Increase Pilot Jet Size?

Increase Pilot Jet Size?

Postby FZBob » Tue Mar 24, 2015 3:34 pm

I am rebuilding the carbs on my '79 CBX after 20 years of sitting in my garage. While I have it apart, I would like to take the best shot at getting the jetting right. Stock airbox, stock paper filter, stock or Pipemaster exhaust.

Back in the early '90s when I was riding it a lot, it always felt a bit cold blooded. The carbs were clean, I had shimmed the needles .020, and mixture screws were 2 turns out. Back in the day, "conventional wisdom" was that bikes of this vintage were jetted lean in the first 1/4 throttle for emissions, and one of the cures was to go up one or two sizes on the pilot jet. I have done this on more than a few bikes with the expected improvement. I would have tried this long ago on my CBX except for the pressed in pilot jets. Of course we now know that we can un-press the pilot jets, and replace them.

I have read hundreds of carb posts over the last month, and the consensus seems to be 2 1/2 turns out, STOCK 35 pilot, and raise the needle .020 or so. My question is this: Is there a reason to NOT go up on the pilot size? Is it a matter of convenience? Has it been tried and found not to work? Is the CBX in fact NOT lean in the first 1/4 throttle? I always wanted to try this back in the day, and now I have the opportunity, so if it is a bad idea, please talk me out of it!

Thanks, Bob
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Re: Increase Pilot Jet Size?

Postby daves79x » Tue Mar 24, 2015 4:47 pm

Just remove your #35 pressed-in jets, clean them well and tap them back in. Don't go bigger there - see my other post about the mains. If you get the carbs totally clean and your accelerator pump circuits are clear and working, the bike will run just fine.

Dave
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Re: Increase Pilot Jet Size?

Postby FZBob » Tue Nov 06, 2018 3:06 pm

Sorry about reviving an old thread, but I wanted to give an update. Back in 2015 when I rebuilt my carbs, I followed my experience on other late '70's and 80's bikes, and installed #37.5 pilot jets, up from the stock #35. This did exactly what I expected, fixing 80% of the flat spot which occurs at 35-40mph under very slight acceleration. I then tried the #40 pilot jets. They are close to perfection! The throttle response is just buttery. :D

This was standard practice back in the day (Thanks, EPA!). I'm slightly puzzled that this is not a common fix on the CBX...

Earlier this year, I just couldn't leave well enough alone, and tried #42.5's. Too Rich! I went back to the #40's and I'm back to perfection.

For the sake of completeness, #68 primary mains, #98 secondary mains, stock needles with no shims, idle 2.25 to 2.5 turns out.

This is one of the best improvements I have made to the bike. (The other one being to degree the cams...)
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Re: Increase Pilot Jet Size?

Postby Larry G. » Sat Nov 17, 2018 12:14 am

FZBob I just bought a 79 cbx a couple of months ago and am in the process of reviving it. Was not running for the last year because the owner had passed away. It was a one owner with 20533 miles. I cleaned the carbs and installed the Randakk's carb kits. I believe i cleaned the carbs very well but it is very cold hearted and stumbles slightly just on take off. I did install the new accelerator pump. I'm wondering if maybe it still has a little trash in the pilot jets, although I sprayed lots of carb cleaner through them and they seem to be clear. Or maybe i need to go richer with them to help like you did. Can I get the pilot jets out without destroying them? How? By the way it's at 2.25 on the pilot screws.
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Re: Increase Pilot Jet Size?

Postby daves79x » Sat Nov 17, 2018 7:04 am

Larry:

You should have removed the pilot jets to clean them in the first place. They are likely still not completely clean. Installing new, #38 pilots will certainly clean it up, but so will properly cleaning the old ones. There are more threads here on this subject than any other. Do a search and you will find lots of reading material.

Dave
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Re: Increase Pilot Jet Size?

