Re-jetting carbs

[pstelter]

Hey...now that riding opportunities are drawing to a close here in southern New England, I am thinking about re-building the carbs with the Randakk kit. But while I was at it, I thought I would put new jets in as well.

A friend on this board directed me to carbjetkits.com. Their prices seem right and seem to have a good rep on other boards. Has anyone here used them? (As I type this, I'm not sure if my buddy was able to get his new jets installed).

Also, I have a VIN question...my VIN is 1HFSC1426FA12xxxx. The folks at carbjetkits.com have two sets of jets for the 85 Aspy, based upon VIN:

Group 1 VIN range: 1HFSC142-FA102502 to 1HFSC142-FA126301
Group 2 VIN ranges: 1HFSC145-FA100116 to 1HFSC145-FA114611
1HFSC144-FA100101 to 1HFSC144-FA101790

I think I have a Group 1, but I'm not sure since my VIN has a '6' where all their VINs have a '-'. Any thoughts?

Hope everyone had a good riding season.

- Pete

[bs175dths]

..... are you talking rejetting as in new jets, same size as OEM?
or are you talking about rejetting as in new jets, larger size?

Just curoius.......

Why are you rejetting?

(little jab time - here in sunny So. Cal., riding season can be 400 days a year which explains the high miles our bikes have.)

[pstelter]

As far as the reason for the re-jetting, I am hoping it will cure the hesitation that I get in the motor at around 2k rpm.

Don't know about the size...the website has a kit for the GL1200's. They also have a jet calculator that tells you what jets to use based upon your riding conditions. What is confusing is that the pre-made kit is different than the calculator.

So, as far as size goes, I guess I'll talk to the guys from the web-shop and find get what they recommend.

[90Blazer]

Group 1 is for 49 state model bikes and Group 2 is for California bikes. Your bike falls in Group 1. It looks like they go up one size on the pilot get and mains. the shims that they supply should move the needle up so that the mains start engaging at a lower rpm so that should help with the stumble.

[KnoxSwift]

You may not need to replace all the jets. To resolve the stumble you can shim the stock needle jet with stainless steal washers in .05in increments.

[Frank]

Just the carb cleaning is probably going to cure your problem

[bs175dths]

... which was my reason for asking, 'why re-jet'.
do a good cleaning, then shim if needed.......

no real need to open the can of worms with re-jetting.

[wallbrad]

Ok, I'm with Pete here in needing some more info. (I was the guy who recommended www.carbjetkits.com to him.) And, no, Pete, I never got around to ordering the new jets.... Now that it's spring, I'm thinking about doing it, but I hate the idea of taking the bike all apart again just to replace the jets!

Has anyone here got experience with the any of the jet sizes they recommend for the "ultimate" kits for Group 1? Pete and I have got virtually identical bikes and ride in the same conditions. His is an 1985 GL1200a & I've got a 1985 GL1200i. We both live at shoreline elevation, and temperatures in New England are in the 40-80 degree range most riding days.

After using their carb jet calculator, I'm now on the fence between their basic Group 1 Ultimate what their calculator recommends. Their Group 1 Ultimate recommends a #110 main jet & 2 shims (w/#38 pilot), but their "calculator tool" for my conditions recommends #112 w/ 1 shim (& #38 pilot). I've got stock exhaust, stock timing, and stock air filter. If I go to a K&N air filter, it recommends upgrading to #115 w/ 1 shim. An all-stock condition is also apparently exactly the same as the Ultimate (i.e., #110 w/ 2 shims), but it doesn't have the #38 pilot jets.

Little confused here, and frankly, I'd rather see advice from the experience of gl1200 owners who might have done this than from the carbjetkit website folks.

FYI, so this thread doesn't get highjacked onto cleaning discussions, I completely rebuilt my entire carb and intake system last summer, and it's now so clean you could eat off it. Pete hasn't taken his apart yet (at least that I know of, because I stole my carb sync tool back from him last fall to lend to another friend...). We both seem to have the 2K hesitation, although mine is noticeably better than before I did the rebuild. I still need a fair amount of choke (enrichment) opening most of the time, and I don't like running so rich all the time.

So, more shims or bigger jets w/ fewer shims?

Advice, people?

Experience anyone?

