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1991 VT600 VLX
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50 Posts
Discussion Starter #1 (Edited)
As you know, winter came 4 months ago and there was not much to do off-season. So i decided to sentence myself with never ending tuning problem. Here it is.

Honda VT600 1991 dual-carb.

What was done:
  • New Cobra fatty shotgun pipes, baffles removed (so now it is straight and loud and lovely)
  • New K&N filter in stock air box
  • Used (but cleaned) DynoJet kit: needles, main jets
  • Carbs rebuilt with K&L kits (gaskets, float valve, float jet). Slide diaphragms unchanged.
  • Pilot jets remained stock
  • New spark plugs from NGK as per specs
  • A/F screws on both carbs backed 1,5 turn out (as per Dynojet recommendation)
  • Carbs not synced yet (but I haven't split them or touched sync screw)
Before the mods, the old spark plugs were all black (rich) with 1 turn out, but that means nothing now.

How it runs now - if you rev it good enough to I guess around 4k+ RPM, it runs smooth, accelerates well and is a baby devil. But the problem appears when you gently spin from low RPM, on 2-3rd gear, from zero to 1/2 accelerator. Then it starts stuttering a little before it again equals out. Maintaining a constant speed is Ok - it runs smooth. In general, it runs really good, but has these occasional stutters. On decel, it gives gentle pops, but something louder to backfire only occasionally. This may be due to exploiting same exhaust gaskets for 4 mounts/unmounts, so I will be replacing them very soon to hopefully solve it.
What I don't like is that stuttering at certain acceleration point. It points to pilot stream, and A/F screws, but I want to get a 2nd opinion before I go there.

I took out one plug on each cylinder, see pics below. They were all new and ran 40 miles in town with a lot of accelerations/deceleration, some idle work in garage etc. Not much of calm country riding. Also outside it was no warmer than 8 degrees C, and super humid.

They look overly rich.

Front cylinder:
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Rear cylinder:
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What I plan to do next:
  • Sync carbs
  • Put in new exhaust gaskets
  • Start messing with A/F screw, but not sure which way. Making i richer to get rid of stutter? But it already looks rich on the plugs... I am not sure how to proceed.
Is this the right thing to proceed with, or am I missing something?

285396
 

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1999 Shadow 750 ACE VT750CD3
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2,215 Posts
Some quick ramblings and a few questions.

To my way of thinking your jetting with that mod selection should be only slightly richer than stock.
The k&n filter is not a huge flow difference in a complete stock air cleaner assembly and the engines don't care too much about exhaust.
The exhaust looks nice though. (-:

By all means sync your carbs, I can't see it solving the problem though.
What DJ main jets are you running? What "Stage" kit.
What are your slow jets 38?
Where are the jet needle clips. Are the needles shimmed too. the little washer that comes with the kit should be on top of the clip so the spring doesn't sit on it.
How are the SE (choke) valves, do they move freely and at the same time?
The rear plugs don't look horrendously bad, the front is definitely too rich.

Let me know what you find and I'll tell you what I think.
 

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2007 VT750DCA7 Spirit - NHB01 Black
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472 Posts
The plugs look a bit rich to me, not horrible but not great either, for issues with part throttle low speed hesitations and stutters I like to refer to the ole carb tuning chart.

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Below 50% throttle the most effective adjustment is from needle profile and clip position, I’d start there if you have some hesitation below and up to 50% like you stated.

Syncing the carbs is the last thing you should do IMO, once the bike runs well in all ranges of throttle and the plugs read well only then do I sync the carbs.
 

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1991 VT600 VLX
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50 Posts
Discussion Starter #4 (Edited)
Thanks for good follow-up and input. Trying to answer the questions:
  • I am doing the jet work only because of exhaust - I'm not expecting a performance increase or anything. Just want a properly tuned fuel system for the open pipes (this is also recommendation from Cobra - either trying to sell more parts, or not.. :) )
  • It is a 1-2 stage. Pics below for tappered needle and main jet (114). These are the only DJ jets I have.
  • Slow jets are stock, so I guess 38
  • The needle clip is on 2nd groove from top - again as a recommendation from DJ. The washer sits on top, against the spring - as you described
  • Choke works fine - no work done there except inspection. Starts good on cold with choke, start driving, get choke inside, and then it idles normally.
So the only problem is some stuttering accelerating from lower RPM, which dissapears once RPMs pick-up. I'm yet to understand if it overfills with fuel, or it is too lean at that particular stage. Lacking a bit experience with that (but that's how you gain it, haha).

Below are the pics for needles and main jet pics, stock vs. Dynojet. Also the bowl with stock pilot.

285403
285404
285405
 

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2007 VT750DCA7 Spirit - NHB01 Black
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472 Posts
Well personally I’d start easy and take the airbox cover off and air filter out and run it. When they’re too rich it’ll run and drive with the air box cover off a lot of times, if you can take it for a real short run and it doesn’t stumble where it does now you’re likely rich in that area, if it gets worse with the airbox cover off you could be too lean.

Lots of trial and error when jets get changed, I had my carbs out a number of times fixing and adjusting minor things.
 

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1999 Shadow 750 ACE VT750CD3
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2,215 Posts
That needle is a whole lot slimmer than stock, so it will allow a lot more fuel at part throttle openings.
As a base starting point I'd be tempted to try to run that needle with the stock main jets and see what the plugs look like.