Postby FZBob » Sat Nov 17, 2018 3:07 pm

Larry G. wrote:FZBob I just bought a 79 cbx a couple of months ago and am in the process of reviving it. Was not running for the last year because the owner had passed away. It was a one owner with 20533 miles. I cleaned the carbs and installed the Randakk's carb kits. I believe i cleaned the carbs very well but it is very cold hearted and stumbles slightly just on take off. I did install the new accelerator pump. I'm wondering if maybe it still has a little trash in the pilot jets, although I sprayed lots of carb cleaner through them and they seem to be clear. Or maybe i need to go richer with them to help like you did. Can I get the pilot jets out without destroying them? How? By the way it's at 2.25 on the pilot screws.


Larry,
If it's stumbling a bit after being fully warmed up, I'm going to second Dave's suggestion. Removing the jets is not too hard using a 4-40 tap and then a jacking screw to lift it out. When I did mine I bought new pilot jets (35, 37, 40 and 42), as the holes are small, old deposits can be difficult to remove, and using mechanical cleaning (drills) might possibly scratch the hole and alter the flow. Also, I wanted clean, consistent data when I tried different pilot sizes...

That being said, there is always the small possibility it could be something entirely different...
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Re: Increase Pilot Jet Size?

Postby FZBob » Sat Nov 17, 2018 4:28 pm

daves79x wrote:Larry:

You should have removed the pilot jets to clean them in the first place. They are likely still not completely clean. Installing new, #38 pilots will certainly clean it up, but so will properly cleaning the old ones. There are more threads here on this subject than any other. Do a search and you will find lots of reading material.

Dave


Dave,

I'm curious if you or others have tried 38 or 40 pilot jets, and if so, what was your experience?

My experience - Changing from 35 to 37 was a fairly obvious improvement. The 35's ran like every other stock late 70's/80's literbike I have owned/ridden - a bit lean on light throttle and feathering into the throttle after an apex. The 37's cleaned this up and really improved around town drivability, as I have seen/done on many other bikes. 37 to 40 was a subtle improvement - the icing on the cake. 42 was clearly too far. I run the stock airbox with a new paper air filter and a variety of exhausts. Fresh rebuilt carbs.

I don't see much about bigger pilot jets on the forums, and I wonder why... Back in the day, EPA lean pilot jets were very well known (and hated). I see lots on raising the needle, and/or increasing the primary main, which for me didn't really fix this problem. My theories are:

1. Been tried many times, didn't work
3. Someone did a bad job drilling pilot jets back in the day, and therefore got bad results
4. Lack of Pilot Jet availability back in the day
5. No procedure to pull pilot jets back in the day
6. Easier to shim the needle - don't need to tilt the motor

I wanted to try this back around 1990, but never got around to it.

Thanks,
Bob

PS - Thanks for all the advice you (and others) have given over the years! It has really helped me with my CBX. I wish this information had been available back in 1990 when I first rebuilt my CBX!
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Re: Increase Pilot Jet Size?

Postby daves79x » Sat Nov 17, 2018 5:11 pm

The only jetting changes I make to ‘79 carbs for stock applications I’d to go from 65 to 68 on the primary mains. I’ve never felt the need to go bigger on the pilots for stock or close-to-stock bikes. I’ve not had driveability problems and I’ve done a bunch of them.

Dave
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Re: Increase Pilot Jet Size?

Postby Larry G. » Mon Nov 19, 2018 10:04 pm

Today I pulled the carbs again to extract the pilot jets. With the 4-40 tap, cap screw,washer, and nut it was easy. I didn't know that trick before or I would have done it. All the holes in the emulsion tubes were clear as well as the jets. I'll try to make sure the jet thru hole doesn't have any hard residue left on the hole walls tomorrow. Soak in some carb cleaner and run them in my sonic cleaner for a while. The rubber intake boots are old and hard, maybe there was a small intake leak, have new ones on order. Also it has the original stock airbox with foam filter.
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Re: Increase Pilot Jet Size?

Postby daves79x » Tue Nov 20, 2018 8:13 am

Guys - the secret for cleaning the pilot jets completely is to heat them red hot with a propane/Mapp torch. The accumulated gunk will all turn to powder and just fall out. Then gauge the jets to make sure no one before you has hogged them out. K&L makes a great set of jet cleaners/gauges. The smallest one is #35.

Dave
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