rough rpms at low speed

[Matthew A.]

Upon having to operate the motor at 2000 rpms or so to establish the 5 mph speed limits, the motor runs rough. By rough I mean the motor tends to vibrate more then normal due to a very light yet, "continuous" [if that makes any sense] hesitation or stumble between the motors normal vibration occillations. I recently gave the carbs a cleaning and reset the air/fuel mix. If one determines proper air/fuel mix by full throttle starts from 0 mph to full plane, the motor has no hesitation, stumble or cough whatso-ever. Air screws are set at 1 1/2 turns out. Fuel bulb stays firm, inline filter between pump and carbs remains full. Plugs are new and after a full day of use as well as after rinsing the motor at home for 10 minutes or so, plugs have very little if any deposits. The carb butterfly valves are all in sync with eachother. Electrical conn. good. Compression good.
The motor starts and idles beautifully. Runs at all other rpms beautifully as well. Only running at 1900 to 2100 rpms does the motor run rougher then it normally does. Almost as if it's just unhappy at that rpm.
I am fairly certain its a fuel circuit issue. Perhaps one carbs air/fuel mix not quite right. Is it advisable to make adjustments to the air/fuel mix while the motor is running and under load at 2000 rpm? Could there be another cause I may be overlooking perhaps? Worn reed pedal?

 

[bman1bpm]

Re: rough rpms at low speed

yeah sounds like you're idle mixtures aren't quite set right, mine was runnin pretty rough at slow acceleration and adjusting the mixture fixed it up for me.

 

[john from md]

Re: rough rpms at low speed

The engine at low speeds is running from a very small hole and varying vacuum. That type of metering circuit can't be tuned like a double pumper Holly. If you have deposits around the jet or surface corrosion on the throttle valve or intake body, it all adds up to interrupt the fuel flow. You should consider yourself lucky that it is only at one particular rpm.

Vertical two stokes without Direct oil injection should not be run clean anyway. The should be run somewhat rich for a couple of reasons.
1. All three cylinders burn differently with the uppermost being the leanest.
2. The idle mixture circuit contributes fuel through the entire throttle opening.

If you have a clean, smooth idle, you will have an excessively lean mixture at WOT. Additionally, if you adjust all three cylinders to run smooth at idle, the upper cylinder will be very lean. This is a good way to melt a piston.

I run my upper cylinder about an 1/8 turn richer than my lower. My idle at 6mph is a little rough but I don't worry about melting anything at WOT or extended high rpm cruise.

Hope this helps,

John

 

[Matthew A.]

Re: rough rpms at low speed

Thank you for the replies everyone.
I decided to remove the head just for good measure. The upper radius of the top cylinder bores on no. 1 and 2 cylinders had very light discoloration from high temperatures. [scorching]. The lower two exhaust ports on all 3 cylinders
had almost zero deposits where the upper exhaust ports had some deposits. Although, I am guessing it is normal for the lower exhaust ports to have less deposits due to lower temps and unburnt fuel/oil mix after shut off. As far as no. 1 and 2 cylinders go I'll give them an additional 1/8 to 1/4 turn on the air/fuel screw.
FYI. Only a few months ago I paid 23 dollars for a head gasket. Now...they go for 45 to 50. Ouch...but still cheaper than a rebuild.

 

[john from md]

Re: rough rpms at low speed

If you didn't damage the gasket on removal, they can be reused. I usually coat both sides with white lithium grease and retorque. Been doing this on bikes, boats and lawn mowers for a long time with good results.

Regards,

John

 

[Matthew A.]

Re: rough rpms at low speed

I had to purchase a new gasket as the old one had areas where the gasket was deteriorating where the gasket material and water ports next to the cylinder sleeves meet. A couple of places had nearly managed to go all the way thru the gasket itself. I am certain it was due to my own inexperience when I rebuilt the powerhead by not putting enough sealant on both gasket surfaces before installing the head. Eh...live and learn.