Carb tuning... The whats its and what nots please

I'm about to attempt a carb tuning on my 1996 for the sake of performance and to hopefully solve the backfiring issue I'm having.

The backfiring only happens when I fully roll off the throttle and the bike begins to engine brake. I get what I think is some carburator back firing. The engine definitely smells like its running rich, especially in the carb/air box vacinity. The problem developed right after I laid my bike down back in June.

Since, I've changed the air filter, spark plugs, exhaust slip on, wiring harness and the problem persists. Some of my research tells me that I need to make some adjustments to the fuel/air mixture. I have 3 main questions about this topic.

Will making these adjustments address my current issue? I still plan to balance the carbs regardless but I'm hoping I'm on the right path.

I know the 3 screws I must adjust to balance the carbs but will I need to make adjustments to the fuel screws under each one of the carbs? I saw a video on a similarly backfiring bike solved by making this adjustment. But I am unsure if these are the screws to make the adjustments with or there is a different idle mixture screw I should be adjusting.

Lastly to do all this I know I need a carb tuner and 2 other types of bent screwdriver things. One with a cross end for the carb balancing and another with some sort of special end to fit the awkwardly designed pilot mixture screw. What are these last 2 tools called?

Thank you in advance

 

Here is a link to the Honda common service manual; there's a good section in this on carbs and adjusting them.

https://www.dropbox.com/s/xz39vanvr66jqeq/Honda Common Service Manual.pdf?dl=0

There's also a great guide to tuning CV carbs at the Factorypro.com website.

Before you start - doesn't it seem odd that you need to break into the carbs after you had a crash? I'd say more likely you've stirred up some crud and have partly blocked jets, so I'd suggest starting with a simple carbs out, bowls open, jets out clean. New carb inlet rubbers are always a good idea under these circumstances.

 

http://www.ebay.com/itm/Motion-Pro-...hash=item2cb9fa6a9f:m:mckE7E9NLmLueNWeKPzsqYw

http://www.ebay.com/itm/CARBTUNE-PR...ash=item4b12cb06a6:g:Aj4AAOxy0zhTPZKM&vxp=mtr

I use something similar to the screwdriver, I have three of these stupid things, one works really well, you gota get the bike high off the ground (have a lift) and it makes life fairly easy, I bench synchronize them and once installed on the bike, I touch the adjusters a few CH's and thats it. Good luck keep us posted

 

I know at least one person who thinks a GOOD bench sync using the paper method is all that's required in the real world for all but serious racing applications by seriously discerning riders.

OK, I got my nomex on. Fire away.

Agree, step one is carbs out and detailed cleaned and all parts examined in detail for anything bent, broken or cracked. Backfiring on deceleration is usually due to a too lean mixture after installing proper headers.

Cheers,
Glenn

 

Does anyone know of a way to download this common service manual
Besides the way described in this thread above?
I have a PDF of the service manual but not a Common Service manual
Are they one in the same?

 

They are not the same. You can download from the link above. I just did to see for sure. I checked the box to accept the terms. Then I clicked on the bottom "No Thanks, view..." whatever it says.

Cheers,
Glenn

 

Thanks for the "finding" manual guidance
I have to admit, my computer skills suck some times

 

Thanks so much for thr ample advice. Okey was hoping not to pull em.

So for my parts/tool list I have:
-Carb balancer (what is bench balancing)
-D shaped pilot/phillips screwdrivers
-New inlet rubbers
-I saw some aftermarket pilot screw hand adjusters. These worth the money? Or stick to stock in regards to performance and reliability?

***I figure since I'm going to bust into the carbs that I'd upgrade my main jets. I found a kit that comes with 4 main jets and 2 shims per a jet and a pilot screwdriver. I was wondering what the shims are used for and weather or not I should buy a stock rebuild kit for it as well for the gaskets, seats needles?***

Also are there any other tools, solvents (besides carb cleaner) that make the job easier? What type of tool is required to remove the jets?

