G'day,

I own a 99 Road Star and have done around 70,000 kilometres on her so far from new. I love it... but

I need help!!!

I have been reading quite a lot of very good info on your site about jetting and mixture screw adjustment here and am impressed by the expertise shown by the posters on this site.

Here is the situation so far:

I have problems with backfiring on my machine this occurs mainly on the rear cylinder with the shorter exhaust and only on closed throttle coasting. The bike is fitted with straight through pipes with little or no back pressure and has a K&N filter in the stock airbox.

I have gone through the following to try and solve the problem to no avail.

Rejetted the carby installing a larger pilot jet The pilot Jet was a #35 but I have drilled it out to 0.5 mm (probably equivelent to a #50), main jet increased to #170, Riased the needle but returned it to position 3 as it made no difference. I have also replaced and sealed up the inlet manifold, removed and applied sealant to the exhaust pipes at the head and rechecked and resealed the slip on mufflers.

I now have the pilot PMS screwed out 6 turns and it still backfires!!!!

In the technical discussions about the carby on the 1600's there was little or no discussion on the operation and adjustment of the coasting enricher circuit and pilot air jet which is a #100

It still appears as if the problem is mixture related as it will not backfire when the choke (enriching circuit) is used on decel

Can anyone give me a clue as to whether I should increase the pilot air and how far and what size I should increase the pilot jet to, or am I totally off the track here?

Thanks in advance

Trev

Post edited by: Bustedbiker, at: 2005/06/12 09:46

Post edited by: Bustedbiker, at: 2005/06/12 10:01<br><br>Post edited by: Bustedbiker, at: 2005/06/12 10:07

From what you tell me I would guess that you're too rich.

You still have the stock airbox. My limited experience tells me that even though you have &quot;straight through pipes with little or no back pressure&quot; (which by the way rob you of torque) the stock airbox will no allow enough flow to use the richer mix.

&quot;Riased the needle but returned it to position 3 as it made no difference&quot; If the carb is setup correctly then moving the needle up and down will make a difference.

The 170 is too BIG (I have a BAK and Nasties and I use a 170). I would go back to the stock main or a 167.5 and a #35.

BTW .... it is impossible to screw the PMS out 6 turns. How are you counting the turns? I mark my screw driver and count a full revolution as one.

Doc

Doc beat me to the punch on this one, and his points are good ones, so consider this an alternative opinion. It would help a lot if you could spec your bike for us

The coasting enricher is not adjustable Trev.

If you are getting backfire on Decel, you are too lean at idle, so changing the air jet to the pilot would be fairly ineffective.

The pilot screw is really only effective to about 4 turns out. Any more than that is wide open as far as the pilot is concerned.

It sounds like you are either undersized on the main, or clip is too high on the needle.

I know you already tried to move the clip on the needle, but you weren't real clear about which direction you moved the clip on the needle. Clip three is pretty tough on most needles, and really only effective on Baron's needle when your main is too big and you can't get a smaller one (see Doc's note above). Even when a main is too large, the clip setting on the needle can make the midrange too lean (thats what third clip works well for, compensating for an oversized main).

You most likely need to move the clip down (toward the tip). If you are using a Baron's needle, I would take it to 5th clip position and see how it does. If it had the problem at 4th clip, and 5th solves the problem, your main is just shy of too small. If it works at 5th clip keep the main.

Even though your bike is not exhibiting problems at any throttle setting other than idle, Road Star's can tolerate a lot of leanness. They almost always lean out in the range between idle and your torque peak (anywhere from 2000 to about 3000 rpm). Backing down from this lean RPM to closed throttle will cause this problem if its just TOO lean in this range. The pilot circuit just can't compensate for this type of condition. The only way to really solve it is to increase the main size or move the clip on the needle closer to the tip.

GRAM<br><br>Post edited by: GRAM, at: 2005/06/12 11:08

Doc, Gram,

Thanks blokes. I'll try and explain the situation a little more.

The bike is stock except for the slip-ons and air filter.

I did raise the needle to the fourth clip position, it is the stock needle (Australian polution laws are such that the stock needle is adjustable and the PMS is screw is not capped) Raising the needle did increase the torque to some extent in the mid range but I set it back before I went overboard with the adjustment of the PMS as I needed to see what effect it had. I have a screwdriver with a pocket clip so I count the turns out from the fully in position noting the position of the clip.

As the Main really only comes in at wide open throttle and my problem occurs on coasting I felt it better to leave it a little rich. The original jet was #165

I have tried a larger pilot jet, drilled out to about 0.7mm this was way too rich at idle, the bike would hunt at small throttle openings and used heaps of fuel, around 150 miles to the tank and never needed the choke to start.

I guess I'll have to screw the PMS back in to 4 turns seeing that it is wide open at that setting (the damn thing will fall out if I don't) and try raising the needle 1 notch at a time.

Pity about the non adjustability of the coasting enricher circuit. I was thinking that if I was to use the 0.7mm pilot jet and maybe a #150 air jet then when the coasting enricher shut off the pilot air jet, I'd have an even richer mixture.

I'll keep you posted on progress.

Thanks again
Trev

Gentlemen,
I am no expert but since this is a 1999 carb maybe the enricher diaphram or cap are cracked and not pulling piston out. Also, could it be the piston is dirty and not moving out as it should?

I recently had my carb off to fix a vacume leak and noticed my enricher was sticking.

Just a thought.
Gary

Thats good thinking Gary.

I have torn more carbs apart than I could possibly count, and on most of the older ones the plastic coasting enricher cap/cover was starting to warp.

Never found one malfunctioning from it, but with the amount of warpage I was seeing, it sure looked like it would happen sooner or later.

GRAM

Gary, Gram,

Thanks for the tip.

I had already cleaned the enricher piston prior to my first post as it was dirty and I thought it might have been sticking too.

Come to think of it the cap was a little warped. (the spring is fairly strong) Maybe that is the problem after all. I will order a new one tomorrow, as lifting the needle has not helped at all.

I also checked the valve clearances and timing. All ok. Set the needle to notch 4 in the end.

Thanks again

Trev<br><br>Post edited by: Bustedbiker, at: 2005/06/14 05:06