said that the fibreglass tank was ruined by ethanol, are you still using this tank?
I've had this same problem with a Pantah and the resins were dissolving and blocking the carb jets, clean the carb and it was good for a few kms then idle and very low throttle opening went bad again. Lined the tank and dump the fuel and no more problems.
Graeme

[Jordan]

The Dellorto tuning guide talks about a "progression or transition" flow in the carb when opening and closing the throttle, with apertures forward of the needle allowing differing fuel-air mixtures.

That description could refer to the slide cutaway?

Does it run better or worse if the choke/enrichener is used?
Could be a pointer to making some changes.

[DucRedux]

Hello, you said that the fibreglass tank was ruined by ethanol, are you still using this tank?
I've had this same problem with a Pantah and the resins were dissolving and blocking the carb jets, clean the carb and it was good for a few kms then idle and very low throttle opening went bad again. Lined the tank and dump the fuel and no more problems.
Graeme

The tank is now about five years old and the paint job has been rippled by the internal leaching of 10% ethanol . The tank was made in Italy and when it was first used, it glued the carburetor needle and slide solid over a weekend!
I've been looking for a metal tank (Anyone going to Imola's mostra-scambio? ) but have resigned myself to lining a new fiberglass replacement when the time comes for a refresher.
Perhaps your analysis is correct. Though I drain the tank and disassemble the carburetor each year to clean it, maybe the residue persists or newly builds.

[DucRedux]

Jordan: As I mentioned earlier, the carburetor acted up even years ago and using the enricher was a quick way of resolving running issues until I could get to the garage.
Thanks for mentioning it and, ethanol clogging issues aside, I'll try a run this weekend with the present settings but then move the choke lever about a touch.

[DucRedux]

Over the past two weekends, I had a chance to test ride and fiddle with the carburetor settings. I put on about 150 miles and as the mileage climbed the partial throttle response improved slightly. Tweaking the choke lever at first ,even minimally, only flooded the engine. As I was riding, I surmised that gas consumption was scouring the carburetor body passageways and improving performance. Between rides and not wanting to fully strip the assembly, I removed the air screw and inserted a small piece of high denier cotton (fine cotton-no lint) into the opening and soaked it with carburetor cleaner which is toluene based. I kept the cotton wet for several hours and then replaced the air screw. On the next ride, the partial throttle response was greatly improved.
I am guessing that the body passageways between idle jet port and air mixture ports and even body apertures still had residual clogging, if not from gas lacquer, certainly from tank and fuel line residues deposited by ethanol gas. These tough deposits might need more thorough, or more frequent cleanings.