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Unstable idle after cleaning the carbs

Auris

Auris

Tuned and Synch'ed
Joined
Apr 18, 2016
Messages
40
Location
Netherlands
Hi guzzi people,

I got my carburators (dellorto phbh 30) cleaned by a local mechanic who cleaned the parts in a bath, replaced the float needles and a few gaskets and o-rings. Since then the idle has been unstable. It starts at around 1200 rpm but quickly goes up to 2000 - 2200. I've tried adjusting the air screws but it seems to make no difference when the idle is high. When the engine is warm it idles between 1200 and 2100 rpm, but is unstable. The throttle seems to be adjusted ok. The mechanic told me there are no vacuum leaks, but at the same time he couldn't find any other the cause of it, so I am not convinced. Moreover he replaced the air filter and I was wondering how much that may affect the idle. I tired disconnecting the spark for individual cylinders and it looks like sometimes one carb would idle high, and some others the other one, doesn't look like a one carb issue.. Is the vacuum leak the main suspect here or can there be other reasons?
 
Have you checked cable routing? Be sure the enriching jets are closing also. There needs to be some slack in the cables to both the slides and enriching plungers. Also check the condition of the rubber pieces on the bottom of the enriching plungers. These can lead to a high idle.
 
john zibell said:
Have you checked cable routing? Be sure the enriching jets are closing also. There needs to be some slack in the cables to both the slides and enriching plungers. Also check the condition of the rubber pieces on the bottom of the enriching plungers. These can lead to a high idle.
Click to expand...

Thanks John, I'll check the cables and report my findings.
 
I have just been adjusting the throttle cables and managed to get to the point where both have the same amount of slack. Didn't run the bike yet because it's late night here, but tomorrow I'll try and see if that made any difference. If not, I'll proceed to the enricher cable.
One question though, shouldn't throttle cable adjustment be done with both air screws being in the same position? If I adjust the cables while the carb cylinders are at different positions it means that difference carry through to the act of accelerating, right?
 
Auris said:
I have just been adjusting the throttle cables and managed to get to the point where both have the same amount of slack. Didn't run the bike yet because it's late night here, but tomorrow I'll try and see if that made any difference. If not, I'll proceed to the enricher cable.
One question though, shouldn't throttle cable adjustment be done with both air screws being in the same position? If I adjust the cables while the carb cylinders are at different positions it means that difference carry through to the act of accelerating, right?
Click to expand...


Not necessarily. There is some variation in the carbs so the fuel mix screws may be in slightly different positions. Once you have adequate slack, you should then use an manometer to balance the carb flow both at idle and open throttle. Be sure to open the throttle manually before you move the idle stop screw in. If you don't you will damage the slide. The screws will not be in the same position if someone had formed a groove in the slide by not opening first when turning the screw in. High speed sync is achieved by adjusting the cables. I usually increase cable slack to the carb with the least vacuum. If you don't have slack before you start, you will never get a good sync. You might want to do a web search for a Del Orto manual so you will know which screw does what.
 
So I checked the tension of the choke cables, looked fine to me, there is a few milimeters of slack as far as I can tell. I also took out the choke plunger of the left side carburetor (to access the right one I need to remove the tank), the rubber has a tiny oval grove from sitting on that little hole, but still looks ok, uncracked or in other way damaged. I replaced the o-ring of the choke plunger top cover, the old one was a bit worn. For the other carb I only adjusted the choke cable, but did not open the choke plunger cover as I wasn't able to access it without removing the tank. Before starting the bike I adjusted the throttle cables again with idle speed screews out this time, after that I put them back in their prior position (2.5 turns from the bottom). It started with rough idle, then I went for a 20 min ride, when the engine warmed up, I adjusted the idle speed by removing one spark plug at a time and adjusting the idle speed screws. After that it idled smoothly at 1200 rpm for quite a while with no roughness. I hope it stays that way...
 
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