To rejet or not to rejet...

[mhooner]

Now that I have replaced my exhaust I notice a lot of popping and backfiring when decelerating or going downhill.
Is it absolutely neccesary to rejet to eliminate the popping or can the same thing be accomplished by leaning out the fuel using the mixture adjustment screw?

Inquiring minds want to know.

[M@Man]

It depends on your new exhaust, but from what you decribed it sounds like you should be able to just adjust your mixture screws, and be fine. You didn't mention any hesitation or performance issues, so I don't think re-jetting is necessary.

M@

[fegs]

You probably will have to re-jet. If I am not mistakin, your ride should have only 1 carb. Rejetting should be rather easy on your ride and will improve the overall performance.

[mhooner]

Thank you for th replies.
No performance issues. It actually feels a little peppier but that could just be the added sound :happy:

Yes, only one carb and I have the jet kit on order but I am going to try leaning it out a bit while I wait for the kit to arrrive.

[M@Man]

If you lean it out your popin' will get worse, you need to richen the mixture.

M@

[Machinehead]

If you lean it out your popin' will get worse, you need to richen the mixture.

M@


M@ is correct, you have a lean condition. Turning the mixture screw/s out until it is removed should work. Most of the time anything over 3.5 turns out will require a larger slow jet. This is because of the travel limit of the needle/seat and spring.

Please keep us posted.

[mhooner]

M@ is correct, you have a lean condition. Turning the mixture screw/s out until it is removed should work. Most of the time anything over 3.5 turns out will require a larger slow jet. This is because of the travel limit of the needle/seat and spring.

Please keep us posted.

Thanks for the info. I was told that the popping was caused by excess fuel igniting in the exhaust system therefore assumed I should lean it out.

[navillustoo]

M@ is correct, you have a lean condition. Turning the mixture screw/s out until it is removed should work. Most of the time anything over 3.5 turns out will require a larger slow jet. This is because of the travel limit of the needle/seat and spring.

Please keep us posted.

Keep in mind, a larger slow jet will richen the mixture up beyond just the idle circuit, resulting in poorer fuel mileage. A little trick recommended by Dynojet and the shop manuals is to raise your float levels a couple of mm. This will richen the idle circuit without affecting the mixture above 1/8 or so throttle opening.

Brian

[Machinehead]

That's true Nav, but the issue is on decel...which tends to point to a lean mix. This is where I am riding to....ehmm.

Mr. Mikuni can elaborate/expound:

http://www.mikuni.com/fs-tuning_guide.html


Fuggin' with the float will/should affect the performance across the board. This prob is only experienced on decel....it may be a solution ...I hope...the simplest is best...
.
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Howwa bout those Spurs?

[mhooner]

Haven't played with the mixture yet but I did go to premium gas octane level 92 and the popping disappeared. I did have to adjust the idle upwards a little as the engine kept wanting to stall when the bike was stopped.

Thanks for the link mh. It really helped a lot. I have a better idea of what's happening.

[HSGear]

I rejetted my Ducati (yeah, I know, not a Shadow) and it worked wonders on the peformance as well as eliminated the backfires on decel that I had from bigger pipes.

I'm about to rejet my Sabre - went to Bub 2:1 pipes and it pops like a sonofabitch when going on hard engine brakes.

Mike

[navillustoo]

That's true Nav, but the issue is on decel...which tends to point to a lean mix. This is where I am riding to....ehmm.

Mr. Mikuni can elaborate/expound:

http://www.mikuni.com/fs-tuning_guide.html


Fuggin' with the float will/should affect the performance across the board. This prob is only experienced on decel....it may be a solution ...I hope...the simplest is best...
.
.
Howwa bout those Spurs?
That's right MH, it's the idle mixture though, and nothing else. Raising the float level only a silly mm or two will not affect the mixture above about 1/8 throttle, but the pilot jet covers up to about 3/8 to 1/2 throttle, where most of your cruising is done. This is why everyone that has gone to a larger pilot jet is also complaining that their gas mileage has taken a big hit.
The only Mikunis that I have messed with are on 2 cycle engines, nothing like these Keihins. I prefer to link folks to :
http://www.motocross.com/motoprof/pdffiles/carb101.pdf
This link provides the best generic explanation I've found to date. We're not riding HDs!

Brian

Edit: See what Dynojet has to say about the pilot circuit here:
http://www.dynojet.com/jetkits/jetkit_info.php

[Machinehead]

Doesn't really matter who made the carb, but I understand and appreciate the distinction and clarrification.

The point I was trying to make is that (in this case) if the a/f mix adjust doesn't do the trick the next step would be the slow/idle jet. More than likely this will be resolved with adjusting the a/f. One doesn't have to understand the mechanics to resolve the issue.

What happened here is we have passed on more information that was warranted...ok?

[navillustoo]

Doesn't really matter who made the carb, but I understand and appreciate the distinction and clarrification.

The point I was trying to make is that (in this case) if the a/f mix adjust doesn't do the trick the next step would be the slow/idle jet. More than likely this will be resolved with adjusting the a/f. One doesn't have to understand the mechanics to resolve the issue.

What happened here is we have passed on more information that was warranted...ok?

MH,
I know what you're saying and I'm trying to disagree with you, but it seems you won't let me! hehehe! I'm saying that if the A/F mixture screws won't do the trick, the larger slow jet is NOT necessarily the next step. Raising the float level slightly will richen the idle mixture enough to allow you to return the idle mixture screws to 2 to 3 turns out. Comprende'?

As far as passing on more info than is warranted, maybe someone else will read this down the road and get something out of it. It's all good!

Brian

[Mr.Sinister]

I think I learned something . Just don't ask me what it was . :happy:

[Ghost_Rider]

Haven't played with the mixture yet but I did go to premium gas octane level 92 and the popping disappeared. I did have to adjust the idle upwards a little as the engine kept wanting to stall when the bike was stopped.

Thanks for the link mh. It really helped a lot. I have a better idea of what's happening.

Hi mhooner,

What octane that Honda recommend to your bike? If you use 92 octane which is higher than the recommended (maybe 87 like my VLX) without any other modification like advanced timing , your engine will run slightly richer. Looks like it helps the popping a bit, but the air & fuel mixture cannot be burnt completey due to the higher flash point of 92 octane. That's why your engine keep wanting to stall, because the unburnt fuel is fouling the plugs. You will have carbon built up faster as well. This was happend to my ride as well last time.

Just a suggestion, I think the best way is still to adjust your A/F mixture screw setting and use the recommended octane level.

[Machinehead]

MH,
I know what you're saying and I'm trying to disagree with you, but it seems you won't let me! hehehe! I'm saying that if the A/F mixture screws won't do the trick, the larger slow jet is NOT necessarily the next step. Raising the float level slightly will richen the idle mixture enough to allow you to return the idle mixture screws to 2 to 3 turns out. Comprende'?

Brian


It's all good bro....as long as the issue is resolved and printed on this board...if you know what I mean. Possible solutions added are welcome....no matter who post them....I will agree to disagree on float level tho....have you ever adjusted the FL on your 750 Holley double-pumper to compensate for a lean condition?

[mhooner]

Ok, so I rejetted using the Dynojet Kit for the750 Aero. It still pops like crazy. I followed the instructions for the stage 1 mod replacing the needle, spring and using the 118 jet. The mixture screw was turned all the way in and then backed out 2 full turns. The only other mod was the replacement of the stock exhaust with Vance and Hines Cruzer exhausts. The baffles are removable but have been left in place with no mods.

I did notice that the stock jet was a 125, larger than the jet recommended for the stage one mod. I assume that the new needle and spring are supposed to make up for the reduced size of the jet.

My feeling is to put in a larger jet or increase the fuel mixture another turn or 2.

Any thoughts?

[navillustoo]

...I will agree to disagree on float level tho....have you ever adjusted the FL on your 750 Holley double-pumper to compensate for a lean condition?


MH,
No, I haven't. But if you can't tell the difference between a Keihin CV carb and a Holley 750 DP, you should have your eyes checked! LOL! While the principles are the same, both carbs use atmospheric pressure and vacuum to operate, they are worlds apart in their fuel and air circuit adjustability.

Mhooner,
Do not be confused by the numbers on the main jets. They are not measured the same. Do not go to a bigger main jet because the engine is popping on deceleration, it will not help the popping one bit. None of the A/F mixture screw recommendations work, at least that's what I have found to be the case. Try backing the mixture screw out more, 1/4 to 1/2 turn at a time. If at 3-1/2 to 4 turns out, you still have the popping, you can raise your float level slightly(1 to 2 mm) and this will allow you to adjust your screws out to less than 4 turns to eliminate the popping. Another way to accomplish this is to go to a larger pilot or slow jet, but be prepared to see your MPG drop about 10% to 15%. The larger slow jet will allow more fuel to flow to the engine from just off idle to about 1/2 throttle. This is the throttle range where most of your riding is done, thus the big increase in fuel consumption.

Brian

[Machinehead]

MH,
No, I haven't. But if you can't tell the difference between a Keihin CV carb and a Holley 750 DP, you should have your eyes checked! LOL! While the principles are the same, both carbs use atmospheric pressure and vacuum to operate, they are worlds apart in their fuel and air circuit adjustability.
Brian


hey Brian...and fellow members, I associated both the circumstances in an attempt to prove a point. If the float level is OFF, the float level will be OFF in ANY circumstance regardless of any modifications. Float level is not an adjustment used to counter/offset a modification, it is listed in most troubleshooting manuals for reference in case the original settings have been altered (for instance the float has hit the floor and the tang has moved). If the float setting(height) is not righteous it doesn't matter what pipe/jet is installed (whether it's OEM or aftermarket) it will have to be a preliminary setting before proceeding if you/us can understand.

Once again I brought Holly to the table as a reference only, hoping to blend the two.

BTW....I haven't been keepin' up with this thread , but more than likely it was solved with an A/F adjustment.....and you know it.....heehee.....carb boystereeni

[navillustoo]

hey Brian...and fellow members, I associated both the circumstances in an attempt to prove a point. If the float level is OFF, the float level will be OFF in ANY circumstance regardless of any modifications. Float level is not an adjustment used to counter/offset a modification, it is listed in most troubleshooting manuals for reference in case the original settings have been altered (for instance the float has hit the floor and the tang has moved). If the float setting(height) is not righteous it doesn't matter what pipe/jet is installed (whether it's OEM or aftermarket) it will have to be a preliminary setting before proceeding if you/us can understand.

Once again I brought Holly to the table as a reference only, hoping to blend the two.

BTW....I haven't been keepin' up with this thread , but more than likely it was solved with an A/F adjustment.....and you know it.....heehee.....carb boystereeni

I certainly am not looking for an arguement, nor do I claim to know more than the engineers at Dynojet, but when They say "PILOT CIRCUIT - This controls 100% of the engine idle and 25% of the transition onto the needle. Dynojet has found that the engine will idle with the standard pilot jet with or without the air-box and with the slides and needles removed from the carburetors;therefore we never change the pilot jet. Doing so is proof that you are not using the other circuits correctly.Idle and off idle is controlled by the mixture screws and the FLOAT LEVEL which have the most positive effect below 4000 R.P.M. On some models the pilot air jet is changed to provide optimum fuel economy. Correct balancing of the carburetors also ensures a smooth idle.", I take their word for it. On their troubleshooting chart that is included in their jet kits, they state that raising the float level slightly will richen the idle mixture enough to return the mixture screws to a manageable adjustment. They claim this is better than using a larger pilot jet. Even my Clymer's shop manual makes this claim. Reading it in two separate publications is good enough for me!

Here's the link again for anyone that wants to read it for themselves:

http://www.dynojet.com/jetkits/jetkit_info.php


Brian

[Machinehead]

I only know what I have tried.....can't really read much more. Only hopeful to meet half way. I will try once more to expound on the float level.

I associate a float level adjustment to an (auto) distributor adjustment. Once it corrected, it's left alone (regardless of the mods). IF an adjustment to the float level rectifies ANY problems RIGHT NOW then there should have been a problem with the height/level long before the mod happened.....a low float level would not only cause a pop, but cause other problems within the range of operation from acceleration to the top of the gear to warm up..


I agree with proper balancing.....but it still remains a decel issue .... and it usually takes a simple adjustment.....right?

[mhooner]

I finally decided to get to the bottom of this problem. I removed the carburetor and replaced the main jet. I had gone to a 130 (DYNjet recommended the 118) so I replaced it with the 120. I then took the tool provided in the kit and turned the mixture screw all the way in only to find that it was already turned in as far as it would go. I found that the tool was too loose on the screw so that when I had tried to turn the screw out it was just spinning on the screw. I took a pair of pliers and adjusted the tool so that it would fit the mixture screw more tightly and actually turned the screw out 3 full turns. Voila! no more popping.

Moral: make sure your tools work properly. :crazy3:

Thanks again for everyones input. :happy:

[Machinehead]

Thank you for updating your post. Glad to hear that it's all good.

[MacKay]

I just bought a used 2002 Honda Shadow 750 ACE with 1500 miles -- $3,000 bucks -- like out of the showroom ... couldn't pass it up. Anyway, the bike popped and backfired like crazy on decel. Everything is stock and I heard Honda loves to impress the EPA by tuning the carbs super lean. I drilled out the pilot screw aluminum plugs on each carb with a 3/16 bit. The factory setting was 2-1/4 turns out. I turned each pilot screw out another full turn and the bike now runs super smooth with no pop or backfire !!