 

Carb balancing involves setting the relative positions of the carb butterflies so they all pass the same airflow at idle; this is what the carb balancer is doing, allowing you to measure and match the vacuum generated through each carb by adjusting the three other butterflies against the fixed reference butterfly. Bench setting is using something thin (e.g. a slip of paper) to set the physical clearance between the butterfly and the carb floor, so not as accurate as using the vacuum method but enough to get a bike started and running. Unbalanced carbs lead to a rough idle.

Unless you absolutely have to, I would leave the pilot screws alone, and focus on making sure the stock carbs are running properly. Once you get the float bowls off, you will be able to remove the low speed jet and the main jet using a flat blade screwdriver and a small wrench. Be wary of spraying carb cleaner around until you have the rubber parts removed especially the main diaphragms, but also the smaller diaphragms in the air cut circuits. A compressed air gun is very handy for blowing passages clean. Don't reassemble the carbs until you know all passages are clean. I also used some strands of fine wire to clear a serious clog in the low speed jet, if you do that don't damage the jets as they are reasonably soft brass.

Honda use screws that are not true Phillips heads, they are actually JIS heads, and unless you are careful, a Phillips head can chew out a stubborn screw. JIS screwdrivers are available on eBay.

Here is a very useful link to cleaning ST1100 carbs; I know not a VFR but the ST1100 is a Honda V4 engine that just happens to have been turned 90 degrees in the frame, and the carb layout will be very similar to what you have.

http://www.st-riders.net/index.php?topic=10249.0

 

eep:

Exhaust system leaks can cause backfiring too, sometimes !

First thing to try, however, would be to bend over and snug up all the carb boot clamps, noting if any feel loose.


Hold a rag over the exhaust pipe end while engine is running to check for exhaust gasket leaks that you might hear. Go around and check header nuts for tightness. Also, check all the sparkplugs for an even, light tan color. Plug color suggests alot about whatever mixture screw adjustments you might need........or might not. eaceful:

Remove the plugs for inspection whenever there's a running problem, and replace any plug too dark or black or sooty with new ones, and try again.......! :eagerness: Quicker and easier than diving into the carbs, maybe unnecessarily.



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Posting your plugs' photos here, showing electrode colors, plug # s, and cylinder positions for diagnosis and comment, might prove useful. :cool-new:

 

In your experience can you determine which cylinder might need a pilot screw adjustment, just based on plug colour?

 

When I changed out all the plugs to try to fix this. None of them looked bad at all in particular. It could be worse now though.

 

Okey nice. Ill dive into that read later tonight.

 

Yes, most of the time, but a few variables need to be accounted for, like weak spark or low cylinder compression plus a couple other factors.

 

Alright fellas. Thanks for the info. I got it all ordered 90 degree screwdriver with changeable bits, d shaped, Phillips ect., carb tuner, Japanese carb drivers, hi flow jet kit to match my current set up of an open exhaust/air box, and a set of carb rebuild kits to replace the existing part, new carb boots. Ill tighten the bolts down on the headers and text for exhaust leaks once I put every back together. If there is a leak in this region ill get to changing the gaskets soon since I plan on replacing my headers. Anything else I might have missed?

 

Oh and carb cleaner.

 

Thanks squirrel master. I think given what @terry smith said and the great condition prior I'm just gonna go ahead and rebuild. I wanted to re-jet anyways and was just being lazy/scared lol. The true reality is ive replaced A TON of parts: exhaust, plugs, chain, sprockets, few electronics, full harness unspliced, air/fuel filter. Might as well knock this out too.

 

You should have mentioned that in your original post !! Sheeesh ! That stuff will almost ALWAYs cause backfiring.

 

Oh I didn't mention it previously because the backfiring was there before the upgrade. I had wondered if my can being slightly dented was causing it or the carb. In the end I changed the slip on. Was still there. I then removed the snorkel from thr airbox and the carb backfiring got worse when warming up the bike. I opted now for new high flow jet system, fresh carb kit. Ill be redoing seats needles float levels and all that and going for the full monty on the carb set. Will be balancing and tuning after. Ill update of course on it all, but for now...

https://youtu.be/EmrgojQBdV0

 

A quicker fix, and cheaper and easier fix, would be to simply reinstall the oem airbox and filter.