Warm weather carb performance
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Cruiser
Joined: Oct 2011
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From: Bellmawr NJ
Warm weather carb performance
So my 78 XLH has a brand new Mikuni VM-38 on it (4 months old, 600 miles). Up until now the nicest weather I have ridden was 55/60 F and down to 25 a few times. It has been starting and running great, a little lean in the cold but that is expected. Today was 60 F when I left for work, Sporty started and ran great but once it was warmed up took awhile to settle into idle, hanging around 1500 rpm for a few moments before going down to idle (1000 rpm).
Ride home was 85 F and the bike started (but would not with enrichment on, just started and ran at 1000 rpm with enrichment off) and idled better than ever before taking off. After it was warm it would not settle to idle, hanging at 1500 rpm and going down to about 1200 rpm. When I stopped to get gas, it did not start very well and took a little turning and a little throttle jab (which the vm38 should not need). Behavior on the road was fine, no differences except when I came to a stop it would idle at 1500 rpm. When I got home, it would not go to the set idle unless I backed the idle down until it shuddered, then turned it back to 1000 rpm. After that if I revved it the engine would fall back down to 1500 rpm.
Is it too rich for the warm weather? Is there something else completely different I should be looking at?
Ride home was 85 F and the bike started (but would not with enrichment on, just started and ran at 1000 rpm with enrichment off) and idled better than ever before taking off. After it was warm it would not settle to idle, hanging at 1500 rpm and going down to about 1200 rpm. When I stopped to get gas, it did not start very well and took a little turning and a little throttle jab (which the vm38 should not need). Behavior on the road was fine, no differences except when I came to a stop it would idle at 1500 rpm. When I got home, it would not go to the set idle unless I backed the idle down until it shuddered, then turned it back to 1000 rpm. After that if I revved it the engine would fall back down to 1500 rpm.
Is it too rich for the warm weather? Is there something else completely different I should be looking at?
Slow or inconsistent return to normal idle is a classic symptom of intake leaks. Probably need to uninstall the carb and the manifold then re-install. Ensure that the manifold lines up well with the cylinder heads; the o-rings are well seated; use T-bolt/aircraft clamps; must use a carb/air cleaner suport bracket.
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Cruiser
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From: Bellmawr NJ
I also thought intake leak, but I thought the bike would do well on the enrichment circuit if that was the case (enrichment kills it).
I do not have a carb support though. When my indie attached the throttle cable to the carb I asked him to fashion a support, and he said that with the air cleaner having a rubber / plastic back it would not be wise to use it to support the carb, and that this carb didn't need a support. I was kind of surprised but it was running fine so I never re-visited it.
I do not have a carb support though. When my indie attached the throttle cable to the carb I asked him to fashion a support, and he said that with the air cleaner having a rubber / plastic back it would not be wise to use it to support the carb, and that this carb didn't need a support. I was kind of surprised but it was running fine so I never re-visited it.
The way these engines vibrate, together with the nature of the o-ring seals at the manifold, means that some kind of support is necessary. I would find a way. What about using the bottom stud of the carb, and run a bracket to ... somewhere?
You can test for intake leaks using propane gas. Just aim it at each of the joints between the heads/manifold/carb with the engine running. Any change in RPM is a leak.
While you are at it, check for exhaust system leaks - pour about 1/2 oz any oil into each cylinder thru the spark plug holes then run the engine. You get lots of smoke for maybe 5 minutes. All smoke should come from the ends of the mufflers. Any other is a leak which must be fixed.
You can test for intake leaks using propane gas. Just aim it at each of the joints between the heads/manifold/carb with the engine running. Any change in RPM is a leak.
While you are at it, check for exhaust system leaks - pour about 1/2 oz any oil into each cylinder thru the spark plug holes then run the engine. You get lots of smoke for maybe 5 minutes. All smoke should come from the ends of the mufflers. Any other is a leak which must be fixed.
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Cruiser
Joined: Oct 2011
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From: Bellmawr NJ
This morning was 65 F, bike started normally on enrichment the idled high from my changing the settings. I set it back to 1000 rpm and the bike idled and held it. Got to work with no issues, still holding 1000 rpm before switching off.
Ride home was 75 F, started and idled as normal, good ride home, settled to 1000rpm every stop. When I got home I gave it a leak test with WD-40 (didn't have propane, don't mind the mess), no change in RPM. Weird. I'm still going to find a solution for supporting the carb though.
Ride home was 75 F, started and idled as normal, good ride home, settled to 1000rpm every stop. When I got home I gave it a leak test with WD-40 (didn't have propane, don't mind the mess), no change in RPM. Weird. I'm still going to find a solution for supporting the carb though.
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