Idle change on sidedraughts
Discussion
This one has me confused....
Running a Dolomite Sprint engine on twin 45's (Dellorto).
Have been running with trumpets and no air filters for a while.
Warm idle is approx 800rpm.
Decide to run air box with filter to be on the safe side.
Start by fitting carbon base plate for air box (full airbox to be fitted later). Re attach trumpets.
Warm idle is now 1100rpm.
Remove air box base plate and re attach trumpets.
Warm idle is now 700rpm.
I don't understand. Is there are any reason that the air box, even though not sealed, could be affecting the idle speed. I am very confused.....
Running a Dolomite Sprint engine on twin 45's (Dellorto).
Have been running with trumpets and no air filters for a while.
Warm idle is approx 800rpm.
Decide to run air box with filter to be on the safe side.
Start by fitting carbon base plate for air box (full airbox to be fitted later). Re attach trumpets.
Warm idle is now 1100rpm.
Remove air box base plate and re attach trumpets.
Warm idle is now 700rpm.
I don't understand. Is there are any reason that the air box, even though not sealed, could be affecting the idle speed. I am very confused.....
We have had occasions where the carbs are displaced by the flter housing as the carbs are not rigidly mounted, this can put pressure on the secondary throttle linkage to the carb linked to the carb with the throttle cable (I suppose it is the indirect/passive carb?) this can increase or lower the idle rpm. It can even be the case you can distort enough to bleed some air in past the misab plates. Try manually lifting/moving/disturbing the carbs and see if you can make the effect happen.
Peter
Peter
Pumaracing said:
Does the filter backplate have the holes required to line up with the holes in the DCOEs and feed air to the float chamber?
Yes, although that must be related.The base plate is one of these - http://www.reverie.ltd.uk/product_detail.php?prod=...
It is exactly the wrong height for the vent hole, so in order to still vent that hole, I have had to cut into the top as well as the black to create an aperture large enough.
So, there is probably less free air around that hole now.
Is that hole for air exiting or entering?
PeterBurgess said:
We have had occasions where the carbs are displaced by the flter housing as the carbs are not rigidly mounted, this can put pressure on the secondary throttle linkage to the carb linked to the carb with the throttle cable (I suppose it is the indirect/passive carb?) this can increase or lower the idle rpm. It can even be the case you can distort enough to bleed some air in past the misab plates. Try manually lifting/moving/disturbing the carbs and see if you can make the effect happen.
Peter
Thanks Pete. The carbs are rock solid though so know way there has been any movement there......Peter
My old Magnum had the same symptom on 45s when I fitted socks on the trumpets. I also noted that the air fuel ration went almost 1/2 a ratio higher on idle with the socks on. The problem was simply that the vacuum increased slightly with the socks on and the slightly lean idle corrected itself causing the idle speed to increase a little.
fatjon said:
My old Magnum had the same symptom on 45s when I fitted socks on the trumpets. I also noted that the air fuel ration went almost 1/2 a ratio higher on idle with the socks on. The problem was simply that the vacuum increased slightly with the socks on and the slightly lean idle corrected itself causing the idle speed to increase a little.
If your filter socks created enough restriction to affect the mixture at idle speeds, that should have been the point at which you removed them and placed them in the nearest bin.Quite right, and I did but I have seen the same on other engines. Notably my sons 4 stroke kart which idles badly and lean without the air cleaner and perfectly with it. It's just one of those little black box filters that come with all these generic Honda style engines used on karts, power washers etc. Something modern with a MAP and EGO sensor would be relatively insensitive to such changes but carbs certainly like to be adjusted with whatever filtration device is going to be used in place.
A strange thing yesterday, I am presuming related to the affected idle.
Engine warm, stopped for petrol. Engine then wouldn't fire. No issue with a spark but no matter what I tried, it just didn't fire. Got bump started with the help of 3 friendly Irish gents. Once I got back home, I adjusted the idle screw to take it back down to 800rpm. Restarted fine.
Can the presence of an open air box back plate really affect the air fuel mixture that much?
Engine warm, stopped for petrol. Engine then wouldn't fire. No issue with a spark but no matter what I tried, it just didn't fire. Got bump started with the help of 3 friendly Irish gents. Once I got back home, I adjusted the idle screw to take it back down to 800rpm. Restarted fine.
Can the presence of an open air box back plate really affect the air fuel mixture that much?
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff