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  #1  
Old 23rd June 2019
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Serious Blac Serious Blac is offline
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Default Occasional carb spit back, am I missing something?

My bike has always done this since I got it but I've managed to reduce it quite a bit but it still "randomly" happens, often in traffic accelerating moderately. It did it when I had the Mikuni VM38 and it does it with the Keihin CV (That is now very nicely set up). Here's what it IS NOT. Manifold leak, too weak idle jet (did it with a # 50 idle jet in the CVK too!), probably not the cams - Q cams. Have had a look down the inlet ports when the carbs were off and there was no tell tale signs of poor valve sealing (Am about to pull the heads for an overhaul soon anyway). I have Ultima ignition and when I went over to that and a VOES it improved things so I'm taking that as an indication of a timing issue. My home brewed curve with more timing on the VOES curve may have made things SLIGHTLY worse. I was looking into this single fire/dual fire thing and wondering if there was something there, to the point I've ordered a single fire coil. It's a long shot as the dual fire coil appears only to be an issue on long duration cams but the description of what happens when there is a wasted spark on the rear cylinder and a combustable mixture in the chamber seems to match my symptoms. As near as I can see things it's the rear cylinder that spits back. Can anybody suggest anything I'm missing. Oh yeah, new plugs (BPR6HIX 1.1MM gap), standard 3ohm coil. New leads short and straight, no cross over.

Motor specs are: STD pistons, thin head gasket, Q cams, Ultima ignition with VOES and home-brewed curve, Keihin CVK, N65C needle, #46 idle jet, #175 main jet (very slightly rich), S&S filter housing with K&N type fliter.

The bike otherwise behaves very sweetly with good fuel economy and enough grunt when I ask for it. The motor is CONSIDERABLY smoother than when I bought the bike.

It's just this one last thing that is annoying me then I'll leave it alone.

Edit: I promise!
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Old 23rd June 2019
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Quote:
Originally Posted by Serious Blac View Post
My bike has always done this since I got it but I've managed to reduce it quite a bit but it still "randomly" happens, often in traffic accelerating moderately. It did it when I had the Mikuni VM38 and it does it with the Keihin CV (That is now very nicely set up).
<SNIP>

Keihin CVK, N65C needle, #46 idle jet, #175 main jet (very slightly rich)
I see you're not using a HD CV40 carb - the CVK has a coasting enrichener port - This may be playing into the issue...

Here's a link to read over:
https://www.nighthawk750.com/maint/keihincarb.html

That 46 pilot jet might be just a tad on the rich side as most CV40 carbs are happy with a 45...

Does the problem occur just off idle? Or farther up the transition function?

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  #3  
Old 23rd June 2019
The Doctor71 The Doctor71 is offline
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I'm interested in your findings (when you get there) on dual fire vs single fire coil & if that proves beneficial.

In the meantime, I'll share what I do, and don't do, to reduce the carb spit back / farts, on my bikes.

What I don't do......
  • I don't mess with my carb.
  • the 2nd thing I don't do, I don't mess with my carb (there is a message in there )
For some history,
  • I run a Tillotson on my '71 & a Bendix on my '74. Both bikes will exhibit carb farts, occasionally, but it's rare. Should the frequency of carb farts increase, for me, it's a sign the timing is off..... usually triggered by the pre-mature wear of the point rubbing block. I liberally lube the point cam every 400-500 miles to ward this off.
What I do......
  • I meticulously address point gap & timing. No short cuts. And then verify that the timing is on the money, with a timing light. (static timing is never close enough for me.)
    • I know what works for my bikes, & this gets the frequency of carb farts on my bikes back to the "rare" category.
In your situation, I can only suggest:
  • put a timing light on your Ultima ignition & VERIFY the timing is where you think it is.
And if a single fire coil eliminates this phenomena for you ..... I'd like to hear about it
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Old 23rd June 2019
gpydeski gpydeski is offline
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I have been chasing that monster for 15 years on my 70 xlch and my 77 shovel. I have changed carbs changed heads, rejetted, adjusted timing and changed points/ condenser to till I turned blue. These 2 engines are 49 and 42 years old and both run from point a to point b. There is bound be some Orange County choppers wanna be crack mechanic on this side who is gonna say his bike never misses a beat. Ignore him and except the occasional spit or backfire as a blessing because it is only once in a while and not every block.
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Old 23rd June 2019
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Hmmmm, not going to give in that easy. It's just another engine, all be it with an unconventional layout. It would seem this is not an uncommon phenomenon. In which case I'm probably on the right track looking at the ignition. If it isn't that then I would think the inlet arrangement would be the culprit. One that I can't do much about though.

Having had a good look at the inlet tract and having read a bit about tuning this engine the inlet tract is too large and it's route is a bit tortuous. Probably conducive to flow reversal in the inlet track so maybe my plan for a D shaped port might help things. We'll see, it will be a couple of weeks at least before I will get to take the heads off and have a play so I'll probably wait until after trying the single fire coil.

Of course it could also be caused by pressure waves flying about the intake side. In which case I can't do much more than experiment with bellmouth length. Anyway, time for another beer......
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Old 2nd July 2019
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OK, new single fire fitted today. Piece of piss to fit until you come to fit the tank and find it won't fit over the coil in the standard place. A quick look in the manuals showed that the Evo has the coil turned 90 degrees so I cobbled up a couple of brackets to do that and lower it about 20mm. I'll try and source the Evo bracket. I'm using exactly the same plugs and leads - everything. This is so I get a valid comparison with nothing else changed.
The bike has done a couple of thousand mile since I last did any maintenance so it now needs a service. This was deliberate as an out-of-tune bike shows up faults easier. The result? Better, about the same as if you adjust the valves and oil the chain. Nicer at low speeds but I don't expect any more top end. General sweeter with slightly better throttle response at low speeds. Nothing spectacular but If I were to buy a kit for another bike I'd go for the single fire. The indications so far are that there is still a fart but it isn't as bad, could be the bike was still a bit cold. I'll be using the bike regularly for the next week then I'll do a service next week before riding over to the other side of Norway to visit the Mrs. I will off course update this thread.
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Old 2nd July 2019
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I think the occasional carb fart is just the nature of the beast. My Ironhead with a VM38-9 is precisely tuned to pretty much a 1 kick bike and it runs nearly perfect.
I have noticed that the occasional carb farts occur more often with cheaper gas from the grocery store pumps. I try to stick with Chevron or Shell. 92 octane is the highest I can get here in the Seattle area. Considering the original manual calls for minimum 95 octane I always thought the issue was from the gas.
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Old 3rd July 2019
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Thanks, I haven't noticed a difference with 98, here 95 is normal. I'll try this after I've serviced the bike
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Old 3rd July 2019
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Took the bike to work so first run from stone cold. A lot sweeter when cold and the instability before the engine warms is almost gone. When warm the idle speed was lower and the exhaust smelt different. Less sharp. When I got to work I re-adjusted the idle mixture. About 1/3 out brought the idle back up. Will be interested to see how the town fuel economy works out. The best news is no spit backs - so far. I wonder if anybody has ever done part throttle Dyno runs comparing dual and single fire coils?
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Old 3rd July 2019
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Yes, Hammer has done just that so you can find info in their area of the site.

Sent with :-)
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