250 scrambler with high idle problem

[amartina75]

Hi,
I am new on here, this is my first post. i need some help with a '66 250 scrambler i just bought. I also have a 450 scrambler that has been a long running project for me.

I bought the 250 scrambler as a running bike, it needs some work but it is mostly all original and in pretty good shape for its age.

when i bought the bike it started and idled fine, it idled a little low if anything, but it was backfiring pretty bad.

what i've done so far to the bike is;

put the factory straight pipe back on, it had a cheap reverse cone on it.

took off the 70's style high bars and put some low bars on it.

flushed the sludge out of the forks, and replace the fork oil.

replaced the manifold gasket.

changed the oil, 2 quarts of 20w-50 lucas motorcycle oil and 1/2 quart of racing 40 wt.

took off the velocity tube, put the factory dellorto air cleaner back on.

took apart and cleaned the petcocks, replaced the fuel lines

cleaned the tank, replaced the fuel with premium gas and added some lead substitute

and took apart the Dell'Orto SSI 27d carb cleaned and reassembled it.

the problem i am having is that i cannot get it to idle at the proper level. when i start it up it races up to around 3k rpm's or so (i am guessing because of course there is no tach)

i have tried just about everything with the carb and i cannot get it to idle any lower.

the carb is set up with; m14 needle, 115 main, 50 idle jet, 80 slide, 265 needle jet.

thanks, I am looking foreword to some expert advice,

Aaron.

[JimF]

When setting up Aaron's account I suggested a check of the slide's position when the throttle is 'closed.'

The slide position in quiescence was one of my many mistakes. On my first 250 restoration I had the slide set up too high when the throttle was not twisted. I could barely get the thing to start (understandably so in retrospect since the carb was operating nowhere near the idle circuit) and once I did get the engine to start the engine ran no lower than a few thousand RPMs.

Jim

[DewCatTea-Bob]

By: amartina75 ...
" it idled a little low if anything, but it was backfiring pretty bad. "

____ I assume the exhaust-popping occurred as the revs fell after the throttle was shut-down... If so, then your fuel/air-mix screw was set too lean.



" changed the oil, 2 quarts of 20w-50 lucas motorcycle oil and 1/2 quart of racing 40 wt. "

____ That's fairly good for getting the motor washed-out but, next oil-change preferably use 2-qts of 40w and top-off (to full-capacity) with the 20w50.
__ Always make-sure that ya drain-out all the used oil by tilting your Duke's heated-up motor side to side (more & more slowly as it cools-down, while the old oil gets drained-out), so as to keep your new oil from getting contaminated and-also make room for a FULL-sump/load (of nothing other than FRESH-oil) !



" took apart the Dell'Orto SSI 27d carb cleaned and reassembled it. "

____ Were you sure to then face the slide's cut-away towards the rear ?



" the problem i am having is that i cannot get it to idle at the proper level. when i start it up it races up to around 3k rpm's or so "

____ Have you checked to be sure that your throttle-cable has sufficient slack ?


Duke-Cheers,
DCT-Bob

[amartina75]

JIm, thanks for the sugestion, I did check the slide position i thought it was low enough, perhaps it was not. i have since taken the carb back off and back apart to look at it again.
I don't think the throttle stop screw was working right because it was slightly bent. also it was not wanting to stay in its seat in the slide. so i switched the slide out with another one that i had, and i straightened the stop screw. it seems to be working right now.

Bob, thanks for your input. I did check that the throttle had slack, if something was holding the slide too high it was not the throttle. i did make sure the slide cut away was facing to the rear, i have made that mistake before. I will follow your advice on the oil for my next change. The exhaust popping was as you say, when the throttle was let off and rpm's falling. i will see what happens with that when i get it started back up.

i read in an older post that to set the fuel level in the carb you rotate the float bowl up or down. and that you want it just below the point where fuel starts leaking out of the overflow holes.
is my understanding of that correct? also how important is the position of the carb and float bowl? how close to verticle do they need to be?
Thanks, Aaron

[JimF]

An SSI with a float bowl which is attached to the base insures your float level is set correctly. Only if you are using a remote float do you have to be concerned with the float level, and you should be very concerned. But your syptoms do not suggest to me that the problem has anything to do with float level as an improper float level would mean the engine would either be starved for fuel or fuel would be essentially dribbling out of the carb (not the float) anytime you parked with the petcocks open.

Do you have a remote float bowl?

My slide presents a very small slot when the throttle is closed. Keep in mind the carb is sucking in atomized fuel, so a little more than nothing is close to correct.

Jim

[amartina75]

Jim, no I do not have a remote fuel bowl. I have the type that is attached to the carb body. what i was wondering is since the float bowl rotates on a different angle then the carb, doesn't rotating the bowl either raise or lower the fuel level in the carb. if it does not then there is no adjustment for fuel level since the float is fixed.
I am not sure exactly what positoin to put the carb and the float bowl.
I did manage to get the Duc to idle just fine after getting the throttle stop screw working right and the slide adjusted right. well i shouldn't say just fine because it has presented other problems now.
it no longer revs high, but it will not stay running long now and if i give it a quick turn of the throttle it bogs, if i turn the throttle slowly it revs ok but sounds a little choppy.
I have an #80 slide in it, and that is what it was running with before i bought it, i also have an extra SSI27 carb from another scrambler parts/project bike i have, it also had a #80 slide in it.
I do not have another one that i can try. I am not sure about the pilot air screw, how far out it should be. also i am not sure about the fuel level being correct. the Duc seems to flood very easily and that makes working on it difficult. I believe it is getting too much fuel for some reason.
Thanks, Aaron

[JimF]

If you loosen the idle adjustment screw you will lean out the mixture, so if you think you are running too rich at idle turn the screw out. I think the rule of thumb is to bottom out the screw and then back it out one and a half turns and start from there.

Maybe your carb is different than the SSI29s that I am familair with, their floats don't change level when they are rotated around the base of the carb.

The quick turn of the throttle may be too aggressive right now. You have to think of the carb as having several circuits. One crude circuit gets the bike started by either flooding a lot of fuel into the carb mouth or by choking off the air to create the same effect, that is an overly rich mixture.

That's the first circuit you work on.

When you can get the bike to start I would suggest you work on the idle circuit next. That's a combination of the idle air screw and the pilot jet if I am not mistaken, as well as where the slide rests when there is no throttle twist.

Once you get the bike to start and idle then you can slowly twist the throttle and diagnose the next circuit which would then be the slide cut. If you work each on each circuit one at a time you're less likely to get confused by different problems in different circuits.

Jim

[amartina75]

Jim,
thanks for your help, I had some time to spend on the Duc today and managed to get her running pretty smooth. no problem with the idle anymore. I guess i found the sweet spot for the air screw and the slide position, finally. i took a ride around the block and it was pretty good, it ran strong and didn't leave me walking it home, so thats good. also the backfiring that it was doing before is gone as well.
Thanks again,
Aaron

[JimF]

Hi Aaron,

I get a lot of help from the people here and once in a great while I manage to pay it forward a little.

Have fun on the Scrambler! If you find anything else you need help with there are plenty here that can answer your questions, but I hope your Duc never has another problem ever.

Jim