Half-throttle to full-throttle stumble

[tpbeebejr]

I finished putting everything back together after cleaning it all. It runs very well now below half-throttle - better than before. But now when I get to half-throttle and above, the bike stumbles until I reduce the throttle to below half.

From my experience on other bikes, the air-mixture screw affects the idle circuit, the pilot jet (I guess it is called the slow jet on the XL350R) controls from idle to half-throttle, the needle height controls from one-quarter to three-quarters throttle, and the main jet controls from half-throttle up to full-throttle.

If this is approximately true for the Honda XL350R, then there must be a problem with my needle height or my main jets. Does anyone have any suggestions?

Thanks,
Tom

 

[Rich Rohrich]

Isn't half throttle the transition point where it goes from a single carb to both carbs? I'd take a close look at the Honda sync/throttle cable adjustment procedure for the carbs before I would touch the jetting.

I never spent enough time with this setup to remember all the specifics.

 

[tpbeebejr]

Good idea

Thanks, Rich. I'll check this out and keep you posted.
Thanks,
Tom

 

[Ol'89r]

Rich is correct about the transition from one carb to the other. If I remember correctly, there is a tab on the bellcrank on one carb. You can bend that tab to make the secondary carb open sooner or later depending upon how you bend it. Adjust it so it opens sooner rather than later.

 

[tpbeebejr]

The secondary carburetor

The parts diagram is a little hard for me to understand. It shows two tubes that connect one carb to the other. I have them circled in red in the attached diagram. When I took things apart, there was only one tube (the lower, shorter one) with o-rings. Looking at the carbs, there is no obvious hole for the connection of a second tube (the longer, upper one) to the secondary carb. How does the secondary carb get its fuel?
Thanks,
Tom (tpbeebejr)

 

[Ol'89r]

Tom. I believe one is the fuel feed to the secondary carb and the other one is a breather tube. I have a '83 XR350, I will take a look at the carbs.

 

[High Lord Gomer]

I have a '83 XR350...

BS, you've got 3 of every motorcycle ever made! Why not just send him a spare?

 

[tpbeebejr]

Tom. I believe one is the fuel feed to the secondary carb and the other one is a breather tube. I have a '83 XR350, I will take a look at the carbs.

I think the lower one in the diagram must be the fuel feel to the secondary carb. But I can't even see a place where a second tube would connect to either carb. Is it possible that this is a generic diagram for the several models of bike that my Clymer manual covers, and that this top tube is simply not present on my XL350R?
Thanks,
Tom

 

[Ol'89r]

BS, you've got 3 of every motorcycle ever made! Why not just send him a spare?

I would but I only have 2 XR350's. :nener:

Tom the carbs on my '83's have a hard plastic tube running between the two carbs to feed fuel to the secondary carb. No breather or flexible tubes. Your '84 carbs may be a little different,,,, kinda like Gomer. ;)

 

[dirt bike dave]

My guess is the connector tubes Tom has circled are an either/or scenario. Your bike either has the longer top one, or the shorter bottom one. Probably varies by year, or whether it is an XL250R or XL350R.

I found some images on the web that suggest the XL350R has the carbs mounted very close together, with the short tube, which is how Tom described his bike.

IMO, the secondary carb is either getting no fuel, or it's not opening. In addition to examining the carb and fuel flow, make sure both throttle cables are working properly. If the cables are intact and the secondary carb has fuel in the float bowl, it must be something to do with the synchronization of the carbs.

The XL especially may have been jetted too lean, but the way he describes it the second carb is not working at all.

 

[tpbeebejr]

Sorry - double posting

Hey guys, sorry for the double posting. I have been sailing for the last few weeks and before I left I tried to get a bunch of stuff taken care of. When I go into this mode I do a whole bunch of stuff in rapid-fire robot mode, and sometimes I forget what I did. I did previously sent out a post about this same issue (http://www.dirtrider.net/forums3/showpost.php?p=1432456&postcount=1) and forgot that I did it. Sorry.

But thanks for all the good suggestions. I am pretty sure that both carb bowls have fuel, and that both throttle cables are working. So how can I learn more about synchronizing the carbs? On my Yamaha V-Star 1100 there is an easy way to do it by using a U-shaped manometer that I hook up to the vacuum port for each cylinder and then adjust a bolt that controls the opening (at idle) of each butterfly valve until the vacuum balances on both cylinders. Since this is one cylinder and 2 carbs that operate at different throttle openings, things are completely different.

Thanks,
Tom