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Carburetor Woes

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First off... I know there are probably a hundred threads on this and i have tried to read but im stumped. Hoping some of the experts can help.

The stats: I have a 96 xr400 with white bros r4 slip on and twin air filter. I ride 500-1000ft elevation Bike has a horrible dead spot at Quarter to mid throttle. I have checked the diaphram and cleaned the carb multiple times. I started reading on here about jets and decided i should be at about 62 pilot to 162 main. I opened up the carb and it had a 62 and 148. Order a 162 and swapped it. Bike ran way rich. Like smoking and barely rideable rich. Swapped back to 148 and it ran better again. I tried flipping choke at dead spot and it seemed to help. That makes me conclude 148 is too lean. I fiddled with the air fuel screw and turned it out as far as 5 turns and it still wasnt idling way up. That makes me feel like pilot is way lean. I checked needle clip and it is on 3rd slot. I almost feel like i need maybe a 65 pilot and 155 or so main but that seems way off of what everyone recommends. Only other thing i noticed is bike still has old needle instead of the newer dual taper.

Sorry for the long rant but trying to give all details. Thanks for any help anyone has.

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It has a dead spot under what kind of throttle activation at those positions? A smooth roll on or a quick whack open from those positions?

I guess the diaphram you speak of is the air diverter valve? Or do you mean to be describing the float?

You should definitely be sure the float level is set to spec of 14.5mm.

Is the airbox snorkle under the seat still installed?

How clean/dirty is the air filter? Be sure its not over saturated with oil.

Did your carb cleaning include removing the fuel screw and cleaning that passage and checking the condition of its oring? The fuel screw has no effect past 3 turns out. And the engine idle doesn't increase as significantly as it sounds like you're expecting, especially when coming out with it(richer).

When setting it, you start with it at 2 turns out, warm up the engine, set idle speed at a reasonable speed that you can easily hear changes in. Turn the screw leaner(clockwise), listening for any subtle change in speed. It should reach a point where it doesn't increase anymore. Stop, turn it back out. As you keep coming out with it, it should decrease again(too rich). Stop and turn it back in to where the idle just picks back up again. That's near best/highest idle on the rich side. Leave it there. Keep track of how many turns out that point is. If 1 or less, pilot jet can go smaller. If near 3 turns out, go bigger.

Be sure you're using the choke lever correctly. All the way up is full choke. All the way down is choke off. Be sure the lever isn't moving on its own because of a loose lever bracket screw.

I agree, a 148 main is too lean. You already know 162 is rich( this is assuming the float isn't set too high) , so drop in a 158.

The needle in the 3rd position is stock, so leave that for now. You may try it in 4th from top once you get the main and pilot circuits tuned right. Try to make one change at a time and evaluate. Doing more than one thing at a time when tuning a carb can get confusing because you don't know what change it was that made any difference you see. I prefer to get the pilot/fuel screw circuit tuned first, then the main, then needle for last.

Edited by Trailryder42

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The dead spot is under load. So whacking the throttle or if under heavy load like on a hill.

The diaphram is the air diverter valve.

I did make sure that the float level is set correctly. Min and max. 14.5mm to 19.5mm.

The snorkel is still still there but seems very open. Maybe gutted?

The filter was not very dirty but did seem a little heavy on the oil. I may give it a good soaking in some 2stroke gas.

I did pull the fuel screw and clean the passage. The o ring was still intact and not dry rotted but was a little flat.

I am using the choke properly and it is tight.

I will follow your directions on the air/fuel screw to make a decision on the pilot then i will order a few more main jets in between what i have.

Is it worth getting the the dual taper needle? Any other tips after reading my answers? Thanks for all of your help!

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I've never had the older needle but those that have say the 49 state A16a needle is an improvement.

It would be easy to tell if the snorkle itself had been gutted. The rubber would be cut up, otherwise its usually pretty restrictive compared to the hole it leaves in the top of the airbox when its pulled out.

Don't overlook things that would seem to be unrelated. Like the exhaust. Does it have a packing that could be blown, causing problems under certain flow conditions?

If when you get to that throttle position where it bogs and you hold the throttle there, does it continue to bog or will it clear up?

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If im not mistaken the snorkle was significantly less restrictive on the 96-97 than the 98+ from the factory. The snorkle on it literally sticks up an inch or so and is wide open so i can see straight into the air box.

Checking the packing is probably a good idea as i dont know how old the exhaust is.

If i stay on it in the bog it will die. I have to let off and ease back into throttle. This causes problem on hills. Lol

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I remember now, I think you're right about the snorkle.