[admin]

Jetting is really an individual bike thing that can only be proven on a dyno using an exhaust gas analyzer or full throttle testing, which means run up to top of second gear, kill the ignition and immediately pull the plugs to see what color they are. But, that's only for full power, drivability must also be considered. If my bike, in excellent condition and I had no exhaust or camshaft mods I would shim the needles up/down and if more/less fuel is needed then increase/decrease the pilot jets size. One main jet size jump is really big, then pilot jet and needle need to match the main jet. Very tedious and complex. How do you know for certain it's the jets causing the problem? Could be, low/high fuel pump pressure, float level, weak spark or could be vacuum leaks, etc. Jetting is a real science and requires a major time investment to get it right. The factory is almost always right on for these bikes, but, if you enjoy tweaking and tuning go ahead and enjoy, but, first make certain everything is per factory specs before you touch the jets. Make sure that each time you remove the carbs that you always use new O-rings for proper sealing and that carbs are sealed and that vacuum hoses do not have leaks. I use an unlit propane torch outdoors on a no wind day to check all vacuum joints for leaks. With engine idling, outdoor, if engine speed changes when the propane finds a leak you will then have found a vacuum leak that needs fixing. Good luck.

[bs175dths]

well put, for there is nothing better than starting with the basics and creating a base point...

or you may be destined for chasing your tail......

[wallbrad]

Thanks, Folks (and especially Admin!).

Now that I'm out riding a lot and have had a really good chance to analyze what's happening to my bike (versus what's happening to Pete's), let's talk some more.

So, that hesitation I get is around the 2000-2500 RPM range and usually only happens after I've let off the throttle first and then try to accelerate at a medium rate, like coming off a stop or when trying to roll on the throttle in a curve. Having the enrichment (choke) on at least some of the way pretty much makes the problem disappear, but at the cost of running that rich the rest of the time, including at idle. When the engine hasn't come up to operating temperature, I need even more choke. I've gotten to the point where I find that performance is so much better when I leave the choke on a bit that I really only turn it off totally if I'm doing a lot of start and stop driving and don't want the engine idling that high. Basically, operating the choke a bit has become a regular operational tool/mode !

Now, let's repeat the baseline of info here: My bike is nearly mint with only about 24K miles (if I remember right...) and I just meticulously rebuilt the carbs last season with new K&L parts (including the troublesome slow-air cut off valve). I haven't done your vacuum leak check, but I did replace most of the rubber when I rebuilt the carbs. Sparks are all new stock. Fuel pump seems to have no problems (evidenced by the fact that I see no problems at wide open throttle and no other surge-surge behavior at any speed). Float heights were measured with calipers to be spot on the spec requirements.

It sounds to me like you guys are saying start with shimming only? As I justify that answer to my engineer brain, see if this logic makes sense: Lifting the needle in the main jet will have the effect of opening that jet more at almost all throttle positions, adding more gas just like the enrichment (choke) circuit effectively does. However, since we have constant velocity carburetors, those slide valves will only maintain the same vacuum at the throat of the slide valve opening under steady state conditions; that isn't true under the transients when the slide valve actually opens (where it momentarily increases the vacuum level at the throat before the slide position catches up). Because the enrichment circuit follows a more complex flow path to create a differential pressure to dump fuel into the air flow down stream of the throttle plate, there will necessarily be a different behavior between the two "fuel adders". I'm guessing that the momentary increase in vacuum (i.e., when the slide hasn't caught up yet) will pull gas into the flow more efficiently than the "across-the-board" way that the choke dumps fuel in. The choke dumps it in all the time, therefore affecting idle speed, where as the main jet won't dump much into the flow when the throttle plate is closed at idle.

So...here's the question to test this thought experiment: Does shimming the needles increase idle speed? I'm guessing it won't impact it appreciably. Anyone have any before/after data on a shimming operation?

BTW, I haven't looked in detail yet--Can you get the forward slide valve covers off without taking the intake housing and carbs off the bike? I looked down today while I was at a stop light, and it looks like the two inner-most screws might be accessible with the faux tank removed? I don't feel like @#$%ing with the throttle cables again to get the carb body out !!!

If you guys are right and only shimming does the trick AND if I can get the tops off all four carbs without taking the carb body out, this will be an AWESOME answer!!!

Thanks for the help! This forum is a great asset!!

[admin]

Thanks, Folks (and especially Admin!).

Now that I'm out riding a lot and have had a really good chance to analyze what's happening to my bike (versus what's happening to Pete's), let's talk some more.

So, that hesitation I get is around the 2000-2500 RPM range and usually only happens after I've let off the throttle first and then try to accelerate at a medium rate, like coming off a stop or when trying to roll on the throttle in a curve. Having the enrichment (choke) on at least some of the way pretty much makes the problem disappear, but at the cost of running that rich the rest of the time, including at idle. When the engine hasn't come up to operating temperature, I need even more choke. I've gotten to the point where I find that performance is so much better when I leave the choke on a bit that I really only turn it off totally if I'm doing a lot of start and stop driving and don't want the engine idling that high. Basically, operating the choke a bit has become a regular operational tool/mode !