One of the reasons why I suggested checking the choke action is sometimes the cables from the splitter can get gummed up and one not push as well and the valves will not shut off at the same time and one cylinder will show as rich.
 

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1999 Shadow 750 ACE VT750CD3
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2,215 Posts
I've been thinking more on this.

The pilot jets are 40's so that is stock for the bike.
The 114 Dynojet main is equivalent to a 120 Keihin so 2 steps up in richness from the stock 115.
The needle clip is on the second notch.

Nothing too crazy there in the jetting I would expect it to run fine.

Open pipes usually hurt low range response and mid range power, I'm thinking that's what we are seeing a rather extreme example of here with the stumble.
It may not really be a carb problem, but a pipe problem.
I think other than reinstalling the baffles the best route might be to lean out the mid-range a bit and see if there is an improvement.
Putting the jet needle clip to the highest (1st) notch and seeing if there is an improvement might be the cheapest, quickest way to test this theory.
Beyond that would require a different needle, for testing purposes maybe the stock needle and maybe a washer shim would be the next step down in that direction.
 

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1991 VT600 VLX
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50 Posts
Discussion Starter #11 (Edited)
CSRoad - thanks for all your thinking! Really appreciated!

Putting the baffles back is a 10 mins job to go out running - they should create some backwards pressure in the pipes (they get the diameter down, but don't have a lollipop effect). I could try it out easily.

I also liked Froth's suggestion with opening up the air inflow for a test drive.

I will start with the safest (the baffles) and post the feedback here.

And then decide if I need to get that bloody airbox out once again - or what specifically needs to done in the carbs.

 

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1991 VT600 VLX
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Discussion Starter #12
Small update. Put the baffles in today. Had a short trip, but tried to provoke stuttering, it didn't happen. At those acceleration and RPM ranges where I was expecting it, it was a soft hump-hump-hump, but all smooth. Hope to finalize "testing" with a longer trip come weekend.
 

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1999 Shadow 750 ACE VT750CD3
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2,215 Posts
Thanks for reporting back and good luck with the testing.
A possible compromise might be one of the can opener style "shorty" type baffles as found in the aftermarket, possibly cheaper in the long run than hacking your expensive Cobra ones.
 

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1999 Shadow 750 ACE VT750CD3
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With enough tweaking of carbs, it can probably be made to run alright, but it will always be a compromise on low and mid-range power with the benefit of loudness and maybe more top end power.
A stock bike will be faster in normal use.

Lollipops might help the low end and mid-range, but they are another variable to try to tune into the system and depending on the angle and clearance will make the bike quieter and will possibly hurt top-end power.

Try unwrapping the packing from the stock baffle, that will make it louder and may actually be a win-win situation with the tuning.
 

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1991 VT600 VLX
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Discussion Starter #16
If stock Cobra baffles prove to solve the issues, I will order some short ones and put those in.

I like it loud. My neighbours don't. My wife is scared.
 

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1991 VT600 VLX
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Discussion Starter #17
Update. Did some riding with the baffles in - no stuttering or hesitation on low RPMs. Had a trip, took them mid-way for a change, and could feel the stuttering coming back on some acceleration scenarios. So looks like the back pressure, wich you can sometimes find in discussions as "not needed", is actually important.

But I like it loud, so will look into smaller baffles as recommended by CSRoad.

Wasn't checking the spark plugs again, but I want it to ride good, and then get back to the sparks.
 

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1991 VT600 VLX
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Discussion Starter #18
Another update. Will keep this log on how it's going.

After running ~1000 miles, I was not satisfied with the drop in mpg (the best I could get was 45, maybe 47 mpg). And I know it was way better before. Also dark spark plugs was a good indication I have to do something. So tonight I took out the carbs, and got rid of DynoJet tappered needle. Got back to stock, with 1 washer shim under.

So current setup:
  • K&N in stock box
  • Stock needles with 1 washer shim under
  • DJ 114 main jets (Keihin 120 equivalents - thanks CSRoad)
  • Stock pilot jets (40)
  • Idle screws 2 turns out (it was popping on decel with 1,5)
  • Cobra fatty shotguns with stock baffles
Had a quick midnight ride - accelerates good, no popping, no stuttering. I didn't feel any difference how it accelerated before - now it even feels smoother all the way to 50 mph in 2nd gear (or placebo). Will do more testing on highway speeds, longer trips, etc. Want to see also the mpg for the next 3-4 fuel tanks, if I start winning the range back.

I can still go back to stock main jets, but let's see how it runs now. I didn't want to be radical going back, if needed will take the carbs out again.

P.S. I think I forgot to set the float height to the specs after I put the new valves from rebuild kit. So will have to there again anyways.
 

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1999 Shadow 750 ACE VT750CD3
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Thanks for reporting back, I'm glad it is running better for you.
Float heights can really change the mixture in the part throttle cruise situation and a 1mm difference either way is a bunch.
That could possible be part of the gas mileage difference.

Good luck and keep winning the battle
 

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1991 VT600 VLX
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Discussion Starter #20
Roses are red,
Violets are blue.
Front cylinder spark plug
I’m looking at you.

Your float may be wrong,
Or the choke not closing.
I’ll make you bright like rear,
So you can get me going.

But my carbs are no longer OEM,
I don’t know how to write a poem.

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