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New "Slingshort"single carb system not working??

P901 Single carb intake, FLAASH Cams, Super Charger, NOS.......
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New "Slingshort"single carb system not working??

PostAuthor: beltdrive » Sun Sep 15, 2019 9:31 am

Hello All great people in here - - ok you can tell I need your help.

I got tired of my double carburetor stock as I could not make it work right and got a " slingshot" air intake , with equal length intake manifold runners for single CV40mm carburetor, as this take time it has been years in the process and now I finally start to ride it I thought.

First I forgot to wash out my new steel fuel tank, so I filled the carburetor bowl with dirt ( yes I know , I should be experienced enough not to do these things), ok I got the hoses of and shaking the bike a little and got a filter on the hoses and no more debris in the carb bowl.

First run was terrible, lots of puffing and huffing sounds, well I got back doing 10 miles an hour?? OK I think it was fuel starvation, checking all fuel systems for dirt, the carb intake was black??, I think air restriction?

Second run the bike had lots of power to start of after 7 -8 seconds it dies of and can only run slow, I think fuel starvation again, now I have 2 hoses going directly to filter and one hose from filter to carb

Yes, it will idle for 7-8 seconds then it cuts out with puff sound? I took the new filter off, so I can see that it puffs out through the carburetor and dies.
With full choke it will idle longer, but it misfires from time to time in the mufflers?

Now the 2 carbs I had on before my single carb fairytale, went to my “new” 1982 920J that I got from Holland with completely clogged carbs and it runs strong like new, I just had 4 days with 2500 km in Norway on it , now I think about it - - - Did I have an issue already before my single carb fairytale???

Then for the final touch, it has this whistling sound just before the engine stand still, is that no oil on the camshaft?? Or the plastic runners, my 1982 920J do not have this sound ?

WHAT AM I MISSING HERE???? I have a new ignition box and a new rectifier, from an electronic co. in Holland, as well as the new single carb setup, the rest was on the bike last time I had it running well.

Thanks, from Erik
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Re: New "Slingshort"single carb system not working??

PostAuthor: extremeodd » Mon Oct 07, 2019 1:12 pm

That is an awesome looking bike and a super unique intake! The whistling sound you are hearing just before it dies could be a bad vacuum leak, have you tried spraying around the connections with starter fluid/carb cleaner? If the bike revs up or bogs down when you spray, you've found your issue. It would also explain it running for just a moment before finally conking out.

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Re: New "Slingshot"single carb system not working??

PostAuthor: beltdrive » Wed Oct 09, 2019 6:27 am

Vacuum leak - OK I`ll have a look - the 1 in 2 intake tube uses a flange to the front cylinder and a pipe end into the rear cylinder carburetor boot maybe it should be changed to go into carb boots on both cylinders? In order to stop leaking?
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Re: New "Slingshort"single carb system not working??

PostAuthor: extremeodd » Wed Oct 09, 2019 11:01 am

It using a flange vs boot shouldn't matter too much as long as the mating surfaces are flat, clean, and if it uses an O ring or gasket, that it's in good condition.

In the first pcture in the first post, the one labeled "completely sooty", the o-ring gasket looks like it's starting to harden and crack. If that one is in that condition, any others found on the intake run the chance of being similar or in even worse shape which could easily cause a vacuum leak.

The stock boots have essentially the same problem, the o-ring where the boot bolts to the block hardens over time and ends up breaking. This causes all sorts of fun running issues since the extra air coming in leans out the mixture quite a bit. Rereading your inital post makes me lean even more towards this being a simple vacuum leak issue:
has been years in the process and now I finally start to ride it

Sounds like the intake has been sitting a while not attached to the bike, this seems to wreak havoc on o rings
lots of puffing and huffing sounds, well I got back doing 10 miles an hour. the carb intake was black??

Signature sign of running lean. The puffing/huffing is carburetor backfires from the overly lean condition, that's where your soot is coming from.
Second run the bike had lots of power to start of after 7 -8 seconds it dies of and can only run slow

it will idle for 7-8 seconds then it cuts out with puff sound? I took the new filter off, so I can see that it puffs out through the carburetor and dies.
With full choke it will idle longer, but it misfires from time to time in the mufflers?

It idling for a short time is from it being able to pull enough fuel during the initial start then leaning out almost immediately from the air leak. Lean conditions frequently cause both backfires and afterfires (a backfire is when the puff out of the carb is ignited, an after fire is the bang in the muffler. However 99% of the time when someone says backfire, they mean the bang in the exhaust pipe) or carb back fires and exhaust backfires. This is caused from the fuel only getting partially burnt then getting ignited in the exhaust, a hallmark of an overly rich or (in your case) overly lean condition.
It running a bit longer on full choke is because the choke is typically an enrichment circuit that just dumps in more fuel, by using it you are helping to offset the air leak enough to get a few more seconds of idling before the bike dies.
it has this whistling sound just before the engine stand still

The final point of evidence that SCREAMS 'vacuum leak'. This is the sound of air getting sucked in through a tiny gap, I'd bet my bike that if you find the source of that squeak and sprayed it with some starter fluid right after starting that it'll rev way up (or bog way down).

Until you address this and the bike will idle on it's own. DO NOT ATTEMPT TO RIDE IT. Lean air fuel mixture = overly hot burn. Running lean is how engines get destroyed, you're always better off running rich vs lean.

If you want to be completely on the safe side, I'd replace any and all o-rings with fresh ones. Or at the very least, replace any that aren't supple and have visible cracking.

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Re: New "Slingshot"single carb system not working??

PostAuthor: beltdrive » Wed Oct 09, 2019 1:18 pm

[size=200]Bingo - it shows a massive vacuum leak, I have now belt sanded it flatter, and hope I can get it flat enough, as I have no tools to measure the "flatness"
I look forward to get time to start up and test it again
Thanks a lot - I keep you posted
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Re: New "Slingshort"single carb system not working??

PostAuthor: extremeodd » Thu Oct 10, 2019 1:21 am

Oh wow that is a bad leak. Were there any o rings in those grooves? If not, that would be your problem. Take it up to your local hardware store and take a look in the little drawers in the plumbing department for some o rings that will fit. It look like it'll hold 2 rings which is 1 more than a stock boot uses.

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