I think I'd be checking for electrical gremlins too. Make sure the electrical grounds are tight and clean.

Check to make sure the spark plug boot is firmly attached to the end of the coil wire.

Make sure the plug is not loose in the head.

Check the kill button.

Is the manual decomp system still in place and working?

Any new, odd engine noises?

Have you verified the oring between the intake manifold and head is there and in good shape?

Header bolts tight at the head, no exhaust leaks?

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The 96-97 XR400 were jetted assuming the owner would remove the intake snorkel and quiet core. Then go race.

The 98-04 XR400 were jetted assuming the owner would leave the bike as delivered. Making the EPA and CARB happy.

In 98, the jetting was changed, the needle was changed, the slide was changed, the exhaust header was closed up. Possibly the CDI too.

This is the intake snorkel, it has never changed.

http://www.4strokes.com/tech/honda/snorkel.asp

The main jet won't matter, until you are nearly full throttle. My XR400 is plated, so I test on the freeway.

Want to know what too big a jet feels like? Put on half choke. Too small, put in a bigger main. Can you go faster without feeling like the choke is on, you are on the right track. Honda put in a 162 for racing.

Read This for setting the pilot and fuel screw.

http://www.thumpertalk.com/topic/98806-xr400-engine-bog-pilot-jet-and-the-fuel-screw/

Baja Designs will cut you slide and sell you a needle. Or you might find used parts or carb.

Ethanol is nasty stuff. It will clog up carbs and turn them into crap in a month. I run SeaFoam and use Stabil.

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I had some time this weekend to dig into it a little. I realized I had an exhaust leak and from the rtv around the intake o ring probably an intake leak as well. When i went to clean the air filter i realized it was a little rough as well. I went ahead and ground down the header welds and ordered exhaust gaskets. Header o ring. Carb gasket kit. Some more main jets. And an air filter. Ill update once I get the lack of maintenance from PO all taken care of.

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Ok. I had some time over the weekend to work on the bike. I put in a new air filter. Carb gasket kit. Intake o-ring. Exhaust header gaskets. Repacked muffler. Changed oil. The only thing maintenance wise i didnt knock out was a new spark plug as i couldnt get a socket fit even after grinding on one for what felt like 30 minutes.

I tried a variety of jets from 148-162. Moved needle clip down one. It still has a dead spot so bad that if i open throttle fast it will. Die it is backfiring on decel and i also noticed that if i let it set at idle it will randomly "miss" and die. Im gonna grind on the socket some more and see if i can the plug changed. Is there any easy way to check the coil and cdi other than swapping out? Anything else i am missing?

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Sounds like an air leak somewhere.  If you just ease the throttle on gradually, can you eventually get to full throttle?  If so, does it run relatively well at that point?  You shouldn't try this test up a hill, a flat section of road is fine.

 

If it runs well at WOT, your problem is in the mid-range - an issue this big tells me it's a hole (or holes) in the diaphragm.  Even pinholes can cause this problem, and they are very difficult to see.  If you still get the problem at WOT, you likely have an air leak on the intake side.

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tr42

Does the diaphragm work by admitting extra air on deceleration?

 

OP Sometimes I have found vacuum leaks by spraying wd40 or carb cleaner around on the intake manifold and various carb parts to see if it helps.

 

I have made up gauges on the hand throttle to tell which range I am in as an aid for tuning. Here is one on a road bike with a slide throttle carb.

throttlemarking_sm.jpg

 

1/4-3/4 is mostly needle. That is what I am reading from your post. If it gets worse from 1/2 on up then your main.

 

Also what did you pack on your muffler? I thought honda units were strictly mechanical baffles.

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Had a similar problem and after trying every jet option found out it was a bad coil. Lesson is you cant jet away an electrical problem!

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I examined the diaphram thorouly and could not find any pin holes. I will try the carb cleaner idea. Id say is is about 1/4 throttle where issue is. I can get through it if i ease the throttle. Coil is kindo of my next suspect. Any way to check the coil?

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Bet your coil and ignition is fine. What happens when you go up 5 on the primary (low speed) jet. Also consider raising your needle. A 1/4th throttle is the range of both of these circuits, that is the range of your misfire/bog.

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Since you don't have a stock exhaust, stock jetting may not be correct, don't be afraid to try something. A primary jet is what 5 dollars and a new honda coil is what? 50 dollars?

 

As far as changing the main jet It is hardly worth doing unless your low speed jetting is close first.

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I am actually running a 162 pilot. It seems to idle well with that. I have tried dropping the needle one clip in effect raising the needle.

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