Now, let's repeat the baseline of info here: My bike is nearly mint with only about 24K miles (if I remember right...) and I just meticulously rebuilt the carbs last season with new K&L parts (including the troublesome slow-air cut off valve). I haven't done your vacuum leak check, but I did replace most of the rubber when I rebuilt the carbs. Sparks are all new stock. Fuel pump seems to have no problems (evidenced by the fact that I see no problems at wide open throttle and no other surge-surge behavior at any speed). Float heights were measured with calipers to be spot on the spec requirements.

It sounds to me like you guys are saying start with shimming only? As I justify that answer to my engineer brain, see if this logic makes sense: Lifting the needle in the main jet will have the effect of opening that jet more at almost all throttle positions, adding more gas just like the enrichment (choke) circuit effectively does. However, since we have constant velocity carburetors, those slide valves will only maintain the same vacuum at the throat of the slide valve opening under steady state conditions; that isn't true under the transients when the slide valve actually opens (where it momentarily increases the vacuum level at the throat before the slide position catches up). Because the enrichment circuit follows a more complex flow path to create a differential pressure to dump fuel into the air flow down stream of the throttle plate, there will necessarily be a different behavior between the two "fuel adders". I'm guessing that the momentary increase in vacuum (i.e., when the slide hasn't caught up yet) will pull gas into the flow more efficiently than the "across-the-board" way that the choke dumps fuel in. The choke dumps it in all the time, therefore affecting idle speed, where as the main jet won't dump much into the flow when the throttle plate is closed at idle.

So...here's the question to test this thought experiment: Does shimming the needles increase idle speed? I'm guessing it won't impact it appreciably. Anyone have any before/after data on a shimming operation?

BTW, I haven't looked in detail yet--Can you get the forward slide valve covers off without taking the intake housing and carbs off the bike? I looked down today while I was at a stop light, and it looks like the two inner-most screws might be accessible with the faux tank removed? I don't feel like @#$%ing with the throttle cables again to get the carb body out !!!

If you guys are right and only shimming does the trick AND if I can get the tops off all four carbs without taking the carb body out, this will be an AWESOME answer!!!

Thanks for the help! This forum is a great asset!!

Hmmm, after reading this I'm leaning more towards the fact that your carbs jets (most likely pilot jets) are dirty.

[pstelter]

So I have completed the cleaning of the carbs and replaced the jets. I ended up using the jet calculator which replaces the #108's with #112's, the #35's with #38's and installing one shim per needle.

Why did I replace the jets? Couple reasons...

1. Time...I have one shot this summer to get this done. I don't want to take stuff apart again because the cleaning didn't fix everything. I'm almost tempted to order new float seats since I am down for repairs.

2. Price...considering the price of some other motorcycle stuff, the 50 bucks for the kit make it an affordable science experiment.

3. Curiosity...I am a sparky by trade. So this is an opportunity for me to get a little mechanical. Should help me fix the leaky carb in my lawn tractor, too.

- Pete

[bs175dths]

also keep in mind that the throttle needle is tapered
using a shim on the throttle needle will effectively increase the jet opening (because the location of taper has been changed) allowing more fuel to pass.
any sort of residue on the inside of the needle jet holder, or on the throttle needle will change the amount and flow (turbulence) affecting that particular carburetors performance.

Complicated? Yes.
Frustrating? It can be.
Which may be the reason why I am seeing 'single carburetor intakes for your GL1200' here and there.

All that can really be said is, you asked for our opinions, and we gave it.
What you do with it, for your application is your final decission, good or bad (you will let us all know) and we leave it at that.

Bottom line is to keep the (s)miles rolling by on our beloved GL1200's.

come back with any updates......

[pstelter]

So here is the after action report for the carb re-jet:

Replacing the jets was actually very anti-climactic. Easy as changing eight threaded fasteners.

As far as the performance goes, the stumble is gone, or close enough to being gone that I no longer consider it an issue, In general, fuel economy is down slightly on average. But considering I have bigger jets (and burn more gas), that is not surprising. However, some tanks I got great mileage and others worse. Since the jets were sized for 40-70 deg F, I think the economy was better in the cooler months. Could have also had varying degrees of ethanol as well.

One problem I had is unrelated to the re-jet. I was unable to complete the idle-drop procedure in the Randakk book. I was able to adjust carbs #1 and #2 and get the vacuum in the range specified in the manual. But, for #3 and #4, the throttle linkage was in the way - I could not see the screws to know if I was engaging them. I installed the carbs with them as close to the factory setting as possible, but don't think they ever moved.

As a result, the balance went ok, but my idle keeps wanting to creep up. It eventually settles in at about 1300 - this could also be hurting the fuel economy. The two options I see are:

1. Reset all the idle screws to the factory position and just balance from there, or
2. Re-do the idle drop using a tool that allows me to get at the screws (i.e., a right angle screwdriver or similar)

Any opinions?

- Pete